Inverted organic photosensitive devices

Abstract

The present disclosure relates to organic photosensitive optoelectronic devices grown in an inverted manner. An inverted organic photosensitive optoelectronic device of the present disclosure comprises a reflective electrode, an organic donor-acceptor heterojunction over the reflective electrode, and a transparent electrode on top of the donor-acceptor heterojunction.

Claims

1. A method for producing an inverted, organic photovoltaic device, said method comprising: providing a metal reflective electrode; performing at least an ultra-violet ozone (UV-O.sub.3) surface treatment on said metal reflective electrode; forming an organic donor-acceptor heterojunction over said metal reflective electrode; and forming a transparent electrode over said organic donor-acceptor heterojunction.

2. The method of claim 1, wherein the reflective electrode is positioned over a substrate.

3. The method of claim 1, wherein the donor of the organic donor-acceptor heterojunction comprises a material selected from phthalocyanines, porphyrins, subphthalocyanines, and derivatives or transition metal complexes thereof.

4. The method of claim 1, wherein the donor of the donor-acceptor heterojunction comprises copper phthalocyanine.

5. The method of claim 1, wherein the acceptor of the organic donor-acceptor heterojunction comprises a material selected from polymeric or non-polymeric perylenes, naphthalenes, and fullerenes.

6. The method of claim 1, wherein the acceptor of the organic donor-acceptor heterojunction comprises 3,4,9,10-perylenetetracarboxylic bis-benzimidazole.

7. The method of claim 1, wherein the transparent electrode comprises a material selected from transparent oxides and metal or metal substitutes.

8. The photosensitive device of claim 1, wherein the transparent electrode permits at least about 50% of ambient electromagnetic radiation to be transmitted through said electrode.

9. The method of claim 1, wherein the transparent electrode comprises a material selected from tin oxide, gallium indium tin oxide, and zinc indium tin oxide.

10. The method of claim 1, further comprising the step of providing an exciton blocking layer.

11. The method of claim 10, wherein the exciton blocking layer is positioned between the reflective electrode and the transparent electrode.

12. The method of claim 10, wherein the exciton blocking layer is positioned between the acceptor of the organic donor-acceptor heterojunction and the transparent electrode.

13. The method of claim 10, wherein the exciton blocking layer comprises a material selected from N,N-diphenyl-N,N-bis-alpha-naphthylbenzidine, aluminum tris (8-hydroxyquinoline), carbazole biphenyl, bathocuproine, and tris(acetylacetonato) ruthenium (III).

14. The method of claim 1, wherein the organic donor-acceptor heterojunction comprises a structure selected from planar heterojunctions, bulk heterojunctions, nanocrystalline bulk heterojunctions, hybrid planar-mixed heterojunctions, and mixed heterojunctions.

15. The method of claim 1, further comprising performing a plasma treatment before performing the UV-O.sub.3 surface treatment on the reflective electrode.

16. The method of claim 15, wherein the plasma surface treatment is selected from oxygen plasma treatment and argon plasma treatment.

17. The method of claim 1, wherein the metal reflective electrode is a metal anode.

18. The method of claim 1, wherein the metal reflective electrode comprises a metal chosen from nickel, silver, aluminum, magnesium, indium, and mixtures or alloys thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an inverted organic PV device comprising a reflective electrode formed over a substrate, an organic donor-acceptor heterojunction on top of said reflective electrode, and a transparent electrode on top of said donor-acceptor heterojunction.

(2) FIG. 2a is a plot of PTCBI thicknesses versus eta (.sub.Ppower conversion efficiency) and responsivity (J.sub.SC/P.sub.0), wherein A/W connotes Amps per Watt and sim connotes simulated.

(3) FIG. 2b is a plot of PTCBI versus V.sub.OC and FF.

(4) FIG. 2c is a plot of PTCBI thickness versus series resistance (R.sub.S) and n.

(5) FIG. 2d is a plot of PTCBI thickness versus reverse saturation current (J.sub.S).

(6) FIG. 3a is a plot of CuPc thicknesses versus eta and J.sub.SC/P.sub.0.

(7) FIG. 3b is a plot of CuPc versus V.sub.OC and FF.

(8) FIG. 3c is a plot of CuPc thickness versus R.sub.S and.

(9) FIG. 3d is a plot of CuPc thickness versus J.sub.S.

(10) FIG. 4a shows calculations from a standard transfer-matrix simulation performed on a control PV device grown on glass: ITO (1550 )/CuPc (200 )/PTCBI (250 )/BCP (100 )/Ag (1000 ). The optical fields at peak absorption of CuPc at 625 nm (squares) and of PTCBI at 540 nm (stars) are shown.

(11) FIG. 4b shows calculations from a standard transfer-matrix simulation performed on an inverted PV device consistent with the embodiments described herein: quartz/Ag (1000 )/BCP (100 )/PTCBI (300 )/CuPc (150 )/ITO (400 ). The optical fields at peak absorption of CuPc at 625 nm (squares) and of PTCBI at 540 nm (stars) are shown.

(12) FIG. 4c shows calculations from a standard transfer-matrix simulation performed on an inverted PV device consistent with the embodiments described herein: quartz/Ni (1000 )/CuPc (400 )/PTCBI (100 )/BCP (100 )/ITO (400 ). The optical fields at peak absorption of CuPc at 625 nm (squares) and of PTCBI at 540 nm (stars) are shown.

(13) FIG. 5a shows current-voltage curves for a control PV device grown on glass: ITO (1550 )/CuPc (200 )/PTCBI (250 )/BCP (100 )/Ag (1000 ) in the dark (filled squares) and under simulated 1-sun illumination (open circles). FIG. 5a also shows current-voltage curves for an inverted PV device consistent with the embodiments described herein: quartz/Ni (1000 )/CuPc (400 )/PTCBI (100 )/BCP (100 )/ITO (400 ) in the dark (filled triangles) and under simulated 1-sun illumination (open triangles). The lines are fit to the dark current curves.

(14) FIG. 5b shows .sub.P (squares), V.sub.OC (stars) and FF (triangles) as a function of incident power density for an inverted PV device consistent with the embodiments described herein: quartz/Ni (1000 )/CuPc (400 )/PTCBI (100 )/BCP (100 )/ITO (400 ) treated with Ar plasma.

(15) FIG. 6a shows the current-voltage characteristics for a control device (glass/ITO (1550 )/CuPc (200 )/PTCBI (250 )/BCP (100 )/Ag (1000 )) in the dark (squares) and under simulated 1 sun, AM1.5G illumination (dashed lines), and for inverted PV device consistent with the embodiments described herein: quartz/Ni (1000 )/CuPc (350 )/PTCBI (100 )/BCP (100 )/ITO (400 ) in the dark (triangles) and under illumination (dashed-dotted line).

(16) FIG. 6b shows the .sub.P (circles), V.sub.OC (triangles), and FF (squares) for an inverted PV device comprising quartz/Ni (1000 )/CuPc (350 )/PTCBI (100 )/BCP (100 )/ITO (400 ).

(17) FIG. 7a shows the simulated (line) and measured (filled squares) photocurrent at one sun intensity of inverted PV devices having varying CuPc thicknesses (x=100 to 400 ) in a structure comprising quartz/Ni (1000 )/CuPc (x )/PTCBI (100 )/BCP (100 )/ITO (400 ).

(18) FIG. 7b shows .sub.P (squares), V.sub.OC (triangles), and FF at one sun AM1.5G illumination of inverted PV devices having varying CuPc thicknesses (x=100 to 400 ) in a structure comprising quartz/Ni (1000 )/CuPc (x )/PTCBI (100 )/BCP (100 )/ITO (400 ).

(19) FIG. 8a shows the simulated (line) and measured (filled squares) photocurrent at one sun intensity of inverted PV devices having varying PTCBI thicknesses (y=0 to 300 ) in a structure comprising quartz/Ni (1000 )/CuPc (400 )/PTCBI (y )/BCP (100 )/ITO (400 ).

(20) FIG. 8b shows .sub.P (squares), V.sub.OC (triangles), and FF at one sun AM1.5G illumination of inverted PV devices having varying PTCBI thicknesses (y=0 to 300 ) in a structure comprising quartz/Ni (1000 )/CuPc (400 )/PTCBI (y )/BCP (100 )/ITO (400 ).

(21) FIG. 9a shows a simulated contour plot for J.sub.SC as a function of CuPc and PTCBI thicknesses.

(22) FIG. 9b shows a simulated contour plot for .sub.P as a function of CuPc and PTCBI thicknesses.

DETAILED DESCRIPTION

(23) Inverted organic photosensitive optoelectronic devices are described herein. The organic devices described may be used, for example, to generate a usable electrical current from incident electromagnetic radiation (e.g., PV devices) or may be used to detect incident electromagnetic radiation. Some embodiments may comprise an anode, a cathode, and a photoactive region between the anode and the cathode. The photoactive region is the portion of the photosensitive device that absorbs electromagnetic radiation to generate excitons that may dissociate in order to generate an electrical current. The devices described herein may also include at least one transparent electrode to allow incident radiation to be absorbed within the device. Several PV device materials and configurations are described in U.S. Pat. Nos. 6,657,378, 6,580,027, and 6,352,777, which are incorporated herein by reference for their disclosure of PV device materials and configurations.

(24) As used herein, the term 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).

(25) The terms electrode and contact are used herein to refer to a layer that provides a medium for delivering photo-generated current to an external circuit or providing a bias current or voltage to the device. That is, an electrode, or contact, provides the interface between the active regions of an organic photosensitive optoelectronic device and a wire, lead, trace or other means for transporting the charge carriers to or from the external circuit. Anodes and cathodes are examples. U.S. Pat. No. 6,352,777, incorporated herein by for its disclosure of electrodes, provides examples of electrodes, or contacts, which may be used in a photosensitive optoelectronic device. In a photosensitive optoelectronic device, it may be desirable to allow the maximum amount of ambient electromagnetic radiation from the device exterior to be admitted to the photoconductively active interior region. That is, the electromagnetic radiation must reach a photoconductive layer(s), where it can be converted to electricity by photoconductive absorption. This often dictates that at least one of the electrical contacts should be minimally absorbing and minimally reflecting of the incident electromagnetic radiation. In some cases, such a contact should be substantially transparent. The opposing electrode may be a reflective material so that light which has passed through the cell without being absorbed is reflected back through the cell. As used herein, a layer of material or a sequence of several layers of different materials is said to be transparent when the layer or layers permit at least about 50% of the ambient electromagnetic radiation in relevant wavelengths to be transmitted through the layer or layers. Similarly, layers which permit some, but less than about 50% transmission of ambient electromagnetic radiation in relevant wavelengths are said to be semi-transparent.

(26) The term cathode is used in the following manner. In a non-stacked PV device or a single unit of a stacked PV device under ambient irradiation and connected with a resistive load and with no externally applied voltage, e.g., a PV device, electrons move to the cathode from the photo-conducting material. Similarly, the term anode is used herein such that in a PV device under illumination, holes move to the anode from the photoconducting material, which is equivalent to electrons moving in the opposite manner. It will be noted that as the terms are used herein, anodes and cathodes may be electrodes or charge transfer layers.

(27) As used herein, top means furthest away from the substrate structure (if present), while bottom means closest to the substrate structure. If the device does not include a substrate structure, then top means furthest away from the reflective electrode. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate structure, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom side closest to the substrate, and a top side further away from the substrate. Where a first layer is described as disposed over or on top of a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is in physical contact with the second layer. For example, a cathode may be described as disposed over or on top of an anode, even though there are various organic layers in between.

(28) FIG. 1 shows inverted organic photosensitive optoelectronic device 100. The figures are not necessarily drawn to scale. Device 100 may include reflective substrate 110, donor layer 115, acceptor layer 120, optional blocking layer 125, and transparent electrode 130. Device 100 may be fabricated by depositing the layers described, in order. In some embodiments, the device described in FIG. 1 may optionally include a very thin, damage inducing metal layer between blocking layer 125 and transparent electrode 130 such that transparency is not impacted. Device 100 may also optionally include substrate structure 135. In some embodiments, the substrate structure may directly support reflective electrode 110.

(29) The specific arrangement of layers illustrated in FIG. 1 is exemplary only, and is not intended to be limiting. For example, some of the layers (such as blocking layers) may be omitted. Other layers (such as reflective electrode or additional acceptor and donor layers) may be added. The order of layers may be altered. Arrangements other than those specifically described may be used. Additionally, the organic PV device may exist as a tandem device comprising one or more additional donor-acceptor layers. A tandem device may have charge transfer layers, electrodes, or charge recombination layers between the tandem donor-acceptor layers. The substrate and reflective electrode may be combined, the substrate may be reflective and the electrode transparent.

(30) Substrate 135, onto which the device may be grown or placed, may be any suitable material that provides the desired structural properties. The substrate may be flexible or rigid, planar or non-planar. The substrate may be transparent, translucent or opaque. Plastic, glass, and quartz are examples of rigid substrate materials. Plastic and metal foils are examples of flexible substrate materials. The material and thickness of the substrate may be chosen to obtain the desired structural and optical properties.

(31) In some embodiments, reflective electrode 110 may comprise an electrode, such as a metal anode. In some embodiments, reflective electrode 110 may comprise a low work function metal selected from steel, Ni, Ag, Al, Mg, In, and mixtures or alloys thereof. In some embodiments, the electrode may comprise one metal as the base and one as the electrode material, such as Ti, stainless steel, or Al sheets, with or without Ag on top.

(32) In some embodiments, reflective electrode 110 and substrate material 135 may be combined or formed of two metals. In some embodiments substrate 135 is reflective and electrode 110 is transparent.

(33) In some embodiments, the electrodes described herein may be composed of metal or metal substitutes. Herein, the term metal is used to embrace 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. Here, 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. Commonly used metal substitutes for electrodes and charge transfer layers would include doped wide-bandgap semiconductors, for example, transparent conducting oxides such as indium tin oxide (ITO), gallium indium tin oxide (GITO), and zinc indium tin oxide (ZITO). In particular, ITO is a highly doped degenerate n+ semiconductor with an optical bandgap of approximately 3.2 eV, rendering it transparent to wavelengths greater than approximately 3900 . Another suitable metal substitute is the transparent conductive polymer polyaniline (PANT) and its chemical relatives.

(34) Metal substitutes may be further selected from a wide range of non-metallic materials, wherein the term non-metallic is meant to embrace a wide range of materials, provided that the material is free of metal in its chemically uncombined form. When a metal is present in its chemically uncombined form, either alone or in combination with one or more other metals as an alloy, the metal may alternatively be referred to as being present in its metallic form or as being a free metal. Thus, the metal substitute electrodes described herein may sometimes be referred to as metal-free, wherein the term metal-free is expressly meant to embrace a material free of metal in its chemically uncombined form. Free metals typically have a form of metallic bonding that results from a sea of valence electrons which are free to move in an electronic conduction band throughout the metal lattice. While metal substitutes may contain metal constituents, they are non-metallic on several bases. They are not pure free-metals nor are they alloys of free-metals. When metals are present in their metallic form, the electronic conduction band tends to provide, among other metallic properties, a high electrical conductivity as well as a high reflectivity for optical radiation.

(35) Transparent electrode 130 may be chosen from transparent oxides and metal or metal substitutes having a thickness sufficient to render them transparent. Commonly used metal substitutes for electrodes and charge transfer layers would include doped wide-bandgap semiconductors, for example, transparent conducting oxides. In some embodiments, transparent electrode 130 may be selected from ITO, GITO, and ZITO. Other exemplary electrodes include highly transparent, non-metallic, low resistance cathodes such as those disclosed in U.S. Pat. No. 6,420,031, to Parthasarathy et al., or a highly efficient, low resistance metallic/non-metallic compound cathode such as those disclosed in U.S. Pat. No. 5,703,436 to Forrest et al., both incorporated herein by reference for their disclosure of cathodes. Each type of cathode is typically prepared in a fabrication process that includes the step of sputter depositing an ITO layer onto either an organic material, such as CuPc, to form a highly transparent, non-metallic, low resistance cathode or onto a thin Mg:Ag layer to form a highly efficient, low resistance metallic/non-metallic compound cathode.

(36) The devices described herein will comprise at least one photoactive region in which light is absorbed to form an excited state, or exciton, which may subsequently dissociate in to an electron and a hole. The dissociation of the exciton will typically occur at the heterojunction formed by the juxtaposition of an donor layer and an acceptor layer. For example, in the device of FIG. 1, the photoactive region may include donor layer 115 and acceptor layer 120. Charge separation may occur predominantly at the organic heterojunction between donor layer 115 and acceptor layer 120. The built-in potential at the heterojunction is determined by the HOMO-LUMO energy level difference between the two materials contacting to form the heterojunction. The HOMO-LUMO gap offset between the donor and acceptor materials produces an electric field at the donor-acceptor interface that facilitates dissociation of excitons created within an exciton diffusion length of the interface into opposite signed carriers (holes and electrons).

(37) Suitable materials comprising acceptor layer 120 may include, for example, polymeric or non-polymeric perylenes, naphthalenes, fullerenes or nanotubules. In some embodiments, acceptor layer 120 may comprise 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). In other embodiments, acceptor layer 120 may comprise a fullerene material as described in U.S. Pat. No. 6,580,027, the description of fullerene material which is incorporated herein by reference in its entirety. In some embodiments, donor layer 115 may comprise squaraines, phthalocyanine, porphyrin, subphthalocyanine (SubPc), copper phthalocyanine (CuPc), or a derivative or transition metal complex thereof such as aluminum phthalocyanine chloride (AlClPc).

(38) Other suitable organic materials for use in the photoactive layers may include cyclometallated organometallic compounds. The term organometallic as used herein is as generally understood by one of ordinary skill in the art and as given, for example, in Inorganic Chemistry (2nd Edition) by Gary L. Miessler and Donald A. Tarr, Prentice Hall (1998). Thus, the term organometallic may refer to compounds which have an organic group bonded to a metal through a carbon-metal bond. Organometallic compounds may comprise, in addition to one or more carbon-metal bonds to an organic species, one or more donor bonds from a heteroatom. The carbon-metal bond to an organic species may refer, for example, to a direct bond between a metal and a carbon atom of an organic group, such as phenyl, alkyl, alkenyl, etc. The term cyclometallated refers to compounds that comprise a bidentate organometallic ligand so that, upon bonding to a metal, a ring structure is formed that includes the metal as one of the ring members.

(39) As alluded to above with respect to the term layer, it should be understood that the boundary of acceptor layer 120 and donor layer 115, as depicted in FIG. 1, may be imperfect, discontinuous, and/or otherwise represent an interpenetrating, entangled or convoluted network of donor and acceptor materials. For example, in some embodiments, while the organic donor-acceptor heterojunction may form a planar heterojunction, in others it may form a bulk heterojunction, nanocrystalline bulk heterojunction, hybrid planar-mixed heterojunction, or mixed heterojunction. In some embodiments, two or more organic donor-acceptor heterojunctions may be used to create a tandem inverted PV device.

(40) Organic layers may be fabricated using vacuum deposition, spin coating, organic vapor-phase deposition, inkjet printing, and other methods known in the art.

(41) Organic photosensitive optoelectronic devices of the embodiments described herein may function as a PV device, photodetector or photoconductor. 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. 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.

(42) The desired result may be achieved by considering several guidelines that may be used in the selection of layer thicknesses. It may be desirable for the layer thickness, L, to be less than or on the order of an exciton diffusion length, L.sub.D, since it is believed that most exciton dissociation will occur within a diffusion length of a donor-acceptor interface. In this description, L is the distance from the exciton formation site and a donor-acceptor interface. If L is greater than L.sub.D, then many excitons may recombine before dissociation. It is further desirable for the total photoconductive layer thickness to be of the order of the electromagnetic radiation absorption length, 1 (where is the absorption coefficient), so that nearly all of the radiation incident on the PV device is absorbed to produce excitons. Furthermore, the photoconductive layer thickness should be as thin as possible to avoid excess series resistance due to the high bulk resistivity of organic semiconductors.

(43) Accordingly, such competing guidelines inherently may require tradeoffs to be made in selecting the thickness of the photoconductive organic layers of a photosensitive optoelectronic cell. Thus, on the one hand, a thickness that is comparable or larger than the absorption length may be desirable in order to absorb the maximum amount of incident radiation. On the other hand, as the photoconductive layer thickness increases, two undesirable effects may be increased. One may be due to the high series resistance of organic semiconductors, as an increased organic layer thickness may increase device resistance and reduce efficiency. Another undesirable effect is that increasing the photoconductive layer thickness may increase the likelihood that excitons will be generated far from the charge-separating interface, resulting in enhanced probability of geminate recombination and, again, reduced efficiency. Therefore, it may be desirable to have a device configuration that balances between such competing effects, in a manner that produces a high external quantum efficiency for the overall device.

(44) The device of FIG. 1 may further include one or more of blocking layer 125, such as the exciton blocking layers (EBLs) described in U.S. Pat. No. 6,097,147, Peumans et al., Applied Physics Letters 2000, 76, 2650-52, and U.S. Pat. No. 6,451,415, Forrest et al., all of which are incorporated herein by reference for their disclosure of blocking layers. In certain embodiments, higher internal and external quantum efficiencies have been achieved by the inclusion of an EBL to confine photogenerated excitons to the region near the dissociating interface and to prevent parasitic exciton quenching at a photosensitive organic/electrode interface. In addition to limiting the volume over which excitons may diffuse, an EBL can also act as a diffusion barrier to substances introduced during deposition of the electrodes. In some circumstances, an EBL can be made thick enough to fill pinholes or shorting defects which could otherwise render an organic PV device non-functional. An EBL can therefore help protect fragile organic layers from damage produced when electrodes are deposited onto the organic materials.

(45) Without being bound to any particular theory, it is believed that the EBLs derive their exciton blocking property from having a LUMO-HOMO energy gap substantially larger than that of the adjacent organic semiconductor from which excitons are being blocked. Thus, the confined excitons are prohibited from existing in the EBL due to energy considerations. While it is desirable for the EBL to block excitons, it is not desirable for the EBL to block all charge. However, due to the nature of the adjacent energy levels, an EBL may block one sign of charge carrier. By design, an EBL will exist between two other layers, usually an organic photosensitive semiconductor layer and an electrode, a charge transfer layer or a charge recombination layer. The adjacent electrode or charge transfer layer will be in context either a cathode or an anode. Therefore, the material for an EBL in a given position in a device will be chosen so that the desired sign of carrier will not be impeded in its transport to the electrode or charge transfer layer. Proper energy level alignment ensures that no barrier to charge transport exists, preventing an increase in series resistance. In certain embodiments, it may be desirable for a material used as a cathode side EBL to have a LUMO energy level closely matching the LUMO energy level of the adjacent acceptor material so that any undesired barrier to electrons is minimized.

(46) It should be appreciated that the exciton blocking nature of a material is not necessarily an intrinsic property of its HOMO-LUMO energy gap. Whether a given material will act as an exciton blocker depends upon the relative HOMO and LUMO energy levels of the adjacent organic photosensitive material. Therefore, it may not be possible to identify a class of compounds in isolation as exciton blockers without regard to the device context in which they may be used. However, with the teachings herein, one of ordinary skill in the art may identify whether a given material will function as an exciton blocking layer when used with a selected set of materials to construct an organic PV device.

(47) In some embodiments, blocking layer 125 may comprise an EBL situated between acceptor layer 120 and transparent electrode 130. Examples of suitable EBL materials include, but are not limited to, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (also called bathocuproin or BCP), which is believed to have a LUMO-HOMO energy level separation of about 3.5 eV, or bis(2-methyl-8-hydroxyquinolinoato)-aluminum(III)phenolate (Alq.sub.2OPH). BCP may be an effective exciton blocker which can easily transport electrons to the cathode from an acceptor layer. In other embodiments, the EBL may be selected from N,N-diphenyl-N,N-bis-alpha-naphthylbenzidine (NPD), aluminum tris (8-hydroxyquinoline) (Alq3), carbazole biphenyl (CBP), and tris(acetylacetonato) ruthenium (III) (Ru(acac).sub.3).

(48) In some embodiments, blocking layer 125 may comprise an EBL doped with a suitable dopant, including but not limited to 3,4,9,10-perylenetracarboxylic dianhydride (PTCDA), 3,4,9,10-perylenetracarboxylic diimide (PTCDI), 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), and derivatives thereof. BCP, as deposited in the devices described herein, may be amorphous. Amorphous BCP exciton blocking layers may exhibit film recrystallization, which may be especially rapid under high light intensities. The resulting morphology change to polycrystalline material results in a lower quality film with possible defects such as shorts, voids or intrusion of electrode material. Accordingly, it has been found that doping of some EBL materials, such as BCP, that exhibit this effect with a suitable, relatively large and stable molecule can stabilize the EBL structure to prevent performance degrading morphology changes. It should be further appreciated that doping of an EBL which is transporting electrons in a given device with a material having a LUMO energy level close to that of the EBL may help to insure that electron traps are not formed which might produce space charge build-up and reduce performance. Additionally, it should be appreciated that relatively low doping densities should minimize exciton generation at isolated dopant sites. Since such excitons are effectively prohibited from diffusing by the surrounding EBL material, such absorptions reduce device photoconversion efficiency.

(49) In some embodiments, the device of FIG. 1 may further comprise one or more transparent charge transfer layers or charge recombination layers. As described herein, charge transfer layers are distinguished from acceptor and donor layers by the fact that charge transfer layers are frequently, but not necessarily, inorganic (often metals) and they may be chosen not to be photoconductively active. The term charge transfer layer is used herein to refer to layers similar to but different from electrodes in that a charge transfer layer only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. The term charge recombination layer is used herein to refer to layers similar to but different from electrodes in that a charge recombination layer allows for the recombination of electrons and holes between tandem photosensitive devices and may also enhance internal optical field strength near one or more active layers. A charge recombination layer can be constructed of semi-transparent metal nanoclusters, nanoparticles or nanorods as described in U.S. Pat. No. 6,657,378, the disclosure of which is incorporated herein by reference.

(50) In other embodiments, a smoothing layer may be situated between reflective electrode 110 (e.g., anode) and donor layer 115. A exemplary material for this layer comprises a film of 3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS). The introduction of the PEDOT:PSS layer between reflective electrode 110 (e.g., anode comprising ITO) and donor layer 115 (e.g., CuPc) may lead to greatly improved fabrication yields. Without being bound to a particular theory, it is believed that the improved fabrication yields may be a result of the ability of the spin-coated PEDOT:PSS film to planarize the ITO, whose rough surface could otherwise result in shorts through the thin molecular layers.

(51) In a further embodiment, one or more of the layers of the FIG. 1 device may undergo surface treatments. For example, one or more of the layers may be treated with plasma prior to depositing the next layer. The layers may be treated, for example, with a mild argon or oxygen plasma. This treatment may be beneficial in reducing the series resistance. It may be advantageous to subject an optional PEDOT:PSS layer to a mild plasma treatment prior to deposition of the next layer. Alternatively, one or more of the layers may be exposed to ultra-violet ozone (UV-O.sub.3) treatment. In at least one embodiment, the reflective electrode (e.g., anode layer) is exposed to a surface treatment.

(52) The embodiments described herein also include a method for producing the organic PV device of FIG. 1, comprising: providing reflective electrode 110, performing at least one surface treatment on reflective electrode 110, forming an organic donor-acceptor heterojunction (e.g., donor layer 115 and acceptor layer 120) over reflective electrode 110, and forming transparent electrode 130 over said organic donor-acceptor heterojunction.

(53) The embodiments described herein also include methods for generating and/or measuring electricity. In some embodiments, that method comprises: providing light to the device of FIG. 1, which comprises reflective electrode 110, organic donor-acceptor heterojunction on top of said reflective electrode (e.g., donor layer 115 and acceptor layer 120), and transparent electrode 130 on top of said donor-acceptor heterojunction.

(54) In some embodiments, a power-generating device is described, which may include at least one device of FIG. 1, comprising: a reflective electrode 110; organic donor-acceptor heterojunction on top of said reflective electrode (e.g., donor layer 115 and acceptor layer 120); and transparent electrode 130 on top of said donor-acceptor heterojunction. In some embodiments, the device may be in the form of a paint, film, or foil. For example, in one embodiment, device 100 can be formed on substrate structure 135, which comprises a film, foil, or the like, or formed directly on the enclosure of a device, such as applying paint. In some embodiments, the device displays a .sub.p in a range from about 0.3 to about 0.4. In some embodiments, the device displays a V.sub.oc in a range from about 0.2 to about 1.5, such as about 0.4 to about 0.5. In some embodiments, the device displays a FF in the range of about 0.4 to about 0.85, such as about 0.5. In some embodiments, the device displays J.sub.sc/P.sub.0 in the range of about 0.002 to about 0.025 A/W, such as 0.02. In some embodiments, the device displays a R.sub.SA in a range from about 5 to about 12. In some embodiments, the device displays a J.sub.S in a range from about 210.sup.7 to about 710.sup.7. In some embodiments, the device displays an of less than about 2, such as approaching about 1.

(55) In further embodiments, the organic photosensitive optoelectronic devices described herein may function as photodetectors. In this embodiment, device 100 may be a multilayer organic device, for example, as described in U.S. Pat. No. 6,972,431, the disclosure of which is incorporated herein by reference. In this case, an external electric field may be generally applied to facilitate extraction of the separated charges.

(56) Coatings may be used to focus optical energy into desired regions of device 100. See, e.g., U.S. Pat. No. 7,196,835; U.S. patent application Ser. No. 10/915,410, which is incorporated by reference to provide examples of such a coating.

(57) The simple layered structure illustrated in FIG. 1 is provided by way of non-limiting example, and it is 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 photosensitive 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. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of 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. Organic layers that are not a part of the photoactive region, i.e., organic layers that generally do not absorb photons that make a significant contribution to photocurrent, may be referred to as non-photoactive layers. Examples of non-photoactive layers include EBLs and anode-smoothing layers. Other types of non-photoactive layers may also be used.

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

EXAMPLES

Example 1

(59) Inverted structures were demonstrated using the archetype donor-acceptor bilayer system formed by CuPc and PTCBI. Optical simulations were employed to predict device performance and optimize the inverted structures. Standard transfer-matrix calculations were performed to predict the J.sub.SC. See, e.g., Appl. Phys. Rev. 93, 3693 (2003) and J. Appl. Phys. 86, 487 (1999), which are incorporated herein by reference for the disclosure of transfer-matrix calculations. Results of the thickness studies are shown in FIGS. 2 and 3.

Example 2

(60) Optical constants of organic films grown on Si substrates were measured using ellipsometry, while those of Ni were taken from the literature. See, e.g., J. Phys. F: Metal Phys. 9, 2491 (1979), which is incorporated herein by reference for this purpose. Exciton diffusion lengths of CuPc and PTCBI were taken to be 80 and 40 , respectively, with lifetimes of 2 ns. See, e.g., Appl. Phys. Rev. 93, 3693 (2003), which is incorporated herein by reference for this purpose. In the simulations, three structures were investigated: one control PV and two inverted PVs. The control PV device was glass/ITO (1550 )/CuPc (200 )/PTCBI (250 )/BCP (100 )/Ag (1000 ). Results for this control PV can be seen in FIG. 4a. The first inverted PV device was quartz/Ag (1000 )/BCP (100 )/PTCBI (300 )/CuPc (150 )/ITO (400 ). Results for this control PV can be seen in FIG. 4b. The second inverted PV device was quartz/Ni (1000 )/CuPc (400 )/PTCBI (100 )/BCP (100 )/ITO (400 ). Results for this control PV can be seen in FIG. 4c.

Example 3

(61) Three different types of the second inverted PV device (quartz/Ni (1000 )/CuPc (400 )/PTCBI (100 )/BCP (100 )/ITO (400 )) were formed by preparing a quartz substrate by solvent cleaning. See, e.g., Organic Electron 6, 242 (2005), which is incorporated herein by references for this purpose. The quartz base structures were loaded into an electron beam evaporator where 1000 Ni were deposited at a rate of 10 /s. See, e.g., Appl. Phys. Lett. 86, 263502 (2005), which is incorporated herein by references for this purpose. The Ni anodes were exposed to three different surface treatments. The first was exposed to 30 minutes of ultra-violet ozone (UV-O.sub.3) treatment. The second was exposed to O.sub.2 plasma at a power of 120 W for 80 s. The third was exposed to Ar plasma at a power of 70 W for 2 minutes, followed by 30 minutes of UV-O.sub.3 treatment. The structures were then loaded in a high vacuum thermal deposition chamber having base pressure of 510.sup.7 Torr. Purified organic sources were grown at a pressure of 110.sup.6 Torr and a rate of 2 /s. See, e.g., Organic Electron 6, 242 (2005), which is incorporated herein by reference for this purpose. The planar double heterojunction solar cell PV structure grown consisted of a 400 thick CuPc donor layer, a 100 thick PTCBI acceptor layer, and a 1000 thick BCP exciton blocking (see, e.g., Appl. Phys. Lett. 76, 2650 (2000), which is incorporated herein by reference for this purpose) and damage absorbing layer. A vacuum break and exposure to air was necessary before attaching a shadow mask in a nitrogen ambient atmosphere. The top contact was formed by a 400 thick ITO cathode layer sputter-deposited at 15 W and 13.56 MHz through the shadow mask defining 1 mm diameter holes.

(62) Current-voltage measurements were used to characterize the performance of the cells in the dark and under simulated AM1.5G solar illumination (uncorrected for solar spectral mismatch) using a 150 W Xenon arc lamp. Performance data for the Ar plasma treated device is shown in FIG. 5. The dark current (filled triangles) and 1 sun illumination (open triangles) current-voltage curves are shown in FIG. 5a, along with the fit to the dark current (line). Performance for this cell as a function of illumination intensity is shown in FIG. 5b.

(63) The control PV was grown under similar conditions, in the same chamber and using the same organic materials, on solvent-cleaned, 10 minute UV-O.sub.3 treated ITO-coated glass, comprising: ITO (1550 )/CuPc (200 )/PTCBI (250 )/BCP (100 )/Ag (1000 ). Under AM1.5G 1 sun solar illumination, the control device displayed a V.sub.OC of 0.44 V, a FF of 0.64, a J.sub.SC/P.sub.0 of 0.44 /W, leading to a .sub.P of 1.20.1%. The dark current-voltage current curve was fit to the modified ideal diode equation:

(64) $J_D=J_S\left\{\exp \left[\frac{q\left(V-J_D{R}_{\mathrm{SA}}\right)}{\mathrm{nkT}}\right]-1\right\}$
giving n of 1.66, R.sub.SA of 0.75 -cm.sup.2, and J.sub.S of 9.810.sup.8 /cm.sup.2. FIG. 5a shows the device current in the dark (filled squares) and under 1 sun illumination (open circles). The line represents the fit to the dark current.

(65) Table 1 (below) lists dark curve fit parameters and AM1.5G 1 sun performance data for devices grown on substrates exposed to the three different surface treatments discussed above.

(66) TABLE-US-00001 TABLE 1 Surface 1 sun .sub.P 1 sun 1 sun J.sub.S/P.sub.O R.sub.SA Treatment (%) V.sub.OC (V) 1 sun FF (A/W) (-cm.sup.2) J.sub.S (A/cm.sup.2) n UV-O.sub.3 0.31 0.39 0.50 0.016 9.1 2.1 10.sup.7 1.88 0.06 0.03 0.05 0.001 4.6 0.9 10.sup.7 0.08 O.sub.2 plasma 0.31 0.37 0.51 0.016 4.9 7.0 10.sup.7 2.00 0.02 0.01 0.01 0.002 1.0 0.5 10.sup.7 0.10 Ar plasma 0.35 0.45 0.48 0.017 11.6 2.7 10.sup.8 1.83 0.04 0.01 0.03 0.001 3.7 0.2 10.sup.8 0.10

Example 4

(67) Inverted PV devices having CuPc and PTCBI layers of varying thickness were prepared as follows. Quartz substrates were solvent cleaned, then loaded into an electron beam evaporator where a 1000 thick layer of Ni was deposited at a rate of 5 /s. The Ni anodes were exposed to ultraviolet ozone treatment for 30 min, then loaded into a high vacuum thermal deposition chamber with a base pressure of 510 Torr. Purified organic sources were grown at a pressure of 110.sup.6 Torr, and a rate of 2 /s. A vacuum break occurred before attaching a shadow mask to the deposited layers and substrate in a high purity (<1 ppm H.sub.2O and O.sub.2) N.sub.2 ambient. The top contact consisted of a 400 thick ITO layer sputter deposited at 20 W with a rate of 0.1 /s through the shadow mask defining an array of 1 mm diameter circular cathodes. Performance data for the devices prepared by this method are disclosed below.

(68) The performance of an inverted PV device comprising quartz/Ni (1000 )/CuPc (350 )/PTCBI (100 )/BCP (100 )/ITO (400 ) is disclosed in FIG. 6. FIG. 6a shows the current-voltage characteristics for a control device (glass/ITO (1550 )/CuPc (200 )/PTCBI (250 )/BCP (100 )/Ag (1000 )) in the dark (squares) and under simulated 1 sun, AM1.5G illumination (dashed lines), and for the inverted device in the dark (triangles) and under illumination (dashed-dotted line). FIG. 6b shows the power conversion efficiency (circles), open-circuit voltage (triangles), and fill factor (squares) for the inverted device.

(69) FIGS. 7 and 8 show the performance of inverted devices having CuPc layers (x=100 to 400 ) and PTCBI layers (y=0 to 300 ) of varying thickness. The structures of FIG. 7 comprise quartz/Ni (1000 )/CuPc (x A)/PTCBI (100 )/BCP (100 )/ITO (400 ). FIG. 7a shows the simulated (line) and measured (filled squares) photocurrent of such devices at one sun intensity. FIG. 7b shows .sub.P (squares), Voc (triangles), and FF at one sun AM1.5G illumination. The structures of FIG. 8 comprise quartz/Ni(1000 )/CuPc (400 )/PTCBI (y A)/BCP (100 )/ITO (400 ). FIG. 8a shows the simulated (line) and measured (filled squares) photocurrent of such devices at one sun intensity. FIG. 8b shows .sub.P (squares), Voc (triangles), and FF at one sun AM1.5G illumination.

(70) FIG. 9 shows a simulated contour plot for a devices having varying CuPc and PTCBI thicknesses. FIG. 9a shows a simulated contour plot for J.sub.SC, while FIG. 9b shows a simulated contour plot for .sub.P. For FIG. 9, it is assumed the ideality factor n=1.78, reverse saturation current J.sub.S=1.4610.sup.7 /cm.sup.2, and the series resistance as a function of the CuPc and PTCBI thicknesses (t.sub.CuPc and t.sub.PTCBI) follows R.sub.SA=0.05[(t.sub.CuPc+t.sub.PTCBI)/]cm.sup.2. The diffusion lengths of CuPc and PTCBI were taken as 8020 and 305 , with a lifetime of 2 ns. Values of Ni anode, ITO cathode. and BCP layers thicknesses were 1000, 400, and 100 , respectively, for these simulations.

(71) 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.