Watching Space Charge Build Up in an Organic Solar Cell

Space charge effects can significantly degrade charge collection in organicphotovoltaics (OPVs), especially in thick-film devices. The two main causesof space charge are doping and imbalanced transport. Although these arecompletely different phenomena, they lead to the same voltage dependenceof the photocurrent, making them difficult to distinguish. Herein, a method isintroduced on how the build-up of space charge due to imbalanced transport canbe monitored in a real operating organic solar cell. The method is based on thereconstruction of quantum efficiency spectra and requires only optical inputparameters that are straightforward to measure. This makes it suitable for thescreening of new OPV materials. Furthermore, numerical and analytical meansare derived to predict the impact of imbalanced transport on charge collection. Itis shown that when charge recombination is sufficiently reduced, balancedtransport is not a necessary condition for efficient thick-film OPVs.