POLYMER PHOTOVOLTAICS EMPLOYING A SQUARAINE DONOR ADDITIVE

Abstract

Disclosed herein are organic photosensitive optoelectronic devices comprising two electrodes in superposed relation, a photoactive region located between the two electrodes, wherein the photoactive region comprises a donor mixture and an organic acceptor material, the donor mixture comprising at least one organic polymer donor material and at least one squaraine donor. Methods of fabricating the organic photosensitive optoelectronic devices are also disclosed.

Claims

1. An organic photosensitive optoelectronic device comprising: two electrodes in superposed relation; a photoactive region located between the two electrodes, wherein the photoactive region comprises a donor mixture and an organic acceptor material, the donor mixture comprising at least one organic polymer donor material and at least one squaraine donor.

2. The device of claim 1, wherein the at least one squaraine donor has a maximum absorptivity at a longer wavelength than a maximum absorptivity of the at least one organic polymer donor material.

3. The device of claim 1, wherein the at least one squaraine donor has an absorptivity of at least 10.sup.3 cm.sup.−1 at one or more wavelengths ranging from 450 to 950 nm.

4. The device of claim 1, wherein the at least one squaraine donor has an absorptivity of at least 10.sup.5 cm.sup.−1 at one or more wavelengths ranging from 450 to 950 nm.

5. The device of claim 1, wherein the donor mixture comprises the at least one organic polymer donor material and the at least one squaraine donor at a polymer donor:squaraine ratio ranging from 1:0.005 to 1:0.2 by weight.

6. The device of claim 5, wherein the polymer donor:squaraine ratio ranges from 1:0.01 to 1:0.1 by weight.

7. The device of claim 1, wherein the donor mixture and the organic acceptor material form a donor-acceptor heterojunction.

8. The device of claim 7, wherein the donor-acceptor heterojunction is chosen from a mixed heterojunction, a bulk heterojunction, a planar heterojunction, and a hybrid planar-mixed heterojunction.

9. The device of claim 1, wherein the at least one squaraine donor is chosen from 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl] (DBSQ), 2,4-bis[4-N-carbazolo-2,6-dihydroxyphenyl]squaraine (CBZSQ), 2,4-bis[4-N-phenothiazino-2,6-dihydroxyphenyl]squaraine (PTSQ), 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine (DPSQ), 2,4-bis[4-(N-Phenyl-1-naphthylamino)-2,6-dihydroxyphenyl]squaraine (1NPSQ), 2,4-bis[4-(N-Phenyl-2-naphthylamino)-2,6-dihydroxyphenyl]squaraine (2NPSQ), {2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-diphenylamino}squaraine (USSQ), {2-[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]-4-diphenylamino}squaraine (DPUSQ), and diphenylamino-squarate (YSQ).

10. The device of claim 1, wherein the at least one organic polymer donor material is chosen from polythiophene, polycarbazole, polyfluorene, polydithienosilole, polybenzodithiophene, and copolymers thereof.

11. The device of claim 1, wherein the at least one organic polymer donor material is chosen from poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene], poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene-alt-4,7-(2,1,3-benzothiadiazole)], poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl), poly{2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione}, poly[2,7-(9,9-didecylfluorene)-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)], and alternating copolymer of thieno[3,4-b]-thiophene and benzodithiophene.

12. The device of claim 1, wherein the organic acceptor material comprises at least one compound chosen from perylenes, naphthalenes, fullerenes, and derivatives thereof.

13. The device of claim 1, wherein the at least one organic polymer donor material is P3HT and the organic acceptor material comprises a fullerene or a derivative thereof.

14. A method of fabricating an organic photosensitive optoelectronic device comprising: depositing a photoactive region over a first electrode; and depositing a second electrode over the photoactive region, wherein the photoactive region comprises a donor mixture and an organic acceptor material, the donor mixture comprising at least one organic polymer donor material and at least one squaraine donor.

15. The method of claim 14, wherein the deposition of a photoactive region over a first electrode comprises co-depositing the at least one organic polymer donor material and the at least one squaraine donor over the first electrode, and depositing the organic acceptor material over the first electrode, wherein the co-deposition of the at least one organic polymer donor material and the at least one squaraine donor occurs before or after the deposition of the organic acceptor material over the first electrode.

16. The method of claim 15, wherein the at least one organic polymer donor material and the at least one squaraine donor are co-deposited at a polymer donor:squaraine ratio ranging from 1:0.005 to 1:0.2 by weight.

17. The method of claim 14, wherein the deposition of a photoactive region over a first electrode comprises co-depositing the at least one organic polymer donor material, the at least one squaraine donor, and the organic acceptor material over the first electrode.

18. The method of claim 17, wherein the at least one organic polymer donor material, the organic acceptor material, and the at least one squaraine donor are co-deposited at a polymer donor:acceptor:squaraine ratio ranging from 1:0.5:x to 1:1.5:x by weight, wherein x represents a number ranging from 0.005 to 0.2.

Description

[0041] The accompanying figures are incorporated in, and constitute a part of this specification.

[0042] FIG. 1 shows a schematic of an organic photosensitive optoelectronic device in accordance with the present disclosure.

[0043] FIG. 2 shows an example of a device schematic wherein the donor mixture is in contact with the acceptor material, forming a donor-acceptor heterojunction.

[0044] FIG. 3A shows ultraviolet-visible absorption spectra of P3HT:PCBM:DBSQ films at varying concentrations of DBSQ; and 3B shows x-ray diffraction spectra of P3HT:PCBM:DBSQ films at varying concentrations of DBSQ.

[0045] FIG. 4 shows atomic force microscopy images of (A) P3HT:PCBM film; (B) P3HT:PCBM:5 wt % DBSQ film; and (C) P3HT:PCBM:10 wt % DBSQ film. All films were annealed at 120° C. for 10 minutes. The scan area was 5 μm×5 μm.

[0046] FIG. 5 shows plots of (A) current density vs. voltage; (B) power conversion efficiency vs. intensity; and (C) external quantum efficiency vs. wavelength for P3HT:PCBM:DBSQ photovoltaic cells at varying concentrations of DBSQ.

[0047] FIG. 6A shows photoluminescence spectra of P3HT, P3HT:PCBM, P3HT:PCBM:5 wt % DBSQ and P3HT:PCBM:10 wt % DBSQ films; and 6B shows a plot of the expansion of the low intensity region between wavelength of 600 and 800 nm.

[0048] As used herein, the term “co-depositing” or “co-deposition” may include simultaneously depositing materials independently (from separate sources) onto a substrate, where the ratio of the materials can be controlled by the rate of deposition of each material. In some cases, materials that are “co-deposited” may be deposited sequentially and subjected to further processing, such as thermal annealing or solvent annealing, to form a mixture. Vapor deposition methods are examples of these approaches. Alternatively, “co-depositing” or “co-deposition” may include mixing the materials at a desired ratio and depositing the mixed materials onto a substrate. Fluid solution deposition methods are examples of this alternative approach.

[0049] 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 reference 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 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.

[0050] 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).

[0051] As used herein, the expression that a material or component is deposited “over” another material or component permits other materials or layers to exist between the material or component being deposited and the material or component “over” which it is deposited. For example, a layer may be described as being deposited “over” an electrode, even though there are various materials or layers in between the layer and the electrode.

[0052] As used herein, the term “absorptivity” refers to the percentage of incident light at a given wavelength that is absorbed.

[0053] In the context of the organic materials of the present disclosure, the terms “donor” and “acceptor” refer to the relative positions of the HOMO and LUMO energy levels of two contacting but different organic materials. If the LUMO energy level of one material in contact with another is lower, 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.

[0054] The organic photosensitive optoelectronic devices of the present disclosure utilize a mixture of donor materials: at least one organic polymer material and at least one small molecule additive, wherein the small molecule additive is a squaraine donor. The donor mixture of at least one organic polymer material and a squaraine can expand the absorption range of an organic photosensitive optoelectronic device, leading to efficient light absorption, for example, from the visible spectrum into the NIR. The increased absorption efficiency can significantly increase the J.sub.SC and PCE of the device.

[0055] As shown in FIG. 1, an organic photosensitive device of the present disclosure comprises two electrodes in superposed relation, and a photoactive region located between the two electrodes. As used herein, the “photoactive region” refers to a region of the device that absorbs electromagnetic radiation to generate excitons. The excitons may dissociate into an electron and a hole in order to generate an electrical current. The photoactive region comprises a donor mixture and an organic acceptor material, wherein the donor mixture comprises at least one organic polymer donor material and at least one squaraine donor.

[0056] Absorption bands of the at least one squaraine donor and the at least one organic polymer donor material may complement one another to expand the light absorption wavelength range of the photosensitive device. In some embodiments, the at least one squaraine donor has a maximum absorptivity at one or more wavelengths, the maximum absorptivity of the at least one squaraine donor being at least twice as large as an absorptivity of the at least one organic polymer donor material at the one or more wavelengths. In some embodiments, the at least one squaraine donor has a maximum absorptivity at a longer wavelength than a maximum absorptivity of the polymer donor material. In some embodiments, the at least one squaraine donor has an absorptivity of at least 10.sup.3 cm.sup.−1 at one or more wavelengths ranging from 450 to 950 nm, 450 to 800 nm, 500 to 750 nm, 650 to 950 nm, 650 to 900 nm or 700 to 850 nm. In some embodiments, the at least one squaraine donor has an absorptivity of at least 10.sup.5 cm.sup.−1 at one or more wavelengths ranging from 450 to 950 nm, 450 to 800 nm, 500 to 750 nm, 650 to 950 nm, 650 to 900 nm or 700 to 850 nm.

[0057] The donor mixture and the organic acceptor material may form a donor-acceptor heterojunction. The donor-acceptor heterojunction may be any heterojunction known in the art for organic photosensitive devices. For example, in some embodiments, the donor-acceptor heterojunction may be chosen from a mixed heterojunction, a bulk heterojunction, a planar heterojunction, and a hybrid planar-mixed heterojunction. FIG. 2 shows an example of a device schematic wherein the donor mixture is in contact with the organic acceptor material, forming a donor-acceptor heterojunction. Although FIG. 2 depicts the donor mixture and the organic acceptor material as neat layers, it should be understood that the surfaces of the layers, such as the interface between the donor mixture and the organic acceptor material, may be imperfect, wherein said surfaces represent an interpenetrating, entangled or convoluted network with other material(s) or layer(s). In some embodiments, the photoactive region comprises a blend of the donor mixture and the organic acceptor material. As one of ordinary skill in the art would appreciate, the location of the donor mixture relative to the organic acceptor material depends on the desired type of donor-acceptor heterojunction for the device.

[0058] The amount of the at least one squaraine donor in the photoactive region may be optimized to achieve peak device performance. The optimization may include balancing increased absorption efficiency at increasing squaraine concentrations with series resistance effects at such increased concentrations. In some embodiments, the donor mixture comprises the at least one polymer donor material and the at least one squaraine donor at a polymer donor:squaraine ratio ranging from 1:0.005 to 1:0.2 by weight. In some embodiments, the polymer donor:squaraine ratio ranges from 1:0.01 to 1:0.1 by weight. In certain embodiments, the polymer donor:squaraine ratio is 1:0.05 by weight.

[0059] In some embodiments, the at least one organic polymer donor material, the acceptor material, and the at least one squaraine donor are present in the photoactive region at a polymer donor:acceptor:squaraine ratio ranging from 1:0.5:x to 1:1.5:x by weight, where x represents a number ranging from 0.005 to 0.2. In some embodiments, x represents a number ranging from 0.01 to 0.1.

[0060] The at least one organic polymer donor material may be any organic polymer donor material known in the art. Non-limiting mention is made to organic polymer donor materials chosen from polythiophene, polycarbazole, polyfluorene, polydithienosilole, polybenzodithiophene, and copolymers thereof. In some embodiments, the at least one organic polymer donor material may be chosen from poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene], poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene-alt-4,7-(2,1,3-benzothiadiazole)], poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], poly(4,4-dioctyldithieno(3,2-b:2′,3′-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl), poly{2,6-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione}, poly[2,7-(9,9-didecylfluorene)-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)], and alternating copolymer of thieno[3,4-b]-thiophene and benzodithiophene.

[0061] The at least one squaraine donor may be any squaraine known in the art. In some embodiments, the at least one squaraine donor is chosen from from 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DBSQ), 2,4-bis[4-N-carbazolo-2,6-dihydroxyphenyl]squaraine (CBZSQ), 2,4-bis[4-N-phenothiazino-2,6-dihydroxyphenyl]squaraine (PTSQ), 2,4-bis[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]squaraine (DPSQ), 2,4-bis[4-(N-Phenyl-1-naphthylamino)-2,6-dihydroxyphenyl]squaraine (1 NPSQ), 2,4-bis[4-(N-Phenyl-2-naphthylamino)-2,6-dihydroxyphenyl]squaraine (2NPSQ), {2-[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-diphenylamino}squaraine (USSQ), {2-[4-(N,N-diphenylamino)-2,6-dihydroxyphenyl]-4-diphenylamino}squaraine (DPUSQ), and diphenylamino-squarate (YSQ). Additional examples of suitable squaraine donors are disclosed in U.S. Patent Publication No. 2012/0248419, which is incorporated herein by reference for its disclosure of squaraines.

[0062] Examples of the organic acceptor material include perylenes, naphthalenes, fullerenes, and derivatives thereof. Non-limiting mention is made to those chosen from C.sub.60, C.sub.70, C.sub.76, C.sub.82, C.sub.84, 3,4,9,10-perylenetetracarboxylicbis-benzimidazole (PTCBI), Phenyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]PCBM), Phenyl-C.sub.71-Butyric-Acid-Methyl Ester ([70]PCBM), Thienyl-C.sub.61-Butyric-Acid-Methyl Ester ([60]ThCBM), and hexadecafluorophthalocyanine (F.sub.16CuPc). In certain embodiments, the acceptor material is chosen from fullerenes and derivatives thereof. The organic acceptor material is not limited to a single material. A combination of acceptors can be used.

[0063] In some embodiments of the present disclosure, the at least one squaraine donor comprises two or more squaraine donors. The squaraine donors may be selected to complement the absorption of the at least one organic polymer donor material, as disclosed herein, to expand the light absorption wavelength range of the photosensitive device. In some embodiments, the two or more squaraine donors comprise at least a first squaraine donor and a second squaraine donor, wherein the absorption ranges of the first and second squaraine donors may fully or at least partially overlap. In some embodiments, the first squaraine donor has a maximum absorptivity at one or more wavelengths, the maximum absorptivity of the first squaraine donor being at least twice as large as an absorptivity of the second squaraine donor and an absorptivity of the at least one organic polymer donor at the one or more wavelengths. In some embodiments, the second squaraine donor has a maximum absorptivity at one or more wavelengths, the maximum absorptivity of the second squaraine donor being at least twice as large as an absorptivity of the first squaraine donor and an absorptivity of the at least one organic polymer donor at the one or more wavelengths.

[0064] One of the electrodes of the present disclosure may be an anode, and the other electrode a cathode. It should be understood that the electrodes should be optimized to receive and transport the desired carrier (holes or electrons). The term “cathode” is used herein such that 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.

[0065] The organic photosensitive optoelectronic devices of the present disclosure may have a conventional or inverted structure. Examples of inverted device structures are disclosed in U.S. Patent Publication No. 2010/0102304, which is incorporated herein by reference for its disclosure of inverted device structures.

[0066] The organic photosensitive optoelectronic devices of the present disclosure may further comprise additional layers as known in the art for such devices. For example, devices may further comprise charge carrier transport layers and/or buffers layers such as one or more blocking layers, such as an exciton blocking layer (EBL). One or more blocking layers may be located between the photoactive region and either or both of the electrodes. With regard to materials that may be used as an exciton blocking layer, non-limiting mention is made to those chosen from bathocuproine (BCP), bathophenanthroline (BPhen), 1,4,5,8-Naphthalene-tetracarboxylic-dianhydride (NTCDA), 3,4,9,10-perylenetetracarboxylicbis-benzimidazole (PTCBI), 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), tris(acetylacetonato) ruthenium(III) (Ru(acac).sub.3), and aluminum(Ill)phenolate (Alq.sub.2 OPH), N,N′-diphenyl-N,N′-bis-alpha-naphthylbenzidine (NPD), aluminum tris(8-hydroxyquinoline) (Alq.sub.3), and carbazole biphenyl (CBP). Examples of blocking layers are described in U.S. Patent Publication Nos. 2012/0235125 and 2011/0012091 and in U.S. Pat. Nos. 7,230,269 and 6,451,415, which are incorporated herein by reference for their disclosure of blocking layers.

[0067] The organic photosensitive optoelectronic devices of the present disclosure may comprise additional buffer layers as known in the art for such devices. For example, the devices may further comprise at least one smoothing layer. A smoothing layer may be located, for example, between the photoactive region and either or both of the electrodes. A film comprising 3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) is an example of a smoothing layer.

[0068] The organic 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 photoactive region having a donor-acceptor heterojunction. When a subcell is used individually as a photosensitive optoelectronic device, it typically includes a complete set of electrodes. A tandem device may comprise charge transfer material, electrodes, or charge recombination material or a tunnel junction between the tandem donor-acceptor heterojunctions. In some tandem configurations, it is possible for adjacent subcells to utilize common, i.e., shared, electrode, charge transfer region or charge recombination zone. In other cases, adjacent subcells do not share common electrodes or charge transfer regions. The subcells may be electrically connected in parallel or in series.

[0069] 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.

[0070] The devices of the present disclosure may be, for example, photodetectors, photoconductors, or organic PV devices, such as solar cells.

[0071] Methods of preparing organic photosensitive optoelectronic devices of the present disclosure are also disclosed herein. In one embodiment, a method of fabricating an organic photosensitive optoelectronic device comprises depositing a photoactive region over a first electrode, and depositing a second electrode over the photoactive region, wherein the photoactive region comprises a donor mixture and an organic acceptor material, the donor mixture comprising at least one organic polymer donor material and at least one squaraine donor.

[0072] In some embodiments, depositing a photoactive region over a first electrode comprises co-depositing the at least one organic polymer donor material and the at least one squaraine donor over the first electrode, and depositing the organic acceptor material over the first electrode, wherein the co-deposition of the at least one organic polymer donor material and the at least one squaraine donor occurs before or after the deposition of the organic acceptor material over the first electrode. In some embodiments, the first electrode is optimized to receive and transport holes, and the at least one organic polymer donor material and the at least one squaraine donor is co-deposited over the first electrode before the deposition of the organic acceptor material over the first electrode. In other embodiments, the first electrode is optimized to receive and transport electrons, and the organic acceptor material is deposited over the first electrode before the co-deposition of the at least one organic polymer donor material and the at least one squaraine donor over the first electrode. The at least one organic polymer donor material and the at least one squaraine donor may be co-deposited at a polymer donor:squaraine ratio ranging from 1:0.005 to 1:0.2 by weight. In some embodiments, the polymer donor:squaraine ratio ranges from 1:0.01 to 1:0.1 by weight. In certain embodiments, the polymer donor:squaraine ratio is 1:0.05 by weight.

[0073] In another embodiment, depositing a photoactive region over a first electrode comprises co-depositing the at least one organic polymer donor material, the at least one squaraine donor, and the organic acceptor material over the first electrode. In some embodiments, the co-deposition is at a polymer donor:acceptor:squaraine ratio ranging from 1:0.5:x to 1:1.5:x by weight, wherein x represents a number ranging from 0.005 to 0.2. In some embodiments, x represents a number ranging 0.01 to 0.1.

[0074] As described herein, additional layers, such as transport layers, blocking layers, smoothing layers, and other buffer layers known in the art for organic photosensitive optoelectronic devices may be deposited during fabrication of the devices.

[0075] Layers and materials may be deposited using techniques known in the art. For example, the layers and materials described herein can be deposited from a solution, vapor, or a combination of both. In some embodiments, the organic materials or organic layers can 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.

[0076] In other embodiments, the organic materials may be deposited or co-deposited using vacuum evaporation, such as vacuum thermal evaporation, organic vapor phase deposition, or organic vapor-jet printing.

[0077] It should be understood that embodiments described herein may be used in connection with a wide variety of other structures. Functional organic optoelectronic devices may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Additional layers not specifically described may also be included. Materials other than those specifically described may be used. The names given to the various layers herein are not intended to be strictly limiting.

[0078] 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.

[0079] 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.

[0080] The devices and methods described herein will be further described by the following non-limiting examples, which are intended to be purely exemplary.

EXAMPLES

[0081] Organic photosensitive optoelectronic devices were fabricated using a 2,4-bis[4-(N,N-diisobutylamino)-2,6-dihydroxyphenyl]squaraine (DBSQ) additive and a poly(3-hexylthiophene) (P3HT) polymer donor material. DBSQ strongly absorbs from a wavelength of λ=520 to 750 nm with a peak optical density of 2.0×10.sup.5 cm.sup.−1 at λ=700 nm, whereas P3HT absorbs from λ=400 to 630 nm. Fabricated devices had the following structure: indium tin oxide (ITO, 50 nm)/poly-(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS, 40 nm)/P3HT: [6,6]-phenyl C61-butyric acid methyl ester (PCBM): DBSQ (140 nm)/LiF 1 nm)/AI (200 nm). The DBSQ concentrations in the photoactive region were varied at 0, 5 wt % and 10 wt %. The relative ratio of P3HT:PCBM was 1:0.7. The P3HT:PCBM:DBSQ blend was dissolved in chlorobenzene at a concentration of 4.0 wt %, and then was spin-coated to a thickness of 140 nm onto the ITO layer with a sheet resistivity of 10 Ω/square. Following spin coating, the P3HT:PCBM:DBSQ layer was baked at 120° C. for 10 minutes inside of a glove box filled with ultrahigh purity N.sub.2 gas.

[0082] The absorption spectrum of the P3HT:PCBM:DBSQ blended film spin-coated on a quartz substrate was measured using an ultraviolet-visible (UV-Vis) spectrophotometer (Shimadzu, UV-2501 PC). The photoluminescence spectrum was recorded using a fluorescence spectrophotometer (Hitachi, F-7000). Organic PV device performance was measured using an Abet solar simulator with a Keithley 2400 source measurement unit under 1 sun (100 mW/cm.sup.2) illumination intensity, after spectral mismatch correction. The morphology of the P3HT:PCBM:DBSQ film was analyzed using atomic force microscopy (AFM; Digital Instrument Co. Multimode Nanoscope IIIa) in the tapping mode with a Si tip. X-ray diffraction spectra of P3HT:PCBM:DBSQ films spin coated on PEDOT:PSS/ITO coated glass substrate were obtained using a Rigaku Ultima IV and a Cu Kα radiation source.

[0083] The UV-Vis absorption spectrum of the P3HT:PCBM:DBSQ blended film was measured as a function of DBSQ concentration, with results shown in FIG. 3A. As the solubility of DBSQ is <4 wt % in chlorobenzene, the DBSQ concentration was kept at ≤10 wt % in the P3HT:PCBM:DBSQ blends. The blend is transparent at wavelengths λ>650 nm, i.e., the blend transmits ≥50% of incident radiation at wavelengths a λ>650 nm, while absorption between λ=650 and 750 nm increases with DBSQ concentration. For example, the peak absorption of DBSQ at λ=680 nm is 40% that of P3HT for a DBSQ concentration of only 10 wt %.

[0084] X-ray diffraction of a spin-coated P3HT:PCBM:DBSQ blend was used to study the effect of DPSQ on film morphology. FIG. 3B shows an X-ray diffraction pattern of P3HT:PCBM:DBSQ films after thermal annealing at 120° C. under N.sub.2 atmosphere for 10 minutes. The blended film exhibited a peak at angle 2θ=6.0° assigned to diffraction from the (100) crystal plane of P3HT. The peak expected for DBSQ at 2θ=8.0° was notably absent. The intensity of the P3HT diffraction peak remained unchanged for the 5 wt % and 10 wt % DBSQ mixed films, indicating that DBSQ does not hinder the crystallization of P3HT.

[0085] FIG. 4 shows atomic force microscope (AFM) images of a series of P3HT:PCBM:DBQ films with different concentrations of DBSQ. The neat P3HT:PCBM film exhibited a smooth surface morphology with a root mean square roughness of RMS=1.1±0.1 nm. The smooth surface morphology of the blend film was roughened by adding DBSQ. For example, the roughness of the film consisting of 10 wt % DBSQ in P3HT:PCBM was RMS=1.6±0.2 nm. Evidence of phase separation in the 10 wt % DBSQ film was also observed, possibly due to precipitation of DBSQ.

[0086] FIG. 5A shows current density-voltage (J-V) characteristics of P3HT:PCBM:DBSQ devices with varying DBSQ concentrations. J.sub.SC increased from 7.3±0.3 mA/cm.sup.2 to 8.9±0.1 mA/cm.sup.2 under 1 sun, AM1.5G illumination by adding DBSQ due to the expended UV-Vis absorption spectral range of the P3HT:PCBM:DBSQ photoactive region. There was a sharp increase to J.sub.SC=8.8±0.1 mA/cm.sup.2 by adding only 5 wt % of DBSQ, whereas a further increase of DBSQ content did not significantly increase J.sub.SC. In contrast, V.sub.OC was not affected by adding DBSQ, leading to only a 0.02 V increase since the highest occupied molecular orbital (HOMO) energy of DBSQ at −5.3 eV is comparable to −5.1 eV for P3HT. The larger HOMO energy of DBSQ contributed to the slight increase observed for V.sub.OC.

[0087] The fill factor (FF) of 0.64±0.01 of the P3HT:PCBM device remained constant up to 5 wt % DBSQ concentration, and then decreased to FF=0.53±0.04 at 10 wt % DBSQ. As shown in the AFM images of P3HT:PCBM:DBSQ films spin-coated on PEDOT:PSS (FIG. 4), large-scale phase separation was observed at 10 wt % DBSQ. This led to an increased series resistance of the device, and hence a concomitant decrease in FF. The specific series resistance of the P3HT:PCBM:DBSQ device calculated from dark J-V characteristics using the modified ideal diode equation was increased from 3.2±0.2 Ω.Math.cm2 for neat P3HT:PCBM films to 11±3 Ω.Math.cm.sup.2 at 10 wt % DBSQ.

[0088] The power conversion efficiency (PCE) of P3HT:PCBM:DBSQ organic PVs was measured as a function of the power intensity of AM1.5G simulated illumination, and the results are shown in FIG. 5B. The PCE of the P3HT:PCBM device was 2.8±0.1% at 1 sun (100 mW/cm.sup.2) intensity, which increased to 3.4±0.3% by with the addition of 5 wt % DBSQ. The 20% increase in PCE obtained by the addition of DBSQ was primarily due to increased absorption at long wavelength in the blended film (See FIG. 3A). The expanded absorption spectral range of P3HT:PCBM:DBSQ resulted in an increase in J.sub.SC. However, upon increasing the DBSQ concentration to 10 wt %, only a marginal further increase in PCE was observed compared to the P3HT:PCBM device due to the reduced FF, as discussed above.

[0089] The external quantum efficiency (EQE) of P3HT:PCBM:5 wt % DBSQ at wavelengths λ>650 nm shown in FIG. 5C was increased compared to that of a neat P3HT:PCBM bulk heterojunction cell due to the additional NIR absorption of the DBSQ blend. However, the EQE in the visible range due to P3HT:PCBM absorption was reduced with the addition of 10 wt % DBSQ. This is due to differences in the charge generation mechanisms in the presence of DBSQ. There are at least three possible routes to charge generation: (i) Exciton dissociation at P3HT:PCBM domain interfaces, (ii) exciton dissociation at DBSQ:PCBM interfaces, and (iii) hole and electron generation by energy transfer from P3HT to DBSQ. In this latter process, excitons generated on P3HT transfer energy via a Förster process to DBSQ before diffusion to an interface with PCBM. This route is energetically allowed since the photoluminescence (PL) emission of P3HT is between λ=600 and 850 nm, which overlaps the peak absorption of DBSQ at λ=700 nm. The DBSQ excitons then dissociate into holes and electrons at the interface with PCBM. The energy transfer from P3HT to DBSQ was confirmed by PL measurements of P3HT:PCBM:DBSQ, with spectra shown in FIG. 6.

[0090] Emission from P3HT is primarily quenched by PCBM, and the P3HT emission from the P3HT:PCBM mixture was further quenched by adding DBSQ, which confirms the existence of energy transfer from P3HT to DBSQ. The relative P3HT intensity of the P3HT:PCBM:5 wt % DBSQ was only 45% of P3HT emission of P3HT:PCBM, indicating that 5 wt % DBSQ can quench 55% of excitons generated in P3HT. For P3HT:PCBM:10 wt % DBSQ, approximately 70% of the P3HT excited states were quenched by energy transfer to DBSQ. It may be inferred from these results that 95% of the excitons generated in P3HT are dissociated at interfaces with PCBM (route (i)), and 3.0% and 3.8% of P3HT excitons are transferred to DBSQ for concentrations of 5 wt % and 10 wt %, respectively (route (iii)).

[0091] In addition, the EQE of the P3HT:PCBM:5 wt % DBSQ device was high relative to the absorption of P3HT:PCBM:5 wt % DBSQ. This indicates that excitons generated on DBSQ were efficiently dissociated. Hence, at this concentration, it is likely that DBSQ molecules were located close (within a Förster radius) to the PCBM. The short exciton diffusion length of DBSQ (<2 nm) and high EQE of DBSQ provide further evidence that DBSQ molecules were within close proximity to PCBM.

[0092] Device performance is summarized in Table 1 below.

TABLE-US-00001 TABLE 1 Device performance of P3HT:PCBM:DBSQ bulk heterojunctionsolar cells J.sub.sc V.sub.oc FF PCE R′, A (mA/cm.sup.2) (V) (%) (%) (Ω .Math. cm.sup.2) P3HT:PCBM 7.3 ± 0.3 0.59 ± 0.01 .sup. 64 ± 0.1 2.75 ± 0.13 3.2 ± 0.2 P3HT:PCBM:5 8.8 ± 0.1 0.61 ± 0.01 64 ± 4 3.38 ± 0.27 6.6 ± 0.4 wt % DPSQ P3HT:PCBM:10 8.9 ± 0.1 0.61 ± 0.01 53 ± 4 2.88 ± 0.25 11.4 ± 2.6  wt % DPSQ

[0093] In sum, the PCEs of P3HT:PCBM organic PVs were increased by greater than 20% through the addition of a small concentration of DBSQ into the mixture, thereby enhancing the NIR absorption of the resulting PV. A power conversion efficiency of 3.4±0.3% and an external quantum efficiency as high as 55% was achieved for a P3HT:PCBM blend that included 5 wt % DBSQ.