Insights from Transient Optoelectronic Analyses on the Open-Circuit Voltage of Organic Solar Cells

In this Perspective, we review recent progress on the use of transient optoelectronic techniques to quantify the processes determining the open-circuit voltage (VOC) of organic solar cells. Most theoretical treatments of VOC include the effects of both material energetics and recombination dynamics, yet most experimental approaches are based on materials energetics alone. We show that by direct measurement of charge carrier dynamics and densities, the rate of nongeminate charge recombination may be determined within working cells and its impact on achievable VOC determined. A simple fit-free device model utilizing these measurements is shown to agree (to within ±5 mV) with experimentally observed open-circuit voltages for devices comprised of a range of different photoactive layer materials and different processing conditions, and utilizing both bulk and bilayer heterojunctions. This agreement is significantly better than that obtainable from analyzing materials energetics alone, even when employing an in situ analysis of effective electronic band gap. We go on to argue that the precision of our VOC calculations derives from implicitly including the impact of film microstructure on open-circuit voltage. We show that this can modulate VOC by up to 200 mV, and thereby account for the limits of energy-based models in accurately predicting achievable performance.