U.S. patent application number 12/748266 was filed with the patent office on 2010-12-30 for molecular semiconductors containing diketopyrrolopyrrole and dithioketopyrrolopyrrole chromophores for small molecule or vapor processed solar cells.
Invention is credited to Tyler Kent, Chunki Kim, Thuc-Quyen NGUYEN, Arnold Bernarte Tamayo, Mananya Tantiwiwat, Bright Walker.
Application Number | 20100326525 12/748266 |
Document ID | / |
Family ID | 43379421 |
Filed Date | 2010-12-30 |
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United States Patent
Application |
20100326525 |
Kind Code |
A1 |
NGUYEN; Thuc-Quyen ; et
al. |
December 30, 2010 |
MOLECULAR SEMICONDUCTORS CONTAINING DIKETOPYRROLOPYRROLE AND
DITHIOKETOPYRROLOPYRROLE CHROMOPHORES FOR SMALL MOLECULE OR VAPOR
PROCESSED SOLAR CELLS
Abstract
Optoelectronic devices, such as photovoltaic devices, comprising
a low band gap, solution processable diketopyrrolopyrrole or
dithioketopyrrolopyrrole chromophore core or cores are disclosed.
Also disclosed are methods of fabricating such optoelectronic
devices.
Inventors: |
NGUYEN; Thuc-Quyen;
(Glendale, CA) ; Tamayo; Arnold Bernarte; (Golden,
CO) ; Walker; Bright; (Goleta, CA) ; Kent;
Tyler; (Newport Beach, CA) ; Kim; Chunki;
(Goleta, CA) ; Tantiwiwat; Mananya; (Goleta,
CA) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
755 PAGE MILL RD
PALO ALTO
CA
94304-1018
US
|
Family ID: |
43379421 |
Appl. No.: |
12/748266 |
Filed: |
March 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61163789 |
Mar 26, 2009 |
|
|
|
Current U.S.
Class: |
136/263 ;
977/734 |
Current CPC
Class: |
B82Y 10/00 20130101;
H01L 51/0071 20130101; H01L 51/0052 20130101; H01L 51/0036
20130101; H01L 51/0062 20130101; H01L 51/4253 20130101; H01L
51/0047 20130101; H01L 51/442 20130101; H01L 51/0046 20130101; H01L
51/0037 20130101; H01L 51/0072 20130101; Y02E 10/549 20130101; H01L
51/0068 20130101 |
Class at
Publication: |
136/263 ;
977/734 |
International
Class: |
H01L 31/04 20060101
H01L031/04 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with Government support under grants
N000140510677 and N000140811226 awarded by the Office of Naval
Research, and grant DE-FG02-06ER46324 awarded by the Department of
Energy. The Government has certain rights in this invention.
Claims
1. A optoelectronic device comprising: a) a first hole-collecting
electrode; b) an optional hole-transporting layer; c) a layer
comprising a mixture of an electron donor material and an electron
acceptor material; and d) a second electron-collecting electrode,
wherein the electron donor material comprises a compound of Formula
(I): ##STR00083## wherein X is oxygen or sulfur; A.sub.1 and
A.sub.2 are independently selected from substituted and
unsubstituted aryl or heteroaryl groups, wherein each individual
A.sub.1 within the (A.sub.1).sub.m moiety can be independently
selected from a substituted or unsubstituted aryl or heteroaryl
group, and each individual A.sub.2 within the (A.sub.2).sub.n
moiety can be independently selected from a substituted or
unsubstituted aryl or heteroaryl group; B.sub.1 is independently
selected from substituted and unsubstituted aryl or heteroaryl
groups; m is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or
9; n is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; p
is independently selected from 0 or 1; E.sub.1 and E.sub.2 are
independently selected from a nonentity, H, or a substituted or
unsubstituted aryl or heteroaryl group or a C.sub.1-C.sub.12 alkyl
group; and R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are independently
selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
2. The device of claim 1, wherein X is oxygen.
3. The device of claim 1, wherein p is 1.
4. The device of claim 1, wherein A.sub.1 and A.sub.2 are
independently selected from the group consisting of thiophene,
bithiophene, terthiophene, thienothiophene, dithienothiophene,
benzothiophene, isobenzothiophene, benzodithiophene,
cyclopentadithiophene, indole, benzene, naphthalene, anthracene,
indene, fluorene, pyrene, azulene, furan, pyrrole, pyridine,
oxazole, thiazole, thiazine, pyrimidine, pyrazine, imidazole,
benzoxazole, benzoxadiazole, benzothiazole, benzimidazole,
benzofuran, isobenzofuran, thiadiazole, pyridothiadiazole, and
carbazole.
5. The device of claim 1, wherein B.sub.1 is independently selected
from the group consisting of thiophene, bithiophene, terthiophene,
thienothiophene, dithienothiophene, benzothiophene,
isobenzothiophene, benzodithiophene, cyclopentadithiophene, indole,
benzene, naphthalene, anthracene, indene, fluorene, pyrene,
azulene, furan, pyrrole, pyridine, oxazole, thiazole, thiazine,
pyrimidine, pyrazine, imidazole, benzoxazole, benzoxadiazole,
benzothiazole, benzimidazole, benzofuran, isobenzofuran,
thiadiazole, pyridothiadiazole, carbazole, ##STR00084##
6. The device of claim 1, wherein A.sub.1 is selected from
##STR00085## where m is independently 1, 2, or 3, and A.sub.2 is
selected from ##STR00086## where n is independently 1, 2, or 3.
7. The device of claim 1, wherein m=n=1, m=n=2, or m=n=3.
8. The device of claim 1, wherein R.sub.1, R.sub.2, R.sub.3, and
R.sub.4 are the same and are selected from hexyl, 2-ethylhexyl,
t-butoxycarbonyl, and trimethylsilyl.
9. The device of claim 1, wherein the first electrode comprises
indium tin oxide and the second electrode comprises aluminum,
silver or magnesium.
10. The device of claim 1, wherein the optional hole transporting
layer is present and comprises poly(3,4-ethylene
dioxythiophene:poly(styrenesulfonate) (PEDOT:PSS).
11. The device of claim 1, wherein the electron acceptor is
selected from the group consisting of fullerene electron acceptors,
[6,6]-phenyl C61-butyric acid methyl ester (PCBM), PC.sub.71BM,
titanium dioxide, and zinc oxide.
12. A optoelectronic device comprising: a) a first hole-collecting
electrode; b) an optional hole-transporting layer; c) a layer
comprising a mixture of an electron donor material and an electron
acceptor material; and d) a second electron-collecting electrode,
wherein the electron donor material comprises a compound of Formula
(II): ##STR00087## wherein X is oxygen or sulfur; A.sub.1 and
A.sub.2 are selected from substituted and unsubstituted aryl or
heteroaryl groups, wherein each individual A.sub.1 within the
(A.sub.1).sub.m moiety can be independently selected from a
substituted or unsubstituted aryl or heteroaryl group, and each
individual A.sub.2 within the (A.sub.2).sub.n moiety can be
independently selected from a substituted and unsubstituted aryl or
heteroaryl group; m is independently selected from 1, 2, 3, 4, 5,
6, 7, 8, or 9; n is independently selected from 1, 2, 3, 4, 5, 6,
7, 8, or 9; E.sub.1 and E.sub.2 are independently selected from a
nonentity, H, or a substituted or unsubstituted aryl or heteroaryl
group or a C.sub.1-C.sub.12 alkyl group; and R.sub.1 and R.sub.2
are independently selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
13. The device of claim 12, wherein X is oxygen.
14. The device of claim 12, wherein A.sub.1 and A.sub.2 are
independently selected from the group consisting of thiophene,
bithiophene, terthiophene, thienothiophene, dithienothiophene,
benzothiophene, isobenzothiophene, benzodithiophene,
cyclopentadithiophene, indole, benzene, naphthalene, anthracene,
indene, fluorene, pyrene, azulene, furan, pyrrole, pyridine,
oxazole, thiazole, thiazine, pyrimidine, pyrazine, imidazole,
benzoxazole, benzoxadiazole, benzothiazole, benzimidazole,
benzofuran, isobenzofuran, thiadiazole, pyridothiadiazole, and
carbazole.
15. The device of claim 12, wherein A.sub.1 is selected from
##STR00088## where m is independently 1, 2, or 3, and A.sub.2 is
selected from ##STR00089## where n is independently 1, 2, or 3.
16. The device of claim 12, wherein m=n=1, m=n=2, or m=n=3.
17. The device of claim 12, wherein R.sub.1 and R.sub.2 are the
same and are selected from hexyl, 2-ethylhexyl, t-butoxycarbonyl,
and trimethylsilyl.
18. The device of claim 12, wherein the first electrode comprises
indium tin oxide and the second electrode comprises aluminum,
silver or magnesium.
19. The device of claim 12, wherein the optional hole transporting
layer is present and comprises poly(3,4-ethylene
dioxythiophene:poly(styrenesulfonate) (PEDOT:PSS).
20. The device of claim 12, wherein the electron acceptor is
selected from the group consisting of fullerene electron acceptors,
[6,6]-phenyl C61-butyric acid methyl ester (PCBM), PC.sub.71BM,
titanium dioxide, and zinc oxide.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority benefit of U.S. Provisional
Patent Application No. 61/163,789, filed Mar. 26, 2009. The entire
content of that application is hereby incorporated by reference
herein in its entirety.
BACKGROUND OF THE INVENTION
[0003] Solar cells based on organic semiconductors are evolving
into a promising cost-effective alternative to silicon-based solar
cells due to low-cost fabrication by solution-processing, ease of
processing, light weight, and compatibility with flexible
substrates. Gunes et al. Chem. Rev. 2007, 107, 1324; Roncali, J.,
Acc. Chem. Res. 2009, 42, 1719. Devices based on these materials
were predicted to have an efficiency of 10% based on theoretical
models. Scharber et al. J. Adv. Mater. 2006, 18, 789. For instance,
solution processed bulk heterojunction (BHJ) solar cells using
poly(3-hexylthiophene) (P3HT) as the electron donor and
[6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the electron
acceptor have been reported to have efficiencies as high as 5.2%.
Kim et al. Nature Mater. 2006, 5, 197. Although this system is
capable of yielding some of the highest reported efficiencies among
organic photovoltaics, it suffers from two significant
shortcomings: i) the P3HT-PCBM mixture does not absorb light with
wavelengths longer than about 650 nm, and thus a large fraction of
sunlight is wasted in this system; and ii) the P3HT must have a
relatively high molecular weight and very high regioregularity.
Most commercially available P3HT yields BHJ devices with
efficiencies much lower than 5%. Synthesizing "good" P3HT can be
problematic as the consistency between different batches of P3HT
can vary.
[0004] Recently, there has been a significant effort to address the
light absorption problem, and a large number of materials have been
designed which have low band gaps and are able to absorb light at
wavelengths up to 800 or 900 nm. Although such materials are
successfully able to absorb a large fraction of the solar spectrum,
most of them do not yield efficient BHJ photovoltaics due to
inadequate electrical properties such as low charge carrier
mobilities, incorrectly aligned HOMO and LUMO levels, or poor solid
state morphologies.
[0005] The most successful low bandgap material reported as of
early 2009 has been
poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dith-
iophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). In this case,
photovoltaics with efficiencies of close to 6% have been reported.
However, the material is polymeric and there have been problems
reproducing results using PCPDTBT from different sources.
[0006] The absorption problem can also be solved by fabricating
"tandem" solar cells, in which two BHJ photovoltaics are stacked on
top of each other, with each layer absorbing a different fraction
of the solar spectrum. Efficiencies as high as 6.5% have been
reported using this type of system; however, fabricating two
photovoltaic layers on top of each other adds a significant amount
of complexity to the device architecture and difficulty in
processing.
[0007] One material that has been used as an organic semiconductor
in electronic devices is a diketopyrrolopyrrole-based polymer;
efficiencies as high as 4.7% were reported for this material. See
U.S. Patent Application Publication No. 2007/0228359; see also WO
2008/000664.
[0008] Solution-processed solar cells based on conjugated small
molecule donors and fullerene acceptors have also been investigated
by a number of research groups. As of March 2009, small
molecule-based solar cell devices have power conversion
efficiencies ranging from 0.3% to 1.3%. Schmidt-Mende et al.
Science 2001, 293, 1119. These efficiencies remain low when
compared to either thermally deposited small molecule bilayer solar
cells or polymer-based solar cells. Despite the lower performance
attained thus far, conjugated small molecules can potentially offer
several advantages over polymeric materials, making them promising
materials for solution-processed solar cells. For example,
molecular organic semiconductors, such as oligothiophenes and
oligoacenes, display higher hole and electron mobilities than their
polymeric analogues as a result of better molecular ordering. Most
important, conjugated small molecules do not suffer from batch to
batch variations, broad molecular weight distributions, end group
contamination, and difficult purification methods, as is the
situation for their polymeric counterparts.
[0009] These devices have distinct advantages over previous
optoelectronic devices. First, the material is not a polymer,
making the synthesis and purification easier and more repeatable
than for similar conjugated polymers. Second, the material has
desirable electronic properties such as favorable HOMO level, LUMO
level, and hole mobility (.about.10.sup.-4 cm.sup.2/Vs). Third, the
material has desirable optical properties, including a very high
optical density and good overlap with the solar spectrum in the
solid state. Fourth, the efficiencies observed in photovoltaics
fabricated using this system are higher than those previously
reported for solution-processed, non-polymeric photovoltaics.
Oligothiophene derivatives functionalized with a
diketopyrrolopyrrolo core have been used in solution-processed
field effect transistors (Tantiwiwat et al., J. Phys. Chem. C,
112:17402 (2008)).
BRIEF SUMMARY OF THE INVENTION
[0010] The present invention is directed to an optoelectronic
device comprising of a mixture of a non-polymeric electron-donating
chromophore based on a diketopyrrolopyrrole (DPP) structure, as
well as an electron-accepting material such as a fullerene or
methanofullerene, and a method of fabricating the device by
solution processing. Also disclosed are non-polymeric electron
donating chromophores based on the DPP structure.
[0011] In one embodiment, the invention embraces an optoelectronic
device comprising a) a first hole-collecting electrode; b) an
optional hole-transporting layer; c) a layer comprising a mixture
of an electron donor material and an electron acceptor material;
and d) a second electron-collecting electrode, wherein the electron
donor material comprises a compound of the following general
Formula I:
##STR00001##
wherein X is oxygen or sulfur; A.sub.1 and A.sub.2 are
independently selected from substituted and unsubstituted aryl or
heteroaryl groups, wherein each individual A.sub.1 within the
(A.sub.1).sub.m moiety can be independently selected from a
substituted or unsubstituted aryl or heteroaryl group, and each
individual A.sub.2 within the (A.sub.2).sub.n moiety can be
independently selected from a substituted or unsubstituted aryl or
heteroaryl group; B.sub.1 is independently selected from
substituted and unsubstituted aryl or heteroaryl groups; m is
independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; n is
independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; p is
independently selected from 0 or 1; E.sub.1 and E.sub.2 are
independently selected from a nonentity, H, or a substituted or
unsubstituted aryl or heteroaryl group or a C.sub.1-C.sub.12 alkyl
group; and R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are independently
selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
[0012] In one embodiment, the device is a photovoltaic device.
[0013] In one embodiment, X is oxygen. In another embodiment, X is
sulfur.
[0014] In one embodiment, A.sub.1 and A.sub.2 are independently
selected from the group consisting of thiophene, bithiophene,
terthiophene, thienothiophene, dithienothiophene, benzothiophene,
isobenzothiophene, benzodithiophene, cyclopentadithiophene, indole,
benzene, naphthalene, anthracene, indene, fluorene, pyrene,
azulene, furan, pyrrole, pyridine, oxazole, thiazole, thiazine,
pyrimidine, pyrazine, imidazole, benzoxazole, benzoxadiazole,
benzothiazole, benzimidazole, benzofuran, isobenzofuran,
thiadiazole, pyridothiadiazole, and carbazole. In another
embodiment, A.sub.1 and A.sub.2 are thiophene. In another
embodiment, the thiophene moieties bonded to the
diketopyrrolopyrrole core are bonded to the core via the 2-position
of the thiophene. In another embodiment, the thiophene moieties
bonded to the E.sub.1 and E.sub.2 moieties are bonded to the
E.sub.1 and E.sub.2 moieties via the 5-position of the thiophenes.
In another embodiment, the thiophene moieties bonded to the
diketopyrrolopyrrole core are bonded to the remainder of the
molecule via the 5-position of the thiophene, the thiophene
moieties bonded to the E.sub.1 and E.sub.2 moieties are bonded to
the remainder of the molecule via the 2-position of the thiophenes,
and, if either or both of m and n are greater than 1, any
thiophene-thiophene bond is bonded via a bond between the 2- and
5-positions of the thiophenes.
[0015] In one embodiment, m=n=1. In another embodiment, m=n=2. In
another embodiment, m=n=3.
[0016] In another embodiment, A.sub.1 is selected from
##STR00002##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00003##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00004##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00005##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00006##
where m is 1, and A.sub.2 is selected from
##STR00007##
where n is 1; or where m=n=2, or where m=n=3.
[0017] In another embodiment, A.sub.1 is selected from
##STR00008##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00009##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00010##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00011##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00012##
where m is 1, and A.sub.2 is selected from
##STR00013##
where n is 1; or where m=n=2, or where m=n=3.
[0018] In one embodiment, B.sub.1 is independently selected from
the group consisting of thiophene, bithiophene, terthiophene,
thienothiophene, dithienothiophene, benzothiophene,
isobenzothiophene, benzodithiophene, cyclopentadithiophene, indole,
benzene, naphthalene, anthracene, indene, fluorene, pyrene,
azulene, furan, pyrrole, pyridine, oxazole, thiazole, thiazine,
pyrimidine, pyrazine, imidazole, benzoxazole, benzoxadiazole,
benzothiazole, benzimidazole, benzofuran, isobenzofuran,
thiadiazole, pyridothiadiazole, carbazole,
##STR00014##
where the bonds ending in wavy lines indicate the attachment of the
B.sub.1 moiety to the remainder of the molecule.
[0019] In one embodiment, R.sub.1, R.sub.2, R.sub.3, and R.sub.4
are the same and are selected from hexyl, 2-ethylhexyl,
t-butoxycarbonyl, and trimethylsilyl.
[0020] In one embodiment, the invention embraces an optoelectronic
device comprising a) a first hole-collecting electrode; b) an
optional hole-transporting layer; c) a layer comprising a mixture
of an electron donor material and an electron acceptor material;
and d) a second electron-collecting electrode, wherein the electron
donor material comprises a compound of the following general
Formula II:
##STR00015##
wherein X is oxygen or sulfur; A.sub.1 and A.sub.2 are selected
from substituted and unsubstituted aryl or heteroaryl groups,
wherein each individual A.sub.1 within the (A.sub.1).sub.m moiety
can be independently selected from a substituted or unsubstituted
aryl or heteroaryl group, and each individual A.sub.2 within the
(A.sub.2).sub.n moiety can be independently selected from a
substituted and unsubstituted aryl or heteroaryl group; m is
independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; n is
independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; E.sub.1
and E.sub.2 are independently selected from a nonentity, H, or a
substituted or unsubstituted aryl or heteroaryl group or a
C.sub.1-C.sub.12 alkyl group; and R.sub.1 and R.sub.2 are
independently selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
[0021] In one embodiment, the device is a photovoltaic device.
[0022] In one embodiment, X is oxygen. In another embodiment, X is
sulfur.
[0023] In one embodiment, A.sub.1 and A.sub.2 are independently
selected from the group consisting of thiophene, bithiophene,
terthiophene, thienothiophene, dithienothiophene, benzothiophene,
isobenzothiophene, benzodithiophene, cyclopentadithiophene, indole,
benzene, naphthalene, anthracene, indene, fluorene, pyrene,
azulene, furan, pyrrole, pyridine, oxazole, thiazole, thiazine,
pyrimidine, pyrazine, imidazole, benzoxazole, benzoxadiazole,
benzothiazole, benzimidazole, benzofuran, isobenzofuran,
thiadiazole, pyridothiadiazole, and carbazole. In another
embodiment, A.sub.1 and A.sub.2 are thiophene. In another
embodiment, the thiophene moieties bonded to the
diketopyrrolopyrrole core are bonded to the core via the 2-position
of the thiophene. In another embodiment, the thiophene moieties
bonded to the E.sub.1 and E.sub.2 moieties are bonded to the
E.sub.1 and E.sub.2 moieties via the 5-position of the thiophenes.
In another embodiment, the thiophene moieties bonded to the
diketopyrrolopyrrole core are bonded to the remainder of the
molecule via the 5-position of the thiophene, the thiophene
moieties bonded to the E.sub.1 and E.sub.2 moieties are bonded to
the remainder of the molecule via the 2-position of the thiophenes,
and, if either or both of m and n are greater than 1, any
thiophene-thiophene bond is bonded via a bond between the 2- and
5-positions of the thiophenes.
[0024] In one embodiment, m=n=1. In another embodiment, m=n=2. In
another embodiment, m=n=3.
[0025] In another embodiment, A.sub.1 is selected from
##STR00016##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00017##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00018##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00019##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00020##
where m is 1, and A.sub.2 is selected from
##STR00021##
where n is 1; or where m=n=2, or where m=n=3.
[0026] In another embodiment, A.sub.1 is selected from
##STR00022##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00023##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00024##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00025##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00026##
where m is 1, and A.sub.2 is selected from
##STR00027##
where n is 1; or where m=n=2, or where m=n=3.
[0027] In one embodiment, R.sub.1 and R.sub.2 are the same and are
selected from hexyl, 2-ethylhexyl, t-butoxycarbonyl, and
trimethylsilyl.
[0028] In one embodiment, the invention embraces an optoelectronic
device comprising a) a first hole-collecting electrode; b) an
optional hole-transporting layer; c) a layer comprising a mixture
of an electron donor material and an electron acceptor material;
and d) a second electron-collecting electrode, wherein the electron
donor material comprises a compound of the following general
Formula IIa:
##STR00028##
wherein X is oxygen or sulfur; m is independently selected from 1,
2, 3, 4, 5, 6, 7, 8, or 9; n is independently selected from 1, 2,
3, 4, 5, 6, 7, 8, or 9; E.sub.1 and E.sub.2 are independently
selected from a nonentity, H, or a substituted or unsubstituted
aryl or heteroaryl group or a C.sub.1-C.sub.12 alkyl group; and
R.sub.1 and R.sub.2 are independently selected from H,
C.sub.1-C.sub.12 alkyl, and --C(.dbd.O)--O--C.sub.1-C.sub.12
alkyl.
[0029] In one embodiment, the device is a photovoltaic device.
[0030] In one embodiment, X is oxygen. In another embodiment, X is
sulfur.
[0031] In one embodiment, m=n=1. In another embodiment, m=n=2. In
another embodiment, m=n=3.
[0032] In another embodiment, A.sub.1 is selected from
##STR00029##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00030##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00031##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00032##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00033##
where m is 1, and A.sub.2 is selected from
##STR00034##
where n is 1; or where m=n=2, or where m=n=3.
[0033] In another embodiment, A.sub.1 is selected from
##STR00035##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00036##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00037##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00038##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00039##
where m is 1, and A.sub.2 is selected from
##STR00040##
where n is 1; or where m=n=2, or where m=n=3.
[0034] In one embodiment, R.sub.1 and R.sub.2 are the same and are
selected from hexyl, 2-ethylhexyl, t-butoxycarbonyl, and
trimethylsilyl.
[0035] In one embodiment, the invention embraces an optoelectronic
device comprising a) a first hole-collecting electrode; b) an
optional hole-transporting layer; c) a layer comprising a mixture
of an electron donor material and an electron acceptor material;
and d) a second electron-collecting electrode, wherein the electron
donor material comprises a compound of the following general
Formula IIb:
##STR00041##
wherein m is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or
9; n is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9;
E.sub.1 and E.sub.2 are independently selected from a nonentity, H,
or a substituted or unsubstituted aryl or heteroaryl group or a
C.sub.1-C.sub.12 alkyl group; and R.sub.1 and R.sub.2 are
independently selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
[0036] In one embodiment, the device is a photovoltaic device.
[0037] In one embodiment, m=n=1. In another embodiment, m=n=2. In
another embodiment, m=n=3.
[0038] In another embodiment, A.sub.1 is selected from
##STR00042##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00043##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00044##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00045##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00046##
where m is 1, and A.sub.2 is selected from
##STR00047##
where n is 1; or where m=n=2, or where m=n=3.
[0039] In another embodiment, A.sub.1 is selected from
##STR00048##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00049##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00050##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00051##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00052##
where m is 1, and A.sub.2 is selected from
##STR00053##
where n is 1; or where m=n=2, or where m=n=3.
[0040] In one embodiment, R.sub.1 and R.sub.2 are the same and are
selected from hexyl, 2-ethylhexyl, t-butoxycarbonyl, and
trimethylsilyl.
[0041] In one embodiment, the invention embraces an optoelectronic
device comprising a) a first hole-collecting electrode; b) an
optional hole-transporting layer; c) a layer comprising a mixture
of an electron donor material and an electron acceptor material;
and d) a second electron-collecting electrode, wherein the electron
donor material comprises a compound of the following general
Formula IIc:
##STR00054##
wherein m is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or
9; n is independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9;
E.sub.1 and E.sub.2 are independently selected from a nonentity, H,
or a substituted or unsubstituted aryl or heteroaryl group or a
C.sub.1-C.sub.12 alkyl group; and R.sub.1 and R.sub.2 are
independently selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
[0042] In one embodiment, the device is a photovoltaic device.
[0043] In one embodiment, m=n=1. In another embodiment, m=n=2. In
another embodiment, m=n=3.
[0044] In another embodiment, A.sub.1 is selected from
##STR00055##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00056##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00057##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00058##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00059##
where m is 1, and A.sub.2 is selected from
##STR00060##
where n is 1; or where m=n=2, or where m=n=3.
[0045] In another embodiment, A.sub.1 is selected from
##STR00061##
where m is 1, 2, or 3. In another embodiment, A.sub.2 is selected
from
##STR00062##
where n is 1, 2, or 3. In another embodiment, A.sub.1 is selected
from
##STR00063##
where m is independently 1, 2, or 3, and A.sub.2 is selected
from
##STR00064##
where n is independently 1, 2, or 3. In another embodiment, A.sub.1
is selected from
##STR00065##
where m is 1, and A.sub.2 is selected from
##STR00066##
where n is 1; or where m=n=2, or where m=n=3.
[0046] In one embodiment, R.sub.1 and R.sub.2 are the same and are
selected from hexyl, 2-ethylhexyl, t-butoxycarbonyl, and
trimethylsilyl.
[0047] In one embodiment of any of the above devices, the first
electrode comprises indium tin oxide.
[0048] In one embodiment of any of the above devices, the optional
hole transporting layer is present and comprises poly(3,4-ethylene
dioxythiophene:poly(styrenesulfonate) (PEDOT:PSS).
[0049] In one embodiment of any of the above devices, the electron
acceptor is a fullerene electron acceptor. In another embodiment of
any of the above devices, the fullerene electron acceptor is
[6,6]-phenyl C61-butyric acid methyl ester (PCBM). In another
embodiment of any of the above devices, the fullerene electron
acceptor is PC.sub.71BM.
[0050] In one embodiment of any of the above devices, the electron
acceptor is a vinazene, a perylenetetracaroxylicacid-dianhydrides,
or a perylenetetracaroxylicacid-diimide.
[0051] In one embodiment of any of the above devices, the electron
acceptor is an inorganic acceptor selected from TiO.sub.2 (titanium
dioxide) and ZnO (zinc oxide). In one embodiment of any of the
above devices, the electron acceptor is TiO.sub.2 (titanium
dioxide). In one embodiment of any of the above devices, the
titanium dioxide is anatase. In another embodiment of any of the
above devices, the titanium dioxide is rutile. In another
embodiment of any of the above devices, the titanium dioxide is
amorphous. In one embodiment of any of the above devices, the
titanium dioxide is prepared by depositing a sol-gel precursor
solution, for example by spincasting or doctorblading, and
sintering at a temperature between about 300.degree. C. and
500.degree. C. The precursor can comprise titanium isopropoxide or
another titanium alkoxide, or a mixture of titanium alkoxides.
[0052] In one embodiment of any of the above devices, the electron
donor:electron acceptor ratio is between about 3:7 to about 7:3
parts by mass.
[0053] In one embodiment of any of the above devices, the layer
comprising a mixture of an electron donor material and an electron
acceptor material is about 50 nm to about 150 nm in thickness.
[0054] In one embodiment of any of the above devices, the second
electrode comprises aluminum, silver or magnesium.
[0055] In one embodiment of any of the above devices, the first
electrode can comprise Au or a material having a work function
higher than the work function of the second electrode. In one
embodiment of any of the above devices, the second electrode can
comprise an ITO substrate modified using a self-assembled monolayer
of 3-aminopropyltrimethoxysiloxane or a material having a work
function lower than the work function of the first electrode. In
one embodiment of any of the above devices, the first electrode can
comprise Au or a material having a work function higher than the
work function of the second electrode, and the second electrode can
comprise an ITO substrate modified using a self-assembled monolayer
of 3-aminopropyltrimethoxysiloxane or a material having a work
function lower than the work function of the first electrode.
[0056] In one embodiment of any of the above devices, the device
exhibits an external quantum efficiency of about 25% or greater
between the wavelengths of about 550 nm to about 750 nm.
[0057] In one embodiment of any of the above devices, the electron
donor:electron acceptor layer is cast from a solution comprising a
solvent and the electron donor and the electron acceptor. In
various embodiments of any of the above devices, the solvent can
comprise chloroform, thiophene, trichloroethylene, or carbon
disulfide, or a mixture of any of the foregoing solvents.
[0058] In another embodiment, the invention embraces a compound of
Formula I.
[0059] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula I
[0060] In another embodiment, the invention embraces a compound of
Formula I where p=0.
[0061] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula I where p=0.
[0062] In another embodiment, the invention embraces a compound of
Formula I where p=1.
[0063] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula I where p=1.
[0064] In another embodiment, the invention embraces a compound of
Formula II.
[0065] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula II.
[0066] In another embodiment, the invention embraces a compound of
Formula IIa.
[0067] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula IIa.
[0068] In another embodiment, the invention embraces a compound of
Formula IIb.
[0069] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula IIb.
[0070] In another embodiment, the invention embraces a compound of
Formula IIc.
[0071] In another embodiment, the invention embraces an electronic
device comprising a compound of Formula IIc.
[0072] In another embodiment, the invention embraces compound
A:
##STR00067##
[0073] In another embodiment, the invention embraces an electronic
device comprising the compound A.
[0074] In another embodiment, the invention embraces
2,5-di-(2-ethylhexyl)-3,6-bis-(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)-
-pyrrolo[3,4-c]pyrrole-1,4-dione (SMDPPEH):
##STR00068##
[0075] In another embodiment, the invention embraces an electronic
device comprising the compound SMDPPEH.
[0076] In another embodiment, the invention embraces compound
(4):
##STR00069##
[0077] In another embodiment, the invention embraces an electronic
device comprising the compound 4.
[0078] In another embodiment, the invention embraces the compound
C6PT1C6.
[0079] In another embodiment, the invention embraces an electronic
device comprising the compound C6PT1C6.
[0080] In another embodiment, the invention embraces the compound
C6PT2C6.
[0081] In another embodiment, the invention embraces an electronic
device comprising the compound C6PT2C6.
[0082] In another embodiment, the invention embraces the compound
C6PT3C6.
[0083] In another embodiment, the invention embraces an electronic
device comprising the compound C6PT3C6.
[0084] In another embodiment, the invention embraces the compound
EHPT2C6.
[0085] In another embodiment, the invention embraces an electronic
device comprising the compound EHPT2C6.
[0086] In another embodiment, the invention embraces the compound
C6PT2.
[0087] In another embodiment, the invention embraces an electronic
device comprising the compound C6PT2.
[0088] In another embodiment, the invention embraces the compound
26 of Example 11.
[0089] In another embodiment, the invention embraces an electronic
device comprising the compound 26 of Example 11.
[0090] In another embodiment, the invention embraces the compound
36 of Example 12.
[0091] In another embodiment, the invention embraces an electronic
device comprising the compound 36 of Example 12.
BRIEF DESCRIPTION OF THE FIGURES
[0092] FIG. 1 depicts the architecture of one embodiment of the
devices of the invention.
[0093] FIG. 2 depicts the solution and film absorption of compound
(4) in solution and on a quartz substrate.
[0094] FIG. 3 depicts tapping mode AFM (10 .mu.m.times.10 .mu.m)
topographical (a) and phase (b) images of pristine film of (4)
spin-coated from chloroform on quartz substrate.
[0095] FIG. 4 depicts absorption of blends containing compound (4)
and PCBM in various ratios, spin-coated on quartz from chloroform
solutions with a 2% (w/v) total concentration.
[0096] FIG. 5 depicts J-V characteristics of organic solar cells
prepared from compound (4):PCBM blends: 30:70 (squares), 50:50
(triangles) and 70:30 (circles) under AM 1.5 irradiation (100
mW/cm.sup.2).
[0097] FIG. 6 depicts external quantum efficiency (EQE) curve for a
device using 70:30 blend of (4):PCBM.
[0098] FIG. 7 depicts AFM topography (a, c and e) and phase images
(b, d and f) of actual devices fabricated from films spun cast from
(4):PCBM blends with the following ratios 30:70 (a,b), 50:50 (c,d)
and 70:30 (e,f).
[0099] FIG. 8 depicts a) optical absorption spectra for SMDPPEH in
a chloroform solution (dotted line) and in a thin film (solid line)
and C.sub.71-PCBM film (dashed line); (b) Optical absorption
spectra of SMDPPEH:C.sub.71-PCBM at various ratios: 30:70 (open
squares), 50:50 (open circles), and 70:30 (open diamonds).
[0100] FIG. 9 depicts (a) current density-voltage characteristics
and (b) EQE of BHJ solar cells as a function of
SMDPPEH:C.sub.71-PCBM donor-acceptor ratio: 30:70 (open squares),
50:50 (open circles), and 70:30 (open diamonds).
[0101] FIG. 10 depicts AFM topographic (a,c,e) and phase (b,d,f)
images of SMDPPEH:C.sub.71-PCBM films spun from 2% (w/v) solution
at various ratios: 30:70 (a and b), 50:50 (c and d), and 70:30 (e
and f). Images are 500 nm.times.500 nm in size.
[0102] FIG. 11 depicts ultra-violet visible absorption spectra. a,
Pure materials in solution and solid state. b, Pure donor films
after annealing at different temperatures. As-cast films composed
of different donor-acceptor ratios. c, As-cast films composed of
different donor-acceptor ratios.
[0103] FIG. 12 depicts AFM Images of 70:30 SMBFu:PC71BM films
spin-coated on ITO/PEDOT substrates and annealed at various
temperatures. a-c, Height images of as-cast film (a), film after
annealing at 90.degree. C. (b) and after annealing at 100.degree.
C. (c). d-f, Phase images of films as-cast (d), annealed at
90.degree. C. (e) and annealed at 100.degree. C. (f). The scan size
for all images is 2 .mu.m.times.2 .mu.m.
[0104] FIG. 13 depicts AFM images of different SMBFu:PC71BM blend
ratios spin-coated on ITO/PEDOT substrates and annealed at
100.degree. C. for 10 minutes. Height images for 30:70 (a), 50:50
(b) and 70:30 (c) blend ratios. The scan size for all images is 5
.mu.m.times.5 .mu.m.
[0105] FIG. 14 depicts J-V characteristics and external quantum
efficiencies of SMBFu:PC71BM devices: a, J-V curves of a 60:40
SMBFu:PC71BM blend ratio annealed at different temperatures for 3
min. b, EQE spectra of a 60:40 SMBFu:PC71BM blend annealed for 3
minutes at different temperatures (solid lines) compared to a
P3HT:PC61BM device (dashes). c, J-V curves of different
SMBFu:PC71BM blend ratios after annealing at 100.degree. C. for 10
min. d, EQE spectra of different SMBFu:PC71BM blend ratios after
annealing at 100.degree. C. for 10 min.
[0106] FIG. 15 shows current-voltage curves for certain devices of
the invention which use a material having two diketopyrrolopyrrole
cores. Device 1, dashed line, prepared with a 50:50 mixture of 36
and PC.sub.71BM and spincast from chloroform; Device 2, line with
triangles, prepared with 50:50 mixture of material 36 and
PC.sub.71BM and spincast from 98% chloroform and 2%
1,8-octanedithiol; Device 3, line with circles, prepared with a
30:70 mixture of material 26 and PC.sub.71BM and spincast from
chloroform.
[0107] FIG. 16 shows current-voltage curves for certain devices of
the invention, where the active layer of the device was cast using
various solvents. Carbon disulfide, 80.degree. C., inverted
triangles; trichloroethylene, 130.degree. C., circles; thiophene,
130.degree. C., squares; chloroform, 130.degree. C., line with no
symbols.
[0108] FIG. 17 shows the structure of Material 4 and energy level
diagrams of various material configurations for devices of the
invention. FIG. 17 A shows the structure of Material 4. FIG. 17B
shows, at left, an energy level diagram comparing HOMO and LUMO
values of C4PT2C6 (Material 3) with PCBM, and at right, an energy
level diagram comparing HOMO and LUMO values of C4PT2C6 (Material
3) with Material 4.
[0109] FIG. 18 shows a current-voltage curve for a device prepared
with a 50:50 mixture of C4PT2C6 and Material 4.
[0110] FIG. 19 shows energy level diagrams for devices of the
invention. FIG. 19A, conventional "normal" device. FIG. 19B,
inverted device.
[0111] FIG. 20 shows current-voltage curves for a conventional
"normal" Device 1 (dashed line, no shapes) of Example 13 versus
inverted devices Device 9 (line with squares) and Device 10 (line
with circles)
DETAILED DESCRIPTION OF THE INVENTION
[0112] Thiophene-based oligomers have not yet been investigated as
donor materials in solar cells, as they do not absorb strongly in
the red and near-infrared part of the solar spectrum where most
photons are concentrated. Excellent overlap between the absorption
of the semiconducting material and the terrestrial solar spectrum
is a key requirement for increasing solar cell efficiencies.
Another reason why oligothiophenes have not been as readily
incorporated into solution-processed devices is due to their low
solubility in common organic solvents, as a result of strong
intermolecular .pi.-.pi. interactions. To make oligothiophenes that
exhibit absorption at long wavelengths, electron donors (e.g.,
arylamines) and acceptors (e.g., cyano, benzothiadiazole) have been
appended onto their backbone. The addition of these groups changes
the redox properties of the materials and thereby their absorption
properties.
[0113] To make oligothiophenes that can be solution-processed, a
variety of functional groups have been incorporated as side chains
such as charged groups, acyl groups, alkylsulfanyl groups,
alkylsilyl groups, ether-based dendrons and straight and branched
alkyl moieties. Other approaches include attaching solubilizing
groups that can be thermally removed after film deposition and to
partially disrupt the strong intermolecular .pi.-.pi. interactions
by designing star-shaped or cross-shaped systems.
[0114] One potential strategy to make suitable donor materials
using oligothiophenes is to incorporate highly absorbing
chromophores that are used to make dyes and pigments. One such
chromophore is
3,6-diaryl-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione, more
commonly known as diketopyrrolopyrrole (DPP). DPP-containing
materials are bright and strongly fluorescent with exceptional
photochemical, mechanical and thermal stability and are therefore
used in industrial applications as high performance pigments in
paints, plastics, and inks. DPP-based molecular materials, however,
are not soluble in most common organic solvents due to the
concurrent strong H-bonding and .pi.-.pi. intermolecular
interactions in the solid state. Soluble derivatives, however, can
be made by attaching solubilizing groups including ionic groups,
charged, or neutral long alkyl chains, and organic protecting
groups such as t-Boc on the 3,4-positions (i.e., the lactam N
atoms) and/or on the 2,5-positions of the DPP moiety. It has been
shown that the solid state packing and optical properties of DPP
containing materials are dependent on the nature of these
substituents. DPP-containing oligothiophenes can self-assemble in
the solid state forming unexpected nanostructures. The
self-assembly process can be controlled by the number of thiophene
rings and the nature of the aliphatic chains attached to N
atoms.
[0115] Abbreviations and Definitions
[0116] Abbreviations used herein are as follows:
5-di-(2-ethylhexyl)-3,6-bis-(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)-p-
yrrolo[3,4-c]pyrrole-1,4-di one (SMDPPEH); Atomic force microscopy
(AFM); cyclic voltametry (CV); diketopyrrolopyrrole (DPP); external
quantum efficiency (EQE); indium tin oxide (ITO); organic
field-effect transistors (OFETs); poly(3-hexylthiophene) (P3HT);
[6,6]-phenyl C61-butyric acid methyl ester (PCBM);
poly(3,4-ethylene dioxythiophene:poly(styrenesulfonate)
(PEDOT:PSS); power conversion efficiency (PCE).
[0117] "(C.sub.1-C.sub.12)-alkyl" is intended to embrace a
saturated linear, branched, cyclic, or a combination of linear
and/or branched and/or cyclic hydrocarbon chain(s) and/or ring(s)
of 1 to 12 carbon atoms. Examples of "(C.sub.1-C.sub.12)-alkyl"
include, but are not limited to, methyl, ethyl, n-propyl,
isopropyl, cyclopropyl, n-butyl, isobutyl, sec-butyl, t-butyl,
cyclobutyl, cyclopropyl-methyl, methyl-cyclopropyl, pentyl, or
2-ethylhexyl, where the point of attachment of the alkyl group to
the remainder of the molecule can be at any chemically possible
location.
[0118] Optoelectronic Device Structure and Fabrication
[0119] In one embodiment, the optoelectronic device of the
invention comprises the following layers:
[0120] a) a first hole-collecting electrode, optionally coated onto
a transparent substrate;
[0121] b) an optional layer or layers adjacent to the first
electrode, such as a hole-transporting layer;
[0122] c) a layer comprising a mixture of an electron donor of the
general Formula I, II, IIa, IIb, or IIc, and an electron acceptor
(donor:acceptor);
[0123] d) an optional layer or layers such as hole-blocking,
exciton-blocking, or electron-transporting layers; and
[0124] e) a second electron-collecting electrode.
[0125] Typically, the first electrode can be transparent, allowing
light to enter the device, but in some embodiments, the second
electrode can be transparent. In some embodiments, both electrodes
are transparent.
[0126] In another embodiment, the optoelectronic device of the
invention comprises the following layers:
[0127] a) indium tin oxide (ITO) coated glass (a first
electrode),
[0128] b) poly(3,4-ethylene dioxythiophene:poly(styrenesulfonate)
(PEDOT:PSS),
[0129] c) a mixture of electron-donating chromophores of the
general Formula I, II, IIa, IIb, or IIc, and an electron-acceptor
(donor:acceptor), and
[0130] e) a metal electrode (a second electrode);
[0131] where layer (d) in the previous embodiment is absent.
[0132] In one configuration, where light passes though a
transparent first electrode (such as ITO-coated glass), it is
absorbed by the donor:acceptor mixture, which results in the
separation of electrical charges and migration of the charges to
the electrodes, yielding a usable electrical potential (FIG.
1).
[0133] The first electrode can be made of materials such as
indium-tin oxide, indium-magnesium oxide, cadmium tin-oxide, tin
oxide, aluminum- or indium-doped zinc oxide, gold, silver, nickel,
palladium and platinum. Preferably the first electrode has a high
work function (4.3 eV or higher).
[0134] The optional layer adjacent to the first electrode is
preferably polystyrenesulfonic acid-doped
polyethylenedioxythiophene (PEDOT:PSS). Other hole transporting
materials, such as polyaniline (with suitable dopants), or
N,N'-diphenyl-N,N'-bis(3-methylphenyl)[1,1'-biphenyl]-4,4'-diamine
(TPD), can be used.
[0135] One method of fabricating the optoelectronic device is as
follows: A conductive, transparent substrate is prepared from
commercially available indium tin oxide-coated glass and
polystyrenesulfonic acid doped polyethylenedioxythiophene using
standard procedures. A solution containing a mixture of the donor
and acceptor materials is prepared so that the ratio of donor to
acceptor is between 1:99 and 99:1 parts by mass; more preferably
between 3:7 and 7:3 parts by mass. The overall concentration of the
solution may range between 0.1 mg/mL and 100 mg/mL, but is
preferably in the range of 10 mg/mL and 30 mg/mL.
[0136] The electron acceptor is preferably [6,6]-phenyl C61-butyric
acid methyl ester (PCBM), but may be a different fullerene
(including, but not limited to, C71-PCBM), a
tetracyanoquinodimethane, a vinazene, a perylene tetracarboxylic
acid-dianhydride, a perylene tetracarboxylic acid-diimide, an
oxadiazole, carbon nanotubes, or any other organic electron
acceptor, such as those compounds disclosed in U.S.
2008/0315187.
[0137] In other embodiments, the electron acceptor is an inorganic
acceptor selected from TiO.sub.2 (titanium dioxide) and ZnO (zinc
oxide). The titanium dioxide can anatase, rutile, or amorphous. A
titanium dioxide layer can be prepared by depositing a sol-gel
precursor solution, for example by spincasting or doctorblading,
and sintering at a temperature between about 300.degree. C. and
500.degree. C. When an inorganic layer is used, component (c) of
the optoelectronic device described above can be comprised of a
layer of electron-donating chromophores of the general Formula II
and an inorganic electron-acceptor layer. Alternatively, the
inorganic material can be dispersed in the electron-donating
chromophores to create a single layer. Preparation of TiO.sub.2 for
use in solar cells is described in Brian O'Regan & Michael
Gratzel Nature 353:737 (1991) and Serap Gunes et al., 2008
Nanotechnology 19 424009.
[0138] Useful solvents include chloroform, toluene, chlorobenzene,
methylene dichloride and carbon disulfide. However, the solvent
used may be any solvent which dissolves or partially dissolve both
donor and acceptor materials and has a non-zero vapor pressure.
[0139] The solution of donor and acceptor is deposited onto the
transparent conductive substrate by spin casting, ink-jet printing,
roll-to-roll coating or any process which yields a continuous film
of the donor-acceptor mixture such that the thickness of the film
is within the range of 10 to 1000 nm, more preferably between 50
and 150 nm.
[0140] In certain embodiments, the layer of the donor and acceptor
is cast from a solution comprising a solvent and the electron donor
and the electron acceptor. The solvent can comprise chloroform,
thiophene, trichloroethylene, or carbon disulfide, or a mixture of
any of the foregoing solvents. The solvent can also include
processing additives, such as those disclosed in US Patent
Application Publication Nos. 2009/0032808, 2008/0315187, or
2009/0108255. For example, 1,8-octanedithiol can be added to the
solvent/donor/acceptor mixture in an amount of about 1% or at least
about 1%, about 2% or at least about 2%, about 3% or at least about
3%, about 4% or at least about 4%, about 5% or at least about 5%,
about 8% or at least about 8%, or about 10% or at least about 10%.
The additive, such as 2% 1,8-octanedithiol, can be added to any
organic solvent used to cast the layer of donor/acceptor, such as
chloroform.
[0141] An additional layer or layers of material (i.e., the
layer(s) adjacent to the second electrode) may optionally be
deposited on top of the donor-acceptor film in order to block holes
or excitons, act as an optical buffer, or otherwise benefit the
electrical characteristics of the device.
2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline can act as a
hole-blocking or exciton-blocking material, while
4,4',4''-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine and
polyethylene dioxythiophene can act as exciton-blocking
materials.
[0142] Finally, a metal electrode is deposited on top of the
structure by thermal evaporation, sputtering, or some other
process. The metal is preferably aluminum, silver or magnesium, but
may be any metal. In some embodiments, the device is annealed
before and/or after evaporation of the metal electrode.
[0143] Hole and electron mobilities are important parameters to
consider in the fabrication/function of bulk heterojunction solar
cells. For optimal device performance, a balance in the mobility of
both charge carriers is desirable. Preferably, the electron and
hole mobilities are both on the order of 10.sup.-4 cm.sup.2/Vs or
higher. More preferably, the electron mobilities are on the order
of 10.sup.-3 cm.sup.2/Vs or higher. In some embodiments, the
electron mobilities are on the order of 10.sup.-4 cm.sup.2/Vs or
higher, and the hole mobilities are between 10.sup.-8 cm.sup.2/Vs
and 10.sup.-4 cm.sup.2/Vs or higher. In other embodiments, the
electron mobilities are on the order of 10.sup.-3 cm.sup.2/Vs or
higher, and the hole mobilities are between 10.sup.-8 cm.sup.2/Vs
and 10.sup.-4 cm.sup.2/Vs or higher.
[0144] Optoelectronic devices of the present invention have
excellent photovoltaic properties. In some embodiments, the power
conversion efficiency (PCE) is at least 0.5%, at least 1.0%, at
least 2.0%, at least 3.0%, or at least 4.0%. In some embodiments,
the short circuit current density is greater than 3.0 mA/cm.sup.2,
and preferably greater than 8 mA/cm.sup.2. In some embodiments, the
open circuit voltage is between 0.6 and 1.0 V or higher. In some
embodiments, the device exhibits an external quantum efficiency of
approximately 25% or greater between 550-750 nm.
[0145] The morphological properties of the donor:acceptor films can
be measured using atomic force microscopy or other
surface-sensitive techniques. Preferably, the films will have a
root-mean-squared surface roughness of less than 1.0 nm, more
preferably less than 0.5 nm.
[0146] For embodiments of the devices using an inverted device
architecture, the first electrode can comprise Au or another
material having a work function higher than the work function of
the second electrode, while the second electrode can comprise an
ITO substrate modified using a self-assembled monolayer of
3-aminopropyltrimethoxysiloxane or another material having a work
function lower than the work function of the first electrode.
[0147] Electron-Donating Chromophores
[0148] Electron-donating chromophores useful in fabricating the
devices described herein can be of the formulas I, II, IIa, IIb, or
IIc as described herein. For example, the electron-donating
chromophores can be of the following general Formula II:
##STR00070##
wherein X is oxygen or sulfur; A.sub.1 and A.sub.2 are selected
from substituted and unsubstituted aryl or heteroaryl groups,
wherein each individual A.sub.1 within the (A.sub.1).sub.m moiety
can be independently selected from a substituted or unsubstituted
aryl or heteroaryl group, and each individual A.sub.2 within the
(A.sub.2).sub.n moiety can be independently selected from a
substituted and unsubstituted aryl or heteroaryl group; m is
independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; n is
independently selected from 1, 2, 3, 4, 5, 6, 7, 8, or 9; E.sub.1
and E.sub.2 are independently selected from a nonentity, H, or a
substituted or unsubstituted aryl or heteroaryl group or a
C.sub.1-C.sub.12 alkyl group; and R.sub.1 and R.sub.2 are
independently selected from H, C.sub.1-C.sub.12 alkyl, and
--C(.dbd.O)--O--C.sub.1-C.sub.12 alkyl.
[0149] In certain embodiments, X is sulfur. In other embodiments, X
is oxygen.
[0150] In certain embodiments, the one or more substituted or
unsubstituted aryl or heteroaryl groups A.sub.1 and A.sub.2 are
independently selected from the group consisting of thiophene,
bithiophene, terthiophene, thienothiophene, dithienothiophene,
benzothiophene, isobenzothiophene, benzodithiophene,
cyclopentadithiophene, indole, benzene, naphthalene, anthracene,
indene, fluorene, pyrene, azulene, furan, pyrrole, pyridine,
oxazole, thiazole, thiazine, pyrimidine, pyrazine, imidazole,
benzoxazole, benzoxadiazole, benzothiazole, benzimidazole,
benzofuran, isobenzofuran, thiadiazole, pyridothiadiazole, and
carbazole. Each individual A.sub.1 group in an (A.sub.1).sub.m
chain, and each individual A.sub.2 in an (A.sub.2).sub.n chain, can
be independently selected. Thus, for example, when m=3, the
(A.sub.1).sub.3 chain can be:
##STR00071##
[0151] The aryl or heteroaryl groups are directly connected to each
other at any valence where a bond to another group is chemically
possible, and in a manner so as to allow conjugation of the
delocalized electrons between the aryl or heteroaryl groups and any
other aryl or heteroaryl groups to which they are bonded, such as
the diketopyrrolopyrrole core moieties.
[0152] In certain embodiments, the moieties E.sub.1 and E.sub.2 are
independently selected from n-hexyl (--C.sub.6H.sub.13) and
benzofuran
##STR00072##
In other embodiments, when E.sub.1 and/or E.sub.2 is benzofuran,
the benzofuran moiety is attached to the remainder of the molecule
at the 2-position of the benzofuran ring.
[0153] In certain embodiments, m=n=3. In other embodiments, m=n=2.
In other embodiments, m=n=1.
[0154] In certain embodiments, the moieties R.sub.1 and R.sub.2 are
independently selected from the group consisting of hexyl,
2-ethylhexyl, t-butoxycarbonyl, and trimethylsilyl. In another
embodiment, the same group selected for R.sub.1 is also used for
the R.sub.2 group.
[0155] In certain embodiments, the electron-donating chromophore
comprises compound A, which has the following formula:
##STR00073##
[0156] In other embodiments, the electron-donating chromophore
comprises
2,5-di-(2-ethylhexyl)-3,6-bis-(5''-n-hexyl-[2,2';5',2'']terthiophen-5-yl)-
-pyrrolo[3,4-c]pyrrole-1,4-dione (SMDPPEH), which has the following
formula:
##STR00074##
[0157] In other embodiments, the electron-donating chromophore
comprises compound (4), which has the following formula:
##STR00075##
[0158] In other embodiments, the electron-donating chromophore
comprises C6PT1C6. In other embodiments, the electron-donating
chromophore comprises C6PT2C6. In other embodiments, the
electron-donating chromophore comprises C6PT3C6. In other
embodiments, the electron-donating chromophore comprises EHPT2C6.
In other embodiments, the electron-donating chromophore comprises
C6PT2. The structures of C6PT1C6, C6PT2C6, C6PT3C6, EHPT2C6, and
C6PT2 are described in Example 10 below. In other embodiments, the
electron-donating chromophore comprises compound 26 of Example 11.
In other embodiments, the electron-donating chromophore comprises
compound 36 of Example 12.
[0159] Excellent solubility and high thermal stability of the
electron-donating chromophores leads to a more robust
optoelectronic device. In some embodiments, the electron-donating
chromophore has a solubility of >20 mg/mL in chloroform,
chlorobenzene or toluene. In some embodiments, the
electron-donating chromophore is thermally stable up to 250.degree.
C.
[0160] Conversion of Diketopyrrolopyrroles to
Dithioketopyrrolopyrroles
[0161] Syntheses of diketopyrrolopyrroles are presented in the
Examples below. Once the diketopyrrolopyrroles have been prepared,
reaction with Lawesson's reagent or other thiation agents converts
the diketopyrrolopyrroles into dithioketopyrrolopyrroles. Exemplary
protocols are provided in Closs and Gompper, Angew. Chem. Int. Ed.
Engl. 26(6):552 (1987), and in Thomsen et al., Organic Syntheses,
Coll. Vol. 7, p. 372 (1990); Vol. 62, p. 158 (1984).
[0162] Use of Diketopyrrolopyrroles and Dithioketopyrrolopyrroles
as Electron Acceptors
[0163] While the diketopyrrolopyrroles and
dithioketopyrrolopyrroles described herein are described primarily
as electron donors, compounds of the formula I, II, IIa, IIb, and
IIc can also be used as electron acceptor materials. For use as an
electron acceptor, the diketopyrrolopyrrole or
dithioketopyrrolopyrrole should have a relatively low LUMO level,
about 0.3 eV or more lower than the LUMO of the electron donor it
will be used with. It should also have a reasonably high electron
mobility to serve as an acceptor, such as an electron mobility
about or on the order of 10.sup.-6 cm.sup.2/Vs or higher,
preferably about or on the order of 10.sup.-4 cm.sup.2/Vs or
higher, more preferably about or on the order of 10.sup.-3
cm.sup.2/Vs or higher. Lowering the LUMO and increasing electron
mobility can be accomplished by using electron withdrawing groups
(such as fluoro, chloro, cyano, trifluoromethyl, etc.) attached to
the conjugated system, or by using electron deficient aryl groups
(such as pyridine or benzothiadiazole); see Prashant Sonar, J.
Mater. Chem., 2010, DOI: 10.1039/b924404b.
[0164] Inverted Device Architecture
[0165] In some cases, it can be advantageous to have the substrate
act as a cathode, while the top electrode acts as the anode. For
example, using the substrate to collect electrons can allow a
stable, high work function metal such as gold or nickel to be used
as the top electrode. This can be achieved by modifying the work
function of the substrate or using an n-type substrate.
[0166] FIG. 19A shows an energy level diagram for a normal device,
while FIG. 19B shows an energy level diagram for an inverted
device. In the normal device, photo-generated holes travel to an
ITO substrate while photo-generated electrons travel to a top
electrode consisting of a relatively low work-function metal such
as Al. In the inverted devices, the charge carriers flow in the
opposite direction, where electrons travel to the substrate while
holes travel to the top electrode and are collected by a relatively
high work function metal such as Au. This configuration has the
advantage that a relatively stable metal is used as the top
electrode, which can increase the lifetime of the device. Such
inverted devices are shown in Example 16 below.
EXAMPLES
Example 1
Synthesis of Compound 4
[0167] Compound 4 was synthesized according to the following
synthetic procedure:
##STR00076##
[0168] Part A. The parent
2,5-dihydro-1,4-dioxo-3,6-dithienylpyrrolo[3,4-c]-pyrrole (1) was
prepared in 55% yield following a previously reported procedure
which comprises the reaction of 2-thiophenecarbonitrile with 0.5 eq
of di-n-butyl succinate ester and an excess of potassium t-butoxide
using 2-methyl-2-butanol as solvent. See Tamayo et al., J. Phys.
Chem. C 112:15543 (2008) and references therein.
[0169] Part B. Compound (2) was synthesized according to a modified
literature procedure. Zambounis et al. Nature 1997, 388, 131. In a
three-necked, oven-dried 100 mL round bottom flask, 1 (3.0 g, 10.0
mmol) was dissolved in 150 mL of anhydrous tetrahydrofuran (THF)
and the resulting solution was purged with argon for ten minutes.
Dimethylaminopyridine (DMAP 3.0 g, 25 mmol) was added and the
reaction mixture was stirred for 15 minutes under argon at room
temperature. Di-tert-butyl-dicarbonate (11.0 g, 50 mmol) was then
added and the mixture was stirred for 24 hours, after which the
solvent was removed in vacuo to obtain a brownish red solid.
Petroleum ether (250 mL) was poured into the crude product and the
resulting suspension was stirred for 1 hour. The solids were then
collected by vacuum filtration and further washed with several
portions of petroleum ether. The crude product was purified by
flash chromatography using chloroform as eluent and the solvent was
removed in vacuo giving 4.0 g of pure product as a shiny
crystalline red powder. (yield: 4.0 g, 80.2%).
[0170] Analysis: NMR (400 MHz, CDCl.sub.3, ppm) .delta.=8.28 (dd,
J=4.0 Hz, 1.2 Hz, 2H), 7.68 (dd, J=4.0 Hz, 1.2 Hz, 2H), 7.19 (dd,
J=8.0 Hz, 4.0 Hz, 4.0 Hz, 2H), 1.60 (s, 18H). MS (ESI-TOF):
Calculated for C.sub.24H.sub.24N.sub.2O.sub.6S.sub.2: 500.11.
Found: 501 (M.sup.++H). CHN Analysis: Calcd. C, 57.58; H, 4.83; N,
5.60. Found: C, 57.23; H, 4.88; N, 5.73.
[0171] Part C. Compound (3) was synthesized according to a modified
literature procedure. Zhang et al. J. Mater. Chem. 2006, 16, 736.
In a three-necked, oven-dried 150 mL round bottom flask covered
with aluminum foil, 2 (2.623 g, 4.0 mmol) was dissolved in 150 mL
of anhydrous CHCl.sub.3, covered with aluminum foil and stirred at
room temperature under argon for 15 minutes. N-bromosuccinimide
(1.500 g, 8.4 mmol) was then added and the reaction mixture was
kept at room temperature and stirred for 48 hours. The reaction
mixture was poured into 400 mL of methanol and the resulting
suspension was stirred at room temperature for 15 minutes. The
solid was then collected by vacuum filtration and was washed with
several portions of hot distilled water and hot methanol to obtain
1.98 g of pure product as shiny dark purple powder (yield: 1.98 g,
60.2%).
[0172] Analysis: .sup.1H NMR (400 MHz, CDCl.sub.3, ppm)
.delta.=8.08 (d, J=4.0 Hz, 2H), 7.15 (d, J=4.0 Hz, 2H), 1.63 (s,
18H). MS (ESI-TOF): Calculated for
C.sub.30H.sub.38Br.sub.2N.sub.2O.sub.2S.sub.2: 655.93. Found 656
(M.sup.++H). CHN Analysis: Calcd. C, 43.78; H, 3.37; N, 4.25.
Found: C, 43.67; H, 3.30; N, 4.21.
[0173] Part D. Compound (4) was synthesized according to a modified
literature procedure. Zrig et al. J. Org. Chem. 2007, 72, 5855. In
an three-necked, oven-dried 100 mL round bottom flask, 3 (0.6825 g,
1.00 mmol) and 5'-hexyl-2,2'-bis-thiophene boronic acid pinacol
ester (2.25 mmol) were dissolved in 15 mL of anhydrous toluene and
20 ml of anhydrous 1,2-dimethoxyethane and the resulting solution
degassed for ten minutes. Tris(dibenzylideneacetone)dipalladium(0)
(14 mg, 0.0153 mmol), and tri-tert-butylphosphonium
tetrafluoroborate (18 mg, 0.0620 mmol) were then added and the
mixture degassed for another five minutes. A degassed 2.0 molar
solution of potassium phosphate (5 mL) was then added and the
reaction mixture stirred and heated to 90.degree. C. under argon
overnight. The reaction mixture was allowed to cool down to room
temperature after which it was poured into 400 mL of methanol and
was then stirred for 30 minutes. The precipitated solid was then
collected by vacuum filtration and was washed with several portions
of distilled water, methanol, isopropanol and petroleum ether. The
crude product was purified by flash chromatography using chloroform
as eluent and the solvent removed in vacuo giving a shiny green
crystalline powder (yield: 0.65 g, 65.0%).
[0174] Analysis: .sup.1H NMR (400 MHz, CDCl.sub.3, ppm)
.delta.=8.28 (d, J=4.0 Hz, 2H), 7.21 (dd, J=4.0 Hz, 4H), 7.04 (dd,
J=4.0 Hz, 4H), 6.71 (d, J=4.0 Hz, 2H), 2.81 (t, J=8.0 Hz, 4H),
1.68-1.72 (m, 4H), 1.66 (s, 18H), 1.22-1.46 (m, 10), 0.90 (t, J=7.2
Hz, 6H). MS (FAB): Calculated for
C.sub.52H.sub.56N.sub.2O.sub.6S.sub.6: 996.25. Found 996. MS (FAB):
Calculated for C.sub.52H.sub.56N.sub.2O.sub.6S.sub.6: 996.25. Found
996. Calcd. C, 62.62; H, 5.66; N, 2.81. Found: C, 62.30; H, 5.64;
N, 2.85.
Example 2
Device Fabrication
[0175] Indium tin oxide (ITO)-coated glass substrates (Thin Film
Devices) were cleaned with detergent and de-ionized water after
which the substrates were sonicated for 10 minutes in soap
solution, de-ionized water, acetone and isopropanol. The ITO
substrates were then treated in a UV ozone cleaner for 30 minutes
followed by spin coating a solution of poly(3,4-ethylene
dioxythiophene:poly(styrenesulfonate) (PEDOT:PSS, Baytron P) (5000
rpm for 40 seconds). The PEDOT:PSS film was dried at 140.degree. C.
inside a glovebox for 15 minutes which yielded a film 60 nm thick.
A 2% (w/v) blend solution of compound (4) and PCBM (Nano-C, USA) in
chloroform (CHCl.sub.3) was filtered through a 0.45 .mu.m
poly(tetrafluoroethylene) (PTFE) filter and spin coated at 1500 rpm
for 60 seconds on top of the PEDOT:PSS layer. Subsequently,
aluminum (1200 .ANG.) was thermally evaporated at a pressure of
1.times.10.sup.-7 Torr at room temperature using a shadow mask.
Illumination was done through the glass slide using light from 150
W Newport-Oriel AM 1.5 G light source operating at 100 mW/cm.sup.2.
Mobility measurements were done using the following diode
structures: ITO/PEDOT:PSS/active material/Au for holes and
Al/active material/Al for electrons. Au (1000 .ANG.) and Al (500
.ANG.) electrodes were thermally evaporated at a pressure of
1.times.10.sup.-7 Torr at room temperature using a shadow mask
(Angstrom Engineering, Inc.). The active layer thicknesses in all
devices obtained by AFM were approximately 100-110 nm. All
fabrications and characterization were performed under
nitrogen.
Example 3
Electrochemical and Photophysical Characterization of (4)
[0176] Cyclic voltammetry (CV) was performed using an EG&G
potentiostat/galvanostat model 283. Anhydrous dichloromethane was
used as the solvent under an inert atmosphere, and 0.1 M solution
tetra-butyl ammonium hexafluorophosphate was used as the supporting
electrolyte. A glassy carbon rod was used as the working electrode,
a platinum wire was used as the counter electrode, and a silver
wire was used as a pseudo reference electrode. The redox potentials
are obtained by taking the average of anodic and cathodic waves and
are reported relative to a ferrocenium/ferrocene
(Cp.sub.2Fe.sup.+/Cp.sub.2Fe, 0.475 V versus SCE in
dichloromethane) redox couple used as an internal reference.
UV-visible absorption spectra were recorded on a Shimadzu UV-2401
PC dual beam spectrometer. Steady-state fluorescence experiments at
room temperature were performed using a PT1 (Lawrenceville, N.J.)
Quantum Master fluorimeter equipped with a Xenon lamp excitation
source and a Hamamatsu (Japan) 928 PMT using 90.degree. angle
detection for solution samples.
[0177] The CV of (4) exhibits one reversible oxidation process and
one reduction process at 0.43 V and at -1.61 V, respectively. Based
on the measured oxidation and reduction potentials, (4) has HOMO
and LUMO energy levels of -5.03 eV and -3.0 eV, respectively. These
values are within the required electronic levels for BHJ solar
cells when PCBM is used as the electron acceptor.
[0178] FIG. 2 shows a plot of the solution and film absorption
spectra of (4) in chloroform and on quartz, respectively. The high
degree of conjugation between the electron donating thiophene rings
and the electron accepting DPP moiety, is demonstrated in solution
where the compound shows a broad and featureless absorption band
with a .lamda..sub.max=616 nm corresponding to the intramolecular
charge transfer (ICT) transition. This absorption band is
bathochromically shifted relative to the .pi.-.pi.* transitions in
water soluble sexithiophenes, which occur between 400-500 nm in
dilute solutions. The film absorption of pure (4) on a quartz
substrate spin-coated from a 2% (w/v) chloroform solution is
significantly broadened and exhibits two absorption bands centered
at 660 nm and 742 nm. These peaks are 44 nm and 126 nm red-shifted
from the main absorption band observed in solution, respectively.
The first absorption band is quite broad and is likely due to the
same charge transfer band seen in solution and the shift in the
peak maximum is due to aggregation in the solid state. The second
sharp absorption band is possibly due to ordered aggregation, which
is confirmed by AFM (see Example 4). The optical gap of the
compound is reduced from 1.72 eV to 1.5 eV based on solution to
solid state absorption. One reason for this significant change
might be the coplanarization of the thiophene rings due to
molecular ordering, similar to what has been observed for spun cast
films of thiophene oligomers and polymers.
Example 4
Film Morphology of (4)
[0179] Tapping mode atomic force microscopy (AFM) topographic and
phase images of pure (4) and the blends were obtained using the
Multi-Mode microscope and the controller NanoScope IIIa (Veeco
Inc.). Images were collected in air using silicon probes with a
typical spring constant of 1-5 nN/m and a resonant frequency of 75
kHz (Budget Sensors).
[0180] FIG. 3 shows the topographic and phase images of (4)
spin-coated from CHCl.sub.3 onto a quartz substrate. Fiber-like
structures are observed in the topographic and phase images
indicating a high degree of molecular order. The hole mobility,
.mu..sub.h, of (4) was measured to be 3.times.10.sup.-6 cm.sup.2/Vs
using the ITO/(4)/Au diode configuration. This value is comparable
with hole mobilities measured for small molecules (i.e.
.about.10.sup.-6 cm.sup.2/Vs, for triphenylamine derivatives) and
conjugated polymers that are used as electron donors for solar
cells (i.e. .about.10.sup.-5 cm.sup.2/Vs for P3HT, as determined by
using the SCLC model). However, it is known that hole mobilities
change when mixed with PCBM as both hole and electron mobilities
depend on factors such as blend ratio and film morphology.
Example 5
Photovoltaic Properties and Film Morphology of (4):PCBM Blend
[0181] To demonstrate the potential of (4) as an electron donor in
photovoltaic devices, BHJ devices were fabricated by spin-coating
from 2% (w/v) chloroform solutions comprising a mixture of (4) and
PCBM in different blend ratios (30:70, 50:50, and 70:30). FIG. 4
shows the film absorption spectra of the various blends. It can be
seen that as the amount of PCBM decreases, the peak at 300 nm
assigned to PCBM decreases in intensity with the simultaneous
appearance of a shoulder band around 740 nm that was previously
assigned to the aggregation of (4). However, the absorption band is
not as intense as the one seen in the pure film probably due to
disruption of the solid state packing of (4) with the addition of
PCBM, as previously observed for conjugated polymers where PCBM
inhibits crystallization. Melzer et al. Adv. Funct. Mater. 2004,
14, 865.
[0182] FIG. 5 shows the current density versus voltage curves for
devices using 30:70, 50:50 and 70:30 blend ratios under AM 1.5
simulated solar illumination at an intensity of 100 mW/cm.sup.2. A
summary of photovoltaic properties is given in Table 1. The best
devices based on 30:70 blend ratios delivered a short circuit
current density of 3.13 mA/cm.sup.2 and an open circuit voltage of
0.63 V. Combined with a fill factor of 0.27, the device gave a
power conversion efficiency of 0.53%. The short circuit current
density increased significantly when the donor-acceptor blend ratio
was changed to 50:50 and 70:30. The open circuit voltage, however,
remained relatively constant. A short circuit current density of
8.42 mA/cm.sup.2 and an open circuit voltage of 0.67 V were
obtained from the device employing a 70:30 blend ratio. The power
conversion efficiency of the device was calculated to be 2.33% with
a fill factor of 0.45. This power conversion efficiency value is
the highest among solution-processed small molecule-based BHJ solar
cells reported in the literature. This device exhibits external
quantum efficiencies close to 25% at 343 nm and close to 30%
between 550-750 nm (see FIG. 6), the former being mainly due to the
PCBM acceptor while the latter is attributed to the donor
material.
TABLE-US-00001 TABLE 1 Effect of (4) and PCBM blend ratio on the
device characteristics. Ratio V.sub.OC J.sub.SC .eta. (4):PCBM
(volts) (mA/cm.sup.2) (%) FF 30:70 0.63 3.13 0.53 0.27 50:50 0.65
5.42 1.20 0.34 70:30 0.67 8.42 2.33 0.45
[0183] AFM topographical and phase images collected in the areas
between the Al electrodes of the devices are shown in FIG. 7.
Overall, there is no micron-sized phase segregation occurred for
the three donor:acceptor ratios used. The 30:70 (4):PCBM film
surface is very smooth, with a RMS .about.0.45 nm. Increasing the
donor:acceptor ratio leads to increased surface roughness
(.about.1.0 nm for 70:30 (4):PCBM film). Some degrees of
intermolecular interactions of (4) is maintained in the blended
film as evidenced by the fiber-like structures in the topographic
and phase images (FIG. 7). The devices fabricated from the 30:70
(FIG. 7 a,b), 50:50 (FIG. 7 b,c), and 70:30 (FIG. 7 e,f) blend
ratios show minimal or no visible phase separation, but at a higher
donor ratio (70:30), nanostructures can be observed from the
topographic image (FIG. 7 e,f). This continuous film morphology
with the absence of large phase segregation combined with the
increase in absorption at longer wavelengths brought by the
increase in donor concentration can account for the increase in
device performance.
[0184] The hole and electron mobilities were measured for 30:30,
50:50 and 70:30 blend ratios using the hole-only and electron-only
diode structures. The mobility values were extracted using the SCLC
model. Blom et al. Phys. Rev. B 1997, 55, 656. It was found that
the electron mobilities do not change significantly as the donor
concentration increases. The electron mobilitites are 3, 5 and
6.times.10.sup.-4 cm.sup.2/Vs for 30%, 50% and 70% acceptor
contents, respectively. In contrast, the hole mobilities exhibited
a more pronounced dependence on the concentration of donor
material, increasing by more than an order of magnitude from
2.times.10.sup.-8 to 5.times.10.sup.7 cm.sup.2/Vs as the
concentration of donor material was decreased from 30% to 70%. It
is evident that the charge transport properties are more balanced
at high donor concentration, resulting in improved device
efficiency. From these results, the electron and hole mobilities
are different by three orders of magnitudes. Thus, higher
efficiency can be achieved if one can further improve the hole
mobility.
[0185] The optimal donor concentration in this system is much
larger than optimal blend ratios previously observed for
solution-processed conjugated polymer/PCBM mixtures. This may be
because large concentrations of (4) do not disrupt the percolation
of the PCBM phase as polymers do. In this system, a high donor
concentration is not seen to adversely affect device properties as
with polymers, but allows for enhanced light absorption by the
chromophore and increased order in film morphology, in addition to
balancing charge transport properties.
Example 6
Electrochemical and Photophysical Characterization of SMDPPEH and
SMDPPEH:PCBM Blend
[0186] The optical properties of SMDPPEH in solution (dotted line)
and in film (solid line) together with that of C.sub.71-PCBM
(dashed line) are shown in FIG. 8(a). The absorption of SMDPPEH
exhibits a broad and intense intramolecular charge transfer band in
solution that onsets around 700 nm with a peak maximum ca. 660 nm
(.epsilon.=.about.10.sup.5 L/mol-cm). The corresponding film
contains an even broader absorption region that extends to 800 nm.
The 720 nm band in the film has been observed previously in highly
ordered oligothiophene-DPP systems and assigned to strong
intermolecular interactions. When mixed with the fullerene
acceptor, which has an absorption band centered at 480 nm, the
resulting blend film gives a very good spectral coverage. The hole
mobility of a pure SMDPPEH film is .about.1.0.times.10.sup.-4
cm.sup.2/Vs based on the SCLC model, which is two orders of
magnitude higher than that of the t-Boc-derivative. (Tamayo et al.,
J. Phys. Chem. C 112:15543 (2008)). The hole mobility of SMDPPEH is
comparable to that of (P3HT).
[0187] FIG. 8(b) shows the film absorption spectra of the various
blend ratios. The thicknesses of the films are around 80 nm
irrespective of the blend ratio. It can be seen that as the amount
of C.sub.71-PCBM decreases, the absorption peak at 480 nm assigned
to C.sub.71-PCBM decreases with the concomitant increase in the
absorption bands at 620 nm and 710 nm, which are assigned to
SMDPPEH. These DPP absorption bands are blue-shifted by 20 nm and
30 nm, respectively, compared to the absorption peaks in the pure
film probably due to disruption of the solid state packing of
SMDPPEH by the addition of C.sub.71-PCBM.
[0188] FIG. 9(a) shows the current density versus voltage (J-V)
characteristics for devices using 30:70, 50:50, and 70:30 blend
ratios of SMDPPEH:C.sub.71-PCBM. A summary of photovoltaic
properties is given in Table 1. The open-circuit voltages
(V.sub.OC)=0.72-0.75 V for the three sets of devices are
independent of the blend ratios. The slight increase in V.sub.OC is
consistent with the lower HOMO energy level of SMDPPEH (5.2 eV as
determined by ultraviolet photoelectron spectroscopy, UPS) compared
to HOMO energy level of the (4) (4.9 eV as determined by UPS). In
contrast, the short-circuit current densities (J.sub.SC) are
dependent on the donor-acceptor ratio. The devices based on 30:70
blend ratios delivered a short circuit current density of 7.7
mA/cm.sup.2 and a fill factor of 0.41 giving a PCE of 2.4%. The
short-circuit current density increases to 9.2 mA/cm.sup.2 when the
donor-acceptor blend ratio was changed to 50:50. Combined with a
slightly increased fill factor of 0.45, a PCE of 3.0% is achieved,
which is the highest reported J.sub.SC and PCE for
solution-processed, small molecule based BHJ solar cells. Changing
the blend ratio to 70:30, however, slightly decreases both J.sub.SC
and fill factor to 8.7 mA/cm.sup.2 and 0.35, respectively, giving a
PCE of 2.2%.
[0189] From the external quantum efficiency (EQE) spectra shown in
FIG. 9(b), it is clear that the absorption bands at 480 nm, 620 nm
and 710 nm significantly contribute to the device photocurrent in
all blend ratios. The EQE spectra show that the contribution of
C.sub.71-PCBM decreases as its concentration changes from 70% to
50% to 30%. This is consistent with the decrease in the amount of
absorption at 480 nm assigned to C.sub.71-PCBM in the blend film
(see FIG. 8b). In contrast, the peaks at 620 nm and 710 nm in the
EQE spectra assigned to SMDPPEH do not significantly change as a
function of blend ratio. The EQE does not track with the film
absorption at 620 nm and 710 nm which double in intensity when the
amount of donor is increased from 30% to 70%. One possible
explanation is that the donor domains increase in size due to
increase in the donor concentration such that some generated
excitons do not reach a donor-acceptor interface.
[0190] The hole and electron mobilities for all the blends are
given in Table 2. The hole mobilities are of the order of
10.sup.-4-10.sup.-5 cm.sup.2/Vs and are not significantly different
from the measured hole mobility of the pure donor film irrespective
of the blend ratio. Similarly, the electron mobilities are also of
the order of 10.sup.-4 cm.sup.2/Vs. These carrier mobilities are
comparable to those measured for high performance BHJs solar cells
based on P3HT:PCBM blends. Importantly, the electron and hole
mobilities of the blends are quite balanced which may help explain
the high PCEs observed for this donor-acceptor system.
TABLE-US-00002 TABLE 2 Summary of hole and electron mobilities and
solar cell characteristics of various SMDPPEH:C.sub.71-PCBM ratios.
SMDPPEH: C.sub.71-PCBM Ratio V.sub.OC J.sub.SC .eta. .mu..sub.hole
.mu..sub.electron (20 mg/ml) (volts) (mA/cm.sup.2) FF (%)
(cm.sup.2/V-s) (cm.sup.2/V-s) 30:70 0.72 7.7 0.41 2.4 0.4 .times.
10.sup.-4 6.3 .times. 10.sup.-4 50:50 0.75 9.2 0.44 3.0 1.0 .times.
10.sup.-4 4.8 .times. 10.sup.-4 70:30 0.72 8.7 0.35 2.2 1.6 .times.
10.sup.-4 1.9 .times. 10.sup.-4
Example 7
Film Morphology of SMDPPEH
[0191] The AFM images of the as-cast films of SMDPPEH:C.sub.71-PCBM
with different blend ratios are given in FIG. 10. All the films are
very smooth independent of the donor-acceptor blend ratio. The
average rms (root-mean-squared) surface roughnesses are .about.0.3
nm, 0.5 nm and 0.3 nm for the 30:70, 50:50, and 70:30 blend ratios,
respectively. No significant macro-scale phase segregation was
observed in any of the films. The 30:70 SMDPPEH:C.sub.71-PCBM blend
shows two distinct domains in both topographical and phase
images--both fiber-like structures and oval-shaped features (FIGS.
10 (a) and (b)). The domain sizes for the fibers increase as the
amount of donor material is increased from 30% to 50% and 70%. The
oval-shaped features (i.e., light spots in the height images and
dark spots in the phase images) become smaller in size when the
amount of C.sub.71-PCBM is decreased. From these images, it can be
inferred that the fiber-like structures are donor-rich domains and
the oval-shaped features are acceptor-rich domains. In all the
ratios, a high degree of ordering is observed. Combined with the
absence of large phase separation, this favorable ordered network
of donor and acceptor domains in the blend film can also help
explain the high PCEs observed for the devices.
Example 8
Synthesis of Compound A,
3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,-
4-c]pyrrole-1,4-dione (SMBFu)
[0192] In a three-necked, oven-dried 100 mL round-bottom flask,
3,6-bis-(5-bromo-thiophen-2-yl)-2,5-di-n-octyl-pyrrolo[3,4-c]pyrrole-1,4--
dione (0.683 g, 1.00 mmol) (Wienk, M. M., Turbiez, M., Gilot, J.
& Janssen, R. A. J., Adv. Mater. 20, 2556-2560 (2008)) was
mixed with 15 mL of anhydrous toluene and 10 ml of 2.0 M potassium
phosphate and the resulting mixture was degassed for 10 min.
Benzofuran-2-boronic acid (0.375 g, 2.25 mmol),
tris(dibenzylideneacetone)dipalladium(0) (14 mg, 0.0153 mmol), and
tri-tert-butylphosphonium tetrafluoroborate (18 mg, 0.0620 mmol)
were then added to the mixture and then degassed again for 5
minutes. The reaction mixture was stirred and heated to 90.degree.
C. under argon overnight. The reaction mixture was allowed to cool
down to room temperature, after which it was poured into 300 mL of
methanol and then stirred for 30 min. The precipitated solid was
then collected by vacuum filtration and washed with several
portions of distilled water, methanol, isopropanol, and petroleum
ether. The crude product was purified by flash chromatography using
chloroform as eluent, and the solvent was removed in vacuo to
obtain a pure product.
3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,-
4-c]pyrrole-1,4-dione is formed as a shiny, dark-green powder
(yield: 67.2%). mp 233.degree. C. .sup.1H NMR (250 MHz, CDCl.sub.3,
ppm) .delta.=9.01 (d, J=4.0 Hz, 2H), 7.48-7.61 (m, 6H), 7.20-7.36
(m, 6H), 7.05 (s, 2H), 4.85 (dd, J=4.0 Hz, 0.5 Hz, 4H) 1.98 (m,
2H), 1.20-1.50 (m, 20H), 0.80-1.00 (m, 12H). MS (LR-EI) m/z:
[M.sup.+] calculated for
C.sub.38H.sub.44N.sub.22N.sub.2O.sub.2S.sub.4: 756.31, found
756.03. CHN analysis: calcd: C, 72.89; H, 6.39; N, 3.70. found: C,
72.35; H, 6.33; N, 3.88.
Example 9
Fabrication of Solar Cells Incorporating SMBFu
[0193] Solar cells were fabricated by spin-casting the active bulk
heterojunction layer onto a 50 nm layer of H.C. Stark Baytron P
4083 poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) on
Corning 1737 active-matrix liquid-crystal-display glass patterned
with 140 nm of indium tin oxide by Thin Film Devices. An
80-nm-thick aluminum cathode was deposited (area 20 mm.sup.2) by
thermal evaporation with no heating of the sample. Unless otherwise
stated, the bulk heterojunction layer was spin-cast at 2,500 r.p.m.
from a solution of SMBFu and PC.sub.71BM in chloroform at a total
solids concentration of 20 mgml.sup.-1. PC.sub.71BM was purchased
from Nano-C. The active layers were determined to be approximately
95 nm thick using an Ambios XP-100 Stylus profilometer. Device
efficiencies were measured with a 150 WNewport-Oriel AM 1.5 G light
source operating at 100 mWcm.sup.-2 and independently cross-checked
using a 300 W AM 1.5 G source operating at 100 mWcm-2 for
verification. Solar-simulator illumination intensity was measured
using a standard silicon photovoltaic with a protective KG5 filter
calibrated by the National Renewable Energy Laboratory. IPCE
spectra measurements were made with a 250 W Xe source, a McPherson
EU-700-56 monochromater, optical chopper and lock-in amplifier, and
a National Institute of Standards and Technology traceable silicon
photodiode for monochromatic power-density calibration. AFM images
were taken on a Veeco Innova AFM. Ultraviolet-visible absorption
spectroscopy was measured on a Shimadzu 2401 diode array
spectrometer. For ultraviolet photoelectron spectroscopy (UPS)
measurements, a 75 nm thick Au film was deposited on a precleaned
Si substrates with a thin native oxide.
[0194] FIG. 11 shows the absorption of pure SMBFu solution and
films, pure PC.sub.71BM film and SMBFu:PC.sub.71BM films. SMBFu
absorbs past 650 nm in solution with a molar absorptivity of 64,000
M-1 cm.sup.-1 at 630 nm. The absorption broadens and extends to 710
nm in the solid state (FIG. 11a). The absorption spectrum of the
pure SMBFu changes considerably after thermal annealing as shown in
FIG. 11b. A significant increase in absorption intensity at 590 nm
occurs at annealing temperatures up to 100.degree. C., followed by
a decrease in intensity above 100.degree. C. Increases in optical
absorption after annealing have been observed for P3HT and are
found to derive from aggregation and an increase in the
crystallinity of the material, which enhances the probability of
optically active .pi.-.pi.* electronic transitions. The increase in
crystallinity of annealed SMBFu is confirmed by X-ray diffraction
(XRD) studies. XRD data shows an increase in the diffraction
intensity which implies higher degree of crystallinity after
thermal annealing at 100.degree. C. At annealing temperatures above
60.degree. C., the absorption peak at 660 nm begins to decrease.
From this information, it is seen that that optimal light
absorption for pure SMBFu is achieved after annealing at
temperatures between 60.degree. C. and 100.degree. C.
[0195] Absorption characteristics of as-cast SMBFu:PC.sub.71BM
films at various blend ratios are shown in FIG. 11c. It can be seen
that mixtures of the two materials absorb strongly throughout the
range of 300 nm to 700 nm. The absorption of the blends is
relatively high in the 500 to 700 nm range for all blend ratios,
reflecting the higher optical density of SMBFu relative to
PC.sub.71BM. The SMBFu absorption (550 to 700 nm) increases with an
increasing donor content while the PC.sub.71BM absorption reduces
with lower acceptor content in the blends.
[0196] Blend (or pure) solutions were then spin coated at spin
speeds of 2,000 rpm and concentration of 0.1%. All films were
fabricated inside a N.sub.2 atmosphere glovebox and were
transferred via an airtight sample holder to the UPS analysis
chamber. Samples were also kept in a high vacuum chamber overnight
to remove solvent residues. The UPS analysis chamber was equipped
with a hemispherical electron energy analyzer (Kratos Ultra
Spectrometer) and was maintained at 1.times.10.sup.-9 Torr. The UPS
measurements were carried out using the He I (hv=21.2 eV) source.
During UPS measurements, a sample bias of -9 V was used in order to
separate the sample and the secondary edge for the analyzer. In
order to confirm reproducibility of UPS spectra, the measurements
were repated twice on each set of samples. Thin film XRD spectra
were recorded using an X'Pert Phillips Material Research
Diffractometer (MRD) at 45 kV and 40 mA with a scanning rate of
0.004 degree per second, and Cu K.alpha. radiation (with wavelength
.lamda.=1.5405 .ANG.) with a 2theta-omega configuration. Carrier
mobilities were measured using the space charge limited current
technique where J-V curves were measured for devices with the
architecture ITO/PEDOT:PSS/SMBFu:PC.sub.71BM/Au for holes and
Mg/SMBFu:PC.sub.71BM/Mg for electrons. Electron mobility
measurements were repeated using the architecture
Al/SMBFu:PC.sub.71BM/Ba/Al. Electron mobilities were measured using
electrodes with different work functions to ensure that the
built-in potential of the devices did not affect the results.
Mobilities were extracted by fitting the current density-voltage
curves using the Mott-Gurney relationship.
[0197] One of the primary reasons that small molecules have not
been investigated more extensively as solution processable
semiconductors is that they tend to either be insoluble or form
rough films. Until now, morphologies have not been achieved which
allow efficient charge separation and collection such as those
observed in P3HT:PCBM systems. Since it is known that the
morphology of the active layer in BHJ solar cells significantly
affects the performance, the morphologies of pure SMBFu films and
blends with PC71BM were investigated using tapping mode atomic
force microscopy (AFM). SMBFu is soluble in chloroform at
60.degree. C. up to .about.20 mg/mL and forms smooth films with an
average surface roughness of 0.7 nm when spin cast onto indium tin
oxide (ITO) coated glass substrates with a 45 nm layer of
poly(styrenesulfonic acid) doped poly(ethylenedioxythiophene)
(PEDOT:PSS). Annealing pure SMBFu at .about.100.degree. C.
increases surface roughness to .about.1.0 nm.
[0198] The solid state structure of mixed donor-acceptor films
influences how the material will perform as a BHJ solar cell. The
donor and acceptor films should phase separate into an
interpenetrating network of donor and acceptor domains, where the
ideal domain size of a donor or acceptor material depends largely
on the exciton diffusion length in each material. When the domain
size of the material is smaller than the exciton diffusion length,
this allows many excitons to diffuse to a donor-acceptor interface
and undergo charge separation.
[0199] AFM was used to examine the surface structure of
SMBFu:PC.sub.71BM films as a function of annealing temperature.
SMBFu forms good quality films when spin cast with PC.sub.71BM.
Mixtures of SMBFu and PC.sub.71BM at 20 mg/mL overall concentration
in chloroform, spin cast at 2,500 rpm yield films approximately 95
nm thick. FIG. 12 shows the topographic and phase images of as-cast
and annealed SMBFu:PC.sub.71BM (70:30) films. The topographic and
phase images of as-cast SMBFu:PC.sub.71BM are featureless with a
surface roughness of .about.0.5 nm (FIG. 12a,d). Thermal annealing
the SMBFu:PC.sub.71BM film at temperatures above 80.degree. C.
result in significant changes in the film morphology. FIGS. 12b and
12c depict the morphologies of a 70-30 SMBFu:PC.sub.71BM blend
ratio after heating at 90.degree. C. and 100.degree. C. for 10 min
in nitrogen, respectively. The topographic image of
SMBFu:PC.sub.71BM annealed at 90.degree. C. comprises small oblong
domains between 10-50 nm in width with the surface roughness of
.about.1.1 nm (FIG. 12b). These oblong domains increase to
.about.100 nm upon thermal annealing at 100.degree. C., and the
surface roughness is .about.2.3 nm (FIG. 12c). Similar changes in
the film morphology upon thermal annealing are observed for other
blend ratios as well.
[0200] FIGS. 12e and 12f show the corresponding phase images of the
annealed SMBFu:PC.sub.71BM. The phase images show two distinct
domains (orange and blue colors) that form continuous networks. The
blue phase was assigned to a donor-rich material and the orange
phase was assigned to an acceptor-rich material because the
blue-phase and the orange-phase domains increase with the donor and
the acceptor contents, respectively. There is a clear dependence
between the annealing temperature and the domain size, where the
average domain size of the donor material increases from several
ten of nanometers when annealing at 90.degree. C. (FIG. 12e) to
.about.100 nm at 100.degree. C. (FIG. 12f). Thus, the domain sizes
at a fixed donor:acceptor ratio can be controlled by varying the
annealing temperature. The observed changes in morphology agree
well with the absorption and XRD results.
[0201] Next, the effect of donor:acceptor ratio on film morphology
was examined. The as-cast SMBFu:PC.sub.71BM films at various blend
ratios are smooth with a surface roughness of less than 1 nm
similar to those of FIGS. 12a and 12d. FIG. 13 shows the
topographic images of SMBFu:PC.sub.71BM films at various blend
ratios annealed at 100.degree. C. for 10 min. under nitrogen. It
can be seen that the 30:70 blend exhibits isolated clusters of
rod-like domains of donor material within a matrix of PC.sub.71BM
(FIG. 13a). The average width and length of the rod-like structures
are .about.80 nm and 440 nm, respectively, while the average
surface roughness is 2.3 nm. At a 50:50 ratio, the entire surface
is covered with rectangular clusters of rod-like domains (FIG.
13b), with a surface roughness of 2.1 nm. The dimensions of the
rod-shaped features are similar to those observed in the 30:70
ratio. The 70:30 ratio shows a morphology similar to the 50:50
ratio, with a somewhat more amorphous nature and smaller acceptor
domain sizes filling in the volume between donor domains (FIG.
13c), with an average surface roughness of 2.3 nm. Although the
size of the domains on the film surface appear to be somewhat
larger than typical exciton diffusion lengths for organic solids,
it was observed that SMBFu:PC.sub.71BM blends exhibit much weaker
photoluminescence (PL) than pure SMBFu, with much shorter PL
lifetimes (<60 ps for a 60:40 blend vs. 1100 ps, for the pure
material), demonstrating that the SMBFu:PC.sub.71BM hetero
structure effectively quenches excitons.
[0202] The charge carrier mobilities of BHJ films have a large
affect on how a material will perform in a solar cell. Low charge
carrier mobilities result in charge accumulation and inefficient
charge collection while un-balanced charge carrier mobilities
decrease the fill factor (FF) and efficiency of BHJ devices by
promoting excessive charge recombination. In order to quantify
carrier mobilities for the materials, current density-voltage (J-V)
characteristics of single-carrier diodes were measured for pure and
blended films. The hole and electron mobilities were extracted
using the space-charge limited current (SCLC) model. Pure SMBFu
films exhibit hole mobilities on the order of
.about.1.times.10.sup.-5 cm.sup.2/Vs before and after annealing at
100.degree. C. s. The average measured hole mobilities for as-cast
30:70, 50:50, 60:40 and 70:30 films were found to be
0.9.times.10.sup.-5 cm2/Vs, 2.times.10.sup.-5 cm.sup.2/Vs,
3.times.10.sup.-5 cm.sup.2/Vs and 3.times.10.sup.-5 cm.sup.2/Vs,
respectively. The hole mobilities did not change significantly upon
annealing. Electron mobilities were found to increase from
2.times.10.sup.-5 cm.sup.2/Vs to 70.times.10.sup.-5 cm.sup.2/Vs to
200.times.10.sup.-5 cm.sup.2/Vs for the 30:70, 50:50 and 70:30
blend ratios, respectively, increasing with the acceptor
concentration. The electron mobilities increased after annealing at
100.degree. C. to 90.times.10.sup.-5 cm.sup.2/Vs to
200.times.10.sup.-5 cm.sup.2/Vs to 300.times.10.sup.-5 cm.sup.2/Vs
for 30:70, 50:50 and 70:30 blend ratios, respectively. The dramatic
increase of the electron mobility at low acceptor concentration
suggests that percolation pathways for electron transport do not
exist in as-cast films with low fullerene concentrations, but are
developed after thermal annealing. The charge carrier mobilities
for all different blend ratios before and after annealing were
compared. The carrier mobilities were found to be more balanced at
high donor concentrations, consistent with the observed increase in
fill factor at high donor concentrations. The measured hole
mobilities for this series of materials were somewhat surprising,
as they did not increase after annealing despite an obvious
increase in the crystallinity of the donor material.
[0203] Finally, the performance of SMBFu:PC.sub.71BM films in solar
cells was examined. FIGS. 14a and 14b show the J-V and the incident
photon conversion efficiency (IPCE) characteristics of as-cast and
annealed SMBFu:PC.sub.71BM (60:40) devices under AM 1.5 G
irradiation at an intensity of 100 mW/cm.sup.2 irradiation. The
devices were annealed at various temperatures for 3 minutes under
nitrogen. The short-circuit current density (J.sub.sc), and
V.sub.oc and FF for all devices were found to increase after
scanning several times. Table 3 summarizes the device results. The
V.sub.oc is .about.0.92 V and remains unchanged for all annealing
temperatures. The short-circuit current density (J.sub.sc) is very
small for as-cast films (1.45 mA/cm.sup.2) and increases
substantially to a value of 8.90 mA/cm.sup.2 for devices annealed
at 80.degree. C. (Table 3). The J.sub.sc reaches the maximum value
of 9.96 mA/cm.sup.2 after annealing at 110.degree. C., a factor of
6 higher than for the as-cast devices. The J.sub.sc. decreases to
8.31 mA/cm.sup.2 for annealing at 150.degree. C. (Table 3). A
slight drop in J.sub.sc likely is a result of decreased charge
separation due to a reduced donor-acceptor interfacial area,
consistent with the observed change in the donor and the acceptor
domain sizes from the AFM images collected at high annealing
temperatures. The FF shows the same trend as the J.sub.sc. The FF
is 0.24 for as-cast devices and increases to 0.38 for devices
annealed at 80.degree. C. (Table 3). The FF reaches a maximum value
of 0.48 for devices annealed at 110.degree. C. and decreases
slightly to 0.46 for those annealed at 150.degree. C. The improved
J.sub.sc and FF of the device annealed at 110.degree. C. leads to
an efficiency of 4.4%, over an order of magnitude higher than the
as-cast device (0.33%) (Table 3).
[0204] The IPCEs of a 60:40 blend ratio at various annealing
temperatures are shown in FIG. 14b. Integrating the IPCE yields the
theoretical J.sub.sc values which equal to the measured values in
the J-V curves.+-.1 mA/cm.sup.2. The IPCE of a P3HT:PC.sub.61BM
device is included for comparison (dashed line). The IPCEs of the
SMBFu:PC.sub.71BM devices extend past 700 nm. The IPCE of the
as-cast SMBFu:PC.sub.71BM device is around 10%. The IPCE of the
60:40 device annealed at 100.degree. C. integrates to 10
mA/cm.sup.2 similar to the measured J.sub.sc (FIG. 14a) and reaches
a maximum of 58% at 585 nm. The shape of the EQE spectrum resembles
the shape of the absorption spectrum of the blended films, with the
notable exception that the relative height of the IPCE plot in the
350-500 nm region is higher than the absorption. This can be
explained by a higher charge separation rate for excitons generated
in the PC.sub.71BM phase, due to the smaller PC.sub.71BM domain
sizes as seen in the AFM images (FIG. 12).
[0205] FIGS. 14c and 14d show the J-V and the IPCE characteristics
of SMBFu:PC.sub.71BM devices annealed at 100.degree. C. for 10
minutes as a function of the blend ratios. The V.sub.oc remains the
same for all the blend ratios studied here. Before annealing,
J.sub.sc is highest (6.7 mA/cm.sup.2) for large acceptor
concentrations (70% by weight,). After annealing at 100.degree. C.,
J.sub.sc. increases with the donor concentration from 5.7
mA/cm.sup.2 for 30:70 blend ratio to 8.4 mA/cm.sup.2 for 50:50
ratio and reaches the highest value of 9.9 mA/cm.sup.2 for the
60:40 blend ratio (Table 3). J.sub.sc drops slightly to 9.0
mA/cm.sup.2 at higher donor concentration (70:30). The 60:40 ratio
appears to result in the best balance of light absorption, exciton
diffusion, charge separation and charge collection for both donor
and acceptor materials. The FF increases with increasing donor
concentration from 0.38 for 30:70 to 0.49 for 70:30 (Table 3),
which is likely due to a greater balance of charge carrier
mobilities at high donor concentrations. Maximum efficiency of 4.4%
occurs at blend ratio of 60:40 or 65:35 at annealing temperatures
between 100 and 110.degree. C. The IPCEs observed for different
blend ratios after annealing at 100.degree. C. for 10 min are
plotted in FIG. 14d. The IPCE increases with the donor
concentration. The relative quantum efficiency of the material at
different wavelengths does not show a strong dependence on blend
ratio as the absorption spectra do, suggesting that the collected
current is not limited by the absorption of photons by different
materials, but more likely influenced by the donor:acceptor
interfacial area and formation of percolation pathways.
[0206] The photovoltaic characteristics for different blend ratios
and annealing temperatures are summarized in Table 3. The highest
performance observed occurred for a 60:40 blend annealed for 3
minutes at 110.degree. C., yielding a short circuit current of 10.0
mA/cm.sup.2, a V.sub.oc of 0.92 V, a FF of 0.48 and a power
conversion efficiency of 4.4%. It is apparent that the annealing
temperature which yields the highest efficiency results in
donor-rich domain sizes on the order of .about.100 nm, higher than
expected for efficient exciton diffusion and is slightly higher
than annealing temperatures which yield the highest optical
absorption. This suggests that percolation paths are underdeveloped
at lower temperatures.
[0207] Table 3 below compares the device characteristics of
different SMBFu:PC.sub.71BM blend ratios annealed at different
temperatures. J.sub.SC is the short circuit current density,
V.sub.OC is the open circuit voltage, FF is the fill factor and
.eta. is the overall power conversion efficiency. The architecture
is ITO/PEDOT:PSS/SMBFu:PC.sub.71BM/Al for all devices.
TABLE-US-00003 TABLE 3 Device Characteristics of Different
SMBFU:PC.sub.71BM Blend Ratios Annealed at Different Temperatures
Blend Annealing J.sub.sc V.sub.oc .eta. Ratio Temp (.degree. C.)
(mA/cm.sup.2) (V) FF (%) 60-40 As-cast 1.45 0.96 0.24 0.33 60-40 80
8.90 0.92 0.38 3.1 60-40 100 9.88 0.92 0.42 3.8 60-40 110 9.96 0.92
0.48 4.4 60-40 150 8.31 0.92 0.46 3.5 30-70 As-cast 6.70 0.90 0.38
2.3 30-70 100 5.70 0.90 0.38 2.0 50-50 As-cast 4.81 0.94 0.35 1.6
50-50 100 8.38 0.90 0.45 3.4 60-40 As-cast 1.45 0.96 0.24 0.3 60-40
100 9.88 0.92 0.42 3.8 70-30 As-cast 0.78 0.88 0.26 0.18 70-30 100
9.01 0.94 0.49 4.2
Example 10
Synthesis of Additional Diketopyrrolopyrrole Compounds C6PT1C6,
C6PT2C6, C6PT3C6, EHPT2C6, and C6PT2
##STR00077##
[0209] Material Synthesis. 5-Hexyl-2-thiophene boronic acid pinacol
ester, 2,2'-bithiophene-5-boronic acid pinacol ester, and
5-hexyl-2,2'-bithiophene-5'-boronic acid pinacol ester were
purchased from Sigma-Aldrich Chemical Co. and used as received.
Other chemicals and solvents were used as received from commercial
sources without further purification. Tetrahydrofuran (THF) was
distilled over sodium/benzophenone.
3,6-bis(4-bromophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrolo-1,4-dione,
11, was synthesized following the procedure in D. Cao et al. J.
Poly. Sci. A 2006, 44, 2395.
[0210]
2,5-Dihexyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]-pyrrole-1,4-dione
(12a). To a solution of 11 (5.0 g, 11, mmol) in
N,N-dimethylformamide (DMF) (50 mL), 1-bromohexane (7.4 g, 45 mmol)
and cesium carbonate (11 g, 34 mmol) were added at 40.degree. C.
After stirring 24 hr, the reaction mixture was filtered to remove
solid. The filtrate was extracted with chloroform and
recrystallized from methanol to yield red needle-like crystal (4.1
g, 60%). .sup.1H NMR (200 MHz, CDCl.sub.3): 7.67 (s, 8H), 3.71 (t,
4H), 1.58 (m, 4H), 1.19 (m, 12H), 0.82 (t, 6H).
[0211]
2,5-Dihexyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]-pyrrole-1,4-dione
(12b). The procedure for the synthesis of 12a was followed to
prepare 12b using 2-ethylhexyl bromide (3.5 g, 18 mmol) instead of
1-bromohexane. The crude product was purified by gradient column
chromatography on a silical gel with using gradient solvent with
dichloromethane/hexane from 1/1 to 2.5/1 (v/v) to yield in 22%
(0.66 g). .sup.1H NMR (200 MHz, CDCl.sub.3): 7.65 (s, 8H), 3.70 (d,
4H), 1.45 (m, 2H), 1.09 (m, 16H), 0.74 (m, 12H).
[0212] 5-hexyl-2,2':5',2''-terthiophene, (15). To a mixture of
2-bromothiophene (1.56 g, 9.57 mmol),
5-hexyl-2,2'-bithiophene-5'-boronic acid pinacol ester (3.0 g, 8.0
mmol), tri (dibenzylidene-acetone)palladium (0)
(Pd.sub.2(dba).sub.3) (0.15 g, 0.17 mmol),
tri-tert-butylphosphonium tetrafluoroborate (0.18 g, 0.65 mmol),
and potassium phosphate (14 g, 64 mmol), degassed THF/water (30
mL/3 mL) was added. After stirring under argon at 80.degree. C.
overnight, the reaction mixture was poured into methanol. The crude
product was collected by filtration and purified by column
chromatography on a silica gel with hexane to obtain 15 (2.1 g,
81%). .sup.1H NMR (200 MHz, CDCl.sub.3): 7.20 (m, 2H), 7.04 (m,
4H), 6.68 (d, 1H), 2.79 (t, 2H), 2.64 (m, 2H), 1.33 (m, 6H), 0.89
(t, 3H).
[0213] 5-hexyl-2,2':5',2''-terthiophene-5''-boronic acid pinacol
ester, (16). To a solution of 15 (1.8 g, 5.4 mmol) in 45 mL of THF,
n-BuLi (2.6 mL, 6.5 mmol, 2.5 M in hexane) was added dropwise at
-78.degree. C. After stirred for 30 min at -78.degree. C., the
solution was warmed to room temperature and stirred for 1 hr. Then,
the mixture was cooled down to -78.degree. C. and add
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dixoaborolane (1.5 g, 8.1
mmol) at once. The mixture was warmed up to room temperature and
stirred overnight. After reaction was quenched with water, the
organic layer was extracted by ethyl acetate and dried over
magnesium sulfate. After solvent was removed under reduced
pressure, the crude product was purified by column chromatography
to obtain 16 (1.9 g, 77%). .sup.1H NMR (200 MHz, CDCl.sub.3): 7.51
(d, 1H), 7.22 (d, 1H), 7.11 (d, 1H), 6.99 (m, 2H), 6.68 (d, 1H),
2.79 (t, 2H), 1.68 (m, 2H), 1.35 (m, 20H), 0.89 (t, 3H).
[0214]
2,5-Dihexyl-3,6-bis[4-(5-hexylthiophene-2-yl)phenyl]pyrrolo[3,4-c]--
pyrrole-1,4-dione, C6PT1C6. To a mixture of 12a (0.75 g, 1.2 mmol),
5-hexyl-2-thiophene boronic acid pinacol ester (0.79 g, 2.7 mmol),
Pd.sub.2(dba).sub.3 (0.056 g, 0.061 mmol),
tri-tert-butylphosphonium tetrafluoroborate (0.11 g, 0.37 mmol),
and potassium phosphate (3.11 g, 14.7 mmol), degassed THF/water (30
mL/3 mL) was added. After stirring under argon at 80.degree. C.
overnight, the reaction mixture was poured into methanol. The crude
product was collected by filtration and purified by gradient column
chromatography on a silica gel with chloroform/hexane from 2/1 to
5/1 (v/v) to obtain C6PT1C6 (0.40 g, 42%). .sup.1H NMR (200 MHz,
CDCl.sub.3): 7.83 (d, 4H), 7.68 (d, 4H), 7.24 (d, 2H), 6.78 (d,
2H), 3.79 (t, 4H), 2.84 (4H), 1.68 (m, 8H), 1.23-1.35 (m, 24H),
0.87 (m, 12H).
[0215]
2,5-Dihexyl-3,6-bis[4-(5-hexyl-2,2'-bithiophene-5-yl)phenyl]pyrrolo-
[3,4-c]-pyrrole-1,4-dione, C6PT2C6. The procedure for the synthesis
of C6PT1C6 was followed using 5-hexyl-2,2'-bithiophene-5'-boronic
ester pinacol ester (0.92 g, 2.4 mmol), instead of 5-hexyl
thiophene boronic acid pinacol ester to yield in 91% (0.85 g).
.sup.1H NMR (200 MHz, CDCl.sub.3): 7.87 (d, 4H), 7.73 (d, 4H), 7.33
(d, 2H), 7.10 (d, 2H), 7.05 (d, 2H), 6.71 (d, 2H), 3.80 (t, 4H),
2.81 (t, 4H), 1.70 (m, 8H), 1.23-1.43 (m, 24H), 0.87 (m, 12H).
[0216]
2,5-Dihexyl-3,6-bis[4-(5-hexyl-2,2':5',2''-terthiophene-5''-yl)phen-
yl]pyrrolo[3,4-c]-pyrrole-1,4-dione, C6PT3C6. The procedure for the
synthesis of C6PT1C6 was followed using
5-hexyl-2,2':5',2''-terthiophene-5''-boronic acid pinacol ester
(0.90 g, 2.0 mmol), instead of 5-hexyl thiophene boronic acid
pinacol ester to yield in 78% (0.68 g). .sup.1H NMR (200 MHz,
CDCl.sub.3): 7.83 (d, 4H), 7.69 (d, 4H), 7.31 (d, 2H), 7.14 (m,
4H), 6.98 (m, 4H), 6.62 (d, 2H), 3.78 (t, 4H), 2.88 (t, 4H), 1.61
(m, 8H), 1.10-1.40 (m, 24H), 0.82 (m, 12H).
[0217]
2,5-Dihexyl-3,6-bis[4-(2,2'-bithiophene-5-yl)phenyl]pyrrolo[3,4-c]--
pyrrole-1,4-dione, C6PT2. The procedure for the synthesis of
C6PT1C6 was followed using 2,2'-bithiophene-5'-boronic acid pinacol
ester (0.89 g, 3.1 mmol), instead of 5-hexyl thiophene boronic acid
pinacol ester to yield in 78% (0.75 g). .sup.1H NMR (200 MHz,
CDCl.sub.3): 7.88 (d, 4H), 7.74 (d, 4H), 7.35 (d, 2H), 7.27 (m,
4H), 7.19 (d, 2H), 7.06 (m, 2H), 3.80 (t, 4H), 1.66 (m, 4H), 1.24
(m, 12H), 0.84 (t, 6H).
[0218]
2,5-Dihexyl-3,6-bis[4-(5-hexyl-2,2'-bithiophene-5-yl)phenyl]pyrrolo-
[3,4-c]-pyrrole-1,4-dione, EHPT2C6. The procedure for the synthesis
of C6PT1C6 was followed using 12b (0.58 g, 0.87 mmol) and
5-hexyl-2,2'-bithiophene-5'-boronic ester pinacol ester (0.81 g,
2.2 mmol), instead of 12a and 5-hexyl thiophene boronic acid
pinacol ester to yield in 86% (0.75 g). .sup.1H NMR (200 MHz,
CDCl.sub.3): 7.83 (d, 4H), 7.70 (d, 4H), 7.31 (d, 2H), 7.08 (d,
2H), 7.04 (d, 2H), 6.70 (d, 2H), 3.80 (d, 4H), 2.81 (t, 4H), 1.71
(m, 4H), 1.34 (m, 14H), 1.11 (m, 16H), 0.90 (t, 6H), 0.77 (m,
12H).
Example 11
Synthesis of Two-Core Diketopyrrolopyrrole Compounds with
Electron-Rich Bridge Moiety
##STR00078## ##STR00079##
[0220] 2,5-Dihexyl-3,6-dithienylpyrrolo[3,4-c]-pyrrole-1,4-dione
(22). To a solution of 21 (10 g, 33 mmol) in N,N-dimethylformamide
(DMF) (100 ml), 1-bromohexane (17 g, 99 mmol) and potassium
carbonate (12 g, 83 mmol) were added at 90.degree. C. After
stiffing 8 hr, the reaction mixture was filtered to remove solid.
The filtrate was extracted with chloroform and recrystallized from
methanol to yield 2 (9.0 g, 58%). .sup.1H NMR (200 MHz,
CDCl.sub.3): 8.94 (d, 2H), 7.65 (d, 2H), 7.29 (d, 2H), 4.08 (t,
4H), 1.75 (m, 4H), 1.22-1.50 (m, 12H), 0.89 (t, 6H).
[0221]
2,5-Dihexyl-3-bromothienyl-6-thienylpyrrolo[3,4-c]-pyrrole-1,4-dion-
e (23). To the solution of 22 (4.5 g, 9.6 mmol) in chloroform (140
ml), N-bromosuccinimide (NBS) (1.14 g, 6.4 mmol) was added. After
stiffing overnight at room temperature, the reaction mixture was
washed with water. The organic layer was dried over magnesium
sulfate and solvent was removed by reduced pressure. The crude
product was purified by gradient column chromatography on silica
gel with chloroform/hexane (from 2/1 to 4/1, v/v) to yield 23 (3.2
g, 91%). .sup.1H NMR (200 MHz, CDCl.sub.3): 8.95 (d, 1H), 8.67 (d,
1H), 7.67 (d, 1H), 7.29 (d, 1H), 7.23 (d, 1H), 4.02 (m, 4H), 1.71
(m, 4H), 1.22-1.50 (m, 12H), 0.89 (t, 6H).
[0222]
2,5-Dihexyl-3-thienyl-6-(5-hexyl-2,2':5',2''-terthienylpyrrolo[3,4--
c]-pyrrole-1,4-dione (24). To a mixture of 23 (1.0 g, 1.8 mmol),
5-hexyl-2,2'-bithiophene-5'-boronic acid pinacol ester (0.82 g, 2.2
mmol), tri (dibenzylidene acetone)palladium (0)
(Pd.sub.2(dba).sub.3) (0.084 g, 0.091 mmol),
tri-tert-butylphosphonium tetrafluoroborate (0.16 g, 0.55 mmol),
and potassium phosphate (3.1 g, 15 mmol), degassed THF/water (35
ml/2 ml) was added. After stirring under reflux overnight, the
reaction mixture was poured into methanol. The crude product was
collected by filtration and purified by column chromatography on a
silica gel with chloroform/hexane (5/1, v/v) to obtain 24 (1.3 g,
99%). .sup.1H NMR (200 MHz, CDCl.sub.3): 8.98 (d, 1H), 8.95 (d,
1H), 7.64 (d, 1H), 7.32 (d, 2H), 7.24 (d, 1H), 7.06 (t, 2H), 6.74
(d, 1H), 4.08 (t, 4H), 2.83 (t, 2H), 1.75 (m, 6H), 1.23-1.53 (m,
18H), 0.90 (m, 9H).
[0223]
2,5-Dihexyl-3-bromothienyl-6-(5-hexyl-2,2':5',2''-terthienylpyrrolo-
[3,4-c]-pyrrole-1,4-dione (25). To the solution of 24 (1.3 g, 1.7
mmol) in chloroform (45 ml), NBS (0.34 g, 1.9 mmol) was added.
After stirring overnight at room temperature, the reaction mixture
was washed with water. The organic layer was dried over magnesium
sulfate and solvent was removed by reduced pressure. The crude
product was purified by column chromatography on silica gel with
chloroform/hexane (4/1, v/v) to yield 5 (1.3 g, 92%). .sup.1H NMR
(200 MHz, CDCl.sub.3): 8.99 (d, 1H), 8.64 (d, 1H), 7.32 (d, 1H),
7.24 (d, 2H), 7.05 (t, 2H), 6.74 (d, 1H), 4.08 (m, 4H), 2.83 (t,
2H), 1.75 (m, 6H), 1.23-1.50 (m, 18H), 0.92 (m, 9H).
[0224]
9,9-Dihexylfluorene-2,7-bis(2,5-dihexyl-3-thienyl-6-(5-hexyl-2,2':5-
',2''-terthienyl pyrrolo[3,4-c]-pyrrole-1,4-dione) (26). To a
mixture of 25 (0.80 g, 1.0 mmol),
9,9-dihexylfluoroene-2,7-bis(boronic ester pinacol ester) (0.26 g,
0.44 mmol), Pd.sub.2(dba).sub.3 (0.070 g, 0.028 mmol),
tri-tert-butylphosphonium tetrafluoroborate (0.053 g, 0.18 mmol),
and potassium phosphate (1.1 g, 5.2 mmol), degassed THF/water (25
ml/2 ml) was added. After stiffing under reflux overnight, the
reaction mixture was poured into methanol. The crude product was
collected by filtration and purified by gradient column
chromatography on a silica gel with chloroform/hexane (from 1/1 to
4/1, v/v) to obtain 26 (0.40 g, 52%). .sup.1H NMR (200 MHz,
CDCl.sub.3): 9.02 (d, 2H), 8.95 (d, 2H), 7.73 (d, 2H), 7.68 (d,
2H), 7.63 (s, 2H), 7.55 (d, 2H), 7.31 (d, 2H), 7.23 (d, 2H), 7.05
(t, 4H), 6.72 (d, 2H), 4.15 (m, 8H), 2.83 (t, 4H), 2.08 (m, 4H),
1.82 (m, 8H), 1.70 (m, 4H), 1.50 (m, 8H), 1.30-1.45 (m, 28H), 1.10
(m, 12H), 0.90 (m, 9H), 0.75 (m, 10H).
Example 12
Synthesis of Two-Core Diketopyrrolopyrrole Compounds with
Electron-Deficient Bridge Moiety
##STR00080## ##STR00081##
[0226]
2,5-Bis(2-ethylhexyl)-3,6-dithienylpyrrolo[3,4-c]-pyrrole-1,4-dione
(32). To a solution of 31 (5.0 g, 17 mmol) in DMF (70 ml),
2-ethylhexyl bromide (13 g, 67 mmol) and potassium carbonate (6.9
g, 50 mmol) were added at 90.degree. C. After stirring 8 hr, the
reaction mixture was filtered to remove solid. The filtrate was
extracted with chloroform and dried over magnesium sulfate. Upon
evaporating solvent under reduced pressure, the crude product was
purified by column chromatography with chloroform/hexane (4/1, v/v)
to yield 32 (2.7 g, 32%). .sup.1H NMR (200 MHz, CDCl.sub.3): 8.90
(d, 2H), 7.63 (d, 2H), 7.29 (d, 2H), 4.04 (d, 4H), 1.83 (m, 2H),
1.22-1.50 (m, 16H), 0.88 (m, 12H).
[0227]
2,5-Bis(2-ethylhexyl)-3-bromothienyl-6-thienylpyrrolo[3,4-c]-pyrrol-
e-1,4-dione (33). To the solution of 32 (2.0 g, 3.8 mmol) in
chloroform (60 ml), NBS (0.45 g, 2.5 mol) was added. After stiffing
overnight at room temperature, the reaction mixture was washed with
water. The organic layer was dried over magnesium sulfate and
solvent was removed by reduced pressure. The crude product was
purified by gradient column chromatography on silica gel with
chloroform/hexane (from 2/1 to 4/1, v/v) to yield 33 (1.7 g, 75%).
.sup.1H NMR (200 MHz, CDCl.sub.3): 8.94 (d, 1H), 8.67 (d, 1H), 7.67
(d, 1H), 7.29 (d, 1H), 7.24 (d, 1H), 4.02 (m, 4H), 1.90 (m, 2H),
1.25-1.52 (m, 16H), 0.89 (m, 12H).
[0228]
2,5-Bis(2-ethylhexyl)-3-thienyl-6-(5-hexyl-2,2':5',2''-terthienylpy-
rrolo[3,4-c]-pyrrole-1,4-dione (34). To a mixture of 33 (1.0 g, 1.7
mmol), 5-hexyl-2,2'-bithiophene-5'-boronic acid pinacol ester (0.8
g, 2.0 mmol), Pd.sub.2(dba).sub.3 (0.076 g, 0.083 mmol),
tri-tert-butylphosphonium tetrafluoroborate (0.14 g, 0.50 mmol),
and potassium phosphate (2.8 g, 13 mmol), degassed THF/water (30
ml/3 ml) was added. After stirring under reflux overnight, the
reaction mixture was poured into methanol. The crude product was
collected by filtration and purified by column chromatography on a
silica gel with chloroform/hexane (5/1, v/v) to obtain 34 (1.2 g,
94%). .sup.1H NMR (200 MHz, CDCl.sub.3): 8.98 (d, 1H), 8.92 (d,
1H), 7.63 (d, 1H), 7.32 (d, 2H), 7.24 (d, 1H), 7.06 (t, 2H), 6.74
(d, 1H), 4.08 (d, 4H), 2.83 (t, 2H), 1.90 (m, 2H), 1.70 (m, 2H),
1.20-1.54 (m, 22H), 0.89 (m, 15H).
[0229]
2,5-Bis(2-ethylhexyl)-3-bromothienyl-6-(5-hexyl-2,2':5',2''-terthie-
nylpyrrolo[3,4-c]-pyrrole-1,4-dione (35). To the solution of 34
(1.2 g, 1.6 mmol) in chloroform (25 ml), NBS (0.30 g, 1.7 mmol) was
added. After stirring overnight at room temperature, the reaction
mixture was washed with water. The organic layer was dried over
magnesium sulfate and solvent was removed by reduced pressure. The
crude product was purified by column chromatography on silica gel
with chloroform/hexane (4/1, v/v) to yield 35 (1.2 g, 91%). .sup.1H
NMR (200 MHz, CDCl.sub.3): 9.00 (d, 1H), 8.66 (d, 1H), 7.32 (d,
1H), 7.25 (d, 2H), 7.07 (t, 2H), 6.74 (d, 1H), 4.08 (m, 4H), 2.83
(t, 2H), 1.90 (m, 2H), 1.70 (m, 2H), 1.20-1.55 (m, 22H), 0.92 (m,
15H).
[0230]
2,1,3-Benzothiadiazole-4,7-bis(2,5-dihexyl-3-thienyl-6-(5-hexyl-2,2-
':5',2''-terthienyl pyrrolo[3,4-c]-pyrrole-1,4-dione) (36). To a
mixture of 35 (0.86 g, 1.0 mmol),
2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol ester), (0.17
g, 0.44 mmol), Pd.sub.2(dba).sub.3 (0.028 g, 0.031 mmol),
tri-tert-butylphosphonium tetrafluoroborate (0.053 g, 0.18 mmol),
and potassium phosphate (0.745 g, 3.51 mmol), degassed THF/water
(20 ml/1 ml) was added. After stirring under reflux overnight, the
reaction mixture was poured into water and extracted with
chloroform. The solution was dried over magnesium sulfate, followed
by evaporating the solvent under reduced pressure. The crude
product was purified by gradient column chromatography on a silica
gel with chloroform/hexane (from 1/1 to 5/1, v/v) to obtain 36
(0.67 g, 91%). .sup.1H NMR (200 MHz, CDCl.sub.3): 9.05 (d, 2H),
8.99 (d, 2H), 7.90 (d, 2H), 7.70 (s, 2H), 7.07 (d, 2H), 7.00 (d,
2H), 6.88 (d, 2H), 6.82 (d, 2H), 6.60 (d, 2H), 3.95 (m, 8H), 2.83
(t, 4H), 1.90 (m, 4H), 1.70 (m, 4H), 1.20-1.55 (m, 44H), 0.92 (m,
30H).
##STR00082##
[0231]
2,5-Dibutyl-3,6-bis[4-(5-hexyl-2,2'-bithiophene-5-yl)phenyl]pyrrolo-
[3,4-c]-pyrrole-1,4-dione, C4PT2C6. To a mixture of
2,5-dibutyl-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]-pyrrole-1,4-dione
(0.40 g, 0.72 mmol), 5-hexyl-2,2'-bithiophene boronic acid pinacol
ester (0.67 g, 1.8 mmol), Pd.sub.2(dba).sub.3 (0.033 g, 0.036
mmol), tri-tert-butylphosphonium tetrafluoroborate (0.062 g, 0.22
mmol), and potassium phosphate (2.4 g, 12 mmol), degassed THF/water
(22 mL/2 mL) was added. After stiffing under reflux overnight, the
reaction mixture was poured into methanol. The crude product was
collected by filtration and purified by gradient column
chromatography on a silica gel with chloroform/hexane from 2/1 to
5/1 (v/v) to obtain C4PT2C6 (0.55 g, 86%). .sup.1H NMR (200 MHz,
CDCl.sub.3): 7.83 (d, 4H), 7.68 (d, 4H), 7.28 (d, 2H), 7.03 (d,
2H), 7.00 (d, 2H), 6.67 (d, 2H), 3.78 (t, 4H), 2.76 (t, 4H), 1.61
(m, 8H), 1.18-1.46 (m, 16H), 0.83 (m, 12H).
Example 13
Characteristics of Devices with Two-Core Diketopyrrolopyrrole
Compounds
[0232]
2,1,3-Benzothiadiazole-4,7-bis(2,5-dihexyl-3-thienyl-6-(5-hexyl-2,2-
':5',2''-terthienyl pyrrolo[3,4-c]-pyrrole-1,4-dione) (36) has a
band gap of approximately 1.3 eV and an absorption spectrum which
extends beyond 900 nm.
9,9-Dihexylfluorene-2,7-bis(2,5-dihexyl-3-thienyl-6-(5-hexyl-2,2'-
:5',2''-terthienyl pyrrolo[3,4-c]-pyrrole-1,4-dione) (26) has a
band gap of approximately 1.6 eV and an absorption spectrum which
extends to 800 nm.
[0233] One example of a device using compound 36 is shown in Device
1 of FIG. 15, which has a device architecture of
ITO/PEDOT:PSS/(donor:acceptor blend)/Al). The device was prepared
in a manner similar to that describe in Example 9 above. A
PEDOT:PSS layer was prepared on an ITO electrode by spincasting a
layer of Baytron Clevios P4083 onto a cleaned ITO substrate at 2500
rpm and annealing at 140.degree. C. for 30 minutes. A 50:50 mixture
of 36 and PC.sub.71BM was dissolved in chloroform at a
concentration of 16.7 mg/mL and spincast onto the ITO/PEDOT:PSS at
a rate of 2000 rpm. After the active layer was deposited, 80-100 nm
of aluminum (Aldrich evaporation grade) was evaporated at a base
pressure of 10.sup.-6 Torr and a rate less than or equal to 1.5
Angstrom/second. This yielded a device with a Jsc of 2.2
mA/cm.sup.2, a Voc of 0.51 V, a FF of 66% and a power conversion
efficiency of 0.74%.
[0234] In another example (Device 2 of FIG. 15), a 50:50 mixture of
material 36 and PC.sub.71BM was dissolved in a solvent mixture
consisting of 98% chloroform and 2% 1,8-octanedithiol and used to
prepare a device as described above. The mixture was prepared at a
solids concentration of 16.7 mg/mL and spincast at a rate of 2000
rpm, yielding a device with a Jsc of 5.3 mA/cm.sup.2, a Voc of 0.55
V, a FF of 53% and a power conversion efficiency of 1.6%.
[0235] In another example (Device 3 of FIG. 15), a 30:70 mixture of
material 26 and PC.sub.71BM was used to prepare a device as
described above. The mixture was dissolved in chloroform at a
concentration of 16.7 mg/mL and spincast at a rate of 2000 rpm and
annealed at 60.degree. C., yielding a device with a Jsc of 6.2
mA/cm.sup.2, a Voc of 0.78V, a FF of 55% and a power conversion
efficiency of 2.7%.
[0236] Current-voltage characteristics under 100 mW/cm.sup.2
simulated solar radiation of these Devices 1-3 are plotted in FIG.
15.
Example 14
Fabrication of Devices Using Different Casting Solvents
[0237] In some cases, it can be advantageous to select new or
different solvents for the fabrication of the active layer. Any
volatile solvent which adequately dissolves both donor and acceptor
material may be used to fabricate the active layer, so long as the
concentration and processing conditions are adjusted to achieve
films with thickness in the range of 50 to 300 nm, preferably
75-125 nm, and the annealing temperature is optimized for the
solvent being used.
[0238] The devices described below were prepared using the method
described for the devices in Example 13. In one example (Device 4),
a 60:40 mixture of SMBFu and PC.sub.71BM were dissolved in
chloroform at a concentration of 16.7 mg/mL, spincast on a
substrate at 1500 rpm and annealed at 130.degree. C. for 10
minutes, yielding a device with a Jsc of 10.1 mA/cm.sup.2, a Voc of
0.88V, a FF of 48% and a power conversion efficiency of 4.3%.
[0239] In another example (Device 5 of FIG. 16), a 60:40 mixture of
SMBFu and PC.sub.71BM were dissolved in thiophene at a
concentration of 16.7 mg/mL, spincast on a substrate at 1500 rpm
and annealed at 130.degree. C. for 10 minutes, yielding a device
with a Jsc of 9.7 mA/cm.sup.2, a Voc of 0.89V, a FF of 45% and a
power conversion efficiency of 3.9%.
[0240] In another example (Device 6 of FIG. 16), a 60:40 mixture of
SMBFu and PC.sub.71BM were dissolved in trichloroethylene at a
concentration of 16.7 mg/mL, spincast on a substrate at 800 rpm and
annealed at 110.degree. C. for 10 minutes, yielding a device with a
Jsc of 10.1 mA/cm.sup.2, a Voc of 0.89V, a FF of 47% and a power
conversion efficiency of 4.2%.
[0241] In another example (Device 7 of FIG. 16), a 60:40 mixture of
SMBFu and PC.sub.71BM were dissolved in carbon disulfide at a
concentration of 16.7 mg/mL, spincast on a substrate at 1500 rpm
and annealed at 80.degree. C. for 10 minutes, yielding a device
with a Jsc of 10.6 mA/cm.sup.2, a Voc of 0.92V, a FF of 44% and a
power conversion efficiency of 4.2%.
[0242] Current-voltage characteristics under 100 mW/cm.sup.2
simulated solar radiation of these Devices 4-7 are plotted in FIG.
16.
Example 15
Fabrication of Devices Using Different Acceptor Materials
[0243] Although fullerenes typically yield significantly higher
efficiencies than other electron accepting materials when used in
BHJ solar cells, they are very expensive to synthesize and refine.
Additionally, it can be difficult to adjust the ionization
potential (HOMO) and electron affinity (LUMO) of fullerene
derivatives in order to adjust device characteristics such as Voc.
Specifically, the Voc of optimized devices is often found to be
about 0.3 V less than the difference between the LUMO of the
acceptor and the HOMO of the donor (A). For these reasons, it may
be advantageous to use electron accepting materials other than
fullerenes.
[0244] In one example, the DPP based electron donating material
C4PT2C6 (indicated as Material 3 of FIG. 17B) and a non-fullerene
electron accepting material (Material 4, shown in FIG. 17A) was
used in place of PCBM. The device described below was prepared
using the method described for the devices in Example 13.
[0245] An energy level diagram comparing HOMO and LUMO values of
C4PT2C6 (Material 3) with PCBM is shown at left in FIG. 17B, while
an energy level diagram comparing HOMO and LUMO values of C4PT2C6
(Material 3) with Material 4 is shown at right in FIG. 17B. The
higher LUMO level of this acceptor relative to PCBM leads to a
larger .DELTA. value and indicates that BHJ solar cells prepared
from this donor acceptor combination should yield higher Voc's
compared to PCBM.
[0246] In one example (Device 8 of FIG. 18), a 50:50 mixture of
C4PT2C6 and Material 4 are dissolved in chloroform at a
concentration of 16.7 mg/mL, spincast on a substrate at 4000 rpm
and annealed at 110.degree. C. for 10 minutes, yielding a device
with a Jsc of 2.5 mA/cm.sup.2, a Voc of 1.10 V, a FF of 34% and a
power conversion efficiency of 1.1%. Current-voltage
characteristics under 100 mW/cm.sup.2 simulated solar radiation of
device 8 are plotted in FIG. 18.
Example 16
Inverted Device Architecture
[0247] Devices having inverted architecture were prepared. In one
example (Device 9), the work function of an ITO substrate is
modified using a self-assembled monolayer of
3-aminopropyltrimethoxysiloxane and measured to be 4.3 eV by UPS. A
60:40 mixture of SMBFu and PC71BM were dissolved in chloroform at a
concentration of 16.7 mg/mL and spincast on the modified substrate
at 2000 rpm. A layer of PEDOT:PSS was then deposited on top by
spincasting a 2:1 mixture of isopropanol:Baytron Clevios P4083 at
3000 rpm. The resulting film was annealed at 110.degree. C. for 30
minutes, followed by the evaporation of Au anodes at a base
pressure of 10.sup.-6 ton and a rate of less than 1
Angstrom/second. This device had a Jsc of 6.4 mA/cm.sup.2, a Voc of
0.74 V, a FF of 43% and a power conversion efficiency of 2.0%.
[0248] In another example (Device 10), an ITO substrate was
modified by spincasting a solution consisting of 4% titanium
isopropoxide, 10% dimethoxyethanol and 86% isopropanol by volume at
3000 rpm in an atmosphere with 20 ppm water concentration. This
substrate was then annealed at 200.degree. C. for 30 minutes in the
same atmosphere. A 60:40 mixture of SMBFu and PC71BM were dissolved
in chloroform at a concentration of 16.7 mg/mL and spincast on the
modified substrate at 2000 rpm. A layer of PEDOT:PSS was then
deposited on top by spincasting a 2:1 mixture of
isopropanol:Baytron Clevios P4083 at 3000 rpm. The resulting film
was annealed at 110.degree. C. for 30 minutes, followed by the
evaporation of Au anodes at a base pressure of 10.sup.-6 ton and a
rate of less than 1 Angstrom/second. This device had a Jsc of 8.8
mA/cm.sup.2, a Voc of 0.88 V, a FF of 36% and a power conversion
efficiency of 2.8%.
[0249] FIG. 19A shows an energy level diagram for a normal device,
while FIG. 19B shows an energy level diagram for an inverted
device. The current-voltage characteristics of a normal device
(Device 1 of FIG. 15 and Example 13) are compared to the inverted
devices (Device 9 and Device 10) in FIG. 20.
[0250] The disclosures of all publications, patents, patent
applications and published patent applications referred to herein
by an identifying citation are hereby incorporated herein by
reference in their entireties.
[0251] Although the foregoing invention has been described in some
detail by way of illustration and example for purposes of clarity
of understanding, it is apparent to those skilled in the art that
certain changes and modifications will be practiced. Therefore, the
description and examples should not be construed as limiting the
scope of the invention.
* * * * *