U.S. patent application number 12/581037 was filed with the patent office on 2010-02-18 for controlled growth of larger heterojunction interface area for organic photosensitive devices.
This patent application is currently assigned to THE TRUSTEES OF THE PRINCETON UNIVERSITY. Invention is credited to Stephen R. Forrest, Fan Yang.
Application Number | 20100041177 12/581037 |
Document ID | / |
Family ID | 38948342 |
Filed Date | 2010-02-18 |
United States Patent
Application |
20100041177 |
Kind Code |
A1 |
Yang; Fan ; et al. |
February 18, 2010 |
CONTROLLED GROWTH OF LARGER HETEROJUNCTION INTERFACE AREA FOR
ORGANIC PHOTOSENSITIVE DEVICES
Abstract
An optoelectronic device and a method of fabricating a
photosensitive optoelectronic device includes depositing a first
organic semiconductor material on a first electrode to form a
continuous first layer having protrusions, a side of the first
layer opposite the first electrode having a surface area at least
three times greater than an underlying lateral cross-sectional
area; depositing a second organic semiconductor material directly
on the first layer to form a discontinuous second layer, portions
of the first layer remaining exposed; depositing a third organic
semiconductor material directly on the second layer to form a
discontinuous third layer, portions of at least the second layer
remaining exposed; depositing a fourth organic semiconductor
material on the third layer to form a continuous fourth layer,
filling any exposed gaps and recesses in the first, second, and
third layers; and depositing a second electrode on the fourth
layer, wherein at least one of the first electrode and the second
electrode is transparent, and the first and third organic
semiconductor materials are both of a donor-type or an
acceptor-type relative to second and fourth organic semiconductor
materials, which are of the other material type.
Inventors: |
Yang; Fan; (Santa Clara,
CA) ; Forrest; Stephen R.; (Ann Arbor, MI) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
2049 CENTURY PARK EAST, 38th Floor
LOS ANGELES
CA
90067-3208
US
|
Assignee: |
THE TRUSTEES OF THE PRINCETON
UNIVERSITY
Princeton
NJ
THE REGENTS OF THE UNIVERSITY OF MICHIGAN
Ann Arbor
MI
|
Family ID: |
38948342 |
Appl. No.: |
12/581037 |
Filed: |
October 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11483641 |
Jul 11, 2006 |
7638356 |
|
|
12581037 |
|
|
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|
Current U.S.
Class: |
438/82 ;
257/E21.09 |
Current CPC
Class: |
Y02P 70/50 20151101;
H01L 51/0008 20130101; H01L 51/4246 20130101; Y02E 10/549 20130101;
Y02P 70/521 20151101; H01L 51/4253 20130101 |
Class at
Publication: |
438/82 ;
257/E21.09 |
International
Class: |
H01L 51/48 20060101
H01L051/48 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with government support under
contract DE-AC36-98-GO10337 and sub-contract XAT-5-33636-03 awarded
by the U.S. Department of Energy, National Renewable Energy
Laboratory. The government has certain rights in the invention.
Claims
1. A method of fabricating a photosensitive optoelectronic device,
comprising: depositing a first organic semiconductor material on a
first electrode to form a continuous first layer having
protrusions, a side of the first layer opposite the first electrode
having a surface area at least three times greater than an
underlying lateral cross-sectional area; depositing a second
organic semiconductor material directly on the first layer to form
a discontinuous second layer, portions of the first layer remaining
exposed; depositing a third organic semiconductor material directly
on the second layer to form a discontinuous third layer, portions
of at least the second layer remaining exposed; depositing a fourth
organic semiconductor material on the third layer to form a
continuous fourth layer, filling any exposed gaps and recesses in
the first, second, and third layers; and depositing a second
electrode on the fourth layer, wherein at least one of the first
electrode and the second electrode is transparent, and the first
and third organic semiconductor materials are both of a donor-type
or an acceptor-type relative to second and fourth organic
semiconductor materials, which are of the other material type.
2.-24. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation application of U.S.
patent application Ser. No. 11/483,641, entitled "Controlled Growth
of Larger Heterojunction Interface Area for Organic Photosensitive
Devices," filed Jul. 11, 2006, attorney docket number
082007-0140.
JOINT RESEARCH AGREEMENT
[0003] The claimed invention was made by, on behalf of, and/or in
connection with one or more of the following parties to a joint
university-corporation research agreement: Princeton University,
The University of Michigan, 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
[0004] The present invention generally relates to organic
photosensitive optoelectronic devices. More specifically, it is
directed to organic photosensitive optoelectronic devices having
bulk donor-acceptor heterojunctions.
BACKGROUND
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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 (for example, if a first layer is
"on" or "over" a second layer), unless it is specified that the
first layer is "in physical contact with" or "directly on" the
second layer; however, this does not preclude surface treatments
(e.g., exposure of the first layer to ultraviolet-ozone or a
plasma).
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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).
[0014] 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.
[0015] 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.
[0016] As used herein, the term "organic" includes polymeric
materials as well as small molecule organic materials that may be
used to fabricate organic opto-electronic devices. "Small molecule"
refers to any organic material that is not a polymer, and "small
molecules" may actually be quite large. Small molecules may include
repeat units in some circumstances. For example, using a long chain
alkyl group as a substituent does not remove a molecule from the
"small molecule" class. Small molecules may also be incorporated
into polymers, for example as a pendent group on a polymer backbone
or as a part of the backbone. Small molecules may also serve as the
core moiety of a dendrimer, which consists of a series of chemical
shells built on the core moiety. 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.
[0017] 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
[0018] A method of fabricating a photosensitive optoelectronic
device according to an embodiment of the present invention includes
depositing a first organic semiconductor material on a first
electrode to form a continuous first layer having protrusions, a
side of the first layer opposite the first electrode having a
surface area at least three times greater than an underlying
lateral cross-sectional area; depositing a second organic
semiconductor material directly on the first layer to form a
discontinuous second layer, portions of the first layer remaining
exposed; depositing a third organic semiconductor material directly
on the second layer to form a discontinuous third layer, portions
of at least the second layer remaining exposed; depositing a fourth
organic semiconductor material on the third layer to form a
continuous fourth layer, filling any exposed gaps and recesses in
the first, second, and third layers; and depositing a second
electrode on the fourth layer, wherein at least one of the first
electrode and the second electrode is transparent, and the first
and third organic semiconductor materials are both of a donor-type
or an acceptor-type relative to second and fourth organic
semiconductor materials, which are of the other material type.
[0019] Portions of the third layer may directly contact the first
layer through gaps in the second layer, and portions of the fourth
layer may directly contact the second layer through gaps in the
third layer.
[0020] Preferably, from any point within the first layer, a
distance to an interface boundary with a layer of an opposite
material type is no more than 2.5 exciton diffusion lengths of the
first organic semiconductor material, and from any point with the
second layer, a distance to an interface boundary with a layer of
an opposite material type is no more than 1.5 exciton diffusion
lengths of the second organic semiconductor material, and from any
point with the third layer, a distance to an interface boundary
with a layer of an opposite material type is no more than 1.5
exciton diffusion lengths of the third organic semiconductor
material, and from any point in the fourth layer, a distance to an
interface boundary with a layer of opposite material type is no
more than 2.5 exciton diffusion lengths of the fourth organic
semiconductor material.
[0021] Preferably, the surface area of the side of the first layer
opposite from the first electrode is at least three times greater
than a surface area of a side of the first layer facing the first
electrode, and/or the side of the first layer opposite the first
electrode may have a surface area at least five times greater than
the underlying lateral cross-sectional area.
[0022] The first organic semiconductor material and the third
organic semiconductor material may be a same material, and the
second organic semiconductor material and the fourth organic
semiconductor material may be a same material. Alternatively, the
first organic semiconductor material and the third organic
semiconductor material may be a different material, and the second
organic semiconductor material and the fourth organic semiconductor
material may be a different material. Or the first and the third
may be the same, and the second and fourth may be different. Or the
first and the third may be different, and the second and fourth may
be the same.
[0023] Depositing the first organic semiconductor material may
include delivery of a vapor of the first organic semiconductor
material in an inert carrier gas. Preferably, the first electrode
has an exposed surface having a root mean square roughness of at
least 30 nm and a height variation of at least 200 nm.
[0024] A photosensitive optoelectronic device according to an
embodiment of the present invention includes a first electrode and
a second electrode and a plurality of organic photoactive layers
disposed between the first electrode and the second electrode. At
least one of the first electrode and the second electrode being
transparent. The plurality of organic photoactive layers include a
first layer consisting essentially of a first organic semiconductor
material, the first layer being continuous and having protrusions,
a side of the first layer that faces a second layer having a
surface area at least three times greater than a lateral
cross-sectional area; the second layer consisting essentially of a
second organic semiconductor material, the second layer being
discontinuous and in direct contact with the first layer, portions
of the first layer coinciding with gaps in the second layer; a
third layer consisting essentially of a third organic semiconductor
material, the third layer being discontinuous and in direct contact
with the second layer, portions of the second layer coinciding with
gaps in the third layer; and a fourth layer consisting essentially
of a fourth organic semiconductor material, the fourth layer being
continuous and on the third layer, the fourth layer filling gaps
and recesses in the other organic photoactive layers if the gaps
and recesses are exposed to the fourth layer. The first and third
organic semiconductor materials are both of a donor-type or an
acceptor-type relative to second and fourth organic semiconductor
materials, which are of the other material type.
[0025] Portions of the third layer may be in direct contact with
the first layer through gaps in the second layer, and portions of
the fourth layer may be in direct contact with the second layer
through gaps in the third layer.
[0026] Preferably, from any point within the first layer, a
distance to an interface boundary with a layer of an opposite
material type is no more than 2.5 exciton diffusion lengths of the
first organic semiconductor material; from any point with the
second layer, a distance to an interface boundary with a layer of
an opposite material type is no more than 1.5 exciton diffusion
lengths of the second organic semiconductor material; from any
point with the third layer, a distance to an interface boundary
with a layer of an opposite material type is no more than 1.5
exciton diffusion lengths of the third organic semiconductor
material; and from any point in the fourth layer, a distance to an
interface boundary with a layer of opposite material type is no
more than 2.5 exciton diffusion lengths of the fourth organic
semiconductor material.
[0027] The surface area of the side of the first layer facing the
second layer is preferably at least three times greater than a
surface area of an opposite side of the first layer, and/or the
side of the first layer that faces the second layer having a
surface area at least five times greater than a lateral
cross-sectional area.
[0028] The first organic semiconductor material and the third
organic semiconductor material may be a same material, and the
second organic semiconductor material and the fourth organic
semiconductor material may be a same material. Alternatively, the
first organic semiconductor material and the third organic
semiconductor material may be a different material, and the second
organic semiconductor material and the fourth organic semiconductor
material may be a different material. Or the first and the third
may be the same, and the second and fourth may be different. Or the
first and the third may be different, and the second and fourth may
be the same.
[0029] One of the first electrode and the second electrode may be
an anode and the other may be a cathode. Among the plurality of
organic photoactive layers, a respective organic semiconductor
material of each donor-type layer that is closer to the anode
preferably has a HOMO no more than 0.026 eV lower than that of an
adjacent donor-type layer further from the anode, and a respective
organic semiconductor material of each acceptor-type layer that is
closer to the cathode preferably has a LUMO no more than 0.026 eV
higher than that of an adjacent acceptor-type material further from
cathode. More preferably, among the plurality of organic
photoactive layers, a respective organic semiconductor material of
each donor-type layer that is closer to the anode has a HOMO that
is equal to or higher than that of an adjacent donor-type layer
further from the anode, and a respective organic semiconductor
material of each acceptor-type layer that is closer to the cathode
has a LUMO that is equal to or lower than that of an adjacent
acceptor-type material further from the cathode.
[0030] Preferably, the first electrode may have an upper surface
having a root mean square roughness of at least 30 nm and a height
variation of at least 200 nm.
[0031] The plurality of organic photoactive layers may further
include a fifth layer and a sixth layer between the third layer and
the fourth layer. The fifth layer consists essentially of a fifth
organic semiconductor material, the fifth layer being discontinuous
and in direct contact with the third layer, portions of the third
layer coinciding with gaps in the fifth layer. The sixth layer
consists essentially of a sixth organic semiconductor material, the
sixth layer being discontinuous and in direct contact with the
fifth layer, portions of the fifth layer coinciding with gaps in
the sixth layer. The fifth organic semiconductor material is of
same material type as the first and third organic semiconductor
materials, the sixth organic semiconductor material being of the
other material type.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIG. 1 is an energy level diagram illustrating a
donor-acceptor heterojunction.
[0033] FIG. 2 illustrates an organic photosensitive device
including a donor-acceptor heterojunction.
[0034] FIG. 3 illustrates a donor-acceptor bilayer forming a planar
heterojunction.
[0035] FIG. 4 illustrates a hybrid heterojunction including a mixed
heterojunction between a donor layer and an acceptor layer.
[0036] FIG. 5 illustrates a bulk heterojunction.
[0037] FIG. 6 illustrates an organic photosensitive device
including a Schottky-barrier heterojunction.
[0038] FIG. 7 illustrates tandem photosensitive cells in
series.
[0039] FIG. 8 illustrates tandem photosensitive cells in
parallel.
[0040] FIGS. 9A-9H illustrate a method of forming a bulk
heterojunction including a plurality of discontinuous donor and
acceptor layers in accordance with an embodiment of the present
invention.
[0041] FIGS. 10A and 10B demonstrate formation of discontinuous
layers by organic vapor phase deposition.
[0042] FIG. 11 demonstrates formation of a discontinuous layer by
organic vapor jet deposition.
[0043] The figures are not necessarily drawn to scale.
DETAILED DESCRIPTION
[0044] 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.
[0045] 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.
[0046] 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 layer is
said to be "transparent" if it transmits at least 50% of the
ambient electromagnetic radiation in a relevant wavelength.
[0047] 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.
[0048] 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.
[0049] 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).
[0050] Organic layers may be fabricated using vacuum deposition,
spin coating, organic vapor-phase deposition, inkjet printing,
organic vapor jet printing and other methods known in the art.
[0051] 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).
[0052] 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.
[0053] 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, Published
U.S. Patent Application 2005/0224113 A1, entitled "High efficiency
organic photovoltaic cells employing hybridized mixed-planar
heterojunctions" by Jiangeng Xue et al., published Oct. 13, 2005,
is hereby incorporated by reference.
[0054] 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.
[0055] 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.
[0056] The devices as illustrated may be 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.
[0057] 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.;
Published U.S. Patent Application 2006-0032529 A1, entitled
"Organic Photosensitive Devices" by Rand et al., published Feb. 16,
2006; and Published U.S. Patent Application 2006-0027802 A1,
entitled "Stacked Organic Photosensitive Devices" by Forrest et
al., published Feb. 9, 2006; 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.
[0058] 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.
[0059] In each of the devices described above, layers may be
omitted, such as the smoothing layer and 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 Published US Patent
Application No. 2005-0266218 A1, entitled "Aperiodic dielectric
multilayer stack" by Peumans et al., published Dec. 1, 2005, 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.
[0060] Efforts have focused on the use of organic photovoltaic
cells to achieve acceptable photovoltaic conversion efficiencies
with economical production costs. Efficient dissociation of
excitons in organic materials occurs under strong electric fields,
or at a donor-acceptor heterojunction where the differences in the
electron affinities and ionization potentials between the
contacting organic materials are sufficiently large to overcome the
exciton binding energy. The latter mechanism has been employed to
form donor-acceptor heterojunction photovoltaic cells with a power
conversion efficiency .eta..sub.P.about.1%, and is limited by
exciton diffusion lengths (L.sub.D.about.10-40 nm) being much
shorter than the optical absorption length (L.sub.A.about.100 nm).
The organic materials used for solar cells usually absorb more than
90% of the incident light with a thickness of 100 nm.
[0061] Excitons generated within an exciton diffusion length of a
donor-acceptor heterojunction have a high probability of
disassociating efficiently at the heterojunction. Conversely,
excitons generated more than an exciton diffusion length from the
donor-acceptor heterojunction generally are unlikely to
disassociate efficiently and to contribute to photocurrent.
[0062] As described above with FIG. 5, bulk heterojunctions were
developed in an effort to maximize the proximity a donor-acceptor
heterojunction to an exciton. Bulk heterojunctions preferably have
a highly interfolded or inter-percolated network of donor and
acceptor materials such that an exciton generated by the absorption
of incident radiation is likely close to a heterojunction, and
therefore, is likely to contribute to photocurrent.
[0063] Early bulk heterojunctions were fabricated by spin coating a
mixture of soluble versions of the donor and acceptor materials,
followed by phase segregation of the mixture by high temperature
annealing. During the spin coating and solvent evaporation, the
donor and acceptor materials phase separate, creating an intricate
inter-penetrating network with a large interfacial area between the
two phases. The morphology of the resulting structure was
controlled by changing the spin conditions, solvents, and relative
material concentrations. Although bulk heterojunctions formed by
spin-coat methods exhibited improved power conversion efficiencies
over traditional bilayer designs, the bulk devices exhibited a high
series resistance due to the disordered structure of the interface
layer. Phase separation during spin coating and annealing-induced
phase segregation are both thermodynamically driven methods
characterized by randomly structured inter-digitation between the
donor and acceptor layers due to the entropy of the interface
formation process.
[0064] The performance of bulk devices was improved by switching
from spin coating to organic vapor phase deposition (OVPD), as
described in U.S. Patent Application Publication 2005/0227390 A1 to
Shtein et al. entitled "Method of Fabricating An Optoelectronic
Device Having A Bulk Heterojunction", and in "Controlled growth of
a molecular bulk heterojunction photovoltaic cell" by Yang et al.,
Nature Materials Vol. 4, 37-41 (2005), both of which are
incorporated herein by reference.
[0065] OVPD is inherently different from the widely used vacuum
thermal evaporation (VTE) in that OVPD uses a carrier gas to
transport vapors into a deposition chamber. Spatially separating
the functions of evaporation and transport leads to precise control
over the deposition process, and enabling control over the organic
surface morphology, e.g., flat with smooth surface or layers with
protrusions. Another feature of OVPD, compared with VTE, is the
large molecular surface diffusivity and the non-ballistic
trajectories followed by the molecules in their arrival at the
surface. OVPD is particularly effective at filling preexisting
voids and other surface non-uniformities, whereas VTE is
ineffective due to the long mean free paths and ballistic
trajectories followed by incident molecules.
[0066] At typical deposition conditions used in OVPD, the flow of
the carrier gas around the substrate creates a hydrodynamic
boundary layer where molecular transport is diffusion-limited. The
deposition rate, deposition efficiency, and film morphology are
controlled by adjusting the organic species concentration, flow
hydrodynamics, and surface diffusivity. Utilizing OVPD,
ordered-bulk heterojunctions grown using small molecule materials
have exhibited improved series resistance over earlier spin-coated
bulk heterojunction designs.
[0067] In addition to improvements in carrier mobility, series
resistance, and overall efficiency over spin-coat designs, the
ordered nature of OVPD-grown bulk heterojunctions can eliminate the
occurrence of pockets of donor and acceptor material not
electrically connected by a percolation pathway to an
electrode.
[0068] However, the absorption efficiency of OVPD-grown bulk
heterojunctions is spatially limited. In general, the absorption
characteristics of a heterojunction are maximized by selecting
donor materials and acceptor materials with different absorption
spectra. If an incident photon has a wavelength near an absorption
peak of the first material but not the second material, and the
incident photon transits through the bulk heterojunction
predominantly via the second material (e.g., passing down the
length of a "finger" of the second material), there is a reduced
likelihood that the photon will contribute to photocurrent.
[0069] It would be beneficial to retain the advantages of an
ordered bulk heterojunction, such as the short distances for
excitons to travel before disassociation, while further increasing
photon-to-exciton conversion and increasing the donor-acceptor
interface area where excitons disassociate.
[0070] A new structure improving upon the efficiency of existing
bulk heterojunction designs includes a plurality of alternating
discontinuous donor and acceptor layers embedded within a bulk
heterojunction. By maximizing the number of discontinuous layers,
the thickness of the structure can be made a multiple of the
absorption length, theoretically enabling the structure to absorb
as much as 90% of incident light of the solar spectrum.
[0071] FIGS. 9A through 9H illustrate construction of an example of
the new structure. The views are of a cross section and are not
drawn to scale. The scale of various features are not proportional,
but are exaggerated for the purpose of demonstration.
[0072] As illustrated in FIG. 9A, the structure begins with
formation of a first layer 951 consisting essentially of a first
organic semiconductor material over a first electrode 920. The
first layer 951 corresponds to one of layer 252 and layer 254 in
FIG. 5. The first layer 951 completely covers the first electrode
920, and includes protrusions 948. The protrusions 948 give the
first layer 951 a surface area at least three times greater than an
underlying cross-sectional area (e.g., the area of the plane of the
underlying substrate 110); or better yet, the top of the first
layer 951 has a surface area at least three times greater than a
bottom of the first layer 951 (i.e., three times greater than the
underlying surface). More preferably, the protrusions 948 give the
first layer 951 a surface area at least five times greater than an
underlying cross-sectional area; or better yet, the top of the
first layer 951 has a surface area at least five times greater than
the bottom of the first layer 951.
[0073] Next, as illustrated in FIG. 9B, a second layer 952a
consisting essentially of a second organic semiconductor material
is formed. The second layer 952a is discontinuous and in direct
contact with the first layer 951. Portions of the first layer 951
remain exposed through the second layer 952a.
[0074] Next, as illustrated in FIG. 9C, a third layer 953a
consisting essentially of a third organic semiconductor material is
formed. The third layer 953a is discontinuous and in direct contact
with the second layer 952a. Portions of the third layer 953a
directly contact the first layer 951 through gaps in the second
layer 952a.
[0075] Odd number organic semiconductor layers of the photoactive
region 950 are of a donor-type or an acceptor type, whereas even
number organic semiconductor layers are of the other type.
Accordingly, the interface between each of layers 951, 952, 953,
and 954 includes at least one donor-acceptor heterojunction.
Similarly, contact points between layers of a same type (e.g.,
951-to-953, 953-to-953, 952-to-952, 952-to-954) provide a
conduction pathway for disassociated charge carriers through the
structure to the respective electrode.
[0076] As shown in FIGS. 9D and 9E, the pattern of alternating
discontinuous layers (e.g., layer 952b of a fourth organic
semiconductor material; layer 953b of a fifth organic semiconductor
material) repeat indefinitely until a desired overall thickness is
reached. Considerations to be balanced in determining how many
discontinuous layers to add include such things as series
resistance, material optical absorption lengths, and whether a
particular optical transparency is desired for the structure (for
example, if designing a tandem structure or a photovoltaic window
film). At some point, the amount of light that will reach a layer
will be small due to the absorption in the preceding layers, such
that further layers would contribute more to series resistance than
photocurrent, thereby reducing overall efficiency.
[0077] Preferably, an even number of discontinuous layers are
formed, as the discontinuous layers are covered by a sixth layer
954 consisting essentially of a sixth organic semiconductor
material, having a material type opposite that of the first layer
951. The sixth layer 954 is continuous, and fills any gaps and
recesses if the gaps and recesses are exposed when the sixth layer
is deposited.
[0078] Thereafter, as illustrated in FIG. 9G, an exciton blocking
layer 956 may be deposited.
[0079] Then, as illustrated in FIG. 9H, a second electrode 970 is
deposited.
[0080] The electrodes 920 and 970 are constructed as described
above (i.e., a single layer or multiple layers; transparent,
semi-transparent, or opaque). At least one of the electrodes 920,
970 is transparent. Electrode 920 can serve as either anode 120, or
as cathode 170, with electrode 970 being the other.
[0081] Instead of electrodes, one or both of electrodes 920 and 970
may be arranged as a charge transfer layer or a charge
recombination zone, such as if the photoactive region 950 serves as
the photoactive region 150, 150' in a tandem device as described
above with FIGS. 7 and 8.
[0082] An exciton blocking layer may be provided between the first
layer 951 and the first electrode 920 (not illustrated). If an
exciton blocking layer is placed between the first electrode 920
and the first layer 951, and the controlled growth method used to
form the protrusions 948 relies on strain (e.g.,
Stranski-Krastanov, discussed further below), then the generation
of interfacial strain between the lower exciton blocking layer and
the first layer 951 is a design consideration when selecting the
materials for the respective layers.
[0083] Interfacial strain is also a design consideration if
including an electrode smoothing layer (e.g., anode smoothing layer
122). However, electrode smoothing layers increase series
resistance. Using the carrier-gas delivery of organic vapor
deposition techniques described herein, a smoothing layer is
generally unnecessary.
[0084] The first layer 951 should be thick enough to assure
continuous coverage of the first electrode 920 (thereby preventing
electrical shorts), and the protrusions 948 should be wide enough
to avoid breakage. At the same time, excess thickness at the base
of the first layer 951 and excessively wide protrusions 948
increase series resistance and decrease the interface surface area.
Preferably, a distance to an interface boundary with a layer of the
opposite material type in the first organic layer 951 is no more
than 2.5 exciton diffusion lengths of the first organic
semiconductor material.
[0085] Similarly, within the continuous "capping" organic
semiconductor layer (i.e., the sixth layer 954), a distance to an
interface boundary with a layer of the opposite material type is
also no more than 2.5 exciton diffusion lengths of the organic
semiconductor material of the layer (e.g., the sixth organic
semiconductor material in FIG. 9H). More preferably, the distance
within the sixth layer 254 to an interface boundary with a layer of
the opposite material type is no more than 1.5 exciton diffusion
lengths.
[0086] The optimal thickness for photoactive region 950 is
estimated to be around 80 nm, if constructing the continuous first
and last layers (251, 254) to each provide the distance of no more
than 2.5 exciton diffusion lengths to an opposite boundary,
constructing the discontinuous layers (252, 253) to each provide
the distance of no more than 1.5 exciton diffusion lengths to the
opposite boundary, and working with donor and acceptor materials
such as CuPc (copper phthalocyanine), PTCBI
(3,4,9,10-perylenetetracarboxylic bis-benzimidazole), and
C.sub.60.
[0087] The structure of each discontinuous organic semiconductor
layer 252, 253 involves a balance between maximizing coverage to
provide volume for photovoltaic conversion and providing sufficient
gaps through the layer to promote conductive pathways, thereby
minimizing series resistance and the occurrence of islands. Too few
pathways increases the series resistance; but too little coverage
reduces the benefit (photoabsorption and interface area) of the
layer. Preferably a distance to an interface boundary with a layer
of the opposite material type in each discontinuous layer 952, 953
is no more than 1.5 exciton diffusion lengths of the respective
organic semiconductor material of the layer. The discontinuous
organic semiconductor layers are preferably arranged so that
portions of a plurality of the discontinuous layers overlap between
each protrusion 948.
[0088] Within the photoactive region 950, each donor layer may be
formed of a same material. However, some or all of the donor layers
may be formed of different materials. If different donor materials
are used, the donor materials should be arranged so that the HOMO
levels are favorable to hole conduction to the anode 120. A donor
layer closer to the anode 120 should have a HOMO level no more than
3 kT lower than an adjacent donor layer further from the anode 120
(k being the Boltzmann constant and T being the operating
temperature). More preferably, a donor layer closer to the anode
120 should have a HOMO level equal to or higher than that of an
adjacent donor layer further from the anode 120.
[0089] Likewise, within the photoactive region 950, each acceptor
layer may be formed of a same material. However, some or all of the
acceptor layers may be formed of different materials. If different
materials are used, the acceptor materials should be arranged so
that the LUMO levels are favorable to electron conduction to the
cathode 170. An acceptor layer closer to the cathode 170 should
have a LUMO level no more than 3 kT higher than an adjacent
acceptor layer further from the cathode 170. More preferably, an
acceptor layer closer to the cathode 170 should have a LUMO level
equal to or lower than that of an adjacent acceptor layer further
from the cathode 170.
[0090] Operating temperatures for organic photosensitive devices
are commonly specified as having an operating range of
T=-40.degree. C. to +100.degree. C., with a nominal operating
temperature approximated as +3000K. Using the nominal operating
temperature, 3 kT=0.078 eV (i.e., 3.times.1.381 E-23(J/K)/1.602
E-19 (J/eV).times.300K).
[0091] Each respective organic semiconductor layer 951, 952, 953,
954 may or may not be doped. The "essential" material of each layer
is whatever component or components define the character of the
respective layer as either donor or acceptor. In other words, the
"essential" material may be a matrix material or materials, a
dopant, or a combination thereof.
[0092] The first organic semiconductor layer 951 may be grown using
any controlled-growth technique, such as those employed to grow
organic bulk heterojunctions.
[0093] Growth of the first organic semiconductor layer 951 can be
initiated with or without a wetting layer of the first organic
semiconductor material. An advantage of using a wetting layer
formed by either OVPD or organic vapor jet deposition (OVJD) is
that both processes enable the reliable and repeatable formation of
a thin conformal layer, making it possible to use rough electrode
materials such as SnO.sub.2 without an appreciable increase in
series resistance and without shorts. There are several advantages
to using a rough electrode material, including lower cost and
increased surface area.
[0094] Cost can be reduced because materials such as SnO.sub.2 are
less expensive than smoother substrate materials (such as indium
tin oxide), and because process steps to smooth the substrate can
be eliminated (e.g., formation of a smoothing layer 122). Surface
area is increased because the conformal nature of the wetting layer
enables the surface roughness to carry through as a feature of the
first organic semiconductor layer 951. Series resistance is lowered
because the conformal layer can reliably be made very thin.
Preferably, the rough electrode has an exposed surface with a root
mean square roughness of at least 30 nm and a height variation of
at least 200 nm.
[0095] Any number of less-expensive transparent conductive oxides
(TCOs) with high surface roughness are available. Examples of other
less-expensive rough transparent conductive oxides include ZnO, and
SnO.sub.2. Preferably, the conductivity of the rough TCO is
increased by doping, such as with aluminum-doped ZnO (ZnO:Al),
antimony-doped SnO.sub.2 (SnO.sub.2:Sb), fluorine-doped ZnO
(ZnO:F), and gallium-doped ZnO (ZnO:Ga). In addition, as an
alternative to TCOs, a transparent rough transparent oxide with
exceptional conductivity properties can be formed from carbon
nanotubes in a small molecule or polymer matrix. If desired, more
expensive rough TCOs may also be used, such as GaO and InGaO.
[0096] Each of vacuum thermal deposition (VTE), organic molecular
beam deposition (OMBD), and inkjet printing have been unreliable
for forming thin (i.e., a few molecules thick) conformal layers
over a large surface areas of a rough electrode material, as known
in the art. Thin layers formed by these methods typically have
uneven thicknesses and are prone to the creation of short circuits
(when an anode directly contacts the acceptor or when a cathode
directly contacts the donor).
[0097] An example of a controlled-growth method by which the
protrusions 948 can be grown is the Stranski-Krastanov
layer-plus-island growth, as described (among other places) in
"Ultrathin Organic Films Grown by Organic Molecular Beam Deposition
and Related Techniques" by Forrest, Chemical Reviews Vol. 97, No.
6, 1793-1896 (1997), which is incorporated herein by reference. A
conformal strained polycrystalline wetting layer is grown to a
thickness where coverage is continuous and the molecules begin to
relax (for most material combinations, this thickness is between 10
and 20 nm). Then the substrate temperature is raised and the
chamber pressure is lowered (relative to conditions used for
conformal growth) to form the protrusions by equilibrium-growth,
taking advantage of the high surface diffusivity possible with OVJD
and OVPD. The increased surface temperature results in molecules
skimming along the surface of the wetting layer until a point of
energy equilibrium is reached (usually toward a middle of one of
the polycrystalline domains where the wetting layer is most
relaxed). The surface energy conditions favor further molecules
bonding to these equilibrium-point nucleation sites, rather than
the wetting layer itself, generating protrusions 948. The higher
the strain in the wetting layer, the narrower the protrusions 948
are for any given material. The resulting protrusions 948 have very
good crystallinity, which is favorable to high carrier mobility and
low series resistance. Protrusions can be grown to heights as long
as a micron, whereas protrusion cross-sections are on the order of
the exciton diffusion length of the material--around fifty
nanometers.
[0098] Another example of a controlled growth process by which
protrusions 948 can be grown is nucleation-by-condensation. Using
OVJD or OVPD, the chamber pressure is increased to cause the
molecules to cluster while still in the gas stream. The surface
temperature of the substrate is lowered to decrease surface
diffusivity, such that the clusters stick where they land. The
result is protrusions having heights up to around one hundred
nanometers, with cross-sections between ten to one hundred
nanometers. The height and width dimensions tend to be similar to
be on order of the exciton diffusion length of the material; for
example, the height and width of the protrusions formed with CuPc
were around 30 to 40 nm. The growth process is easier to control
than the Stranski-Krastanov method, but crystallinity in the
protrusions 948 is reduced (mobility for a same material is
approximately 1/10 that achieved with Stranski-Krastanov). This
method can be performed with or without a conformal wetting layer,
although the wetting layer is preferred (especially with a rough
electrode). Mushroom-shaped protrusions that maximize surface area
are easily formed with this method, since by morphology control,
incoming clusters can be made to clump to the first surface they
contact.
[0099] The discontinuous layers are made discontinuous by selective
(patterned) deposition, by angling the substrate during deposition,
by making the layers very thin (e.g., a few molecules thick), or a
combination thereof. The discontinuous layers may be formed using
most any method, including OVPD, OVJD, VTE, OMBD, or inkjet
printing. However, OVJD and OVPD are preferred, since VTE and
inkjet printing provide imprecise control over film-thickness, and
since OMBD can be prohibitively expensive.
[0100] OVJD can be performed in a same chamber as OVPD (e.g.,
hot-walled chamber, carrier gas delivery, similar pressures) and
provides the highest degree of control. Whereas molecules have
fairly random vectors in OVPD, OVJD delivers collimated jets of
organic vapor and carrier gas (similar to the directional nature of
VTE and OMBD, but having a hydrodynamic flow at the deposition
surface). For a background discussion of OVJD, see U.S. Patent
Application Publication 2004/0048000A1 by Shtein, entitled "Device
and Method For Organic Vapor Jet Deposition," incorporated herein
by reference.
[0101] OVJD and OVPD can be used for the deposition of small
molecule materials, monomers, oligomers, and dendrimers. Since OVJD
and OVPD heat the molecular source, these processes are generally
unsuitable for use with polymers (which thermally decompose when
heated).
[0102] A further consideration when choosing the deposition method
for depositing the discontinuous layers is the desired to avoid
isolated islands of donor/acceptor material that do not contribute
to photocurrent (but do increase series resistance). Selective
deposition (e.g., using a shadow mask placed close to the
substrate; targeted pulsing, angling and/or positioning the
molecular beam or nozzles), and tilting the substrate between
layers all provide a degree of control that can help minimize the
occurrence of islands. Selectivity may also include changing the
position of the substrate relative to the gas flow during the
deposition process.
[0103] Another consideration when choosing the deposition method
for the discontinuous layers is the desire to prevent voids in the
finished device. Although FIGS. 9A-9H present the protrusions 948
as neat columns, this is a design choice: more complex structures
are readily obtainable. If the protrusions are made tall (e.g.,
approaching a micron), the protrusions may twist and bend, forming
covered pockets. Similarly, if in an effort to maximize surface
area, the protrusions are made in mushrooms shapes (i.e., by
morphology control during growth), covered pockets are created that
are at risk for becoming voids in a finished device. While VTE and
OMBD may be used to form the discontinuous layers if the shape of
the protrusions 948 are simple, a shortcoming of both VTE and OMBD
is the poor ability to fill pockets that exist with more complex
protrusions 948. Filling pockets is also a shortcoming of ink-jet
(solvent-based) printing. However, the carrier-gas flow dynamics of
both OVPD and OVJD are able to provide excellent coating and
coverage of such pockets.
[0104] In general, OVPD is less selective than OVJD and is better
at filling voids. However, the flow dynamics of OVJD can be
modified to promote void filling by inter alia lowering the
pressure in the deposition chamber, lowering flow rate of the
carrier gas, moving the target away from the nozzles, and
increasing the surface temperature of the target. While locally,
this makes an OVJD deposition less selective, a high degree of
overall selectivity can be retained by controlling over which areas
of the target the nozzles are activated.
[0105] FIGS. 10A and 10B illustrate examples of selectively
depositing discontinuous layers by OVPD. The deposition chamber
1000 includes an inlet 1001 connected to a gas-delivery head 1003,
and outlets 1002. The platform 1004 on which the substrate 110 is
mounted includes temperature coils 1006 through which fluid is
passed to heat or cool the substrate 110. The platform 1004 is
mounted on a hub 1005 that allows the platform to be raised,
lowered, rotated, and/or tilted. Although not illustrated, the
walls of the deposition chamber 1000 are heated, and other
apparatus may be included to control the temperature of the
substrate (e.g., piezo-electric cooling elements; infrared
heaters).
[0106] In view of the short mean-free path with OVPD in the gas
phase (<1 cm), selectivity can be improved by decreasing the
distance between the substrate 110 and the gas-delivery head 1003.
FIGS. 10A and 10B also demonstrate tilting the substrate to promote
selective deposition and discontinuous coverage.
[0107] FIG. 11 illustrates an example of selectively depositing a
discontinuous layer using OVJD. For OVJD, the jet nozzle 1103 may
be configured to move and/or angle. With the ability to selectively
target and pulse the collimated jet of material, the jet can be
rastered across the substrate. Although shown with only one jet
nozzle, the chamber 1000 can also be configured with a plurality of
jet nozzles 1103 to increase throughput. In addition, as noted
above, a chamber may be configured with both an OVPD head 1003 and
one-or-more OVJD nozzles 1103.
[0108] 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). The materials, structures
and fabrications approaches described herein may have applications
in devices other than organic photovoltaic cells. For example,
other optoelectronic devices such as organic photodetectors,
organic photosensors, and organic light-emitting diodes may employ
the materials, structures and fabrication approaches. More
generally, organic devices, such as organic transistors, may employ
the materials, structures and fabrication approaches
[0109] 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.
* * * * *