Effects of oligothiophene π-bridge length on physical and photovoltaic properties of star-shaped molecules for bulk heterojunction solar cells

The preparation of four different star-shaped donor (D)-π-acceptor (A) small molecules (N(Ph-1T-DCN-Me)₃, N(Ph-2T-DCN-Me)₃, N(Ph-2T-DCN-Hex)₃and N(Ph-3T-DCN-Hex)₃) possessing various oligothiophene π-bridge lengths and their use in solution-processed bulk heterojunction small molecule solar cells is reported. Optical and electrochemical data show that increasing oligothiophene π-bridge length leads to a decrease of the optical band gap due to a parallel increase of the highest occupied molecular orbital (HOMO) level. Furthermore, subtle modifications of a molecular π-bridge length strongly affect the thermal behavior, solubility, crystallization, film morphology and charge carrier mobility, which in turn significantly change the device performance. Although the moderately increasing oligothiophene π-bridge length uplifts the HOMO level, it nevertheless induces an increase of the efficiency of the resulting solar cells due to a simultaneous improvement of the short circuit current (J_sc) and fill factor (FF). The study demonstrates that such an approach can represent an interesting tool for the effective modulation of the photovoltaic properties of the organic solar cells (OSCs) at a moderate cost.