Space charge effects in the active layer usually lead to a degraded device performance of organic solar cells. In principle, there are two possible reasons for the build-up of space charge: unintentional doping and imbalanced charge transport. Although these are fundamentally different phenomena, they lead to the very same degradation of the device current-voltage curve, which makes them difficult to distinguish in practive.

In a recent publication that goes back on Sebastian’s previous research at the University of Oldenburg, Germany, and at Åbo Akademi University, Finland, a new method is introduced how the how the build‐up of space charge due to imbalanced charge 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 organic absorber materials. Furthermore, numerical and analytical means are derived to predict the impact of imbalanced transport on charge collection. It is shown that when charge recombination is sufficiently reduced, balanced transport is not a necessary condition for efficient thick‐film devices.

Watching Space Charge Build Up in an Organic Solar Cell
Build-up of space charge due to imbalanced charge transport in a thick-film organic solar cell

The results from the work are also relevant for the ReMorphOPV project. They clearly indicate that reducing the bimolecular recombination coefficient in the active layer is key to enable the transition towards thick-film organic solar cells. In the course of ReMorphOPV, strategies how such a reduction can be facilitated will be systematically investigated using quasi-atomistic modeling.

S. Wilken, O. Sandberg, D. Scheunemann, R. Österbacka, Watching Space Charge Build Up in an Organic Solar Cell, Sol. RRL 4, 1900505 (2020)