Achieving a high fill factor for organic solar cells

Organic photovoltaics (OPVs) have developed rapidly in the last decades due to their potential for providing cost-efficient, low-energy consumption, and environmentally friendly renewable energy sources. Some research reports have focused on the device physics of organic photovoltaics that governs open circuit voltage (V-oc) and short circuit current (J(sc)) to improve their performance. In this review, we focus on the third parameter, fill factor (FF), that is equally important in determining the power conversion efficiency. We discuss the mathematical calculation of the FF and the relationship between the FF and equivalent circuit model elements, namely, shunt resistance, series resistance, and diode ideal factor. In order to provide a strategy toward a high FF for OPVs from the viewpoints of device design and material synthesis, we review important device features and BHJ features that have a large impact on the device FF, including preventing shorting, buffer layer design, domain size or purity, gradated BHJ structures, pi-pi stacking distance or direction, etc. We hope this article can provide a comprehensive insight into elements controlling the FF of OPVs and give a valuable direction for better device and material design.