Both transient and steady state photoluminescence PL have been frequently used to analyze the properties of halide perovskite films¹ and recently also layer stacks, i.e. films with interfaces.^2-4 Here, we present our current level of understanding of how to analyze the data. In the case of films, long decays in transient PL correlate well with strong steady state PL. The shape of the decays allows us to determine bimolecular and monomolecular recombination coefficients, the former of which is clearly affected by photon recycling.^1,5 In the case of films with one interface, we show that high luminescence is still beneficial for high open-circuit voltages in devices and still correlates with long photoluminescence decays.⁶ We show by simulation how the combination of steady state PL with tr-PL can be used to better understand band alignment at interfaces and how it provides an estimate of the surface recombination velocities. Finally, we discuss the case of layer stacks with two contacts and of full devices. Here, additional effects such as the conductivity and capacitance of contact layers become important. In addition, the size of the laser spot becomes relevant because it affects how much the lateral redistribution of charge carriers via the conductive contact layers affects the result. Finally, we discuss how transient PL and methods like transient photovoltage differ at least quantitatively in the perovskite solar cell. The difference between the two is that transient PL measures the internal voltage, i.e. the quasi-Fermi level splitting, and transient photovoltage measures the external voltage that builds up at the external terminals of the cell. While both decays are affected by the contact layers, the impact is substantially different. The external voltage first has to be build up by charging up the capacitance of the interfacial layers, the internal voltage peaks immediately after the laser pulse and then decays quickly.

   (1)    Staub, F.; Hempel, H.; Hebig, J. C.; Mock, J.; Paetzold, U. W.; Rau, U.; Unold, T.; Kirchartz, T. Beyond Bulk Lifetimes: Insights into Lead Halide Perovskite Films From Time-Resolved Photoluminescence. Phys. Rev. Applied 2016, 6, 044017.

   (2)    Krogmeier, B.; Staub, F.; Grabowski, D.; Rau, U.; Kirchartz, T. Quantitative Analysis of the Transient Photoluminescence of CH₃NH₃PbI₃/PC₆₁BM Heterojunctions by Numerical Simulations. Sustainable Energy Fuels 2018, 2, 1027-1034.

   (3)    Stolterfoht, M.; Wolff, C. M.; Marquez, J. A.; Zhang, S.; Hages, C. J.; Rothhardt, D.; Albrecht, S.; Burn, P. L.; Meredith, P.; Unold, T. et al. Visualization and Suppression of Interfacial Recombination for High-Efficiency Large-Area Pin Perovskite Solar Cells. Nat. Energy 2018, 3, 847-854.

   (4)    Stolterfoht, M.; Caprioglio, P.; Wolff, C. M.; Marquez, J. A.; Nordmann, J.; Zhang, S.; Rothhardt, D.; Hörmann, U.; Redinger, A.; Kegelmann, L.; Albrecht, S.; Kirchartz, T.; Saliba, M.; Unold, T.; Neher, D. The perovskite/transport layer interfaces dominate non-radiative recombination in efficient perovskite solar cells. Unpublished Work, 2018.

   (5)    Staub, F.; Kirchartz, T.; Bittkau, K.; Rau, U. Manipulating the Net Radiative Recombination Rate in Lead Halide Perovskite Films by Modification of Light Outcoupling. The Journal of Physical Chemistry Letters 2017, 8, 5084-5090.

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