Organic Solar Cells: Concepts towards The Single Junction Limit and Beyond

Thanks to the development of novel electron acceptor materials, power conversion efficiencies (PCE) of organic photovoltaic (OPV) devices are now approaching 20%. Further improvement of PCE is complicated by the need for a driving force to split strongly bound excitons into free charges, causing voltage losses. This presentation discusses recent approaches to find efficient OPV systems with minimal driving force, combining near unity quantum efficiency (maximum short circuit currents) with optimal energy efficiency (maximum open circuit voltages). We discuss apparently contradicting results on the amount of exciton binding in recent literature, and approaches to harmonize the findings. We then present a comprehensive view on motifs providing a driving force for charge separation, namely hybridization<u> </u><u>at the </u><u>donor:acceptor</u><u> interface and</u>, polarization<u> </u><u>effects in the bulk</u>. Apart from controlling the energies of the involved states, these motifs also control the dynamics of recombination processes, essential to avoid voltage and fill factor losses. Importantly, all motifs are shown to depend on both molecular structure and process conditions. The resulting high dimensional search space advocates for high throughput (HT) workflows. The final part of the presentation presents recent concepts for architectures allowing to bypass the single junction limit for organic photovoltaics.