Understanding the Photovoltaic Behavior of A-D-A Molecular Semiconductors through a Permutation of End Groups

The facile synthesis of a series of benzodithiophene (BDT)- and indacenodithiophene (IDT)-based A-D-A oligomers with different end groups is reported, and their properties are studied by optical spectroscopy, electrochemistry, and density functional theory calculations. The permutation of central and terminal units tunes the optoelectronic properties and photovoltaic device characteristics in a predictable way, aiding in the rational design of small molecule semiconducting materials. Among the three rhodanine-derived terminal groups, N-alkylthiazolonethione revealed the strongest electron-withdrawing character, resulting in the lowest band gap, the highest stability, and the best photovoltaic device performance. The crystallographic analysis of two IDT derivatives yielded a highly unusual three-dimensional packing of the conjugated backbone, which is likely responsible for the remarkable photovoltaic performance of such A-D-A semiconductors.