  Space charge effects can significantly degrade charge collection in organic
photovoltaics (OPVs), especially in thick-film devices. The two main causes of
space charge are doping and imbalanced transport. Although these are completely
different phenomena, they lead to the same voltage dependence of the
photocurrent, making them difficult to distinguish. In this work, a method is
introduced how the build-up of space charge due to imbalanced transport can be
monitored in a real operating organic solar cell. The method is based on the
reconstruction of quantum efficiency spectra and requires only optical input
parameters that are straightforward to measure. This makes it suitable for the
screening of new OPV materials. Furthermore, numerical and analytical means are
derived to predict the impact of imbalanced transport on the charge collection.
It is shown that when charge recombination is sufficiently reduced, balanced
transport is not a necessary condition for efficient thick-film OPVs.
