Hole transport in poly[2,7-(9,9-dihexylfluorene)- alt -bithiophene] and high-efficiency polymer solar cells from its blends with PCBM

Abstract

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 (Mw = 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-C61-butyric acid methyl ester (PCBM) (1:3 weight ratio) as the active layer present a high open-circuit voltage (V oc ∼0.9 V) and a promising power conversion efficiency of 2.4% under simulated solar light AM1.5G (100 mW/cm2). Furthermore, F6T2 shows sufficient hole mobility [ca. 8.4 - 10?5 cm2/(V s) at 310 K and 2.5 - 105 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. © 2009 American Chemical Society.