Fill Factor Losses and Deviations from the Superposition Principle in Lead-Halide Perovskite Solar Cells

The enhancement of the fill factor in the current generation of perovskite solar cells is the key for further efficiency improvement. Thus, methods to quantify the fill factor losses are urgently needed. A classical method to quantify Ohmic and non-Ohmic resistive losses in solar cells is based on the comparison between the voltage in the dark and under illumination analysed at equal recombination current density. Applied to perovskite solar cells, we observe a combination of an Ohmic series resistance with a voltage-dependent resistance that is most prominent at short circuit and low forward bias. The latter is most likely caused by the poor transport properties of the electron and/or hole transport layers. By measuring the photoluminescence of perovskite solar cells as a function of applied voltage, we provide direct evidence for a high quasi-Fermi level splitting at low and moderate forward bias that substantially exceeds the externally applied voltage. This quasi-Fermi level splitting causes recombination losses and, thus, reduces both the short-circuit current and the fill factor of the solar cell.