HK1108764B - High efficiency organic photovoltaic cells employing hybridized mixed-planar heterojunctions - Google Patents

Description

High efficiency organic photovoltaic cells using mixed hybrid-planar heterojunctions

This application is a continuation-in-part application of U.S. application No. 10/822,744, filed on 13.4.2004, which is incorporated by reference in its entirety.

The invention disclosed herein is supported by the government, who has certain rights in the invention.

Technical Field

The present invention relates to efficient organic photosensitive devices.

Background

Opto-electronic devices using organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential to be cost-effective over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications, such as fabrication on flexible substrates. Examples of organic opto-electronic devices include Organic Light Emitting Devices (OLEDs), organic transistors/phototransistors, organic photovoltaic cells, and organic photodetectors, for OLEDs organic materials have performance advantages over traditional (i.e., inorganic) materials. For example, the wavelength of light emitted by the organic light-emitting layer is generally easily adjusted with an appropriate dopant. For organic transistors/phototransistors, the substrates on which they are constructed may be flexible, which provides broader applications in industry and commerce.

The term "organic" as used herein includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic devices, including optoelectronic devices. "Small molecule" refers to any organic material that is not a polymer, and "small molecules" can be quite large in nature. In some cases, the small molecule may include a repeat unit. For example, the use of long chain alkyl groups as substituents does not remove the molecule from the "small molecule" class. Small molecules can also be incorporated into polymers, for example as a side group on the polymer backbone or as part of the backbone. Small molecules can also be used as the core moiety of dendrimers, which consist of a series of chemical shells built on the core moiety. Small molecules generally have a defined molecular weight, whereas polymers generally do not.

General background information on Small Molecular Weight Organic Thin film photodetectors and Solar Cells can be found in Peumans et al, "Small Molecular Weight Organic Thin-film photoresists and Solar Cells," journal of Applied Physics Reviews-Focused Review, Vol. 93, No. 7, pp. 3693-3723 (4. 2003).

The "fill factor" (FF) of a solar cell is P_maxV (Jsc Voc), where P_maxIs the maximum power of the solar cell, determined by finding the point on the I-V curve where the product of current and voltage is the maximum. A high FF indicates how "square" the I-V curve of the solar cell looks.

Optoelectronic devices rely on the optical and electrical properties of materials to electrically generate or detect electromagnetic radiation or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Photovoltaic (PV) devices or solar cells are a class of photosensitive optoelectronic devices, particularly for generating electricity. PV devices, which can generate power from sources other than sunlight, are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment, such as computers or remote monitoring or communication equipment. These power generation applications also typically involve the charging of batteries or other energy storage devices so that device operation can continue when direct illumination is not available from the sun or other ambient light sources. The term "resistive load" as used herein refers to any device, apparatus or system that consumes or stores energy. Another type of photosensitive optoelectronic device is a photoconductive cell. In this function, the signal detection circuit monitors the resistance of the device to detect changes due to absorbed light. Another type of photosensitive optoelectronic device is a photodetector. In operation, the light detector has an applied voltage and the current detection circuit measures the resulting current when the light detector is exposed to electromagnetic radiation. The detection circuit described herein is capable of providing a bias voltage to the photodetector and measuring the electrical response of the photodetector to ambient electromagnetic radiation. These three photosensitive optoelectronic devices can be characterized according to the presence or absence of a rectifying junction as defined below and also according to whether the device is operated with an applied voltage, also referred to as a bias voltage or a bias voltage. The photoconductive cell has no rectifying junction and is typically operated under bias. The PV device has at least one rectifying junction and operates under no bias. The photodetector has at least one rectifying junction and is typically, but not always, operated at a bias voltage.

There is a need for higher efficiency organic photovoltaic cells.

Disclosure of Invention

A device is provided having a first electrode, a second electrode, and a photoactive region disposed between the first electrode and the second electrode. The photoactive region includes a first organic layer comprising a mixture of an organic acceptor material and an organic donor material, wherein the first organic layer has a thickness of no greater than 0.8 characteristic charge transport length (charge transport length hs), and a second organic layer in direct contact with the first organic layer, wherein: the second organic layer includes an unmixed layer of the organic acceptor material or the organic donor material of the first organic layer, and the thickness of the second organic layer is not less than about 0.1 optical absorption length. Preferably, the thickness of the first organic layer is no greater than 0.3 characteristic charge transport length. Preferably, the thickness of the second organic layer is not less than about 0.2 optical absorption length. Embodiments of the present invention can have a power efficiency of 2% or greater, and preferably 5% or greater.

Drawings

Fig. 1 is a schematic diagram of an organic photovoltaic cell according to an embodiment of the present invention.

Fig. 2 is a schematic diagram of another organic photovoltaic cell according to an embodiment of the present invention.

Fig. 3 is a schematic diagram of yet another organic photovoltaic cell according to an embodiment of the present invention.

Fig. 4 shows a method of manufacturing an organic photovoltaic cell according to an embodiment of the present invention.

Fig. 5 shows an energy level diagram of the device.

FIG. 6 shows the J-V characteristics of a hybrid device (hybrid device).

Fig. 7 shows other photovoltaic characteristics of the device described with reference to fig. 6.

FIG. 8 shows CuPc: C with various mixture ratios deposited on ITO₆₀The absorption spectrum of (1).

Fig. 9 shows corrected photocurrent-voltage characteristics at various light intensities for the device described with reference to fig. 6.

Fig. 10 shows current density versus voltage (J-V) characteristics in the dark for planar HJ devices and hybrid HJ devices.

FIG. 11 shows a method for the production of a catalyst withMixed HJ cell of (1), n and J_sFor the thickness d of the mixed layer_mThe dependency of (c).

FIG. 12 shows that for hybrid devices with various hybrid layer thicknesses, 120mW/cm is used at Po²Photocurrent density J under illumination intensity of_Ph。

FIG. 13 shows the thickness of the layer for a mixture of layersExperimental J-V characteristics at various Po.

FIG. 14 shows a thickness of the mixed layer ofAbsorption spectra of the planar HJ device and the hybrid HJ device of (1).

FIG. 15 shows illumination intensity vs. η for hybrid HJ devices and planar HJ devices_PFF and V_OCThe relationship (2) of (c).

FIG. 16 shows homogeneous and mixed CuPc and C₆₀X-ray diffraction results of the film.

Detailed Description

Organic Photovoltaic (PV) cells have attracted a great deal of attention due to their potential for low-cost solar or ambient energy conversion. The early results using organic PV cells based on a single donor-acceptor (D-a) heterojunction yielded thin films with 1% efficiency. See c.w.tang, appl.phys.lett.48, 183 (1986). From this point on, power conversion efficiency, η, by using new materials and device structures_PHas steadily increased. See p.peumans et al, j.appl.phys.93, 3693 (2003); yakimov and s.r.forrest, appl.phys.lett.80, 1667 (2002); p.peumans and s.r.forrest, appl.phys.lett.79,126 (2001); s.e. shaheen et al, appl.phys.lett.78, 841 (2001); p. Peumans et al, Nature (London)425, 158 (2003). Especially in the double heterojunction copper phthalocyanine (CuPc)/C₆₀The solar cell realizes the solar energy of 1 sun (100 mW/cm)²) Eta under AM1.5G simulated sunlight illumination_P(3.6 ± 0.2)%. P.peumans and s.r.forrest, appl.phys.lett.79,126 (2001). However, these single heterojunction devices are limited in that the "active region" of the device, i.e., the region where absorbed photons can be used for photocurrent, is limited to the region from which excitons excited by the photons can diffuse with reasonable probability to the single heterojunction.

Donor (D) -acceptor (a) bulk (bulk) heterojunctions (BHJs) can be used to improve the efficiency of polymer and small molecule based Photovoltaic (PV) cells. Due to the fact thatExternal quantum efficiency (. eta.) for organic D-A bilayer structures_EQE) Usually limited by short exciton diffusion lengths, BHJ has been proposed as a way to overcome this limitation, resulting in increased η_EQEAnd power conversion efficiency (η)_P). Such BHJ may be composed of donor-type phthalocyanine (Pc) and acceptor-type C₆₀The mixed film of (1). More recently, in mixed ZnPc: C₆₀In a PV cell, at 0.1 sun (10 mW/cm)²AM1.5) has been reported to be eta_P3.37%. See d.gebeyehu et al, Solar Energy mater.solar Cells, 79, 81 (2003). Unfortunately that device has a large cell series resistance (R)_s) Resulting in a reduced short circuit current density (J)_sc) Therefore, the power efficiency is reduced to 1.04% at 1 sun intensity. Such a large R_sThe reason for (a) may be due to the presence of a resistive organic layer comprising poly (3, 4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS), and more importantly their contact resistance. On the other hand, recent results have shown very low R_sCuPc/C of₆₀The two-layer device exhibits eta_PIs significantly improved, in particular at higher illumination intensities, under 4 to 12 suns ═ c) % maximum power efficiency. See Xue et al, appl. phys. lett., 84, 3013 (2004).

Referring now in detail to the drawings, there is shown in FIG. 1a schematic diagram of an organic photovoltaic cell 100 according to an embodiment of the present invention. The device 100 may include a first electrode 102, a first organic layer 106, a second organic layer 108, a third organic layer 114, and a second electrode 104, sequentially disposed on a substrate. The first organic layer 106 comprises a mixture of an organic acceptor material and an organic donor material. The second organic layer 108 includes the organic acceptor material of the first organic layer 106 but does not include the organic donor material of the first organic layer 106. The thickness of the second organic layer 108 is between about 0.5 exciton diffusion length and about 10 exciton diffusion lengths. Preferably, the organic layer 108 has a thickness of about 1-10 exciton diffusion lengths. As a result, the first organic layer 106 functions as an integral heterojunction, wherein photogenerated excitons may separate into electrons and holes. The second organic layer 108 may be photoactive in the sense that it absorbs photons to generate excitons that may later contribute to photocurrent, but these excitons may first diffuse into the heterojunction of the first organic layer 106. The third organic layer 114 comprises an exciton blocking layer comprised of a material selected to block excitons from the second organic layer 108 from entering the third organic layer 114. The third organic layer 114 may be referred to as a non-photoactive organic layer because it does not act to absorb photons that contribute significantly to photocurrent.

Fig. 2 is a schematic diagram of another organic photovoltaic cell 200 according to an embodiment of the present invention. The device 200 may include a first electrode 202, a first organic layer 206, a second organic layer 208, and a second electrode 204 disposed in that order on a substrate. The first organic layer 206 comprises a mixture of an organic acceptor material and an organic donor material. A second organic layer 208, a third organic layer 214, and a second electrode 204. The first organic layer 206 comprises a mixture of an organic acceptor material and an organic donor material. The second organic layer 208 comprises the organic donor material of the first organic layer 206, but does not include the organic acceptor material of the first organic layer 206. The thickness of the second organic layer 208 is between about 0.5 exciton diffusion length and about 10 exciton diffusion lengths, and preferably between about 1-10 exciton diffusion lengths. As a result, the first organic layer 206 acts as an integral heterojunction, wherein photogenerated excitons may separate into electrons and holes. The second organic layer 208 absorbs photons to generate excitons that may later contribute to photocurrent, which may be photoactive in this sense, but these excitons may first diffuse into the heterojunction of the first organic layer 206. The third organic layer 214 contains an exciton blocking layer comprised of a material selected to block excitons from the second organic layer 208 from entering the third organic layer 214.

Examples of diffusion lengths for various acceptor and donor materials are shown in table 1 below:

TABLE 1 exciton diffusion Length reported

^aPpii ═ perylene bis (phenethylimide), Alq₃Tris (8-quinolinolato) aluminum.

^bUse for SnO₂The results of the surface were quenched and an infinite surface recombination velocity was assumed. Result inThe results of (a) may be affected by quencher diffusion and morphology change during solvent evaporation assisted annealing.

^cOptical interference effects are not considered.

It is clear that the organic materials listed in table 1 above are exemplary and not meant to be limiting. Other materials having similar or different diffusion lengths may be used without departing from the scope of the present invention. Furthermore, it is clear that the diffusion lengths listed in table 1 are not meant to limit the invention disclosed herein to only those listed lengths. Other lengths, whether obtained by using other materials or by different determination, calculation or measurement methods of the diffusion length of the materials as indicated above, may be used without departing from the scope of the present invention.

In one embodiment, the mixture of organic acceptor material and organic donor material in a mixed organic layer, such as the first organic layer 106 (or 206), may be present in a ratio of from about 10: 1 to about 1: 10 by weight, respectively. In one embodiment, the organic layer comprising a mixture of acceptor and donor materials (e.g., first organic layer 106), and the organic layer comprising only acceptor material or donor material (e.g., second organic layer 108 or 208), each contribute 5% or more, and preferably 10% or more, of the total energy output of the photoactive deviceAnd is larger. In one embodiment, the organic layer comprising a mixture of acceptor and donor materials (e.g., first organic layer 106 or 206) and the organic layer comprising only acceptor material or donor material (e.g., second organic layer 108 or 208) each absorb 5% or more, and preferably 10% or more, of the energy incident on the photoactive device. Layers with lower percentages of energy and/or absorption contributions are not considered to participate significantly as part of the photoactive region of the device. In one embodiment, the organic acceptor material may be selected from: fullerene, perylene, back-condensed (catenated) conjugated molecular systems such as linear polyacenes (including anthracene, naphthalene, tetracene and pentacene), pyrene, coronene and functionalized variants thereof. In one embodiment, the organic donor material may be selected from: metal-containing porphyrins, metal-free porphyrins, rubrene, metal-containing phthalocyanines, metal-free phthalocyanines, diamines (e.g., NPD), and functionalized variants thereof, including naphthalocyanines (naphthalocyanines). This enumeration is not comprehensive and other suitable acceptor and donor materials may be used. In one embodiment, the first organic layer 206 may consist essentially of CuPc and C₆₀The composition of the mixture. In one embodiment, the photoactive device 100, 200 may further comprise a third organic layer 114, 214 that may be disposed between the second electrode 104, 204 and the second organic layer 108, 208 and may be a non-photoactive layer. In one embodiment, the third organic layer 114, 214 may comprise 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP). In one embodiment, the third organic layer 114, 214 may be an exciton blocking layer. In one embodiment, the first electrode 102, 202 may be comprised of indium tin oxide or other conductive oxide. In one embodiment, the second electrode 104, 204 may be composed of Ag, LiF/Al, Mg: Ag, Ca/Al, and other metals. Other material choices may be used.

When a layer is referred to as an "unmixed" acceptor or donor layer, the "unmixed" layer may include a very small amount of the opposite material as an impurity. A material may be considered an impurity if the concentration is significantly lower than the amount required for penetration in the layer, i.e. less than about 5 wt%. Preferably, any impurities are present in lower amounts, for example less than 1 wt% or most preferably less than about 0.1 wt%. Depending on the process and process parameters used to fabricate the device, some contamination of the material in the immediately adjacent layers is unavoidable.

Preferably, the blocking layer is transparent to the wavelength of light absorbed by the photoactive region. The blocking layer is preferably susceptible to injection and conduction of a type of charge carrier that can pass therethrough-for example, a blocking layer disposed on the acceptor side of the photoactive region, disposed between the acceptor material and the electrode, should be susceptible to electron injection from the acceptor and should be susceptible to conduction of electrons.

A layer is said to be "optically active" if photons absorbed by the layer make a significant contribution to the photocurrent of the device. The device may have a photoactive region comprising several photoactive layers. In many embodiments of the present invention, the photoactive region comprises a plurality of photoactive layers, including layers that are mixtures of acceptor and donor materials and layers that include only either the acceptor material or the donor material, but not both (although impurities may be present as described above). A device that combines a mixed photoactive layer with one or more unmixed photoactive layers may be referred to as a hybrid device because it combines the advantageous properties of a planar HJ device (a D-a interface without a mixed layer) with the advantageous properties of a mixed layer device (a mixed D-a layer without unmixed a or D layers, or an unmixed layer with only minimal a and D materials).

Fig. 3 is a schematic diagram of yet another organic photovoltaic cell 300 according to an embodiment of the present invention. The device 300 may include a first electrode 302, a third organic layer 310, a first organic layer 306, a second organic layer 308, a fourth organic layer 314, and a second electrode 304, sequentially disposed on a substrate. The first organic layer 306 comprises a mixture of an organic acceptor material and an organic donor material. The second organic layer 308 comprises the organic acceptor material of the first organic layer 306, but does not include the donor material of the first organic layer 306. The thickness of the second organic layer 308 is between about 0.5 exciton diffusion length and about 10 exciton diffusion lengths, and preferably between about 1-10 exciton diffusion lengths. The third organic layer 310 includes the organic donor material of the first organic layer 306, but does not include the acceptor material of the first organic layer 306. The thickness of the second organic layer 310 is between about 0.5 exciton diffusion length and about 10 exciton diffusion lengths, and preferably between about 1-10 exciton diffusion lengths. As a result, the first organic layer 306 functions as an integral heterojunction, wherein photogenerated excitons may dissociate into electrons and holes. The second organic layer 308 and the third organic layer 310 may be photoactive in the sense that they absorb photons to generate excitons that may later contribute to photocurrent, but these excitons may first diffuse into the heterojunction of the first organic layer 306. The fourth organic layer 314 comprises an exciton blocking layer composed of a material selected to block excitons from the second organic layer 308 from entering the third organic layer 314. The fourth organic layer 314 may be referred to as a non-photoactive organic layer because it does not produce absorption of photons that contribute significantly to photocurrent.

Preferred parameters of the embodiment of fig. 3, such as layer thickness, material selection, proportion of materials in the first organic layer 306 (hybrid layer), relative amount of incident energy absorbed, and relative amount of total energy output, are similar to those in fig. 1 and 2.

In many embodiments of the present invention, there are organic layers (e.g., layers 106, 206, and 306) that include a mixture of acceptor and donor materials, and at least one layer (e.g., layers 108, 208, 308, and 310) that includes only donor or acceptor materials from the mixed layer. When the device absorbs photons, excitons may be generated. Then, if the exciton is able to reach a properly designed heterojunction, it may dissociate and contribute to photocurrent. The layer comprising the mixture of acceptor and donor materials provides an integral heterojunction and thus advantageously has a large volume in which such dissociation is likely to occur. However, such layers may have lower electrical conductivity than the unmixed layers, and low electrical conductivity is not desirable. Thicker layers exacerbate conductivity problems, so there are limits to the thickness such mixed layers can have if reasonable conductivity is desired.

Layers that include only acceptors or donors may advantageously have higher conductivities than the mixed layers. However, there is no heterojunction in such a layer, so that excitons formed by absorption of photons need to be moved into the heterojunction for efficient dissociation. As a result, there is also a limit to the useful thickness of the unmixed layer in solar cells, but unlike conductivity issues, this limit is more related to the diffusion length of the excitons.

In addition, thick photoactive regions are preferred because thicker photoactive layers can absorb more photons that can contribute to photocurrent than thinner photoactive layers.

Many embodiments of the present invention provide a combination of the advantageous properties of devices having a bulk heterojunction (e.g., mixed layer 106, 206, or 306), but no unmixed layer, and devices without a bulk heterojunction-i.e., devices having a pure acceptor layer that forms a planar junction with a pure donor layer. The mixed and unmixed layers are each part of the photoactive region, so that an increased thickness allows more photons to be absorbed. Thus, greater layer thicknesses contributing to photocurrent can be achieved than if the photoactive region included only a mixed layer or only an unmixed layer, or if the majority of the thickness came from a device with only mixed or only unmixed layers. Alternatively, a device with lower resistance for a given thickness of the photoactive region may be achieved.

In a preferred embodiment of the present invention, one or more layers, such as layers 108, 208, 308, and 310, that include only a single acceptor or donor material, rather than two mixtures, may be selected to have high conductivity while being able to contribute to photocurrent. In order to contribute to photocurrent, excitons formed by absorption of photons in such a layer must diffuse into the heterojunction. As a result, the thickness of such a layer is preferably from about 0.5 exciton diffusion length to about 10 exciton diffusion lengths, and more preferably from about 1-10 exciton diffusion lengths. For layers with thicknesses greater than about 10 diffusion lengths, any additional thickness cannot contribute significantly to the photocurrent because photons absorbed too far from the heterojunction cannot reach the heterojunction.

In the absence ofAt the lower limit of the hybrid photoactive layer, optical absorption is a more important parameter than exciton diffusion length. The "optical absorption length" of a material is the length at which the incident light intensity is reduced to (1/e), or about 37%. Typical absorption lengths of organic photoactive materials are 500-Within the range of (1). For CuPc, the optical absorption length is for wavelengths of 500nm-700nmFor C₆₀The optical absorption length for a wavelength of 450nm isIn order for the layer to contribute significantly to the photocurrent, the layer thickness should be at least a large fraction of the absorption length. Preferably, the thickness of the photoactive layer, e.g., the unmixed organic photoactive layer, is not less than about 0.1 absorption length, and more preferably not less than about 0.2 absorption length. For smaller thicknesses, the layer may not contribute significantly to photocurrent.

In a preferred embodiment of the present invention, a layer comprising a mixture of acceptor and donor materials, for example layers 106, 206 and 306, comprises 10% or more acceptor material and 10% or more donor material. It is believed that 10% is the lower limit for sufficient material penetration. Penetration in both acceptor and donor materials is preferred because it allows photogenerated electrons and holes originating from dissociation anywhere in the mixed layer to reach the appropriate electrode by passing through the acceptor and donor, respectively, but not through the opposite (donor or acceptor) layer. Preferably, the unmixed layer in the photoactive region comprises one of the materials present in the mixed layer, thereby avoiding charge carrier HOMO/LUMO mismatch that penetrates through the mixed layer and reaches the unmixed layer.

D-a phase separation is required for efficient carrier collection in both polymer and small molecule based BHJ solar cells. C, CuPc₆₀The hybrid layer exhibits a large optimized bilayer device phase as compared to the use of the same materialEta when_PThis is in contrast to CuPc: the hybrid layer device of 3, 4, 9, 10-perylenetetracarboxylic acid dibenzoimidazole is the reverse. See Peumans et al, Nature, 425, 158 (2003). In fact, for CuPc C₆₀Performing a similar annealing procedure on a hybrid layer battery results in η_PIs significantly reduced. Assuming the concentrations of both materials are above the percolation threshold, this indicates mixed CuPc: C₆₀The system itself may undergo phase separation during deposition, so that the intermixed layer is a percolating network of the two materials.

Unmixed organic donor-acceptor heterojunctions can be used to provide efficient photo-generation of charge carriers upon absorption of incident light. The efficiency of such cells may be limited by the poor ability of excitons (i.e., the combined electron-hole pairs) to diffuse onto the donor-acceptor interface. The use of a mixed layer, i.e., a donor-acceptor mixture, can alleviate this problem by creating a donor-acceptor interface that is spatially distributed for each photogenerated exciton generated in the mixed layer. However, because charge mobility is significantly reduced in the mixture compared to a homogeneous film, recombination of photogenerated holes and electrons is more likely to occur in the mixture, resulting in incomplete collection of carriers.

In one embodiment of the present invention, a preferred microstructure for a molecular donor-acceptor mixture is provided. Mixed layers having preferred microstructures may be used in photosensitive devices with or without one or more unmixed photoactive layers. For CuPc and C, although other donor and acceptor materials may be used₆₀To illustrate examples of preferred microstructures. Preferred microstructures include permeation channels for hole and electron transport through the mixed donor-acceptor layer, each channel being only one or a few molecules wide. Preferably, the width of the channel is 5 molecules wide or less, and more preferably 3 molecules wide or less. Photo-generated charges can be efficiently transported along such channels to their respective electrodes without significant recombination with their counter charges. The interpenetrating network of the donor and acceptor materials forms a nanostructured, spatially distributed donor-acceptor interface for use inEfficient exciton diffusion and subsequent dissociation.

1: 1 weight ratio of CuPc: C prepared by vacuum thermal evaporation₆₀The preferred microstructure is exhibited in the mixture. In this mixture, the charge transport length, i.e. the average distance the charge travels before recombining with its counter charge, was found to be about 40nm in the absence of an applied bias, on the same order of magnitude as the optical absorption length. It is believed that there is no pure donor or acceptor region present in the CuPc mixture. Regions lacking such purity are preferred. By increasing C in a layer₆₀The content of (b) reduces the tendency of CuPc to aggregate.

X-ray diffraction to study homogeneous and mixed CuPc and C₆₀The crystal structure of the film is shown in fig. 16. The homogeneous CuPc film was found to be polycrystalline, while the homogeneous C₆₀The film is amorphous. 1: 1 weight ratio of mixed CuPc: C₆₀The film was also amorphous, indicating that no significant phase separation occurred. By "without significant phase separation" is meant that there is no measurable aggregation by currently available measurement techniques. It is believed that the most sensitive of these current techniques is the measurement using a synchrotron X-ray source (e.g., Brookhaven) capable of measuring aggregates 5 molecules wide and above. Note that these definitions of "no apparent phase separation" and "aggregates" do not exclude the possibility of intersecting interpenetrating molecular lines that may be many molecules long.

For mixed CuPc: C at different mixing ratios₆₀The film measured the optical absorption spectrum as shown in fig. 8. From the relative intensities of the two CuPc absorption peaks (about 620nm and 690nm) in relation to the mixing ratio, it was found that CuPc molecules appear to follow C₆₀The content increases and the tendency to aggregate decreases.

Manufacture in homogeneous phase of CuPc and C₆₀With mixed CuPc: C sandwiched between layers₆₀Organic photovoltaic cells of layers to form mixed planar-mixed heterojunction photovoltaic cells and tested under simulated am1.5g solar illumination. The photoactive region of the cell has 15nm CuPc/10nm CuPc: C₆₀(1: 1 weight ratio)/35 nm C₆₀. Photocurrent of the cell andthe cell with a single 33nm thick mixed photoactive layer was as high and the charge collection efficiency was as high as the cell without the mixed layer (i.e., a planar heterojunction cell). A maximum power conversion efficiency of 5.0% was obtained under simulated am1.5g sunlight illumination of 1-4 suns compared to 3.5% of the mixed layer under 1-4 suns (3.6% under 1 suns) and 4.2% under 4-12 suns for the planar heterojunction cell. A charge transfer length of 40nm (as shown in fig. 13) was obtained for the cell under short circuit conditions using a model based on charge transfer length to fit the current-voltage characteristics of the hybrid planar-hybrid heterojunction cell under illumination, which is in the same order as the optical absorption wavelength. In contrast, the charge transport length of the CuPc: PTCBI (3, 4, 9, 10-perylenetetracarboxylic-bisbenzimidazole) mixed layer is estimated to be less than 5-10 nm.

Although many of the embodiments are described in terms of undoped organic layers, it is understood that dopants may be added to the various organic layers in order to increase conductivity and/or alter the light absorption characteristics of the doped organic layers, thereby beneficially affecting the performance of the device or layer.

It will be appreciated that the embodiments shown in fig. 1-3 are merely exemplary, and that other embodiments may be used in accordance with the present invention. Any photovoltaic cell having both a mixed organic layer including both an acceptor material and a donor, and an adjacent layer including only either an acceptor or donor material, where both the mixed and unmixed layers contribute significantly to photocurrent, would be within the scope of embodiments of the present invention. For example, the order of the layers shown in FIGS. 1-3 may be changed. For example, in fig. 1 and 2, the positions of the photoactive layers, i.e., the first organic layer 106 (or 206) and the second organic layer 108(208), may be swapped, and the blocking layers, etc., rearranged as appropriate. Additional layers, such as blocking layers, charge recombination layers, etc., may or may not also be present. For example, the barrier layer, i.e., the third organic layer 114 or the fourth organic layer 314, may be removed, and/or an additional barrier layer (e.g., a barrier layer between the first organic layer 106 and the underlying first electrode 104) may be present. Various solar cell configurations may be used, such as tandem solar cells. Materials other than those specifically described may be used. For example, a device can be made in which all electrodes are ITO so that the device is transparent to some extent. Alternatively, the device may be fabricated on a substrate and then applied to a support surface such that the last deposited electrode is closest to the support surface. Although many embodiments are described in terms of solar cells, other embodiments may be used in other types of photosensitive devices having a D-a heterojunction, such as photodetectors.

Fig. 4 shows a method of manufacturing an organic photovoltaic cell according to an embodiment of the invention. The method starts in step 400. In step 402, a first organic layer may be deposited over the first electrode. The first organic layer may be a mixed layer including both an organic acceptor material and an organic donor material. In step 404, a second organic layer may be deposited over the first organic layer. The second organic layer may be an unmixed layer including the organic acceptor material or the organic donor material of the first organic layer, but not both. The organic layer may be deposited by any suitable method, including thermal evaporation (or co-evaporation of multiple materials) and OVPD. In step 406, a second electrode may be deposited over the second organic layer. The method ends at step 408.

In one embodiment of the present invention, donor-acceptor copper phthalocyanine (CuPc): C is provided with vacuum codeposition₆₀A high efficiency organic solar cell with a hybrid layer. Manufacture of a semiconductor device having indium tin oxideCuPc:C₆₀(1∶1)/1002, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline/Ag. The series resistance of the device is only R_s＝0.25Ω·cm²Results in a 1A/cm forward bias of +1V²Current density of and 10 at + -1V⁶The rectification ratio of (1). Short circuit electricity under simulated solar illumination (all simulated solar spectra described herein are AM1.5G simulated solar spectra)The flow density increases linearly with light intensity reaching 2.4 suns. Measuring maximum power conversion efficiency eta under 0.3 suns_PEqual to (3.6 ± 0.2)%, and η under 1 sun_P(3.5 ± 0.2)%. Although the fill factor decreases with increasing intensity, up to η is observed at 2.4 solar intensities_PPower efficiency of (3.3 ± 0.2)%.

In another embodiment of the present invention, a high efficiency solar cell is provided. The device has the following structure: indium tin oxide-CuPc/CuPc:C₆₀(1: 1 weight ratio)/3502, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthrolineThe photovoltaic cell exhibits (5.0 ± 0.2)% power conversion efficiency over 1-4 suns illuminated by simulated am1.5g sunlight.

The power efficiency achieved by embodiments of the present invention is higher than that achieved by any other previous organic solar cell. These surprising results are due to the interaction between several features of embodiments of the present invention, including the use of an unmixed organic photoactive layer in conjunction with a mixed organic photoactive layer, with the thickness being selected according to the desired efficiency. Embodiments of the present invention are capable of achieving power efficiencies of 2%, 3.5%, or 5% or greater. It is expected that even higher power efficiencies may be achieved with fine tuning and optimization of devices according to embodiments of the present invention.

One parameter to be considered in selecting the thickness of the mixed layer is the characteristic charge transport length L, which can be considered as the average distance an electron or hole travels in the mixed layer before recombining under an electric field. If the thickness of the mixed layer is too large, many charge carriers will recombine, as opposed to generating a photocurrent. Therefore, the choice of the thickness of the hybrid layer is a trade-off among several factors, including the need for thick layers to increase absorption and the need for thin layers to avoid recombination. Preferably, the thickness of the hybrid layer is no greater than about 0.8 characteristic charge transport length, and more preferably no greater than about 0.3 characteristic charge transport length. C Using CuPc for some of the examples described herein₆₀(1: 1) embodiment of the mixed layer, the mixed layer has a characteristic charge transport length of about 45 nm. For a mixed layer thickness ofAndthe device of (3) achieves excellent efficiency.

The device disclosed in FIG. 1 of high-layered organic solar cell with an active interlayer of doped pigments, applied. Phys. Lett.58(10) (1991) has a hybrid layer with a characteristic charge transport length of about 40nm and a layer thickness of about 1 characteristic charge transport length. As a result, recombination in the mixed layer of the device may partially account for low device efficiency.

In Table 2, the MPc of various structures is summarized₆₀Photovoltaic characteristics of the hybrid device.

TABLE 2

In table, P₀Is the intensity of incident light, J_scIs short-circuit current density, V_ocIs the open circuit voltage, FF is the fill factor,η_Pis the power conversion efficiency, MPP is N, N '-dimethyl-3, 4, 9, 10-perylene (carboximide), and m-MTDATA is 4, 4' -tris (3-methylphenylphenylamino) triphenylamine.

The simplest mixing structure ITO-CuPc:C₆₀/BCP/Ag exhibits a large J at 1 sun comparable to optimized bilayer devices using the same donor and acceptor material combination_sc＝(12.0±0.6)mA/cm². See Xue et al, appl. physlett, 84, 3013 (2004). However, due to the reduced fill factor, FF<0.5 vs. FF. about.0.6 in the bilayer structure), eta observed in the hybrid_PLess than optimal bilayer structure (2.8 ± 0.1)% is used. See above. By mixing in CuPc: C₆₀And between the BCP layers add thin (C))C₆₀The layer further improves J_scAnd η_PAnd both. It is believed that by moving the photoactive region further away from the reflective metal cathode, additional C₆₀The layer results in an increased optical field at the D-a interface. See Peumans et al, j.appl.phys., 93, 3693 (2003). With optimisationThick CuPc: C₆₀The device showed J under 1 sun_sc＝(15.2±0.7)mA/cm²And η_P(3.5 ± 0.2)%. In this case, J_scAbout 20% greater than a two-layer device under 1 sun, and η_PApproximately equal to that of a two-layer device under 1 sun.

Experiments and calculations

On a glass substrate precoated withPhotovoltaic devices were fabricated on thick Indium Tin Oxide (ITO) layers. Exposing solution cleaned ITO surface to UV/O before deposition₃The following steps. Organic source materials were also purified by thermal gradient sublimation prior to use as described in Forrest, chemrev, 97, 1793 (1997): CuPc, C₆₀And 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP). Under high vacuum (<10^-6Torr) all organic materials were thermally evaporated and a quartz crystal monitor was used to determine film thickness and deposition rate. Unless otherwise indicated, the weight% based CuPc and C will be measured using a thickness monitor₆₀The mixing ratio of (A) was fixed at 1: 1. The Ag cathode was evaporated through a metal shadow mask with 1mm diameter openings. The current density-voltage (J-V) characteristics were measured in the dark and under am1.5g simulated solar spectral illumination from a filtered Xe arc lamp light source. The illumination intensity was measured using a calibrated power meter.

Fig. 5 shows an energy level diagram of the device. In addition to efficient exciton dissociation, the homogeneous D: the a-hybrid film allows both electrons and holes to migrate to the contact. By depositing Ag cathodes on BCP, electrons are generated from C₆₀Efficiently migrate to the cathode while effectively blocking defect states of hole and exciton migration. At the anode, adding CuPc to C₆₀The hybrid layer is deposited directly onto the previously cleaned ITO surface.

FIG. 6 shows a table withCuPc:C₆₀/100Hybrid devices of BCP/Ag structure, J-V characteristics at various illumination intensities of the dark and AM1.5G simulated solar spectrum. Specifically, J-V characteristics under darkness and under 0.01, 0.03, 0.08, 0.3, 0.9, and 2.4 sun are provided. The dark J-V characteristic shows-10 at. + -. 1V⁶The rectification ratio of (a) to (b),and a forward current at +1V>1A/cm²Indicating a low series resistance R_s＝0.25Ω·cm²Which is obtained by fitting the J-V characteristic curve according to the modified ideal diode equation. See Xue et al, appl. phys. lett, 84, 3013 (2004).

Fig. 7 shows further photovoltaic characteristics of the device described with reference to fig. 6. J. the design is a square_scWith incident light intensity (P)₀) The linear increase is realized, and the response rate is (0.15 +/-0.07) A/W. In addition, with P₀Increase of V_ocIncrease and FF decrease. As a result, η at all intensities between 0.01-2.4 suns_PAlmost constant and maximum η_P(3.6 ± 0.2)%, and under 0.3 solar illumination, J_sc＝(4.2±0.1)mA/cm²，V_oc0.47V and FF 0.49. At higher intensities, FF decreases to 0.42, resulting in η at 2.4 suns_P＝(3.3±0.2)％。

Although R is_sMay affect the J-V characteristics at high intensity, but the small R of the hybrid device_s＝0.25Ω·cm²Under 2.4 suns, under short circuit conditions, results in J only_sc·R_sA voltage drop of 10 mV. And ideal device (R)_s＝0Ω·cm²) By contrast, estimating the corresponding let η of such a voltage drop_PThe reduction is less than 0.1%. Using ZnPC: C₆₀Recently reported results for hybrid layer structures, see d.gebehu et al, Solar Energy matrix.solar Cells, 79, 81(2003), and ZnPc entry in table 2 show eta at lower (-1/10 sun) intensities comparable to some hybrid layer devices_PAnd similar photovoltaic characteristics, but under 1 sun J_scAnd FF is significantly reduced, resulting in a smaller η_P(see Table 2). Eta_PMay be due to the large R of the former device_s(40-60Ω·cm²)。

More recently, structures similar to those in table 2 have been reported in Sullivan et al, appl.phys.lett., 84, 1210(2004), although those devices are about 3 times less efficient than certain devices disclosed herein. Peumans et al, j.appl.phys.,93, 3693 has shown that once the layer exceeds the "detrimental thickness" caused during contact deposition, the efficiency decreases exponentially with the thickness of the barrier layer (BCP). The BCP layer of Sullivan isSignificantly exceeding the detrimental thickness. In addition, we have found that the purity of the material is very important in determining the efficiency of the PV cell. For the devices made by the inventors and disclosed herein, all of the material sources are sublimated at least three times before being used to make the devices.

FIG. 8 shows CuPc: C deposited on ITO with various mixing ratios₆₀Absorption spectrum of the film. The concentration of CuPc in the mixed film was (a) 100% (CuPc single layer), (b) 62%, (c) 40%, (d) 33%, and (e) 21%. Pure CuPc films have two peaks centered at wavelengths of 620nm and 690 nm. The longer wavelength peak is due to Frenkel exciton generation by the molecule, while the shorter wavelength features are attributed to the formation of CuPc aggregates. The long wavelength peak is dominant in the gas phase or dilute solution. FIG. 8 shows the intensity of the long wavelength peak with increasing C₆₀The content is increased. Thus, with increasing C₆₀The CuPc content molecules show a lower tendency to aggregate. This indicates C₆₀The increase in concentration suppresses the aggregation of CuPc thereof, thereby reducing hole migration in the mixed film, possibly resulting in low carrier collection efficiency. This is reflected in CuPc: C₆₀(1: 2) reduced power efficiency (η) of hybrid layer PV cells_P(2.6 ± 0.1)%, see table 2). However, at a concentration of 1: 1, there may be sufficient (although not measurable) aggregation of CuPc molecules, and/or the formation of CuPc "lines" or percolation channels, allowing low resistance hole transport. C of higher symmetry₆₀The molecules may also form permeation pathways for efficient electron transport to the cathode. At present, it is believed that 1.2: 1 (weight ratio) of CuPc/C, although other concentrations are possible₆₀Is most preferred.

Fig. 9 shows the calibrated photocurrent-voltage characteristics at various light intensities for the device described with reference to fig. 6. The current density was calibrated by subtracting the dark current and then dividing by the am1.5g light intensity. Figure 9 also shows the proposed photovoltaic process for both the two-layer and hybrid layer devices. In the bilayer device 910, photogenerated excitons migrate (1) to the D-a interface where they dissociate into charge carriers (2) in a built-in depletion region, followed by a sweep through the neutral region (3) by diffusion assisted by a carrier concentration gradient. In mixed layer device 920, the exciton immediately separates into charge carriers (4) at the D-a pair. The charge carriers then travel towards the electrode by drifting under the built-in electric field (5), and some carriers are lost by recombination (6).

In a bi-layer cell, photons may not contribute to photocurrent if absorbed too far from the D-a interface. Distance "too far" and exciton diffusion length (L)_D) It is related. External quantum efficiency (η) of a two-layer device_EQE) And efficiency of absorption of excitons diffusing to the D-A junction (eta)_ED) The limit of (2). On the other hand, in the hybrid device, η is easy to dissociate because all excitons are generated at the D-a molecular pair and thus_EDIs high (-100%). This indicates that the hybrid device is not subject to the organic thin film small L_DThe limitation of the characteristics. Thus, the hybrid device J under 1 Sun_sc＝15.4 mA/cm²J larger than optimized bilayer device_sc＝11.3mA/cm². See Xue et al, appl. phys. lett., 84, 3013 (2004). However, the hybrid device exhibits a large electric field dependence in the J-V characteristic (see fig. 9), resulting in a smaller FF and thus a smaller power conversion efficiency than the dual layer device.

Electron-hole recombination is more likely in mixed layer devices because the high resistance of the mixed layer makes charge separation far from the exciton dissociation site difficult. However, the J-V characteristics at different illumination intensities in fig. 9 indicate that the collimated photocurrent is not significantly reduced even at high intensities (and therefore higher carrier concentrations), indicating that bimolecular recombination of the photogenerated carriers is not significant in the mixed layer. Since the generation of carriers occurs throughout the mixed layer, the carrier concentration gradient is very small, indicating that the diffusion portion to the total current is also small. Thus, electricity in the mixed layerThe flow is mainly drift driven and can be strongly influenced by an applied electric field (see fig. 9, device 910). On the other hand, in a bilayer device, photogenerated carriers at the D-a interface diffuse through the neutral region (see fig. 9, device 920). The large charge concentration gradient extending from the D-a interface to the electrodes aids this process, resulting in relatively little electric field dependence. Another hybrid photovoltaic cell was fabricated having the following structure: indium tin oxide-CuPc/CuPc:C₆₀(1: 1 weight ratio)/3502, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthrolineAnd Ag. This photovoltaic cell exhibited (5.0 ± 0.2)% maximum power conversion efficiency under 1-4 simulated am1.5g sunlight illuminated sunlight. The device was fabricated as follows: e &Organic hybrid HJ PV cells were fabricated on thick transparent, conductive ITO anode pre-coated glass substrates with a thin Film resistance of 15 Ω/sq and obtained from Applied Film Corp, Boulder, CO, 80301. The substrate is washed in a solvent as described in Burrows et al, J.appl.Phys.79, 7991 (1996). The substrate was then treated with UV-ozone for 5 minutes as described in Xue et al, j.appl.phys.95, 1869 (2004). At a base pressure of-2X 10^-7Organic layers and metal cathodes are deposited by thermal evaporation in a high vacuum chamber. Depositing a layer of CuPc on an ITO anode, followed by co-depositing a uniformly mixed CuPc: C₆₀(1: 1 weight ratio) layer, followed by C₆₀And (3) a layer. Various devices having organic layers of different thicknesses are fabricated. The thickness of the CuPc layer is d_D～50 To change between. Co-deposited homogeneously mixed CuPc: C₆₀The thickness of the (1: 1 weight ratio) layer is dm-0-To change between. C₆₀The thickness of the layer is d_A～250-To change between. At the deposition of C₆₀After the layer, depositingA thick BCP exciton blocking layer. Finally, evaporation is carried out through a shadow mask having 1mm diameter openingsThick Ag cathode. For d_mA device larger than 0, the device looks as described in device 1010, i.e. the device is similar to the device in fig. 3, where the third organic layer 310 is CuPc and the first organic layer 306 is CuPc and C₆₀The second organic layer 308 is C₆₀And the fourth organic layer 314 is a BCP.

The current-voltage characteristics of the PV cell at 25 ℃ were measured in the dark or under simulated am1.5g sunlight illumination from a 150W Xe arc lamp (Oriel Instruments) using an HP4155B semiconductor parameter analyzer. The illumination intensity was varied using a neutral density filter and measured using a calibrated broadband optical power meter (Oriel Instruments). To measure the external quantum efficiency, a monochromatic light beam was used, which was generated by passing the white light of a Xe arc lamp through a 0.3m monochromator (Acton Research SpectraPro-300i) and its intensity was determined using a calibrated Si photodetector (Newport 818-UV). The photocurrent was then measured at a chopping frequency of 400Hz using a lock-in amplifier (stanford research sr830) as a function of the wavelength of the incident light and the applied voltage.

FIG. 10 shows the alignment for a planeHJ(And is，d_m0) and mixed HJ () Battery, current density versus voltage (J-V) characteristics in the dark. Both batteries exhibited rectification ratios at + -1V>10⁶And shunt resistance>1MΩ·cm². Forward bias characteristics can be fitted using the modified diode equation:

in the formula J_sIs the reverse bias saturation current density, n is the ideality factor, R_sIs the series resistance, q is the electronic charge, k is the boltzmann constant, and T is the temperature. When R is_sAbout the same for both cells, 0.25. omega. cm²When n is reduced from 1.94 + -0.08 of the planar HJ cell to mixed1.48. + -. 0.05 for HJ cell, and J_sAlso from (4. + -. 1). times.10^-7A/cm²(plane HJ) is reduced to (1.0. + -. 0.3). times.10^-8A/cm²(mixed HJ).

FIG. 11 shows a method for the production of a catalyst withMixed HJ cell of (1), n and J_sThickness d of mixing layer_mThe relationship (2) of (c). With increasing d_mIn aLower n (open circle) and J_s(solid squares) are all significantly reduced and are inThe lower tends to saturate.

For cells with mixed layers, lower n and J_sCan be attributed to the reduction of recombination current in the depletion region of these cells. For the planar HJ battery, the current density is CuPc/C₆₀At the interface, the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) are large energy shifts (-1 eV), the diffusion-emission current is negligible; the dark current is therefore mainly the recombination current in the depletion region, which includes the entire mixed layer and the partially unmixed photoactive layer in contact with the mixed layer, resulting in n ≈ 2. J of composite current according to Shockley-Hall-Read composite model_sCan be expressed as follows:

in the formula, n_iIs the intrinsic electron/hole concentration, W' is the effective depletion width, τ ═ 1/(N)_tσνth) Is the lifetime of the excess carriers, N_tIs the total density of recombination centers, σ is the electron/hole trapping cross-section, and v_thIs the thermal velocity of the carriers. In disordered semiconductors, where the carriers migrate by means of a hopping process, Paasch et al, Synth. Met.132, 97(2002) have been shown for μ<1cm²/V·s，ν_th∝μ^1.1Where μ is the carrier mobility. Therefore, J may occur in the mixed layer as a result of a decrease in μ in the mixed layer as compared with the unmixed layer_sIs reduced. At more reduced recombination currents, the contribution of the diffusion-emission current to the dark current becomes significant, resulting in 1 in a battery with a mixed layer<n<2. By providing for planar HJ cells andcomparative example J of hybrid Battery_sIt can be inferred that CuPc and C are mixed with each other at a weight ratio of 1: 1₆₀Hole mobility in CuPc and C₆₀The electron mobility in (a) is reduced by about 1.5 orders of magnitude.

FIG. 12 shows that the thickness is 0 for the mixed layerIn P of₀＝120mW/cm²Photocurrent density at illumination intensity of J_Ph. In the same way as above, the first and second,and isAt 0V (short, solid square), forJ_PhWith d_mIs increased when d is_mIs further increased toThe time remains almost unchanged. When a bias of-1V (open circle) is applied, J_PhSignificantly increased, more for cells with thicker hybrid layers. For planar HJ cells, this may be due to field-assisted exciton dissociation away from the D-a interface. However, for hybrid HJ cells, especially those with thicker hybrid layers ((ii)))，J_PhThe significant increase in (n) can be attributed to the increased charge collection efficiency (η) due to the increased electric field in the mixed layer_CCOr the fraction of photo-generated charge collected at the electrode), which is directly related to the poor migration properties of the mixed layer.

Based on the model described by Peumans et al, j.appl.phys.93, 3693(2003) which takes into account both the optical interference effect and exciton diffusion, it is assumed that the excitons in the mixed layer are completely dissociated and that the charge collection (. eta.) is ideal_CC1), J of a hybrid HJ cell can be simulated as a function of the thickness of the hybrid layer_Ph. As shown by the solid line 1210 in fig. 12, for use in CuPc and C₆₀In each case 70The exciton diffusion length, the model's prediction is reasonably consistent with the experimental data at-1V.The difference below can be attributed to field-assisted exciton dissociation in the mixed layer, which is used to generate line 1210There is no consideration in the model.

To account for the limited η in hybrid HJ cells_CCThe following model can be used: provided in a mixed layer/C₆₀(or CuPc) the probability that an electron (or hole) in the mixed layer at a distance x from the unmixed layer interface reaches the mixed layer/unmixed layer interface is p (x) exp (-x/L), where the electron (or hole) migrates through the unmixed layer and is collected at the electrode. L is the characteristic length of carrier migration. Then, the total charge collection efficiency is:

if p (x) is constant (3)

Wherein p (x) is the hole concentration. By multiplying η by the result of the model described in the preceding paragraph_CCThe photocurrent density J can be obtained_PhAnd used to generate a line 1210, which corresponds to η_CC1. Fitting J at 0V Using the model described in this paragraph_PhExperimental data, dashed line 1220 was generated and characteristic electricity was obtainedLength of charge transfer

The characteristic charge transport length L can be viewed as the average distance an electron or hole travels in the mixed layer before recombining under an electric field. Thus, L can be expressed as follows:

L＝τμ(V_bi-V)/W≈L₀(V_bi-V)/V_bi (4)

where τ is the carrier lifetime, μ is the carrier mobility, V_biIs built-in potential, W is depletion width, and L₀＝τμV_biL (V ═ 0). If W does not change significantly with bias, an approximation is made. Now charge collection efficiency η_CCThe voltage relationship through L becomes a function of V as follows:

J_Ph(V)＝P₀R₀η_CC(V) (5)

in the formula R₀Is corresponding to η_CCResponse rate of 1. Total current density is J_PhAnd the sum of the dark current densities described by equation (1). FIG. 13 shows a table forMixed HJ cell of (1), at different P₀The following experiment J-V characteristics. Using J from dark current analysis_sN and R_sResult of (A) and V_bi0.6V by applying a voltage of-1V<V<The fitting data at 0.6V can be calculatedAnd R is₀(0.22 ± 0.02) a/W. L obtained herein₀Consistent with the results of the fit to the short circuit current density.

FIG. 14 shows a planar HJ cell (solid line) andthe absorption spectrum of the hybrid HJ cell (dashed line) of (1). Absorption efficiency eta_AWhere R is the reflection of incident light by a glass substrate with an Ag cathode over the organic layer (see structure 1410). The slight difference in the absorption spectra of the two devices, in addition to the different aggregation states of CuPc in MCL and PCL, can also be attributed to different material density distributions and interference-induced inhomogeneous distribution of the optical field intensity in the thickness of the organic layer.

Also shown in FIG. 14 are the external quantum efficiencies, η, at 0V for planar HJ (solid line) and hybrid HJ (dashed line)_ext. The hybrid HJ cell has a higher eta in the spectral region between 550nm and 750nm, corresponding to the CuPc absorption_extAnd in C₆₀In the absorption region (380nm-530nm), due to a slightly lower eta_A，η_extSlightly lower in the hybrid HJ cell. Thus, internal quantum efficiency, η, in the CuPc absorption region for a hybrid HJ cell, as compared to a planar HJ cell_int＝η_ext/η_AIs significantly increased, and in C₆₀It is almost identical in the spectral region where absorption is dominant. It is considered that in the planar HJ cell,whileThis is in contrast to CuPc (L)_D～) And C₆₀(L_D～) The different exciton diffusion lengths are uniform. Both the quantum efficiency and the absorption spectrum of the hybrid HJ cell show long wavelength peaks extending from 800nm-900nm, well beyond the absorption edge of CuPc (-750 nm). With Zn phthalocyanine C₆₀The similarity observed in the hybrid system is due to CuPc: C₆₀Absorption of charge transfer states in the mixture. See GRuani et al, J.ChemPhys.116, 1713 (2002).

FIG. 15 shows a graph for a graph having ITO/CuPc (150)/CuPc:C₆₀(1: 1 weight ratio)/C₆₀()/BCP()/Ag() Hybrid HJ cell of structure (open circle), illumination intensity and η_PFF and V_ocThe relationship (2) of (c). The results of the previously reported planar HJ cell (solid squares) in Xue et al, appl. physlett.84, 3013(2004) and the hybrid HJ cell (solid triangles) in fig. 6 are also shown. All three cells used the full range P in the experiment₀All show J therein_scTo P₀The linear relationship of (c). In 1 sun (═ 100 mW/cm)²) Below, J for planar, monolithic and hybrid HJ cells, respectively_sc(11.8. + -. 0.5), (15.5. + -. 0.5) and (15.0. + -. 0.5) mA/cm². The higher photocurrents obtained in both bulk and planar HJ cells may be a more favorable result of exciton diffusion in the mixed layer than in the unmixed layer. The hybrid HJ cell has almost the same J as the bulk HJ cell, although only a very thin hybrid layer is used_sc. Except at the highest intensity, V for all three cells_ocWith P₀The logarithm increases, which can be explained using the p-n junction theory. See Xue et al, appl. Phys Lett.84, 3013 (2004). V_ocLog (P)₀) Due to the different ideality factors of these diodes: n ≈ 2 for planar HJ cells and n ≈ 1.5 for body and planar HJ cells.

Due to the unmixed layerLow R of_sAnd good migration properties, planar HJ cells have a high FF of 0.6. FF is significantly reduced for the overall HJ cell, especially at high intensities, e.g., 0.45 FF under 1 sun compared to 0.62 FF for the planar HJ cell. Under a much thinner mixed layer than the bulk HJ structure (To pair) Hybrid HJ cell in P₀Under 1 sun showed FF ≥ 0.6 and only slightly decreased to 0.53 under strong illumination of-10 suns, indicating more improved charge transport properties.

In a word, the composite HJ battery is 120mW/cm²≤P₀≤380mW/cm²Lower maximum efficiency η_P(5.0 ± 0.2%) (see diagram 1510). Due to V_ocReducing the illumination intensity below 1 sun results in η_PIs reduced. Increasing intensity above 4 suns also causes η due to reduced FF_PA slight decrease in. Such a V_ocAnd FF and P₀The interaction between the relationships may result in a maximum η at a certain illumination intensity_PAnd the illumination intensity can be adjusted between a fraction of sunlight to several suns by changing the thickness of the mixed layer. FF follows P with thicker mixed layer in mixed HJ structure₀More significantly reduced, resulting in η_PWith a peak at lower intensities. Very thin for the mixed layer () The series resistance of the battery may be a factor limiting FF under strong lighting. For example, forEta of hybrid HJ battery_PAt P₀4-10 suns to a maximum, and forThe peak value of the cell is less than or equal to P and less than or equal to 0.4 sun₀Less than or equal to 1.2 suns.

While the invention has been described with respect to specific examples and preferred embodiments, it will be clear that the invention is not limited to these examples and embodiments. Therefore, it is apparent to one skilled in the art that the invention according to the claims includes variations from the specific examples and preferred embodiments disclosed herein.

Claims (36)

1. An organic opto-electronic device for generating a photocurrent by absorption of a photon, the device comprising:

a first electrode;

a second electrode;

a photoactive region disposed between the first electrode and the second electrode, the photoactive region further comprising:

a first photoactive organic layer comprising a mixture of a small molecule organic acceptor material and a small molecule organic donor material, the first photoactive organic layer for generating excitons by absorption of photons, wherein the first photoactive organic layer has a thickness of no greater than 0.8 characteristic charge transport length; and

a second photoactive organic layer in direct contact with the first photoactive organic layer, the second photoactive organic layer for generating excitons by absorbing photons, wherein: the second photoactive organic layer comprises an unmixed layer of the small molecule organic acceptor material of the first photoactive organic layer, and the thickness of the second photoactive organic layer is not less than 0.1 optical absorption length, wherein excitons generated by the absorption of photons by the first and second photoactive organic layers dissociate into electrons and holes that contribute to photocurrent.

2. The organic optoelectronic device of claim 1, wherein the first organic layer has a thickness of no more than 0.3 characteristic charge transport length.

3. The organic optoelectronic device of claim 1, wherein the device has a power efficiency of 2% or greater.

4. The organic optoelectronic device of claim 1, wherein the device has a power efficiency of 5% or greater.

5. The organic optoelectronic device of claim 1, wherein the thickness of the second photoactive organic layer is not less than 0.2 optical absorption length.

6. The organic optoelectronic device of claim 1, wherein the mixture ratio of the organic acceptor material and the organic donor material in the first photoactive organic layer is in the range of from 10: 1 to 1: 10, respectively.

7. The organic optoelectronic device of claim 1, wherein the first and second photoactive organic layers each contribute at least 5% of the total energy output of the photoactive device.

8. The organic optoelectronic device of claim 7, wherein the first and second photoactive organic layers each contribute at least 10% of the total energy output of the photoactive device.

9. The organic optoelectronic device of claim 1, wherein each of the first and second photoactive organic layers absorbs at least 5% of the energy absorbed by the photoactive region.

10. The organic optoelectronic device of claim 9, wherein each of the first and second photoactive organic layers absorbs at least 10% of the energy absorbed by the photoactive region.

11. The organic optoelectronic device of claim 1, wherein said organic acceptor material is selected from the group consisting of: fullerenes, perylenes, back-condensed conjugated molecular systems, pyrenes, coronenes and functionalized variants thereof.

12. The organic optoelectronic device of claim 11, wherein the back-condensed conjugated molecular system comprises a linear polyacene.

13. The organic opto-electronic device of claim 1, wherein the organic donor material is selected from the group consisting of: metal-containing porphyrins, metal-free porphyrins, rubrene, metal-containing phthalocyanines, metal-free phthalocyanines, diamines, and functionalized variants thereof.

14. The organic opto-electronic device of claim 13, wherein the functionalized variant of the metal-free phthalocyanine comprises a naphthalocyanine.

15. The organic optoelectronic device of claim 1, wherein the first photoactive organic layer consists essentially of CuPc and C₆₀The composition of the mixture.

16. The organic optoelectronic device of claim 1, further comprising a first non-photoactive layer disposed between the first electrode and the second photoactive organic layer.

17. The organic optoelectronic device of claim 16, wherein the first non-photoactive layer comprises 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline.

18. The organic optoelectronic device of claim 16, wherein the first non-photoactive layer is an exciton blocking layer.

19. The organic optoelectronic device of claim 1, wherein the first electrode is comprised of indium tin oxide.

20. The organic optoelectronic device of claim 1, wherein the second electrode is comprised of a metal selected from the group consisting of Ag, LiF/Ag, Mg: Ag, and Ca/Al.

21. The organic optoelectronic device of claim 1, comprising a third photoactive organic layer disposed between the first electrode and the second electrode, the third photoactive organic layer being in direct contact with the first photoactive organic layer, wherein the third photoactive organic layer comprises an unmixed layer of the organic donor material of the first photoactive organic layer, and the third photoactive organic layer has a thickness not less than 0.1 optical absorption length.

22. The organic optoelectronic device of claim 1, wherein the device is a tandem solar cell.

23. The organic optoelectronic device of claim 1, wherein the device is a solar cell.

24. The organic optoelectronic device of claim 1, wherein the device is a photodetector.

25. The organic optoelectronic device of claim 1, wherein the photoactive region consists of a mixture of two organic materials, and whereinThe series resistance between the first and second electrodes is 0.25 Ω -cm²±0.15Ω.cm²Within the range of (1).

26. The organic opto-electronic device of claim 1, wherein there is no significant phase separation between the organic acceptor material and the organic donor material of the first photoactive organic layer.

27. The organic optoelectronic device of claim 1, wherein a side of the first photoactive organic layer opposite the second photoactive organic layer is in direct contact with the second electrode.

28. The organic optoelectronic device of claim 1, wherein the mixture of small molecule organic acceptor material and small molecule organic donor material in the first photoactive organic layer is homogeneous.

29. The organic optoelectronic device of claim 1, wherein the second photoactive organic layer is positioned between the first photoactive organic layer and the second electrode.

30. The organic optoelectronic device of claim 1, further comprising: a non-photoactive organic layer disposed between the second photoactive organic layer and the second electrode.

31. An organic opto-electronic device for generating a photocurrent by absorption of a photon, the device comprising:

a first electrode;

a second electrode;

a photoactive region disposed between the first electrode and the second electrode, the photoactive region further comprising:

a first organic layer comprising a homogeneous mixture of an organic acceptor material and an organic donor material, wherein the first organic layer has a thickness no greater than 0.8 characteristic charge transport length;

a second organic layer in direct contact with the first organic layer, the second organic layer for generating excitons by absorption of photons, wherein: the second organic layer comprises an unmixed layer of the organic acceptor material of the first organic layer, and the thickness of the second organic layer is not less than 0.1 absorption length, wherein excitons generated by the absorption of photons by the first and second photoactive organic layers separate into electrons and holes that contribute to photocurrent; and

a third organic layer disposed between the first electrode and the second electrode, the third organic layer for generating excitons by absorbing photons, and the third organic layer being in direct contact with the first organic layer, wherein: the third organic layer comprises an unmixed layer of the organic donor material of the first organic layer, and the thickness of the third organic layer is not less than 0.1 optical absorption length, wherein excitons generated by the absorption of photons by the first and second photoactive organic layers separate into electrons and holes that contribute to photocurrent.

32. The organic optoelectronic device of claim 31, wherein the first organic layer has a thickness no greater than 0.3 characteristic charge transport length.

33. The organic optoelectronic device of claim 31, wherein the device has a power efficiency of 2% or greater.

34. The organic optoelectronic device of claim 31, wherein the device has a power efficiency of 5% or greater.

35. The organic optoelectronic device of claim 31, wherein the thickness of the second organic layer is not less than 0.2 optical absorption length.

36. The organic opto-electronic device of claim 31, wherein the organic acceptor and donor materials are small molecule materials.