Cross-Reference to Related Applications
Contractual Origin
[0002] The United States Government has rights in this invention under Contract No. DE-AC36-08GO28308
between United States Department of Energy and the Alliance for Sustainable Energy,
LLC, the Manager and Operator of the National Renewable Energy Laboratory.
Background
[0003] Optoelectronic devices are an increasingly central part of everyday life. Smart phones,
computers, televisions, handheld electronics, radio-frequency ID tags (RFIDs), 'smart'
appliances, photovoltaic devices, and more, include such optoelectronic devices. Examples
of such devices include: displays, such as liquid crystal displays (LCDs) and organic
light emitting diode displays (OLEDs); photovoltaic (PV) devices, including crystalline
silicon, inorganic thin-film, and organic photovoltaic (OPV); and field-effect transistors
(FETs), which are a key element in many electronic devices. The trend is to reduce
the size and/or cost of these optoelectronic devices in order to enable widespread
commercial adoption. Once low enough thresholds are reached for size and/or cost,
such devices are expected to become nearly ubiquitous in everyday life.
[0004] In order to make these devices more cost-effective, techniques that allow high-throughput
large-area manufacturing are needed to reduce the cost per unit device to reasonable
levels. While optoelectronic devices are diverse, and thus the materials and manufacturing
techniques involved vary quite a bit, there are a number of common elements in a variety
of optoelectronic devices. Many such devices require the controlled transport of electrons
and/or holes (i.e., electron vacancies) into or out of the device, in order to precisely
control the flow (e.g., in FETs), separation (e.g., in PV), or recombination (e.g.,
in OLEDs) of such particles in the device, enabling the desired device properties.
The materials used to enable such controlled flow of electrons or holes in a device
are referred to as electron transport layers or hole transport layers (ETLs or HTLs),
respectively. An ETL will allow the transport (flow, collection, or injection, depending
on the device) of electrons, while blocking the transport of holes in a device, while
a HTL will do the opposite.
[0005] While there are a variety of ETL and HTL materials used in the many various types
and versions of optoelectronic devices in existence, many common transport layers
are based upon metal oxide thin films. Metal oxide thin films have a number of advantages
over alternative materials, such as thin polymer films and self-assembled monolayers
(SAMs). Metal oxide thin films are relatively well-studied and understood materials
and are generally physically, thermally, and chemically robust. The variety of metals
that form useable oxides ensure a broad range of such device-important physical properties,
such as n-type or p-type material, work function, conductivity, electron/hole mobility,
optical transparency and reflectivity. In contrast to metal oxides, thin polymer film
transport layers are generally much less well studied and understood materials, often
have low mobilities, which require very thin films (~5 nm) to ensure adequate performance,
and as such often have poor physical robustness. Additionally, thin polymer films
are generally much less thermally stable than metal oxides. Similarly, SAM transport
layers are poorly studied materials, and are not currently well understood. Their
monolayer nature ensures very fragile films with high potential for pinholes/shorts
and often exhibit poor thermal and chemical stability.
[0006] Metal oxide thin films can be produced via a variety of techniques, including: sputtering,
chemical vapour deposition (CVD), pulsed-laser deposition (PLD), atomic layer deposition
(ALD), thermal evaporation, and sol-gel chemistry methods. These techniques share
a common disadvantage in that they either require a vacuum based process to enable
the film deposition or they require subjecting materials to high temperatures for
extended time periods. Vacuum-based process significantly increases the time and cost
of depositing metal oxide thin films, as samples are pumped down to the desired vacuum
levels, the deposition performed, and then the samples returned to atmospheric pressure
levels. High temperature techniques, which often require temperatures in excess of
300 °C, add significant cost due to the high energy demands on obtaining and maintaining
such temperatures. Furthermore, such high temperatures significantly limit the range
of substrates that can be used. For example, temperatures above 150 °C for extended
periods prevent the use of many polymer foils, such as polyethylene terephthalate
(PET) and polyethylene naphthalate (PEN), often used in high-throughput roll-to-roll
manufacturing lines. Additionally, elevated temperatures tend to cause damage to any
other underlying layers exposed to the high temperatures. Meanwhile, nanoparticle
techniques produce materials with diminished transport and hole blocking characteristics
as compared metal oxide thin films produced using the sol-gel or vacuum deposition
methods, and their use is complicated by wetting and aggregation issues that hinder
large-scale production.
[0008] US2012/132272 (A1) discloses a method for forming a thin film charge selective transport layer.
[0009] US2008/032443 (A1) discloses the formation of ZnO film in FET device from Diethylzinc in toluene and
THF at 110°C.
[0010] The foregoing examples of the related art and limitations related therewith are intended
to be illustrative and not exclusive. Other limitations of the related art will become
apparent to those of skill in the art upon a reading of the specification and a study
of the drawings.
Brief Description of the Drawings
[0011] Exemplary embodiments are illustrated in referenced figures of the drawings. It is
intended that the embodiments and figures disclosed herein are to be considered illustrative
rather than limiting. Those skilled in the art will understand that the drawings,
described herein, are for illustration purposes only. The drawings are not intended
to limit the scope of the present disclosure.
Figures 1, and 1A -1E illustrate fabrication of a photovoltaic device having a charge
selective transport layer of one example embodiment of the present disclosure.
Figures 2 and 2A - 2E illustrate methods for producing a charge selective transport
layer of one example embodiment of the present disclosure.
Figure 3 is a graph of current density vs. voltage comparing spin-cast vs. slot-die
coated zinc oxide ETL according to exemplary embodiments disclosed in the present
disclosure.
Figure 4 is a graph illustrating the performance of a photovoltaic module of one embodiment
of the present disclosure.
Detailed Description
[0012] Embodiments of the present disclosure present low temperature solution based methods
according to the claims for fabrication charge selective transport layers for use,
for example, in photovoltaic and other optoelectronic devices. More particularly,
the present disclosure describes methods of generating thin films suitable for use
as charge selective transport layers from precursor solutions. These charge selective
transport layers include both electron transport layers and hole transport layers.
In the various embodiments described below, precursor solutions suitable for forming
charge selective transport layers may be produced by dissolving a metal containing
reactive precursor material into a complexing solvent. The resulting solution is then
deposited and annealed to form either an electron transport layer or a hole transport
layer. As explained below, whether the charge selective transport layer functions
as a hole transport layer or an electron transport layer will depend at least in part
on the composition of the metal containing reactive precursor material and the location
of the resulting material layer within the device.
[0013] As further detailed below, an electron transport layer is a layer formed between
an active layer (such as an active semiconductor hetero-junction layer) and a conductive
layer designed to function as an electron emitting terminal (i.e., a cathode) for
a device. The presence of an electron transport layer serves two functions: it will
have a low enough work function to help provide the built-in field necessary to assist
in charge collection, and it will have a proper energy level to efficiently transport
electrons while blocking holes. For example, a charge selective transport layer having
a work function in the range of 3-4.5 eV would be considered suitable for functioning
as an electron transport layer in most applications. This range is however provided
as a general guideline because how a work function is measured will cause the measured
value to vary. At the same time, one of ordinary skill in the art after reading this
disclosure would readily be able to determine, for their particular application, whether
a resulting material layer has a work function sufficient for providing an electron
transport layer.
[0014] A number of different materials can serve as an electron transport layer, including
but not limited to such metal oxides as zinc oxide (ZnO) and titanium oxide (TiO
x) as well as caesium carbonate (Cs
2CO
3), thin polymer dielectrics such as poly[(9,9-bis(3'-(N,N-dimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)
(PFN), or self-assembled monolayers (SAMs) based on molecules such as N-propyl trimethoxysilane
or aminopropyl triethoxysilane.
[0015] Similarly, and as further detailed below, a hole transport layer is a layer formed
between an active layer and a conductive layer that is designed to function as the
hole emitting terminal (i.e., an anode) for a device. A hole transport layer should
have a high enough work function to help provide the built-in field and have proper
energy levels to efficiently transport holes while blocking electrons. For example,
a charge selective transport layer having a work function in the range of 4.9-6 eV
would be considered suitable for functioning as a hole transport layer in most applications.
This range is however provided as a general guideline because how a work function
is measured will cause the measured value to vary. One of ordinary skill in the art
after reading this disclosure would readily be able determine for their particular
application whether the resulting material layer has a work function sufficient for
providing a hole transport layer. As such, a number of different materials can act
as a hole transport layer, including but not limited to such metal oxides as: molybdenum
oxide (MoO
3), tungsten oxide (WO
3), vanadium oxide (V
2O
5), and nickel oxide (NiO). HTLs may also include doped organic polymeric materials
such as polyethylenedioxythiophene:polystyrene sulfonate (PEDOT:PSS).
[0016] Figures 1 and 1A-1E, are diagrams illustrating fabrication of a device comprising
charge selective transport layers for example embodiments of the present disclosure.
As shown generally at
150, an exemplary structure begins with a substrate
110. The substrate material
110 can be any one or more of a number of substrates suitable for deposition of a conducting
film. In some embodiments, the substrate comprises one or more of a glass, ceramic,
plastic and other organic polymers, semiconductor material, a silicon wafer or other
wafer material, or similar materials. In other embodiments, the substrate itself may
comprise a photovoltaic cell. Examples of organic polymers, like plastics, include,
but are not limited to polyesters such as polyethylene terephthalate (PET) and polyethylene
naphthalate (PEN).
[0017] Referring next to
152, a first conductor layer
112 is deposited onto the substrate
110. The first conductor layer
112 can be any one or more of a number of conducting materials suitable for collecting
charge. In some embodiments, first conductor layer
112 comprises a transparent conductor. For example, such a transparent conductor may
be implemented using a transparent conducting oxide (TCO), which may include one or
more doped metal oxides with considerable conductivity. In one exemplary embodiment,
the first conductor layer
112 comprises a doped TCO such as indium tin oxide (ITO). In other embodiments, other
materials suitable for fabricating a conductor layer may comprise metal oxides including
one or more of many doped metal oxides, including but not limited to: gallium-doped
zinc oxide (GZO), indium-doped zinc oxide (IZO), tin-doped indium-oxide (TIO), aluminium-doped
zinc-oxide (AZO), indium-doped cadmium-oxide, fluorine-doped tin-oxide (FTO), and
combinations thereof.
[0018] In certain embodiments, the first conductor layer
112 can include one or more non-oxide conducting materials, including but not limited
to: silver nanowire or carbon nanotube meshes; continuous graphene sheets or small
overlapping graphene sheets; highly doped organic semiconducting polymers, including
but not limited to poly(ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS);
and TCO nanoparticle films; or any combination thereof.
[0019] In some implementations, the first conductor layer
112 may be employed as a conducting material deposited on top of the substrate
110 (such as shown at
152) or the first conductor layer
112 may be a layer of conductive material embedded within the substrate
110. In still other implementations, the first conductor layer
112 may comprise a layer that is adjacent to the region where the precursor solution
114 is deposited.
[0020] Referring next to
154, a precursor solution
114 is deposited onto one or both of substrate
110 and first conductor layer
112. Precursor solution
114 is deposited in such a way as to facilitate the electrical coupling of what will
become the electron transport layer to the first conductor layer
112. Some embodiments may optionally include one or more intervening layers (such as a
conducting buffer material layer) between the deposited precursor solution
114 and the first conductor layer
112. In such embodiments, the resulting charge selective transport layer will be electrically
coupled to the first conductor layer through the buffer material layer.
[0021] The precursor solution
114 is a solution that comprises a metal-containing reactive precursor material that
has been mixed with a complexing solvent. As used herein, a metal containing reactive
precursor material is a reactive precursor material which includes compounds having
a metal atom, M, which is bonded to one or more species, X. The nature of the M-X
bond is such that the molecule readily reacts with water and/or oxygen in an ambient
environment in such a way as to convert the M-X bond to a M-O bond, where O represents
an oxygen atom, and the H represents a hydrogen atom. This reaction may be represented
by: M-X
n + H
2O/O
2 → M-O
n(H). The O atom may be, but does not necessarily need to be, bonded to a hydrogen
atom, H. For example, the metal containing reactive precursor may comprise a pyrophoric
precursor material, but it need not be pyrophoric. The complexing solvent serves to
stabilize the reactive metal-containing precursor material, mitigating its reactive
nature, enabling the handling of the precursor solution in ambient environments containing
oxygen and water without compromising the integrity of the precursor material. The
precursor material will not react with oxygen or water, and instead will remain complexed
with the solvent, over an extended period of time (commonly referred to as the 'shelf-life'
of the precursor material). For example, in one embodiment, the precursor solution
114 may contain a pyrophoric precursor material that is stabilized by dissolving it in
a complexing solvent under mild conditions. That is, for such embodiments the precursor
solution
114 can be handled at room temperature under normal atmospheric conditions because the
complexing solvent renders the solution non-pyrophoric. Utilizing such a precursor
solution eliminates the need to provide rigorous oxygen and water-free environments,
significantly reducing costs.
[0022] Precursor solution
114 comprises organozinc. In some embodiments, the reactive metal-containing precursor
material is diethylzinc, while in other cases it is dimethylzinc.
[0023] A complexing solvent, as the term is used herein, is defined as a solvent wherein
the constituent molecules are capable of donating electron density, generally in the
form of an electron lone pair, to an electron deficient molecule, such as in a reactive
metal-containing precursor material, thus stabilizing it. In such a case, the complexing
solvent generally donates the electron density directly to the otherwise electron-deficient
metal atom. The complexing solvent comprises at least one of diethyl ether, tetrahydrofuran,
diglyme, pyridine, acetonitrile, tetramethylethylenediamine.
[0024] Several methods are available for applying a precursor solution onto the preceding
layers of the device that avoids any need for high vacuum and high temperature deposition
techniques. These methods include, but are not limited to: slot-die coating, spin-casting,
drop-casting, dip-coating, knife coating, spray-coating, ink-jet printing, screen
printing, Mayer rod coating, Gravure coating, Flexo printing, or curtain coating.
[0025] For example, in some embodiments precursor solution
114 is deposited onto the substrate
110 and first conductor layer
112 via spin-casting. The thickness of the resulting electron-transport layer
116 can be controlled by adjusting the concentration of the precursor in the complexing
solvent, and by controlling the spin speed. In some embodiments, the precursor solution
114 is deposited onto the substrate
110 and first conductor layer
112 by using a technique that is compatible with high-throughput roll-to-roll manufacturing,
such as slot-die coating. In that case, the thickness of the electron transport layer
116 can be controlled by adjusting the concentration of the precursor in the complexing
solvent, by controlling the web speed, and/or by controlling the flow rate at which
the precursor solution
114 is provided into the process.
[0026] Referring next to
156, the precursor solution
114 is annealed resulting in the formation of an electron transport layer
116, as shown generally at
158. Depending on the nature of the reactive metal-containing precursor material and the
target metal, the annealing step can vary in both temperature and time, but due to
the reactive nature of the precursor material, the conversion process is generally
accomplished at low temperatures that are compatible with a wide variety of substrate
and conductor materials. That is, at the temperatures applied, the annealing does
not result in adverse degradation of the substrate, conductor or any other prior layer.
Both the time and temperature of the annealing can be tailored to the materials utilized.
[0027] The annealing can occur for a period of time ranging from about 1 minute to about
15 minutes. The annealing can occur at a temperature ranging from about 100 °C to
about 120 °C, which is advantageous for use with sensitive substrate materials, such
as polymer substrates in high-throughput roll-to-roll manufacturing conditions, or
in the case of sensitive first conductor materials. It should be considered within
the skill of one of ordinary skill in the art who has studied the teachings of this
disclosure to choose appropriate annealing times and temperatures for the particular
precursor, substrate and conductor layer materials they have selected.
[0028] In Figure 1, the electron transport layer
116 is positioned between the first conductor layer
112 and an active layer
118. Exemplary embodiments of the electron transport layer
116 may have a thickness between 25-200nm. In the laboratory, exemplary embodiments such
as those described herein have shown uniformity of the films produced to be at least
90% over large areas. It should be noted that the resulting electron transport layer
116 in some embodiments will comprise the same base materials as a transparent conductor.
For example, a ZnO electron transport layer may be formed onto a doped ZnO transparent
conductor, such as aluminium-doped zinc oxide (AZO).
[0029] Figures 1A and 1B provide illustrations of alternate implementations of the process
illustrated in Figure 1 where one or both of the precursor depositions shown at
154 and the annealing shown at
156 are performed through two are more interactive steps.
[0030] For example, referring to Figure 1A, an alternate embodiment is illustrated at
170 and
171 where the precursor solution
114 is deposited as a first precursor solution layer
114-1 followed by deposition of at least one subsequent precursor solution layer
114-2. Although Figure 1A illustrates two precursor layers
(114-1 and
114-2), any number of multiple depositions may be performed to form any number of precursor
solution layers. The plurality of precursor solution layers are then annealed at
156 to form the electron transport layer
116.
[0031] In Figure 1B another embodiment is illustrated where precursor deposition and annealing
steps are alternated to form electron transport layer
116. That is, a first precursor solution layer
114-1 is deposited as shown generally at
175. This is followed by an annealing step (shown at
176), which forms a first layer
116-1 of material for the electron transport layer
116 (shown at
177). Then a second precursor solution layer
114-2 is deposited on top of the previously formed electron transport material
116-1 as shown generally at
178. This is followed by another annealing step (shown at
179), which forms a second layer
116-2 of material for the electron transport layer
116 (shown at
180). Multiple iterations of these alternating deposition and annealing steps may be performed
until an electron transport layer
116 of the desired thickness is achieved.
[0032] Forming the electron transport layer
116 through multiple depositions of precursor material, as shown by either Figures 1A
or 1B, has the benefit of avoiding pinhole voids that otherwise may form and penetrate
completely through the electron transport layer
116. Conductive materials from later applied layers can enter these pinholes degrading
the effectiveness of electron transport layer
116 in blocking hole transport. When such pinholes occur, they do so with a random distribution
with respect to the upper surface of the layer
116. Deposition of layer
116 in multiple passes (either as shown in Figure 1A or 1B) results in different sub-layers
of the electron transport layer
116 with different randomly located pinholes, substantially reducing the probability
of any pinhole completely penetrating through the entirety of electron transport layer
116.
[0033] As shown in Figure 1 at
160, once formation of the electron transport layer
116 is completed, subsequent layers may be deposited based on the desired function of
the completed device. In some embodiments, the resulting device will further include
an active layer
118 and a second conductor layer
120. It may also include an optional second charge selective transport layer that functions
as a hole transport layer
126. To complete fabrication, in one embodiment a first conductive lead
124 is coupled to the first conductor layer
112 and a second conductive lead
122 coupled to the second conductor layer
120.
[0034] Depending on the type of device being implemented, the active layer
118 may comprise an appropriate semiconductor junction. For example, for an organic photovoltaic
device (OPV) active layer 118 may comprise a heterojunction, such as a bulk or planar
heterojunction, for example. For an OPV device, the materials used to fabricate active
layer
118 may include, but are not limited to, poly(3-hexylthiophene):phenyl C
61 butyric acid methyl ester (P3HT:PCBM) blends, PCPDTBT:PCBM blends, Si-PCPDTBT:PCBM
blends, PCDTBT:PCBM blends, PTB7:PC
71BM blends, or any combinations thereof. The second conductor layer
120 can be any one or more of a number of conducting materials suitable for collecting
charge such as those mentioned for 1
st conducting layer
112. Further, either of the conductor layers
112 and
120 may comprise one or more metals including, but not limited to magnesium, silver,
gold, aluminium, copper, palladium, cadmium, nickel, or zinc. Other optoelectronic
devices may be implemented such as, but not limited to: inorganic thin-film PV devices
and modules, organic or inorganic light-emitting diodes (LEDs), and organic or inorganic
field-effect transistors (FETs).
[0035] As mentioned above, the embodiment shown in Figure 1 at
160, further comprises a hole-transport layer
126 positioned between the active layer
118 and the second conductor layer
120. This second charge selective transport layer may be fabricated in the same way as
described above for an electron transport layer, but using a second precursor solution
having a different selection of metal containing reactive precursor material and a
complexing solvent appropriate for that material. For example, a precursor solution
for a hole transport layer may be prepared by dissolving a metal containing reactive
precursor material, such as one containing nickel. This precursor solution may be
deposited onto an active layer and annealed into a nickel oxide (NiO) hole transport
layer. A number of other materials can also be used to provide a hole transport layer,
including but not limited to such metal oxides as: molybdenum oxide (MoO
3), tungsten oxide (WO
3), and vanadium oxide (V
2O
5).
[0036] As illustrated in Figure 1C, after deposition of the active layer
118 (shown at
161), the second precursor solution
121 is deposited onto the surface of the active layer
118 (shown at
162). Any of the solution deposition methods described above for the first precursor material
114 may be utilized to deposit the second precursor material
121, again avoiding the need for high vacuum or high temperature deposition techniques
to create this layer. The second precursor solution
121 is then annealed (shown at
163) to form a hole transport layer
126 (shown at
164). The second conductor layer
120 would then be deposited on the hole transport layer
164 and used to electrically couple an anode for the device to the active layer
118.
[0037] Figures 1D and 1E provide illustrations of alternate implementations of the process
illustrated in Figure 1C where one or both of the second precursor depositions shown
at
162 and the annealing shown at
163 are performed through two or more interactive steps.
[0038] For example, referring to Figure 1D, an alternate embodiment is illustrated at
165 and
166 where the precursor solution
121 is deposited as a first precursor solution layer
121-1 followed by deposition of at least one subsequent precursor solution layer
121-2. Although Figure 1D illustrates two precursor layers
(121-1 and
121-2), any number of multiple depositions may be performed to form any number of precursor
solution layers. The plurality of precursor solution layer are then annealed at
163 to form the hole transport layer
126.
[0039] In Figure 1E another embodiment is illustrated where precursor deposition and annealing
steps are alternated to form hole transport layer
126. That is, a first layer of the second precursor solution
121-1 is deposited as shown generally at
194. This is followed by an annealing step (shown at
195), which forms a first layer
126-1 of material that will form the hole transport layer
126 (shown at
196). Then a second layer of the second precursor solution
121-2 is deposited on top of the previously formed hole transport material
126-1 as shown generally at
197. This is followed by another annealing step (shown at
198), which forms a second layer
126-2 of material for the hole transport layer
126 (shown at
199). Multiple iterations of these alternating deposition and annealing steps may be performed
until a hole transport layer
116 of the desired thickness is achieved.
[0040] As discussed with respect to the electron transport layer
116, forming the hole transport layer
126 through multiple depositions of precursor material, as shown by either Figures 1D
or 1E, has the benefit of avoiding pinhole voids that otherwise may form and penetrate
completely through the hole transport layer
126. Deposition of layer
126 in multiple passes (either as shown in Figure 1D or 1E) results in different sub-layers
of the hole transport layer
126 with different randomly located pinholes, substantially reducing the probability
of any pinhole completely penetrating through the entirety of hole transport layer
126.
[0041] In different embodiments, the ETL and HTL layers produced as described above can
be used in other optoelectronic devices, including but not limited to: inorganic thin-film
PV devices and modules, organic or inorganic light-emitting diodes (LEDs), and organic
or inorganic field-effect transistors (PETs).
[0042] Figure 2 is flow chart illustrating a method
200 for fabricating a charge selective transport layer via one embodiment of the present
disclosure. In one embodiment, the method of Figure 2 is utilized in conjunction with
fabrication of a photovoltaic device such as the one described above with respect
to any of Figures 1 and 1A-1E.
[0043] Method
200 begins at
210 with providing a precursor solution according to the claims that comprises a metal
containing reactive precursor material dissolved into a complexing solvent. As explained
above, the metal containing reactive precursor material is a reactive precursor material
which includes compounds having a metal atom, M, which is bonded to one or more species,
X. The nature of the M-X bond is such that the molecule readily reacts with water
and/or oxygen in an ambient environment to convert the M-X bond to a M-O bond. The
O atom may be, but does not necessarily need to be, bonded to a hydrogen atom, H.
For example, the metal containing reactive precursor may comprise a pyrophoric precursor
material, but it need not be pyrophoric. Also as described above, the complexing solvent
is a solvent where constituent molecules are capable of donating electron density,
generally in the form of an electron lone pair, to an electron deficient molecule.
Examples of complexing solvents which may be used at
210 include, diethyl ether, tetrahydrofuran (THF), and diglyme, pyridine, acetonitrile,
tetramethylethylenediamine. Such combinations of a metal containing reactive precursor
material with a complexing solvent may be used to form the precursor solution
114 shown in Figure 1.
[0044] In one example embodiment, a ZnO electron transport layer is fabricated from a metal
containing reactive precursor material such as diethylzinc dissolved into the complexing
solvent THF. In one implementation of such an embodiment, a 10 mL of a 15 wt. % solution
of diethylzinc in toluene is mixed with 50 mL of THF in the absence of water and oxygen
to produce a precursor solution of 2.5 wt. % diethylzinc/THF/toluene solution. In
one alternate implementation, the diethylzinc can be mixed directly with THF, without
the use of a toluene solution, in the absence of water and oxygen, to produce a precursor
solution of 2.5 wt. % diethylzinc/THF solution. Either of such precursor solutions
may be used, for example, for formulating precursor solution
114.
[0045] Method
200 proceeds to
220 with depositing the precursor solution onto a surface of a substrate to form a film.
In one embodiment, the film at least in part contacts a first conductor. As discussed
above, the first conductor's ultimate purpose is to conduct electrons received from
the electron transport layer that will be created from the precursor solution. As
such, the precursor solution is deposited in such a way as to facilitate the electrical
coupling of the electron transport layer with the first conductor. In one embodiment,
the first conductor comprises a conductor layer deposited on top of a substrate surface.
Figure 1 illustrates one such embodiment at
154 where first conductor layer
112 is deposited on top of substrate
110. In other implementations, the conductor layer may be a layer of conductive material
embedded within the substrate. In still other implementations, the first conductor
may comprise a layer that is adjacent to the region where the precursor solution is
deposited. Further, in some embodiments, one or more intervening layers may exist
between the deposited precursor solution
114 and the first conductor. However, in such embodiments, the one or more intervening
layers either are, or will become, conductive layers that will establish electrical
coupling between the electron transport layer and the first conductor. For example,
the one or more intervening layers may comprise a buffer layer that indirectly couples
the electron transport layer to the first conductor.
[0046] Examples of deposition methods which may be used at
220 include, but are not limited to, slot-die coating, spin-casting, drop-casting, dip-coating,
knife coating (also known as doctor blading), spray-coating, ink-jet printing, screen
printing, Mayer rod coating (also known as metering rod coating), Gravure coating,
Flexo printing, and curtain coating. The particular method used may be selected based
on its compatibility with other manufacturing processes being used. For example, in
one implementation, the precursor solution is deposited using a technique that is
compatible with high-throughput roll-to-roll manufacturing, such as slot-die coating.
In that case, the thickness of the electron transport layer can be controlled, for
example, by varying the concentration of the metal containing reactive precursor material
mixed with the complexing solvent, by controlling the web speed and/or the flow rate
at which the precursor solution is applied to the substrate.
[0047] Method 200 proceeds to
230 with annealing the film to transform the precursor film into an electron transport
layer. Such annealing is represented in
Figure 1 generally at
156 to form the electron transport layer
116 shown generally at
158 and
160. Both the time and temperature of annealing at
230 may be controlled so as to not adversely affect the substrate material or other previously
deposited materials. Depending on the particular material used, typical annealing
can be achieved within a period of time ranging from about 1 minute to about 15 minutes
and temperatures ranging from about 100 °C to about 120 °C. Such duration and temperatures
would, for example, be advantageous for use with substrates, such as polymer substrates
used in high-throughput roll-to-roll manufacturing conditions.
[0048] In the above-mentioned embodiment where method
200 is utilized to produce a ZnO electron transport layer, the depositing performed at
220 may comprise slot-die coating of the precursor solution. For example, in one implementation,
the precursor solution produced from the diethylzinc and THF is slot-die coated onto
the substrate at room temperature, in normal ambient atmosphere, to form a film. The
resulting thin film, which will include zinc oxides and hydroxides, is annealed at
230, at a temperature of 100 °C for 5 minutes to produce an electron transport layer comprising
a ZnO thin film. Slot-die coating of such ZnO thin films from stabilized diethylzinc
solutions on substrates as large as 6" x 6" have produced very high quality thin and
uniform films, which demonstrates the potential for very large area roll-to-roll fabrication.
Optical profilometry of such films show very smooth films with root-mean-squared surface
roughness (R
q) values of ~ 1.65 nm. The thickness of the ZnO thin film electron transport layer
can be varied depending on the precursor concentration (1-10 wt.%), solution flow
rate (0.1-5 mL/min), and coating speed (0.5-5 m/min), with typical thickness values
of 25-200 nm, as determined via stylus profilometry. The uniformity of the films produced
has been estimated via UV-visible absorption mapping to be as high as 90% over large
areas. The work function of the ZnO produced has been measured to be between 4.0 and
4.5 eV by Kelvin probe (referenced to a gold film).
[0049] Forming the electron transport layer
116 through multiple depositions of precursor material, as shown by either Figures 1A
or 1B, has the benefit of avoiding pinhole voids that otherwise may form and penetrate
completely through the electron transport layer
116. Conductive materials from later applied layers can enter these pinholes degrading
the effectiveness of electron transport layer
116 in blocking hole transport. When such pinholes occur, they do so with a random distribution
with respect to the upper surface of the layer
116. Deposition of layer
116 in multiple passes (either as shown in Figure 1A or 1B) results in different sub-layers
of the electron transport layer
116 with different randomly located pinholes, substantially reducing the probability
of any pinhole completely penetrating through the entirety of electron transport layer
116.
[0050] Figures 2A and 2B provide illustrations of alternate implementations of the process
illustrated in Figure 2 where one or both of the precursor deposition shown at
220 and the annealing shown at
230 are performed through two are more interactive steps. Forming a charge selective
transport layer through multiple depositions of precursor material, as shown by either
Figures 2A or 2B or elsewhere in this description, has the benefit of avoiding pinhole
voids that otherwise may form an penetrate completely through the electron transport
layer for the reasons previously described above.
[0051] For example, referring to Figure 2A, an alternate embodiment is illustrated at
202 where the deposit of block
220 and annealing at block
230 are further defined. In this embodiment,
221 comprises depositing a precursor solution onto a surface of a substrate to form a
first film layer, where the first film at least in part contracts a 1
st conductor. The process then proceeds to
222 with depositing the precursor solution onto a surface of the first film to form a
second film layer. The process then proceeds to
231 with annealing the first film layer and the second film layer to transform the first
film layer and second film layer into an electron transport layer. It would be appreciated
that depositing one or more additional layers of the precursor solution onto the previously
deposited precursor solutions may be performed before proceeding to
231. The plurality of precursor solution layers are then annealed at
231 to form the electron transport layer.
[0052] In Figure 2B another embodiment is illustrated where precursor deposition (block
220) and annealing (block
230) steps are alternated to form the electron transport layer. That is, a first deposition
is shown generally at
235 with depositing a precursor solution onto a surface of a substrate to form a first
film layer. This deposition may lay the first film so that it at least in part contracts
a 1
st conductor, or it may be deposited on an intervening layer that provides a conductive
path to the 1
st conductor. The method proceeds with annealing the first film layer to transform the
first film layer into a first electron transport layer at
235. Then another layer of precursor solution is applied and annealed. At
226, the process proceeds with depositing a precursor solution onto a surface of the first
electron transport layer to form a second film layer and then to
236 with annealing the second film layer to transform the second film layer and the first
electron transport layer into single electron transport layer. Multiple iterations
of these alternating deposition and annealing steps may be performed until an electron
transport layer of the desired thickness is achieved.
[0053] Also as shown in
Figure 2, method
200 may further comprise one or more additional steps to achieve various optional or
alternate embodiments for specific applications. For example, in one embodiment, method
200 proceeds to
240 with forming an active layer on the electron transport layer. The particular materials
for forming the active layer are selected based on the desired function of the device
as already described above for active layer
118 and apply to this method, and as such are not repeated here in detail.
[0054] Regardless of the particular application, the placement of the electron transport
layer between the active layer and the
first conductor will result in a device where electrons are permitted to flow from the
active layer to the
first conductor, but holes are not, so that an electrical lead coupled to the
first conductor will function as the cathode lead for the device (such as mentioned at
250). In one embodiment (illustrated at
247), a second conductor is electrically coupled to the active layer such that an electrical
lead coupled to the second conductor will function as the anode lead for the device
(such as mentioned at
250). Further, in one embodiment illustrated by
245, a hole transport layer is formed on the active layer prior to the second conductor
(such as hole transport layer
126 shown in
Figure 1). Analogous in function to the electron transport layer, the placement of a hole transport
layer between the active layer and the second conductor will result in a device where
hole current is permitted to flow from the active layer to the second conductor, but
electrons are not.
[0055] As shown in Figure 2C, the forming of the hole transport layer is consistent with
the process described above with respect to Figures 1 and 1A-1E. In one embodiment,
the step at
260 comprises depositing a second precursor solution onto a surface of an active layer
to form a film. This is followed at
261 with annealing the film to transform the film into a hole transport layer. Also as
shown in Figures 2D and 2E, the deposition and annealing can be subdivided into interactive
steps.
[0056] In Figure 2D at
208, the process may comprise depositing a second precursor solution onto a surface of
an active layer to form a third film layer (at
262), depositing the second precursor solution onto a surface of the third film layer to
form a fourth film layer (at
263) and annealing the third film layer and the fourth film layer to transform the third
film layer and fourth film layer into a hole transport layer (at
264, Figure 2E). It would be appreciated that depositing one or more additional layers
of the precursor solution onto the previously deposited precursor solutions may be
performed before proceeding to
264. The plurality of precursor solution layers is then annealed at
264 to form the electron transport layer.
[0057] In Figure 2E another embodiment is illustrated at
209 where precursor deposition and annealing steps are alternated to form the hole transport
layer. That is, a first deposition is shown generally at
264 with depositing the second precursor solution onto a surface of a substrate to form
a third film layer. This deposition may lay the third film layer so that it at least
in part contracts the previously deposited active layer, or it may be deposited on
an intervening layer that provides a conductive path to the active layer. The method
proceeds with annealing the first film layer to transform the third film layer into
a first hold transport layer at
265. Then another layer of the second precursor solution is applied and annealed. At
266, the process proceeds with depositing the second precursor solution onto a surface
of the first hole transport layer to form a fourth film layer and then to
267 with annealing the fourth film layer to transform the fourth film layer and the first
hole transport layer into a single hole transport layer. Multiple iterations of these
alternating deposition and annealing steps may be performed until a hole transport
layer of the desired thickness is achieved.
[0058] In alternate embodiments, one or both of the
first conductor and the second conductor may be implemented as transparent conductor layers.
For example, in one example embodiment where the ZnO electron transport layer is produced
from
230, a single-cell photovoltaic device may be produced by coating the electron transport
layer with an active layer that provides a bulk heterojunction (BHJ) solution, followed
by a hole-transport layer, and finally depositing a high work-function top electrode,
such as silver, for the second conductor. In such devices, the electron transport
layer serves to facilitate electron extraction and block hole extraction, which helps
to maximize the open-circuit voltage (V
OC) from the photovoltaic device. Representative current density-voltage (J-V) traces
for 0.11 cm
2 devices fabricated based upon such a slot-die coated ZnO electron transport layer
with a P3HT:PCBM active layer and a PEDOT:PSS-based hole transport layer are shown
in
Figure 3, along with average device performance characteristics in
Table 1 (below), demonstrating comparable performance of a device with a spin-cast ZnO electron
transport layer.
Table 1
|
Voc (mV) |
Jsc (mA/cm^2) |
Fill Factor |
Efficiency (%) |
R Shunt (Ohms) |
R Rect. (Ohms) |
R Series (Ohms) |
Number of Suns |
Spin-Cast ZnO |
500 |
11.3 |
44.1 |
2.53 |
6.04E+04 |
75 |
15 |
0.984 |
Slot-Die ZnO |
482 |
10.2 |
45.3 |
2.26 |
7.91E+04 |
108 |
20 |
0.983 |
[0059] For the production of large-area modules based upon the slot-die coated ZnO electron
transport layers, a ZnO electron transport layer and first conductor layer (which
may be a transparent conducting oxide, like indium tin oxide (ITO)), can be patterned
to permit formation of discrete cells that are then connected in either a serial or
paralleled fashion to yield voltage or current addition, respectively. Examples using
a convention P1, P2, P3 scribing process, a ZnO/ITO, BHJ, and PEDOT/Ag layers, respectively,
can be patterned to produce a serial interconnection of cell stripes to produce modules
with additive voltage.
[0060] Figure 4 illustrates a performance plot, certified by the National Renewable Energy
Laboratory (NREL), of a large-area Organic Photovoltaic Module incorporating a Zinc
Oxide electron transport layer fabricated via an embodiment of the method described
herein. As demonstrated by this plot, this Zinc Oxide electron transport layer is
quite suitable for use in large-area, high-performance Organic Photovoltaic Modules.
The favourable V
oc demonstrates the effective nature of the ZnO electron transport layer produced from
a precursor solution comprising the stabilized diethylzinc solution, and the potential
this technology has to enable low-temperature, atmospheric production of inverted-architecture
OPV modules in a high-throughput roll-to-roll compatible process.