Supramolecular Association of Pyrrolidinofullerenes Bearing Chelating Pyridyl Groups and Zinc Phthalocyanine for Organic Solar Cells Pavel A. Troshin,* ,² Robert Koeppe,* ,‡ Alexander S. Peregudov, § Svetlana M. Peregudova, § Martin Egginger, ‡ Rimma N. Lyubovskaya, ² and N. Serdar Sariciftci ‡ Institute of Problems of Chemical Physics of RAS, ChernogoloVka, Moscow Region, 142432, Russia, Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler UniVersity Linz, Altenbergerstrasse 69, A-4040 Linz, Austria, and A. N. NesmeyanoV Institute of Organoelement Compounds, VaVyloVa St. 28, B-334, Moscow, 119991, Russia ReceiVed May 8, 2007. ReVised Manuscript ReceiVed August 7, 2007 We investigated donor-acceptor bilayer heterojunctions formed by deposition of solution-processed pyrrolidinofullerenes bearing chelating pyridyl groups (PyFs) on vacuum-evaporated films of zinc phthalocyanine (ZnPc). It is shown that coordination complexes are formed at the interface between these donor and acceptor components; such association facilitates photoinduced charge separation and results in improved performance of the photovoltaic devices. Thus, the bilayer photovoltaic cells fabricated from different pyrrolidinofullerenes and ZnPc exhibit short circuit current (I sc ) densities in the range of 3-5 mA/cm 2 , open circuit voltages (V oc ) of 400-600 mV, and fill factors (FF) of 40-50% that correspond to power conversion efficiencies (η) of up to 1.5% under 100 mW/cm 2 simulated AM1.5 illumination. The reference cells based on the nonchelating fullerene derivative [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) as acceptor component yield lower power conversion efficiencies (0.4-0.6%); the performance of such devices can be increased significantly by mixing PCBM with a small amount (4% w/w) of PyF in the acceptor layer. A novel multicomponent organic solar cell architecture is suggested in order to expand the active layer absorption by formal combination of the solution-processed bulk heterojunction polymer/fullerene cells with evaporated bilayer ZnPc/fullerene devices. For this purpose, a blend of the fullerene derivatives (PCBM and PyF mixed in different ratios) with the polyconjugated polymer poly((2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene) vinylene (MDMO-PPV) is spin-coated on the ZnPc film sublimed on an indium tin oxide (ITO) substrate. Evaporation of the top aluminum electrodes yields photovoltaic devices that demonstrate power conversion efficiencies of up to 2% and efficient photocurrent generation in the full range from 350 to 820 nm. 1. Introduction Many research efforts are focused on the development of low-cost flexible organic solar cells using fullerenes or their derivatives as acceptor components. 1-3 The two main concepts in the design of organic solar cells consist of blending donor and acceptor components together in solution and casting into thin films to yield “bulk hetero- junction” type devices, 4,5 whereas the other route is based on thermal evaporation/sublimation of the small molecular semiconductors as thin films on a substrate in high vacuum. 6,7 The basic working principle of organic solar cells is the dissociation of photogenerated excitations at the interface between electron donor and acceptor phases by a photoin- duced charge-transfer process with subsequent transport of the charge carriers in the respective phases to the electrodes. 8 Critical parameters for the photocurrent generation are therefore the active layer absorption, the efficiency of the charge transfer, and the transport of charge carriers in the materials involved. A severe limitation for the efficiency of the bulk hetero- junction devices is a relatively narrow absorption spectrum. Thus, the best solar cells made from the polyconjugated polymer poly(2-methoxy-5-{3′,7′-dimethyloctyloxy}-p-phe- nylene vinylene) (MDMO-PPV) and 1-(3-methoxycarbo- nyl)propyl-1-phenyl[6,6]C 61 (PCBM) demonstrate power conversion efficiencies of 2.5% under AM1.5 conditions. 9 Even better suited for building solar cells is a combination of PCBM with regioregular poly(3-hexylthiophene) (P3HT), which possesses better hole transport properties and a ² Institute of Problems of Chemical Physics of RAS. ‡ Johannes Kepler University Linz. § A. N. Nesmeyanov Institute of Organoelement Compounds. (1) Shaheen, S. E.; Ginley, D. S.; Jabbour, G. E. MRS Bull. 2005, 30, 10. (2) Hoppe, H.; Sariciftci, N. S. J. Mater. Res. 2004, 19, 1924. (3) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45. (4) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. 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