High Open Circuit Voltage Solution-Processed Tandem Organic Photovoltaic Cells Employing a Bottom Cell Using a New Medium Band Gap Semiconducting Polymer

Abstract

Two donor–acceptor (D–A) copolymers, based on the donor unit TIPS substituted benzodithiophene (TIPSBDT) and the acceptor quinoxaline-based units with or without fluorine substitution (PTIPSBDT-DTQX and PTIPSBDT-DFDTQX), were designed and synthesized as a donor material for bulk-heterojunction (BHJ) photovoltaic cells. The introduction of F atoms with high electron affinity to be quinoxailine moieties is effective in further lowering both the HOMO and LUMO energy levels of PTIPSBDT-DFDTQX to attain higher open-circuit voltage (Voc). Single junction photovoltaic cells were fabricated, and the polymers:PC71BM active layer morphology was optimized by adding 1,8-diiodooctane (DIO) as an additive. In a single layer photovoltaic device, they showed power conversion efficiencies (PCEs) of 2–6%. The solution process inverted tandem photovoltaic cells, in which two photovoltaic cells with different absorption characteristics are linked to use a wider range of the solar spectrum, were fabricated with each layer pro...