Dithienogermole-containing small-molecule solar cells with 7.3% efficiency: In-depth study on the effects of heteroatom substitution of Si with Ge

Two small molecule donor materials (DTGe(FBTTh₂)₂ and DTGe(FBTBFu)₂) incorporating the dithienogermole (DTGe) moiety with fluorobenzothiadiazole (FBT) and bithiophene (Th₂) or benzofuran (BFu) end-capping groups are synthesized and their properties as donor materials in small molecule bulk heterojunction type (BHJ) solar cells are 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 voltage (V_OC) at the expense of short circuit current (J_SC) and fill factor (FF). In this study, the DTGe-based molecules are systematically compared to the dithienosilole (DTSi)-based analogues, which are currently among the highest power conversion efficiency (PCE) small molecule solar cell donor materials known. The J_SC produced by the DTGe molecule is found to be similar to, or slightly higher than the Si analogue, despite similar absorption characteristics, however, the PCE is similar to the Si analogues due to small decreases in V_OC and FF. This report marks the first small molecule BHJ based on a Ge-containing heterocycle with PCE over 7%. A series of solution-processed small-molecules based on dithienogermole (DTGe) units is synthesized and fully optimized with a high power conversion efficiency (PCE) of 7.3%. This is a record for germanium (Ge)-based small molecules in devices and the PCE value reported is, for the first time, certified for this class of small-molecule bulk heterojunction solar cells.