Hole Transport in Poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] and High-Efficiency Polymer Solar Cells from Its Blends with PCBM

We report herein a detailed study of the thermal and hole-transport properties of poly[2,7-(9,9-dihexylfluorene)-alt-bithiophene] (F6T2) and its photovoltaic performance in a bulk-heterojunction (BHJ) solar cell. This crystalline polymer has a high weight-average molecular weight (M(w) = 52 400) with a polydispersity index of 1.99. With a band gap of 2.36 eV, F6T2 exhibits strong absorption in the 300-500 nm region. BHJ solar cells blending F6T2 with [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) (1:3 weight ratio) as the active layer present a high open-circuit voltage (V(oc) approximately 0.9 V) and a promising power conversion efficiency of 2.4% under simulated solar light AM1.5G (100 mW/cm(2)). Furthermore, F6T2 shows sufficient hole mobility [ca. 8.4 x 10(-5) cm(2)/(V s) at 310 K and 2.5 x 10(5) V/cm applied electric field] by a time-of-flight transient photocurrent technique, allowing efficient charge extraction and a good fill factor for solar cell application. Nanoscale phase separation was observed in F6T2/PCBM films with a surface roughness lower than 60 nm.