Organic optoelectronics with electrode buffer layers

Abstract

There is disclosed an organic optoelectronic device comprising two electrodes in superposed relation comprising an anode and a cathode, at least one donor material and at least one acceptor material located between the two electrodes forming a donor-acceptor heterojunction, an anode buffer layer adjacent to the anode and a cathode buffer layer adjacent to the cathode, and an intermediate layer adjacent to at least one of the anode and cathode buffer layers, wherein when the intermediate layer is adjacent to the anode buffer layer, the intermediate layer is chosen to facilitate the transport of holes to the anode buffer layer, and when the intermediate layer is adjacent to the cathode buffer layer, the intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer. Also disclosed are methods of making the same.

Claims

1. An organic optoelectronic device comprising: two electrodes in superposed relation comprising an anode and a cathode; at least one donor material and at least one acceptor material located between the two electrodes forming a donor-acceptor heterojunction; an anode buffer layer adjacent to the anode and a cathode buffer layer adjacent to the cathode, wherein the anode buffer layer and the cathode buffer layer are independently chosen from transition metal oxides and conductive polymers; and an intermediate layer chosen from elementally pure metals and metal alloys composed of two or more elementally pure metals, wherein the intermediate layer is adjacent to the anode buffer layer and between the anode buffer layer and the at least one donor material, or wherein the intermediate layer is adjacent to the cathode buffer layer and between the cathode buffer layer and the at least one acceptor material, wherein when the intermediate layer is adjacent to the anode buffer layer, the intermediate layer is chosen to facilitate the transport of holes to the anode buffer layer, and when the intermediate layer is adjacent to the cathode buffer layer, the intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer.

2. The device of claim 1, wherein the anode buffer layer and the cathode buffer layer are independently chosen from transition metal oxides.

3. The device of claim 1, wherein the transition metal oxides are MoO.sub.3, V.sub.2O.sub.5, WO.sub.3, CrO.sub.3, Co.sub.3O.sub.4, NiO, ZnO, and TiO.sub.2.

4. The device of claim 2, wherein the anode and cathode buffer layers comprise the same transition metal oxide.

5. The device of claim 4, wherein the same transition metal oxide is MoO.sub.3.

6. The device of claim 1, wherein the intermediate layer is chosen from Ni, Ag, Au, Al, Mg, Pt, Pd, Cu, Ca, Ti, and In.

7. The device of claim 1, wherein the intermediate layer comprises metal nanoparticles, nanoclusters, or nanorods.

8. The device of claim 1, wherein the intermediate layer has a thickness of 5 nm or less.

9. The device of claim 1, wherein the intermediate layer has an average thickness of 1 nm or less.

10. The device of claim 1, wherein the intermediate layer is adjacent to the anode buffer layer and between the anode buffer layer and the at least one donor material, the device further comprising a second intermediate layer adjacent to the cathode buffer layer and between the cathode buffer layer and the at least one acceptor material, wherein the second intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer.

11. The device of claim 1, further comprising an exciton blocking layer located between at least one of the anode and the donor material and the cathode and the acceptor material.

12. The device of claim 1, wherein the two electrodes are chosen from metals, metal substitutes, conducting oxides, conductive polymers, graphene, and carbon nanotubes.

13. The device of claim 12, wherein at least one of the two electrodes is transparent.

14. The device of claim 13, wherein the electrode opposing the transparent electrode is reflective.

15. The device of claim 13, wherein the electrode opposing the transparent electrode is at least semi-transparent.

16. The device of claim 12, wherein the two electrodes are at least semi-transparent.

17. The device of claim 11, wherein the at least one exciton blocking layer comprises a material chosen from BCP, BPhen, NTCDA, PTCBI, TPBi, Ru(acac)3, and Alq2 OPH.

18. An organic optoelectronic device comprising: two electrodes in superposed relation comprising an anode and a cathode; at least one donor material and at least one acceptor material located between the two electrodes forming a donor-acceptor heterojunction; at least one buffer layer chosen from an anode buffer layer adjacent to the anode and a cathode buffer layer adjacent to the cathode, wherein the at least one buffer layer is independently chosen from transition metal oxides and conductive polymers; and an intermediate layer chosen from elementally pure metals and metal alloys composed of two or more elementally pure metals, wherein the intermediate layer is adjacent to the anode buffer layer and between the anode buffer layer and the at least one donor material, or wherein the intermediate layer is adjacent to the cathode buffer layer and between the cathode buffer layer and the at least one acceptor material, wherein when the intermediate layer is adjacent to the anode buffer layer, the intermediate layer is chosen to facilitate the transport of holes to the anode buffer layer, and when the intermediate layer is adjacent to the cathode buffer layer, the intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer.

19. The device of claim 18, wherein the intermediate layer is chosen from Ni, Ag, Au, Al, Mg, Pt, Pd, Cu, Ca, Ti, and In.

20. The device of claim 18, wherein the intermediate layer comprises metal nanoparticles, nanoclusters, or nanorods.

Description

(1) FIG. 1 shows schematics of example optoelectronic devices in accordance with the present disclosure having (a) two buffer layers and one intermediate layer, (b) two buffer layers and two intermediate layers, and (c) one buffer layer and one intermediate layer.

(2) FIG. 2 shows a specific, non-limiting example of a device in accordance with the present disclosure.

(3) FIG. 3(a) shows a linear plot of the J-V characteristics of devices with various electrodes under one-sun simulated illumination, and FIG. 3(b) shows a semilog plot of the same devices in the dark.

(4) FIG. 4 shows external quantum efficiencies (EQEs) for devices with various electrodes.

(5) FIG. 5 shows a plot of V.sub.OC versus J.sub.SC for the devices with varying electrodes.

(6) FIG. 6 shows an equilibrium energy level diagram depicting the energy level alignment of intermediate layer Ag and cathode buffer layer MoO.sub.3.

(7) As used and depicted herein, a layer refers to a member or component of a photosensitive device whose primary dimension is X-Y, i.e., along its length and width. It should be understood that the term layer is not necessarily limited to single layers or sheets of materials. In addition, it should be understood that the surfaces of certain layers, including the interface(s) of such layers with other material(s) or layers(s), may be imperfect, wherein said surfaces represent an interpenetrating, entangled or convoluted network with other material(s) or layer(s). Similarly, it should also be understood that a layer may be discontinuous, such that the continuity of said layer along the X-Y dimension may be disturbed or otherwise interrupted by other layer(s) or material(s).

(8) As used herein, the expression that a material or component is deposited over another material or component permits other materials or layers to exist between the material or component being deposited and the material or component over which it is deposited. For example, a buffer layer may be described as deposited over donor and acceptor materials, even though there are various materials or layers in between the buffer layer and the donor and acceptor materials.

(9) As used herein, an electrode is said to be transparent when it permits at least 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through it. An electrode is said to be semi-transparent when it permits some, but less that 50% transmission of ambient electromagnetic radiation in relevant wavelengths.

(10) As used herein, the term metal embraces both materials composed of an elementally pure metal, e.g., Mg, and also metal alloys which are materials composed of two or more elementally pure metals, e.g., Mg and Ag together, denoted Mg:Ag. Herein, the term metal substitute refers to a material that is not a metal within the normal definition, but which has the metal-like properties that are desired in certain appropriate applications.

(11) As shown in FIG. 1, the optoelectronic devices of the present disclosure may comprise one or more buffer layers and one or more intermediate layers. For example, there is disclosed an organic optoelectronic device comprising two electrodes in superposed relation comprising an anode and a cathode, at least one donor material and at least one acceptor material located between the two electrodes forming a donor-acceptor heterojunction, at least one buffer layer chosen from an anode buffer layer adjacent to the anode and a cathode buffer layer adjacent to the cathode, and an intermediate layer adjacent to the at least one buffer layer, wherein when the intermediate layer is adjacent to the anode buffer layer, the intermediate layer is chosen to facilitate the transport of holes to the anode buffer layer, and when the intermediate layer is adjacent to the cathode buffer layer, the intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer.

(12) There is also disclosed an organic optoelectronic device comprising two electrodes in superposed relation comprising an anode and a cathode, at least one donor material and at least one acceptor material located between the two electrodes forming a donor-acceptor heterojunction, an anode buffer layer adjacent to the anode and a cathode buffer layer adjacent to the cathode, and an intermediate layer adjacent to at least one of the anode and cathode buffer layers, wherein when the intermediate layer is adjacent to the anode buffer layer, the intermediate layer is chosen to facilitate the transport of holes to the anode buffer layer, and when the intermediate layer is adjacent to the cathode buffer layer, the intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer.

(13) In accordance with the present disclosure, by employing anode and cathode buffer layers, such as transition metal oxide buffer layers, adjacent to the anode and cathode, respectively, the choice of anode and cathode is arbitrary with respect to work function/energy levels. In addition, the inventors have discovered that by inserting a disclosed intermediate layer adjacent to at least one of the anode and cathode buffer layers, the choice of the adjacent buffer layer is also arbitrary with respect to work function.

(14) In accordance with the present disclosure, the intermediate layer, if adjacent to the anode buffer layer, is chosen to facilitate the transport of holes to the anode buffer layer, and if adjacent to the cathode buffer layer, is chosen to facilitate the transport of electrons to the cathode buffer layer. In accordance with the present disclosure, the intermediate layer may facilitate the respective charge transport by aligning the energy transport levels of an adjacent organic material with the respective buffer layer. An adjacent organic material may be a donor or acceptor material, an electron or hole transporting material, or an exciton-blocking, electron or hole transporting material.

(15) As a non-limiting example of the aligning effect of the intermediate layer, FIG. 6 shows an equilibrium energy level diagram depicting the energy level alignment of intermediate layer Ag and cathode buffer layer MoO.sub.3. The top line is the vacuum level shift, which denotes the shift when the energy levels in two materials, (e.g., Ag and MoO.sub.3) shift to align.

(16) In some embodiments, the intermediate layer is chosen with a work function to align with the highest occupied molecular orbital (HOMO) of an adjacent organic material to facilitate the transport of holes to the anode buffer layer. In some embodiments, the intermediate layer is chosen with a work function to align with the lowest unoccupied molecular orbital (LUMO) of an adjacent organic material to facilitate the transport of electrons to the cathode buffer layer.

(17) Thus, in accordance with the present disclosure, by using an intermediate layer adjacent to at least one of the anode and cathode buffer layers, the choice of electrode and buffer layer adjacent to the intermediate layer is arbitrary with respect to work function/energy levels. For example, the inventors have discovered that MoO.sub.3, which is typically used as an anode buffer layer, can also be used as the cathode buffer layer when used in conjunction with the intermediate layer of the present disclosure, because the intermediate layer will align the energy transport levels of an adjacent organic material with the cathode buffer layer. In addition, organic PVs using an intermediate layer with symmetric electrodes consisting of Ag/MoO.sub.3 or ITO/MoO.sub.3 functioned comparably to a device with archetypal ITO/MoO.sub.3 anode and Ag cathode.

(18) These discoveries suggest a new, flexible design criteria for organic optoelectronic devices wherein electrode/buffer materials may be selected with arbitrary work functions/energy levels. Thus, in accordance with the present disclosure devices can be fabricated with electrode/buffer combinations that are appropriate for the specific application of a device, whether of typical or inverted orientation. That is, suitable anode and cathode combinations can be selected from electrodes that are transparent, semi-transparent, reflective, etc. in order to optimize the performance of the device based upon its specific application.

(19) The anode and cathode buffer layers as disclosed herein may be independently chosen from transition metal oxides and conductive polymers. In certain embodiments, the anode and cathode buffer layers are independently chosen from transition metal oxides. In certain embodiments, the transition metal oxides are MoO.sub.3, V.sub.2O.sub.5, WO.sub.3, CrO.sub.3, Co.sub.3O.sub.4, NiO, ZnO, and TiO.sub.2. In certain embodiments, the conductive polymers are polyanaline (PANI) and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS).

(20) In some embodiments, the intermediate layer is chosen from metals. In certain embodiments, the metals are Ni, Ag, Au, Al, Mg, Pt, Pd, Cu, Ca, Ti, and In.

(21) In some embodiments, the intermediate layer comprises metal nanoparticles, nanoclusters, or nanorods.

(22) In certain embodiments, the intermediate layer has a thickness of 15 nm or less, 10 nm or less, or 5 nm or less. In some embodiments, the intermediate layer has an average thickness of 1 nm or less.

(23) In some embodiments, the intermediate layer is adjacent to the anode buffer layer, and the device further comprises a second intermediate layer adjacent to the cathode buffer layer, wherein the second intermediate layer is chosen to facilitate the transport of electrons to the cathode buffer layer.

(24) In some embodiments, the second intermediate layer is chosen from metals. In certain embodiments, the metals are Ni, Ag, Au, Al, Mg, Pt, Pd, Cu, Ca, Ti, and In.

(25) In some embodiments, the second intermediate layer comprises metal nanoparticles, nanoclusters, or nanorods.

(26) In certain embodiments, the second intermediate layer has a thickness of 15 nm or less, 10 nm or less, or 5 nm or less. In certain embodiments, the second intermediate layer has an average thickness of 1 nm or less.

(27) In addition to being chosen from metals, the intermediate layers may be chosen from transition metal oxides. When a buffer layer is chosen from transition metal oxides, the adjacent intermediate layer should not be the same transition metal oxide.

(28) Non-limiting examples of transition metal oxides as disclosed herein are MoO.sub.3, V.sub.2O.sub.5, WO.sub.3, CrO.sub.3, Co.sub.3O.sub.4, NiO, ZnO, and TiO.sub.2.

(29) In some embodiments, the anode and cathode buffer layers comprise the same material. In certain embodiments, the anode and cathode buffer layers comprise the same transition metal oxide. In certain embodiments, the same transition metal oxide is MoO.sub.3.

(30) In some embodiments, the two electrodes comprise materials chosen from metals, metal substitutes, conducting oxides, conductive polymers, graphene, carbon nanotubes. In some embodiments, at least one of the two electrodes comprises a conducting oxide, such as indium tin oxide (ITO), tin oxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZnO), and zinc indium tin oxide (ZITO), or a conductive polymer, such as polyanaline (PANI) or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT-PSS). In some embodiments, at least one of the two electrodes comprises a metal, such as Ag, Au, Ti, Sn, and Al. In some embodiments, the anode comprises a conducting oxide. In some embodiments, the anode comprises ITO. In some embodiments, the cathode comprises a metal. In some embodiments, the cathode comprises a metal chosen from Ag, Au, Ti, Sn, and Al.

(31) The optoelectronic devices disclosed herein can be grown or placed on any substrate that provides desired structural properties. Thus, in some embodiments, the device further comprises a substrate. For example, the substrate may be flexible or rigid, planar or non-planar. The substrate may be transparent, translucent or opaque. The substrate may be reflective. Plastic, glass, metal, and quartz are examples of rigid substrate materials. Plastic and metal foils and thin glass are examples of flexible substrate materials. The material and thickness of the substrate may be chosen to obtain the desired structural and optical properties. In some embodiments, the substrate is stainless steel, such as a stainless steel foil (SUS). SUS substrates are relatively low cost compared to conventional materials, and provide better heat sinks during growth of layers.

(32) In accordance with the present description, the optoelectronic devices, such as organic PVs, may have a conventional or inverted structure. Examples of inverted device structures are disclosed in U.S. Patent Publication No. 2010/0102304, which is incorporated herein by reference for its disclosure of inverted device structures.

(33) With regard to donor materials that may be used in the present disclosure, non-limiting mention is made to those chosen from phthalocyanines, such as boron subphthalocyanine (SubPc), copper phthalocyanine (CuPc), chloroaluminium phthalocyanine (ClAlPc), tin phthalocyanine (SnPc), zinc phthalocyanine (ZnPc), and other modified phthalocyanines, naphthalocyanines, merocyanine dyes, boron-dipyrromethene (BODIPY) dyes, thiophenes, such as poly(3-hexylthiophene) (P3HT), pentacene, tetracene, diindenoperylene (DIP), and squaraine (SQ) dyes.

(34) Non-limiting embodiments of the squaraine donor material that may be used are those chosen from 2,4-bis[4-(N,N-dipropylamino)-2,6-dihydroxyphenyl] squaraine, 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] squaraine, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl] squaraine (DPSQ) and salts thereof. Examples of suitable squaraine materials are disclosed in U.S. Patent Publication No. 2012/0248419, which is incorporated herein by reference for its disclosure of squaraine materials.

(35) In one embodiment, the donor materials may be doped with a high mobility material, such as one that comprises pentacene or metal nanoparticles.

(36) Examples of acceptor materials that may be used in the present disclosure include polymeric or non-polymeric perylenes, polymeric or non-polymeric naphthalenes, and polymeric or non-polymeric fullerenes. Non-limiting mention is made to those chosen from fullerenes (for example, C.sub.60, C.sub.70, C.sub.84) and functionalized fullerene derivatives (e.g., PCBMs, ICBA, ICMA, etc.), 3,4,9,10-perylenetetracarboxylicbis-benzimidazole (PTCBI), Phenyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]PCBM), Phenyl-C.sub.71-Butyric-Acid-Methyl Ester ([70]PCBM), Thienyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]ThCBM), and hexadecafluorophthalocyanine (F.sub.16CuPc).), 3,4,9,10-perylenetetracarboxylicbis-benzimidazole (PTCBI), Phenyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]PCBM), Phenyl-C.sub.71-Butyric-Acid-Methyl Ester ([70]PCBM), Thienyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]ThCBM), and hexadecafluorophthalocyanine (F.sub.16CuPc).

(37) The at least one donor material and the at least one acceptor material of the present disclosure form at least one donor-acceptor heterojunction. The heterojunction may be formed by a planar, bulk, mixed, hybrid-planar-mixed, or nanocrystalline bulk heterojunction.

(38) The organic optoelectronic device according to the present disclosure may further comprise one or more blocking layers, such as an exciton blocking layer (EBL). With regard to materials that may be used as an exciton blocking layer, non-limiting mention is made to those chosen from bathocuproine (BCP), bathophenanthroline (BPhen), 1,4,5,8-Naphthalene-tetracarboxylic-dianhydride (NTCDA), 3,4,9,10-perylenetetracarboxylicbis-benzimidazole (PTCBI), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), tris(acetylacetonato) ruthenium(III) (Ru(acac).sub.3), and aluminum(III)phenolate (Alq.sub.2 OPH), N,N-diphenyl-N,N-bis-alpha-naphthylbenzidine (NPD), aluminum tris(8-hydroxyquinoline) (Alq.sub.3), and carbazole biphenyl (CBP).

(39) In some embodiments, the one or more blocking layers are located between one or both of the anode and the donor material and the cathode and the acceptor material. Examples of blocking layers are described in U.S. Patent Publication Nos. 2012/0235125 and 2011/0012091 and in U.S. Pat. Nos. 7,230,269 and 6,451,415, which are incorporated herein by reference for their disclosure of blocking layers.

(40) The organic optoelectronic devices of the present disclosure may exist as a tandem device comprising two or more subcells. Subcell, as used herein, means a component of the device which comprises at least one donor-acceptor heterojunction. When a subcell is used individually as a photosensitive optoelectronic device, it typically includes a complete set of electrodes. A tandem device may comprise charge transfer material, electrodes, or charge recombination material or a tunnel junction between the tandem donor-acceptor heterojunctions. In some tandem configurations, it is possible for adjacent subcells to utilize common, i.e., shared, electrode, charge transfer region or charge recombination zone. In other cases, adjacent subcells do not share common electrodes or charge transfer regions. The subcells may be electrically connected in parallel or in series.

(41) In some embodiments, the charge transfer layer or charge recombination layer may be chosen from Al, Ag, Au, MoO.sub.3, Li, LiF, Sn, Ti, WO.sub.3, indium tin oxide (ITO), tin oxide (TO), gallium indium tin oxide (GITO), zinc oxide (ZO), or zinc indium tin oxide (ZITO). In another embodiment, the charge transfer layer or charge recombination layer may be comprised of metal nanoclusters, nanoparticles, or nanorods.

(42) Organic optoelectronic devices of the present disclosure may function, for example, as PV devices, such as solar cells, photodetectors, or photoconductors.

(43) Whenever the organic photosensitive optoelectronic devices described herein function as a PV device, the materials used in the photoconductive organic layers and the thicknesses thereof may be selected, for example, to optimize the external quantum efficiency of the device. Similarly, the thicknesses of the anode and cathode buffer layers can be selected to optimize the external quantum efficiency of the device. For example, appropriate thicknesses can be selected to achieve the desired optical spacing in the device and/or reduce resistance in the device. The thickness of the intermediate layers, for example, may also be optimized to reduce resistance. Whenever the organic photosensitive optoelectronic devices described herein function as photodetectors or photoconductors, the materials used in the photoconductive organic layers and the thicknesses thereof may be selected, for example, to maximize the sensitivity of the device to desired spectral regions.

(44) There is also disclosed a method of preparing an organic optoelectronic device comprising depositing a first buffer layer on a first electrode comprising an anode or cathode, depositing at least one donor material and at least one acceptor material over the first buffer layer, depositing a second buffer layer over the donor and acceptor materials, and depositing a second electrode on the second buffer layer, wherein the second electrode is a cathode when the first electrode is an anode, or an anode when the first electrode is a cathode; wherein an intermediate layer is deposited adjacent to at least one of the first and second buffer layers; and wherein the intermediate layer is chosen to facilitate the transport of one of holes and electrons to the adjacent buffer layer.

(45) In some embodiments, the intermediate layer is deposited adjacent to the first buffer layer, and the method further comprises depositing a second intermediate layer adjacent to the second buffer layer, wherein the second intermediate layer is chosen to facilitate the transport of one of holes and electrons to the adjacent second buffer layer.

(46) In another disclosed method of preparing an optoelectronic device, the method comprises depositing at least one donor material and at least one acceptor material over a first electrode comprising an anode or cathode, and depositing a second electrode over the donor and acceptor materials, wherein the second electrode is a cathode when the first electrode is an anode, or an anode when the first electrode is a cathode; wherein at least one buffer layer is deposited adjacent to at least one of the anode and cathode; wherein an intermediate layer is deposited adjacent to the at least one buffer layer and is chosen to facilitate the transport of one of holes and electrons to the adjacent buffer layer.

(47) The materials comprising the optoelectronic devices of the present disclosure may be deposited using methods known in the art.

(48) In some embodiments, the organic materials or organic layers, or organic thin films, can be applied via solution processing, such as by one or more techniques chosen from spin-coating, spin-casting, spray coating, dip coating, doctor-blading, inkjet printing, or transfer printing. For molecules which degrade at temperatures lower than the evaporation or sublimation point, solution-processing technique can be used to achieve uniform, high-quality thin films for electronic purposes.

(49) In other embodiments, the organic materials may be deposited using vacuum evaporation, such as vacuum thermal evaporation, organic vapor phase deposition, or organic vapor-jet printing.

(50) The anode and cathode buffer layers and intermediate layers may be deposited, for example, by vacuum evaporation, such as vacuum thermal evaporation, vapor phase deposition techniques, such as OVPD, or by solution processing that utilizes orthogonal solvents to previous and subsequent layers.

(51) Schematics of example organic optoelectronic devices according to the present disclosure are shown in FIG. 1. Electrode 110 comprises an anode or cathode. Electrode 135 comprises a cathode when electrode 110 comprises an anode. Electrode 135 comprises an anode when electrode 110 comprises a cathode. Organic layers 120 and 125 form a donor-acceptor heterojunction as described herein. Organic layer 120 comprises at least one donor material or at least one acceptor material. Organic layer 125 comprises at least one donor material when layer 120 comprises at least one acceptor material. Organic layer 125 comprises at least one acceptor material when layer 120 comprises at least one donor material.

(52) In some embodiments, buffer layers 115 and 130 may be independently chosen from transition metal oxides and conductive polymers as described herein. Buffer layer 115 is an anode buffer layer when electrode 110 is an anode, and is a cathode buffer layer when electrode 110 is a cathode. Similarly, Buffer layer 130 is an anode buffer layer when electrode 135 is an anode, and is a cathode buffer layer when electrode 135 is a cathode. In some embodiments, as in device A, the device includes an intermediate layer 140 adjacent to a buffer layer 130. In some embodiments, as in device B, the device includes two intermediate layers, one adjacent to buffer layer 130 and one adjacent to buffer layer 115.

(53) In some embodiments, as in device C, the device includes only one buffer layer 130, and one intermediate layer 140. In certain embodiments, buffer layer 130 may be chosen from transition metal oxides and conductive polymers, and is an anode buffer layer when electrode 135 is an anode, and is a cathode buffer layer when electrode 135 is a cathode.

(54) As described herein, additional layers may be included, such as blocking layers or transport layers. For example, FIG. 2 is provided as a specific, non-limiting embodiment of the present disclosure. In FIG. 2, the anode buffer layer is MoO.sub.3 (20 nm) and the cathode buffer layer is MoO.sub.3 (30 nm). The donor material is DPSQ. The acceptor material is C.sub.70. PTCBI is used as an exciton-blocking, electron-transport layer, and an intermediate layer comprises Ag nanoparticles, nanoclusters, or nanorods. The two electrodes comprising the anode and cathode may be chosen as described herein.

(55) It should be understood that embodiments described herein may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional organic optoelectronic devices may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Additional layers not specifically described may also be included. Materials other than those specifically described may be used. Although the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting.

(56) Other than in the examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, analytical measurements and so forth, used in the specification and claims are to be understood as being modified in all instances by the term about. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.

(57) Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, unless otherwise indicated the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

(58) The methods and devices described herein will be further described by the following non-limiting examples, which are intended to be purely exemplary.

EXAMPLES

(59) Organic PV devices having anode and cathode buffer layers adjacent to an anode and cathode, respectively, were fabricated. Devices were fabricated with the following structure: glass/anode/20 nm MoO.sub.3/13 nm DPSQ/10 nm C.sub.70/5 nm PTCBI/0.1 nm Ag/30 nm MoO.sub.3/cathode, where the anode was either ITO (pre-coated) or 20 nm Ag, and the cathode consisted of 100 nm Ag, 20 nm Ag, or 40 nm sITO. FIG. 2 shows a schematic of the devices. Although MoO.sub.3 typically has an electronegative lowest unoccupied molecular orbital (LUMO), the Ag nanocluster layer deposited on top of PTCBI has the effect of aligning the Fermi level of PTCBI and MoO.sub.3. The result is efficient electron transport from PTCBI to MoO.sub.3 to the cathode.

(60) Devices were grown on either 100 nm thick layers of ITO pre-coated onto glass substrates or 20 nm thick layers of Ag on glass. Prior to deposition, the ITO or glass surface was cleaned in a surfactant and a series of solvents and then exposed to ultraviolet-ozone for 10 minutes before loading into a high vacuum chamber (base pressure <10.sup.?7 Torr) where MoO.sub.3 was thermally evaporated at ?0.1 nm/s.

(61) Substrates were then transferred to a N.sub.2 glovebox where DPSQ films were spin-coated from filtered 1.6 mg/ml solutions in chloroform. Substrates were again transferred into the high vacuum chamber for deposition of purified organics at 0.1 nm/s, followed by transfer back into the glovebox and exposure to saturated chlorophorm vapors for 10 minutes to create a favorable film morphology. After a transfer back to the vacuum chamber, a 0.1 nm Ag silver nanocluster layer and a MoO.sub.3 transport layer were deposited.

(62) The cathode material (Ag or ITO) was deposited through a shadow mask with an array of 1 mm diameter openings. Sputtered ITO (sITO) was deposited at 0.01 nm/s with 20 W DC power. Current density versus voltage (J-V) characteristics were measured in an ultra-pure N.sub.2 ambient, in the dark and under simulated AM1.5G solar illumination from a filtered 300 W Xe lamp. Lamp intensity was varied using neutral density filters. Optical intensities were referenced using an NREL-calibrated Si detector, and photocurrent measurements were corrected for spectral mismatch.

(63) Device characteristics under 1 sun AM1.5G simulated illumination and in the dark are shown in FIGS. 3(a) and 3(b), respectively, and performance is summarized in Table 1 as follows:

(64) TABLE-US-00001 TABLE 1 Device performance at one sun illumination. J.sub.SC V.sub.OC PCE R.sub.S Anode Cathode (mA/cm.sup.2) (V) FF (%) (? cm.sup.2) ITO 100 nm Ag 6.0 0.92 0.67 3.7 0.87 20 nm Ag 20 nm Ag 2.8 0.89 0.65 1.6 0.64 20 nm Ag 100 nm Ag 3.7 0.90 0.67 2.2 0.29 ITO sITO 1.4 0.86 0.52 0.8 14.1

(65) The difference in short-circuit current (J.sub.SC) can be attributed to the difference in reflectivity of the electrodes used. In the case of an ITO anode, reflectivity is low, leading to more light absorbed at the active layers. Using 20 nm Ag as the anode is more reflective, decreasing responsivity. For the cathode, using 100 nm Ag reflects light back though the active layers, further increasing responsivity, while 20 nm Ag and sITO are increasingly transparent, leading to lower responsivity. This transparency, however, can be advantageous when designing a semi-transparent organic PV. These differences in responsivity can also be seen in the external quantum efficiency data shown in FIG. 4. For the device with 20 nm Ag as the anode and 100 nm Ag as the cathode, the peak at 500 nm can be attributed to microcavity effects, which can be turned to enhance certain wavelengths.

(66) The difference in V.sub.OC between these four devices can be correlated to the difference in J.sub.SC. There is a known relationship between V.sub.OC and J.sub.SC:
qV.sub.OC=?E.sub.m+k.sub.ST ln(J.sub.SC/J.sub.C).(1)
where q is the electron charge, ?E.sub.HL is the energy difference between the donor highest occupied molecular orbital (HOMO) and the donor LUMO, k.sub.b is Boltzmann's constant, T is the temperature and J.sub.0 is the saturation dark current. It can be seen from Eq. 1 that there is a logarithmic dependence of V.sub.OC and J.sub.SC. By plotting V.sub.OC as a function of J.sub.SC in FIG. 5, it is observed that the data for all four devices falls on a line. This indicates that all four devices are operating similarly, and the differences in V.sub.OC are only due to differences in the amount of light absorbed by the active layers.

(67) By fitting the dark J-V data to the ideal diode equation, the series resistance (R.sub.S) can be extracted for each device:

(68) $\begin{array}{cc}J=J_S\left[\exp \left(\frac{q\left(V-{\mathrm{JR}}_S\right)}{{\mathrm{nk}}_{b}T}\right)-1\right].& \left(2\right)\end{array}$
where J.sub.S is the reverse saturation current, n is the ideality factor, and T is the temperature. As shown in Table 1, devices with ITO and Ag have very low R.sub.S, <1 ?cm.sup.2. For the device with sITO, it is much higher, with R.sub.S=14.1 ?cm.sup.2. This is due to the fact that sITO is of lower quality than ITO (sheet resistance ?200?/? vs. 15?/?), leading to increased R.sub.S and decreased FF for this device.

(69) Although the present disclosure is described with respect to particular examples and embodiments, it is understood that the devices described herein are not limited to these examples and embodiments. The embodiments as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art.