U.S. patent application number 12/005773 was filed with the patent office on 2008-06-05 for low resistance thin film organic solar cell electrodes.
Invention is credited to Stephen Forrest, Jiangeng Xue.
Application Number | 20080131993 12/005773 |
Document ID | / |
Family ID | 38322592 |
Filed Date | 2008-06-05 |
United States Patent
Application |
20080131993 |
Kind Code |
A1 |
Forrest; Stephen ; et
al. |
June 5, 2008 |
Low resistance thin film organic solar cell electrodes
Abstract
A method which lower the series resistance of photosensitive
devices includes providing a transparent film of a first
electrically conductive material arranged on a transparent
substrate; depositing and patterning a mask over the first
electrically conductive material, such that openings in the mask
have sloping sides which narrow approaching the substrate;
depositing a second electrically conductive material directly onto
the first electrically conductive material exposed in the openings
of the mask, at least partially filling the openings; stripping the
mask, leaving behind reentrant structures of the second
electrically conductive material which were formed by the deposits
in the openings of the mask; after stripping the mask, depositing a
first organic material onto the first electrically conductive
material in between the reentrant structures; and directionally
depositing a third electrically conductive material over the first
organic material deposited in between the reentrant structures,
edges of the reentrant structures aligning deposition so that the
third electrically conductive material does not directly contact
the first electrically conductive material, and does not directly
contact the second electrically conductive material.
Inventors: |
Forrest; Stephen;
(Princeton, NJ) ; Xue; Jiangeng; (Piscataway,
NJ) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
38322592 |
Appl. No.: |
12/005773 |
Filed: |
December 27, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11205122 |
Aug 17, 2005 |
7314773 |
|
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12005773 |
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Current U.S.
Class: |
438/82 ;
257/E51.012 |
Current CPC
Class: |
H01L 51/441 20130101;
H01L 51/424 20130101; Y02E 10/549 20130101; H01L 27/301 20130101;
Y02P 70/50 20151101; Y02P 70/521 20151101 |
Class at
Publication: |
438/82 ;
257/E51.012 |
International
Class: |
H01L 51/44 20060101
H01L051/44 |
Goverment Interests
UNITED STATES GOVERNMENT RIGHTS
[0001] This invention was made with U.S. Government support under
Contract No. 339-6002 awarded by the U.S. Air Force Office of
Scientific Research and under Contract No. 341-4141 awarded by U.S.
Department of Energy, National Renewable Energy Laboratory. The
government has certain rights in this invention.
Claims
1. A method comprising: providing a transparent film comprising a
first electrically conductive material, arranged on a transparent
substrate; depositing and patterning a mask over the first
electrically conductive material, such that openings in the mask
have sloping sides which narrow approaching the substrate;
depositing a second electrically conductive material directly onto
the first electrically conductive material exposed in the openings
of the mask, at least partially filling the openings; stripping the
mask, leaving behind reentrant structures of the second
electrically conductive material which were formed by said
depositing in the openings of the mask; after said stripping,
depositing a first organic material onto the first electrically
conductive material in between the reentrant structures; and
depositing a third electrically conductive material over the first
organic material deposited in between the reentrant structures,
edges of the reentrant structures aligning deposition so that the
third electrically conductive material does not directly contact
the first and second electrically conductive materials.
2. The method of claim 1, the second electrically conductive
material being more conductive than the first electrically
conductive material.
3. The method of claim 1, wherein the first electrically conductive
material is an oxide or a polymer.
4. The method of claim 3, wherein the oxide is a degenerate
semiconductor oxide.
5. The method of claim 3, wherein the second electrically
conductive material is a metal.
6. The method of claim 1, wherein the first electrically conductive
material of the transparent film is a transparent coating of a
metal.
7. The method of claim 1, wherein the second electrically
conductive material is a metal.
8. The method of claim 1, further comprising electrically
interconnecting the third electrically conductive material
deposited between the reentrant structures.
9. The method of claim 1, further comprising depositing a second
organic material over the first organic material, the first and
second organic materials forming a planar, bulk, or hybrid
donor-acceptor heterojunction.
10. The method of claim 1, further comprising depositing a second
organic material with the first organic material, the first and
second organic materials forming a mixed, bulk or hybrid
donor-acceptor heterojunction.
11. The method of claim 1, wherein the first electrically
conductive material is metal, the transparent film and the first
organic material forming a Schottky-barrier heterojunction.
12. The method of claim 1, wherein the third electrically
conductive material is metal, the third electrically conductive
material and the first organic material forming a Schottky-barrier
heterojunction.
13. The method of claim 1, wherein said transparent film is a sheet
in physical contact with the transparent substrate.
Description
JOINT RESEARCH AGREEMENT
[0002] 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: Princeton University,
The University of Southern California, and Global Photonic Energy
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
[0003] The present invention generally relates to the manufacturing
of organic photosensitive optoelectronic devices. More
specifically, it is directed to a method for building an organic
photosensitive optoelectronic devices having a low resistance
transparent electrode.
BACKGROUND
[0004] 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.
[0005] Photosensitive optoelectronic devices convert
electromagnetic radiation into an electrical signal or electricity.
Solar cells, also called photovoltaic ("PV") devices, are a type of
photosensitive optoelectronic device that is specifically used to
generate electrical power. Photoconductor cells are a type of
photosensitive optoelectronic device that are used in conjunction
with signal detection circuitry which monitors the resistance of
the device to detect changes due to absorbed light. Photodetectors,
which may receive an applied bias voltage, are a type of
photosensitive optoelectronic device that are used in conjunction
with current detecting circuits which measures the current
generated when the photodetector is exposed to electromagnetic
radiation.
[0006] These three classes of photosensitive optoelectronic devices
may be distinguished 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.
[0007] As used herein, the term "rectifying" denotes, inter alia,
that an interface has an asymmetric conduction characteristic,
i.e., the interface supports electronic charge transport preferably
in one direction. The term "semiconductor" denotes materials which
can conduct electricity when charge carriers are induced by thermal
or electromagnetic excitation. The term "photoconductive" generally
relates to the process in which electromagnetic radiant energy is
absorbed and thereby converted to excitation energy of electric
charge carriers so that the carriers can conduct (i.e., transport)
electric charge in a material. The term "photoconductive material"
refers to semiconductor materials which are utilized for their
property of absorbing electromagnetic radiation to generate
electric charge carriers. As used herein, "top" means furthest away
from the substrate, while "bottom" means closest to the substrate.
There may be intervening layers, unless it is specified that the
first layer is "in physical contact with" the second layer.
[0008] When electromagnetic radiation of an appropriate energy is
incident upon an organic semiconductor material, a photon can be
absorbed to produce an excited molecular state. In organic
photoconductive materials, the generated molecular state is
generally believed to be an "exciton," i.e., an electron-hole pair
in a bound state which is transported as a quasi-particle. An
exciton can have an appreciable life-time before geminate
recombination ("quenching"), which refers to the original electron
and hole recombining with each other (as opposed to recombination
with holes or electrons from other pairs). To produce a
photocurrent, the electron-hole forming the exciton are typically
separated at a rectifying junction.
[0009] In the case of photosensitive devices, the rectifying
junction is referred to as a photovoltaic heterojunction. Types of
organic photovoltaic heterojunctions include a donor-acceptor
heterojunction formed at an interface of a donor material and an
acceptor material, and a Schottky-barrier heterojunction formed at
the interface of a photoconductive material and a metal.
[0010] FIG. 1 is an energy-level diagram illustrating an example
donor-acceptor heterojunction. In the context of organic materials,
the terms "donor" and "acceptor" refer to the relative positions of
the Highest Occupied Molecular Orbital ("HOMO") and Lowest
Unoccupied Molecular Orbital ("LUMO") energy levels of two
contacting but different organic materials. If the LUMO energy
level of one material in contact with another is lower, then that
material is an acceptor. Otherwise it is a donor. It is
energetically favorable, in the absence of an external bias, for
electrons at a donor-acceptor junction to move into the acceptor
material.
[0011] As used herein, a first HOMO or LUMO energy level is
"greater than" or "higher than" a second HOMO or LUMO energy level
if the first energy level is closer to the vacuum energy level 10.
A higher HOMO energy level corresponds to an ionization potential
("IP") having a smaller absolute energy relative to a vacuum level.
Similarly, a higher LUMO energy level corresponds to an electron
affinity ("EA") having a smaller absolute energy relative to vacuum
level. On a conventional energy level diagram, with the vacuum
level at the top, the LUMO energy level of a material is higher
than the HOMO energy level of the same material.
[0012] After absorption of a photon 6 in the donor 152 or the
acceptor 154 creates an exciton 8, the exciton 8 disassociates at
the rectifying interface. The donor 152 transports the hole (open
circle) and the acceptor 154 transports the electron (dark
circle).
[0013] A significant property in organic semiconductors is carrier
mobility. Mobility measures the ease with which a charge carrier
can move through a conducting material in response to an electric
field. In the context of organic photosensitive devices, a material
that conducts preferentially by electrons due to a high electron
mobility may be referred to as an electron transport material. A
material that conducts preferentially by holes due to a high hole
mobility may be referred to as a hole transport material. A layer
that conducts preferentially by electrons, due to mobility and/or
position in the device, may be referred to as an electron transport
layer ("ETL"). A layer that conducts preferentially by holes, due
to mobility and/or position in the device, may be referred to as a
hole transport layer ("HTL"). Preferably, but not necessarily, an
acceptor material is an electron transport material and a donor
material is a hole transport material.
[0014] How to pair two organic photoconductive materials to serve
as a donor and an acceptor in a photovoltaic heterojunction based
upon carrier mobilities and relative HOMO and LUMO levels is well
known in the art, and is not addressed here.
[0015] As used herein, the term "organic" includes polymeric
materials as well as small molecule organic materials that may be
used to fabricate organic optoelectronic 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. The core moiety of a dendrimer may
be a fluorescent or phosphorescent small molecule emitter. A
dendrimer may be a "small molecule." In general, a small molecule
has a defined chemical formula with a molecular weight that is the
same from molecule to molecule, whereas a polymer has a defined
chemical formula with a molecular weight that may vary from
molecule to molecule. As used herein, "organic" includes metal
complexes of hydrocarbyl and heteroatom-substituted hydrocarbyl
ligands.
[0016] For additional background explanation and description of the
state of the art for organic photosensitive devices, including
their general construction, characteristics, materials, and
features, U.S. Pat. No. 6,657,378 to Forrest et al., U.S. Pat. No.
6,580,027 to Forrest et al., and U.S. Pat. No. 6,352,777 to Bulovic
et al. are incorporated herein by reference.
SUMMARY OF THE INVENTION
[0017] A method and structure are disclosed which lower the series
resistance of photosensitive devices.
[0018] The method includes providing an electrode comprising a
first electrically conductive material, arranged on a transparent
substrate; depositing and patterning a mask over the first
electrically conductive material, such that openings in the mask
have sloping sides which narrow approaching the substrate;
depositing a second electrically conductive material directly onto
the first electrically conductive material exposed in the openings
of the mask, at least partially filling the openings; stripping the
mask, leaving behind reentrant structures of the second
electrically conductive material which were formed by the deposits
in the openings of the mask; after stripping the mask, depositing a
first organic material onto the first electrically conductive
material in between the reentrant structures; and depositing a
third electrically conductive material over the first organic
material deposited in between the reentrant structures, edges of
the reentrant structures aligning deposition so that the third
electrically conductive material does not directly contact the
first electrically conductive material, and does not directly
contact the second electrically conductive material.
[0019] The second electrically conductive material may have a
similar conductivity to the first electrically conductive material,
or may be more conductive.
[0020] The first electrically conductive material of the
transparent electrode may be a metal or a metal substitute. Such
metal substitutes include, among other things, oxides and polymers.
An example conductive oxide conductive oxide is a degenerate
semiconductor oxide.
[0021] The second electrically conductive material may be a metal
or metal substitute.
[0022] The method may further include electrically interconnecting
the third electrically conductive material deposited between the
reentrant structures. This may be accomplished, among other ways,
by forming interconnects/vias as part of a wiring step, or by
structuring the reentrant structures so that the third electrically
conductive material forms a single continuous layer when
deposited.
[0023] The steps described above may be used to create a
donor-acceptor heterojunction or a Schottky-barrier heterojunction.
To form a donor-acceptor heterojunction, the method may further
include depositing a second organic material over the first organic
material, the first and second organic materials forming a planar,
bulk, or hybrid donor-acceptor heterojunction. Alternatively, the
method may further include depositing a second organic material
with the first organic material, the first and second organic
materials forming a mixed, bulk or hybrid donor-acceptor
heterojunction.
[0024] To form a Schottky-barrier heterojunction, the first
electrically conductive material may be metal, the transparent
electrode and the first organic material forming a Schottky-barrier
heterojunction. Alternatively, the third electrically conductive
material may be metal, the third electrically conductive material
and the first organic material forming a Schottky-barrier
heterojunction. An example of a structure that may be used for the
transparent electrode is a sheet in physical contact with the
transparent substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is an energy level diagram illustrating a
donor-acceptor heterojunction.
[0026] FIG. 2 illustrates an organic photosensitive device
including a donor-acceptor heterojunction.
[0027] FIG. 3 illustrates a donor-acceptor bilayer forming a planar
heterojunction.
[0028] FIG. 4 illustrates a hybrid heterojunction including a mixed
heterojunction between a donor layer and an acceptor layer.
[0029] FIG. 5 illustrates a bulk heterojunction.
[0030] FIG. 6 illustrates an organic photosensitive device
including a Schottky-barrier heterojunction.
[0031] FIG. 7 illustrates tandem photosensitive cells in
series.
[0032] FIG. 8 illustrates tandem photosensitive cells in
parallel.
[0033] FIGS. 9A-9F illustrate a method for forming an organic
photovoltaic device having low resistance contacts.
[0034] FIGS. 10A and 10B illustrate a mask that can be used for the
step illustrated in FIG. 9B that simplifies creation of a common
anode and cathode. FIG. 10A is a cross section of the structure,
and FIG. 10B is an overhead view, with line A-A' providing a
frame-of-reference between the figures.
[0035] FIGS. 11A and 11B illustrate the conductor patterns
resulting from the usage of the mask illustrated in FIGS. 10A and
10B. FIG. 11A is a cross section of the structure, and FIG. 11B is
an overhead view, with line B-B' providing a frame-of-reference
between the figures.
[0036] The figures are not necessarily drawn to scale.
DETAILED DESCRIPTION
[0037] An organic photosensitive device comprises at least one
photoactive region in which light is absorbed to form an exciton,
which may subsequently dissociate into an electron and a hole. FIG.
2 shows an example of an organic photosensitive optoelectronic
device 100 in which the photoactive region 150 comprises a
donor-acceptor heterojunction. The "photoactive region" is a
portion of a photosensitive device that absorbs electromagnetic
radiation to generate excitons that may dissociate in order to
generate an electrical current. Device 100 comprises an anode 120,
an anode smoothing layer 122, a donor 152, an acceptor 154, an
exciton blocking layer ("EBL") 156, and a cathode 170, over a
substrate 110.
[0038] Examples of EBL 156 are described in U.S. Pat. No. 6,451,415
to Forrest et al., which is incorporated herein by reference for
its disclosure related to EBLs. Additional background explanation
of EBLs may also be found in Peumans et al., "Efficient photon
harvesting at high optical intensities in ultrathin organic
double-heterostructure photovoltaic diodes," Applied Physics
Letters 76, 2650-52 (2000). EBLs reduce quenching by preventing
excitons from migrating out of the donor and/or acceptor
materials.
[0039] The terms "electrode" and "contact" are used interchangeably
herein to refer to a layer that provides a medium for delivering
photo-generated current to an external circuit or providing a bias
current or voltage to the device. As illustrated in FIG. 2, anode
120 and cathode 170 are examples. Electrodes may be composed of
metals or "metal substitutes." Herein the term "metal" is used to
embrace both materials composed of an elementally pure metal, and
also metal alloys which are materials composed of two or more
elementally pure metals. The term "metal substitute" refers to a
material that is not a metal within the normal definition, but
which has the metal-like properties such as conductivity, such as
doped wide-bandgap semiconductors, degenerate semiconductors,
conducting oxides, and conductive polymers. Electrodes may comprise
a single layer or multiple layers (a "compound" electrode), may be
transparent, semi-transparent, or opaque. Examples of electrodes
and electrode materials include those disclosed in U.S. Pat. No.
6,352,777 to Bulovic et al., and U.S. Pat. No. 6,420,031, to
Parthasarathy, et al., each incorporated herein by reference for
disclosure of these respective features. As used herein, a material
is said to be "transparent" if it transmits at least 50% of the
ambient electromagnetic radiation in a relevant wavelength.
[0040] The substrate 110 may be any suitable substrate that
provides desired structural properties. The substrate may be
flexible or rigid, planar or non-planar. The substrate may be
transparent, translucent or opaque. Rigid plastics and glass are
examples of preferred rigid substrate materials. Flexible plastics
and metal foils are examples of preferred flexible substrate
materials.
[0041] An anode-smoothing layer 122 may be situated between the
anode layer 120 and the donor layer 152. Anode-smoothing layers are
described in U.S. Pat. No. 6,657,378 to Forrest et al.,
incorporated herein by reference for its disclosure related to this
feature.
[0042] In FIG. 2, the photoactive region 150 comprises the donor
material 152 and the acceptor material 154. Organic materials for
use in the photoactive region may include organometallic compounds,
including cyclometallated organometallic compounds. The term
"organometallic" as used herein is as generally understood by one
of ordinary skill in the art and as given, for example, in Chapter
13 of "Inorganic Chemistry" (2nd Edition) by Gary L. Miessler and
Donald A. Tarr, Prentice Hall (1999).
[0043] Organic layers may be fabricated using vacuum deposition,
spin coating, organic vapor-phase deposition, inkjet printing and
other methods known in the art.
[0044] Examples of various types of donor-acceptor heterojunctions
are shown in FIGS. 3-5. FIG. 3 illustrates a donor-acceptor bilayer
forming a planar heterojunction. FIG. 4 illustrates a hybrid
heterojunction including a mixed heterojunction 153 comprising a
mixture of donor and acceptor materials. FIG. 5 illustrates an
idealized "bulk" heterojunction. A bulk heterojunction, in the
ideal photocurrent case, has a single continuous interface between
the donor material 252 and the acceptor material 254, although
multiple interfaces typically exist in actual devices. Mixed and
bulk heterojunctions can have multiple donor-acceptor interfaces as
a result of having plural domains of material. Domains that are
surrounded by the opposite-type material (e.g. a domain of donor
material surrounded by acceptor material) may be electrically
isolated, such that these domains do not contribute to
photocurrent. Other domains may be connected by percolation
pathways (continuous photocurrent pathways), such that these other
domains may contribute to photocurrent. The distinction between a
mixed and a bulk heterojunction lies in degrees of phase separation
between donor and acceptor materials. In a mixed heterojunction,
there is very little or no phase separation (the domains are very
small, e.g., less than a few nanometers), whereas in a bulk
heterojunction, there is significant phase separation (e.g.,
forming domains with sizes of a few nanometers to 100 nm).
[0045] Small-molecule mixed heterojunctions may be formed, for
example, by co-deposition of the donor and acceptor materials using
vacuum deposition or vapor deposition. Small-molecule bulk
heterojunctions may be formed, for example, by controlled growth,
co-deposition with post-deposition annealing, or solution
processing Polymer mixed or bulk heterojunctions may be formed, for
example, by solution processing of polymer blends of donor and
acceptor materials.
[0046] If a photoactive region includes a mixed layer (153) or bulk
layers (252, 254) and one or both of the donor (152) and acceptor
layers (154), the photoactive region is said to include a "hybrid"
heterojunction. The arrangement of layers in FIG. 4 is an example.
For additional explanation of hybrid heterojunctions, U.S.
application Ser. No. 10/910,371 entitled "High efficiency organic
photovoltaic cells employing hybridized mixed-planar
heterojunctions" by Jiangeng Xue et al., filed Aug. 4, 2004, is
hereby incorporated by reference.
[0047] In general, planar heterojunctions have good carrier
conduction, but poor exciton dissociation; a mixed layer has poor
carrier conduction and good exciton dissociation, and a bulk
heterojunction has good carrier conduction and good exciton
dissociation, but may experience charge build-up at the end of the
material "cul-de-sacs," lowering efficiency. Unless otherwise
stated, planar, mixed, bulk, and hybrid heterojunctions may be used
interchangeably as donor-acceptor heterojunctions throughout the
embodiments disclosed herein.
[0048] FIG. 6 shows an example of a organic photosensitive
optoelectronic device 300 in which the photoactive region 350 is
part of a Schottky-barrier heterojunction. Device 300 comprises a
transparent contact 320, a photoactive region 350 comprising an
organic photoconductive material 358, and a Schottky contact 370.
The Schottky contact 370 is typically formed as a metal layer. If
the photoconductive layer 358 is an ETL, a high work function metal
such as gold may be used, whereas if the photoconductive layer is
an HTL, a low work function metal such as aluminum, magnesium or
indium may be used. In a Schottky-barrier cell, a built-in electric
field associated with the Schottky barrier pulls the electron and
hole in an exciton apart. Generally, this field-assisted exciton
dissociation is not as efficient as the disassociation at a
donor-acceptor interface.
[0049] The devices as illustrated are connected to an element 190.
If the device is a photovoltaic device, element 190 is a resistive
load which consumes or stores power. If the device is a
photodetector, element 190 is a current detecting circuit which
measures the current generated when the photodetector is exposed to
light, and which may apply a bias to the device (as described for
example in Published U.S. Patent Application 2005-0110007 A1,
published May 26, 2005 to Forrest et al.). If the rectifying
junction is eliminated from the device (e.g., using a single
photoconductive material as the photoactive region), the resulting
structures may be used as a photoconductor cell, in which case the
element 190 is a signal detection circuit to monitor changes in
resistance across the device due to the absorption of light. Unless
otherwise stated, each of these arrangements and modifications may
be used for the devices in each of the drawings and embodiments
disclosed herein.
[0050] An organic photosensitive optoelectronic device may also
comprise transparent charge transfer layers, electrodes, or charge
recombination zones. A charge transfer layer may be organic or
inorganic, and may or may not be photoconductively active. A charge
transfer layer is similar to an electrode, but does not have an
electrical connection external to the device and only delivers
charge carriers from one subsection of an optoelectronic device to
the adjacent subsection. A charge recombination zone is similar to
a charge transfer layer, but allows for the recombination of
electrons and holes between adjacent subsections of an
optoelectronic device. A charge recombination zone may include
semi-transparent metal or metal substitute recombination centers
comprising nanoclusters, nanoparticles, and/or nanorods, as
described for example in U.S. Pat. No. 6,657,378 to Forrest et al.;
U.S. patent application Ser. No. 10/915,410 entitled "Organic
Photosensitive Devices" by Rand et al., filed Aug. 11, 2004; and
U.S. patent application Ser. No. 10/979,145 entitled "Stacked
Organic Photosensitive Devices" by Forrest et al., filed Nov. 3,
2004; each incorporated herein by reference for its disclosure of
recombination zone materials and structures. A charge recombination
zone may or may not include a transparent matrix layer in which the
recombination centers are embedded. A charge transfer layer,
electrode, or charge recombination zone may serve as a cathode
and/or an anode of subsections of the optoelectronic device. An
electrode or charge transfer layer may serve as a Schottky
contact.
[0051] FIGS. 7 and 8 illustrate examples of tandem devices
including such transparent charge transfer layers, electrodes, and
charge recombination zones. In device 400 in FIG. 7, photoactive
regions 150 and 150' are stacked electrically in series with an
intervening conductive region 460. As illustrated without external
electrical connections, intervening conductive region 460 may be a
charge recombination zone or may be a charge transfer layer. As a
recombination zone, region 460 comprises recombination centers 461
with or without a transparent matrix layer. If there is no matrix
layer, the arrangement of material forming the zone may not be
continuous across the region 460. Device 500 in FIG. 8 illustrates
photoactive regions 150 and 150' stacked electrically in parallel,
with the top cell being in an inverted configuration (i.e.,
cathode-down). In each of FIGS. 7 and 8, the photoactive regions
150 and 150' and blocking layers 156 and 156' may be formed out of
the same respective materials, or different materials, depending
upon the application. Likewise, photoactive regions 150 and 150'
may be a same type (i.e., planar, mixed, bulk, hybrid) of
heterojunction, or may be of different types.
[0052] In each of the devices described above, layers may be
omitted, such as the exciton blocking layers. Other layers may be
added, such as reflective layers or additional photoactive regions.
The order of layers may be altered or inverted. A concentrator or
trapping configuration may be employed to increase efficiency, as
disclosed, for example in U.S. Pat. No. 6,333,458 to Forrest et al.
and U.S. Pat. No. 6,440,769 to Peumans et al., which are
incorporated herein by reference. Coatings may be used to focus
optical energy into desired regions of a device, as disclosed, for
example in U.S. patent application Ser. No. 10/857,747 entitled
"Aperiodic dielectric multilayer stack" by Peumans et al., filed
Jun. 1, 2004, which is incorporated herein by reference. In the
tandem devices, transparent insulative layers may be formed between
cells, with the electrical connection between the cells being
provided via electrodes. Also in the tandem devices, one or more of
the photoactive regions may be a Schottky-barrier heterojunction
instead of a donor-acceptor heterojunction. Arrangements other than
those specifically described may be used.
[0053] The prevalent designs in contemporary small molecule and
polymer photosensitive cells utilize a transparent non-conductive
material for substrate 110 and a transparent conductive film for
anode 120, with incident light being radiated onto the exposed
surface of the substrate. Such designs are limited in efficiency by
large series resistance, much of it due to the anode 120, which is
generally made thin for reasons of optical transmissivity. In order
to boost efficiency, there is a need to produce low cost modules
with lower resistance contacts.
[0054] Embodiments of the present invention include a method for
forming an organic photosensitive device having a transparent
substrate-side electrode with improved conductivity, as well as the
resulting device.
[0055] FIGS. 9A-9F illustrate a method for improving conductivity.
In FIG. 9A, a mask layer 930 is formed on a transparent film
comprising a first electrically conductive material 920 arranged on
an electrically non-conductive, transparent substrate 110.
Depending upon the particular application, the transparent film may
be continuous or patterned. Examples of a continuous film include a
sheet or a coating in physical contact with the substrate 110. The
transparent film may serve as an electrode layer, or may serve as
one layer of a compound electrode. The mask layer 930 may be made
of photoresist or any other material that is easily patterned and
appropriate for use as a sacrificial layer.
[0056] The first material 920 may comprise a metal or metal
substitute, as described above for electrodes and Schottky
contacts. Among other things, examples include a conductive oxide
or polymer, a thin coating of metal (as defined above to include
metal alloys), or a doped semiconductor. An example of a conductive
oxide is a degenerate semiconductor oxide, such as indium tin oxide
(ITO), gallium indium tin oxide (GITO), and zinc indium tin oxide
(ZITO). In particular, ITO is a highly doped degenerate n.sup.+
semiconductor with a bandgap of approximately 3.2 eV, rendering it
transparent to wavelengths greater than approximately 3900 .ANG..
An example of suitable transparent polymer is polyaniline (PANI)
and its chemical relatives.
[0057] As described above, the transparent substrate 110 may be
rigid or flexible, but rigid is generally preferred for this
structure to avoid peeling or separation of layers and
short-circuits, in the finished device. The substrate may also be
planar or non-planar.
[0058] In FIG. 9B, the mask layer 930 is patterned (e.g.,
photolithography) to create reentrant openings 932 having sloping
sides which narrow approaching substrate 110. An example pattern
for mask layer 930 is illustrated in FIGS. 10A and 10B. Although it
is not required, a benefit of forming the openings 932 as a
connected pattern is that wiring is simplified in the finished
device.
[0059] In FIG. 9C, a second electrically conductive material 924 is
deposited directly onto the first material 920 exposed in the
reentrant openings 932 of the mask 930, at least partially filling
the openings 932. The second material 924 may have a conductivity
that is the same, or similar to the conductivity of the first
material 920, but preferably is more conductive. Examples of the
second material 924 include the metal and metal substitutes as
described above. The second material 924 may be the same as the
first material 920, or may be different.
[0060] In FIG. 9D, the sacrificial mask layer 930 is stripped away,
leaving behind reentrant structures of the second material 924.
Reentrant, as used herein, refers to a geometry where the sides of
a structure are directed inward, a cross-section of the structure
narrowing toward its base (i.e., substrate 110).
[0061] In FIG. 9E, one or more organic layers 950 are deposited.
These organic layers 950 include the layer or layers which will be
the photoactive region in the finished device. If a donor-acceptor
heterojunction is to be formed, organic layers 950 may include
donor layers 152, 252, acceptor layers 154, 254, and/or a mixed
layer 153, forming a planar, mixed, bulk, or hybrid heterojunction.
If a Schottky junction is to be formed, the organic layers 950 may
include the photoconductive material 358.
[0062] In FIG. 9F, a third electrically conductive material 970 is
directionally deposited over the organic material(s) 950 deposited
between the reentrant structures 924. Examples of the third
material 970 include metal and metal substitutes as described
above. By directionally depositing between the reentrant structures
of the second material 924, deposition is self aligning. The
resulting gaps 975 prevent shorts between the transparent film and
reentrant structures and the third material 970. FIGS. 11A and 11B
illustrate the corresponding results if using the mask from FIGS.
10A and 10B.
[0063] Even if the first material 920 and the second material 924
are the same, the reentrant structures of the second material 924
lowers resistance and provide for increased current densities, in
comparison to using the transparent film of the first material 920
alone.
[0064] It is not necessary to cap the device. The cells may be
wired up by overlaying the electrodes during patterning and
deposition. However, if capping or encapsulating is desired, any
number of techniques may be used. For example, a coating (e.g.
epoxy) may be added after the electrodes are wired. Preferably, the
processes used to deposit and pattern any capping material is
selected to minimize the thermal and reactive impact on the organic
layers 950.
[0065] As described above, additional layers not illustrated in the
figures may also be deposited. For example, a reflective coating
may be deposited over the third conductive material 970; a
smoothing layer (122) may be deposited over the first conductive
material 920; an anti-reflective coating may be added to the bottom
of the substrate 110 and/or or between the substrate 110 and the
first conductor 920; and an exciton blocking layer (156) may be
formed above or below a donor-acceptor heterojunction as part of
the organic layers 950. The third conductive material 970 may serve
as an electrode layer, or may serve as one layer of a compound
electrode.
[0066] The self-aligning deposition steps may be used to create a
tandem cell. The reentrant structures 924 may be made tall enough
(much thicker than the total organic layer thickness in the tandem
cell) to prevent shorts. By iteratively repeating process steps,
additional photoactive regions and one or more intervening charge
recombination layers or charge transfer layers may be added over
the organic layer(s) 950, creating a tandem cell including layers
as discussed above with FIG. 7.
[0067] Similarly, a self-aligning iterative process may be used to
include an intermediate electrode and an additional photoactive
region, as discussed above with FIG. 8, utilizing the self-aligning
process to form the intermediate electrode so as to not have direct
contact with the reentrant structures 924. However, as opposed to
the normal serial-stacking tandem cell, forming a
parallel-connected tandem cell includes creation of an external
contact to the intermediate electrode. Additional reentrant
structures may optionally be created to contact the intermediate
electrode, depending on the thickness and conductivity of the
material used for this intermediate electrode.
[0068] Conventionally, the first conductive material 920 becomes
the anode and the third conductive material 970 becomes the cathode
in the finished device. However, an inverted device may be formed
by the same process, with the anode on top and the cathode on the
bottom. Although described above in the anode-context, smoothing
layer 122 may be included in an inverted device to smooth the
cathode.
[0069] As described above, organic photosensitive devices of the
present invention may be used to generate electrical power from
incident electromagnetic radiation (e.g., photovoltaic devices) or
may be used to detect incident electromagnetic radiation (e.g., a
photodetector or photoconductor cell).
[0070] Specific examples of the invention are illustrated and/or
described herein. However, it will be appreciated that
modifications and variations of the invention are covered by the
above teachings and within the purview of the appended claims
without departing from the spirit and scope of the invention.
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