Synthesis and photovoltaic properties of silafluorene copolymers substituted by carbazole and triphenylamine pendants

In this paper carbazole and triphenylamine substituted 2,7-silafluorene-based copolymers with 5,6-bis(octyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DBT), namely Cz-PSF-DBT and TPA-PSF-DBT, respectively, were synthesized and their photovoltaic device performance was investigated with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor. These two polymers show nearly identical absorption spectra in the long wavelength range, which can be ascribed to the identical backbones consisting of silafluorene and DBT moieties. Cz-PSF-DBT shows a higher open circuit voltage of 1.0 V due to its higher oxidation onset coming from the σ conjugation between the silica atoms at 9-position and carbazole substituents compared to TPA-PSF-DBT of 0.93 V. However, the power conversion efficiency of TPA-PSF-DBT is much higher than that of Cz-PSF-DBT. The AFM images show the finer morphology of TPA-PSF-DBT:PC71BM film than that of Cz-PSF-DBT:PC71BM film with a rough surface and a large phase separation, which greatly influence the exciton separation at the donor/acceptor interface. The PL spectra indicate that the smoother morphology and smaller domain sizes can inhibit the radiative loss, which is in accordance with the corresponding device performance. It is demonstrated that different substituents in silafluorene-based polymers play a key role in determining the photovoltaic performance.