WO2007040601A1

WO2007040601A1 - Architecture for high efficiency polymer photovoltaic cells using an optical spacer

Info

Publication number: WO2007040601A1
Application number: PCT/US2006/009987
Authority: WO
Inventors: Kwanghee Lee; Alan J. Heeger
Original assignee: The Regents Of The University Of California
Priority date: 2005-03-17
Filing date: 2006-03-17
Publication date: 2007-04-12
Also published as: JP2008533745A; EP1859495A1

Abstract

High efficiency polymer photovoltaic cells have been fabricated with an optical spacer between the active layer and the electron-collecting electrode. Such cells can exhibit approximately 50% enhancement in power conversion efficiency. The spacer layer increases the efficiency by modifying the spatial distribution of the light intensity inside the device, thereby creating more photogenerated charge carriers in the bulk heterojunction layer.

Description

ARCHITECTURE FOR HIGH EFFICIENCY POLYMER PHOTOVOLTAIC CELLS USINGAN OPTICAL SPACER

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Nos. 60/663,398, 11/326,130, and 11/347,111 filed March 17, 2005, January 4, 2006, and

February 2, 2006, respectively. The contents of these applications are hereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to improved architecture for polymer-based photovoltaic cells and methods for the production of cells having the improved architecture.

Background Information

Photovoltaic cells having active layers based on organic polymers, in particular polymer-fullerene composites are of interest as potential sources of renewable electrical energy. (See references 1-4 in the references listed at the end of the text of this application. References are identified throughout this application by the numbers provided in this list. All the references listed herein are incorporated by reference in their entirety.) Such cells offer the advantages implied for polymer-based electronics, including low cost fabrication in large sizes and low weight on flexible substrates. This technology enables efficient "plastic" solar cells which would have major positive impacts on the world's energy needs. Although encouraging progress has been made in recent years with 3-4% power conversion efficiencies reported under AM 1.5 (AM=air mass) illumination (5, 6), this efficiency is not sufficient to meet realistic

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SVGA 31688.1 specifications for commercialization. The need to improve the light-to-electricity conversion efficiency requires the implementation of new materials and the exploration of new device architectures.

Polymer-based photovoltaic cells may be described as thin film devices fabricated in the metal-insulator-metal (MIM) configuration sketched in Fig. IA.

Devices of the art have had the configuration shown in Fig. IAl as device 10. In this configuration, an absorbing and charge-separating bulk heterojunction layer 11, (or "active layer") with thickness of approximately 100 ran is sandwiched between two charge-selective electrodes 12 and 14. These electrodes differ from one another in work function. The work function difference between the two electrodes provides a built-in potential that breaks the symmetry thereby providing a driving force for the photo-generated electrons and holes toward their respective electrodes with the higher work function electrode 12 collecting holes and the lower work function electrode 14 collecting electrons. As shown in Fig. IAl, these devices of the art also include a substrate 15 upon which the MIM structure is constructed. Alternatively, the positions of the two electrodes relative to the support can be reversed. In the most common configurations of such devices, the substrate 15 and the electrode 12 are transparent and the electrode 14 is opaque and reflective such that the light which gives rise to the photoelectric effect enters the device through support 15 and electrode 12 and reflects back through the device off of electrode 14.

Because of optical interference between the incident light 17 and back-reflected light 18 (light is incident from the electrode 12 side), the optical electric field goes to zero at electrode 14 (7-9). Thus, as sketched in Fig. 1A3, in devices of the art a relatively large fraction of the active layer is in dead-zone 16 in which the photogeneration of carriers is significantly reduced. Moreover, this effect causes more electron-hole pairs to be produced near electrode 12, a distribution which is known to reduce the photovoltaic conversion efficiency (10, 11). This Optical interference effect' is especially important for thin film structures where layer thicknesses are comparable to the absorption depth and the wavelength of the incident light 17, as is the case for photovoltaic cells fabricated from semiconducting polymers.

SVGA 31688.1 In order to overcome these problems, one might simply increase the thickness of the active layer 11 to absorb more light. Because of the low mobility of the charge carriers in the polymer-based active layers, however, the increased internal resistance of thicker films will inevitably lead to a reduced fill factor.

STATEMENT OF THE INVENTION

We have now found an alternative approach to solving this problem of internal reflection within polymer-based photovoltaic devices. This approach is to change the device architecture with the goal of spatially redistributing the light intensity inside the device by introducing an optical spacer 19 between the active layer 11 and the reflective electrode 14 as shown in device 20 depicted in Fig.s 1 A2 and 1 A4. Since spacer 19 is located within the light path and electrical circuit of device 20 it needs to be compatible with both the light and electrical flows. Thus, the prerequisites for an ideal optical spacer layer 19 include the following: First, the layer 19 should be constructed of a material which is a good acceptor and an electron transport material with a conduction band edge lower in energy than that of the highest occupied molecular orbital (HOMO) of the material making up the active layer; Second, the layer 19 should be constructed of a material having the energy of its conduction band edge above (or close to) the Fermi energy of the adjacent electron-collecting electrode: and Third, it should be transparent over a significant portion of the solar spectrum. In addition and preferably, the layer 19 should be of a thickness which, taking into consideration the material from which the layer is formed and that material's index of refraction, provides a redistribution of a significant portion of the internal reflection within the device. As shown in Fig. 1 A4 this configuration can reduce or eliminate the dead zone 16 in active layer 11.

Thus, this invention, in one embodiment, provides an improved photovoltaic cell. This cell includes an organic polymer active layer having two sides. One side is bounded by a transparent first electrode through which light can be admitted to the active layer. The second side is adjacent to a light-reflective second electrode which is separated from the second side by an optical spacer layer.

The spacer layer is substantially transparent in the visible wavelengths. It

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SVCA 31688.1 increases the efficiency of the device by modifying the spatial distribution of the light intensity within the photoactive layer, thereby creating more photogenerated charge carriers in the active layer.

In preferred embodiments the spacer layer is constructed of a material that is a good acceptor and an electron transport material with a conduction band lower in energy than that of the highest occupied molecular orbital of the organic polymer making up the photoactive layer.

Also in preferred embodiments the spacer layer is further characterized as being constructed of a material having the energy of its conduction band edge above or close to the Fermi energy of the adjacent electron-collecting electrode.

Good results are attained when the spacer layer has an optical thickness equal to about a quarter of the wavelength of at least a portion of the incident light. The term "optical thickness" refers to the actual physical thickness of the layer multiplied by the index of refraction of the material from which the layer is formed.

Good results are attained when the spacer layer is constructed of a metal oxide, in particular a substantially amorphous metal oxide and especially substantially amorphous titanium oxide or zinc oxide. When the term "titanium oxide" is used to describe a material of construction for the layer 19 it is intended to refer not only to amorphous titanium dioxide but also, and generally preferably, to titanium suboxide. "Titanium suboxide" is titanium oxide in which the titanium is less than completely oxidized and which is referred to herein as TiO_x with the understanding that "x" in this formula is generally less than 2, for example from about 0.7 to just less than 2.

It will be appreciated, however, that these metal oxide materials, while preferred, are merely representative. Other materials meeting the optical and electrical selection criteria just recited may be used as well. These other materials can include conductive organic polymers meeting the criteria set out above. Other representative inorganic materials include amorphous silicon oxide; SiO_x, where x is the same as x in TiO_x oxide, and indium-zinc and lithium-zinc mixed oxides (InZn oxide and LiZn oxide) for example.

SVGA 31688.1 In preferred embodiments the hole-collecting electrode is a bilayer electrode and the active layer comprises an organic polymer in admixture with a fullerene.

In another embodiment this invention provides an improved method of preparing an organic polymer-based photovoltaic cell comprising a transparent substrate, a transparent hole-collecting electrode on the support, and an organic polymer-based active layer on the hole-collecting electrode. The improvement comprises casting a layer of a titanium oxide precursor solution onto the active layer and thereafter heating the cast layer of titanium oxide precursor to convert the precursor to titanium suboxide to provide a spacer layer.

DETAILED DESCRIPTION OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be further described with reference to the accompanying drawings in which:

Fig. IAl is a schematic cross-sectional view of a photovoltaic cell device of the prior art;

Fig. 1 A2 is a schematic cross-sectional view of a photovoltaic cell device of this invention with its added spacer layer;

Fig. 1 A3 is a schematic view of a photovoltaic cell device of the prior art presenting the distribution of the squared optical electric field strength (E²) inside a representative device of the prior art which lacks an optical spacer The dark region in the right hand portion of the active layer denotes the dead-zone as explained in the text;

Fig. 1 A4 is a schematic view of a photovoltaic cell device of the invention illustrating the distribution of the squared optical electric field strength (E²) inside a representative device of the invention which includes an optical spacer;

Fig. IBl is a schematic illustration of a representative thin film photovoltaic cell of the present invention in which the device consists of a composite of conjugated

SVGA 31688.1 poly(3-hexylthiophene) and the follerene derivative [6,6]-phenyl-C₆₁ butyric acid methyl ester ("P3HT:PCBM") active layer sandwiched between an Al electrode and a transparent indium-tin mixed oxide ("ITO") electrode coated with poly(3,4- ethylenedioxythiophene) doped with poly(styrenesulfonate) ("PEDOT:PSS"). A TiO_x optical spacer layer is inserted between the active layer and the Al electrode. A brief flow chart of the chemical steps involved in a representative preparation of a TiO_x spacer layer is also included in this figure;

Fig. 1B2 illustrates the energy levels of the single components of the representative photovoltaic cell shown in Fig. IBl, which show that this device exhibits excellent band matching for cascading charge transfer;

Fig. 2 A is a tapping mode atomic force microscope image which shows the surface morphology of a representative TiO_x spacer film;

Fig. 2B is a graph showing X-ray diffraction patterns of a representative relatively amorphous TiO_x spacer layer formed at room temperature (bottom curve) and OfTiO₂ powder that has been calcined at 50O⁰C (top curve) and exhibits a much more pronounced crystalline structure;

Fig. 2C is the absorption spectrum of a spin coated TiO_x film which can serve as a representative spacer layer in the photovoltaic cells of this invention. This spectrum shows that the TiO_x film is transparent in the visible range;

Fig. 3 A is a graph in which the incident monochromatic photon to current collection efficiency (IPCE)] spectra are compared for the two representative devices with and without a TiO_x optical spacer layer;

Fig. 3B is a pair of absorption spectra obtained from reflectance measurements in which the lower curve depicts the absolute value of the absorbance of the P3HT:PCBM active layer composite and the upper curve depicts the ratio of the intensity of reflectance observed with devices of this invention with their spacer layers divided by the intensity of reflection under the same conditions in devices of the prior art which do not include the spacer layer. The inset is a schematic description of the optical beam path in the samples used to determine the upper curve in Fig. 3B; and

SVGA 31688.1 Fig. 4A. is a pair of graphs showing the current density- voltage characteristics of representative polymer photovoltaic cells with and without a representative TiO_x optical spacer illuminated with 25 mW/cm2 at 532 nm. The conventional device (upper curve) exhibits Voc = 0.60 V, Jsc = 8.41 mA/cm2, and FF = 0.40 with η& = 8.1 %, while the new device with the TiO_x spacer layer (lower curve) exhibits Voc = 0.62 V, Jsc = 11.80 mA/cm2, and FF = 0.45 with ηe = 12.6%.

Fig. 4B is a pair of graphs showing the current density- voltage characteristics of representative polymer photovoltaic cells with and without a representative TiO_x optical spacer illuminated under AMI .5 conditions with a calibrated solar simulator with radiation intensity of 90 mW/cm2. The conventional device (upper curve) exhibits Voc = 0.56 V, Jsc = 10.1 mA/cm2, and FF = 0.55 with ηe = 3.5 %, while the new device with the TiO_x spacer layer (lower curve) exhibits Voc = 0.61 V, Jsc = 11.1 niA/cm2, and FF = 0.66 with ηe = 5.0%.

Fig. 5. is a series of graphs showing the current density- voltage characteristics of representative polymer photovoltaic cells with and without representative zinc oxide optical spacers illuminated with 25 mW/cm² at 532 nm. The conventional device (upper curve) exhibits Voc = 0.58 V, Jsc = 7.26 mA/cm², and FF = 0.41 with ηe = 2.2%, while the new devices with the ZnO spacer layers (lower curves) exhibit Voc = 0.58 V, Jsc = 7.68, 7.89, 7.76 mA/cm², and FF = 0.54 with ηe = 3.0- 3.1%.

DESCRIPTION OF PREFERRED EMBODIMENTS

This Description of Preferred Embodiments begins with a brief description of the materials and configurations of the photovoltaic cells which benefit from the spacers of this invention. This is followed by a more detailed examination of the spacer layers and their function.

As shown in Fig. 1 A2, the present photovoltaic cells to which the spacer is added include the following elements: a substrate/support; a hole-collecting electrode; an active layer; and an electron-collecting electrode. These elements will be described and then

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SVCA 31688.1 the spacer layer which improves these devices will be discussed.

The Substrate/Support

The substrate provides physical support for the photovoltaic device. In most configurations, light enters the cell through the substrate such that the substrate is transparent. A material is "transparent" when it provides at least 70% and preferably at least 80% average transmission over the visible wavelengths of about 400 nm to about 750 nm, and preferably significant transmission in the infrared and ultraviolet regions of the solar spectrum, as well.

Examples of suitable transparent substrates include rigid solid materials such as glass or quartz and rigid and flexible plastic materials such as polycarbonates and polyesters for example poly(ethylene terphthalate) ("PET").

The Hole-Collecting Electrode

This electrode is very commonly on or adjacent to the substrate and is in the transmission path of light into the cell. Thus, it should be "transparent" as defined herein, as well. This electrode is a high work function electrode.

The high work function electrode is typically a transparent conductive metal- metal oxide or sulfide material such as indium-tin oxide ("ITO") with resistivity of 20 ohm/square or less and transmission of 89% or greater @ 550 nm. Other materials are available such as thin, transparent layers of gold or silver. A "high work function" in this context is generally considered to be a work function of about 4.5eV or greater. This electrode is commonly deposited on the solid support by thermal vapor deposition, electron beam evaporation, RF (radio frequency) or Magnetron sputtering, chemical deposition or the like. These same processes can be used to deposit the low work-function electrode as well. The principal requirement of the high work function electrode is the combination of a suitable work function, low resistivity and high transparency.

In preferred embodiments, the hole-collecting electrode is accompanied by a hole-transport layer located between the high work function electrode and the active layer. This provides a "bilayer electrode".

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SVCA 31688.1 When a hole-transport layer is present to provide a bilayer electrode, it is typically 20 to 30 am thick and is cast from solution onto the electrode. Examples of materials used in the transport layer include semiconducting organic polymers such as PEDOT:PSS cast from a polar (aqueous) solution or the precursor of poly(bis tetraphenyldiamino^iphenyl-perfluorocylcobutane) ("ρoly(BTPD-Si-PFCB)") [S. Liu, X. Z. Jiang, H. Ma, M. S. Liu, A. K.-Y. jen, Macro., 2000, 33, 3514; X. Gong, D. Moses, A. J. Heeger, S. Liu and A. K.-Y. Jen, Appl. Phys. Lett., 2003, 83, 183]. PEDOT.-PSS is preferred. On the other hand, by using poly(BTPD-Si-PFCB) as hole injection layer, many processing issues existing in polymer-based light-emitting diodes ("PLEDs"), brought about by the use of PEDOT:PSS, such as the undesirable etching of active polymer, undesirable etching of ITO electrodes, and the formation of micro-shorts can be avoided [G. Greczynski, Th. Kugler and W. R. Salaneck, Thin Solid Films, 1999, 354, 129; M. P. de Jong, L. J. van Ijzendoorn, M. J. A. de Voigt, Appl. Phys. Lett. 2000, 77, 2255].

The Active Layer

The active layer is made of two components — a conjugated polymer which serves as an electron donor and a second component which serves as an electron acceptor. The second component can be a second conjugated organic polymer but better results are achieved if a fullerene is used.

It will be appreciated that the organic active layer defined as "a polymer" or as "conjugated" can also contain small organic molecules as described by P. Peumans, S. Uchida and S.R. Forrest, NATURE, 2003, 425, 158. (Incorporated by reference.)

Conjugated polymers include polyphenylenes, polyvinylenes, polyanilines, polythiophenes and the like. We have had our best results with poly(3- hexylthiophene), ("P3HT"), as conjugated polymer.

By using fullerenes, particularly buckminsterfullerene ("C₆₀"), as electron acceptors (U.S. Pat. No. 5,454,880), the charge carrier recombination otherwise typical in the photoactive layer may be largely avoided, which leads to a significant increase in efficiency.

SVCA 31688.1 Fullerenes and especially fullerene derivatives such as [6,6]-phenyl-C₆₁- butyric acid methyl ester ("PCBM") are thus preferred. These active layers can be laid down using solution processes such as spin-casting and the like.

The Electron-Collecting Electrode

This electrode is a reflective low work function electrode, most commonly a metal and particularly an aluminum electrode. This electrode can be laid down using vapor deposition methods.

The Spacer Layer

The spacer layer is made from organic or inorganic materials meeting the electrical and optical criterion set forth in the Statement of the Invention above.

Substantially amorphous titanium oxide (TiO_x) and silicon oxide SiO_x and zinc oxide give good results.

Titanium dioxide (TiO₂) might be considered a promising candidate as an electron acceptor and transport material as confirmed by its use in dye-sensitized Grazel cells (12,13), hybrid polymer/TiO₂ cells (14-16), and multilayer Cu- phthalocyanine/dye/TiO₂ cells (9,17). Typically, however, crystalline TiO₂ is used either in the anatase phase or the rutile phase both of which require treatment at high temperatures (T > 450°C) that are inconsistent with the device architecture shown in Fig. IB. The polymeric photoactive layers such as those made of polymer/C₆₀ composite cannot survive such high temperatures. We have used a solution-based sol- gel process to fabricate a substantially amorphous titanium oxide (TiO_x) layer on top of the polymer-fullerene active layer (Fig. IB). By introducing the TiO_x optical spacer, we demonstrate polymer photovoltaic cells with power conversion efficiencies that are increased by approximately 50% compared to those obtained without the optical spacer.

Dense TiO_x films were prepared using a TiO_x precursor solution or suspension, as described in detail elsewhere (18). A layer of the precursor solution/suspension was spin-cast in air on top of the polymer-fullerene composite layer. The sample with its layer of precursor solution/suspension was then heated

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SVGA 31688.1 under vacuum at 90⁰C for 10 minutes during which time residual solvent is removed and the TiO_x layer forms. As shown in Fig. 2A, the resulting TiO_x films are transparent and smooth with surface features less than a few nm.

The spacer layer can be from about 8 nm to about 1000 nm in physical thickness, especially from about 20 nm to about 500 nm. Ideally, the layer should present a smooth continuous layer with an "optical thickness" on the general order of ¹A the wavelength of at least a portion of the light being directed onto the cell. As noted previously, "optical thickness" is the product of the physical thickness and the index of refraction. Indices of refraction for the materials from which the spacer layer is prepared run from a high of about 2.75 for some of the inorganic materials down to about 1.32 for the lowest index organic spacer layer materials. Amorphous titanium oxide (TiO_x) has an index of about 2.5-2.6. The wavelengths of "light" should be considered to include not only the visible spectrum (about 400 nm to about 750 nm) but also the infrared (about 750 nm to about 2500 nm) and ultraviolet (100 nm to about 400 nm) portions of the solar spectrum. These considerations lead to physical thicknesses for the spacer layer which can range anywhere from about 9 nm for the highest index (2.75) inorganic materials when considering the shortest ultraviolet wavelengths (100 nm wavelength) up to about 500 nm thickness when using the lowest index (1.32) organic materials and considering the longest infrared wavelengths (2500 nm wavelength).

When focusing on the visible wavelengths (400-750 nm) this range of indexes leads to thickness of from about 35 nm to about 150 nm.

Scanning Electron Microscope (SEM) and separate Photon Correlation (Light Scattering) Spectroscopy measurements confirm that the average size of the TiO_x particles in the films is about 6 nm. However, since the layer was treated at temperatures below about 100⁰C, the film is amorphous as confirmed by the X-ray diffraction (XRD) analysis (Fig. 2B). The typical XRD peaks of the anatase crystalline form appear only after sintering the spin-cast films at 500°C for 2 hours.

Analysis of representative samples by X-ray Photoelectron Spectroscopy (XPS) reveals the oxygen deficiency in the thin film samples with Ti : O ratio in the

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SVCA 31688.1 range from 35-75% and especially 42.13% - 56.38% that of stoichiometric TiO₂; hence TiO_x. In this formula, x is less than 2 such that the material is a "suboxide" usually x is from 0.75 to 1.96, preferably 0.8 to 1.9 and especially 0.9 to 1.9. These values also represent from 35% to 98% full oxidation, preferably 40% to 95% and especially 45% to 95% full oxidation.

While any compatible processing method may be used to apply the TiO_x layers, solvent processing is preferred. In solvent processing, a layer of a solution or suspension (such as a colloidal suspension) of TiO_x or more commonly one or more TiO_x precursors is applied. The precursor can be converted to TiO_x such as by hydrolysis and condensation processes as follows:

Ti(OR)₄ + 2H₂O -> TiO_x + 4 ROH.

It will be appreciated that the equation does not balance perfectly. It is furnished to illustrate the reactants and products in a general sense. This conversion can be carried out before the layer of solution is laid down, in which case it is more accurately a suspension of nanometer scale particles of TiO_x that is being deposited, or it can be conducted after the layer of precursor solution has been laid down.

After the solution/suspension layer is applied, solvent is removed, most commonly by evaporation under mild heating and/or vacuum conditions to yield a continuous thin spacer layer of precursor or TiO_x.

Preferred solutions of TiO_x precursor are based on include solutions of titanium(rV) lower alkoxides such as 1-4 carbon alkoxides including the titanium(IV) butoxides, titanium(IV) propoxides, titanium(IV) ethoxide, and titanium(rV) methoxide. Such titanium materials are commonly available and soluble in lower alkanols such as 1-4 carbon alkanols which are liquids that are generally compatible with and nondestructive to other organic polymer layers commonly found in microelectronic devices. Alkoxyalkanols such as methoxy-ethanol and the like can be used as well as solvent for these solutions. Other titanium sources including titanium salts such as Ti(S O₄)₂ and TiCl₄, and higher alkoxides, and other organometallic compounds and complexes of titanium can be used .

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SVGA 31688.1 The solvent selected should dissolve and suspend the TiO_x precursor but should not appreciably react with the TiO_x precursor. This suggests that care should be used if aqueous solvents or mixed aqueous/organic solvents are used as a substantial concentration of the water component could cause premature reaction such as hydrolysis of the TiO_x precursor or more complete reaction of the precursor to TiO_x than is preferred. The reaction postulated for the formation of TiO_x uses water but these are small-stoichiometric scale amounts of water, i.e. for say 2 to 10 moles of water per mole of titanium and not gross quantities as would be present in a mixed solvent. Such small amount of water can enter the reaction zone from ambient conditions, i.e. routine humidity, during the formation of the precursor solution/suspension and for the spin-casting of the layer.

Another factor to be considered in selecting a titanium source/solvent combination is the ability of the combination to wet the substrate or underlayer upon which the solution is being spread. The lower alkanol-based solutions/suspensions set forth above have given good wetting with the organic polymer substrate layers found in organic polymer-based photovoltaic cells.

Titanium concentration in the solution/suspension can vary from as low as 0.01 % by weight to as much as 10% by weight or greater. While this has not been optimized, concentrations of from about 0.5 to 5% by weight have given good results.

The TiO_x precursor solution/suspension is spread using conventional methods. Spin casting has given good results.

The precursor solution is formed by heating the solution of starting materials for a time and at a temperature suitable to react the starting material but not so high as to cause conversion of the starting material to a full stiochiometric oxide. Temperatures of from about 50°C to about 150°C and times of from about 0.1 hour (at the higher temperatures) to about 12 hours (at the lower temperatures) can be employed. Preferred temperature and time ranges are from about 80°C to about 120°C for from 1 to 4 hours, again with the higher temperatures using the shorter times and the lower temperatures using the longer times.

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SVGA 31688.1 It is a good idea to exclude gross amount of oxygen and water during the heating of the solution of the TiO_x precursors to the extent needed to prevent premature conversion of the precursor to TiO_x or the conversion of the TiO_x precursor to the full TiO₂ oxide. This can be accomplished by carrying out the solution preparation in an inert (non oxygen) atmosphere such as an argon or nitrogen atmosphere. After the solution/suspension has been cast into a layer, solvent is removed by evaporation. This can be carried out at temperatures of from ambient temperature to about 120°C. This drying can be carried out at ambient moisture conditions which complete the formation of a smooth continuous layer of TiO_x.

Additional information about the handling and use of titanium-based solutions and suspensions can be found in the following references which are incorporated by reference:

1. T. Sugimooto, et al., J. Colloid Interface Sci. 259, 43-52 (2003).

2. W. Shangguan, et al., Sol. Energy Mater. Sol. Cells 80, 433-441 (2003).

3. S. Lee, et al., Chem. Mater. 16, 4292-4295 (2004).

4. Z. Zhong, et al., Chem. Mater. 17, 6814-6818 (2005).

In spite of the amorphous nature of the TiO_x layer, the physical properties are excellent. The absorption spectrum of the film shows a well-defined absorption edge at Eg w 3.7 eV. Although this value is somewhat higher than that of the bulk anatase samples (Eg « 3.2 eV), the value is consistent with the calculation of the modified particle in a sphere model for the size dependence of semiconductor band gaps (19). Using optical absorption and Cyclic Voltammetry (CV) data, the energies of the bottom of the conduction band (LUMO) and the top of the valence band (HOMO) of the TiO_x material were determined; see Fig. IB. This energy level diagram demonstrates that the TiO_x layer satisfies the electronic structure requirements of the optical spacer.

Utilizing this TiO_x layer as the optical spacer, as described in Examples 1 and 2 we fabricated donor/acceptor composite photovoltaic cells using the phase separated "bulk heterojunction" material comprising poly(3-hexylthiophene) (P3HT) as the electron donor and the fullerene derivative, [6,6]-phenyl-C₆₁ butyric acid methyl ester

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SVGA 31688.1 (PCBM) as the acceptor. The device structure is shown in Fig. IB.

Fig. 3 A compares the incident photon to current collection efficiency spectrum (IPCE) of devices fabricated with and without the TiO_x optical spacer. (Representative devices are fabricated in the examples.) The IPCE is defined in terms of the number of photo-generated charge carriers contributing to the photocurrent per incident photon. The conventional device (without the TiO_x layer) shows the typical spectral response of the P3HT:PCBM composites with a maximum IPCE of -60% at 500nm, consistent with previous studies (3-6). For the device with the TiO_x optical spacer, the results demonstrate substantial enhancement in the IPCE efficiency over the entire excitation spectral range; the maximum reaches almost 90% at 500nm, corresponding to a 50% increase in IPCE.

We attribute this enhancement to the TiO_x optical spacer; the increased photo- generation of charge carriers results from the spatial redistribution of the light intensity. In order to further clarify the role of the TiO_x layer, we measured the reflectance spectrum from a "device" with glass/P3HT:PCBM/TiO_x/Al geometry using a glass/P3HT:PCBM/Al "device" as the reference (the P3HT:PCBM composite film thickness was about 100 nm in both). Note that the ITO/PEDOT layers were omitted to avoid any complication arising from the conducting layers. Since the two "devices" are identical except for TiO_x optical spacer layer, comparison of the reflectance yields information on the additional absorption in the P3HT:PCBM composite film as a result of the spatial redistribution of the light intensity by the TiO_x layer (20)

Δα(ω) « - (l/2d)ln[I'_0Ut(ω)/ W«)] (1) where r_out(ω) is the intensity of the reflected light from the device with the optical spacer and I_out(ω) is the intensity of the reflected light from an identical device without the optical spacer.

The data demonstrate a clear increase in absorption over the entire spectrum. Moreover, since the spectral features of the P3HT:PCBM absorption are evident in both spectra, the increased absorption arises from a better match of the spatial distribution of the light intensity to the position of the P3HT:PCBM composite film. We conclude that the higher absorption is caused by the TiO_x layer as an optical spacer as sketched in Fig.

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SVGA 31688.1 IA.. As a result, the TiO_x optical spacer increases the number of carriers per incident photon collected at the electrodes.

As shown in Fig: 4A, the enhancement in the device efficiency that results from the optical spacer can be directly observed in the current density vs. voltage (J-V) characteristics under monochromatic illumination with 25 mW/cm2 at 532 nm. The conventional device (without the TiO_x layer) shows typical photovoltaic response with device performance comparable to that reported in previous studies; the short circuit current (Jsc) = 8.41 mA/cm2, the open circuit voltage (Voc) = 0.60 V, and the fill factor (FF) = 0.40. These values correspond to a power conversion efficiency (ηp) = 8.1% (under 25 mW/cm2 monochromatic illumination at 532 nm). For the device with the TiO_x layer, the results demonstrate substantially improved device performance; Jsc increases to 11.80 mA/cm2, the FF increases slightly to 0.45, while Voc remains at 0.62 V. The corresponding power conversion efficiency is ηe = 12.6%, which corresponds to ~50% increase in the device efficiency, consistent with the IPCE measurements.

Under AMI .5 illumination from a calibrated solar simulator with irradiation intensity of 100 mW/cm2, we observed a consistent enhancement in the device efficiency using the TiO_x optical spacer. While the conventional device (without the TiO_x layer) again shows typical photovoltaic responses with a device efficiency of typically 3%, devices fabricated identically, but with the TiO_x layer, demonstrate substantially improved device performance with efficiency of 4 %, which corresponds to ~33% increase.

The additional data obtained under AM 1.5 illumination from a calibrated solar simulator with irradiation intensity of 90 mW/cm² are shown in Fig. 4B. The device without the TiO_x layer again shows typical photovoltaic response with device performance comparable to that reported in previous studies; J_sc = 10.1 mA/cm², V_oc = 0.56 V, FF = 0.55 and η_e= 3.5%. For the device with the TiO_x layer, the results demonstrate substantially improved device performance; J_sc = 11.1 mA/cm², V₀₀ ⁼ 0.61 V, FF = 0.66. The corresponding power conversion efficiency is η_e= 5.0%, which corresponds to ~40% increase in the device efficiency. Postproduction annealing at 15O⁰C improves the morphology and crystallinity of the bulk heterojunction layer with a corresponding increase in solar conversion efficiency to 5% (T). Thus, we anticipate

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SVCA 31688.1 that by using the optical spacer architecture described here, one should be able to improve the performance to efficiencies in excess of 7%.

The results presented in detail in this document utilized TiO_x and ZnO as the materials for the optical spacer layer. Other inorganic spacer materials meeting the criteria set forth herein can be used . Examples of such materials include amorphous silicon oxide, (SiO_x) where x is similar to x in TiO_x). As shown in Fig. 5 and Example 3, we have also successfully demonstrated the use of ZnO (in the form of nanoparticles cast from aqueous solution) as the material for the optical spacer.

Organic spacer layers can be used as well. Such organic spacer materials can be dissolved in water and/or methanol for coating this material on top of the organic layer without damage. Thus, candidates for organic spacer materials are recently-developed water-soluble ionic polymers such as anionic poly (fluorene) ("anion-PF"), cationic poly (fluorine)("cation-PF"), poly{[9,9-bis(6'-(N,N,N-tri-methylammonium)-fluorene- 2,7-diyl]-α/t-[2,5-bisθ7-phenylene)-l,3,4-oxadiazole]}("PFON⁺(CH₃)₃rPBD"), poly(vinylcarbazole)sulfonate lithium salt ("PVK-SO₃Li"), t-butyl-2,3,4-oxadiazole sulfonate sodium salt ('V-Bu-PBD-SO₃Na"), and the like.

The semiconducting polymer used in the active layers in these studies, P3HT, has a relatively large energy gap (approx. 2 eV). As a result, almost half of the energy in the solar spectrum is at wavelengths in the near infra-red at wavelengths too long to be absorbed. We anticipate that utilizing both a semiconducting polymer with energy gap well matched to the solar spectrum and the optical spacer concept described here will result in polymer solar cells with approximately 10% efficiency for conversion of sunlight to electricity. Low cost plastic solar cells with power conversion efficiencies approaching 10% could have major impact on the energy needs of our society.

While the scope of the invention is defined solely by the claims herein, the following examples explain the manufacture and testing of devices of the invention in more detail.

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SVCA 31688.1 Example 1

The sol-gel procedure for producing TiO_x is as follows; titanium isopropoxide (Ti[OCH(CH₃)₂]₄, Aldrich, 97%, 1OmL) was prepared as a precursor, and mixed with 2- methoxyethanol (CH₃OCH₂CH₂OH, Aldrich, 99.9+%, 15OmL) and ethanolamine (H₂NCH₂CH₂OH, Aldrich, 99.5+%, 5mL) in a three-necked flask equipped with a condenser, thermometer, and argon gas inlet/outlet. Then, the mixed solution was heated to 8O⁰C for 2 hours in silicon oil bath under magnetic stirring, followed by heating to 120°C for 1 hour. The two-step heating (8O⁰C and 12O⁰C) was then repeated. The typical TiO_x precursor solution was prepared in isopropyl alcohol.

For the preparation of the polymer-fullerene composite solar cells in the structure shown in Fig.s 1 A4 and IBl and 1B2, we used regioregular poly(3- hexylthiopene) (P3HT) as the electron donor, and the fullerene derivative, [6,6]-phenyl- C61 butyric acid methyl ester (PCBM) as the electron acceptor. The P3HT:PCBM composite weight ratio was 1:1. After spin casting poly(3,4- ethylenedioxylenethiophene)-polystyrene sulfonic acid (PEDOT:PSS) on ITO glass substrates, with subsequent drying for a period of 30 minutes at 12O⁰C, a thin layer of P3HT:PCBM was spin-cast onto the PEDOT:PSS with a thickness of 100 nm. Then, the TiO_x precursor layer (30 nm) was spin-cast onto the P3HT:PCBM composite from the precursor solution followed by annealing at 9O⁰C for 10 minutes. This casting and heating were carried out under ambient moisture conditions so that the small amounts of water needed to complete the conversion of precursor to TiO_x were present. Finally, the Al electrode was thermally evaporated onto the TiO_x layer in vacuum at pressures below 10-6 Torr.

Example 2

In a second, more optimized device fabrication, the sol-gel procedure for producing titanium oxide (TiO_x) is as follows; titanium isopropoxide (Ti[OCH(CH₃)₂J₄, Aldrich, 99.999%, 1OmL) was used as a precursor and mixed with 2-methoxyethanol (CH₃OCH₂CH₂OH, Aldrich, 99.9+%, 5OmL) and ethanolamine (H₂NCH₂CH₂OH, Aldrich, 99+%, 5mL) in a three-necked flask equipped connected with a condenser, thermometer, and argon gas inlet/outlet. Then, the mixed solution was heated to 80°C

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SVGA 31688.1 for 2 hours in a silicon oil bath under magnetic stirring, followed by heating to 12O⁰C for 1 hour. The two-step heating (80 and 120°C) was then repeated. The typical TiO_x precursor solution was prepared in isopropyl alcohol.

The bulk heterojunction solar cells using poly(3-hexylthiophene) (P3HT) as the electron donor and [6,6]-phenyl-C₆₁butyric acid methyl ester (PCBM) as the acceptor were fabricated in the structure shown in Fig. IB.

Solvent: For achieving optimum performance, we used chlorobenzene as the solvent for the P3HT/PCBM.

P3HT/PCBM Ratio and Concentration: The best device performance is achieved when the mixed solution had a P3HT/PCBM ratio of 1.0 : 0.8; i.e. with a concentration of P3HT(lwt%) plus PCBM(0.8wt%) in chlorobenzene.

Device Fabrication: Polymer solar cells were prepared according to the following procedure: An ITO-coated glass substrate was first cleaned with detergent, then ultrasonicated in acetone and isopropyl, and subsequently dried in an oven overnight. Highly conducting poly(3,4-ethylenedioxylenethiophene)-polystyrene sulfonic acid (PEDOT:PSS, Baytron P) was spin-cast (5000 rpm) with thickness -40 nm from aqueous solution (after passing a 0.45 μm filter). The substrate was dried for 10 minutes at 140°C in air, and then moved into a glove box for spin-casting the photoactive layer. The chlorobenzene solution comprised of P3HT (lwt%) plus PCBM (0.8wt%) was then spin-cast at 700 rpm on top of the PEDOT layer. Then the TiO_x precursor solution in isopropanol was spin-cast in air on top of the polymer-fullerene composite layer. Subsequently, during drying for one hour in air at room temperature, the precursor converts to TiO_x by hydrolysis in the presence of ambient moisture. The sample was then heated at 150⁰C for 10 minutes inside a glove box filled with nitrogen. Subsequently the device was pumped down in vacuum (<10-7 torr), and a ~100 nm Al electrode was deposited on top.

Example 3

A suitable ZnO nanoparticle suspension can be formed using a sol-gel synthesis procedure for producing zinc oxide (ZnO) is as follows; zinc acetate dihydrate

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SVCA 31688.1 [Zn(CH₃CO₂)₂-2H₂O, Aldrich, 98+%, lOmg] was dehydrated using about one hour in vacuum 120⁰C and mixed with 2-methoxyethanol (CH₃OCH₂CH₂OH, Aldrich, 99.9+%, 5OmL) and ethanolamine (H₂NCH₂CH₂OH, Aldrich, 99+%, 5mL) in a three-necked flask each connected with a condenser, thermometer, and argon gas inlet/outlet. Then, the mixed solution was heated to 8O⁰C for 2 hours in a silicon oil bath under magnetic stirring, followed by heating to 120°C for 1 hour. The two-step heating (80°C and 120⁰C) is then repeated. The typical ZnO precursor solution was prepared in isopropyl alcohol. The thin film coating technology using this ZnO precursor solution is more or less similar to that of sol-gel processed TiO_x. The energy of the bottom of the conduction band of ZnO is also well matched to the LUMO of C60 (PCBM). Fig. 5 shows a series of graphs showing the current density- voltage characteristics of representative polymer photovoltaic cells with and without representative zinc oxide optical spacers illuminated with 25 mW/cm² at 532 nm. The conventional device (upper curve) exhibits Voc = 0.58 V, Jsc = 7.26 mA/cm², and FF = 0.41 with ηe = 2.2%, while the new devices with the ZnO spacer layers (lower curve) exhibit Voc = 0.58 V, Jsc = 7.68-76 mA/cm², and FF = 0.54-59 with ηe = 3.06, 3.49, 3.11%.

Calibration and Measurement: For calibration of our solar simulator used in these experiments, we first carefully minimized the mismatch of the spectrum (the simulating spectrum) obtained from the Xenon lamp (150 W Oriel) and the solar spectrum using an AMI .5 filter. We then calibrated the light intensity using carefully calibrated silicon photovoltaic (PV) solar cells. In detail, we used several calibrated silicon solar cells and silicon photodiodes and measured both the short-circuit current and the open-circuit voltage. In order to confirm the accuracy of the solar simulator at Univ. of California at Santa Barbara (UCSB), we carried out a cross-calibration between the solar simulator at UCSB and the solar simulator at Konarka Technologies (Lowell, MA). The accuracy of the solar simulator at Konarka is based on standard cells traced to the National Renewable Energy Laboratory (NREL). Measurements were done with the solar cells inside the glove box by using a high quality optical fiber to guide the light from the solar simulator (outside the glove box). Current density- voltage curves were measured with a Keithley 236 source measurement unit.

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SVCA 31688.1 References

1. N. S. Sariciftci, L. Smilowitz, A. J. Heeger, R Wudl, Science 258, 1474 (1992).

2. N.S. Sariciftci and AJ. Heeger, U.S. Patent 5,454,880

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(1995).

4. C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 11, 15 (2001).

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Hummelen, Appl. Phys. Lett. 78, 841 (2001).

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8. L. A. A. Pettersson, L. S. Roman, O. Inganas, J. Appl. Phys. 86, 487 (1999).

9. T. Stubinger and W. Briitting, J. Appl. Phys. 90, 3632 (2001).

10. H. Hansel, H. Zettl, G. Krausch, R. Kisselev, M. Thelakkat, and H-W. Schmidt, Adv. Mater 15, 2056 (2003).

11. H. J. Snaith, N. C. Greenham, and R. H. Friend, Adv. Mater 16, 1640 (2004).

12. C. Melzer, E. J. Koop, V. D. Mihaletchi, and P. W. M. Blom, Adv. Func. Mater. 14, 865 (2004).

13. B. O'Regan, M. Grazel, Nature 353, 737 (1991).

14. U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissδrtel, J. Salbeck, H. Spreitzel, M. Grazel, Nature 395, 583 (1998).

15. C. Arango, L. R. Johnson, V. N. Bliznyuk, Z. Schlesinger, Sue A. Carter, H-

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SVGA 31688.1 H., Horhold, Adv. Mater. 12, 1689 (2000).

16. J. Breeze, Z. Schlesinger, S. A. Carter, P. J. Brock, Phys. Rev. B 64, 125205 (2001).

17. P. A. van Hal, M. M. Wienk, J. M. Kroon, W. J. H. Verhees, L. H. Slooff, W. J. H. Van Gennip, P. Jonkheijm, R. A. Janssen, Adv. Mater. 15, 118 (2003).

18. M. Thelakkat, C. Schmitz, and H.-W. Schmidt, Adv. Mater. 14, 577 (2002).

19. H. H. Lee, S. H. Kim, J. Y. Kim, K. Lee (unpublished).

20. Y. I. Kim, S. W. Keller, J. S. Krueger, E. H. Yonemoto, G. B. Saupe, T. E. Malouk, J. Phys. Chem. B 101, 2491 (1997).

21. K. Lee, Y. Chang, J. Y. Kim, Thin Solid Films 423, 131 (2003).

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SVCA 31688.1

Claims

WHAT IS CLAIMED IS:

1. In a photovoltaic cell which includes an organic polymer-based photoactive layer having two sides, one side bounded by a transparent first electrode through which incident light can be admitted to the photoactive layer and the second side adjacent to a light-reflective second electrode, the improvement comprising an optical spacer layer separating the photoactive layer from the reflective second electrode.

2. The photovoltaic cell of claim 1 wherein the spacer layer is substantially transparent over a portion of the solar spectrum.

3. The photovoltaic cell of claim 2 wherein the spacer layer is substantially transparent in visible wavelengths.

4. , The photovoltaic cell of claim 2 or 3 wherein the spacer layer increases the efficiency of the device by modifying the spatial distribution of the light intensity within the photoactive layer, thereby creating more photogenerated charge carriers in the active layer.

5. The photovoltaic cell of claim 4 wherein the reflective second electrode is an electron- collecting electrode and wherein the transparent first electrode is a hole- collecting electrode.

6. The photovoltaic cell of claim 5 wherein the spacer layer is constructed of a material that is a good acceptor and an electron transport material with a conduction band lower in energy than that of the highest occupied molecular orbital of the organic polymer making up the photoactive layer.

7. The photovoltaic cell of claim 6 wherein the spacer layer is constructed of a material having the energy of its conduction band edge above or close to the Fermi energy of the adjacent electron-collecting electrode.

8. The photovoltaic cell of claim 2 or claim 7 wherein the spacer layer has an optical thickness about a quarter of the wavelength of some portion of the incident light admitted to the photoactive layer.

9. The photovoltaic cell of claim 8 wherein the spacer layer comprises a metal

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SVGA 31688.1 oxide.

10. The photovoltaic cell of claim 9 wherein the spacer layer comprises a substantially amorphous metal oxide.

11. The photovoltaic cell of claim 9 wherein the spacer layer comprises a material selected from the group consisting of amorphous titanium oxide, amorphous silicon oxide, zinc oxide, indium-zinc oxide and lithium-zinc oxide.

12. The photovoltaic cell of claim 10 wherein the spacer layer comprises TiO_x wherein x has a value less than 2.

13. The photovoltaic cell of claim 10 wherein the spacer layer comprises SiO_x wherein x has a value less than 2.

14. The photovoltaic cell of claiml2 or 13 wherein x has a value from 0.75 to 1.96.

15. The photovoltaic cell of claim 10 wherein the spacer layer comprises zinc oxide.

16. The photovoltaic cell of claim 7 wherein the spacer layer comprises an organic polymer.

17. The photovoltaic cell of claim 1 wherein the hole-collecting electrode is a bilayer electrode.

18. The photovoltaic cell of claim 1 wherein the active layer comprises an organic polymer in admixture with fullerene.

19. A photovoltaic cell comprising a transparent substrate, an ITO -.PEDOTrPSS bilayer hole-collecting electrode on the substrate, an organic polymer-based active layer comprising P3HT:PCBM on the hole-collecting electrode, an amorphous titanium oxide spacer layer on the active layer and a reflective metal electron-collecting electrode on the spacer layer.

20. In a method of preparing an organic polymer-based photovoltaic cell comprising a transparent substrate, a transparent hole-collecting electrode on the support, an organic polymer-based active layer on the hole-collecting electrode, the

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SVCA 31688.1 improvement comprising casting a layer of a titanium oxide precursor solution onto the active layer.

21. The method of claim 20 additionally comprising the step of heating the cast layer of titanium oxide precursor to convert the precursor to a layer of amorphous titanium oxide.

22. In a method of preparing an organic polymer-based photovoltaic cell comprising a transparent substrate, a transparent hole-collecting electrode on the support, an organic polymer-based active layer on the hole-collecting electrode, the improvement comprising casting a layer of a suspension of nanometer scale particles of amorphous titanium oxide in a liquid onto the active layer.

23. The method of claim 20 additionally comprising the step of removing liquid from the layer of suspension.

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SVCA 31688.1