An Optical Route to Increase The Open-Circuit Voltage in Organic Solar Cells

The open-circuit voltage (V_oc) of the solar cells based on organic materials remains lower than their inorganic counterparts with comparable bandgaps. Radiative and non-radiative recombination as well as energy losses during charge separation at the donor-acceptor (D:A) interface are the main causes of the large voltage losses in organic solar cells. Various approaches have been explored to improve the V_oc such as adding a ternary component into the binary D:A photoactive blend, tuning the morphology to minimize non-radiative recombination losses, and developing new photoactive blends with reduced energy offset. Optical-based approaches have been successfully used in the past to boost the performance of solar cells but, in most of the cases, the focus has been on enhancing the short-circuit current and only in very few occasions on modifying the V_oc. In here, we report the use an optical strategy to reduce the large mismatch between the absorption and emission cones in cells with a sufficiently high radiative recombination. In such way, Boltzmann losses can be reduced allowing for clearly measurable power conversion efficiency (PCE) and voltage increases. We will present and discuss the implementation of an organic solar cell architecture where radiative recombination can be clearly reduced. This reduction leads to a V_oc increase of values around 30 mV, achieved solely through optical means. Our work establishes a new pathway for planar geometry single-junction solar cells, surpassing the PCE values set by the Shockley-Queisser limit.