5504
Chem. Mater. 2006, 18, 5504-5509
Nanostructured Organic Layers via Polymer Demixing for Interface-Enhanced Photovoltaic Cells Fernando A. Castro,†,‡ Hadjar Benmansour,† Carlos F. O. Graeff,§ Frank Nu¨esch,† Eduard Tutis,| and Roland Hany*,† Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Functional Polymers, U ¨ berlandstr. 129, CH-8600 Du¨bendorf, Switzerland, Departamento de Fı´sica e Matema´ tica, Faculdade de Filosofia, Cieˆ ncias e Letras de Ribeira˜ o Preto, UniVersidade de Sa˜ o Paulo, AV. Bandeirantes 3900, 14040-901, Ribeira˜ o Preto, SP, Brazil, Departamento de Fı´sica, Faculdade de Cieˆ ncias, UNESP, AV. Luiz Edmundo Carrijo Coube 14-01, 17033-360 Bauru, Brazil, and Institute of Physics, P.O. Box 304, HR-1000 Zagreb, Croatia ReceiVed July 17, 2006. ReVised Manuscript ReceiVed August 31, 2006
Significant progress is being made in the photovoltaic energy conversion using organic semiconducting materials. One of the focuses of attention is the morphology of the donor-acceptor heterojunction at the nanometer scale, to ensure efficient charge generation and loss-free charge transport at the same time. Here, we present a method for the controlled, sequential design of a bilayer polymer cell architecture that consists of a large interface area with connecting paths to the respective electrodes for both materials. We used the surface-directed demixing of a donor conjugated/guest polymer blend during spin coating to produce a nanostructured interface, which was, after removal of the guest with a selective solvent, covered with an acceptor layer. With use of a donor poly(p-phenylenevinylene) derivative and the acceptor C60 fullerene, this resulted in much-improved device performance, with external power efficiencies more than 3 times higher than those reported for that particular material combination so far.
Introduction The emerging field of organic semiconductor materials stimulates new approaches to the production of efficient lowcost photovoltaic devices.1-4 State-of-the-art organic solar cells are commonly fabricated from a combination of an electron-donor and -acceptor material with suitable redox energy levels, sandwiched as a thin (e200 nm) film between metallic electrodes.5,6 Photoexcitation of the organic material leads to an exciton, or a bound electron-hole pair. The exciton is then dissociated into free carriers in a strong electric field or at the donor-acceptor interface.7,8 Subsequently, electrons and holes are transported via drift and diffusion processes to the electrodes, where they are collected, giving rise to an electric current.3,9 The yield of charge generation and charge collection determine the device * To whom correspondence should be addressed. Phone: +41 44 8234084. Fax: +41 44 8234012. E-mail:
[email protected]. † Empa, Swiss Federal Laboratories for Materials Testing and Research. ‡ Universidade de Sa ˜ o Paulo. § UNESP. | Institute of Physics.
(1) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 45334542. (2) Brabec, C. J. Sol. Energy Mater. Sol. Cells 2004, 83, 273-292. (3) Gledhill, S. E.; Scott, B.; Gregg, B. A. J. Mater. Res. 2005, 20, 31673179. (4) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924-1945. (5) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183-185. (6) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. Nat. Mater. 2005, 4, 864-868. (7) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. AdV. Mater. 2005, 17, 66-71. (8) Peumans, P.; Yakimov, A.; Forrest, S. R. J. Appl. Phys. 2003, 93, 3693-3723. (9) Snaith, H. J.; Greenham, N. C.; Friend, R. H. AdV. Mater. 2004, 16, 1640-1645.
efficiency. The small (∼10 nm) exciton diffusion length of organic materials requires that the donor and acceptor materials interpenetrate on a small scale since excitons that are generated far away from the interface will not be able to generate charges before recombination. However, the transport of the separated charges must be ensured as well, and each material must provide a continuous path along which the charges can be readily transported to their respective contacts. The small geometrical interface of the planar double-layer heterojunction solar cell5,10 is not optimized for charge generation; however, the created charges are spatially separated and largely confined to the donor and acceptor side of the interface, and charge recombination losses are reduced. The bulk heterojunction consists of an interpenetrating network of donor and acceptor material.6,11-14 This configuration decreases the distance an exciton must travel to reach an interface, diminishing the loss mechanism of exciton recombination. However, while on transit to the electrodes, charges have the possibility of meeting an opposite charge, resulting in enhanced recombination and reduced current. Carrier mobility in the blend is usually reduced,7,8,10 and the (10) Alam, M. M.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4647-4656. (11) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789-1791. (12) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498500. (13) Watkins, P. K.; Walker, A. B.; Verschoor, G. L. B. Nano Lett. 2005, 5, 1814-1818. (14) Kim. Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197-203.
10.1021/cm061660r CCC: $33.50 © 2006 American Chemical Society Published on Web 10/07/2006
Polymer Demixing for Organic PhotoVoltaic Cells
Chem. Mater., Vol. 18, No. 23, 2006 5505
Figure 1. Schematic diagram describing the fabrication process of an interface-enhanced bilayer film, and the final photovoltaic device. (a) An immiscible semiconducting polymer/guest polymer mixture is spin-coated from a common solvent. (b) Polymer demixing during spin coating results in a vertically segregated bilayer thin film with a rough, nanostructured interface. Subsequently, the guest polymer is removed using a selective solvent. The remaining semiconducting polymer is covered with a layer of a second, active component. This can be done either via spin coating (c), again from a selective solvent, or via thermal evaporation (d). (e) Cross section of the actual device structure. The guest polymer was polystyrene, which was selectively removed with the solvent cyclohexane. Interfaces are represented as experimental cross sections from atomic force microscope images.
cell series resistance is high due to the occurrence of charge traps, space charge buildup, and charge recombination at high illumination intensites.8,9,15 The controlled self-organization at the nanometer scale during and after blend film formation depends strongly on a number of fabrication parameters that need to be optimized for each and every new material combination.3,4,16 In the best bulk heterojunction devices, a soluble C60 derivative (PCBM) has been used with conjugated polymers. For a poly(p-phenylenevinylene) derivative, power-conversion efficiencies of 2.5% have been reported, 17 and with use of poly(3-hexylthiophene) as the electron donor, solar cells approaching the 5% efficiency benchmark have been demonstrated.6,14,18 Recent efforts in manipulating and controlling the structure of organic thin films tried to combine the high efficiency of charge generation of the bulk heterojunction with the low resistance to charge transport of the planar heterojunction in a single-device configuration. For polymer-based systems, laminated double-layer arrangements, heat-induced interdiffusion, or selective dissolution has been used to create bilayers with highly folded, or intercalated, heterojunctions.19-21 With use of small organic molecules, device concepts consisting of a mixed layer of donor and acceptor molecules sandwiched between homogeneous layers, such as the (15) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37-41. (16) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45-61. (17) Shaheen, S. E.; Brabec, C. J.; Sariciftci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841-843. (18) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617-1622. (19) Granstro¨m, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.; Friend, R. H. Nature 1998, 395, 257-260. (20) Drees, M.; Davis, R. M.; Heflin, J. R. Phys. ReV. B 2004, 69, 165320. (21) Fujii, A.; Mizukami, H.; Umeda, T.; Shirakawa, T.; Hashimoto, Y.; Yoshino, K. Jpn. J. Appl. Phys. 2004, 43, 8312-8315.
controlled growth of a molecular bulk heterojunction using organic vapor-phase deposition, have proven exceptionally successful.7,15 Here, we apply the phenomenon of polymer demixing during spin coating to produce nanostructured, semiconducting polymer films and demonstrate their use to produce efficient heterojunction organic solar cells that consist of a large interface area between the donor and acceptor with connecting paths to the respective electrodes for both materials at the same time. A scheme of the fabrication process is shown in Figure 1. Blends of semiconducting polymer and polystyrene (PS) were used to spin coat active thin structured films. Unlike previously22 where PS was used as an additive, its role in this work is a sacrificial one since after spin coating PS was removed to take advantage of the structured semiconducting film. To verify the concept, we used the archetypal material combination20,21,23-27 MEHPPV (poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene]) and C60 to produce solar cells in both the planar and structured, interface-enhanced configuration. Structured devices showed white light efficiencies up to 0.64%, and the monochromatic power conversion efficiency reached 2.97% at 480 nm. These values are ∼3 times higher than (22) Brabec, C. J.; Padinger, F.; Hummelen, J. C.; Janssen, R. A. J.; Sariciftci, N. S. Synth. Met. 1999, 102, 861-864. (23) Krebs, F. C.; Carle´, J. E.; Cruys-Bagger, N.; Andersen, M.; Lilliedal, M. R.; Hammond, M. A.; Hvidt, S. Sol. Energy Mater. Sol. Cells 2005, 86, 499-516. (24) Davenas, J.; Alcouffe, P.; Ltaief, A.; Bouazizi, A. Macromol. Symp. 2006, 233, 203-209. (25) Sariciftci, N. S.; Braun, D.; Zhang, C.; Srdanov, V. I.; Heeger, A. J.; Stucky, G.; Wudl, F. Appl. Phys. Lett. 1993, 62, 585-587. (26) Gao, J.; Hide, F.; Wang, H. Synth. Met. 1997, 84, 979-980. (27) Hayashi, Y.; Yamada, I.; Takagi, S.; Takasu, A.; Soga, T.; Jimbo, T. Jpn. J. Appl. Phys. 2005, 44, 1296-1300.
5506 Chem. Mater., Vol. 18, No. 23, 2006
results for planar devices and the highest efficiencies reported for that material combination so far, including bulk heterojunctions. Experimental Section Photovoltaic devices were fabricated in a sandwich structure between indium tin oxide (ITO, anode) and aluminum (cathode). ITO-coated glass substrates (Merck, sheet resistance 30 Ω square-1) were cleaned in a laminar flow hood by successive ultrasonic treatment in acetone, ethanol, detergent (Hellma), and Milli-Q water. A 80 nm thick layer of poly(ethylene dioxythiophene) doped with polystyrene sulfonic acid (PEDOT:PSS, Bayer) was spin-coated on top of ITO. After being heated on a hot plate for 1 h at 120 °C under vacuum, the substrates were transferred to a nitrogen-filled glovebox (
