Side chain triphenylamine-based conjugated polymers for the preparation of efficient heterojunction solar cells

A series of novel benzothiadiazole based triphenylamine polymers containing different electron-rich side chains (dithienosilole, dithienopyrrole, dithieno-cyclopentadiene derivatives) have been prepared and their photoelectric properties explored. These polymers have been characterized using UV–Vis absorption and fluorescence spectroscopy and their photo-electrochemical properties characterized using cyclic voltammetry. These new polymers exhibit high thermal decomposition temperatures and broad absorption spectra that enable them to absorb light across the entire visible region. The UV absorption response range electron of these polymers could be tuned by modifying their electron donating units, with the best polymer found to exhibit a long fluorescent lifetime. An P3:PC71BM device containing dithienocyclopentadiene electron donating side-chains was shown to give 3.6% photoelectric conversion efficiency for a high short-circuit current of 14.25 mA/cm2. The fluorescence intensity of this copolymer decayed rapidly, implying that effective charge transfer processes were occurring at its interface, which were associated with a relatively high short circuit current. This work clearly demonstrates that use of an appropriate electron donor unit can effectively broaden the polymers spectroscopic response range, thus increasing the rate of carrier transport at the interface, which leads to improved photoelectric conversion efficiency.