ИСТИНА

Donor-acceptor molecules having thiophene fragments as the π-bridge represent a promising class of materials for organic photovoltaics. However, the impact of the oligothiophene π-bridge length on physicochemical properties, photophysics, charge transport and photovoltaic performance of these materials has not been thoroughly addressed. Here we report on the comprehensive investigation of the series of star-shaped donor-acceptor molecules (0T – 4T) having triphenylamine as a donor core linked through an oligothiophene π-bridge of variable length to the terminal hexyl-dicyanovinyl electron-withdrawing groups. We found that variation of the π-bridge length from 0 to 4 thiophene units strongly impacts their properties such as the solubility, highest occupied molecular orbital energy, optical absorption and luminescence spectra, band gap, film morphology, phase behavior and molecular packing as well as the charge-carrier mobility. As a result, the molecules showed different performance in single-component (SC OSCs) and bulk-heterojunction (BHJ OSCs) organic solar cells. The power conversion efficiency of the SC OSCs jumped from 0.06% for 1T to the remarkable 1.09% for 4T with π-bridge elongation, which is in a good agreement with their charge transport, generation and recombination properties, as supported by the ultrafast photoluminescence spectroscopy and Onsager model calculations. The photovoltaic performance in BHJ OSC with PC71BM as acceptor increased significantly with elongation of the π-bridge length from 0T to 2T; however, further elongation of the oligothiophene length did not promote performance in BHJ OSCs. The results obtained provide an insight on how to fine-tune and predict properties and photovoltaic performance of small molecules for organic solar cells.