Dithienogermole-Cored Small Molecules as Donor Materials in Bulk Heterojunction Solar Cells

Abstract

Two small molecule donor materials (DTGe(FBTTh2)2 and DTGe(FBTBFu)2) incorporating the dithienogermole (DTGe) moiety with each two end-capping groups(FBTTh2 or BFu) have been synthesized. Their properties as donor materials in bulk heterojunction type (BHJ) solar cells were investigated. The DTGe(FBTTh2)2 with Th2 end groups shows outstanding solar cell characteristics with efficiencies up to 6.4% using a standard BHJ architecture and 7.3% using a ZnO optical spacer, while the BFu end-capped DTGe(FBTBFu)2 has slightly wider band gaps and yields slightly higher open circuit voltages (VOC) at the expense of short circuit currents (JSC) and fill factors (FF). In this study, the DTGe-based molecules were compared to the dithienosilole (DTSi)-based analogues, which are currently among the highest power conversion efficiency (PCE) small molecule solar cell donor materials known. This report marks the first small molecule BHJ based on a Ge-containing heterocycle with PCE over 7%.