U.S. patent application number 14/695347 was filed with the patent office on 2015-09-24 for high efficiency organic photovoltaic cells employing hybridized mixed-planar heterojunctions.
This patent application is currently assigned to THE TRUSTEES OF PRINCETON UNIVERSITY. The applicant listed for this patent is THE TRUSTEES OF PRINCETON UNIVERSITY. Invention is credited to Stephen Forrest, Barry P. Rand, Soichi UCHIDA, Jiangeng XUE.
Application Number | 20150270497 14/695347 |
Document ID | / |
Family ID | 35005712 |
Filed Date | 2015-09-24 |
United States Patent
Application |
20150270497 |
Kind Code |
A1 |
XUE; Jiangeng ; et
al. |
September 24, 2015 |
HIGH EFFICIENCY ORGANIC PHOTOVOLTAIC CELLS EMPLOYING HYBRIDIZED
MIXED-PLANAR HETEROJUNCTIONS
Abstract
A device is provided, having a first electrode, a second
electrode, and a photoactive region disposed between the first
electrode and the second electrode. The photoactive region includes
a first photoactive organic layer that is a mixture of small
molecule organic acceptor material and a small molecule organic
donor material, wherein the first photoactive organic layer has a
thickness not greater than 0.8 characteristic charge transport
lengths; a second photoactive organic layer in direct contact with
the first organic layer, wherein the second photoactive organic
layer is an unmixed layer of the small molecule organic acceptor
material of the first photoactive organic layer, and the second
photoactive organic layer has a thickness not less than about 0.1
optical absorption lengths; and a third photoactive organic layer
disposed between the first electrode and the second electrode and
in direct contact with the first photoactive organic layer. The
third photoactive organic layer is an unmixed layer of the small
molecule organic donor material of the first photoactive organic
layer and has a thickness not less than about 0.1 optical
absorption lengths.
Inventors: |
XUE; Jiangeng; (Gainesville,
FL) ; UCHIDA; Soichi; (Yokohama, JP) ; Rand;
Barry P.; (Princeton, NJ) ; Forrest; Stephen;
(Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THE TRUSTEES OF PRINCETON UNIVERSITY |
Princeton |
NJ |
US |
|
|
Assignee: |
THE TRUSTEES OF PRINCETON
UNIVERSITY
Princeton
NJ
|
Family ID: |
35005712 |
Appl. No.: |
14/695347 |
Filed: |
April 24, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14068430 |
Oct 31, 2013 |
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14695347 |
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10910371 |
Aug 4, 2004 |
8586967 |
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14068430 |
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10822774 |
Apr 13, 2004 |
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10910371 |
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Current U.S.
Class: |
136/255 ;
257/40 |
Current CPC
Class: |
H01L 51/0072 20130101;
H01L 51/4246 20130101; Y02E 10/549 20130101; H01L 51/0078 20130101;
B82Y 10/00 20130101; H01L 51/4253 20130101; H01L 51/0037 20130101;
H01L 2251/301 20130101; H01L 2251/308 20130101; H01L 51/0073
20130101; H01L 51/0087 20130101; H01L 51/0059 20130101; H01L
51/0046 20130101; H01L 51/0038 20130101; H01L 51/0081 20130101;
H01L 31/0256 20130101 |
International
Class: |
H01L 51/00 20060101
H01L051/00 |
Goverment Interests
GOVERNMENT RIGHTS
[0002] This invention was made with government support under Grant
No. F49620-03-1-0410 awarded by the U.S. Air Force Office of
Scientific Research and Grant No. ACQ-1-30619-05 (Prime
DE-AC36-99G010337) awarded by the National Renewable Energy
Laboratory of the U.S. Department of Energy. The government has
certain rights in the invention.
Claims
1. A device for generating photocurrent by absorbing photons,
comprising: a first electrode; a second electrode; a photoactive
region disposed between the first electrode and the second
electrode, the photoactive region comprising: a first photoactive
organic layer for generating excitons by absorbing photons
comprising a homogenous mixture of a small molecule organic
acceptor material and a small molecule organic donor material,
wherein the first photoactive organic layer has a thickness not
greater than 0.8 characteristic charge transport lengths; a second
photoactive organic layer for generating excitons by absorbing
photons in direct contact with the first photoactive organic layer,
wherein the second photoactive organic layer comprises an unmixed
layer of the small molecule organic acceptor material of the first
photoactive organic layer, and the second photoactive organic layer
has a thickness not less than about 0.1 absorption lengths; and a
third photoactive organic layer for generating excitons by
absorbing photons disposed between the first electrode and the
second electrode, the third photoactive organic layer being in
direct contact with the first photoactive organic layer, wherein
the third photoactive organic layer comprises an unmixed layer of
the small molecule organic donor material of the first photoactive
organic layer, and the third photoactive organic layer has a
thickness not less than about 0.1 optical absorption lengths.
2. The device of claim 1, wherein the first photoactive organic
layer has a thickness not greater than 0.3 characteristic charge
transport lengths.
3. The device of claim 1, wherein the device has a power efficiency
of 2% or greater.
4. The device of claim 1, wherein the device has a power efficiency
of 5% or greater.
5. The device of claim 1, wherein the second photoactive organic
layer has a thickness not less than about 0.2 optical absorption
lengths.
6. The device of claim 1, wherein the mixture of the small molecule
organic acceptor material and the organic donor material in the
first photoactive organic layer occurs in a ratio ranging from
about 10:1 to about 1:10, respectively.
7. The device of claim 1, wherein the small molecule organic
acceptor material is selected from the group consisting of:
fullerenes, perylenes, catacondensed conjugated molecular systems,
pyrene, coronene, and functionalized variants thereof.
8. The device of claim 7, wherein the catacondensed conjugated
molecular systems include linear polyacenes.
9. The device of claim 1, wherein the small molecule organic donor
material is selected from the group consisting of: metal containing
porphyrins, metal-free porphyrins, rubrene, metal containing
phthalocyanines, metal-free phthalocyanines, diamines, and
functionalized variants thereof, including naphthalocyanines.
10. The device of claim 1, wherein the first photoactive organic
layer consists essentially of a mixture of CuPc and C.sub.60.
11. The device of claim 1, further comprising a first
non-photoactive organic layer disposed between the second electrode
and the second organic layer.
12. The device of claim 11, wherein the first non-photoactive
organic layer comprises
2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolin (BCP).
13. The device of claim 11, wherein the first non-photoactive
organic layer is an exciton blocking layer.
14. The device of claim 1, wherein the first electrode is comprised
of indium tin oxide.
15. The device of claim 1, wherein the second electrode is
comprised of metal or metals selected from the group consisting of
Ag, LiF/Al, Mg:Ag, and Ca/Al.
16. The device of claim 1, wherein the device is a tandem solar
cell.
17. The device of claim 1, wherein the device is a solar cell.
18. The device of claim 1, wherein the device is a photodetector.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is continuation of co-pending U.S.
application Ser. No. 14/068,430, filed Oct. 31, 2013, which is a
divisional application of U.S. application Ser. No. 10/910,371,
filed on Aug. 4, 2004, now U.S. Pat. No. 8,586,967 issued on Nov.
19, 2013, which is a continuation-in-part of U.S. application Ser.
No. 10/822,774, filed on Apr. 13, 2004, the disclosures of which
are incorporated herein by reference in their entirety.
JOINT RESEARCH AGREEMENT
[0003] The claimed invention was made by, on behalf of, and/or in
connection with one or more of the following parties to a joint
university corporation research agreement: Regents of the
University of Michigan, Princeton University, University of
Southern California, Global Photonic Energy Corporation, and
Universal Display Corporation. The agreement was in effect on and
before the date the claimed invention was made, and the claimed
invention was made as a result of activities undertaken within the
scope of the agreement.
FIELD OF THE INVENTION
[0004] The present invention relates to efficient organic
photosensitive devices.
BACKGROUND
[0005] Opto-electronic devices that make use of organic materials
are becoming increasingly desirable for a number of reasons. Many
of the materials used to make such devices are relatively
inexpensive, so organic opto-electronic devices have the potential
for cost advantages over inorganic devices. In addition, the
inherent properties of organic materials, such as their
flexibility, may make them well suited for particular applications
such as fabrication on a flexible substrate. Examples of organic
opto-electronic devices include organic light emitting devices
(OLEDs), organic transistors/phototransistors, organic photovoltaic
cells, and organic photodetectors. For OLEDs, the organic materials
may have performance advantages over conventional (i.e., inorganic)
materials. For example, the wavelength at which an organic emissive
layer emits light may generally be readily tuned with appropriate
dopants. For organic transistors/phototransistors, the substrates
upon which they are constructed may be flexible, providing for
broader applications in industry and commerce.
[0006] As used herein, the term "organic" includes polymeric
materials as well as small molecule organic materials that may be
used to fabricate organic devices including opto-electronic
devices. "Small molecule" refers to any organic material that is
not a polymer, and "small molecules" may actually be quite large.
Small molecules may include repeat units in some circumstances. For
example, using a long chain alkyl group as a substituent does not
remove a molecule from the "small molecule" class. Small molecules
may also be incorporated into polymers, for example as a pendent
group on a polymer backbone or as a part of the backbone. Small
molecules may also serve as the core moiety of a dendrimer, which
consists of a series of chemical shells built on the core moiety.
Small molecules generally have a well defined molecular weight,
whereas polymers generally do not have a well defined molecular
weight.
[0007] General background information on small molecular weight
organic thin-film photodetectors and solar cells may be found in
Peumans et al., "Small Molecular Weight Organic Thin-Film
Photodetectors and Solar Cells," Journal of Applied Physics-Applied
Physics Reviews-Focused Review, Vol. 93, No. 7, pp. 3693-3723
(April 2003).
[0008] The "fill factor" (FF) of a solar cell is
P.sub.max/(Jsc*Voc), where P.sub.max is the maximum power of the
solar cell, determined by finding the point on the I-V curve for
which the product of the current and voltage is a maximum. A high
FF is an indication of how "square" the I-V curve for a solar cell
appears.
[0009] Optoelectronic devices rely on the optical and electronic
properties of materials to either produce or detect electromagnetic
radiation electronically or to generate electricity from ambient
electromagnetic radiation. Photosensitive optoelectronic devices
convert electromagnetic radiation into electricity. Photovoltaic
(PV) devices or solar cells, which are a type of photosensitive
optoelectronic device, are specifically used to generate electrical
power. PV devices, which may generate electrical power from light
sources other than sunlight, are used to drive power consuming
loads to provide, for example, lighting, heating, or to operate
electronic equipment such as computers or remote monitoring or
communications equipment. These power generation applications also
often involve the charging of batteries or other energy storage
devices so that equipment operation may continue when direct
illumination from the sun or other ambient light sources is not
available. As used herein the term "resistive load" refers to any
power consuming or storing device, equipment, or system. Another
type of photosensitive optoelectronic device is a photoconductor
cell. In this function, signal detection circuitry monitors the
resistance of the device to detect changes due to the absorption of
light. Another type of photosensitive optoelectronic device is a
photodetector. In operation a photodetector has a voltage applied
and a current detecting circuit measures the current generated when
the photodetector is exposed to electromagnetic radiation. A
detecting circuit as described herein is capable of providing a
bias voltage to a photodetector and measuring the electronic
response of the photodetector to ambient electromagnetic radiation.
These three classes of photosensitive optoelectronic devices may be
characterized according to whether a rectifying junction as defined
below is present and also according to whether the device is
operated with an external applied voltage, also known as a bias or
bias voltage. A photoconductor cell does not have a rectifying
junction and is normally operated with a bias. A PV device has at
least one rectifying junction and is operated with no bias. A
photodetector has at least one rectifying junction and is usually
but not always operated with a bias.
[0010] A need exists for an organic photovoltaic cells with a
higher efficiency.
SUMMARY OF THE INVENTION
[0011] A device is provided, having a first electrode, a second
electrode, and a photoactive region disposed between the first
electrode and the second electrode. The photoactive region includes
a first organic layer comprising a mixture of an organic acceptor
material and an organic donor material, wherein the first organic
layer has a thickness not greater than 0.8 characteristic charge
transport lengths, and a second organic layer in direct contact
with the first organic layer, wherein the second organic layer
comprises an unmixed layer of the organic acceptor material or the
organic donor material of the first organic layer, and the second
organic layer has a thickness not less than about 0.1 optical
absorption lengths. Preferably, the first organic layer has a
thickness not greater than 0.3 characteristic charge transport
lengths. Preferably, the second organic layer has a thickness of
not less than about 0.2 optical absorption lengths. Embodiments of
the invention can be capable of power efficiencies of 2% or
greater, and preferably 5% or greater.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a schematic diagram of an organic photovoltaic
cell in accordance with an embodiment of the invention.
[0013] FIG. 2 is a schematic diagram of another organic
photovoltaic cell in accordance with an embodiment of the
invention.
[0014] FIG. 3 is a schematic diagram of yet another organic
photovoltaic cell in accordance with an embodiment of the
invention.
[0015] FIG. 4 illustrates a method of making an organic
photovoltaic cell in accordance with an embodiment of the
invention.
[0016] FIG. 5 shows an energy level diagram of a device.
[0017] FIG. 6 shows J-V characteristics of a hybrid device
[0018] FIG. 7 shows additional photovoltaic characteristics of the
device described with reference to FIG. 6.
[0019] FIG. 8 shows absorption spectra of CuPc:C.sub.60 films with
various mixture ratios, deposited on ITO.
[0020] FIG. 9 shows normalized photocurrent--voltage
characteristics under various light intensities for the devices
described with respect to FIG. 6.
[0021] FIG. 10 shows the current density vs voltage (J-V)
characteristics in the dark for a planar HJ device and a hybrid HJ
device.
[0022] FIG. 11 shows the dependences of n and J.sub.s on the mixed
layer thickness d.sub.m, for hybrid HJ cells with
d.sub.D=d.sub.A-200 .ANG.=200 .ANG.-d.sub.m/2.
[0023] FIG. 12 shows the photocurrent density, J.sub.Ph, at an
illumination intensity of P.sub.O=120 mW/cm.sup.2 for hybrid
devices having various mixed layer thicknesses.
[0024] FIG. 13 shows experimental J-V characteristics at various
P.sub.O for a hybrid device with a mixed layer thickness of 200
.ANG..
[0025] FIG. 14 shows absorption spectra of a planar HJ device and a
hybrid HJ device with a mixed layer thickness of 200 .ANG..
[0026] FIG. 15 shows the illumination intensity dependences of
.eta..sub.P, FF, and V.sub.OC for hybrid HJ devices and a planar HJ
device.
[0027] FIG. 16 shows X-ray diffraction results for homogeneous and
mixed CuPc and C.sub.60 films.
DETAILED DESCRIPTION
[0028] Organic photovoltaic (PV) cells have attracted considerable
attention due to their potential for low cost solar or ambient
energy conversion. Early results, with an organic PV cell based on
a single donor-acceptor (D-A) heterojunction, resulted in a
1%-efficient thin-film. See C. W. Tang, Appl. Phys. Lett. 48, 183
(1986). The power conversion efficiency, .eta..sub.P, has steadily
improved since then through the use of new materials and device
structures. See P. Peumans et al., J. Appl. Phys. 93, 3693 (2003);
A. Yakimov and S. R. Forrest, Appl. Phys. Lett. 80, 1667 (2002); P.
Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 126 (2001); S. E.
Shaheen et al., Appl. Phys. Lett. 78, 841 (2001); P. Peumans et
al., Nature (London) 425, 158 (2003). In particular,
.eta..sub.P=(3.6.+-.0.2)% under 1 sun (100 mW/cm.sup.2) AM1.5G
simulated solar illumination was achieved in a double
heterostructure copper phthalocyanine (CuPc)/C.sub.60 thin-film
cell. P. Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 126
(2001). However, these single heterojunction devices are limited in
that the "active region" of the device, i.e. the region in which
absorbed photons may contribute to photocurrent, is limited to the
region from which excitons excited by photons can diffuse with a
reasonable probability to the single heterojunction.
[0029] Donor (D)--acceptor (A) bulk heterojunctions (BHJs) may be
used to improve the efficiencies of both polymer and small
molecule-based photovoltaic (PV) cells. Because the external
quantum efficiency (.eta..sub.EQE) of an organic D-A bilayer
structure is often limited by a short exciton diffusion length, the
BHJ has been suggested as a means to overcome this limitation,
resulting in improved .eta..sub.EQE and power conversion efficiency
(.eta..sub.P). Such a BHJ can consist of a blended thin film of a
donor-like phthalocyanine (Pc) and the acceptor-like C.sub.60.
Recently, .eta..sub.P=3.37% has been reported under 0.1 sun (10
mW/cm.sup.2, AM1.5) illumination in a mixed ZnPc:C.sub.60 PV cell.
See D. Gebeyehu et al., Solar Energy Mater. Solar Cells, 79, 81
(2003). Unfortunately, that device had a large cell series
resistance (R.sub.S), resulting in a reduced short circuit current
density (J.sub.SC), and hence the power efficiency fell to =1.04%
at 1 sun intensity. The reason for this large R.sub.S may be
attributed to the presence of resistive organic layers including
poly(3,4-ethylenedioxythiophene):poly(styrenesul-fonate)
(PEDOT:PSS) and, more importantly, their contact resistances. On
the other hand, recent results show that a CuPc/C.sub.60 bilayer
device with a very low R.sub.S shows a significant improvement in
.eta..sub.P, especially at higher illumination intensity, achieving
a maximum power efficiency of =(4.2.+-.0.2) % at 4 to 12 suns. See,
Xue et al., Appl. Phys. Lett., 84, 3013 (2004).
[0030] Referring now in detail to the drawings, there is
illustrated in FIG. 1 a schematic diagram of an organic
photovoltaic cell 100 in accordance with an embodiment of the
invention. Device 100 may include a first electrode 102, a first
organic layer 106, a second organic layer 108, a third organic
layer 114, and a second electrode 104, disposed in that order over
a substrate. First organic layer 106 comprises a mixture of an
organic acceptor material and an organic donor material. Second
organic layer 108 comprises the organic acceptor material of first
organic layer 106, but does not include the donor material of first
organic layer 106. Second organic layer 108 has a thickness of
between about 0.5 exciton diffusion length and about 10 exciton
diffusion lengths. Preferably, organic layer 108 has a thickness of
about 1 to 10 exciton diffusion lengths. As a result, first organic
layer 106 acts a bulk heterojunction, in which photogenerated
excitons may dissociate into electrons and holes. Second organic
layer 108 may be photoactive in the sense that it absorbs photons
to produce excitons that may later contribute to photocurrent, but
these excitons may first diffuse to the heterojunction of first
organic layer 106. Third organic layer 114 comprises an exciton
blocking layer, comprised of materials selected to prevent excitons
from exiting second organic layer 108 into third organic layer 114.
Third organic layer 114 may be referred to as a non-photoactive
organic layer, because it may not be responsible for absorbing
photons that contribute significantly to photocurrent.
[0031] FIG. 2 is a schematic diagram of another organic
photovoltaic cell 200 in accordance with an embodiment of the
invention. Device 200 may include a first electrode 202, a first
organic layer 206, a second organic layer 208, and a second
electrode 204, disposed in that order over a substrate. First
organic layer 206 comprises a mixture of an organic acceptor
material and an organic donor material. Second organic layer 208
comprises the organic donor material of first organic layer 206,
but does not include the acceptor material of first organic layer
206. Second organic layer 208 has a thickness of between about 0.5
exciton diffusion length and about 10 exciton diffusion lengths,
and preferably between about 1 and 10 exciton diffusion lengths. As
a result, first organic layer 206 acts a bulk heterojunction, in
which photogenerated excitons may dissociate into electrons and
holes. Second organic layer 208 may be photoactive in the sense
that it absorbs photons to produce excitons that may later
contribute to photocurrent, but these excitons may first diffuse to
the heterojunction of first organic layer 206. Third organic layer
214 comprises an exciton blocking layer, comprised of materials
selected to prevent excitons from exiting second organic layer 208
into third organic layer 214.
[0032] Examples of diffusion lengths for various organic acceptor
and donor materials are illustrated in Table 1, below:
TABLE-US-00001 TABLE 1 Reported Exciton Diffusion Lengths.
Diffusion Length, Material.sup.a L.sub.D (.ANG.) Technique Ref.
Small Molecule Systems PTCBI 30 .+-. 3 PL quenching P. Peumans, A
Yakimov and S. Forrest, J. App. Phys., vol. 93, no. 7, Apr. 1,
2003, p. 3702 (Peumans et al.) PTCDA 880 .+-. 60 from .eta..sub.EQE
V. Bulovic and S. R. Forrest, Chem. Phys. 210, 13, 1996. PPEI .sup.
~700.sup.b PL quenching B. A. Gregg e al., J. Phys. Chem. B 101,
5362, 1997. CuPc 100 .+-. 30 from .eta..sub.EQE Peumans et al. 680
.+-. 200 from .eta..sub.EQE T. Stubinger and W. Brutting, J. Appl.
Phys. 90, 3632, 2001. ZnPc 300 .+-. 100 from .eta..sub.EQE H. R.
Kerp and E. E. van Faassen, Nord. Hydrol. 1, 1761, 1999. C.sub.60
400 .+-. 50 from .eta..sub.EQE Peumans et al. 141 from
.eta..sub.EQE L. A. A. Pettersson et al., J. Appl. Phys. 86, 487,
1999. Alq.sub.3 200 A. L. Burin and M. A. Ratner, J. Phys. Chem. A
104, 4704, 2000. ~200 V. E. Choong et al. J. Vac. Sci. Technol. A
16, 1838, 1998. Polymer Systems PPV 70 .+-. 10 from .eta..sub.EQE
J. J. M. Halls et al., Appl. Phys. Lett. 68, 3120, 1996. 120 .+-.
30.sup.c from .eta..sub.EQE T. Stubinger and W. Brutting, J. Appl.
Phys. 90, 3632, 2001. PEOPT 47 from .eta..sub.EQE L. A. A.
Pettersson et al., J. Appl. Phys. 86, 487, 1999. 50 PL quenching M.
Theander et al. Phys. Rev. B 61, 12 957, 2000. .sup.aPPEI =
perylene bis(phenethylimide), Alq.sub.3 = tris(8-hydroxyquinoline)
aluminum. .sup.bUsing the result for the SnO.sub.2 quenching
surface and assuming infinite surface recombination velocity. The
results leading to L.sub.D.sup.PPEI = 2.5 .+-. 0.5 .mu.m are likely
influenced by quencher diffusion and morphological changes during
solvent vapor assisted annealing. .sup.cOptical interference
effects not considered.
[0033] It will be understood that the listing of organic materials
in Table 1, above, is exemplary and not meant to be limiting. Other
materials having similar or different diffusion lengths may be used
without departing from the scope of the invention. Furthermore, it
will be understood that the diffusion lengths listed in Table 1 are
not meant to restrict the invention disclosed herein to only those
listed lengths. Other lengths, whether by virtue of the use of
other materials or by virtue of different methods of determination,
calculations, or measurements of diffusion lengths of materials
identified hereinabove, may be used without departing from the
scope of the invention.
[0034] In one embodiment, the mixture of the organic acceptor
material and the organic donor material in a mixed organic layer,
such as first organic layer 106 (or 206) may occur in a ratio
ranging from about 10:1 to about 1:10 by weight, respectively. In
one embodiment, an organic layer including a mixture of acceptor
and donor materials (such as first organic layer 106), and an
organic layer that includes only an acceptor material or a donor
material (such as second organic layer 108 or 208) may each
contribute 5 percent or more, and preferably 10 percent or more, of
the total energy output of the photoactive device. In one
embodiment, an organic layer including a mixture of acceptor and
donor materials (such as first organic layer 106 or 206), and an
organic layer that includes only an acceptor material or a donor
material (such as second organic layer 108 or 208) may each absorb
5 percent or more, and preferably 10 percent or more, of the energy
incident on the photoactive device. A layer that has a lower
percentage of contribution to energy and/or absorption may not be
considered as significantly participating as a part of the
photoactive region of the device. In one embodiment, the organic
acceptor material may be selected from a group consisting of:
fullerenes; perylenes; catacondensed conjugated molecular systems
such as linear polyacenes (including anthracene, napthalene,
tetracene, and pentacene), pyrene, coronene, and functionalized
variants thereof. In one embodiment, the organic donor material may
be selected from a group consisting of: metal containing
porphyrins, metal-free porphyrins, rubrene, metal containing
phthalocyanines, metal-free phthalocyanines, diamines (such as
NPD), and functionalized variants thereof, including
naphthalocyanines This listing is not meant to be comprehensive,
and other suitable acceptor and donor materials may be used. In one
embodiment, the first organic layer 206 may consist essentially of
a mixture of CuPc and C.sub.60. In one embodiment, the photoactive
device 100, 200 may further comprise a third organic layer 114, 214
that may be disposed between the second electrode 104, 204 and the
second organic layer 108, 208, and may be a non-photoactive layer.
In one embodiment, third organic layer 114, 214 may comprise
2,9-dimethyl-,7-diphenyl-1,10-p-henanthrolin (BCP). In one
embodiment, the third organic layer 114, 214 may be an exciton
blocking layer. In one embodiment, the first electrode 102, 202 may
be comprised of indium tin oxide or other conductive oxide. In one
embodiment, the second electrode 104, 204 may be comprised of Ag,
LiF/Al, Mg:Ag, Ca/Al, and other metals. Other material selections
may be used.
[0035] Where a layer is described as an "unmixed" acceptor or donor
layer, the "unmixed" layer may include very small amounts of the
opposite material as an impurity. A material may be considered as
an impurity if the concentration is significantly lower than the
amount needed for percolation in the layer, i.e., less than about
5% by weight. Preferably, any impurity is present in a much lower
amount, such as less than 1% by weight or most preferably less than
about 0.1% by weight. Depending upon the processes and process
parameters used to fabricate devices, some impurities of the
materials in immediately adjacent layers be unavoidable.
[0036] Preferably, blocking layers are transparent to the
wavelengths of light absorbed by the photoactive region. Blocking
layers preferably readily accept injection of and conduct the type
of charge carrier that may be traveling through them--for example,
a blocking layer disposed on the acceptor side of a photoactive
region, disposed between the acceptor material and an electrode,
should readily accept injection of electrons from the acceptor, and
should readily conduct electrons.
[0037] A layer is described as "photoactive" if photons absorbed by
that layer make a significant contribution to the photocurrent of
the device. A device may have a photoactive region comprising
several photoactive layers. In various embodiments of the
invention, the photoactive region comprises a plurality of
photoactive layers, including a layer that is a mixture of acceptor
and donor materials, as well as a layer that includes only an
acceptor or a donor material, but not both (although impurities may
be present as discussed above). A device that combines a mixed
photoactive layer with one or more unmixed photoactive layer may be
referred to as a hybrid device, because it combines favorable
properties of planar HJ devices (a D-A interface with no mixed
layer), with favorable properties of a mixed layer device (a mixed
D-A layer with no unmixed A or D layer, or only minimal unmixed
layers of the A and D materials).
[0038] FIG. 3 is a schematic diagram of yet another organic
photovoltaic cell 300 in accordance with an embodiment of the
invention. Device 300 may include a first electrode 302, a third
organic layer 310, a first organic layer 306, a second organic
layer 308, a fourth organic layer 314, and a second electrode 304,
disposed in that order over a substrate. First organic layer 306
comprises a mixture of an organic acceptor material and an organic
donor material. Second organic layer 308 comprises the organic
acceptor material of first organic layer 306, but does not include
the donor material of first organic layer 306. Second organic layer
308 has a thickness of between about 0.5 exciton diffusion length
and about 10 exciton diffusion lengths, and preferably between
about 1 and 10 exciton diffusion lengths. Third organic layer 310
comprises the organic donor material of first organic layer 306,
but does not include the acceptor material of first organic layer
306. Second organic layer 310 has a thickness of between about 0.5
exciton diffusion length and about 10 exciton diffusion lengths,
and preferably between about 1 and 10 exciton diffusion lengths. As
a result, first organic layer 306 acts as a bulk heterojunction, in
which photogenerated excitons may dissociate into electrons and
holes. Second organic layer 308 and third organic layer 310 may be
photoactive in the sense that they absorbs photons to produce
excitons that may later contribute to photocurrent, but these
excitons may first diffuse to the heterojunction of first organic
layer 306. Fourth organic layer 314 comprises an exciton blocking
layer, comprised of materials selected to prevent excitons from
exiting second organic layer 308 into third organic layer 314.
Fourth organic layer 314 may be referred to as a non-photoactive
organic layer, because it may not be responsible for absorbing
photons that contribute significantly to photocurrent.
[0039] Preferred parameters for the embodiment of FIG. 3, such as
layer thicknesses, material selections, proportions of materials in
first organic layer 306 (the mixed layer), relative amounts of
incident energy absorbed, and relative amount of total energy
output, are similar to those for FIGS. 1 and 2.
[0040] In various embodiments of the invention, there is an organic
layer that includes a mixture of an acceptor and a donor material
(such as layers 106, 206, and 306), and at least one layer that
includes only the donor or acceptor material from the mixed layer
(such as layers 108, 208, 308 and 310). When the device absorbs a
photon, an exciton may be created. The exciton may then dissociate
and contribute to photocurrent if it is able to reach an
appropriately designed hetero-junction. A layer that includes a
mixture of acceptor and donor material provides a bulk
heterojunction, such that there is favorably a large volume over
which such dissociation may occur. However, such a layer may have
lower conductivity than an unmixed layer, and lower conductivity is
undesirable. Conductivity issues are aggravated by thicker layers,
so there is a limit on the thickness that such a mixed layer may
have if a reasonable conductivity is desired.
[0041] A layer that includes only an acceptor or a donor may
favorably have a higher conductivity than a mixed layer. However,
there is no heterojunction in such a layer, such that excitons
formed by the absorption of a photon need to travel to a
heterojunction in order to efficiently dissociate. As a result,
there is also a limit on the useful thickness of unmixed layers in
a solar cell, but the limit may be related more to the diffusion
length of excitons as opposed to conductivity issues.
[0042] In addition, a thick photoactive region is desirable,
because a thicker photoactive layer may absorb more photons that
may contribute to photocurrent than a thinner photoactive
layer.
[0043] Various embodiments of the invention provide a device that
combines the favorable properties of a device having a bulk
heterojunction (such as mixed layer 106, 206 or 306), but no
unmixed layer, with the favorable properties of a device that does
not have a bulk heterojunction--i.e., a device having a pure
acceptor layer that forms a planar junction with a pure donor
layer. The mixed and the unmixed layers are each a part of the
photoactive region, such that the thicknesses add for purposes of
absorbing more photons. Greater thicknesses of layers that
contribute to photocurrent may therefore be achieved than with a
device where the photoactive region includes only a mixed layer or
only unmixed layers, or where most of the thickness is due to only
a mixed layer or only unmixed layers. Or, a device with a lower
resistance for a given thickness of the photoactive region may be
achieved.
[0044] In a preferred embodiment of the invention, a layer or
layers that include only a single acceptor or donor material, but
not a mixture of the two, such as layers 108, 208, 308 and 310, may
be selected to have high conductivity, while being able to
contribute to the photocurrent. Excitons that are formed by a
photon absorbed in such a layer must diffuse to a heterojunction in
order to contribute to photocurrent. As a result, a thickness for
such a layer that is about 0.5 exciton diffusion lengths to about
10 exciton diffusion lengths is preferred, and more preferably
about 1 to 10 exciton diffusion lengths. For layers having a
thickness that is greater than about 10 diffusion lengths, any
additional thickness may not make a significant contribution to
photocurrent, because photons absorbed too far from a
heterojunction are unable to reach a heterojunction.
[0045] At the lower boundary of the unmixed photoactive layers,
optical absorption is a more important parameter than exciton
diffusion length. The "optical absorption length" of a material is
the length in which incident light intensity is reduced to (1/e),
or about 37%. Typical absorption lengths for organic photoactive
materials are in the range 500-1000 .ANG.. For CuPc, the optical
absorption length is 500 .ANG.. for wavelengths in the range 500
nm-700 nm. For C.sub.60, the optical absorption length is 1000
.ANG.. for a wavelength of 450 nm. In order for a layer to
contribute significantly to photocurrent, the layer thickness
should be at least a significant fraction of an absorption length.
Preferably, the thickness of a photoactive layer, such as an
unmixed organic photoactive layer, is not less than about 0.1
absorption lengths, and more preferably is not less than about 0.2
absorption lengths. For smaller thicknesses, the layer may not make
a significant contribution to photocurrent.
[0046] In a preferred embodiment of the invention, a layer than
includes a mixture of acceptor and donor materials, such as layers
106, 206 and 306, include 10% or more of an acceptor material and
10% or more of a donor material. It is believed that 10% is the
lower limit at which there is enough material for percolation.
Percolation is desirable in both the acceptor and donor materials,
because it allows photogenerated electrons and holes that result
from dissociation anywhere in the mixed layer to reach the
appropriate electrodes by traveling through the acceptor and donor,
respectively, without traveling through the opposite (donor or
acceptor) layer. Preferably, the unmixed layers in the photoactive
region comprise one of the materials that is present in the mixed
layer, to avoid any HOMO/LUMO mismatch for charge carriers that are
percolating through the mixed layer and reach an unmixed layer.
[0047] D-A phase separation is needed for efficient carrier
collection in both polymer and small molecule-based BHJ solar
cells. On the other hand, the CuPc:C.sub.60 mixed layer shows a
large .eta..sub.P comparable to optimized bilayer devices employing
the same materials, contrary to CuPc:
3,4,9,10-peryrenetetracarboxylic bis-benzimidazole mixed layer
devices that required annealing and phase separation to improve
efficiency. See, Peumans et al., Nature, 425, 158 (2003). Indeed,
following a similar annealing procedure for CuPc:C.sub.60 mixed
layer cells results in a significant reduction in .eta..sub.P. This
suggests that a mixed CuPc:C.sub.60 system may undergo phase
separation during the deposition process itself, such that the
mixed layer is a percolating network of both materials, provided
that the concentrations of both materials is above the percolation
threshold.
[0048] Unmixed organic donor-acceptor heterojunctions may be used
to provide efficient photo-generation of charge carriers upon
absorption of incident light. The efficiency of this type of cell
may be limited by the poor ability of excitons (i.e., bound
electron-hole pairs) to diffuse to the donor-acceptor interface. A
mixed layer, i.e., a donor-acceptor mixture, may be used to
alleviate this problem by creating a spatially distributed
donor-acceptor interface that is accessible to every photogenerated
exciton generated in the mixed layer. However, since charge
mobility may be significantly reduced in a mixture as compared to a
homogeneous film, recombination of photogenerated holes and
electrons is more likely to happen in a mixture, leading to
incomplete collection of charge carriers.
[0049] In one embodiment of the invention, a preferred
microstructure for a molecular donor-acceptor mixture is provided.
A mixed layer having the preferred microstructure may be used in
photosensitive devices that either have or do not have one or more
unmixed photoactive layers. An example of the preferred
microstructure is described with respect to a mixture of CuPc and
C.sub.60, although other donor and acceptor materials may be used.
The preferred microstructure includes percolating paths for hole
and electron transport through the mixed donor-acceptor layer, with
each path only one or a few molecules wide. Preferably, the width
of the path is 5 molecules wide or less, and more preferably 3
molecules wide or less. Photogenerated charges may be efficiently
transported along such paths to their respective electrodes without
significant recombination with their countercharges. The
interpenetrating network of donor and acceptor materials forms a
nanostructured, spatially distributed donor-acceptor interface for
efficient exciton diffusion and subsequent dissociation.
[0050] The preferred microstructure was demonstrated in a
CuPc:C.sub.60 mixture, 1:1 ratio by weight, prepared by vacuum
thermal evaporation. In the mixture, it was found that the charge
transport length, i.e., the mean distance that charges travel
before recombination with their counter charges, when no bias was
applied, was about 40 nm, on the same order of the optical
absorption length. It is believed that no pure donor or acceptor
domains exist in the CuPc mixture. The lack of such pure domains is
preferred. The tendency of CuPc aggregation was, reduced by
increasing the content of C.sub.60 in the layer.
[0051] X-ray diffraction was performed to study the crystal
structure of homogeneous and mixed CuPc and C.sub.60 films, as
shown in FIG. 16. It was found that a homogeneous CuPc film is
polycrystalline, while a homogeneous C.sub.60 film is amorphous. A
mixed CuPc:C.sub.60 film, 1:1 ratio by weight, is also amorphous,
indicating that no significant phase separation occurs. By "no
significant phase separation," it is meant that there is no
aggregation measurable by presently available measurement
techniques. The most sensitive of these techniques at the present
time is believed to be measurement with a synchrotron x-ray source
(e.g., Brookhaven), which is capable of measuring aggregates 5
molecules wide and up. Note that these definitions of "no
significant phase separation" and "aggregation" does not exclude
the possibility of interpercolating strings of molecules that may
be many molecules long.
[0052] Optical absorption spectra were measured for mixed
CuPc:C.sub.60 films with different mixing ratios, as shown in FIG.
8. From the dependence of the relative intensities of the two CuPc
absorption peaks (around 620 nm and 690 nm) on the mixing ratio, it
was found that CuPc molecules show a reduced tendency to aggregate
with increasing C.sub.60 content.
[0053] Organic photovoltaic cells with a mixed CuPc:C.sub.60 layer
sandwiched between homogeneous CuPc and C.sub.60 layers were
fabricated, to form a hybrid planar-mixed heterojunction
photovoltaic cell, and tested under simulated AM1.5G solar
illumination. The photoactive region of the cell had 15 nm CuPc/10
run CuPc:C.sub.60 (1:1 ratio by weight)/35 nm C.sub.60. The cell
had a photocurrent as high as a cell having a single 33 nm thick
mixed photoactive layer, and a charge collection efficiency as high
as a cell without a mixed layer (i.e., a planar heterojunction
cell). A maximum power conversion efficiency of 5.0% under 1 to 4
suns simulated AM1.5G solar illumination was obtained, compared to
3.5% for the mixed layer cell under 1 to 4 suns (3.6% under 1 sun),
and 4.2% under 4 to 12 suns for the planar heterojunction cell.
Fitting the current-voltage characteristics of the hybrid
planar-mixed heterojunction cells under illumination using a model
based on the charge transport length, a charge transport length of
40 nm was obtained for the cells under short-circuit conditions (as
shown in FIG. 13), which is on the same order of magnitude as the
optical absorption length. A CuPc:PTCBI
(3,4,9,10-perylenetetracarboxyloc bis-benzimidazole) mixed layer
has a charge transport length estimated at less than 5-10 nm, for
comparison.
[0054] Although various embodiments are described with respect to
undoped organic layers, it is understood that dopants may be added
to the various organic layers in order to increase conductivity
and/or to modify the light absorption characteristics of the doped
organic layer to advantageously impact device or layer
performance.
[0055] It is understood that the embodiments illustrated in FIGS.
1-3 are exemplary only, and that other embodiments may be used in
accordance with the present invention. Any photovoltaic cell having
both a mixed organic layer that includes both an acceptor material
and a donor material, as well as an adjacent layer that includes
only an acceptor material or a donor material, where both the mixed
layer and the unmixed layer contribute significantly to
photocurrent, would be within the scope of embodiments of the
invention. For example, the order of the layers illustrated in
FIGS. 1-3 may be altered. For example, in FIGS. 1 and 2, the
positions of the photoactive layers, i.e., first organic layer 106
(or 206) and second organic layer 108 (or 208) may be switched,
with appropriate repositioning of blocking layers, etc. Additional
layers may or may not also be present, such as blocking layers,
charge recombination layers, etc. For example, blocking layers may
be removed, i.e., third organic layer 114 or fourth organic layer
314, and/or additional blocking layers may be present (such as a
blocking layer between first organic layer 106 and underlying first
electrode 104). Various solar cell configurations may be used, such
as tandem solar cells. Different materials than those specifically
described may be used. For example, a device where all of the
electrodes are ITO may be fabricated such that the device may be
transparent to some degree. Additionally, the device could be
fabricated onto a substrate, and then applied to a supporting
surface, such that the last electrode deposited is closest to the
supporting surface. Although many embodiments are described with
respect to solar cells, other embodiments may be used in other
types of photosensitive devices having a D-A heterojunction, such
as a photodetector.
[0056] FIG. 4 illustrates a method of making an organic
photovoltaic cell in accordance with an embodiment of the
invention. The method begins at step 400. At step 402, a first
organic layer may be deposited over a first electrode. The first
organic layer may be a mixed layer, including both an organic
acceptor material and an organic donor material. At step 404, a
second organic layer over may be deposited over the first organic
layer. The second organic layer maybe an unmixed layer, including
either the organic acceptor material or the organic donor material
of the first organic layer, but not both. The organic layers may be
deposited by any suitable method, including thermal evaporation (or
coevaporation for multiple materials) and OVPD. At step 406, a
second electrode may be deposited over the second organic layer.
The method may end at step 408.
[0057] In one embodiment of the invention, an efficient organic
solar cell with a vacuum co-deposited donor-acceptor copper
phthalocyanine (CuPc):C.sub.60 mixed layer is provided. A device
with a structure of indium tin oxide/330 .ANG. CuPc:C.sub.60
(1:1)/100 .ANG. C.sub.60/75 .ANG..
2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolin/Ag was fabricated. The
device had a series resistance of only R.sub.S=0.25
.OMEGA.cm.sup.2, resulting in a current density of .about.1
A/cm.sup.2 at a forward bias of +1 V, and a rectification ratio of
10.sup.6 at .+-.1 V. Under simulated solar illumination (all
simulated solar spectra described herein were AM1.5G simulated
solar spectrum), the short circuit current density increases
linearly with light intensity up to 2.4 suns. A maximum power
conversion efficiency was measured of .eta..sub.P=(3.6.+-.0.2)% at
0.3 suns and .eta..sub.P=(3.5.+-.0.2)% at 1 sun. Although the fill
factor decreases with increasing intensity, a power efficiency as
high as .eta..sub.P=(3.3.+-.0.2) % is observed at 2.4 suns
intensity.
[0058] In another embodiment of the invention, an efficient solar
cell is provided. A device is provided with the structure:
indium-tin-oxide/150 .ANG. CuPc/100 .ANG. CuPc:C.sub.60 (1:1 by
weight)/350 .ANG. C.sub.60/100 .ANG. bathocuproine/1000 .ANG. Ag.
This photovoltaic cell exhibited a maximum power conversion
efficiency of (5.0.+-.0.2)% under 1 to 4 suns of simulated AM1.5G
solar illumination.
[0059] The power efficiencies achieved by embodiments of the
invention are higher than any other previously achieved for organic
solar cells. These surprising results are due to interactions
between several features of embodiment of the invention, including
the use of an unmixed organic photoactive layer in connection with
a mixed organic photoactive layer, with thicknesses selected with
efficiency in mind. Embodiments of the invention are capable of
power efficiencies of 2%, 3.5%, or 5%, or greater. It is expected
that with refinement and optimization of devices consistent with
embodiments of the invention, even higher power efficiencies may be
achieved.
[0060] One parameter to consider in selecting the thickness of the
mixed layer is the characteristic charge transport length L, which
can be considered as the average distance an electron or a hole
travels in the mixed layer under an electric field before being
recombined. If the thickness of the mixed layer is too great, many
of the charge carriers will recombine as opposed to generating
photocurrent. Selecting the thickness of the mixed layer is
therefore a tradeoff among several factors, including the desire
for a thick layer to increase absorption, and the desire for a thin
layer to avoid recombination. It is preferred that the thickness of
the mixed layer be not greater than about 0.8 characteristic charge
transport lengths, and more preferably not greater than about 0.3
characteristic charge transport lengths. For some of the specific
embodiments described herein that use a CuPc:C.sub.60 (1:1) mixed
layer, the characteristic charge transport length of the mixed
layer is about 45 nm. Excellent efficiencies were obtained for
devices having mixed layer thicknesses of 330 .ANG. and 100
.ANG..
[0061] A device disclosed in FIG. 1 of Hiromoto, Three-layered
organic solar cell with a photoactive interlayer of codeposited
pigments, Appl. Phys. Lett. 58 (10) (1991) has a mixed layer with a
characteristic charge transport length of about 40 nm, and the
layer thickness is about 1 characteristic charge transport length.
As a result, recombination in the mixed layer of that device may
account in part for the low device efficiency.
[0062] Photovoltaic characteristics of MPc:C.sub.60 mixed devices
of various structures are summarized in Table 2.
TABLE-US-00002 TABLE 2 P.sub.0 J.sub.SC J.sub.SC/P.sub.0 V.sub.OC
.eta..sub.P Structure (.ANG.) (mW/cm.sup.2) (mA/cm.sup.2) (A/W) (V)
FF (%) ITO/370 CuPc:C.sub.60 (1:1)/ 100 12.3 .+-. 0.6 0.12 0.53
0.43 2.8 .+-. 0.1 75 BCP/Ag ITO/330 CuPc:C.sub.60 (1:1)/ 10 1.6
.+-. 0.1 0.16 0.43 0.51 3.5 .+-. 0.2 100 C.sub.60/75 BCP/Ag 27 4.2
.+-. 0.2 0.16 0.47 0.49 3.6 .+-. 0.2 100 15.4 .+-. 0.7 0.15 0.50
0.46 3.5 .+-. 0.2 ITO/300 CuPc:C.sub.60 (1:2)/ 100 11.1 .+-. 0.5
0.11 0.54 0.44 2.6 .+-. 0.1 100 C.sub.60/75 BCP/Ag ITO/150 CuPc/
100 0.5 0.6 5.0 .+-. 0.2 00 CuPc:C.sub.60 (1:1)/350 C.sub.60/ 100
BCP/1000 Ag ITO/PEDOT:PSS/ 10 1.5 0.15 0.45 0.5 3.37 500 m-MTDATA/
500 ZnPc:C.sub.60(1:2)/500 MPP/ 100 6.3 0.063 0.50 0.33 1.04 10
LiF/Al where P.sub.O is incident light intensity, J.sub.SC is short
circuit current density, V.sub.OC is open circuit voltage, FF is
fill factor .eta..sub.P is power conversion efficiency, MPP is
N,N'-dimethyl-3,4:9,10-perylene bis(dicarboximde), m-MTDATA is
4,4',4''-tris(3-methylphenylphenylamino)triphenylamine.
[0063] The simplest mixed structure of ITO/370 .ANG.
CuPc:C.sub.60/75 .ANG. BCP/Ag shows a large J.sub.SC=(12.0.+-.0.6)
mA/cm.sup.2 at 1 sun, comparable to an optimized bilayer device
using the same combination of donor and acceptor materials. See Xue
et al., Appl. Phys Lett., 84, 3013 (2004). However,
.eta..sub.P=(2.8.+-.0.1) % observed in this mixed device is smaller
than in an optimized bilayer due to a reduced fill factor,
FF<0.5, vs FF.about.0.6 in the bilayer device. See, id. Both
J.sub.SC and .eta..sub.P are further improved with the addition of
a thin (100 .ANG.) C.sub.60 layer between the CuPc:C.sub.60 and BCP
layers. It is believed that, by displacing the active region
farther from the reflective metal cathode, the additional C.sub.60
layer results in an increased optical field at the D-A interface.
See, Peumans et al., J. Appl. Phys., 93, 3693 (2003). A device with
an optimized CuPc:C.sub.60 thickness of 330 .ANG. shows that
J.sub.SC=(15.2.+-.0.7) mA/cm.sup.2 and .eta..sub.P=(3.5.+-.0.2) %
at 1 sun. In this case, J.sub.SC is approximately 20% larger than
that of the bilayer device at 1 sun, and .eta..sub.P is roughly
equal to that of the bilayer device at 1 sun.
[0064] Experimental and Calculations
[0065] Photovoltaic devices were fabricated on 1300 .ANG. thick
layers of indium tin oxide (ITO) precoated onto glass substrates.
The solution cleaned ITO surface was exposed to ultraviolet/O.sub.3
prior to deposition. The organic source materials: CuPc, C.sub.60
and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are
purified by thermal gradient sublimation, also prior to use, as
described in Forrest, Chem Rev., 97, 1793 (1997). All organic
materials were thermally evaporated in high vacuum (<10.sup.-6
Torr) using quartz crystal monitors to determine film thickness and
deposition rate. The mixture ratio of CuPc to C.sub.60 based on the
wt % measured using the thickness monitor is fixed at 1:1, unless
otherwise noted. The Ag cathodes were evaporated through a metal
shadow mask with 1 mm diameter openings. The current
density--voltage (J-V) characteristics were measured in the dark
and under illumination of AM1.5G simulated solar spectrum from a
filtered Xe arc lamp source. Illumination intensities were measured
using a calibrated power meter.
[0066] FIG. 5 shows an energy level diagram the device. A
homogeneous D:A mixed film allows for both electron and hole
transport to the contacts, in addition to efficient exciton
dissociation. By deposition of a Ag cathode on BCP, defect states
are created that transport electrons efficiently from C.sub.60 to
the metal cathode, while effectively blocking hole and exciton
transport. At the anode, the CuPc:C.sub.60 mixed layer was
deposited directly onto the pre-cleaned ITO surface.
[0067] FIG. 6 shows J-V characteristics of a hybrid device with a
structure of ITO/330 .ANG. CuPc:C.sub.60/100 .ANG. C.sub.60/75
.ANG. BCP/Ag, in the dark and under various illumination
intensities of AM1.5G simulated solar spectrum. Specifically, J-V
characteristics are provided for in the dark, and at light
intensities of 0.01 suns, 0.03 suns, 0.08 suns, 0.3 suns, 0.9 suns,
and 2.4 suns. The dark J-V characteristics show a rectification
ratio of .about.10.sup.6 at .+-.1 V, and the forward current at +1
V is >1 .ANG./cm.sup.2, indicating a low series resistance of
R.sub.S=0.25 .OMEGA.cm.sup.2 as obtained by fitting the J-V
characteristics according to a modified ideal diode equation. See,
Xue et al., Appl. Phys. Lett., 84, 3013 (2004).
[0068] FIG. 7 shows additional photovoltaic characteristics of the
device described with reference to FIG. 6. J.sub.SC linearly
increases with incident light intensity (P.sub.O), with a
responsivity of (0.15.+-.0.07) A/W. Also, V.sub.OC increases and FF
decreases with increasing P.sub.O. As a result, .eta..sub.P is
almost constant at all light intensities between 0.01 and 2.4 suns,
with a maximum of .eta..sub.P=(3.6.+-.0.2) %, and
J.sub.SC=(4.2.+-.0.1) mA/cm.sup.2, V.sub.OC=0.47 V and FF=0.49,
under 0.3 sun illumination. At higher intensities, FF decreases to
0.42, resulting in .eta..sub.P=(3.3.+-.) % at 2.4 suns.
[0069] Although R.sub.S may affect the J-V characteristics at high
intensities, the small R.sub.S=0.25 .OMEGA.cm.sup.2 for the mixed
device results in a voltage drop under short circuit conditions of
only J.sub.SCR.sub.S=10 mV at 2.4 suns. This voltage drop, in turn,
is estimated to reduce .eta..sub.P by smaller than 0.1% compared to
an ideal device (R.sub.S=0 .OMEGA.cm.sup.2). Recently reported
results employing a ZnPc:C.sub.60 mixed layer structure, see D.
Gebehu et al., Solar Energy Mater. Solar Cells, 79, 81 (2003) and
the ZnPc entry in Table 2, shows comparable .eta..sub.P with
similar photovoltaic characteristics to some devices with mixed
layers under lower (.about.1/10 sun) intensity, but with a
significant decrease of J.sub.SC and FF at 1 sun, resulting in a
smaller .eta..sub.P (see Table 2). This reduction in .eta..sub.P
may be due to the large R.sub.S (40-60 .OMEGA.cm.sup.2) of the
former device.
[0070] Recently, structures similar to those in Table 2 have been
reported by Sullivan, et al., Appl. Phys. Lett., 84, 1210 (2004),
although the efficiencies of those devices are .about.3 times lower
than certain devices disclosed herein. Peumans, et al., J. Appl.
Phys., 93, 3693, have shown that efficiency decreases exponentially
with blocking (BCP) layer thickness once the layer exceeds the
"damage thickness" induced during contact deposition. The BCP
layers of Sullivan are 120 A, apparently exceeding the damage
thickness. Furthermore, we have also found that material purity is
extremely important in determining PV cell efficiency. For devices
fabricated by the inventors and described herein, all sources of
materials have been sublimed at least three times prior to use in
fabricating the devices.
[0071] FIG. 8 shows absorption spectra of CuPc:C.sub.60 films with
various mixture ratios, deposited on ITO. The concentrations of
CuPc in mixed films are (a) 100% (CuPc single layer), (b) 62%, (c)
40%, (d) 33% and (e) 21%. The pure CuPc film has two peaks centered
at wavelengths of 620 nm and 695 nm. The longer wavelength peak is
due to molecular Frenkel exciton generation, whereas the shorter
wavelength feature is attributed to the formation of CuPc
aggregates. The longer wavelength peak is dominant in the gas phase
or dilute solution. FIG. 8 shows that the magnitude of the longer
wavelength peak increases with increasing C.sub.60 content.
Accordingly, CuPc molecules show a lower tendency to aggregate with
increasing C.sub.60 content. This suggests that an increase in
C.sub.60 concentration inhibits CuPc aggregation, thereby reducing
hole transport in the mixed film, perhaps leading to a low carrier
collection efficiency. This is reflected in the reduced power
efficiency (.eta..sub.P=(2.6.+-.0.1) %, see Table 2) of a
CUPC:C.sub.60 (1:2) mixed layer PV cell. However, at a
concentration of 1:1, there may be sufficient aggregation (albeit
not measurable aggregation) of CuPc molecules, and/or the formation
of CuPc "strings" or percolation paths, to allow for low resistance
hole transport. The much higher symmetry C.sub.60 molecules may
also form a percolation path for efficient electron transport to
the cathode. At the present time, it is believed that a ratio of
1.2:1 (by weight) CuPc/C.sub.60 is most preferred, although other
concentrations may be used.
[0072] FIG. 9 shows normalized photocurrent--voltage
characteristics under various light intensities for the devices
described with respect to FIG. 6. The current densities are
normalized by subtracting the dark current, and then dividing the
AM1.5G light intensity. FIG. 9 also shows proposed photovoltaic
processes for both bilayer and mixed layer devices. In a bilayer
device 910, photogenerated excitons migrate to a D-A interface (1),
where they separate into charge carriers in the built-in depletion
region (2), followed by sweep out through the neutral region by
diffusion assisted by the carrier concentration gradient (3). In a
mixed layer device 920, excitons are separated immediately into
charge carriers at the D-A couple (4). The charge carriers then
proceed towards the electrodes by drift under the built-in electric
field (5), with some undergoing loss due to recombination (6).
[0073] In a bilayer cell, photons may not contribute to the
photocurrent if they are absorbed too far from a D-A interface. The
distance that is "too far" is related to the exciton diffusion
length (L.sub.D). The external quantum efficiency (.eta..sub.EQE)
as well as the absorption efficiency of a bilayer device are
limited by the efficiency for exciton diffusion to the D-A junction
(.eta..sub.ED). In a mixed device, on the other hand, .eta..sub.ED
is high (.about.100%) because all excitons are generated at the D-A
molecular couple, and hence readily dissociate. This suggests that
mixed devices are not restricted by the small L.sub.D
characteristic of organic thin films. Therefore, J.sub.SC=15.4
mA/cm.sup.2 of a mixed device at 1 sun is larger than J.sub.SC=11.3
mA/cm.sup.2 of the optimized bilayer device. See, Xue et al., Appl.
Phys. Lett., 84, 3013 (2004). However, the mixed device shows a
large electric field dependence in the J-V characteristics (see
FIG. 9), resulting in a smaller FF, and hence a smaller power
conversion efficiency than the bilayer device.
[0074] Electron-hole recombination may be more likely in a mixed
layer device since charge separation away from the exciton
dissociation site is made difficult by the high resistance of the
mixed layer. However, the J-V characteristics under different
irradiation intensities in FIG. 9 show that the normalized
photocurrent is not significantly reduced, even at high intensity
(and hence higher carrier concentrations), suggesting that
bimolecular recombination of photogenerated carriers is not
significant in the mixed layer. Because carrier generation occurs
across the entire mixed layer, the carrier concentration gradient
is very small, suggesting that the diffusion component to the total
current is also small. Thus the current within the mixed layer is
primarily driven by drift and may be strongly affected by an
applied electric field (see FIG. 9, device 910). On the other hand,
in a bilayer device, photogenerated carriers at the D-A interface
diffuse across the neutral region (See FIG. 9, device 920). This
process is assisted by a large charge concentration gradient
extending from the D-A interface to the electrodes, resulting in a
relatively small electric field dependence.
[0075] Another hybrid photovoltaic cell was fabricated, having the
structure: indium-tin-oxide/150 .ANG. CuPc/100 .ANG.
CUPC:C.sub.60(1:1 by weight)/350 .ANG. C.sub.60/100 .ANG.
bathocuproine/1000 .ANG. Ag. This photovoltaic cell exhibited a
maximum power conversion efficiency of (5.0.+-.0.2)% under 1 to 4
suns of simulated AM1.5G solar illumination.
[0076] Devices were fabricated as follows: Organic hybrid HJ PV
cells were fabricated on glass substrates precoated with a
.about.1500 .ANG. thick transparent, conducting ITO anode with a
sheet resistance of 15.OMEGA./sq, obtained from Applied Film Corp,
Boulder, Colo., 80301. The substrates were cleaned in solvent as
described in Burrows et al., J. Appl. Phys. 79, 7991 (1996). The
substrate were then treated by UV-ozone for 5 minutes, as described
in Xue et al., J. Appl. Phys. 95, 1869 (2004). The organic layers
and a metal cathode were deposited via thermal evaporation in a
high vacuum chamber with a base pressure .about.2.times.10.sup.-7
Torr. A CuPc layer was deposited on the ITO anode, followed by a
co-deposited, homogenously mixed layer of CuPc:C.sub.60 (1:1 by
weight), followed by a C.sub.60 layer. Various devices were
fabricated, having different thicknesses of the organic layers. The
CuPc layer thickness was varied between d.sub.D.about.0-200 .ANG..
The co-deposited, homogenously mixed layer of CuPC:C.sub.60 (1:1 by
weight) thickness was varied between d.sub.m.about.0-300 .ANG.. The
C.sub.60 layer thickness was varied between d.sub.A.about.250-400
.ANG.. After the C.sub.60 was deposited, a 100 .ANG.. thick
exciton-blocking layer of BCP was deposited. Finally, a 1000 .ANG..
thick Ag cathode was evaporated through a shadow mask with 1 mm
diameter openings. For devices having d.sub.m greater than zero,
the devices appear as illustrated in device 1010, i.e., the devices
are similar to those of FIG. 3, where third organic layer 310 is
CuPc, first organic layer 306 is a mixture of CuPc and C.sub.60,
second organic layer 308 is C.sub.60, and fourth organic layer 314
is BCP.
[0077] Current-voltage characteristics of the PV cells at
25.degree. C. in the dark or under simulated AM1.5G solar
illumination from a 150 W Xe-arc lamp (Oriel Instruments) were
measured using an HP 4155B semiconductor parameter analyzer. The
illumination intensity was varied using neutral density filters and
measured with a calibrated broadband optical power meter (Oriel
Instruments). To measure the external quantum efficiency, a
monochromatic beam of light was used, which was generated by
passing the white light from the Xe-arc lamp through a 0.3 m
monochrometer (Acton Research SpectraPro-300i) and whose intensity
was determined using a calibrated Si photodetector (Newport
818-UV). With a chopping frequency of 400 Hz, the photocurrent was
then measured using a lock-in amplifier (Stanford Research SR830)
as a function of the incident light wavelength and the applied
voltage.
[0078] FIG. 10 shows the current density vs voltage (J-V)
characteristics in the dark for a planar HJ (d.sub.D=200 .ANG. and
d.sub.A=400 .ANG., d.sub.m=0) and a hybrid HJ (d.sub.D=100 .ANG.,
d.sub.m=200 .ANG., and d.sub.A=300 .ANG.) cell. Both cells exhibit
rectification ratios >10.sup.6 at .+-.1 V, and shunt resistances
>1 M.OMEGA.cm.sup.2. The forward-bias characteristics can be fit
using the modified diode equation
J = J s { exp [ q ( V - JR S ) nkT ] - 1 } , ( 1 ) ##EQU00001##
[0079] where J.sub.s is the reverse-bias saturation current
density, n the ideality factor, R.sub.S the series resistance, q
the electron charge, k the Boltzmann's constant, and T the
temperature. While R.sub.S is approximately the same for both
cells, .about.0.25 .OMEGA.cm.sup.2, n is reduced from 1.94.+-.0.08
for the planar HJ cell to 1.48.+-.0.05 for the hybrid HJ cell,
whereas J.sub.S is also reduced from (4.+-.1).times.10.sup.-7
.ANG./cm.sup.2 (planar HJ) to (1.0.+-.0.3).times.10.sup.-8
.ANG./cm.sup.2 (hybrid HJ).
[0080] FIG. 11 shows the dependences of n and J.sub.S on the mixed
layer thickness d.sub.m, for hybrid HJ cells with
d.sub.D=d.sub.A-200 .ANG.=200 .ANG.-d.sub.m/2. With increasing
d.sub.m, both n (open circles) and J.sub.s (filled squares)
decrease significantly at d.sub.m.ltoreq.100 .ANG., and tend to
saturate at d.sub.m.gtoreq.100 .ANG..
[0081] The lower n and J.sub.s for cells with a mixed layer can be
attributed to the decrease in the recombination current in the
depletion region of these cells. For a planar HJ cell, due to the
large energy offset (.about.1 eV) of the highest occupied and
lowest unoccupied molecular orbitals (HOMO and LUMO, respectively)
at the CuPc/C.sub.60 interface, the diffusion-emission current is
negligible; therefore, the dark current is dominated by the
recombination current in the depletion region, which includes the
entire mixed layer and part of the unmixed photoactive layers in
contact with the mixed layer, leading to n.apprxeq.2. According to
the Shockley-Hall-Read recombination model, J.sub.S for the
recombination current can be expressed as:
J s , rec = qn i W ' 2 .tau. = 1 2 qn i W ' N i .sigma. v th , ( 2
) ##EQU00002##
[0082] where n.sub.i is the intrinsic electron/hole concentration,
W.sup.+ is the effective depletion width, .tau.=1/(N.sub.t .sigma.
v.sub.th) is the excess carrier lifetime, N.sub.t is the total
density of recombination centers, .sigma. s the electron/hole
capture cross section, and v.sub.th is the carrier thermal
velocity. In disordered semiconductors where charge carriers
transport via hopping processes, it has been shown by Paasch et
al., Synth. Met. 132, 97 (2002), that v.sub.th.varies..mu..sup.1.1
for .mu.<1 cm.sup.2/Vs, where .mu. is the carrier mobility.
Therefore, a reduction in J.sub.S in a mixed layer may occur as a
result of the reduced .mu. in a mixed layer as compared with an
unmixed layer. With a much reduced recombination current, the
contribution of the diffusion-emission current to the dark current
becomes appreciable, leading to 1<n<2 in cells with a mixed
layer. By comparing J.sub.S for the planar HJ cell and for the
hybrid HJ cells with d.sub.m.gtoreq.200 .ANG., it can be inferred
that the hole mobility in CuPc and the electron mobility in
C.sub.60 are reduced by approximately one and a half orders of
magnitude by intermixing CuPc and C.sub.60 at a ratio of 1:1 by
weight.
[0083] FIG. 12 shows the photocurrent density, J.sub.Ph, at an
illumination intensity of P.sub.O=120 mW/cm.sup.2 for cells with a
mixed layer having a thickness of 0 .ANG..ltoreq.d.sub.m.ltoreq.300
.ANG.. Again, d.sub.D=200 .ANG.-d.sub.m/2 and d.sub.A=400
.ANG.-d.sub.m/2. At 0 V (short circuit, filled squares), J.sub.Ph
increases with d.sub.m for d.sub.m.ltoreq.200 .ANG., while
remaining nearly constant as d.sub.m is further increased to 300
.ANG.. Upon applying a bias of -1 V (open circles), J.sub.Ph
increases significantly, more for cells with a thicker mixed layer.
For the planar HJ cell, this may be attributed to field-assisted
exciton dissociation away from the D-A interface. However, for the
hybrid HJ cells, especially those with a thick mixed layer
(d.sub.m.gtoreq.150 .ANG.), the significant increase in J.sub.Ph
may be attributed to an increased charge collection efficiency
(.eta..sub.CC, or fraction of photogenerated charge being collected
at the electrodes) due to an increased electric field in the mixed
layer, which is directly related to the poor transport property of
the mixed layer.
[0084] Based on a model described by Peumans et al., J. Appl. Phys.
93, 3693 (2003), which considers both the optical interference
effect and exciton diffusion, J.sub.Ph of hybrid HJ cells can be
simulated as a function of the mixed layer thickness, assuming full
dissociation of excitons in the mixed layer and ideal charge
collection (.eta..sub.CC=1). As shown by the solid line 1210 in
FIG. 12, using an exciton diffusion length of 70 .ANG. and 300
.ANG. in CuPc and C.sub.60, respectively, the model prediction is
in reasonable agreement with the experimental data at -1 V. The
discrepancy at d.sub.m.ltoreq.150 .ANG. may be attributed to the
field-assisted exciton dissociation in the mixed layers, which is
not taken into consideration in model used to generate line
1210.
[0085] To account for the limited .eta..sub.CC in hybrid HJ cells,
a model may be used that assumes an electron (or a hole) in the
mixed layer at a distance x away from the mixed layer/C.sub.60 (or
CuPc) unmixed layer interface has a probability of P(x)=exp(-x/L)
reaching the mixed layer/unmixed layer interface, where it is
transported through the unmixed layer and collected at the
electrode. L is a characteristic length for carrier transport.
Then, the overall charge collection efficiency is:
.eta. cc = .intg. p ( x ) P ( x ) x / .intg. p ( x ) x = L d m [ 1
- exp ( - d m L ) ] , if p ( x ) = constant , ( 3 )
##EQU00003##
where p(x) is the hole concentration. The photocurrent density
J.sub.Ph can be obtained by multiplying .eta..sub.CC with the
results from the model described in the previous paragraph and used
to generate line 1210, which corresponds to .eta..sub.CC=1. Fitting
the experimental data of J.sub.Ph at 0 V using the model described
in this paragraph, dashed line 1220 is generated, and a
characteristic charge transport length of L=450 .ANG..+-.50 .ANG.
is obtained.
[0086] The characteristic charge transport length L can be
considered as the average distance an electron or a hole travels in
the mixed layer under an electric field before being recombined.
Hence, L can be expressed as
L=.tau..mu.(V.sub.bi-V)/W.apprxeq.L.sub.0(V.sub.bi-V)/V.sub.bi,
(4)
[0087] where .tau. is the carrier lifetime, .mu. is the carrier
mobility, V.sub.bi is the built-in potential, W is the depletion
width, and L.sub.0=.tau..mu.V.sub.bi/W=L(V=0). The approximation is
made if W does not change significantly with the bias voltage. The
charge collection efficiency .eta..sub.CC now becomes a function of
V through the voltage dependence of L, such that:
J.sub.Ph(V)=P.sub.OR.sub.0.eta..sub.CC(V), (5)
[0088] where R.sub.0 is the responsivity corresponding to
.eta..sub.CC=1. The total current density is a sum of J.sub.Ph and
the dark current density described by Eq. (1). FIG. 13 shows the
experimental J-V characteristics at various P.sub.O for a hybrid HJ
cell with d.sub.m=200 .ANG.. Using the results for J.sub.s, n, and
R.sub.S from the dark current analysis and V.sub.bi=0.6 V, it may
be calculated that L.sub.0=400 .ANG..+-.50 .ANG. and
R.sub.0=(0.22.+-.0.02) A/W by fitting the data at -1 V<V<0.6
V. L.sub.0 obtained here is in agreement with the fitting result on
the short-circuit current density.
[0089] FIG. 14 shows absorption spectra of the planar HJ cell
(solid line) and the hybrid HJ cell with d.sub.m=200 .ANG. (dashed
line). The absorption efficiency .eta..sub.A=1-R, where R is the
reflectance of light incident through the glass substrate with a Ag
cathode on top of the organic layers (see structure 1410). The
slight difference in the absorption spectra for these two devices
can be attributed to the different material density profile and the
interference-induced non-uniform distribution of the optical field
intensity across the thickness of the organic layers, in addition
to the different aggregation states of CuPc in the MCL and PCL.
[0090] Also shown in FIG. 14 are the external quantum efficiencies,
.eta..sub.ext, at 0 V for a planar HJ (solid line) and a hybrid HJ
(dashed line). The hybrid HJ cell has a much higher .eta..sub.ext
in the spectral region between 550 nm and 750 nm, corresponding to
CuPc absorption, whereas in the C.sub.60 absorption region (380 nm
to 530 nm), next is slightly lower in the hybrid HJ cell as a
result of a slightly lower .eta..sub.A. Therefore, the internal
quantum efficiency, .eta..sub.int=.eta..sub.ext/.eta..sub.A, is
significantly enhanced in the CuPc absorption region for the hybrid
HJ cell as compared to the planar HJ cell, while it is nearly the
same in the spectral region where C.sub.60 absorption dominates.
This is consistent with the different exciton diffusion lengths in
CuPc (L.sub.D.about.100 .ANG.) and C.sub.60 (L.sub.D.about.400
.ANG.), considering that in the planar HJ cell, d.sub.D=200
.ANG..about.2L.sub.D, while d.sub.A=400 .ANG..about.L.sub.D. Both
the quantum efficiency and the absorption spectra of the hybrid HJ
cell show a long-wavelength tail extending from 800 nm to 900 nm,
far beyond the absorption edge of CuPc (.about.750 nm). This is
attributed to charge transfer state absorption in the CuPc:C.sub.60
mixture, similar to that observed in the Zn phthalocyanine:C.sub.60
mixed system. See G Ruani et al., J Chem Phys. 116, 1713
(2002).
[0091] FIG. 15 shows the illumination intensity dependences of
.eta..sub.P, FF, and V.sub.OC for a hybrid HJ cell (open circles)
with the structure of ITO/CuPc(150 .ANG.)/CuPc:C.sub.60(100 .ANG.,
1:1 by weight)/C.sub.60(350 .ANG.)/BCP(100 .ANG.)/Ag(1000 .ANG.).
Also shown are previously reported results for a planar HJ cell
from Xue et al., Appl. Phys Lett. 84, 3013 (2004) (filled squares)
and the hybrid HJ cell of FIG. 6 (filled triangles). All three
cells show a linear dependence of J.sub.SC on P.sub.O over the
entire range of P.sub.O used in the experiments. At 1 sun (=100
mW/cm.sup.2), J.sub.SC=(11.8.+-.0.5), (15.5.+-.0.5), and
(15.0.+-.0.5) mA/cm.sup.2 for the planar, bulk, and hybrid HJ cell,
respectively. The higher photocurrent obtained in the bulk and
planar HJ cells may be a result of more favorable exciton diffusion
in the mixed layer compared with the unmixed layers. The hybrid HJ
cell has almost the same J.sub.SC as the bulk HJ cell despite only
using a very thin mixed layer. Except at the highest intensities,
V.sub.OC increases logarithmically with P.sub.O for all three
cells, which can been explained using p-n junction theory. See, Xue
et al., Appl. Phys. Lett., 84, 3013 (2004). The different slope of
V.sub.OC to log(P.sub.O) is due to the different ideality factor of
these diodes: n.apprxeq.2 for the planar HJ cell, and n.apprxeq.1.5
for both the bulk and planar HJ cells.
[0092] The planar HJ cell has a high FF.about.0.6 as a result of
the low R.sub.S and good transport property of the unmixed layers.
The FF is significantly reduced for the bulk HJ cell, especially
under high intensities, e.g., FF=0.45 at 1 sun, compared with
FF=0.62 for the planar HJ cell. With a much thinner mixed layer
than in the bulk HJ structure (100 .ANG. vs 330 .ANG.), the hybrid
HJ cell shows FF.gtoreq.0.6 at P.sub.O.ltoreq.1 sun and only
slightly reduced to 0.53 at an intense illumination of .about.10
suns, indicating the much improved charge transport property.
[0093] Overall, the hybrid HJ cell has a maximum efficiency of
.eta..sub.P=(5.0.+-.0.2)% at 120
mW/cm.sup.2.ltoreq.P.sub.O.ltoreq.380 mW/cm.sup.2 (see panel 1510).
Decreasing the illumination intensity below 1 sun leads to a
decrease in .eta..sub.P due to the reduction in V.sub.OC.
Increasing the intensity above 4 suns also causes a slight
reduction in .eta..sub.P as a result of the reduced FF. Such
interplay between the dependences of V.sub.OC and FF on P.sub.O
leads to a maximum of .eta..sub.P at an illumination intensity that
can be tuned between a fraction of a sun and a few suns by varying
the mixed layer thickness. With a thicker mixed layer in the hybrid
HJ structure, the FF decreases more significantly with P.sub.O,
leading to .eta..sub.P peaking at lower intensities. For cells with
a very thin mixed layer (d.sub.m.ltoreq.50 .ANG.), the cell series
resistance may be factor that limits FF under intense
illuminations. For example, .eta..sub.P for a hybrid HJ cell with
d.sub.m=50 .ANG. reaches the maximum at P.sub.O 4-10 suns, whereas
it peaks at 0.4 sun.ltoreq.P.sub.O.ltoreq.1.2 sun for a cell with
d.sub.m=150 .ANG..
[0094] While the present invention is described with respect to
particular examples and preferred embodiments, it is understood
that the present invention is not limited to these examples and
embodiments. The present invention as claimed therefore includes
variations from the particular examples and preferred embodiments
described herein, as will be apparent to one of skill in the
art.
* * * * *