Two Thieno[3,2-b]thiophene-Based Small Molecules as Bifunctional Photoactive Materials for Organic Solar Cells

Two bifunctional photovoltaic materials, named DRCN3TT and DRCN5TT with acceptor-donor-acceptor (A-D-A) type chemical structures based on thieno[3,2-b]thiophene central unit and 2-(1,1-dicyanomethylene)rhodanine terminal group, are designed and synthesized. Their thermal, optical and electrical properties are systematically investigated. The appropriate energy levels, both the electron and the hole transport properties and the appropriate molecular conformation make them work bifunctionally as both electron donor and acceptor for photovoltaic application. As donors, DRCN5TT and DRCN3TT based devices with [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) as acceptors show power conversion efficiencies (PCEs) of 7.03 and 3.85%, respectively. As acceptors, blended with the most profoundly studied donor materials such as poly(3-hexylthiophene) (P3HT) or its analogue PDCBT (poly[(4,4'-bis(2-butyloctoxycarbonyl-[2,2'-bithiophene]-5,5-diyl)-alt-(2,2'-bithiophene-5,5'-diyl)]), DRCN3TT and DRCN5TT show PCEs of 3-4%. The rarely studied bifunctional photovoltaic materials might provide another strategy to design new materials for high performance photovoltaic applications.