The effect of alkylthio side chains in oligothiophene-based donor materials for organic solar cells

Abstract

With respect to the molecular system of donor materials for organic solar cells (OSCs), the π-conjugated backbone is the most important component in determining their OSC-related physicochemical properties. On the other hand, selecting side chains is essential in designing photovoltaic materials, since side chains (especially alkylthio side chains) have a considerable effect, not only on enabling the solubility of materials for solution-processed device fabrication, but also on physicochemical properties, molecular packing and thus the OSC device performance. In this work, we introduce alkylthio side-chains in the molecular π-conjugated backbone to develop a new oligothiophene-based donor material (namely BDT-S-TRS) for OSCs. The theoretical calculations and molecular simulation with density functional theory (DFT) and cyclic voltammetry results reveal that BDTT-S-TRS showed a lower HOMO energy level than its alkyl side-chain counterpart. Especially, BDTTT-S-TRS has a linear backbone with better planarity. The OSC device based on BDT-S-TRS exhibited a power conversion efficiency (PCE) of 8.07%, with a notable fill factor (FF) of 0.705.