Negligible Energy Loss During Charge Generation in Small-Molecule/Fullerene Bulk-Heterojunction Solar Cells leads to Open-Circuit Voltage over 1.10 V

Solution-processable small molecules (SM) that can serve as donors in bulk-heterojunction (BHJ) solar cells are practical alternatives to their polymer counterparts. However, SM-fullerene blends commonly suffer severe voltage losses. In general, devices that reach open-circuit voltages (VOC) >1 V – yield low photocurrents in BHJ solar cells with fullerene acceptors (e.g. PC71BM) and modest power conversion efficiencies (PCEs). In this contribution, we report on the design, synthesis and BHJ device characteristics of a new SM donor – 2F-DRCN5T – yielding a VOC of up to 1.10 V with PC71BM as the fullerene acceptor, while main-taining PCEs >7% (over 8% achieved upon solvent-vapor annealing (SVA) treatment). The negligible energy loss during charge generation (ΔECT), the deep-lying HOMO of 2F-DRCN5T inferred from its large ionization potential (IP), the high charge transfer state energy (ECT) of the blend and a reduced non-radiative voltage loss account for the high VOC achieved in BHJ solar cells.