In order to further improve the efficiencies and in particular the open-circuit voltages of perovskite solar cells it is important to understand non-radiative recombination and their relation to defect densities. In this talk, I will discuss our recent findings on how to measure and quantify both defect densities and decay times as an assay of the strength of recombination. Defect densities are often measured using methods that rely on measuring the charge density on these defects. Examples for such methods are capacitance-based methods as well as the measurement of unipolar current voltage curves (often called space charge limited current measurements) where the voltage onset of the trap-filled limit is evaluated. All these methods are typically done on sandwich-type devices where the volume charge densities have to compete with the charge per area on the electrodes. If the volume densities are too low to cause a Debye length that is shorter than the thickness of the semiconductor between the electrodes, the volume charge of the defects would not affect the measurement. I show that this is likely often the case in thin-film devices but not in single crystals. Therefore, these methods create an artificial bias towards assigning higher defect densities to thin films than to single crystals.^1-2

A striking observation in the context of measuring recombination lifetimes is the huge discrepancies that are typically reported for lifetimes measured on films and on devices. Typically, the devices show longer lifetimes despite the fact that they have more interfaces and typically higher recombination rates at a given Fermi level splitting than well passivated films. This initially surprising feature can be understood by studying the differences between actual lifetimes due to recombination and a more general concept of decay times that may be due to recombination, extraction, injection and capacitive charging and discharging of electrodes. We develop a reasonably simple analytical set of equations to describe decay times in electrical and optical transient measurements.^3-4

1. Ravishankar, S.; Unold, T.; Kirchartz, T., Comment on “Resolving spatial and energetic distributions of trap states in metal halide perovskite solar cells”. Science 2021, 371 (6532), eabd8014.
2. Siekmann, J.; Ravishankar, S.; Kirchartz, T., Apparent Defect Densities in Halide Perovskite Thin Films and Single Crystals. ACS Energy Letters 2021, 6, 3244-3251.
3. Krückemeier, L.; Krogmeier, B.; Liu, Z.; Rau, U.; Kirchartz, T., Understanding Transient Photoluminescence in Halide Perovskite Layer Stacks and Solar Cells. Advanced Energy Materials 2021, 11, 202003489.
4. Krückemeier, L.; Liu, Z.; Krogmeier, B.; Rau, U.; Kirchartz, T., Consistent Interpretation of Electrical and Optical Transients in Halide Perovskite Layers and Solar Cells. Advanced Energy Materials 2021, 11 (n/a), 2102290.