Diphenylmethanofullerenes: New and Efficient Acceptors in Bulk-Heterojunction Solar Cells** By Ingo Riedel,* Elizabeth von Hauff , Jürgen Parisi, Nazario Martín, Francesco Giacalone, and Vladimir Dyakonov 1. Introduction Conjugated polymers exhibit a high potential for production of efficient and, at the same time, low-cost, flexible optoelec- tronic devices with the option for large-area applications. [1] One approach for achieving an efficient charge-carrier genera- tion in polymer-based light absorbers is to blend them with well-suited acceptors. Upon photoexcitation, an ultrafast elec- tron transfer between the donor and the proximate acceptor takes place. This ensures an efficient charge generation with a quantum efficiency close to unity. [2,3] One of the most exten- sively studied device concepts so far is based on the bulk-het- erojunction approach. [4,5] Here, acceptor-type fullerene mole- cules are dispersed in a polymer matrix, giving a photoactive film with a large geometrical interface between donor and ac- ceptor. The thin absorber film is sandwiched between two elec- trodes with asymmetric workfunctions, with each electrode forming an ohmic contact with the respective p- or n-type semi- conductor. For solar cells based on the polymer±fullerene bulk-heterojunction concept, power conversion efficiencies of up to 5 % have been reported. [6] Among the most frequently investigated material combinations in polymer photovoltaics are the p-type conjugated polymers, such as poly[2-methoxy-5- (3¢,7¢-dimethyloctyloxy)-p-phenylene vinylene] (OC 1 C 10 -PPV) and poly(alkyl thiophene)s, for example, poly[3-hexyl thio- phene-2,5-diyl] (P3HT), each blended with the electron accep- Adv. Funct. Mater. 2005, 15, 1979±1987 DOI: 10.1002/adfm.200500097  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1979 ± [*] Dr. I. Riedel,Prof. V. Dyakonov Bavarian Centre for Applied Energy Research (ZAE-Bayern) Division Thermal Insulation and Heat Transfer Am Hubland, D-97074 Würzburg (Germany) E-mail: ingo.riedel@physik.uni-wuerzburg.de Dr. I. Riedel, E. von Hauff, Prof. J. Parisi Institute of Physics Energy and Semiconductor Research Laboratory University of Oldenburg D-26111 Oldenburg (Germany) Prof. N. Martín, Dr. F. Giacalone Department of Organic Chemistry Universidad Complutense de Madrid (UCM) ES-28040 Madrid (Spain) Prof. V. Dyakonov Experimental Physics VI (Energy Research) Department of Physics and Astronomy University of Würzburg D-97074 Würzburg (Germany) [**] The authors gratefully thank D. Vanderzande and L. Lutsen (LUC Bel- gium) for providing OC 1 C 10 -PPV, J. C. Hummelen for donation of PCBM, and C. Waldauf, P. Schilinsky, and C. J. Brabec (Konarka, Aus- tria) for discussions and simulations of dark J±V characteristics. Many thanks go to M. Pientka, M. Knipper, D. Chirvase, and H. Koch (University of Oldenburg) for their contribution in various dis- cussions and technical assistance. This work was supported by the European Commission, EC (project HPRN-CT-2000-00127), by the Bundesministerium für Bildung und Forschung, BMBF (BMBF projects 01SF0019 and 01SF0026), and by the Spanish Ministry of Sciences and Technology (Ministerio Ciencia y Tecnología, McyT) of Spain project (BQU2002-00855). G. Wittstock (Univ. Oldenburg, Ger- many) is acknowledged for providing the AFM setup. A novel fullerene derivative, 1,1-bis(4,4¢-dodecyloxyphenyl)-(5,6) C 61 , diphenylmethanofullerene (DPM-12), has been investi- gated as a possible electron acceptor in photovoltaic devices, in combination with two different conjugated polymers poly[2- methoxy-5-(3¢,7¢-dimethyloctyloxy)-para-phenylene vinylene] (OC 1 C 10 -PPV) and poly[3-hexyl thiophene-2,5-diyl] (P3HT). High open-circuit voltages, V OC =0.92 and 0.65 V, have been measured for OC 1 C 10 -PPV:DPM-12- and P3HT:DPM-12-based devices, respectively. In both cases, V OC is 100 mV above the values measured on devices using another routinely used fullerene acceptor, [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM). This is somewhat unexpected when taking into account the identi- cal redox potentials of both acceptor materials at room temperature. The temperature-dependent V OC reveals, however, the same effective bandgap (HOMO Polymer ±LUMO Fullerene ; HOMO = highest occupied molecular orbital, LUMO = lowest unoccu- pied molecular orbital) of 1.15 and 0.9 eV for OC 1 C 10 -PPV and P3HT, respectively, independent of the acceptor used. The higher V OC at room temperature is explained by different ideality factors in the dark-diode characteristics. Under white-light illumination (80 mW cm ±2 ), photocurrent densities of 1.3 and 4.7 mA cm ±2 have been obtained in the OC 1 C 10 -PPV:DPM-12- and P3HT:DPM-12-based devices, respectively. Temperature-dependent current density versus voltage characteristics reveal a thermally activated (shallow trap recombination limited) photocurrent in the case of OC 1 C 10 -PPV:DPM-12, and a nearly temperature-independent current density in P3HT:DPM-12. The latter clearly indicates that charge carriers traverse the active layer without significant recombination, which is due to the higher hole-mobility±lifetime product in P3HT. At the same time, the field-effect electron mobility in pure DPM-12 has been found to be l e =210 ±4 cm 2 V ±1 s ±1 , that is, forty-times lower than the one measured in PCBM (l e =810 ±3 cm 2 V ±1 s ±1 ). FULL PAPER