The Cost of Converting Excitons into Free Charge Carriers in Organic Solar Cells.

Efficient exciton dissociation and subsequent generation of free charge carriers at the organic donor-acceptor interface requires a number of electron transfer processes. It is a common view that these steps result in an unavoidable energy loss in organic photovoltaic devices, not present in other types of solar cells. The currently best performing organic solar cells with power conversion efficiencies over 16% challenge this view and no interfacial charge-transfer states with significantly lower energy than the strongly absorbing singlet states are detected within the gap of the used donor and acceptor materials. This perspective will discuss implications, the remaining sources of energy loss and open questions to be solved to achieve power conversion efficiencies over 20%.