Synthesis, hole mobility, and photovoltaic properties of cross-linked polythiophenes with vinylene-terthiophene-vinylene as conjugated bridge

A class of cross-linked polythiophenes with different ratio (2%, 4%, 8%) of vinylene-terthiophene-vinylene (VTThV) conjugated bridges (PT - VTThV2, PT-VTThV4, and PT-VTThV8) were synthesized. The cross-linking influenced the absorption spectra of the polymer solutions very little, but it resulted in blue-shift of the absorption spectra of the polymer films by ca. 28-39 nm. Cyclic voltammograms display that the p-doping/dedoping and n-doping/dedoping processes of all the cross-linked polythiophenes are reversible and the electrochemical bandgaps increased a little with the increase of the content of the conjugated bridges. The hole mobility values determined from the space-charge-limited current (SCLC) model reached 4.70 × 10^-3 for PT-VTThV2, 2.58 × 10^-3 for PT-VTThV4, and 9.48 × 10^-4 cm²/(V s) for PT-VTThV8, respectively. The hole mobility of PT- VTThV2 with 2% VTThV conjugated bridges is about three orders higher than that of the corresponding polymer P1 without the conjugated bridges. The power conversion efficiency of the polymer solar cell based on the blend of PT-VTThV2 and PCBM (1:1, w/w) reached 1.72% under the illumination of AM 1.5, 100 mW/cm², which is two times of that of the device based on P1. The results indicate that the cross-linking with the VTThV conjugated bridges obviously improved charge transportation the photovoltaic properties of the conjugated polymers.