Most photovoltaic technologies exhibit high photon-to-charge-carrier conversion yields, approaching 90%-100%. However, a solar cell only generates power when an additional photo-voltage is produced. This photo-voltage is ideally as close as possible to the open-circuit voltage (Voc) of the device.

Most photovoltaic technologies exhibit high photon-to-charge-carrier conversion yields, approaching 90%-100%. However, a solar cell only generates power when an additional photo-voltage is produced. This photo-voltage is ideally as close as possible to the open-circuit voltage (Voc) of the device. When a solar cell is held at open-circuit, no charges flow in the external circuit, hence they all recombine within the photovoltaic material. Suppressing the recombination of charge carriers will result in increased Voc and power conversion efficiencies. Recombination can result in the emission of a photon or can occur non-radiatively, with the former being unavoidable and thus limiting Voc. In this lecture, we will consider general relations between photon absorption, emission and photovoltaic action, valid for established and emerging photovoltaic technologies. We will derive upper limits for Voc and summarize strategies to suppress recombination and reach those limits. Organic photovoltaics, where charge-transfer states between electron donating and accepting materials play a crucial role, will be discussed in more detail.