U.S. patent application number 13/301389 was filed with the patent office on 2012-05-31 for solution processed metal oxide thin film hole transport layers for high performance organic solar cells.
This patent application is currently assigned to ALLIANCE FOR SUSTAINABLE ENERGY, LLC.. Invention is credited to Joseph J. Berry, Jordan P. Chesin, Calvin J. Curtis, David S. Ginley, Matthew T. Lloyd, Alex Miedaner, Dana C. Olson, K. Xerxes Steirer, Nicodemus Edwin Widjonarko.
Application Number | 20120132272 13/301389 |
Document ID | / |
Family ID | 46125825 |
Filed Date | 2012-05-31 |
United States Patent
Application |
20120132272 |
Kind Code |
A1 |
Steirer; K. Xerxes ; et
al. |
May 31, 2012 |
SOLUTION PROCESSED METAL OXIDE THIN FILM HOLE TRANSPORT LAYERS FOR
HIGH PERFORMANCE ORGANIC SOLAR CELLS
Abstract
A method for the application of solution processed metal oxide
hole transport layers in organic photovoltaic devices and related
organic electronics devices is disclosed. The metal oxide may be
derived from a metal-organic precursor enabling solution processing
of an amorphous, p-type metal oxide. An organic photovoltaic device
having solution processed, metal oxide, thin-film hole transport
layer.
Inventors: |
Steirer; K. Xerxes; (Golden,
CO) ; Berry; Joseph J.; (Boulder, CO) ;
Chesin; Jordan P.; (Cambridge, MA) ; Lloyd; Matthew
T.; (Boulder, CO) ; Widjonarko; Nicodemus Edwin;
(US) ; Miedaner; Alex; (Boulder, CO) ;
Curtis; Calvin J.; (Lakewood, CO) ; Ginley; David
S.; (Evergreen, CO) ; Olson; Dana C.;
(Boulder, CO) |
Assignee: |
ALLIANCE FOR SUSTAINABLE ENERGY,
LLC.
Golden
CO
|
Family ID: |
46125825 |
Appl. No.: |
13/301389 |
Filed: |
November 21, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61415612 |
Nov 19, 2010 |
|
|
|
Current U.S.
Class: |
136/256 ;
136/263; 257/E31.003; 438/82 |
Current CPC
Class: |
Y02E 10/549 20130101;
H01L 51/4253 20130101; H01L 51/442 20130101; H01L 51/0036 20130101;
H01L 51/4273 20130101 |
Class at
Publication: |
136/256 ;
136/263; 438/82; 257/E31.003 |
International
Class: |
H01L 51/46 20060101
H01L051/46; H01L 31/0216 20060101 H01L031/0216; H01L 31/20 20060101
H01L031/20; H01L 31/0224 20060101 H01L031/0224 |
Goverment Interests
[0002] The United States Government has rights in this invention
under Contract No. DE-AC36-08G028308 between the United States
Department of Energy and the Alliance for Sustainable Energy, LLC,
the Manager and Operator of the National Renewable Energy
Laboratory.
Claims
1. A method for fabricating an organic photovoltaic device, the
method comprising: forming a substrate; forming an ITO layer on the
substrate, wherein the ITO layer comprises an anode; forming an HTL
layer on the ITO layer, wherein the HTL layer comprises an
amorphous, p-type metal oxide; forming an active layer on the HTL
layer, wherein the active layer comprises a donor region and an
acceptor region; and forming a contact on the active layer, wherein
the contact comprises a cathode.
2. The method according to claim 1, wherein the donor region of the
active layer comprises a polymer and wherein the acceptor region of
the active layer comprises a fullerene.
3. The method according to claim 2, wherein the donor region of the
active layer comprises P3HT and wherein the acceptor region of the
active layer comprises PCBM.
4. The method according to claim 1, wherein the HTL layer comprises
p-NiO.
5. The method according to claim 1, wherein the HTL layer comprises
sNiO.
6. The method according to claim 5, wherein the sNiO is formed by
solution deposited NiO followed by a low temperature anneal.
7. The method according to claim 6, wherein the sNiO is formed by
spin-coating a diluted nickel ink followed by a low temperature
anneal.
8. The method according to claim 6, wherein the sNiO is formed by
spin-coating a diluted nickel ink at approximately 4000 rpm for
approximately 60 seconds, followed by annealing at approximately
250.degree. C. for approximately 1 hour.
9. The method according to claim 6, wherein the sNiO is formed by
spin-coating a nickel ink at approximately 4000 rpm, followed by
annealing on a low temperature hot plate in air.
10. The method according to claim 5, wherein the sNiO comprises a
thin-film.
11. The method according to claim 10, wherein the thin-film sNiO is
approximately 10 nm.
12. The method according to claim 1, wherein the HTL layer
comprises p-NiO and the active layer comprises a PCDTBT/PCBM bulk
heterojunction.
13. The method according to claim 1 further comprising: exposing
the HTL layer to oxygen plasma after the HTL layer is formed.
14. The method according the claim 1 further comprising: exposing
the HTL layer to an O.sub.2-plasma treatment after the HTL layer is
formed.
15. A method for fabricating an organic photovoltaic device, the
method comprising: forming a substrate; forming an TCO layer on the
substrate, wherein the TCO layer comprises an anode; forming an HTL
layer on the TCO layer, wherein the HTL layer comprises an
amorphous, p-type metal oxide, wherein the HTL layer is formed from
a metal-precursor via solution processing of the amorphous, p-type
metal oxide; forming an active layer on the HTL layer, wherein the
active layer comprises a donor region and an acceptor region; and
forming a contact on the active layer, wherein the contact
comprises a cathode.
16. The method according to claim 15, wherein the HTL layer is
formed via solution processing of a metal-organic ink:
17. The method according to claim 15, wherein the HTL layer is
formed via solution processing of a metal-organic ink using ink-jet
or continuous flow printing.
18. The method according to claim 17, wherein the metal-organic ink
comprises a complex in which a metal in solution coordinates to one
or more diamine groups suspended in a solvent.
19. The method according to claim 16, further comprising annealing
the metal-organic ink in air at elevated temperatures after the
solution processing.
20. The method according to claim 15, wherein the TCO comprises a
ZnO-based material.
21. The method according to claim 20, wherein the TCO comprises
gallium-doped ZnO.
22. The method according to claim 20, wherein the TCO comprises
aluminum-doped ZnO.
23. The method according to claim 15 further comprising: exposing
the HTL layer to an O.sub.2-plasma treatment after the HTL layer is
formed.
24. An organic photovoltaic device comprising: a substrate; a TCO
layer on the substrate, wherein the TCO is configured to act as an
anode; an HTL layer on the TCO layer, wherein the HTL layer
comprises an amorphous, p-type metal oxide thin film; an active
layer on the TCO layer, wherein the active layer comprises a donor
region and an acceptor region; and a cathode on the active
layer.
25. The organic photovoltaic device according to claim 24, wherein
the HTL layer comprises amorphous, p-type NiO.
26. The organic photovoltaic device according to claim 24, wherein
the HTL layer comprises a spinel structure.
27. The organic photovoltaic device according to claim 26, wherein
the HTL layer comprises Co(Ni)Zn2O4.
28. The organic photovoltaic device according to claim 24, wherein
the HTL layer comprises a delafossite structure.
29. The organic photovoltaic device according to claim 28, wherein
the HTL layer comprises CuAlOx.
30. The organic photovoltaic device according to claim 24, wherein
the HTL layer is formed from a metal-organic ink using direct write
solution processing via ink-jet or continuous flow printing
followed by annealing in air at elevated temperatures.
31. The organic photovoltaic device according to claim 24, wherein
the TCO layer comprises a ZnO-based material and the HTL comprises
amorphous, p-type NiO.
32. The organic photovoltaic device according to claim 24, wherein
the active layer comprises a PCDTBT:PC.sub.70BM bulk
heterojunction.
33. The organic photovoltaic device according to claim 24, wherein
the active layer comprises P3HT:PCBM.
34. The organic photovoltaic device according to claim 24, wherein
the HTL layer comprises an oxygen plasma treated thin film NiO
layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/415,612, filed Nov. 19, 2010, which is
incorporated herein by reference in its entirety.
BACKGROUND
[0003] The present disclosure generally relates to organic solar
cells and similar electronic devices. Today's increasing demand for
renewable energy resources, especially solar power, is driving
researchers to develop low cost, efficient photovoltaic devices.
Organic photovoltaics (OPVs) are an attractive route toward solving
the terawatt energy problem. In addition to the potential low cost
of this technology, bulk heterojunction (BHJ) based solar cells can
offer other advantages, such as flexibility, lightweight, and high
throughput manufacturing, such as roll-to-roll and other similar
techniques. BHJ systems have shown power conversion efficiencies
(PCEs) of 4% to >7%. Organic solar cells have undergone a
three-fold increase in PCE between 2001 and 2010, from about 2.5%
to about 7.7%. These rapid gains may be a consequence of enhanced
performance in polymer photovoltaic materials in BHJ solar cells. A
benchmark goal for OPV researchers is to achieve a PCE in excess of
10%, which would help to make OPV competitive with other thin-film
photovoltaic technologies. The ability to control the multiple
interfaces within an OPV device may help achieve this and other
objectives.
[0004] The foregoing examples of the related art and limitations
related therewith are intended to be illustrative and not
exclusive. Other limitations of the related art will become
apparent to those of skill in the art upon a reading of the
specification and a study of the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0006] FIG. 1 illustrates a schematic diagram of an exemplary
device structure of an organic photovoltaic device.
[0007] FIG. 2 is a schematic diagram of an energy level diagram
depicting the components of an organic photovoltaic device of FIG.
1.
[0008] FIG. 3a illustrates an energy diagram of a NiO BHJ organic
photovoltaic device.
[0009] FIG. 3b illustrates a graph of transmission versus
wavelength for sNiO and PEDOT:PSS on quartz substrates.
[0010] FIG. 4 illustrates the work function (.phi..sub.W) response
of sNiO films synthesized from Ni ink to O.sub.2-plasma
treatment.
[0011] FIG. 5 illustrates the current-voltage characteristics of
PEDOT:PSS and sNiO-based devices.
[0012] FIG. 6a illustrates the UPS spectra (Hel) of the
photoemission cut-off showing an increasing work function after
O.sub.2-plasma treatment of the MO.
[0013] FIG. 6b illustrates the combined UPS and IPES spectra of the
NiO near the valence and conduction band edge.
[0014] FIG. 6c illustrates the energy level diagrams of NiO before
and after O.sub.2-plasma treatment.
[0015] FIG. 7a illustrates an OPV device layer structure utilizing
a PCDTBT:PC.sub.70BM BHJ of 100 nm thickness interfaced with a
solution deposited HTL.
[0016] FIG. 7b illustrates a vacuum energy level diagram for each
layer of the OPV device of FIG. 7a.
[0017] FIG. 8a illustrates J-V curves for one sun illumination of a
solar cell performance for BHJ devices utilizing NiO of PEDOT:PSS
as the HTL.
[0018] FIG. 8b illustrates dark J-V measurements and corresponding
fits for BHJ devices.
[0019] FIG. 9a illustrates a normalized EQE plot for
PCDTBT:PC.sub.70BM BHJ devices with HTLs of NiO or PEDOT:PSS.
[0020] FIG. 9b illustrates optical constants n and k obtained for
NiO, PEDOT:PSS, and PCDTBT:PC.sub.70BM (1:4) BHJ thin films in
order to model the optical field in the solar cell.
[0021] FIG. 9c illustrates an optical field plot of 550 nm
irradiation for a 100 nm BHJ layer shown for HTLs of NiO and
PEDOT:PSS.
[0022] FIG. 10 illustrates degradation plots comparing NiO to
PEDOT:PSS HTLs in ITO/HTL/PCDTBT:PC.sub.70BM/Al photovoltaic
devices.
[0023] Corresponding reference characters and labels indicate
corresponding elements among the view of the drawings. The headings
used in the figures should not be interpreted to limit the scope of
the claims.
DETAILED DESCRIPTION
[0024] The working principle of bulk heterojunction (BHJ) solar
cells relies on the intimate bi-continuous phase mixing of an
electron donor and acceptor network. In order to improve charge
generation in BHJ layers, a molecular engineering approach is
employed to increase light absorption in the donor phase by
reducing the effective band-gap. In addition to light harvesting,
the open-circuit voltage (V.sub.OC) of the solar cell may be
improved by tailoring the polymeric repeat unit, such that the
highest occupied molecular orbital (HOMO) level is pushed further
from vacuum and the HOMO/lowest unoccupied molecular orbital (LUMO)
energy offset of the electron donor/acceptor blend is increased.
However, driving the donor HOMO level deeper may require a
simultaneous change of the anode energy levels to ensure good
energy level alignment and minimal loss at the hole collecting
contact. Ideally, the hole transport layer (HTL) facilitates a low
resistance, charge selective contact between the anode and the HOMO
of the donor material, reducing a possible loss in built-in
potential across the device.
[0025] Charge selective contacts may be utilized in active layer
materials to enhance device performance.
Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) or
PEDOT:PSS is a standard anodic contact for high efficiency organic
photovoltaic (OPV) devices. PEDOT:PSS may function as an effective
HTL by improving energy level alignment with the donor material, as
well as limiting reverse electron transfer from the acceptor phase.
However, PEDOT:PSS is acidic (pH of 1.2) and precludes the use of
many pH sensitive transparent conducting oxides, such as ZnO-based
electrodes containing aluminum or gallium. Furthermore, charge
transport in PEDOT:PSS exhibits high resistivity that contributes
to increased series resistance in polymer solar cells. Values for
the work function of PEDOT:PSS vary between about -4.7 eV and -5.2
eV and may vary with the formulation of the supplier. For donor
materials, such as P3HT, where the HOMO level is .about.-5.0 eV,
PEDOT:PSS modifies the work function of the ITO (4.7 eV) to
establish an improved contact between the electrode and the donor
material. However, in higher performance active layer systems, such
as PCDTBT:PC.sub.70BM, the HOMO level of the donor lies further
from the vacuum level (.about.-5.4 eV) in order to minimize energy
losses in the electron transfer process from the donor to accepter
and maximize the photovoltage. The anode contact material should
then have a deeper work function to minimize interfacial
recombination and maximize the power conversion efficiency (PCE) of
the device. In addition to forming an improved electronic interface
with the BHJ, the ideal anodic contact will be resistant to
degradation in the presence of oxygen and water to help improve
device stability.
[0026] Amorphous, p-type metal oxides may be utilized as the hole
transport layers (HTL) in solar cells, rather than PEDOT:PSS.
Amorphous, p-type metal oxides are wide band gap (E.sub.g>3 eV),
p-type materials that are transparent to the visible or near
infrared portions of the solar spectrum. These metal oxides may
minimize transparent conducting oxide (TCO)/donor contact
resistance and the dark current by providing a large energy barrier
to electron transfer while maximizing charge transfer between the
TCO and the highest occupied molecular orbital (HOMO) of the donor
material. Intrinsic stability and chemical compatibility with the
active layer and the indium thin oxide (ITO) or other ZnO-based TCO
materials may be promising for long device lifetimes. The basic
approach also may have potential utility in the broader range of
organic electronics, including organic light emitting devices
(OLEDs), organic transistors, and other similar electronic
devices.
[0027] One embodiment may comprise the application of solution
deposited, p-type, amorphous metal oxide hole transport layers
(HTL) for enhanced performance in organic photovoltaic (OPV)
devices and related organic electronics. The p-type amorphous oxide
may be derived from a metal-organic precursor that permits solution
processing of the amorphous, p-type metal oxide. FIG. 1 illustrates
a device structure, such as that of an OPV device 100. The OPV
device 100 may include a substrate 110, an ITO layer 120, an HTL
layer 130, an active layer 140, a calcium layer 150, and an
aluminum layer 160. The active layer 140 may comprise a
polymer:fullerene.
[0028] The active layer in BHJ cells may utilize a conjugated
polymer as an electron donor, such as poly(3-hexylthiophene)(P3HT),
that is intimately blended with a fullerene, as the electron
acceptor, such as [6,6]-phenyl-C.sub.61-butryric acid methyl ester
(PCBM). Electrodes may consist of a transparent conducting oxide
(TCO) anode, such as indium tin oxide (ITO), and a low work
function metal cathode, such as calcium. The donor and acceptor
phases may be in electrical contact with both of the electrodes,
which may create a need for selective contacts to ensure proper
device operation. A hole transport layer (HTL), such as
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS), may serve as an electron blocking layer (EBL) when
deposited on the ITO electrode to suppress the dark current in
these devices, in addition to serving as a lower resistance contact
for hole extraction.
[0029] PEDOT:PSS may serve as both an HTL and as an EBL in normal
and inverted device architectures, due to its band structure, which
has a lower electron affinity (or higher lowest unoccupied
molecular orbital (LUMO)) than P3HT, providing an energy barrier
for electron transfer to the anode. The band offset of the HTL in a
BHJ is important to establish the device polarity. However,
PEDOT:PSS has been shown to degrade or limit device performance in
multiple ways. Although the reported energy levels for PEDOT:PSS
and P3HT vary dependent upon material preparation and measurement
technique, the LUMO of PEDOT:PSS is essentially fixed at about 0.6
eV above the LUMO of P3HT, which may be insufficient as an electron
barrier. The aqueous PEDOT:PSS suspension is also highly acidic,
with a measured pH of 1.2. This prohibits use of PEDOT:PSS as an
HTL on a number of high performance and low cost transparent
conductors that are easily etched in acidic solutions. In addition,
due to the hygroscopic nature of PEDOT:PSS films, water may be
absorbed resulting in proton release and corrosion of the ITO anode
subsequently allowing indium diffusion throughout the device. This,
combined with the possible release of water from PEDOT:PSS into the
active layer or the cathode can substantially degrade devices,
lowering lifetimes and overall performance. These limitations have
led to investigations of other materials to effectively replace
PEDOT:PSS as the HTL in OPV devices.
[0030] An ideal candidate to replace PEDOT:PSS as the HTL would be
a wide band-gap (E.sub.9>3 eV), p-type material that is
transparent to most of the solar spectrum. It should minimize
TCO/donor contact resistance and the dark current by providing a
large energy barrier to electron transfer, while maximizing charge
transfer between the TCO and the highest occupied molecular orbital
(HOMO) of the donor material. The HTL performance may also depend
upon conductivity, but may vary with the junction type and loss
mechanisms. Intrinsic stability and chemical compatibility with the
active layer and the ITO or other ZnO-based TCO materials may also
be important for long lifetimes. Metal oxide thin films may be an
effective replacement for the typical PEDOT:PSS for the HTL layer
in OPV systems, resulting in both increased device performance and
lifetime. The metal oxide HTL materials may permit tunable contact
properties through changes in composition, as well as through
post-processing with an O.sub.2-plasma of varied power and
time.
[0031] In a device structure, the HTL may act as both a low
resistance (Ohmic) hole contact to the donor material, as well as
an electron blocking contact to reduce recombination. An energy
level diagram is shown in FIG. 2, depicting the relative location
of the energy levels in the device stack. The p-type metal oxide
thin films achieve work function values (>5.3 eV for NiO) that
are higher than that for PEDOT:PSS (.about.5.0 eV), thus resulting
in a reduced barrier for hole extraction at the anode, as well as
achieving enhanced electric field in the device for improved charge
collection from the bulk of the active layer. The p-type nature of
the amorphous metal oxide may permit the formation of a selective
contact for holes, as the electron levels in the acceptor lie in
the gap of the p-type oxide, thereby preventing electron transfer
to the anode and reducing interfacial recombination. Additionally,
the metal oxide HTL materials demonstrate improved transmittance
properties compared to PEDOT:PSS, which may permit more light to
reach the active layer and thereby improve the photon harvesting of
the solar cells. In addition to the improved device characteristics
resulting in improved efficiency, the inorganic HTL also result in
improved device lifetimes, compared to a device employing
PEDOT:PSS. The improvements in the contact properties of the
amorphous, p-type metal oxide HTL materials may further lead to
improved performance in deep energy level donors in organic solar
cells. By improving the contact properties, the photovoltaic and
photocurrent may be increased in these systems beyond what may be
achieved with PEDOT:PSS contacts.
[0032] The HTL of a PCDTBT/PCBM bulk heterojunction may also be
p-NiO. Several materials have been identified as alternative HTL
materials that may be applied by solution means. These include
spinal structures, such as Co(Ni)Zn204 and delafossite structures,
such as CuAlOx. Other p-types oxides are also anticipated in these
applications. Since the layer is very thin film, it is not required
to be entirely transparent. Accordingly, materials such as CuO may
be suitable in these applications. Based upon various
prerequisites, such as p-type, appropriate work function, solution
processability, and relatively chemically inert, there are various,
but not unlimited potential materials for this application. An
appropriate HTL may be designed based upon the actual cell
components. It is anticipated that a precursor solution may be
employed to synthesize the amorphous metal oxide thin films. The
NiO may be based on a metal-organic ink for the deposition of metal
utilizing write solution processing contacts via ink-jet or
continuous flow printing. This method may result in material with
conductivities rivaling that produced by vacuum deposition. The ink
is a complex in which the metal in solution coordinates to two
diamine groups and is suspended in the solvent. At increased
temperatures, the solvent and the metal organic decomposes leaving
behind a high quality metal contact. The metal-organic precursor
thin films may subsequently be annealed in air at elevated
temperatures, in order to form the metal oxide thin films. Similar
results have been obtained from solution precursors that do not
require diamine complex in solution. Other ink chemistries may be
employed for the diverse set of transition metals discussed above
to obtain functional HTL layers.
[0033] The application of amorphous, p-type metal oxide thin films
as contacts has numerous benefits for organic solar cells. For
example, improved performance in terms of both efficiency and
lifetime may be observed in these systems, relative to PEDOT:PSS,
Additionally, the materials and processing costs may be very
inexpensive, thereby permitting roll-to-roll deposition of these
contacts at high speeds and high volumes. Still further, the metal
oxide HTLs are chemically compatible with underlying transparent
electrodes, thus, permitting the deposition of such materials on
ZnO-based TCO materials, such as gallium-doped ZnO or
aluminum-doped ZnO, which are sensitive to the acidic suspension
that PEDOT:PSS is deposited from thereby prohibiting its use on
these electrode materials.
[0034] As shown in the energy band diagram of FIG. 3a, a p-type
metal oxide, such as NiO, has a wide band-gap and valence band
energy close to the HOMO of P3HT. The high conduction band energy
of NiO inhibits electron recombination with the anode, while the
valence band permits efficient conduction of holes to the anode.
Favorable vacuum energy level alignment and material stability
indicate that NiO has many of the required characteristics of an
efficient HTL material in OPV devices.
[0035] Furthermore, NiO may be deposited via pulsed laser
deposition (PLD) and works well as an HTL for polymer solar cells,
with devices exhibiting improved open-circuit voltage (V.sub.OC),
short-circuit current density (J.sub.SC), and fill factor (FF)
relative to a PEDOT:PSS control. The NiO is chemically stable and
inert in relation to ITO and P3HT:PCBM. The work function
(.phi..sub.W) of the NiO film may be tailored by controlling the
oxygen partial pressure during the deposition. Additionally, large
increases in the work function may be induced by O.sub.2-plasma
treatment of the NiO prior to deposition of the active layer.
Pulsed laser deposition is currently neither scalable nor a cost
effective deposition method. A more cost effective and scalable
deposition method may be solution deposited NiO (sNiO) followed by
a lower temperature anneal, as an HTL to improve device efficiency
and stability, while maintaining low-cost methods of
production.
[0036] A sNiO film from a nickel metal organic ink precursor
achieves similar OPV device performance to results seen with NiO
films from PLD. The device structure, ITO/NiO/P3HT:PCBM/Ca/Al, is
shown schematically in FIGS. 3a and 3b, which show that sNiO
thin-films to be an effective replacement for PEDOT:PSS in OPV
devices. As shown in FIGS. 3a and 3b, NiO provides favorable HTL
properties, such as (1) a wide band-gap for transparency, (2)
sufficient electron blocking energy, and (3) Fermi-level lineup
with the HOMO of the donor for hole collection.
[0037] The work function is tunable in the sNiO films by
post-processing with an O.sub.2-plasma of varied power and time.
The change in the work function with plasma treatment may be
greater in magnitude and have a faster decay time than PLD based
materials. The precursor solution used to synthesize the sNiO films
may be based upon a nickel ink for the deposition of Ni using
direct write solution processing contacts via ink-jet or continuous
flow printing, resulting in material with conductivities rivaling
that produced by vacuum deposition. The ink is a complex in which
the Ni coordinates to two diamine groups,
[Ni(en).sub.2](HCO.sub.2).sub.2:
en=H.sub.2N(CH.sub.2).sub.2NH.sub.2, and is suspended in the
solvent. At increased temperatures, the solvent and the metal
organic decomposes, leaving behind a high quality metal contact.
The complex may be hydrated by mixing 50% by volume Dl water and
may have a final Ni ink concentration of 0.34 M, as measured by
inductively coupled plasma (ICP).
[0038] All films may be deposited onto patterned ITO (.about.10
.OMEGA./sq, thin-film devices) substrates after approximately 5
minutes of ultrasonic cleaning in acetone, followed by isopropyl
alcohol. Substrates may be plasma-cleaned prior to HTL deposition
in approximately 0.8 Torr of O.sub.2 at approximately 155 W. The
sNiO films may be synthesized by spin-coating the diluted nickel
ink at approximately 4000 rpm for approximately 60 seconds,
followed by annealing on a hot plate in air at approximately
250.degree. C. for approximately 1 hour, resulting in approximately
10 nm thick films.
[0039] The structural, optical, and electrical properties of sNiO
films may be characterized. The grain-size and surface morphology
may be examined. Reflectance and transmittance spectra were
collected for the sNiO and PEDOT:PSS films on ITO over the UV,
visible, and Near-Infrared (NIR) regions using a Shimadzu UV-3600
UV-Vis-NIR Spectrophotometer. Surface potential measurements were
conducted using an Electrostatic Voltmeter (Monroe Electronics
ISOPROBE 244A) in an inert atmosphere of N.sub.2. The work function
(.phi..sub.W) of a material is often assumed to approximate the
Fermi energy, but may include effects from both surface dipoles an
changes in carrier concentration via doping or changes in defect
densities. All .phi..sub.W measurements were taken relative to the
Inconel.TM. stainless steel reference, which was measured to a
.phi..sub.W of -4.33 eV, as determined from ultraviolet
photoelectron spectroscopy (UPS). To examine the effect of surface
treatments and processing procedures on the .phi..sub.W of NiO
films, they were plasma treated using an RF O.sub.2-plasma at
various powers and treatment times with no bias applied to the
substrate.
[0040] PEDOT:PSS films were used as the control HTL, two coats of
PEDOT:PSS (Baytron P VP AL 4083 filtered to 0.45 .mu.m using a
nylon filter) were spin cast at 6000 rpm for 50 seconds, followed
by a 1 hour anneal at 130.degree. C. in air. For the O.sub.2-plasma
treated sNiO films, the active layer was immediately spun on in an
inert atmosphere. A solution of 1:1 P3HT:PCBM (used as received
from Rieke Metals and Nano-C, respectively) in 1,2-dichlorobenzene
(50 mg/mL total) was stirred at 60.degree. C. and 600 rpm for
several hours prior to deposition at room temperature. The active
layer was spun at 600 rpm for one minute, then allowed to slow dry
in a covered petri dish for approximately 1 hour. After drying, the
active layer was annealed at 110.degree. C. fo 10 minutes in
N.sub.2. Top electrodes were then deposited with 20 nm of Ca,
followed by 100 nm of Al via thermal evaporation through a shadow
mask to form six 0.11 cm.sup.2 devices or one 1.0 cm.sup.2 device
on each substrate at a pressure of less than 7.times.10.sup.-8
Torr.
[0041] Devices were characterized using a solar simulator housed in
an inert atmosphere. Contact was made to both the ITO and Al
electrodes, from which voltage was sourced and the resulting
current was measured. The short-circuit current (J.sub.SC) was
calculated based on measured device areas of 0.11 cm.sup.2 or 1.0
cm.sup.2. Shunt resistance and series resistance (R.sub.s) were
calculated at O V and O mA/cm.sup.2, respectively. External quantum
efficiency (EQE) was measured without white light biasing the
device on a system calibrated with a Si photo-diode illuminated
with a Xe 300 W lamp chopped to create an excitation signal
detected by lock-in amplification. Integrated EQE spectra were used
to verify J.sub.SC values.
[0042] The sNiO films were characterized by atomic force microscopy
(AFM) to evaluate the surface morphology and roughness. The
root-mean square surface roughness of the films synthesized form
the hydrated ink was 2.47 nm, as determined by AFM, which is
similar to roughness values typically obtained for commercially
available ITO. The optical transmittance and reflectance over the
UV and visible spectrum of the sNiO films are comparable to
PEDOT:PSS. The transmittance of the two films, shown in FIG. 3b,
indicates that PEDOT:PSS is more transparent in the UV and at lower
wavelengths, while sNiO is more transparent approaching the NIR
wavelengths. The reflectance, however, indicates that PEDOT:PSS is
slightly less reflective over most of the UV and visible compared
to the sNiO film. Overall, the transmitted light that reaches the
active layer is similar for both HTLs.
[0043] To better determine the dependence of the work function with
plasma treatment, the work function of sNiO films was measured in
N.sub.2 immediately after O.sub.2-plasma treatment at 155 W. The
O.sub.2-plasma treatment power and exposure time were varied for
sNiO films revealing significant differences in the measured work
function. The main reactive components of the plasma, O.sup.- and
O.sup.2+ were assumed to drive oxidation of the Ni films. The
results, shown in FIG. 4, demonstrate that sNiO films respond to
plasma treatment by increasing the work function (.phi..sub.W). The
sNiO films display a saturation behavior at longer treatment times
(.about.5 minutes), after which the .phi..sub.W no longer increased
with further plasma treatment. The solar cells were stored in a
dark inert environment and retested with sNiO and PEDOT:PSS over
two weeks after fabrication. All OPV devices showed similar
V.sub.OC degradation. This may be due to the sensitive Ca electrode
dominating the degradation of the devices. Thus, the sNiO device
HTL is, at a minimum, comparable to the PEDOT:PSS.
[0044] In order to determine the effect of the plasma treatment on
sNiO, a time dependent work function study was conducted by
measuring the work function every 20 seconds over a time-span of 2
hours in an inert atmosphere. It was observed that the work
function (.phi..sub.W) of films degraded quickly, as shown in the
insert in FIG. 4. This would appear to indicate a change in the
surface chemistry of the plasma treated sNiO. Because of the
magnitude of the transient behavior in the work function, the
experimental technique used to evaluate the work function, and the
lack of measurable conductivity in the sNiO, this effect may be
associated with a change in either the density of oxygen vacancies
or the oxidation state of the Ni in the sNiO.
[0045] In order to compare the performance of solution deposited
sNiO in OPV devices, BHJ solar cells were fabricated and tested
under simulated one-sun conditions. The best sNiO-based devices
were those O.sub.2-plasma treated, so that the observed work
function, immediately before deposition of the active layer was in
the range of approximately -5.0 eV up to -5.6 eV. The optimized
sNiO devices, resulted in PCE that are equivalent to PEDOT:PSS
control devices. These devices utilized a sNiO HTL that has a work
function of -5.6 eV before deposition of the active layer. Table 1
summarizes the results obtained via current-voltage characteristics
of the solar cells with a spectral mismatch of 1.0.
TABLE-US-00001 TABLE 1 Solution NiO and PEDOT:PSS control device
summaries. HTL V.sub.OC (mV) J.sub.SC (mA/cm.sup.2) FF PCE (%)
R.sub.shunt (.OMEGA. cm.sup.2) R.sub.series (.OMEGA. cm.sup.2) sNiO
583 .+-. 5 -8.6 .+-. 0.1 0.71 .+-. 0.004 3.6 .+-. 0.1 1600 .+-. 400
6.0 .+-. 0.2 PEDOT:PSS 578 .+-. 3 -9.4 .+-. 0.3 0.66 .+-. 0.02 3.6
.+-. 0.1 2200 .+-. 300 8.1 .+-. 0.2
[0046] The thickness of the sNiO films may also have important
effects. For example, thicker films may produce devices with lower
V.sub.OC, lower J.sub.SC, and higher R.sub.s. Thinner films may
fail to produce uniform devices on a substrate, causing
inconsistent performance, perhaps due to incomplete surface
coverage. A thickness of approximately 10 nm was found to be
satisfactory for PLD deposited NiO films with an active layer
thickness of about 220 nm. This thickness of the active layer
blend, however, may be different for devices with sNiO, as opposed
to PEDOT:PSS. Referring again to Table 1, the sNiO devices showed
higher FF, slightly lower J.sub.SC and similar V.sub.OC, in
relation to the PEDOT:PSS control device. This leads to similar PCE
of 3.6% for both sNiO and PEDOT:PSS HTLs. Series and shunt
resistances were calculated by taking the inverse slopes at
V.sub.OC and J.sub.SC, respectively. The series resistance is lower
in the sNiO devices, as compared to PEDOT:PSS. The decrease in
series resistance in the sNiO OPV devices may be due to reduced
contact resistance at the ITO/sNiO and sNiO/active layer
interfaces.
[0047] FIG. 5 illustrates the current-voltage characteristics of
PEDOT:PSS and sNiO-based devices. An unexpected aspect of the
device performance is the sNiO films with much deeper observed work
function than the assumed optimal -5.0 eV worked best. The plasma
treatment of these films may produce better functioning devices
with non-treated films exhibiting poor FF and low V.sub.OC. It is
possible that the plasma treatment may be cleaning off any organic
contaminants or remaining solvent at the surface, revealing a
higher work function sNiO beneath. Or it may be that the surface
oxidation state is shifted during O.sub.2-plasma treatment, either
reducing a tunnel barrier or pinning the P3HT HOMO level closer to
the NiO valence band. However, higher than expected work function
indicates that there may be charging of the surface from the
plasma, and it is subsequently degrading quickly over time. It is
most likely that there is surface oxidation, which will change the
work function and the doping of the p-type NiO thin-film, modestly
increasing conductivity, while facilitating good ohmic contact,
hole collection, and electron blocking characteristics.
[0048] Other mechanisms may not be ruled out, which may result in
effective charge transport at this interface, such as Fermi-level
pinning. However, these types of interfacial mechanisms suggest
that more relevance be placed on the electric field, and therefore,
surface doping in the sNiO layer than the measured work function of
the layer. Before recombination may occur, a high electric field
may accelerate the already dissociated charge carrier toward the
proper electrode. Higher electric fields may, therefore, correspond
to enhanced current collection. Solution process NiO may be used in
lieu of PLD NiO thin-films in BHJ OPV devices. Furthermore, sNiO is
an effective replacement for PEDOT:PSS as an HTL. There are similar
efficiencies for solar cells with sNiO compared to PEDOT:PSS
controls. Both the precursor ink formulation and the processing
conditions produced 10 nm sNiO HTL films that enable both electron
blocking and hole collection indicative of high-quality, selective
contacts essential for BHJ devices.
[0049] In another embodiment, a solution deposited NiO HTL to
modify the transparent ITO electrode may enable a PCE of 6.7%. The
organic active layer system may attain high V.sub.OC (.about.880
mV) due to the low-lying HOMO level of a PCDTBT donor material.
Improved performance of this system may therefore require contact
layers with deeper work function than PEDOT:PSS. Ultraviolet and
inverse photoelectron spectroscopy (UPS, IPES) show the work
function of NiO is well matched to the HOMO level of the PCDTBT
donor material. The PCDTBT organic solar cells utilizing NiO HTLs
outperform PEDOT:PSS solar cells in every device metric. An optical
model utilizing multilayer matrix theory (MMT) is employed to
assess gains in short circuit current (J.sub.SC). The V.sub.OC
increase analysis is made from diode modeling of the dark current.
Additionally, lifetime measurements demonstrate the enhanced
stability under constant illumination of non-encapsulated devices
utilizing a NiO HTL.
[0050] NiO demonstrates a deep work function HTL, where NiO surface
treatments and the NiO/organic interface play a role in determining
device performance. When used as and HTL, the surface of the
solution processed NiO film, exclusively employed here, may benefit
from exposure to oxygen plasma to establish an appropriate work
function immediately before deposition of the active layer. A
similar mechanism may take place for the surface of the ITO.
[0051] Ultraviolet and inverse photoelectron spectroscopy (UPS,
IPES) measurements of the density of states near the valence band
edge and conduction band edge and resulting band energies are shown
in FIGS. 6a, 6b, and 6c, which illustrate the effects of
O.sub.2-plasma on MO. The photoemission cut-off shown in FIG. 6a
indicates an increase of the work function (.phi..sub.W) from -4.75
eV to -5.32 eV upon O.sub.2-plasma treatment of MO. These results
are consistent with observed changes in the work function
difference for NiO films measured in either air or nitrogen using
Kelvin probe techniques. The ionization energy (IE), defined as the
energy difference between valence band edge and vacuum level,
increases by 0.5 eV following O.sub.2-plasma treatment to a value
of -5.7 eV. In contrast, the O.sub.2-plasma does not affect the
electron affinity (EA), which remains constant at -2.1 eV. In
addition, FIG. 6b shows that the valence band edge is placed very
close (0.4 eV) below the Fermi level, indicating that NiO is a
p-type material: A summary of the UPS and IPES results in the form
of energy level positions is given in FIG. 6c, which shows the
energy level diagrams of NiO before and after O.sub.2-plasma
treatment.
[0052] The energy diagram indicates three attributes of NiO that
may result in a high performance HTL. First, O.sub.2-plasma treated
NiO has a high work function, facilitating contact with the deep
HOMO of the donor material. Second, the position of the conduction
band edge, 2,1 eV from vacuum, may permit NiO to serve as an
effective electron blocking layer to prevent electron recombination
at the anode. Third, NiO is a wide band-gap (3.6 eV, when
O.sub.2-plasma treated) HTL leading to high transmission throughout
the absorption spectrum of the active layer. In contrast to
PEDOT:PSS, the transmission of NiO is higher for wavelengths above
500 nm. Thus, employing NiO as an HTL affords high optical
transparency, low resistivity, and excellent energy level alignment
with the HOMO level of the donor.
[0053] Thin films of NiO deposited from physical vapor or solution
processes produce PCE values similar to PEDOT:PSS in canonical OPVs
with BHJ layers of poly(3-hexylthiophene) (P3HT):PC.sub.60BM. For
comparison, power conversion efficiencies of 6.1% have been
reported for PCDTBT:PC.sub.70BM solar cells when PEDOT:PSS was used
as the HTL in conjunction with TiO.sub.X as the electron transport
layer (ETL) and optical spacer.
[0054] FIG. 7a illustrates an OPV device layer structure utilizing
a PCDTBT:PC.sub.70BM BHJ of 100 nm thickness interfaced with a
solution deposited HTL (either NiO with a work function of -5.3 eV
and thickness of 6 nm or PEDOT:PSS HTL with a work function of -5.1
eV and a thickness of 34 nm). The active layer thickness is 100 nm
for all devices, as determined by stylus profilometry. The vacuum
energy level diagram for each material in the OPV device is shown
schematically in FIG. 7b. Hole transport to the ITO is improved
utilizing NiO as the HTL, whereas reverse transfer of electrons is
blocked by the high conduction band level of NiO.
[0055] Current density versus voltage (J-V) measurements were taken
in the dark and under simulated one-sun illumination. A comparison
of the light and dark J-V measurements for both NiO and PEDOT:PSS
based devices is presented in FIGS. 8a-8b and Table 2. The
PCDTBT:PC.sub.70BM devices employing a PEDOT:PSS HTL exhibit an
average V.sub.OC of 845 mV, J.sub.SC of 11.01 mA/cm.sup.2, and a FF
of 0.60 to yield a PCE of 5.7%. With the NiO HTL there is a 17.3%
increase in device performance with an average V.sub.OC of 879 mV,
J.sub.SC of 11.5 mA/cm.sup.2, FF of 0.65, and PCE of 6.7%. This
result differs from studies of P3HT:PC.sub.60BM solar cells that
demonstrated similar V.sub.OC values for NiO and PEDOT:PSS. This
difference may be explained by comparing the HOMO levels of the
donors and energy level alignment with the HTL, resulting in the
Fermi level pinning in NiO to PCDTBT, whereas there would be none
with PEDOT:PSS. Previous studies indicate that P3HT has an HOMO
level between -4.8 and -5.1 eV. UPS measurements of the work
function far ITO/PEDOT:PSS and ITO/NiO films lead to values of -5.1
eV and -5.3 eV, respectively. In the case of PCDTBT, however, the
device performance clearly demonstrates that PEDOT:PSS fails to
achieve the maximum V.sub.OC for the lower-lying HOMO level of
PCDTBT. The work function of NiO is more closely aligned with the
-5.45 eV HOMO level of PCDTBT which may mitigate the loss
mechanisms that are suffered with the PEDOT:PSS HTL.
TABLE-US-00002 TABLE 2 Device characteristics and ITO/HTL
.PHI..sub.w for PCDTBT:PC.sub.70BM BHJ solar cells with NiO and
PEDOT:PSS HTLs V.sub.OC HTL (mV) J.sub.SC (mAcm.sup.2) FF PCE (%)
.PHI..sub.w (eV) sNio 879 .+-. 7 -11.5 .+-. 0.4 0.65 .+-. 0.01 6.7
.+-. 0.1 -5.4 PEDOT:PSS 845 .+-. 8 -11.1 .+-. 0.1 0.60 .+-. 0.01
5.7 .+-. 0.1 -.51
[0056] In order to elucidate the cause for increased V.sub.OC in
the devices with NiO, a standard diode model was employed, which
includes the parallel and serial resistance of the diode in
Equation 1.
J=(V-JR.sub.s)/R.sub.p-J.sub.satexp(V-JR.sub.s)/nkT (Equation
1)
[0057] In Equation 1, J is the measured current density, V is the
applied bias, R.sub.R is the parallel resistance, J.sub.sat is the
saturation current, n is the diode ideality factor, T is ambient
temperature, and k is the Boltzmann constant. When fitting with
this model, the changes in V.sub.OC may be inferred following
Equation 2.
V.sub.OC.apprxeq.(nkT/q)In(J.sub.so/J.sub.sat) (Equation 2)
[0058] In Equation 2, J.sub.sc is the short circuit current. Fits
of the data to the model are shown in FIG. 8a and resultant
parameters, including relative errors are reported in Table 3. The
predicted increase in V.sub.OC (16 mV) from Equation 2 is due to a
large decrease in J.sub.sat and is verified by the measured J-V
data under illumination. With the NiO contact, the calculated diode
ideality factor, n improves from 2.3 to 1.6, with a corresponding
decrease of several orders of magnitude in J.sub.sat from
2.6.times.10.sup.-6 to 2.3.times.10.sup.-9 mAcm.sup.-2. For organic
solar cells, improvements in the exponential regime of the diode,
where J.sub.sat and n dominate, has been correlated with increases
in V.sub.OC. Changes in J.sub.sat are indicative of the ability of
current to overcome the energy barrier at the HTL/BHJ interface in
the reverse direction. The J.sub.sat is likely reduced with
selective harvesting of positive charges and reduced electron back
transfer. Hence anode enhanced forward charge transfer may account
for increases to the J.sub.SC, while the carrier selectivity
improves the V.sub.OC. A reduction in the carrier recombination and
an increase in charge collection combine to give the improvement in
FF observed for the NiO devices.
TABLE-US-00003 TABLE 3 Dark diode model fit results with errors of
one standard deviation and predicted V.sub.OC for each HTL
calculated for Equation 2. HTL n R.sub.p (.OMEGA.cm.sup.2) R.sub.s
(.OMEGA.cm.sup.2) J.sub.sat (mAcm.sup.-2) Predicted V.sub.OC (mV)
NiO 1.6 .+-. 0.1 1.4E+5 .+-. 1E4 1.40 .+-. 0.15 2.3E-9 .+-. 1E-9
924 PEDOT:PSS 2.3 .+-. 0.3 1.5E+4 .+-. 1E3 1.43E .+-. 0.19 2.6E-6
.+-. 2E-6 908
[0059] In addition to improved V.sub.OC and FF, there is also a
modest increase of J.sub.SC, for NiO. Normalized external quantum
efficiency (EQE) for BHJ devices with PEDOT:PSS and NiO HTLs are
shown in FIGS. 9a, 9b and 9c, along with HTL optical properties and
the MMT results. To analyze changes in J.sub.SC with MMT modeling,
the EQE spectra for wavelengths between 330 nm and 800 nm are
calculated and the photon absorption is integrated over the entire
BHJ thickness. Assuming identical internal quantum efficiencies,
and integrating the optical fields over these wavelengths result in
similar energy dissipation for both HTLs with only a slightly
higher value for NiO. This indicates that the NiO HTL in place of
the PEDOT:PSS layer does not sufficiently alter the optical
resonance within the BHJ layer to account for the experimentally
observed increase in J.sub.SC.
[0060] Differences in EQE curve shapes result from a combination of
changes in thickness and optical properties of the HTLs and are
displayed as a peak redshift from 450 nm to 480 nm in FIG. 9a.
However, as the shift in peak resonance is not sufficient to
entirely account for the increased photocurrent, the enhanced
J.sub.SC is attributed to improved transmission coupled to
decreased recombination at the NiO/BHJ interface. As seen in FIG.
9a, peaks at 450 nm and 550 nm are clearly visible for devices with
NiO. A redshifted peak at 480 nm is shown for PEDOT:PSS due to the
thicker HTL. FIG. 9b shows optical constants n and k obtained for
NiO, PEDOT:PSS, and PCDTBT:PC.sub.70BM (1:4) BHJ think films, in
order to model the optical field in the solar cell. Optical
constants were calculated from spectroscopic ellipsometry
measurements. FIG. 9c shows an optical field plot of 550 nm
irradiation for a 100 nm BHJ layer shown for HTLs of NiO and
PEDOT:PSS. Small differences in field strength account for changes
to EQE, but do not entirely explain the increase in J.sub.SC.
[0061] In order to investigate the effects of the NiO HTL on device
stability, the lifetime of the non-encapsulated PCDTBT:PC.sub.70BN
solar cells was assessed. An all aluminum top electrode was used in
these experiments to reduce effects related to the oxidation of Ca
that would otherwise dominate the device evolution. Devices were
exposed to constant .about.0.7 sun illumination near the maximum
power point, and J-V characteristics were recorded at 30 minute
intervals. Device parameters for 450 hours of testing initiated on
the day of device fabrication are shown in FIG. 10. Instabilities
within the active layer and top electrode clearly contribute to the
overall degradation.
[0062] In the data shown in FIG. 10, the HTL should be largely
responsible for the different rates found in the non-encapsulated
device lifetimes. All device parameters for the NiO HTL, including
V.sub.OC, J.sub.SC, FF, and PCE exhibit lower decay rates than
those for PEDOT:PSS. The exponential drop in V.sub.OC over the
initial 10-50 hours found in both PEDOT:PSS and NiO samples is
likely the result of oxidation of aluminum electrode after the
devices have been exposed to air. As the aluminum oxidizes, a
corresponding increase in the work function (.phi..sub.W) would act
to reduce the internal electric field in the device, resulting in a
lower V.sub.OC and reduced driving force for current collection.
The relative loss in V.sub.OC over time is significantly reduced in
the case of the NiO, which is again consistent with the larger
.phi..sub.W of NiO that acts to maintain the asymmetric electric
field in the device after the rate for the organic HTL stems from
the hygroscopic nature of PEDOT:PSS. Uptake of atmospheric water
vapor has been shown to increase the resistivity of the PEDOT:PSS
and was reported as a main cause of rapid degradation in OPV
devices. The low pH of PEDOT:PSS has also been implicated in
degrading the hole collecting ITO/PEDOT:PSS interface due to
etching ITO, an effect not replicated at the ITO/NiO interface.
[0063] In summary, NiO significantly outperforms PEDOT:PSS as an
HTL for anodic contact in a PCDTBT:PC.sub.70BM BHJ solar cell in
both device performance and stability. UPS/IPES show that the
oxygen plasma treatment of the solution deposited MO increases the
.phi..sub.W enabling the formation of a charge selective contact
with the PCDTBT:PC.sub.70BM active layer. By comparing solar cell
device performance to the PEDOT:PSS, comprehensive improvements are
observed, resulting in a PCE of 6.7%. This is largely the result of
a reduced diode factor (n), a 10.sup.3 reduction of J.sub.sat, and
enhanced transmission leading to increase in V.sub.OC, J.sub.SC,
and FF. Additionally, the stability of device performance is
increased with MO relative to PEDOT:PSS, likely due to the
preservation of the internal electric field with the higher
.phi..sub.W HTL and the deterioration of PEDOT:PSS after exposure
to water vapor. The 17.3% net increase in PCE coupled to improved
environmental stability resulting from the optimized contact
demonstrates the role of the HTL/BHJ interface for maximizing
performance of organic solar cells.
[0064] While a number of exemplary aspects and embodiments have
been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions and sub combinations
thereof. It is therefore intended that the following appended
claims and claims hereafter introduced are interpreted to include
all such modifications, permutations, additions and
sub-combinations as are within their true spirit and scope.
* * * * *