Thienyl-substituted methanofullerene derivatives for organic photovoltaic cells

A series of thienyl-substituted methanofullerenes as electron acceptors for bulk-heterojuction solar cells with poly(3-hexylthiophene) (P3HT) were synthesized and characterized with respect to electrochemical and photophysical properties. The first one-electron reduction potentials of the higher adducts are shifted toward more negative values by ∼100 mV as compared to the monoadduct. As a result, the solar cells composed of the bisadduct (2) and trisadduct exhibit a larger open-circuit voltage (V_oc) than the solar cell composed of P3HT and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), as they have higher LUMO energy levels. Photophysical studies on spin-coated films by femtosecond laser flash photolysis indicate that ultrafast electron transfer from the P3HT donor polymer to all of the fullerene derivatives occurs to form the bound radical pair (BRP) state. No decay of the BRP state of P3HT:thienyl-substituted monoadduct (1) films was observed in the time range of 3 ns. The AFM investigation on P3HT:1 and P3HT:higher adduct films after thermal annealing showed fine donor and acceptor domains and larger domains, respectively. The bulk-heterojunction (BHJ) solar cells based on P3HT:1 exhibited a power conversion efficiency (PCE) of 3.97%, which is comparable with that of the P3HT:PCBM cell. The P3HT:2 based cell showed a PCE value of 1.72% with a higher open-circuit voltage of 0.72 V.