An Efficient Tin‐Free Route to Small Molecules Based on Silole‐Modified Pentathiophenes for Solution‐Processed Organic Solar Cells

Abstract

A acceptor–donor–acceptor (A–D–A) molecule based on a silole‐modified pentathiophene (SiPTA) electron‐donor unit and two diketopyrrolopyrrole (DPP) acceptor units, DPP2(SiPTA), was designed and synthesized. An unsymmetrical molecule with D–A structure, DPP‐SiPTA, was also synthesized for comparison. Both molecules were synthesized using an environmentally friendly Suzuki coupling protocol, which was modified to avoid ring‐opening of the silole under basic conditions. The D–A design has an obvious effect on the electronic structure of these compounds. The D–A–D‐structured DPP2(SiPTA) has a much lower band gap and HOMO than D–A‐structured DPP‐SiPTA. Furthermore, the molecular symmetry significantly influenced crystallinity. Organic solar cells (OSCs) based on DPP2(SiPTA)/PC71BM (1:5, w/w; PC71BM=[6,6]‐phenyl C71‐butyric acid methyl ester) have a power conversion efficiency of 2.36 % with an open‐circuit voltage of up to 0.84 V, which, as expected, is much higher than that of the recently reported OSCs based on DPP2(NPTA) (0.71 V), in which pyrrole‐modified pentathiophene (NPTA) is the donor unit.