Defects are an important topic for every photovoltaic material, because they reduce the open-circuit voltage and thereby also the power conversion efficiency. While metal-halide perovskites are considered to be defect tolerant materials, they are not immune to the influence of defects. The talk attempts to answer three questions, namely do defects matter in lead-halide perovskite solar cells, do they matter in the bulk or at interfaces and finally, where in the band gap are these defects? The first question of the importance of defects in general is easily studied by looking at current high V_oc and high luminescence perovskite solar cells.^1-3 So far perovskite solar cells haven’t beaten the level of 10% external luminescence quantum efficiency, which implies that still 90% of recombination events eventually produce heat. Therefore, non-radiative recombination is still highly relevant, at least at one sun conditions. The second question of where the defects matter has no generic answer. However, for many practical situations, the interfaces between absorber and electron or hole transfer layers limit the open-circuit voltage.^4-5 This implies that it is defects at these interfaces that are most important for further improving efficiencies. The final question deals with the question of where the defects are in energy. For a defect to be highly recombination active, it has to capture electrons and holes efficiently with the slower of the two capture processes limiting the total rate. The classical models of recombination assuming a parabolic potential energy surface (harmonic approximation) predict that defects with energy levels extremely close to midgap should be the most recombination active.^6-7 However, to fully understand the situation in perovskite solar cells, calculations including anharmonicity⁸ have to be performed in the future. 

1.  Krückemeier, L.; Rau, U.; Stolterfoht, M.; Kirchartz, T., How to Report Record Open-Circuit Voltages in Lead-Halide Perovskite Solar Cells. Advanced Energy Materials 2020, 10 (1), 1902573.

2.  Liu, Z.; Krückemeier, L.; Krogmeier, B.; Klingebiel, B.; Marquez, J. A.; Levcenko, S.; Öz, S.; Mathur, S.; Rau, U.; Unold, T.; Kirchartz, T., Open-Circuit Voltages Exceeding 1.26 V in Planar Methylammonium Lead Iodide Perovskite Solar Cells. ACS Energy Letters 2019, 4, 110-117.

3.  Jiang, Q.; Zhao, Y.; Zhang, X.; Yang, X.; Chen, Y.; Chu, Z.; Ye, Q.; Li, X.; Yin, Z.; You, J., Surface passivation of perovskite film for efficient solar cells. Nature Photonics 2019, 13, 460-466.

4.  Stolterfoht, M.; Caprioglio, P.; Wolff, C. M.; Marquez, 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 2019, 12 (9), 2778-2788.