U.S. patent application number 14/948027 was filed with the patent office on 2016-03-17 for fullerene surfactants and their use in polymer solar cells.
This patent application is currently assigned to University of Washington through its Center for Commercialization. The applicant listed for this patent is University of Washington through its Center for Commercialization. Invention is credited to Kwan-Yue Jen, Chang-zhi Li, Hin-Lap Yip.
Application Number | 20160079541 14/948027 |
Document ID | / |
Family ID | 48653362 |
Filed Date | 2016-03-17 |
United States Patent
Application |
20160079541 |
Kind Code |
A1 |
Jen; Kwan-Yue ; et
al. |
March 17, 2016 |
FULLERENE SURFACTANTS AND THEIR USE IN POLYMER SOLAR CELLS
Abstract
Fullerene surfactant compounds useful as interfacial layer in
polymer solar cells to enhance solar cell efficiency. Polymer solar
cell including a fullerene surfactant-containing interfacial layer
intermediate cathode and active layer.
Inventors: |
Jen; Kwan-Yue; (Kenmore,
WA) ; Yip; Hin-Lap; (Seattle, WA) ; Li;
Chang-zhi; (Seattle, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Washington through its Center for
Commercialization |
Seattle |
WA |
US |
|
|
Assignee: |
University of Washington through
its Center for Commercialization
Seattle
WA
|
Family ID: |
48653362 |
Appl. No.: |
14/948027 |
Filed: |
November 20, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13706230 |
Dec 5, 2012 |
9214574 |
|
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14948027 |
|
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|
61566943 |
Dec 5, 2011 |
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Current U.S.
Class: |
136/263 ;
977/738; 977/755; 977/948 |
Current CPC
Class: |
B82Y 30/00 20130101;
C07D 207/08 20130101; Y10S 977/755 20130101; Y10S 977/948 20130101;
H01L 51/0043 20130101; Y10S 977/738 20130101; H01L 51/0036
20130101; H01L 51/4273 20130101; B82Y 10/00 20130101; H01L 51/0047
20130101; Y02E 10/549 20130101; H01L 51/442 20130101; H01L 31/02167
20130101; B82Y 20/00 20130101; H01L 51/4253 20130101; H01L 51/0037
20130101; C07D 209/70 20130101 |
International
Class: |
H01L 51/00 20060101
H01L051/00; H01L 51/44 20060101 H01L051/44; H01L 51/42 20060101
H01L051/42 |
Goverment Interests
STATEMENT OF GOVERNMENT LICENSE RIGHTS
[0002] This invention was made with Government support under
Contract Nos. FA2386-11-1-4072 awarded by the Air Force Office of
Scientific Research, N00014-11-1-0300 awarded by the Office of
Naval Research, and DE-FC3608GO18024/A000 awarded by the Department
of Energy. The Government has certain rights in the invention.
Claims
1. A photovoltaic device comprising: (a) a first electrode; (b) an
active layer disposed on a surface of the first electrode; (c) a
layer comprising a compound disposed on a surface of the active
layer opposite the first electrode, wherein the compound comprises
(i) a fullerene group; (ii) one or more cationic nitrogen centers
covalently coupled to the fullerene group; (iii) one or more
hydrophilic groups covalently coupled to the fullerene group; (iv)
one or more counter ions associated with the cationic nitrogen
center; and (d) a second electrode disposed on a surface of the
layer comprising the compound opposite the active layer.
2. The photovoltaic device of claim 1, wherein the fullerene group
is selected from the group consisting of C.sub.60, C.sub.70,
C.sub.76, C.sub.78, C.sub.82, C.sub.84, and C.sub.92 fullerene
groups.
3. The photovoltaic device of claim 1, wherein the cationic
nitrogen center is a quaternary amine group.
4. The photovoltaic device of claim 1, wherein the hydrophilic
group is a polyether group.
5. The photovoltaic device of claim 4, wherein the polyether group
is a polyalkene oxide group.
6. The photovoltaic device of claim 5, wherein the polyalkene oxide
group is a polyethylene oxide group having the formula
--(CH.sub.2CH.sub.2O).sub.n--, where n is from 1 to about 20.
7. The photovoltaic device of claim 1, wherein the compound further
comprises an anionic center.
8. The photovoltaic device of claim 7, wherein the anionic center
is selected from the group consisting of sulfonate
(SO.sub.3.sup.2-) and carboxylate (--CO.sub.2.sup.-) groups.
9. The photovoltaic device of claim 1, wherein the fullerene group
is selected from the group consisting of a mono-fulleropyrrolidium
group and a bis-fulleropyrrolidium group.
10. The photovoltaic device of claim 1, wherein the compound has
the structure: ##STR00011## wherein F is a fullerene group; B is a
N-containing ring having from 5-7 ring atoms; R.sub.1 and R.sub.2
are independently selected from the group consisting of a
polyalkylene oxide and a C1-C20 alkyl optionally substituted with
an anionic center; Ar is --C.sub.6H.sub.5-PEO, wherein
--C.sub.6H.sub.5-PEO is selected from the group consisting of
mono-, di-, tri-, and tetra-PEO substituted phenyl; and A.sup.- is
a counter ion associated with the cationic nitrogen center.
11. The photovoltaic device of claim 1, wherein the compound has
the structure: ##STR00012## wherein F is a fullerene group; B is a
N-containing ring having from 5-7 ring atoms; R.sub.1 and R.sub.2
are independently selected from the group consisting of a
polyalkylene oxide and a C1-C20 alkyl optionally substituted with
an anionic center; Ar is --C.sub.6H.sub.5-PEO, wherein
--C.sub.6H.sub.5-PEO is selected from the group consisting of
mono-, di-, tri-, and tetra-PEO substituted phenyl; and A.sup.- is
a counter ion associated with the cationic nitrogen center.
12. The photovoltaic device of claim 1, wherein the compound has
the structure: ##STR00013##
13. The photovoltaic device of claim 1, wherein the compound has
the structure: ##STR00014## wherein R is independently selected
from the group consisting of C1-C20 straight chain and branched
alkyl; PEO is an alkylene oxide group independently selected from
the group consisting of polyethylene oxide having the formula
--(CH.sub.2CH.sub.2O).sub.n--, where n is from 1 to about 20 or
polypropylene oxide having the formula
--(CH(CH.sub.3)CH.sub.2O).sub.n--, where n is from 1 to about 20;
C.sub.6H.sub.5-PEO is selected from the group consisting of mono-,
di-, tri-, and tetra-PEO substituted phenyl; and A.sup.- is a
counter ion selected from the group consisting of fluoride,
chloride, bromide, iodide, trifluoromethyl sulfonyl
(CF.sub.3SO.sub.3.sup.-), tetrakis(imidazolyl)borate
(BIm.sub.4.sup.-), and
tetrakis(3,5-bis(trifluoromethyl)phenyl]borate (TFPB.sup.-).
14. The photovoltaic device of claim 1, wherein the compound has
the structure: ##STR00015## wherein R is independently selected
from the group consisting of C1-C20 straight chain and branched
alkyl; PEO is an alkylene oxide group independently selected from
the group consisting of polyethylene oxide having the formula
--(CH.sub.2CH.sub.2O).sub.n--, where n is from 1 to about 20 or
polypropylene oxide having the formula
--(CH(CH.sub.3)CH.sub.2O).sub.n--, where n is from 1 to about 20;
C.sub.6H.sub.5-PEO is selected from the group consisting of mono-,
di-, tri-, and tetra-PEO substituted phenyl; and A.sup.- is a
counter ion selected from the group consisting of fluoride,
chloride, bromide, iodide, trifluoromethyl sulfonyl
(CF.sub.3SO.sub.3.sup.-), tetrakis(imidazolyl)borate
(BIm.sub.4.sup.-), and
tetrakis(3,5-bis(trifluoromethyl)phenyl]borate (TFPB.sup.-).
15. The photovoltaic device of claim 1 further comprising a charge
transport layer intermediate the first electrode and the active
layer.
16. The photovoltaic device of claim 1, wherein the active layer
comprises an active fullerene material.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 13/706,230, filed Dec. 5, 2012, which claims the benefit of
U.S. Application No. 61/566,943, filed Dec. 5, 2011, each
application is expressly incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0003] Effective control of organic-metal interfaces is critical
for achieving high-performance polymer solar cells (PSCs). Ideally,
the work-function (F) of the cathode and anode should be aligned
with the energy of the photo-excited quasi-Fermi levels (E.sub.F)
of organic semiconductors to create Ohmic contact for maxing
achievable open-circuit voltage (V.sub.oc) and minimized energy
barrier for charge-extraction. Although low .PHI. metal such as Ca
(.PHI.=2.9 eV) has been proved to form good contact with bulk
heterojunction (BHJ) layer as cathode, its vulnerability to
environmental conditions undermines its use for practical
applications. More stable metals like Al (.PHI.=4.28 eV) and Ag
(.PHI.=4.57 eV) have been used as cathode, but their relatively
high .PHI. often cause energy mismatch between BHJ blends and
themselves, which results in lower V.sub.oc and device
performance.
[0004] To alleviate this problem, proper interfacial engineering by
inserting a thin layer between cathode and active layer has been
vigorously explored. For example, inorganic materials such as LiF
and Cs.sub.2CO.sub.3 and metal oxides (TiO.sub.x, ZnO.sub.x), and
organic materials such as insulating poly(ethylene oxide) (PEO) and
conjugated polyelectrolyte (CPE) have also been proved to be
effective in improving Al cathode based device performance. In a
recent study, 8.37% of PCE was reported by inserting polyfluorene
derivative (PFN) between the high performance PTB7:PC71BM BHJ and
Ca/Al. In addition, self-assembled fullerenes (e.g., PCBM capped
PEG and fluorocarbon modified PCBM (F-PCBM)) have also been
reported to increase P3HT:PCBM based device performance.
[0005] Despite that interface engineering has been performed for
conventional PSCs, the performances obtained from Ag-based devices
were usually lower than those using Ca/Al and Al cathode. This
significantly limits the utilization of stable and reflective Ag as
cathode for improving performance and stability of devices, though
it is well-known Ag anode can be advantageous in inverted PSCs to
facilitate the printing process.
[0006] On the other hand, fullerene-based materials not only can
match well with the energy level of the lowest unoccupied molecular
orbital (LUMO) of commonly used acceptor (e.g., PCBM), but also
possess sufficiently deep highest occupied molecular orbital (HOMO)
energy level, which make them as energetically ideal candidates for
electron transport layer (ETL) to facilitate electron-selecting and
hole-blocking in PSCs.
[0007] Despite the advances in the development of materials to
enhance solar cell performance, a need exists to provide effective
interfacial materials that are capable of adjusting the .PHI. of
cathode to improve the contact with the BHJ layer, possess
reasonable electron mobility to minimize electrical resistance
across the interfacial layer, and have sufficient orthogonal
solvent-processability and film forming properties to avoid eroding
into the BHJ layer. The present invention seeks to fulfill this
need and provides further related advantages.
SUMMARY OF THE INVENTION
[0008] The present invention provides fullerene surfactant
compounds that can be incorporated into polymer solar cells as an
interfacial layer intermediate the cells' active layer and cathode
to enhance solar cell efficiency.
[0009] In one aspect the invention provides a fullerene compound,
comprising:
[0010] (a) a fullerene group;
[0011] (b) one or more cationic nitrogen centers covalently coupled
to the fullerene group;
[0012] (c) one or more hydrophilic groups covalently coupled to the
fullerene group; and
[0013] (d) one or more counter ions associated with the cationic
nitrogen center.
[0014] Representative fullerene groups include C.sub.60, C.sub.70,
C.sub.76, C.sub.78, C.sub.82, C.sub.84, and C.sub.92 fullerene
groups. In one embodiment, the fullerene group is a C.sub.60
fullerene group. In one embodiment, the cationic nitrogen center is
a quaternary amine group. Suitable hydrophilic groups include
polyether and polyol groups. In certain embodiments, the polyether
group is a polyalkene oxide group such as a polyethylene oxide
group having the formula --(CH.sub.2CH.sub.2O).sub.n--, where n is
from 1 to about 20. In certain embodiments, the fullerene compound
further includes comprising an anionic center. Representative
anionic centers include sulfonate (SO.sub.3.sup.2-) and carboxylate
(--CO.sub.2.sup.-) groups. In one embodiments, the fullerene
compound is a mono-fulleropyrrolidium. In other embodiment, the
fullerene compound is a bis-fulleropyrrolidium.
[0015] In one embodiment, the compound has the structure:
##STR00001##
[0016] In another embodiment, the compound has the structure:
##STR00002##
[0017] In these embodiments, F is a fullerene group; B is a
N-containing ring having from 5-7 ring atoms; R.sub.1 and R.sub.2
are independently selected from the group consisting of a
polyalkylene oxide and a C1-C20 alkyl optionally substituted with
an anionic center; Ar is --C.sub.6H.sub.5-PEO, wherein
--C.sub.6H.sub.5-PEO is selected from the group consisting of
mono-, di-, tri-, and tetra-PEO substituted phenyl; and A.sup.- is
a counter ion associated with the cationic nitrogen center.
[0018] In another aspect of the invention, photovoltaic devices are
provided. In certain embodiments, the photovoltaic device includes
an interfacial layer intermediate the cathode and active layer,
wherein the interfacial layer includes one or more fullerene
surfactant compounds of the invention. In one embodiment, the
photovoltaic device includes:
[0019] (a) a first electrode;
[0020] (b) an active layer disposed on a surface of the first
electrode;
[0021] (c) a layer comprising a fullerene compound of the invention
disposed on a surface of the active layer opposite the first
electrode; and
[0022] (d) a second electrode disposed on a surface of the layer
comprising the fullerene compound of the invention opposite the
active layer.
[0023] In another embodiment, the device further includes a hole
transport layer intermediate the first electrode and the active
layer.
DESCRIPTION OF THE DRAWINGS
[0024] The foregoing aspects and many of the attendant advantages
of this invention will become more readily appreciated as the same
become better understood by reference to the following detailed
description, when taken in conjunction with the accompanying
drawings.
[0025] FIG. 1 illustrates the structures of representative
fullerene surfactants of the invention.
[0026] FIG. 2 is a schematic illustration of the preparation of two
representative fullerene surfactants of the invention, ETL-1 and
ETL-2.
[0027] FIG. 3 is a cross-sectional view of a representative
photovoltaic device of the invention incorporating a fullerene
surfactant-containing interfacial layer intermediate the cathode
and the active layer.
[0028] FIG. 4A is a schematic illustration of the use of two
representative fullerene surfactants of the invention, ETL-1 and
ETL-2, as interfacial layers in a PIDT-PhanQ:PC.sub.71BM polymer
solar cell device. FIG. 4B is a schematic energy diagram of the
device shown in FIG. 4A:
PIDT-PhanQ:poly(indacenodithiophene-co-phananthrene-quinoxaline)
PC.sub.71BM: [6,6]-phenyl C71-butyric acid methyl ester.
[0029] FIGS. 5A-5F compare the current density-voltage (J-V)
characteristics of devices under illumination of AM 1.5 G at 100 mW
cm.sup.-2 for Al, Ca/Al, and Ag cathodes (FIGS. 5A, 5C, and 5E),
respectively, and their corresponding external quantum efficiency
(EQE) spectra (FIGS. 5B, 5D, and 5F).
[0030] FIG. 6 compares surface morphology (5 .mu.m.times.5 .mu.m)
and surface profile (10 nm to -10 nm) of PIDT-PhanQ:PC.sub.71BM BHJ
based device: (a) BHJ only, (b) ETL-1 on BHJ, (c) ETL-2 on BHJ. RMS
roughness for (a) 0.733 nm, (b) 1.01 nm, (c) 0.763 nm,
respectively.
[0031] FIG. 7 shows the chemical structure of a representative
fullerene surfactant of the invention: ETL-2 ("C.sub.60-bis").
[0032] FIG. 8A is a schematic illustration of the architecture of a
representative photovoltaic device of the invention. FIG. 8B is a
schematic illustration of the energy level diagram of the device of
FIG. 8A.
[0033] FIGS. 9A-9D compare performance data for
PIDT-PhanQ:PC.sub.71BM devices fabricated with different choice of
cathode metal with and without a C.sub.60-bis interlayer. The
current density-voltage curves (9A) and external quantum efficiency
spectra (9B) show increases in V.sub.OC and J.sub.SC respectively.
Capacitance-voltage (9C) and Mott-Schottky (9D) analysis explain
increased V.sub.OC in terms of the V.sub.BI of the Schottky
contact.
[0034] FIG. 10 compares normalized PCE for Al, Ag, and Cu devices
with and without C.sub.60-bis under ambient conditions.
[0035] FIG. 11 is a secondary electron cutoff spectrum and first
derivative of Ar.sup.+ ion sputter-cleaned Au foil represented on
the kinetic energy scale. The vertical line through the center of
the first derivative is a guide for reading the work function
directly from the kinetic energy scale.
[0036] FIGS. 12A-12C compare secondary electron cutoff spectra of
Al (12A), Ag (12B), and Cu (12C) metal films with and without
C.sub.60-bis. Films without C.sub.60-bis were Ar.sup.+
sputter-cleaned in vacuo prior to measurement. The Cu spectrum
includes that of clean Au foil as a reference.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The present invention provides fullerene surfactants and
their use to modify the interface of the cathode and bulk
heterojunction layer in organic solar cells. The incorporation of
an interfacial layer including a fullerene surfactant of the
invention in a conventional polymer solar cell enhances the
efficiency of the solar cell.
[0038] In one aspect, the invention provides a fullerene
surfactant. As used herein, the term "fullerene surfactant" refers
to a fullerene that includes hydrophilic group sufficient to render
the fullerene solution processable in the fabrication of polymer
solar cells. The fullerene surfactant includes a fullerene group,
one or more cationic amine centers, one or more hydrophilic groups,
and one or more counter ions. The cationic amine group is
covalently coupled to the fullerene group. The hydrophilic group is
covalently the fullerene group. In certain embodiments, the
hydrophilic group is covalently coupled to the fullerene group
through the cationic amine group. In certain embodiments, the
fullerene surfactant further includes an anionic center.
[0039] Representative fullerene groups include C.sub.60, C.sub.70,
C.sub.76, C.sub.78, C.sub.82, C.sub.84, and C.sub.92 fullerene
groups. In one embodiment, the fullerene surfactant of the
invention includes a C.sub.60 group.
[0040] The cationic amine group is a positively-charged amine
center. Suitable cationic amine groups include quaternary amine
groups prepared by quaternizing amine precursor compounds. In
certain embodiments, the fullerene surfactants of the invention are
prepared by quaternization of precursor fullerene amine
compounds.
[0041] Representative hydrophilic groups include one or more
hydrophilic substituents such as ether and alcohol groups. In
certain embodiments, the hydrophilic group is a polyether group.
Representative polyether groups include polyalkylene oxides with as
polyethylene oxide (PEO) groups, polypropylene oxide (PPO) groups,
and groups that include ethylene oxide and propylene oxide groups.
Suitable polyethylene oxide groups have the formula
--(CH.sub.2CH.sub.2O).sub.n--, where n is from 1 to about 20, and
--(CH(CH.sub.3)CH.sub.2O).sub.n--, where n is from 1 to about 20.
In other embodiments, the hydrophilic group is a polyol.
[0042] In embodiments of the fullerene surfactants that include
anionic centers, representative anionic centers include sulfonate
(SO.sub.3.sup.2-) and carboxylate (--CO.sub.2.sup.-) groups. The
anionic centers are covalently coupled to the fullerene group.
[0043] Representative fullerene surfactants of the invention are
illustrated in FIG. 1. Referring to fullerene surfactant compounds
F1-F14 in FIG. 1, the fullerene group may be any one of C.sub.60,
C.sub.70, C.sub.76, C.sub.78, C.sub.82, C.sub.84, and C.sub.92
fullerene groups; R is independently selected from the group
consisting of C1-C20 straight chain and branched alkyl; PEO is an
alkylene oxide group independently selected from the group
consisting of polyethylene oxide having the formula
--(CH.sub.2CH.sub.2O).sub.n--, where n is from 1 to about 20 or
polypropylene oxide having the formula
--(CH(CH.sub.3)CH.sub.2O).sub.n--, where n is from 1 to about 20;
--C.sub.6H.sub.5-PEO is selected from the group consisting of
mono-, di-, tri-, and tetra-PEO substituted phenyl; and A.sup.- is
a counter ion selected from the group consisting of fluoride,
chloride, bromide, iodide, trifluoromethyl sulfonyl
(CF.sub.3SO.sub.3.sup.-), tetrakis(imidazolyl)borate
(BIm.sub.4.sup.-), and
tetrakis(3,5-bis(trifluoromethyl)phenyl]borate (TFPB.sup.-).
[0044] In one embodiment, the fullerene surfactant compounds of the
invention have formula (IA):
##STR00003##
[0045] In another embodiment, the fullerene surfactant compounds of
the invention have formula (IB):
##STR00004##
[0046] In a further embodiment, the fullerene surfactant compounds
of the invention have formula (IIA):
##STR00005##
[0047] In another embodiment, the fullerene surfactant compounds of
the invention have formula (IIB):
##STR00006##
[0048] In one embodiment, the fullerene surfactant compounds of the
invention have formula (III):
##STR00007##
[0049] In another embodiment, the fullerene surfactant compounds of
the invention have formula (IV):
##STR00008##
[0050] In one embodiment, the fullerene surfactant compounds of the
invention have formula (V):
##STR00009##
[0051] In another embodiment, the fullerene surfactant compounds of
the invention have formula (VI):
##STR00010##
[0052] For fullerene surfactant compounds noted above (i.e.,
compounds of formula (IA)-(VI)), F is a fullerene group (e.g.,
C.sub.60, C.sub.70, C.sub.76, C.sub.78, C.sub.82, C.sub.84, and
C.sub.92); B is a N-containing ring fused to the fullerene group
and having from 5-7 ring atoms (e.g., pyrrolidine, a 5-membered
ring); R.sub.1 and R.sub.2 are independently selected from the
group consisting of a polyalkylene oxide (e.g., PEO or PPO), as
described above, and a C1-C20 alkyl optionally substituted with an
anionic center (e.g., sulfonyl or carboxyl); Ar, Ar.sub.1, and
Ar.sub.2 are independently selected from the group consisting of
--C.sub.6H.sub.5-PEO,
--C.sub.6H.sub.5--N.sup.+(PEO).sub.2R.sub.1|A.sup.-; and
--C.sub.5H.sub.4N.sup.+--R.sub.1|A.sup.-, wherein
--C.sub.6H.sub.5-PEO is selected from the group consisting of
mono-, di-, tri-, and tetra-PEO substituted phenyl, wherein
--C.sub.6H.sub.5--N.sup.+(PEO).sub.2R.sub.1 is a substituted
aniline, and wherein --C.sub.5H.sub.4N.sup.+--R.sub.1 is a
substituted pyridinium; L.sub.1 and L.sub.2 are linkers having from
1 to 20 carbon atoms (e.g., C1-C20 alkylene) optionally including
one or more heteroatoms (e.g., O, N, or S) and/or one or more
functionalized carbon atoms (e.g., C.dbd.O); and A.sup.- is a
counter ion associated with the cationic nitrogen center.
[0053] The preparation of two representative fullerene surfactants
of the invention, ETL-1 and ETL-2, is illustrated schematically in
FIG. 2 and described in Example 1.
[0054] By virtue of its component groups, the fullerene surfactant
of the invention is advantageously soluble in a solvent orthogonal
to the device active layer. In practice of the method of the
invention, device fabrication includes forming a layer intermediate
the active layer and cathode. Application of the fullerene
surfactant to the active layer provides a fullerene surfactant
layer onto which the cathode is formed.
[0055] FIG. 3 is a cross-sectional view of a typical heterojunction
photovoltaic device in accordance with one embodiment of the
invention. Referring to FIG. 3, photovoltaic device 100 includes
first electrode 110 (anode), hole transport layer 120 (also
referred to as a charge transport or charge selective layer) formed
on first electrode 110, photovoltaic layer 130 (also referred to as
the active layer) formed on charge transport layer 120, fullerene
surfactant-containing layer 140 (also referred to as electron
transport or electron selective layer and also referred to herein
as the "interfacial layer") formed on photovoltaic layer 130, and
second electrode 150 (cathode) formed on fullerene
surfactant-containing layer 140. Photovoltaic layer 130 is the
active layer, such as a BHJ layer.
[0056] In the devices of the invention, the hole transport and
electron transport layers define the charge collection properties
in the devices. The best devices reported to date are composed of a
layer of polymer donor and fullerene acceptor bulk-heterojunction
(BHJ) film sandwiched between a transparent electrode, such as
indium tin oxide (ITO), and a metal electrode. Under illumination,
photo-generated excitons will dissociate at the donor-acceptor
interface, driven by the difference in energy levels between the
two semiconductors. The separated charges will then drift under the
inherent electric field created by the work-function difference
between the asymmetric electrodes and ultimately, will be collected
by the corresponding electrodes. The PCE is defined by the product
of three parameters including short-circuit current density
(J.sub.sc), open-circuit voltage (V.sub.oc), and fill factor
(FF).
[0057] The nature of electrical contact between the active BHJ
layer and the electrodes can significantly affect all three
device-related parameters and modification of those interfaces by
inserting appropriate interfacial layers can significantly alter
the contact properties to improve the PCE of OPVs. The interfacial
layer of the invention serves multiple functions that include:
[0058] (a) tuning the energy level alignment at the
electrode/active layer interface;
[0059] (b) defining polarity of electrodes and improving charge
selectivity;
[0060] (c) controlling surface properties to alter the morphology
of the active layer;
[0061] (d) introducing optical spacer and plasmonic effects to
modulate light absorption in the active layer; and
[0062] (e) improving interfacial stability between the active layer
and electrodes.
[0063] The photovoltaic layer (or active layer) can include any one
of a variety of materials and mixtures of materials as known in the
art. Representative useful materials include P3HT, PIDT-PhanQ,
PECz-DTQx, PCDTBT, PDTSTPD, PDTGTPD, PTB7. Representative active
fullerene materials include PCBM and ICBA. Other representative
active fullerene materials suitable for inclusion in a photoactive
layer include those described in U.S. Patent Application
Publication No. US 2011/0132439, incorporated herein by reference
in its entirety.
[0064] The following is a description of representative fullerene
surfactants of the invention and their use in interfacial layers to
enhance the efficiency of polymer solar cells.
[0065] The present invention provides fullerene surfactants, ETL-1
and ETL-2, that can be readily dissolved in alcoholic solvents and
applied as interfacial layer for cathode (see FIG. 4A), which
exhibited effective tuning of cathode .PHI., extraction of
electrons, and photocurrent generation in devices. These two
fullerene surfactants intrinsically help forming interfacial
contact between metal (in either high or low F) and BHJ to improve
device performance. Recently, the mechanism of using fullerene
surfactant to enhance device V.sub.oc has been elucidated that the
metal E.sub.F is pinned to the LUMO energy level of interfacial
layer, thus increasing the device's V.sub.oc regardless of the
choice of different cathode metal. The present invention provides
an organic interfacial material to realize Ag cathode based OPV
with superior performance (as high as 6.63%) to those of Ca/Al and
Al based devices due to the solvent-processed fullerene ETL
simultaneously enhanced V.sub.oc, J.sub.sc, and FF of device.
[0066] ETL-1 and ETL-2 having compact integration of both ionic
moieties and polar ethylene oxide chains onto a C.sub.60 core were
prepared by quaternizing the tertiary nitrogen of
fulleropyrrolidines with methyl iodide (FIG. 2). Comparing to the
poor solubility of most fullerene derivatives (e.g., Mono 1, Bis 2
and PCBM) in polar solvents, fulleropyrrolidiniums (ETL-1 and
ETL-2) exhibit amphiphilic properties that can be dissolved in both
chloroform and methanol, which provides great flexibility for
processing in orthogonal-solvents to prevent eroding bottom BHJ
layer.
[0067] The energy levels of ETL-1 and ETL-2 were estimated by
cyclic voltammogram measurements. As shown in Table 1, the LUMOs of
ETLs _exhibit small energy-gradient compared to that of PCBM due to
that the electron-deficient cationic nitrogen is in close vicinity
of the fullerene core, which made this interfacial material
energetically favor electron collection and transport from PCBM to
cathode.
TABLE-US-00001 TABLE 1 Reduction Potentials and estimated LUMO for
fullerene surfactants..sup.a E.sub.1/2.sup.red vs. Fc/Fc.sup.+
LUMO.sup.b LUMO.sup.c E.sub.1(V) E.sub.2(V) E.sub.3(V) (eV) (eV)
ETL-1 0.87 1.39 2.05 3.93 4.44 ETL-2 0.90 1.43 -- 3.90 4.39
PC.sub.71BM 0.99 1.53 2.04 3.81 4.30 .sup.aPotential in volt vs. a
ferrocene/ferrocenium couple. .sup.bThe LUMO levels were estimated
using the following equation: LUMO level = -(4.8 +
E.sub.1/2.sup.red1) eV. .sup.ccorrelated LUMOs according to PCBM
standard (LUMO = -4.30 eV).
[0068] Both ETL surfactants possess reasonable electron motilities
(2.18.times.10.sup.-4 cm.sup.2 V.sup.-1 s.sup.-1 for ETL-1 and
4.91.times.10.sup.-6 cm.sup.2 V.sup.-1 s.sup.-1 for ETL-2), and
show negligible absorbance to visible light, which qualify them as
proper electron-transporting layer (ca. 10 nm). ETL-1 and ETL-2
bearing cationic nitrogen and PEO linkage effectively up-shifted
the .PHI. of Al and Ag, around 0.8 eV by X-ray photoelectron
spectroscopy (XPS) studies. It may be due to the polar interaction
between fullerene surfactants and metal facilitate pinning of the
metal E.sub.F to that of the ETLs upon equilibration, which reduced
energy barrier between BHJ layer and cathodes. This, in turn,
increases V.sub.oc and charge extraction efficiency.
[0069] The presence of these fullerene layers creates only minimal
energy barrier height for electron extraction from PCBM (due to
matched ETL LUMOs to that of PCBM). This is different from using
the insulating PEO and p-type CPE process that have unfavored
energy level and charge-transporting properties. Moreover, the
n-type nature of fullerene surfactant layer creates an extra
acceptor-donor junction that can potentially enhance exciton
dissociation and prevent cathode from forming direct contact with
active layer to quench excitons. These rationale are supported,
vide infra, by the enhanced performance of PSCs with spun
interfacial layers.
[0070] PSCs with fullerene surfactant-modified Al were studied.
Device configuration of ITO/PEDOT:PSS/PIDT-PhanQ:PC.sub.71BM/ETL/Al
(FIG. 4A) showed significantly improved V.sub.oc, J.sub.sc and FF
compared to those from the reference device without the interfacial
layer (FIGS. 5A, 5B, and Table 2). The reference device A with bare
Al as cathode showed a relatively low PCE of 3.54% with a V.sub.oc
of 0.61 V, a J.sub.sc of 10.55 mA cm.sup.-2, and a FF of 0.55. The
Schottky-barrier at the active layer/Al interface caused low
V.sub.oc, J.sub.sc, and FF. However, when a thin layer (about 10
nm) of ETL-1 or ETL-2 was inserted between the BHJ layer and Al,
higher PCE of 5.96% (device B) and 6.03% (device C) could be
achieved, which accounts for a 70% improvement for device C
compared to the reference device. All the parameters (J.sub.sc,
V.sub.oc and FF) increased significantly for devices B and C,
because a better interfacial contact was created between BHJ and Al
when the fullerene ETL was applied, which lowered the .PHI. of Al,
thus giving higher V.sub.oc, and efficient electron extraction to
give higher J.sub.sc and FF.
TABLE-US-00002 TABLE 2 Characteristics of Devices A-F. V.sub.oc
J.sub.sc PCE Device Cathode [V] [mA/cm.sup.2] FF [%] A Al 0.61
10.55 (10.34) 0.55 3.54 B ETL-1/Al 0.86 11.17 (11.09) 0.62 5.96 C
ETL-2/Al 0.86 11.31 (11.27) 0.62 6.03 D Ca/Al 0.86 11.08 (10.64)
0.63 6.00 E ETL-1/Ca/Al 0.87 11.12 (11.07) 0.64 6.19 F ETL-2/Ca/Al
0.88 11.36 (11.20) 0.65 6.50 G Ag 0.74 10.92 (10.96) 0.57 4.61 H
ETL-1/Ag 0.87 11.28 (11.11) 0.64 6.28 I ETL-2/Ag 0.88 11.41 (11.30)
0.66 6.63
[0071] The values in parentheses were calculated from EQE
spectrum.
[0072] To further understand the effect on surfactant-modified
cathodes, the commonly adopted Ca/Al cathode based device were also
studied, in the device configuration of
ITO/PEDOT:PSS/PIDT-PhanQ:PC.sub.71BM/ETL/Ca/Al. Good contact
between Ca/Al cathode and BHJ can give essentially
high-performance, 6% PCE of device D (V.sub.oc=0.86 V,
J.sub.sc=11.08 mA cm.sup.-2, and FF=0.63). Slight improvements of
device characteristics could be observed when the ETL layer was
applied (FIG. 5C, 5D, and Table 2). Improved PCE of 6.19% (ETL-1,
device E) and 6.50% (ETL-2, device F) were achieved, which
correlated to the slightly enhanced V.sub.oc, J.sub.sc, and FF.
These results indicated that the additional fullerene ETL layer
helped optimize the contact between Ca/Al and BHJ layer leading to
increased PCE. Although being widely used in PSCs as electrodes, Al
and Ca/Al are sensitive to air and moisture, which cause device
degradation in ambient. Ag shows relatively good stability toward
ambient condition. However, the energy mismatch between high .PHI.
of Ag and LUMO of PCBM usually resulted in poor device performance.
To alleviate this problem, devices were fabricated with the
configuration of ITO/PEDOT:PSS/PIDT-PhanQ:PC.sub.71BM/ETL/Ag
(devices G-I). A distinctly improved PCE (44%) could be achieved
for device I compared to that of reference device G due to enhanced
V.sub.oc, J.sub.sc, and FF (FIGS. 5E, 5F, and Table 2). PCE of
6.63% from device I using ETL-2 is one of the highest values
achieved from conventional PSC with Ag cathode.
[0073] The external quantum efficiency (EQE) spectra of devices A-I
(FIGS. 5B, 5D, and 5F) were compared. The calculated J.sub.sc
obtained from integration of EQE spectrum match well with measured
one, with variation of less than 5% (Table 2). With an ETL-1 or
ETL-2, the EQE of devices are higher in part of the spectrum
compared to those of reference device, which correlate well with
the results of higher photocurrents.
[0074] In all devices, ETL-1 show slightly lower PCE and relevant
parameters (J.sub.sc and FF) than those of ETL-2, which may be due
to the difference of film quality of these two ETLs on top of the
BHJ layer. The topography and surface profile of devices with and
without ETL layer were characterized by atomic force microscopy
(AFM) and is shown in FIG. 6. All the interfacial layers covered
well on top of BHJ layer. The surface of ETL-2 is relatively smooth
as indicated by the lower root-mean-square (RMS) roughness, 0.763
nm (FIG. 6(c)) and is similar to 0.733 nm of BHJ surface (FIG.
6(a)). ETL-1 on BHJ exhibited a RMS of 1.01 nm with relatively
rough surface (FIG. 6(b)).
[0075] In one aspect, the invention provides representative
fullerene surfactants, ETL-1 and ETL-2, which can be readily
processed in orthogonal solvents (e.g., methanol) on a BHJ layer in
PSCs. These materials possess proper electron mobility and the
capability of tuning cathode .PHI. to improve electron extraction
and photocurrent generation. Upon the insertion of a thin ETL-1 or
ETL-2 between various metal cathodes and BHJ layer (device A-I),
simultaneously improved V.sub.oc, J.sub.sc, and FF could be
achieved for these devices compared to those without using
surfactant. The performance of PSCs is significantly improved (70%
for Al cathode and 40% for Ag cathode) when surfactant-modified
cathode was applied. High performance PSCs using fullerene ETL
modified Ag cathode were realized (as high as 6.63%) which is
superior to those of Ca/Al and Al based devices.
[0076] The following is a description of the use of a
representative fullerene surfactant of the invention, ETL-2
("C.sub.60-bis") (FIG. 7), in an interfacial layer to enhance the
efficiency of polymer solar cells. FIG. 8A is a schematic
illustration of the architecture of a representative photovoltaic
device of the invention. FIG. 8B is a schematic illustration of the
energy level diagram of the device of FIG. 8A.
[0077] Devices were fabricated with higher WF metals less prone to
oxidation, which are shown to perform better than Al devices over
time. Remarkably, the V.sub.OC appears to be independent of the
choice of cathode metal when C.sub.60-bis is used as a buffer
layer.
[0078] FIG. 9A shows the J-V characteristics for devices fabricated
with different cathode metals both with and without a C.sub.60-bis
buffer layer. The V.sub.OC for devices with an Al cathode is
consistently lower than that of Cu and Ag devices, which can be
attributed to the rapid oxidation of Al in air. The non-ideal
nature of this interface also manifests in a modest fill factor
(FF) of 0.51 and an overall PCE of 3.22%. In contrast, when a layer
of C.sub.60-bis is used, the PCE increases to 5.87% as a result of
an increase in J.sub.SC, FF and most notably V.sub.OC. In addition,
the shunt resistance is shown to increase for all metals in the
case of C.sub.60-bis, which provides evidence of lower leakage
current under illumination. Performance data for all devices are
summarized in Table 3.
TABLE-US-00003 TABLE 3 Performance data for PIDT-PhanQ:PC.sub.71BM
devices with different cathode metals, with and without
C.sub.60-bis. V.sub.OC J.sub.SC PCE R.sub.SH Device [V] [mA
cm.sup.-2] FF [%] [.OMEGA. cm.sup.-2] Al 0.62 10.28 0.51 3.22
309.33 Al/C.sub.60-bis 0.88 11.19 0.60 5.87 773.33 Ag 0.73 10.83
0.53 4.22 351.63 Ag/C.sub.60-bis 0.88 11.50 0.61 6.22 662.86 Cu
0.67 9.58 0.51 3.32 386.67 Cu/C.sub.60-bis 0.87 10.13 0.61 5.37
795.43
[0079] To investigate the improvement in J.sub.SC, external quantum
efficiency (EQE) spectra (FIG. 9B) were obtained for Al, Ag, and Cu
devices. The spectra exhibit an almost constant increase across the
entire wavelength range for each case when the surfactant layer was
inserted. This indicates the improvement in J.sub.SC is due
entirely to the inclusion of the surfactant and a concurrent
decrease in recombination resistance at the organic/electrode
interface, rather than a change in bulk morphology.
[0080] To further demonstrate the utility of C.sub.60-bis as an
interfacial layer, the PCE of devices with different cathode metals
were tracked over a period of time under exposure to ambient
conditions. FIG. 10 shows the normalized PCE for unencapsulated
devices with and without C.sub.60-bis over 100 h in air. As
expected the performance of Al devices drops off rapidly, even with
the inclusion of the fullerene surfactant, which is likely due to
the uptake of oxygen and water molecules and their subsequent
diffusion to the metal/organic interface. The Ag and Cu devices
remain very stable, however, with the Cu/C.sub.60-bis retaining
nearly 90% of its original PCE after the entire period of ambient
exposure.
[0081] By far the most obvious benefit of C.sub.60-bis is a
strongly enhanced V.sub.OC. To further investigate the dramatic
increase in V.sub.OC when C.sub.60-bis is used, capacitance-voltage
characteristics (C-V) were obtained and devices were analyzed via
Mott-Schottky (MS) analysis. It has previously been shown that, due
to the intrinsic p-doped nature of semiconducting polymers, a
Schottky contact is formed upon deposition of the cathode onto the
photoactive layer. The depletion zone formed at this interface is
modulated by the applied voltage under reverse and low (<1.5V)
forward bias. Band-bending has been shown to result in the vicinity
of the cathode, allowing extraction of the built-in potential
(V.sub.BI) and impurity concentration (N) of the region by
application of C.sup.-2=(2/qN)(V.sub.BI-V) to the appropriate bias
voltage range.
[0082] FIG. 9C shows the capacitance behavior of all devices as a
function of bias voltage. The low capacitance region up to about
0.5 V has been attributed to the capacitance of the depletion
layer, whereas a further increase in forward bias voltage yields a
peak in the capacitance related to the storage of minority carriers
in the bulk. FIG. 9D shows the MS plot for all fabricated devices.
At moderate to high reverse bias, C.sup.-2 tends to reach a steady
value related to the geometric capacitance of the organic material
which has become fully depleted of majority carriers and can be
viewed as a classical dielectric. The linear region under low
forward bias is related to the formation of a Schottky contact and
can be fitted to a plot of C.sup.-2 versus bias voltage.
Extrapolation of the linear fit line to the intercept on the bias
axis directly yields V.sub.BI for the device. Once a value for
V.sub.BI has been obtained, an impurity concentration N and
depletion width w=(2.di-elect cons.V.sub.BI/qN).sup.1/2
corresponding to zero applied bias can be extracted. A dielectric
permittivity of 3 has been assumed for calculations involving these
equations. MS analysis data, along with the relative shifts in
V.sub.OC and V.sub.BI, are summarized in Table 4.
TABLE-US-00004 TABLE 4 Built-in potential V.sub.BI, dopant
concentration N, and depletion width w of the organic/cathode
Schottky contact from Mott-Schottky analysis. The work functions
and relative shifts in V.sub.OC and V.sub.BI for all devices are
also included. (.DELTA.V.sub.OC, V.sub.OC V.sub.BI .DELTA.V.sub.BI)
N w .PHI..sub.cathode Device [V] [V] [V] [10.sup.16 cm.sup.-3] [nm]
[eV] Al 0.619 0.636 -- 2.25 97 4.25 Al/C.sub.60-bis 0.877 0.940
(0.26, 0.30) 3.32 97 3.66 Ag 0.734 0.808 -- 3.39 89 4.57
Ag/C.sub.60-bis 0.879 0.959 (0.15, 0.15) 3.77 92 3.97 Cu 0.672
0.712 -- 2.51 97 4.70 Cu/C.sub.60-bis 0.875 0.957 (0.20, 0.25) 4.02
89 3.96
[0083] The depletion width extracted from the capacitance-voltage
data extends over almost the entire thickness of the active layer.
When taken with the N values obtained from the same data, this
indicates a consistent doping profile across the entire layer that
changes negligibly by inclusion of C.sub.60-bis. Because the change
in the Fermi level of the active layer (E.sub.F.sup.p) can be
approximated by .DELTA.E.sub.F.sup.p=k.sub.bT ln(N.sub.b/N.sub.a),
where N.sub.b and N.sub.a are the dopant concentrations of the
device with and without C.sub.60-bis, respectively, it is
reasonable to conclude that E.sub.F.sup.p does not change more than
ca. 10 meV. When a semiconductor is placed in intimate contact with
a metal, their respective E.sub.F come into equilibrium by
electrons being transferred "downhill" in energy. Referencing
V.sub.BI to E.sub.F.sup.p by
V.sub.BI=(E.sub.F.sup.p-.PHI..sub.cathode), where .PHI..sub.cathode
is the cathode WF, then the difference in V.sub.BI with and without
C.sub.60-bis can be attributed to a modification of
.PHI..sub.cathode by the surfactant. Furthermore, because the
relative shifts in V.sub.BI closely follow those of V.sub.OC for
all three metals we can conclude that the observed increase in
V.sub.OC upon inclusion of C.sub.60-bis is due to a dipole-induced
shift in .PHI..sub.cathode at the interface.
[0084] To further investigate the energetics at the interface, WFs
were obtained for Al, Ag, and Cu with and without C.sub.60-bis
spin-coated on top and are summarized in Table 4. WFs of in-situ,
sputter-cleaned Al, Ag, and Cu films were measured to be 4.25 eV,
4.57 eV and 4.70 eV, respectively (FIGS. 12A-12C). The WFs of Ag
and Cu with C.sub.60-bis yield nearly the same value. Because
sampling of the substrate at normal emission is highly surface
sensitive, it is reasonable to assume these WF values correspond to
the C.sub.60-bis. As the material is an n-type semiconductor, one
would expect E.sub.F to be closer to the LUMO level than mid-gap.
It should be noted that the WFs of the organic overlayer may not be
measured in the flat-band condition, but are rather subject to any
band bending occurring at the metal/organic interface as a result
of E.sub.F equilibration. Additionally, it is likely that an
unavoidable thin oxide layer formed on the Al sample when it was
removed from the glovebox for C.sub.60-bis deposition, as evidenced
by a comparison of O1s peak intensity in XPS survey spectra for
bare Al before and after sputter-cleaning with Ar.sup.+ ions. These
considerations might explain the lower WF of the modified Al
cathode as compared to Ag and Cu.
[0085] It should be stressed that these conditions do not prevail
for regular device fabrication since the cathode is deposited under
high vacuum after spin-coating the C.sub.60-bis layer outside the
glovebox. Regardless, at a distance sufficiently far into the bulk
of the photoactive layer only the effective WF of the C.sub.60-bis
modified cathode can be seen by the rest of the device. This
ensures a constant difference between E.sub.F.sup.p and
.PHI..sub.cathode, and explains why V.sub.BI, and consequently
V.sub.OC, is nearly the same for all three metals when C.sub.60-bis
is employed.
[0086] A C.sub.60 bis-adduct surfactant was used to modify the
energy level alignment at the organic/cathode interface in
conventional structure, bulk-heterojunction OSC devices. A
well-defined interface between the photoactive layer and the
surfactant was ensured by virtue of process solvent orthogonality.
The large increase in device V.sub.OC is independent of the choice
of cathode metal due to pinning of the metal E.sub.F to that of the
C.sub.60-bis upon equilibration. Mott-Schottky analysis of the
interface formed between the photoactive layer and the cathode
yields a built-in potential defined by the difference between the
Fermi level of the bulk-heterojunction E.sub.F and the effective
cathode work function .PHI..sub.cathode. The observed changes in
V.sub.BI are reflected in the magnitude of the change in V.sub.OC.
Further, EQE data reveal the overall device performance enhancement
to be due entirely to the inclusion of the surfactant, rather than
a beneficial change in photoactive layer morphology.
[0087] The following examples are provided for the purpose of
illustrating, not limiting, the invention.
EXAMPLES
Example 1
The Preparation, Characterization, and Use of Representative
Fullerene Surfactants: ETL-1 and ETL-2
[0088] In this example, the preparation, characterization, and use
of representative fullerene surfactants, ETL-1 and ETL-2, is
described. The fabrication and characterization of devices that
include the surfactants is also described.
[0089] All reactions dealing with air- or moisture-sensitive
compounds were carried out using standard Schlenk technique. All
.sup.1H (500 MHz) and .sup.13C (125 MHz) spectra were recorded on
Bruker AV500 spectrometers. Spectra were reported in parts per
million from internal tetramethylsilane (.delta. 0.00 ppm) or
residual protons of the deuterated solvent for .sup.1H NMR and from
solvent carbon (e.g., .delta. 77.00 ppm for chloroform) for
.sup.13C NMR. The matrix for MALDI-TOF-MS used 2:1 mixture of
alpha-cyano-4-hydroxycinnamic acid (CHCA)/2,5-dihydroxybenzoic acid
(DHB) in acetonitrile. Elemental analyses were performed by QTI,
Whitehouse, N.J. (www.qtionline.com). AFM images under tapping mode
were taken on a Veeco multimode AFM with a Nanoscope III
controller. 2,3,4-Tris(2-(2-methoxyethoxy)ethoxy)benzaldehyde and
fulleropyrrolidines were synthesized according to literature
methods (Benzaldehyde: Nielsen, C B.; Johnsen, M.; Arnbjerg, J.;
Pittelkow M.; Mclroy, S P.; Ogilby, P R.; Jrgensen, M. J Org. Chem.
2005, 70:7065. Fulleropyrrolidines and fulleropyrrolidiums: Bosi,
S.; Feruglio, L.; Milic, D.; Prato, M. Eur. J. Org. Chem. 2003,
4741). C.sub.60 was purchased from American Dye Source. Unless
otherwise noted, materials were purchased from Aldrich Inc., and
used after appropriate purification.
[0090] Synthesis of Fulleropyrrolidiums
[0091] A solution of C.sub.60 (300 mg, 0.35 mmol),
2,3,4-tris(2-(2-methoxyethoxy)ethoxy)benzaldehyde (478 mg, 1.04
mmol) and sarcosinic acid (111 mg, 1.25 mmol) in chlorobenzene (100
mL) was refluxed under N.sub.2 for 4 h. After evaporation of the
solvent, the residue was subjected to chromatograph purification on
a silica gel column. Elution with toluene gave little unchanged
C.sub.60. Fraction containing mono adduct was collected with
PhMe/EtOAc (1:2) eluent. One fraction of bisadducts consisting
mixture of regioisomers was then collected with EtOAc eluent. Each
sample was precipitated from toluene solution with methanol or
hexane, and gave monofulleropyrrolidine (115 mg, 27%),
bisfulleropyrrolidine (90 mg, 15%).
[0092] Quaternization of neutral fulleropyrrolidines was achieved
by heating a solution of mono or bis fulleropyrrolidine (0.05 mmol)
in chloroform (2 mL) and MeI (1.5 mL) in a screw-topped Schlenk
tube under N.sub.2. Reaction mixture was kept at 80 OC for 40 h.
After evaporation of the solvent, the product was dissolve in
chloroform and precipitated with hexane. After thoroughly washed
with n-hexane, black fulleropyrrolidiums, ETL-1 or ETL-2, were
obtained in quantitative yield.
[0093] Monofulleropyrrolidine.
[0094] .sup.1H NMR (500 MHz, CDCl.sub.3): .delta. 2.78 (s, 3H,
NCH.sub.3), 3.34 (s, 3H, OCH.sub.3), 3.36 (s, 3H, OCH.sub.3), 3.40
(s, 3H, OCH.sub.3), 3.48-3.50 (m, 2H, OCH.sub.2), 3.53-3.58 (m, 4H,
OCH.sub.2), 3.63-3.80 (m, 10H, OCH.sub.2), 3.86 (t, J=5.5 Hz, 2H,
OCH.sub.2), 3.05-4.18 (m, 4H, OCH.sub.2), 4.27-4.32 (m, 2H,
OCH.sub.2), 4.37-4.40 (m, 2H, OCH.sub.2), 4.94 (d, J=9.5 Hz, 1H,
NCH.sub.2), 5.56 (s, 1H, NCH.sub.2), 6.77 (d, J=8.5 Hz, 1H, Ar--H),
7.63 (d, J=8.5 Hz, 1H, Ar--H). .sup.13C NMR (125 MHz, CDCl.sub.3):
.delta. 39.89, 58.99, 59.00, 59.03, 59.05, 59.18, 59.19, 68.36,
69.25, 69.76, 69.83, 70.21, 70.35, 70.62, 70.72, 70.74, 71.95,
71.98, 72.05, 72.23, 73.18, 75.74, 77.20, 109.34, 123.31, 124.57,
134.82, 136.06, 136.49, 136.59, 139.47, 139.53, 140.12, 140.14,
141.19, 141.58, 141.67, 141.89, 141.99, 142.08, 142.11, 142.16,
142.28, 142.29, 142.54, 142.57, 142.63, 142.65, 143.00, 143.08,
144.36, 144.45, 144.61, 145.11, 145.12, 145.18, 145.23, 145.26,
145.31, 145.55, 145.76, 145.94, 146.07, 146.09, 146.12, 146.20,
146.27, 146.77, 146.95, 147.30, 152.20, 152.47, 154.15, 154.33,
155.16, 156.87. MALDI-TOF-MS (+): calcd. for
[C.sub.84H.sub.41NO.sub.9].sup.-, 1208.225. found. [M].sup.-,
1207.893.
[0095] Bisfulleropyrrolidine.
[0096] .sup.1H NMR (500 MHz, CDCl.sub.3): .delta. 2.55-2.88 (m,
NCH.sub.3), 3.29-3.40 (m, OCH.sub.3), 3.42-4.00 (m,
OCH.sub.2&OCH.sub.3), 4.06-4.68, 4.92-5.57, 5.74-5.75,
6.52-6.98, 7.35-7.49, 7.59-7.69, 7.73-7.88, 8.00-8.03; .sup.13C NMR
(125 MHz, CDCl.sub.3): .delta. 39.66-39.83 (m, NCH.sub.3),
53.21-53.43 (m), 58.97-59.22 (m), 68.17-68.43, 69.42, 69.70-69.94,
70.14-70.91, 71.90-72.35, 72.92-73.50, 75.34-76.00, 77.40-77.66,
109.12-109.48, 123.58-123.98, 124.42-124.61, 134.87, 136.53,
139.39, 140.76-141.93, 142.14, 142.00, 142.23, 142.30, 142.37,
142.51, 142.58, 142.95, 142.97, 143.38, 143.39, 143.58, 144.12,
144.36, 144.85, 144.96, 145.08, 145.21, 145.26, 145.44-145.74,
146.05, 146.07, 147.25, 147.47, 147.72, 147.84, 148.64, 148.77,
149.03, 150.75-151.39, 151.97-152.83, 153.66, 154.28-154.98,
155.54; MALDI-TOF-MS (+): calcd. for
[C.sub.108H.sub.82N.sub.2O.sub.18], 1695.809. found. [M-I].sup.+,
1695.929.
[0097] Fulleropyrrolidium ETL-1.
[0098] .sup.1H NMR (500 MHz, CDCl.sub.3): .delta. 3.36 (s, 3H,
OCH.sub.3), 3.38 (s, 3H, OCH.sub.3), 3.52-3.56 (m, 7H,
OCH.sub.2&OCH.sub.3), 3.67-3.77 (m, 8H, OCH.sub.2), 3.80-3.84
(m, 2H, OCH.sub.2), 3.89 (m, 2H, OCH.sub.2), 3.97 (s, 3H,
NCH.sub.3), 4.02 (d, J=8.5 Hz, 2H, OCH.sub.2), 4.20-4.39 (m, 4H,
OCH.sub.2), 4.48 (s, 3H, NCH.sub.3), 4.66-4.68 (m, 2H, OCH.sub.2),
5.80 (d, J=12.5 Hz, 1H, NCH.sub.2), 6.84 (d, J=13.0 Hz, 1H,
NCH.sub.2), 6.88 (d, J=9.0 Hz, 1H, Ar--H), 7.28 (d, J=13.0 Hz, 1H,
NCH.sub.2), 7.71 (d, J=8.5 Hz, 1H, Ar--H); .sup.13C NMR (125 MHz,
CDCl.sub.3): .delta. 45.69, 53.44, 59.04, 59.07, 59.08, 59.28,
67.89, 68.44, 69.38, 69.96, 70.43, 70.54, 70.69, 70.72, 71.42,
71.65, 71.93, 71.97, 72.53, 72.56, 73.13, 73.64, 78.60, 108.66,
111.57, 127.48, 134.10, 134.75, 135.52, 136.11, 139.03, 139.87,
139.98, 140.26, 140.93, 141.24, 141.38, 141.43, 141.45, 141.62,
141.82, 142.03, 142.09, 142.11, 142.13, 142.35, 142.39, 142.51,
142.52, 142.76, 142.84, 142.96, 143.01, 143.12, 143.33, 144.19,
144.23, 144.36, 144.42, 144.82, 144.89, 145.14, 145.26, 145.30,
145.45, 145.54, 145.61, 145.66, 145.77, 145.82, 145.96, 146.02,
146.13, 146.18, 146.35, 146.40, 147.42, 147.56, 149.32, 150.51,
151.18, 152.66, 153.66, 153.83, 155.77; MALDI-TOF-MS (+): calcd.
for [C.sub.85H.sub.44NO.sub.9].sup.+.I.sup.-, 1350.16. found.
[M-I].sup.+, 1222.144; Anal. Calcd for C.sub.85H.sub.44NO.sub.9: C,
75.61; H, 3.28; N, 1.04. Found: C, 73.29; H, 2.76; N, 0.76.
[0099] Fulleropyrrolidium ETL-2 (Mixture of Regioisomers).
[0100] .sup.1H NMR (500 MHz, CDCl.sub.3/CD.sub.3OD): .delta.
3.32-3.40 (m, OCH.sub.3), 3.42-4.03 (m, OCH.sub.2&OCH.sub.3),
4.12-4.50 (m, OCH.sub.2), 4.56-4.61 (m, OCH.sub.2), 4.70-4.72 (m,
OCH.sub.2), 5.36-5.69 (m, NCH.sub.2), 6.02-6.07, 6.68-6.97,
7.04-7.13, 7.20-7.21, 7.32-7.34, 7.37-7.61, 7.77-7.89, 7.98-8.04,
8.10-8.14, 8.27-8.31; .sup.13C NMR (125 MHz, CDCl.sub.3): .delta.
45.28-46.47 (m), 53.21-53.43 (m), 58.99-59.49 (m), 66.06, 66.83,
68.45, 68.49-69.41, 69.51-70.89, 71.27-71.52, 71.96-72.06,
72.46-72.67, 73.55-73.84, 78.65, 78.75, 109.24, 109.35, 109.48,
111.29, 111.44, 136.24, 136.67, 140.04, 140.44, 140.84, 140.96,
141.13, 141.35, 141.58, 141.60, 141.67, 141.73, 141.77, 141.81,
141.83, 141.94, 142.14, 142.17, 142.21, 142.32, 142.38, 142.40,
142.51, 142.61, 145.38, 145.48, 145.59, 146.14, 146.20, 147.21,
147.40, 147.53, 147.79, 147.96-148.09, 148.40, 148.70-148.82,
149.08-149.32, 150.06, 151.57, 153.67-153.76, 155.68-155.89;
MALDI-TOF-MS: calcd. for
[C.sub.110H.sub.88N.sub.2O.sub.18].sup.2+.2I.sup.-, 1979.69. found
[M-2I-NMe.sub.3].sup.+ 1666.278; Anal. Calcd for
C.sub.110H.sub.88I.sub.2N.sub.2O.sub.18: C, 66.74; H, 4.48; N,
1.42. Found: C, 66.07; H, 4.23; N, 1.35.
[0101] CV Measurements
[0102] Cyclic voltammetry (CV) measurements were carried out in a
one-compartment cell under N.sub.2, equipped with a glassy-carbon
working electrode, a platinum wire counter electrode, and an
Ag/Ag.sup.+ reference electrode. Measurements were performed in THF
solution containing tetrabutylammonium hexafluorophosphate (0.1 M)
as a supporting electrolyte with a scan rate of 100 mV/s. All
potentials were corrected against Fc/Fc.sup.+. Due to close
vicinity of the electron-deficient cationic nitrogen to the
fullerene core, the LUMO level of ETL-1 to that of ETL-2 has a
small difference in 0.03 eV.
[0103] Fabrication and Characterization of PSCs
[0104] [6,6]-Phenyl-C61 (or C71)-butyric methyl ester was purchased
from American Dye Source. PEDOT:PSS (Baytron P VP AI 4083) was
purchase from H. C. Stark. Materials were used as received. The
fullerene surfactant solutions in methanol were sonicated for 2 hrs
prior to spin-coating in ambient at 5000 RPM. The surfactant layer
thickness was about 8-10 nm as measured by AFM. The ITO substrates
were cleaned by ultrasonication in acetone for 15 min, followed by
manual scrubbing with detergent and deionized water, then
sonication in deionized water and isopropanol for 15 min each. The
substrates were blown dry under a nitrogen stream and immediately
exposed to air plasma for 20 seconds. A 40 nm thick layer of
PEDOT:PSS was spin coated onto each substrate and subsequently
annealed in air at 140.degree. C. for 30 min. The mixture of
PIDT-PhanQ:PC.sub.71BM in o-dichlorobenzene (20 mg/ml, 1:3, w:w)
was then spin-coated on the PEDOT:PSS layers at 800 RPM, and
subsequently annealed at 110.degree. C. for 10 min under nitrogen
atmosphere to obtain a film thickness approximately 80 nm. After
fullerene surfactant solutions was spin coated on the BHJ layer.
The substrates were then transferred back into the glovebox and
annealed at 110.degree. C. for 5 min. Finally, aluminum (100 nm) or
calcium (30 nm) topped with aluminum (100 nm), or silver (100 nm)
was thermally evaporated onto the active layer through shadow
masks.
[0105] Photocurrent-voltage (J-V) measurements were performed using
a Keithley 4200 in a nitrogen-filled glove box under AM1.5
illumination conditions at intensity of 100 mW/cm.sup.2. A NREL
certified silicon photodiode with a KG5 filter was used to
calibrate. Device EQE spectra were obtained in air by comparison to
a known AM1.5 reference spectrum for a calibrated silicon
photodiode.
[0106] Organic Field-Effect Transistors
[0107] Top contact OFETs were fabricated as typical top contact,
bottom gate devices on silicon substrates. Heavily doped p-type
silicon <100> substrates from Montco Silicon Technologies
INC. with a 300 nm (.+-.5 nm) thermal oxide layer acted as a common
gate with a dielectric layer. After cleaning the substrate by
sequential ultrasonication in acetone, methanol, and isopropyl
alcohol for 15 min flowed by air plasma treatment, the different
fullerene surfactant films were spin-coated from a 0.5 wt %
chloroform solution in ambient. Interdigitated source and drain
electrodes (W=1000 .mu.m, L=12 .mu.m) were defined by evaporating a
50 nm Au film through a shadow mask from the resistively heated Mo
boat at 10.sup.-6 Torr. OFET characterization was carried out in a
N.sub.2-filled glovebox using an Agilent 4155B semiconductor
parameter S6 analyzer. The field-effect mobility was calculated in
the saturation regime from the linear fit of (I.sub.ds).sub.1/2 VS
V.sub.gs. The threshold voltage (V.sub.t) was estimated as the x
intercept of the linear section of the plot of (I.sub.ds).sub.1/2
VS V.sub.gs. The sub threshold swing was calculated by taking the
inverse of the slope of I.sub.ds VS V.sub.gs in the region of
exponential current increase.
[0108] Work Function Measurements by XPS
[0109] Samples for work function analysis were prepared on glass
substrates coated with ITO to ensure good electrical contact. Work
functions were measured with a PHI Versa Probe X-ray photoelectron
spectrometer (ULVAC-PHI, Kanagawa, Japan) employing a monochromatic
focused Al--K.sub..alpha. X-ray source and hemispherical analyzer.
The Au 4f.sub.7/2 (84.00 eV) and Cu 2p.sub.3/2 (932.66 eV)
photoemission peaks were used to calibrate the binding energy
scale. A bias voltage (-5 V) was applied to the sample, and the
location of the secondary electron cut-off was determined at normal
emission by a linear extrapolation to the background level. To
account for the instrument width, 0.14 eV were added to the work
function values thus obtained. This procedure gives a work function
for argon ion sputtered gold foil of 5.17 eV.
TABLE-US-00005 TABLE 5 Comparison of WF of cathodes. Al ETL-1/Al
ETL-2/Al Secondary electron emission (eV) 1477.54 1478.16 1478.32
Work-Function (eV) 4.20 3.66 3.42
Example 2
The Preparation and Characterization of Representative Photovoltaic
Devices with Fullerene Surfactant-Containing Interfacial Layer
[0110] In this example, the preparation and characterization of
representative photovoltaic devices with a fullerene
surfactant-containing layer intermediate the active layer and
cathode is described.
[0111] Fabrication of Photovoltaic Devices
[0112] ITO-coated glass substrates (15 .OMEGA.sq.sup.-1) were
cleaned sequentially by sonication in detergent and deionized
water, acetone and isopropanol. After drying under a N.sub.2
stream, substrates were air-plasma treated for 30 s. A about 35 nm
layer of PEDOT:PSS (Baytron.RTM. P VP Al 4083, filtered through a
0.45 .mu.m nylon filter) was spin-coated onto the clean substrates
at 5 kRPM and annealed at 140.degree. C. for 10 min. The substrates
were transferred to a N.sub.2-filled glovebox where a homogeneously
blended solution of PIDTPhanQ:PC.sub.71BM (40 mg/ml in
o-dichlorobenzene stirred overnight in glovebox, 1:3
polymer:fullerene by weight) was spin-coated at 2 k RPM, producing
an active layer about 100 nm thick, and annealed at 110.degree. C.
for 10 min. Substrates requiring a layer of fullerene surfactant
were briefly transferred out of the glovebox (total ambient
exposure<10 min) and about 2-5 nm thick film of C.sub.60-bis
surfactant (1 mg/ml in methanol) was spin-coated at 5 k RPM. The
substrates were then transferred back into the glovebox and
annealed at 110.degree. C. for 5 min to drive off any remaining
solvent prior to metal deposition. Metal electrodes were deposited
at a base pressure<1.times.10.sup.-6 Torr through a shadow mask,
defining an active device area of 4.64 mm.sup.2. Ag and Cu were
deposited at a rate of 1 .ANG. s.sup.1 and Al was deposited at a
rate of 4 .ANG. s.sup.-1.
[0113] Preparation of XPS Samples
[0114] ITO-coated glass substrates were prepared as above without
air-plasma treatment. Al, Ag, and Cu were deposited over the entire
substrate surface at a rate of 1 .ANG. s.sup.-1. Substrates
requiring a thin layer of fullerene surfactant were transferred out
of the glovebox and a solution of C.sub.60-bis surfactant was
spin-coated from methanol using the same conditions as above. After
transfer back into the glovebox, all substrates were heated at
70.degree. C. for 5 min to evaporate any remaining methanol prior
to being sealed with parafilm in 20 ml glass vials under N.sub.2
for transport to the XPS.
[0115] Measurement and Characterization
[0116] J-V characteristics of the unencapsulated devices were
measured in ambient conditions using a Keithley 2400 source meter
under AM 1.5 G (100 mW cm.sup.-2) irradiation simulated by an Oriel
xenon lamp (450 W). AM 1.5 G illumination was confirmed by means of
calibration to a standard silicon photodiode (Hammamatsu) which can
be traced to the National Renewable Energy Laboratory. External
quantum efficiency spectra were obtained by measuring the
photocurrent response of the device using chopped, monochromated
light from the same xenon lamp in conjunction with a Stanford
Research Systems SR830 lock-in amplifier under ambient conditions.
Mott-Schottky analysis was performed in a N.sub.2-filled glovebox
in the dark using a Signatone probe station interfaced with a
Hewlett-Packard HP4284A LCR meter. The 1 kHz AC field applied
during measurement was kept at an amplitude of 25 mV to maintain
response linearity. Capacitance-voltage characteristics measured
thusly were obtained using devices prepared as above with an active
area of 10.08 mm.sup.2. Work function determination via XPS is
described below. Briefly, the secondary electron cutoff (SEC)
spectrum of each sample was measured under ultra-high vacuum
(<5.times.10.sup.-9 Torr) using a PHI 5000 VersaProbe
(Ulvac-Phi, Inc.) employing a focused, monochromated Al K-.alpha.
x-ray source and a hemispherical analyzer. Proper referencing of
the SEC edge to that of Ar.sup.+ ion sputter-cleaned,
polycrystalline gold allowed for accurate determination of the
sample work functions with a reproducibility of about 0.05 eV.
[0117] Cyclic Voltammetry Measurements
[0118] Cyclic voltammetry measurements were carried out under
N.sub.2 in a one-compartment cell equipped with a glassy carbon
working electrode, a platinum wire counter electrode, and an
Ag/Ag.sup.+ reference electrode. Measurements were performed in THF
solution containing tetrabutylammonium hexafluorophosphate (0.1 M)
as a supporting electrolyte with a scan rate of 100 mV/s. All
potentials were corrected against the Fc/Fc.sup.+ couple and LUMO
levels were estimated using the following equation:
LUMO=-(4.8+E.sub.1/2.sup.red1) eV.
[0119] Work Function Determination
[0120] Work function values were obtained following a modified
method previously described (M. M. Beerbom et al., Journal of
Electron Spectroscopy and Related Phenomena 152, 2006, 12-17). The
spectrometer's analyzer was calibrated according to the
manufacturer's guidelines to yield photoemission lines of Ar.sup.+
ion sputter-cleaned Cu and Au foils for Cu 2p 3/2 and Au 4f 7/2 at
932.62 eV and 83.96 eV, respectively, following ISO 15472 (M. P.
Seah, Surf. Interface Anal., 31, 2001, 721-723). This procedure
ensures the linearity of the binding energy scale for the
instrument, extrapolated out to the secondary electron cutoff (SEC)
near the photon energy of the system (1486.6 eV for monochromated
Al K-.alpha. x-rays). SEC spectra were measured at an x-ray power
of 25 W and 15 kV acceleration at normal emission. For all SEC
spectra a bias of -15V was applied during measurement to ensure
sufficient separation of the sample SEC and that of the analyzer.
Under these conditions a SEC value of 1466.24 eV for clean,
polycrystalline gold was obtained, corresponding to a work function
of 5.36 eV. Because the Cu and Au core level spectra mentioned
above are referenced to the Fermi level, set at zero binding
energy, the work function of Au was obtained by
.PHI..sub.Au=(h.upsilon.-qV.sub.app-E.sub.SEC) where h.upsilon. is
the x-ray photon energy, V.sub.app is the applied bias and
E.sub.SEC is the position of the secondary electron cutoff on the
binding energy scale. Ideally, the SEC edge should be a step
function at 0 K, however experimental conditions include thermal
and instrumental broadening. Hence, the position of the SEC is
taken as the local maximum of the first derivative of the SEC
feature. FIG. 11 shows the SEC spectrum of clean, polycrystalline
Au foil and its corresponding first derivative. Once the work
function of clean Au has been obtained thusly, all other sample
work functions can be derived simply from their SEC positions
obtained via the first derivative method as
.PHI..sub.sample=(E.sub.SEC, Au-E.sub.SEC, sample)+.PHI..sub.Au.
FIGS. 12A-12C show the SEC spectra for Al, Ag, and Cu with and
without C.sub.60-bis. FIG. 14C includes the SEC spectrum of clean
Au foil as a reference.
[0121] While illustrative embodiments have been illustrated and
described, it will be appreciated that various changes can be made
therein without departing from the spirit and scope of the
invention.
* * * * *