Exciton-blocking treatments for buffer layers in organic photovoltaics

Abstract

Disclosed herein are exciton-blocking treatments for buffer layers used in organic photosensitive optoelectronic devices. More specifically, the organic photosensitive optoelectronic devices described herein include at least one self-assembled monolayer disposed on the surface of an anode buffer layer. Methods of preparing these devices are also disclosed. The present disclosure further relates to methods of forming at least one self-assembled monolayer on a substrate.

Claims

1. An organic photosensitive optoelectronic device comprising: an anode and a cathode in superposed relation; a photoactive region comprising at least one organic donor material and at least one organic acceptor material disposed between the anode and the cathode forming a donor-acceptor heterojunction; an anode buffer layer disposed between the anode and the photoactive region, wherein the anode buffer layer has a bottom surface closer to the anode and a top surface further from the anode; and at least one self-assembled monolayer disposed on the top surface of the anode buffer layer between the anode buffer layer and the photoactive region, wherein the at least one self-assembled monolayer comprises a layer of molecules having head groups that bond with the top surface of the anode buffer layer and tail groups comprising carbon-based structures, and wherein at least one head group comprises at least one phosphonic acid.

2. The device of claim 1, wherein the anode buffer layer comprises a transition metal oxide.

3. The device of claim 2, wherein the transition metal oxide is chosen from MoO.sub.3, V.sub.2O.sub.3, ReO.sub.3, WO.sub.3 TiO.sub.2, Ta.sub.2 O.sub.3, ZnO, NiO, and alloys thereof.

4. The device of claim 3, wherein the transition metal oxide is chosen from MoO.sub.3, NiO, and alloys thereof.

5. The device of claim 3, wherein the at least one self-assembled monolayer comprises benzylphosphonic acid or a functionalized derivative thereof.

6. The device of claim 1, wherein the at least one phosphonic acid is chosen from alkylphosphonic acids, arylphosphonic acids, and functionalized derivatives thereof.

7. The device of claim 6, wherein the arylphosphonic acids are chosen from phenylphosphonic acid, benzylphosphonic acid, propylphenyl phosphonic acid, naphthylmethylphosphonic acid, and functionalized derivatives thereof.

8. The device of claim 6, wherein the alkylphosphonic acids are chosen from ##STR00004## wherein n is chosen from 0 to 15.

9. The device of claim 8, wherein the alkylphosphonic acids are chosen from methylphosphonic acid, ethylphosphonic acid, propylphosphonic acid, butylphosphonic acid, and functionalized derivatives thereof.

10. The device of claim 1, wherein the at least one self-assembled monolayer comprises benzylphosphonic acid, butylphosphonic acid, or a functionalized derivative thereof.

11. The device of claim 1, wherein the at least one self-assembled monolayer has a thickness in a range from about 0.4 nm to 1 nm.

12. The device of claim 1, wherein the anode buffer layer in the device exhibits less exciton quenching behavior compared to the anode buffer layer in the device without the at least one self-assembled monolayer.

13. The device of claim 1, wherein the donor-acceptor heterojunction is chosen from a planar heterojunction, a mixed heterojunction, a bulk heterojunction and a planar-mixed heterojunction.

14. The device of claim 1, further comprising a cathode buffer layer disposed between the photoactive region and the cathode.

15. A method of forming an organic photosensitive optoelectronic device comprising: depositing an anode buffer layer over an anode, wherein the anode buffer layer has a bottom surface closer to the anode and a top surface further from the anode; depositing at least one self-assembled monolayer on the top surface of the anode buffer layer, wherein the at least one self-assembled monolayer comprises a layer of molecules having head groups that bond with the top surface of the anode buffer layer and tail groups comprising carbon-based structures, and wherein at least one head group comprises at least one phosphonic acid; depositing a photoactive region over the anode buffer layer, wherein the photoactive region comprises at least one organic donor material and at least one organic acceptor material forming a donor-acceptor heterojunction; and depositing a cathode over the photoactive region; wherein the at least one self-assembled monolayer is disposed between the anode buffer layer and the photoactive region.

16. The method of claim 15, wherein the at least one self-assembled monolayer is deposited by physical vapor deposition.

17. The method of claim 15, wherein the step of depositing at least one self-assembled monolayer comprises applying a solution to at least the top surface of the anode buffer layer.

18. The method of claim 17, wherein the solution comprises a solvent and the at least one phosphonic acid.

19. The method of claim 18, wherein the solvent comprises an alcohol or tetrahydrofuran (THF).

20. The method of claim 18, wherein the solution is applied using a technique chosen from spin coating, soaking, spray coating, blade coating, and slot dye coating.

21. The method of claim 18, wherein the anode buffer layer comprises a transition metal oxide.

22. The method of claim 17, wherein the step of depositing at least one self-assembled monolayer further comprises heating the anode buffer layer.

23. The method of claim 22, wherein the anode buffer layer is heated at a temperature in a range from 40° C. to 200° C.

24. The method of claim 22, wherein the step of depositing at least one self-assembled monolayer further comprises rinsing at least the top surface of the anode buffer layer with a solvent.

25. The method of claim 24, wherein the anode buffer layer comprises NiO or an alloy thereof.

26. The method of claim 15, wherein the step of depositing at least one self-assembled monolayer comprises applying a phosphonic acid solution to at least the top surface of the anode buffer layer, wherein the phosphonic acid solution is applied using a technique chosen from spin coating, soaking, spray coating, blade coating, and slot dye coating.

27. The method of claim 15, wherein the anode buffer layer comprises a transition metal oxide.

28. The method of claim 27, wherein the transition metal oxide is chosen from MoO.sub.3, V.sub.2O.sub.3, ReO.sub.3, WO.sub.3 TiO.sub.2, Ta.sub.2 O.sub.3, ZnO, NiO, and alloys thereof.

29. The method of claim 15, wherein the phosphonic acid is chosen from benzylphosphonic acid or a functionalized derivative thereof and butylphosphonic acid or a functionalized derivative thereof.

30. The method of claim 15, wherein the at least one self-assembled monolayer has a thickness in a range from 0.4 nm to 1 nm.

31. The method of claim 15, further comprising depositing a cathode buffer layer between the photoactive region and the cathode.

Description

(1) The accompanying figures are incorporated in, and constitute a part of this specification.

(2) FIG. 1 shows a schematic of an exemplary organic photosensitive optoelectronic device in accordance with the present disclosure.

(3) FIG. 2A shows HOMO and LUMO (transport) energy levels of materials based on literature values. HOMO energies were confirmed by UPS for NiO, SubPc, and DPSQ. All energy values are in eV relative to the absolute vacuum level, except for MoO.sub.3 which is shifted to lower energy by ˜1 eV to allow for Fermi energy (E.sub.F) alignment with the donor material. FIG. 2B shows HOMO ionization energy (IE), work function (WF), and vacuum level offsets (ΔE.sub.VAC) in eV for NiO and BPA treated NiO, as well as for SubPc and DPSQ deposited on each buffer. FIG. 2C shows PL intensity versus wavelength of a 32 nm-thick SubPc layer fabricated on untreated and BPA treated MoO.sub.3 and NiO. A photoluminescence (PL) response similar to the BPhen reference indicates a fully blocking interface while a response similar to the C.sub.60 buffer indicates a perfectly exciton quenching interface. Inset: Molecular structural formula of BPA.

(4) FIG. 3 shows PL of quartz/test layer (8 nm)/SubPc (40 nm)/BPhen (8 nm). The test layers were C.sub.60 (˜8 nm), BPhen (8 nm), MoO.sub.3 (16 nm), and benzylphosphonic acid (BPA) treated MoO.sub.3 where the BPA treatment resulted in the addition of one or more self assembled monolayers on the MoO.sub.3 having the following thicknesses: a spin coated 1 g BPA/I solution (˜5.4 nm thick), a spin coated 0.5 g BPA/I solution (˜2.2 nm thick), spin coated 0.25 g BPANI solution (˜0.6 nm thick), and a BPA soak (˜0.5 nm thick). The emitted light was measured at a wavelength of λ=710 nm (top graph), and the excitation wavelength at λ=525 nm (bottom graph).

(5) FIG. 4 shows PL of quartz/test layer (8 nm)/SubPc (40 nm)/BPhen (8 nm). The test layers were C.sub.60 (˜8 nm), BPhen (8 nm), MoO.sub.3 (16 nm), BPA-treated MoO.sub.3, octylphosphonic acid (OPA)-treated MoO.sub.3 where the treatments resulted in the addition of one or more self assembled monolayers on the MoO.sub.3 having the following thicknesses: spin coated 0.5 g BPA/I solution (˜2.2 nm thick), spin coated 0.5 g OPA/I solution (˜3.5 nm thick). The emitted light was measured at a wavelength of λ=710 nm (top graph), and the excitation wavelength at λ=525 nm (bottom graph).

(6) FIG. 5 shows PL of quartz/test layer (8 nm)/SubPc (47 nm)/BPhen (8 nm). The test layers were C.sub.60 (˜8 nm), BCP (8 nm), NiO (8 nm), BPA-treated NiO, OPA-treated NiO, methylphosphonic acid (MePA)-treated NiO, and phosphorous acid (Phosphorous)-treated NiO where the treatments resulted in the addition of one or more self-assembled monolayers on the NiO having the following thicknesses: spin coated 1 g BPA/I solution (˜0.5 nm thick), spin coated 1 g OPA/l solution (˜0.5 nm thick), spin coated MePA and Phosphorous (too thin to measure with VASE). An NiO sample soaked in BPA was also generated. The emitted light was measured at a wavelength of λ=710 nm (top graph), and the excitation wavelength at λ=525 nm (bottom graph).

(7) FIG. 6 shows EQE as a function of wavelength for DPSQ/C.sub.60 based photovoltaic devices with various anode buffers. All devices had a PTCBI cathode buffer, except for the device labeled NiO+BPA+BPhen:C.sub.60, which has a NiO+BPA anode buffer and a BPhen:C.sub.60/PTCBI cathode buffer.

(8) FIG. 7 shows current density vs. voltage characteristics for devices in FIG. 6 under 1 sun, AM 1.5G illumination, showing the increased photoresponse for the phosphonic acid treated buffers. The fill factor and open circuit voltage were not affected by the phosphonic acid treatment.

(9) FIG. 8 shows EQE as a function of wavelength for SubPc/Co/BPhen and blended squarine (bSQ)/C.sub.60/PTCBI cells fabricated on MoO.sub.3 and BPA-treated NiO buffers.

(10) FIG. 9 shows EQE (top) and current-voltage performance (bottom) for devices with the structure: glass/ITO/buffer/DPSQ (9 nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm). MoO.sub.3 represents the reference untreated device and “1 g/l,” “0.5 g/l,” and “0.25 g/l” indicate the concentration of BPA spin coated onto MoO.sub.3. OPA at 0.5 g/l also was spin coated onto MoO.sub.3 for one device.

(11) FIG. 10 shows EQE (top) and current-voltage performance (bottom) for devices with the structure: glass/ITO/buffer/DPSQ (9 nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm). MoO.sub.3 represents the reference untreated device. “0.5 g/l CM” and “0.5 g/l THF” indicate that BPA was spin coated onto MoO.sub.3 from a chloroform methanol solution or THF, respectively, and “THF soak” indicates that the MoO.sub.3 anode buffer was soaked for 30 minutes in BPA dissolved in THF.

(12) FIG. 11 shows EQE (top) and current-voltage performance (bottom) for devices with the structure: glass/ITO/buffer/DPSQ (9 nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm), where the MoO.sub.3 buffer was soaked in phosphonic acid solutions for various soak times.

(13) FIG. 12 shows EQE (top) and NIR EQE as a function of DPSQ thickness for devices with the structure: ITO/MoO.sub.3 (20 nm)/DPSQ (x nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm), where MoO.sub.3 indicates the reference untreated device and “+BPA” indicates that the MoO.sub.3 buffer was soaked in a 1 mM solution of BPA in THF for ˜24 hours.

(14) FIG. 13 shows EQE for devices with the structure: glass/ITO/buffer/DPSQ (9 nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm). Various untreated and phosphonic acid treated buffers were used.

(15) FIG. 14 shows EQE (top) and current-voltage performance (bottom) for the devices with the MoO.sub.3 reference buffer, the BPA-soaked MoO.sub.3 buffer, the BPA spin coated NiO buffer, the OPA spin coated NiO buffer, the MePA spin coated NiO buffer, and the phosphorous acid spin coated NIO buffer.

(16) FIG. 15 shows EQE (top) and current-voltage performance (bottom) for devices with the structure: glass/ITO/buffer/DPSQ (9 nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm), where BPA was bonded to the NiO buffer at various heating temperatures.

(17) FIG. 16 shows chemical structures for BPA (left), 4-fluoro BPA (4F-BPA, center), and pentafluoro BPA (F.sub.5BPA, right). EQE is shown for devices with the structure: glass/ITO/buffer/DPSQ (9 nm)/C.sub.60 (40 nm)/PTCBI (5 nm)/Ag (100 nm), where the anode buffers were treated with the various modified BPA compounds.

(18) FIG. 17 shows the photoresponse (top) and forward bias dark current (bottom) for the devices in FIG. 16.

(19) FIG. 18 shows chemical structures for (i) phenylphosphonic acid (Phenyl), (ii) BPA (Benzyl), (iii) propylphenylphosphonic acid (ProPhen), and (iv) 2-naphthylmethylphosphonic acid (Naphth). EQE (top) and current-voltage performance (bottom) are shown for devices with the various arylphosphonic acid treatments applied to an NiO buffer.

(20) FIG. 19 shows EQE (top) and current-voltage performance for devices with phosphorous acid treatment (H) and various alkylphosphonic acid treatments applied to an NiO buffer, including n-butylphosphonic acid (butyl), n-octylphosphonic acid (octyl), and n-hexadecylphosphonic acid treatments.

(21) As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic photosensitive devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone.

(22) In the context of the organic materials of the present disclosure, the terms “donor” and “acceptor” refer to the relative positions of the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energy levels of two contacting but different organic materials. If the LUMO energy level of one material in contact with another is further from the vacuum level, then that material is an acceptor. Otherwise it is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material, and for holes to move into the donor material.

(23) Herein, 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 solar cell, electrons move to the cathode from the adjacent photoconducting material.

(24) Similarly, the term “anode” is used herein such that in a solar cell under illumination, holes move to the anode from the adjacent photoconducting material, which is equivalent to electrons moving in the opposite manner. It is noted that the “anode” and “cathode” electrodes may be charge transfer regions or recombination zones, such as those used in tandem photovoltaic devices. 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 photoconductive 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 transparent or at least semi-transparent. 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. 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.

(25) As used herein, a “photoactive region” refers to a region of the device that absorbs electromagnetic radiation to generate excitons. Similarly, a layer is “photoactive” if it absorbs electromagnetic radiation to generate excitons. The excitons may dissociate into an electron and a hole in order to generate an electrical current.

(26) 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).

(27) In one aspect of the present disclosure, an organic photosensitive optoelectronic device comprises an anode and a cathode in superposed relation, a photoactive region comprising at least one organic donor material and at least one organic acceptor material disposed between the anode and the cathode forming a donor-acceptor heterojunction, an anode buffer layer disposed between the anode and the photoactive region, wherein the anode buffer layer has a bottom surface closer to the anode and a top surface further from the anode, and at least one SAM disposed on the top surface of the anode buffer layer.

(28) A non-limiting example of an organic photosensitive device according to the present disclosure is shown in FIG. 1. The device comprises anode 105 and cathode 130 in superposed relation. Anode buffer layer 110 is disposed between anode 105 and the photoactive region, which comprises at least one organic donor material 120 and at least one organic acceptor material 125 forming a donor-acceptor heterojunction. As used herein, the term donor-acceptor heterojunction refers to the interface between a donor material and an acceptor material for dissociating excitons into holes and electrons. FIG. 1 shows a bilayer photoactive region where the at least one organic donor material 120 and the at least one organic acceptor material 125 form a planar heterojunction. It should be understood that the devices of the present disclosure are not limited to planar heterojunctions. The donor and acceptor materials may be arranged in any manner known in the art for organic photosensitive optoelectronic devices. For example, the donor and acceptor materials may form a planar heterojunction, mixed heterojunction, bulk heterojunction, or planar-mixed heterojunction.

(29) Anode buffer layer 110 has a bottom surface B.sub.Sur closer to anode 105 and a top surface T.sub.Sur further from anode 105. At least one SAM 115 is disposed on the top surface T.sub.Sur of anode buffer layer 110.

(30) As used herein, the term “self-assembled monolayer” refers to a layer of molecules assembled on a substrate surface, wherein the molecules include “head groups” that attach the molecules (e.g., by chemical bonds) to the surface, and “tail groups” comprising one or more of a wide variety of carbon containing “organic” groups such as alkyl chains, aryl groups, such as benzene, and/or modified groups, such as fluorinated benzene. The head groups may comprise functional groups having an affinity to the substrate surface capable of anchoring the molecules to the surface. Suitable “head groups” include, but are not limited to, phosphonic acid, carboxylic acid, silanes (such as trichlorosilanes or trimethoxysilanes), and thiols.

(31) A “self-assembled monolayer” as used herein may, but need not, cover the entire top surface T.sub.Sur of the anode buffer layer (i.e., the self-assembled monolayer need not be a continuous layer across the entire substrate surface). For example, the at least one self-assembled monolayer may cover at least 50% of the surface, such as covering at least 75%, at least 85%, at least 95%, or even 100% of the surface.

(32) As used herein, the language “at least one self-assembled monolayer” allows for one or more self-assembled monolayers to be disposed on the top surface T.sub.Sur of the anode buffer layer. For example, a second SAM may be stacked on top of a first SAM. In some embodiments, two or more self-assembled monolayers are disposed on the top surface T.sub.Sur of the anode buffer layer, such as two SAMs, three SAMs, four SAMs, or five SAMs.

(33) In some embodiments, the at least one SAM 115 comprises molecules chosen from phosphonic acids, carboxylic acids, silanes, and thiols. Phosphonic acids, carboxylic acids, silanes, thiols, and other molecules for forming SAMs have been described in the art and may be used to form SAMs in accordance with the present disclosure. Disclosure of more specific molecules for forming SAMs is set forth below.

(34) Phosphonic acids have the general formula

(35) ##STR00001##
In some embodiments, the phosphonic acids are chosen from alkylphosphonic acids or functionalized derivatives thereof and arylphosphonic acids. As used herein, the term “alkylphosphonic acids” refers to phosphonic acids where the R group comprises a straight-chain or branched saturated hydrocarbyl group. As used herein, the term “arylphosphonic acids” refers to phosphonic acids where the R group comprises at least one aromatic ring. For example, the term “arylphosphonic acids” includes phosphonic acids containing carbon chains of various lengths terminated with at least one aromatic ring.

(36) In some embodiments, the arylphosphonic acids are chosen from phenylphosphonic acid, benzylphosphonic acid (BPA), propylphenyl phosphonic acid, napthylmethylphosphonic acid, and functionalized derivatives thereof.

(37) In some embodiments, the alkylphosphonic acids are chosen from

(38) ##STR00002##
wherein n is chosen from 0 to 15. In certain embodiments, the alkylphosphonic acids are chosen from methylphosphonic acid, ethylphosphonic acid, propylphosphonic acid, butylphosphonic acid, and functionalized derivatives thereof. In some embodiments, the phosphonic acids are chosen from BPA or a functionalized derivative thereof and butylphosphonic acid or a functionalized derivative thereof.

(39) Carboxylic acids have the general formula

(40) ##STR00003##
In some embodiments, the carboxylic acids are chosen from alkylcarboxylic acids or functionalized derivatives thereof and arylcarboxylic acids. As used herein, the term “alkylcarboxylic acids” refers to carboxylic acids where the R group comprises a straight-chain or branched saturated hydrocarbyl group. As used herein, the term “arylcarboxylic acids” refers to carboxylic acids where the R group comprises at least one aromatic ring. For example, the term “arylcarboxylic acids” includes carboxylic acids containing carbon chains of various lengths terminated with at least one aromatic ring. In some embodiments, the carboxylic acids are chosen from benzoic acid, phenylacetic acid, and derivatives thereof.

(41) Thiols have the general formula R—SH. Sulfur is affinitive to various metals (e.g. gold, silver and copper). In some embodiments, the thiols are chosen from alkylthiols or functionalized derivatives thereof and arylthiols. As used herein, the term “alkylthiols” refers to thiols where the R group comprises a straight-chain or branched saturated hydrocarbyl group. As used herein, the term “arylthiols” refers to thiols where the R group comprises at least one aromatic ring. For example, the term “arylthiols” includes thiols containing carbon chains of various lengths terminated with at least one aromatic ring. In some embodiments, the thiols are chosen from thiophenol, benzythiol, and derivatives thereof.

(42) Examples of suitable silanes include trichlorosilanes, such as alkyltrichlorosilanes and aryltrichlorosilanes, and trimethoxysilanes, such as alkyltrimethoxysilanes and aryltrimethoxysilanes.

(43) In accordance with the present disclosure, when the at least one self-assembled monolayer is said to comprise molecules chosen from phosphonic acids, carboxylic acids, silanes, thiols, etc., this includes where the phosphonic acid, carboxylic acid, silane, thiol, etc. Is bonded to the anode buffer layer. For example, the at least one self-assembled monolayer may comprise a phosphonic acid when the self-assembled monolayer includes a phosphonic acid that is bonded to the anode buffer layer via, for example, one, two, or all three oxygens in the general formula to metal sites on the anode buffer layer surface.

(44) The thickness of the at least one self-assembled monolayer can depend on the type of molecules that form the SAM. For example, one SAM of methylphosphonic acid may result in a thickness of about 0.3 nm, whereas one SAM of benzylphosphonic acid and one SAM of hexadecylphosphonic acid may result in thicknesses of about 0.5-0.6 nm and about 2 nm, respectively. Thus, thickness may be optimized for peak device performance, for example, based on the molecules chosen to form the at least one SAM. Thickness optimization may be performed by balancing the beneficial reduction of exciton quenching with the potential for increased series resistance. In some embodiments, the at least one SAM has a thickness in a range from about 0.1 nm to about 5 nm, such as from about 0.1 nm to about 3 nm, from about 0.2 nm to about 2 nm, from about 0.3 nm to about 1.5 nm, from about 0.4 nm to about 1 nm, or from about 0.5 nm to about 0.8 nm.

(45) In some embodiments, the anode buffer layer comprises a transition metal oxide. Suitable transition metal oxides include, but are not limited to, MoO.sub.3, V.sub.2O.sub.3, ReO.sub.3, WO.sub.3, TiO.sub.2, Ta.sub.2O.sub.3, ZnO, NiO, and alloys thereof. In certain embodiments, the transition metal oxide is chosen from MoO.sub.3, NiO, and alloys thereof. In certain embodiments, the anode buffer layer comprises a transition metal oxide, and the at least one SAM comprises at least one phosphonic acid, such as BPA or a functionalized derivative thereof.

(46) In some embodiments of the present disclosure, the anode buffer layer in the device exhibits less exciton quenching behavior compared to the anode buffer layer in the device without the one or more self-assembled monolayers. In some embodiments, the device exhibits a greater power conversion efficiency compared to the device without the one or more self-assembled monolayers.

(47) The devices of the present disclosure may include additional buffer layers. For example, although not shown in FIG. 1, the device may include a cathode buffer layer disposed between the photoactive region and the cathode. The cathode buffer layer should be chosen to block excitons and to facilitate electron transport to the cathode. In some embodiments, the cathode buffer layer comprises a material 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)3), and aluminum(III)phenolate (Alq2 OPH), N,N′-diphenyl-N,N′-bis-alpha-naphthylbenzidine (NPD), aluminum tris(8-hydroxyquinoline) (Alq3), and carbazole biphenyl (CBP). In some embodiments, the cathode buffer layer further comprises an electron-transporting material. In certain embodiments, the electron-transporting material comprises the same material as the at least one acceptor material. In certain embodiments, the electron-transporting material is a fullerene, such as Coo.

(48) The present disclosure is not limited to any particular combination of organic donor and acceptor materials. In some embodiments, the at least one organic acceptor material comprises a material chosen from subphthalocyanines, subnaphthalocyanines, dipyrrin complexes, such as zinc dipyrrin complexes, BODIPY complexes, perylenes, naphthalenes, fullerenes and fullerene derivatives (e.g., PCBMs, ICBA, ICMA, etc.), and polymers, such as carbonyl substituted polythiophenes, cyano-substituted polythiophenes, polyphenylenevinylenes, or polymers containing electron deficient monomers, such as perylene diimide, benzothiadiazole or fullerene polymers. Non-limiting mention is made to those chosen from C.sub.60, C.sub.70, C.sub.76, C.sub.82, C.sub.84, or derivatives thereof such as Phenyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]PCBM), Phenyl-C.sub.71-Butyric-Acid-Methyl Ester ([70]PCBM), or Thienyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]ThCBM), and other acceptors such as 3,4,9,10-perylenetetracarboxylic-bisbenzimidazole (PTCBI), hexadecafluorophthalocyanine (F.sub.16CuPc), and derivatives thereof.

(49) In some embodiments, the at least one organic donor material comprises a material chosen from phthalocyanines, such as copper phthalocyanine (CuPc), chloroaluminium phthalocyanine (ClAlPc), tin phthalocyanine (SnPc), zinc phthalocyanine (ZnPc), and other modified phthalocyanines, subphthalocyanines, such as boron subphthalocyanine (SubPc), naphthalocyanines, merocyanine dyes, boron-dipyrromethene (BODIPY) dyes, thiophenes, such as poly(3-hexylthiophene) (P3HT), low band-gap polymers, polyacenes, such as pentacene and tetracene, diindenoperylene (DIP), squaraines (SQ), tetraphenyldibenzoperiflanthene (DBP), and derivatives thereof. Examples of squaraine donor materials include, but are not limited to, 2,4-bis[4-(N,N-dipropylamino)-2,6-dihydroxyphenyl]squaraine, 2,4-bis[4-(N,Ndiisobutylamino)-2,6-dihydroxyphenyl]squaraine, 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine (DPSQ).

(50) In certain embodiments, the at least one organic donor material is chosen from squaraines or functionalized derivatives thereof, and the at least one acceptor material is chosen from fullerenes or functionalized derivatives thereof. In certain embodiments, the squaraine is DPSQ and the fullerene is C.sub.60.

(51) The organic photosensitive 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.

(52) The organic photosensitive optoelectronic devices of the present disclosure may exist as a tandem device comprising two or more subcells. A 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 (i.e., an anode and a cathode). 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.

(53) 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.

(54) Materials may be deposited using techniques known in the art. For example, materials described herein may be deposited or co-deposited from a solution, vapor, or a combination of both. Depending on the material and the desired deposition technique, materials may be deposited or co-deposited 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. Similarly, materials may be deposited or co-deposited using vacuum evaporation, such as vacuum thermal evaporation, electron beam evaporation, organic vapor phase deposition, or organic vapor-jet printing.

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

(56) 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. For example, appropriate thicknesses can be selected to achieve the desired optical spacing in the device and/or reduce resistance in 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.

(57) Also disclosed is a method of forming an organic photosensitive optoelectronic device comprising depositing an anode buffer layer over an anode, wherein the anode buffer layer has a bottom surface closer to the anode and a top surface further from the anode, depositing at least one SAM on the top surface of the anode buffer layer, depositing a photoactive region over the anode buffer layer, wherein the photoactive region comprises at least one organic donor material and at least one organic acceptor material forming a donor-acceptor heterojunction, and depositing a cathode over the photoactive region.

(58) In some embodiments, the step of depositing the at least one SAM comprises applying a solution or vapor to at least the top surface of the anode buffer layer. For example, in some embodiments, the at least one SAM is deposited on the top surface of the anode buffer layer by physical vapor deposition. If applying a solution, the solution comprises a solvent and the material for forming the at least one SAM. In some embodiments, the solution comprises a solvent and molecules chosen from phosphonic acids, carboxylic acids, silanes and thiols. Concentration in the solvent should be sufficient to coat the top surface of the anode buffer layer for forming the at least one SAM. For example, in some embodiments, the concentration of a phosphonic acid in the solution ranges from 0.25 mg/ml to 1 mg/ml.

(59) In some embodiments, the solvent comprises an alcohol, such as methanol or ethanol, or tetrahydrofuran (THF). In certain embodiments, the solvent comprises a mixture of an alcohol with a less polar solvent, such as a mixture of ethanol and chloroform.

(60) In some embodiments, the solution comprises at least one phosphonic acid. In some embodiments, the solution comprises a THF solvent and at least one phosphonic acid. In some embodiments, the at least one phosphonic acid is a phosphonic acid described herein. In certain embodiments, the at least one phosphonic acid is BPA or a functionalized derivative thereof.

(61) Suitable techniques for applying the solution to at least the top surface of the anode buffer layer include, but are not limited to, spin coating, soaking, spray coating, blade coating, and slot dye coating.

(62) In some embodiments, the anode buffer layer comprises a transition metal oxide. Examples of suitable transition metal oxides are MoO.sub.3, V.sub.2O.sub.3, ReO.sub.3, WO.sub.3 TiO.sub.2, Ta.sub.2O.sub.3, ZnO, NiO, and alloys thereof. In some embodiments, the anode buffer layer comprises a transition metal oxide and the solution is applied by spin coating or soaking. In certain of these embodiments, the transition metal oxide is chosen from MoO.sub.3, NiO, and alloys thereof.

(63) The step of depositing the at least one SAM may further comprise heating the anode buffer layer. Heating may occur, for example, during the application of the solution to at least the top surface of the anode buffer layer and/or after applying the solution. Heating can ensure that the at least one SAM bonds to the surface of the anode buffer layer (i.e., the “head groups” as described above may bond to the surface of the anode buffer layer). In some embodiments, the anode buffer layer is heated at a temperature in a range from 40° C. to 200° C., such as from 80° C. to 160° C., from 100° C. to 140° C., from 110° C. to 130° C., or from 115° C. to 125° C.

(64) The step of depositing at least one SAM may further comprise rinsing at least the top surface of the anode buffer layer with a solvent. In some embodiments, the solvent used for rinsing is the same solvent used in the solution, such as THF. In certain embodiments, the solution comprises at least one phosphonic acid and THF, and the solvent used for rinsing is THF.

(65) In some embodiments, the anode buffer layer comprises NiO or an alloy thereof. In some of these embodiments, the solution is applied by spin coating. In certain embodiments, the solution comprises at least one phosphonic acid, such as BPA. In certain embodiments, the phosphonic acid solution comprises THF as the solvent.

(66) In some embodiments, the anode buffer layer comprises a transition metal oxide chosen from MoO.sub.3 V.sub.2O.sub.3, ReO.sub.3, WO.sub.3, Ta.sub.2O.sub.3 and alloys thereof. In certain of these embodiments, the solution is applied by soaking. In certain embodiments, the solution comprises at least one phosphonic acid, such as BPA. In certain embodiments, the phosphonic acid solution comprises THF as the solvent.

(67) As described above, the thickness of the at least one SAM may be optimized for peak device performance. In some embodiments, the at least one SAM has a thickness in a range from about 0.1 nm to about 5 nm, such as from about 0.1 nm to about 3 nm, from about 0.2 nm to about 2 nm, from about 0.3 nm to about 1.5 nm, from about 0.4 nm to about 1 nm, or from about 0.5 nm to about 0.8 nm. It is noted that when the solution is applied to the top surface of the anode buffer layer for forming the at least one SAM, the resulting thickness disposed on the top surface of the anode buffer layer may be greater than the thickness ranges described above. For example, applying the solution to the anode buffer layer may result in a film approximately 1 nm to 10 nm thick. Upon further processing, as described above, the thickness of the film may be reduced to the scale of the at least one SAM.

(68) The photoactive region is deposited to form a donor-acceptor heterojunction. As discussed above, the present disclosure is not limited to any particular donor-acceptor heterojunction. In some embodiments, the photoactive region is deposited to form a donor-acceptor heterojunction chosen from a planar heterojunction, a mixed heterojunction, a bulk heterojunction, and a planar-mixed heterojunction.

(69) The method of forming an organic photosensitive optoelectronic device may further comprise depositing a cathode buffer layer located between the photoactive region and the cathode. As described herein, the cathode buffer layer is chosen to block excitons and to facilitate electron transport to the cathode.

(70) Further disclosed is method of treating a transition metal oxide substrate with a solution, comprising providing a substrate comprising a transition metal oxide, and applying a solution to at least one surface of the substrate, wherein the solution comprises a tetrahydrofuran (THF) solvent and molecules chosen from phosphonic acids, carboxylic acids, silanes and thiols.

(71) In some embodiments, the transition metal oxide is chosen from transition metal oxides described herein. In some embodiments, the step of applying a solution comprises a technique chosen from spin coating, soaking, spray coating, blade coating, and slot dye coating.

(72) In some embodiments, the phosphonic acids are chosen from those described herein.

(73) The method may further comprise heating the substrate. Heating may occur during the application of the solution to the substrate and/or after applying the solution. In some embodiments, the substrate is heated at a temperature in a range from 40° C. to 200° C., from 80° C. to 160° C., from 100° C. to 140° C., from 110° C. to 130° C., or from 115° C. to 125° C.

(74) The method may further comprise rinsing the at least one surface of the substrate with a solvent. In some embodiments, the solvent used for rinsing is THF.

(75) There is also disclosed a method of treating a NiO substrate with a solution, comprising providing a substrate comprising NiO or an alloy thereof, and spin coating a solution onto at least one surface of the substrate, wherein the solution comprises a solvent and molecules chosen from phosphonic acids, carboxylic acids, silanes, and thiols.

(76) In some embodiments, the phosphonic acids are chosen from phosphonic acids described herein. In some embodiments, the solvent is chosen from those described herein.

(77) The method may further comprise heating the substrate. Heating may occur during and/or after the spin-coating. In some embodiments, the substrate is heated at a temperature in a range from 40° C. to 200° C., from 80° C. to 160° C., from 100° C. to 140° C., from 110° C. to 130° C., or from 115° C. to 125° C.

(78) The method may further comprise rinsing the at least one surface of the substrate with a solvent. In some embodiments, the solvent in the solution and the solvent used for rinsing are the same. In some embodiments, the solvent in the solution and the solvent used for rinsing are both THF.

(79) It should be understood that embodiments described herein may be used in connection with a wide variety of structures. Functional organic photovoltaic 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. The names given to the various layers herein are not intended to be strictly limiting.

(80) 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.

(81) 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.

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

EXAMPLES

(83) The following examples demonstrate the exciton blocking effect of treating an anode buffer layer with a phosphonic acid. Benzylphosphonic acid (BPA), for example, includes phosphonic acid “head groups” which bond the phosphonic acid molecules to the surface of the anode buffer layer. It is believed that the organic benzyl “tail group” provides an exciton-blocking effect to the anode buffer layer, resulting in improved device performance. Although only phosphonic acids are shown in the Examples, other SAMs, such as carboxylic acids, silanes, and thiols described above, having “head groups” to anchor the molecules to the buffer layer surface and having organic “tail groups” will provide an exciton-blocking effect to the anode buffer layer. Thus, as described above, the present disclosure is not limited to only phosphonic acid SAMs.

Example 1

(84) A sensitive method for determining the extent to which a material quenches or blocks excitons is to quantitatively compare the photoluminescence (PL) intensity from the optically pumped donor deposited on various surfaces to its intensity when deposited on perfectly quenching and blocking reference layers. Samples for PL quenching experiments were made with the structure: quartz/test layer (8 nm)/SubPc (40 nm)/BPhen (8 nm), where the test layer was an 8 nm thick layer of perfectly quenching C.sub.60, perfectly blocking BPhen, MoO.sub.3, NiO, or BPA treated MoO.sub.3 or NiO. The organic layers and the MoO.sub.3 were deposited by high vacuum (base pressure of ˜10.sup.−7 Torr) thermal evaporation (VTE). Depositing phosphonic acids on MoO.sub.3 is substantially different than for ITO. When MoO.sub.3 is soaked in a solution containing an alcohol (e.g., methanol and ethanol), the MoO.sub.3 dissolves, precluding the most common deposition techniques of soaking and the tethering by aggregation and growth (TBAG) known in the art. By replacing the mixture of polar and alcohol solvents with THF (a solvent with similar properties to mixtures of chloroform and methanol), however, no loss in MoO.sub.3 thickness was observed with variable angle spectroscopic ellipsometry (VASE). Thus, phosphonic acid self-assembled monolayers (SAMs) were formed by soaking the MoO.sub.3-coated substrate for 30 minutes in a 1 mM solution of BPA dissolved in THF forming a 0.4-0.5 nm (˜1 monolayer) thick BPA layer, as measured by VASE. The substrates were then heated in an ultrahigh-purity N.sub.2 ambient at 120° C. for 30 minutes, resulting in a chemical bond between the BPA and MoO.sub.3.

(85) NiO buffer layers were deposited using e-beam evaporation at a rate of 0.5 Å/s and a deposition pressure of ˜2×10.sup.−5 Torr and then transferred through the ambient to a Glen 1000P Asher where they were treated for 90 s with a remote oxygen plasma at 25 W and 150 mTorr.

(86) When NiO was soaked in a 1 mM solution of various phosphonic acids in THF, the thickness of the NiO decreased with time, suggesting that phosphonic acids themselves are corrosive to the NiO. To deposit monolayers on NiO therefore, phosphonic acid solutions were spin coated onto the NiO surface. The buffer was then heated at 80 to 170° C. to form bonds between the acid and the NiO surface. The NiO surface was then rinsed with a solvent to remove any unreacted phosphonic acid. In particular, the NiO coated samples were transferred to an ultrahigh-purity N.sub.2 filled glovebox where an ˜5 to 6 nm thick layer of BPA was deposited via spin coating from a 1 g/l solution in THF. It was found that a 1 mg/ml solution of BPA in THF spun cast with a ramprate of 1000 revolutions per minute (rpm)/sec and a final speed of 3000 rpm resulted in a BPA thickness of about 6 nm. The samples were then heated for 30 minutes at 120° C. and then rinsed twice in pure THF, again leaving a 0.4-0.5 nm (˜1 monolayer) thick residue of BPA as measured by VASE. The deposition of phosphonic acid SAMs was performed in a high purity nitrogen glovebox, but can also be performed in air.

(87) X-ray photoemission spectra were obtained using a Thermo VG Scientific Clam 4MCD photoelectron emission analyzer and an Al-Kα source (1489 eV). Spectra were collected with a 20 eV constant pass energy, and showed phosphorous 2p ionization and an increase in the C 1 s response after the BPA treatment, indicating the presence of phosphonic acid on the NiO surface. Ultraviolet photoemission spectra (UPS) were obtained using a He—I emission lamp (21.218 eV) excitation, an accelerating voltage 9V, and a collector pass energy of 2.5 eV to determine the work functions, ionization energies, and vacuum level shifts between the NiO, BPA, and the organic materials.

(88) Photoluminescence was measured by illuminating the films though the quartz substrate at 30° from normal with λ=400 to 650 nm wavelength light from a monochromated Xe arc lamp. The PL intensity (at λ=710 nm) was measured at 60° from normal, also through the substrate.

(89) FIG. 2A shows valence and conduction band energies for MoO.sub.3 and NiO, as well as HOMO an LUMO energies for SubPc, DPSQ, C.sub.60 and BPhen, relative to vacuum, using literature values. Ionization (HOMO) energies for NiO, SubPc, and DPSQ were verified using UPS. A large interface dipole existed between MoO.sub.3 and donor materials to allow for Fermi level alignment, resulting from significant charge transfer or molecular reorganization. Relevant HOMO ionization energies, work functions, and vacuum level offsets are shown in FIG. 2B, as measured by UPS. Organics deposited directly onto the NiO showed a 0.5 eV (for SubPc) and a 0.9 eV (for DPSQ) interface dipole energy shift suggesting the presence of significant molecular reorganization and/or charge transfer. Deposition of BPA on NiO was accompanied by a 0.6±0.1 eV shift in the vacuum level, and the subsequent dipole shift between the BPA-treated NiO and SubPc and DPSQ was negligible and within experimental error (±0.1 eV).

(90) The PL quenching results are shown in FIG. 2C, providing a comparison of the blocking vs. quenching efficiencies of the various layers. The MoO.sub.3 buffers had similar PL intensities to samples with C.sub.60 buffers, indicating that excitons were efficiently quenched at the MoO.sub.3/donor interface. The NiO buffer structures also had similar exciton quenching behavior (ranging from 85% to 100% quenching, depending on NiO deposition and plasma treatment process), despite the fact that the transport energy levels did not provide an obvious path to recombination or exciton dissociation with SubPc. The quenching likely occurred via recombination at surface states or defects within the band gap of the NiO. When BPA was applied to either the MoO.sub.3 or NiO surfaces, the PL intensity increased, suggesting that approximately 15% and 70% of the excitons were blocked without quenching, respectively. The difference in blocking efficiency for buffers consisting of MoO.sub.3 and NiO treated with SAMs of similar mean thickness likely resulted from different exciton quenching mechanisms. For BPA-treated MoO.sub.3, the exciton dissociation remained relatively efficient (e.g., ˜85% of excitons were quenched) and likely occurred when a hole in the donor HOMO tunneled to the conduction band of the MoO.sub.3. In the case of BPA-treated NiO, however, only ˜30% of the excitons reaching the interface were quenched.

Example 2

(91) PL quenching results were also obtained for MoO.sub.3 coated substrates having various thicknesses of phosphonic acid SAMs deposited via different techniques. The test structures were quartz/test layer (8 nm)/SubPc (40 nm)/BPhen (8 nm). The test layers were Co (˜8 nm), BPhen (8 nm), MoO.sub.3 (16 nm), and BPA treated MoO.sub.3. For the BPA treated MoO.sub.3, the MoO.sub.3-coated substrate was soaked in a 1 mM solution of BPA in THF, resulting in approximately one monolayer of BPA (˜0.5 nm thickness) attached to the surface of the MoO.sub.3-coated substrate, as measured with VASE. In addition, layers of various phosphonic acids were spin coated from THF-typical concentrations ranged from 0.25 to 1 mg/ml in THF-resulting in an ˜1-6 nm-thick film of phosphonic acid SAMs. A single solvent was chosen to prevent inhomogeneity in films caused by the incongruent evaporation of the solvent mixture. After heating at 120° C. for 30 minutes and rinsing in THF, the thickness remained the same as initially deposited (as measured by VASE). This suggested that the MoO.sub.3 was diffusing into and reacting with the phosphonic acid SAMs.

(92) As shown in FIG. 3, the MoO.sub.3 had similar PL to the reference quencher Coo, indicating strong quenching. By coating the MoO.sub.3 surface with various thicknesses of BPA, as described above, improved blocking (50% for the 1 g/l sample) for thicker layers (e.g., spin coated from 1 g/l BPA in THF) was observed compared to the thinner layers (e.g., BPA soak or BPA spin coated from 0.25 g/l solutions). As shown in FIG. 4, an improved blocking efficiency was observed from octylphosphonic acid (OPA) compared to BPA, both applied at 0.5 g/l, even though the thickness of the OPA was thinner.

(93) SAMs of various phosphonic acids at various concentrations were also deposited on NiO-coated substrates via spin coating, heating, and rinsing, as described above. A sample soaked in BPA was also generated. Results are shown in FIG. 5. As shown, PL for the OPA and BPA spin coated surfaces was similar to the blocking reference BCP, indicating near perfect blocking capabilities. SAMs of methylphosphonic acid (MePA) and phosphorous acid (Phosphorous) were also deposited similarly and the blocking efficiency of the MePA was found to be ˜50% and the PL of the phosphorous acid was similar to the quenching reference. The NiO sample soaked in BPA showed a blocking efficiency mid-way between blocking and quenching.

Example 3

(94) Bilayer OPVs were fabricated on detergent, solvent, and CO.sub.2 snow-cleaned ITO substrates (Bayview Optics, sheet resistance: 20Ω/□) with the structure: glass/ITO/anode buffer/DPSQ (10 nm)/C.sub.60 (40 nm)/cathode buffer/Ag (100 nm). Anode buffer layers were 20 nm and 8 nm thick for the MoO.sub.3 and NiO, respectively. Cathode buffers were either PTCBI (5 nm) or BPhen:C.sub.60 (1:1, 10 nm)/PTCBI (5 nm). The 10 nm-thick DPSQ donor was deposited by spin coating from a chloroform solution; all other organic layers were deposited via VTE and subsequently solvent-vapor-annealed in dichloromethane. Similar devices but with a 16 nm-thick 4:6 blend of asymmetric [2-[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]-4-[4-diphenyliminio]squaraine](DPASQ) and the symmetric DPSQ were also made. SubPc donor devices were also fabricated with structure glass/ITO/buffer/SubPc (10 nm)/C.sub.60 (35 nm)/BPhen (8 nm)/Ag (100 nm). All thicknesses were calibrated using VASE in the transparent region of the material between wavelengths of λ=1000 and 1600 nm.

(95) BPA SAMs were formed by soaking the MoO.sub.3-coated substrate for 30 min in a 1 mM solution of BPA dissolved in THF, as described above, forming a 0.4-0.5 nm (˜1 monolayer) thick BPA layer. The substrates were then heated in an ultrahigh-purity N.sub.2 ambient at 120° C. for 30 min as described above. BPA SAMs were deposited on the NiO buffer layers via spin coating from a 1 g/l solution in THF in an ultrahigh-purity N.sub.2 filled glovebox, as described above, resulting in an ˜5 nm thick layer of BPA. The samples were then heated for 30 min at 120° C. and then rinsed twice in pure THF, leaving a 0.4-0.5 nm (˜1 monolayer) thick residue of BPA.

(96) η.sub.P was measured under 1 sun, AM 1.5G illumination and was corrected for spectral mismatch. Experimental errors in the V.sub.OC and FF for the samples were ±0.01V and ±1%, as determined by sample-to-sample variation, while errors in short circuit current density (J.sub.SC) were dominated by uncertainty in measuring the spectrum and intensity of the solar simulator which, in-turn, dominated errors in η.sub.P.

(97) FIG. 6 shows the EQE for DPSQ/C.sub.60 OPVs with the BPA-treated MoO.sub.3 and NiO buffers. The reference device with a MoO.sub.3 buffer and an untreated NiO buffer have EQE≈29% at λ=715 nm, while the BPA-treated MoO.sub.3 and NiO have maximum EQE≈33% and 36%, respectively. The buffers with the highest PL yield and thus blocking efficiency, had the highest NIR EQE associated with absorption by the DPSQ donor. When the PTCBI cathode buffer layer was replaced with BPhen:C.sub.60/PTCBI, the EQE at λ=400-600 nm was increased based on the reduced exciton-polaron quenching in the fullerene layer. Optical modeling of the structure employing a transfer matrix algorithm showed that the spectral shift in the response of the C.sub.60 acceptor resulted from changes to the optical cavity incurred on inserting 10 nm of transparent BPhen:C.sub.60 between the reflective cathode and the active layers. The model indicated no significant changes in the optical modes when inserting a monolayer of BPA between the NiO and the donor materials, which was anticipated given its extreme thinness.

(98) FIG. 7 shows the current density as a function of applied bias for the devices in FIG. 6. The NiO buffer had a reduced J.sub.SC and V.sub.OC compared to the other buffers tested, but the performance varied depending on deposition and plasma processing conditions. An increase in J.sub.SC and no changes in FF or V.sub.OC was observed for the BPA-treated MoO.sub.3 and NiO buffers compared with the MoO.sub.3 control device. As shown in Table I below, the device with the MoO.sub.3 buffer had η.sub.P=4.8±0.2%, the BPA-treated MoO.sub.3 buffer layer had η.sub.P=5.1±0.3%, and the BPA-treated NiO buffer had η.sub.P=5.4±0.3%; a 13% increase compared to the untreated MoO.sub.3 buffer. The efficiency was further increased to η.sub.P=5.9±0.3% by incorporating the exciton blocking BPhen:C.sub.60/PTCBI cathode buffer.

(99) TABLE-US-00001 TABLE I Photo voltaic performance for DPSQ/C.sub.60 junctions with variuos buffer layers. Cathode J.sub.SC Anode buffer buffer [mA/cm.sup.2] V.sub.OC [V] FF [%] η.sub.P [%] MoO.sub.3 PTCBI 7.6 ± 0.4 0.93 68 4.8 ± 0.2 MoO.sub.3 + BPA PTCBI 8.2 ± 0.4 0.93 68 5.1 ± 0.3 NiO PTCBI 7.2 ± 0.4 0.87 66 4.1 ± 0.2 NiO + BPA PTCBI 8.5 ± 0.4 0.93 68 5.4 ± 0.3 NiO + BPA BPhen:C.sub.60 9.1 ± 0.5 0.93 69 5.9 ± 0.3

(100) To demonstrate the general applicability of the phosphonic acid buffer treatments, BPA-treated NiO was compared to MoO.sub.3 buffers for both SubPc/C.sub.60 and bSQ/C.sub.60 devices in FIG. 8 and Table II below. The SubPc/C.sub.60 device showed an ˜50% improvement in the SubPc response at λ=590 nm, which resulted in an increase in efficiency from η.sub.P=3.1±0.2% to η.sub.P=3.8±0.2%. The bSQ device showed a smaller improvement in EQE of ˜6% due to interdiffusion of the donor and acceptor materials during solvent vapor annealing, resulting in a bulk heterojunction-like morphology conducive to more efficient exciton dissociation. Nonetheless, the efficiency improved from η.sub.P=5.5±0.3% to η.sub.P=6.0±0.3% (Table II).

(101) TABLE-US-00002 TABLE II Photovoltaic performance for SubPc/C.sub.60/BPhen and bSQ/C.sub.60/PTCBI junctions with MoO.sub.3 and BPA treated NiO buffer layers. Donor Anode buffer material J.sub.SC [mA/cm.sup.2] V.sub.OC [V] FF [%] η.sub.P [%] MoO.sub.3 SubPc 4.7 ± 0.2 1.10 60 3.1 ± 0.2 NiO + BPA SubPc 6.0 ± 0.3 1.10 56 3.8 ± 0.2 MoO.sub.3 bSQ 8.7 ± 0.4 0.87 70 5.5 ± 0.3 NiO ± BPA bSQ 9.2 ± 0.5 0.93 70 6.0 ± 0.3

Example 4

(102) FIG. 9 shows the effect of blocking efficiency on BPA and OPA treated MoO.sub.3 surfaces where the BPA or OPA was spin coated onto the MoO.sub.3 and heat treated to bond the acid to surface. The photocurrent and near infrared (NIR) EQE response from the donor increased for all treatments, saturating for concentrations above 0.25 g/l BPA. An increase in series resistance (and decrease in FF), however, was observed for the devices treated with 0.5 g/l OPA and 1 g/l BPA which reduced the overall efficiency.

Example 5

(103) FIG. 10 shows device performance data for DPSQ/C.sub.60 devices with a BPA-treated MoO.sub.3 anode buffer that was soaked for 30 minutes in BPA dissolved in THF or that was spin coated from either a chloroform methanol solution (CM) or THF. The NIR and total photo response and consequently FF were highest for the MoO.sub.3 soaked in the BPA solution. The soaking method also allowed for a single monolayer of phosphonic acid to be deposited on MoO.sub.3.

Example 6

(104) FIG. 11 shows the effects of treating the MoO.sub.3 anode buffer with different soak durations in BPA or OPA dissolved in THF. The longer soak time resulted in slightly higher photocurrent and power conversion efficiency (η.sub.P) for BPA treatments. The OPA treatment resulted in slightly reduced FF and J.sub.sc.

Example 7

(105) FIG. 12 shows the effects of various donor layer thicknesses in DPSQ/C.sub.60 junctions. The NIR quantum efficiency was maximized for a DPSQ thickness of ˜13 nm on the MoO.sub.3 anode buffer surface, while the NIR quantum efficiency was highest at 8 nm DPSQ thickness on the BPA-treated MoO.sub.3 surface. The device yield, however, for DPSQ thicknesses of <10 nm was significantly reduced.

Example 8

(106) FIG. 13 compares the quantum efficiency of MoO.sub.3 and BPA-treated MoO.sub.3 (30 min soak) anode buffers to NiO and BPA-, OPA-, MePA-, and phosphorous acid-treated NiO (applied by spin coating, heating and rinsing as described above). The DPSQ donor has a response from λ=550 nm to 800 nm. The NIR EQE was highest for the OPA-treated surface and all phosphonic acid treatments show Improved EQE. The EQE corresponded to the exciton blocking efficiency in FIGS. 3-5. An approximate 30% increase in EQE was observed for the best blocking interfaces. FIG. 14 shows that the highest η.sub.P was observed for the BPA-treated NiO and that the OPA treatment, while having a high J.sub.sc, had a reduced FF due to an increase in series resistance. In particular, an infection in the current starting at 10.sup.−3 Acm.sup.−2 for the OPA treated NiO sample and at 2×10.sup.−2 Acm.sup.−2 for the MePA treated NiO was observed. This extra resistance to current extraction was seen in the illuminated curves. The power conversion efficiencies for the samples with the various buffer layers were as follows: the MoO.sub.3 reference was 4.8%, the BPA soaked MoO.sub.3 was 5.2%, the BPA treated NiO was 5.6%, the OPA treated NiO was 5.5%, the MePA treated NiO 5.5%, and the phosphorous acid treated NiO was 4.0%.

Example 9

(107) FIG. 15 shows the effect of varying the temperature at which the BPA was bonded to the NiO. At 80° C., a NIR EQE of about 32% was observed, but at a bonding temperature of 170° C. the NIR EQE increased to about 38%. As the temperature was increased, and specifically when the temperature increased above ˜120° C., however, the current-voltage curves degraded and, specifically, an S-kink emerged indicating a loss in FF. S-kinks are generally attributed to a reverse diode in the system, which likely occurred from a decrease in the work function originating from a change in the dipole associated with the phosphonic acid, resulting in a decrease in the effective work function and effective frontier orbital energy levels of the NiO. The ideal heating temperature for bonding the phosphonic acids to the anode buffer surfaces for the tested samples was around 120° C.

Example 10

(108) FIG. 16 shows the effect of using modified BPA acid groups with different dipole moments, e.g. comparing unmodified BPA, BPA fluorinated in the 4-position of the benzyl ring, and BPA fluorinated in all 5 positions of the benzyl ring as depicted in FIG. 16. The EQE from DPSQ/C.sub.60 devices is shown in FIG. 16 where the highest NIR EQE (and thus blocking efficiency) was achieved by the unmodified BPA treatment. The fluorinated compounds had a reduced EQE at λ=700 nm (from the DPSQ donor) compared to the unmodified BPA sample.

(109) FIG. 17 shows the current-voltage response under illumination and in the dark for the devices in FIG. 16. The unmodified BPA-treated sample exhibited a small inflection in dark current (indicating small reverse diode), while the J-V curves for the fluorinated BPAs did not, but the J.sub.SC was reduced for the fluorinated BPAs. FIG. 17 shows that the work function of the BPA treated NiO was too shallow and that choosing a phosphonic acid with a dipole moment, such as fluorinated BPAs to induce a dipole, can act to lower the work function of the NiO and improve the extraction efficiency of the contact.

Example 11

(110) FIG. 18 compares EQE and J-V performance data for devices with different lengths of aryl terminated phosphonic acids. All of the variants reduced exciton quenching, but the best results were observed for the BPA-treated NiO surface. The series resistance increased for napthylmethylphosphonic acid and propylphenylphosphonic acid, the longer acids tested.

Example 12

(111) FIG. 19 compares EQE and J-V performance data for devices with various alkyl chain lengths from zero carbons (phosphorous acid) to a 16-carbon, hexadecyl phosphonic acid on an NiO surface. These alkylphosphonic acid SAMs all led to improved EQE. The highest EQE occurred for the longest chains, but these chains showed significantly reduced fill factor and power conversion efficiency compared to the shorter chains. The best power conversion efficiency was observed for butylphosphonic acid, where shorter chains resulted in less exciton quenching and low resistance and the longer chains had a higher NIR EQE but an increased resistance or an S-kink in the J-V curve.