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

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 (V-oc). 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 processed from solution with the use of BHJ materials comprising semiconducting polymers and fullerene derivatives. We first report here on the design of PTIPSBDT-DFDTQX equivalent poly(3-hexylthiophene), the current medium band gap polymer of choice, which thus is a viable candidate for use in the highly efficient bottom layer in inverted tandem cells.