Stolterfoht, M.; Caprioglio, P.; Wolff, C.; Márquez, J.A.; Nordmann, J.; Zhang, S.; Rothhardt, D.; Hörmann, U.; Amir, Y.; Redinger, A.; Kegelmann, L.; Zu, F.; Albrecht, S.; Koch, N.; Kirchartz, T.; Saliba, M.; Unold, T.; Neher, D.: The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells. Energy & Environmental Science 12 (2019), p. 2778-2788

Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V_OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin and nip type cells including high performance devices with power conversion efficiencies of up to 21.4 %. Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, Spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V_OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V_OC of the device. Importantly, the V_OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells while in poor performing devices, the V_OC is substantially lower than the QFLS, likely due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline several important considerations to maximize the V_OC. This work shows that the real challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in the suppression of defect recombination at the interfaces and proper energy level alignment.