Understanding Power-Law Photoluminescence Decays and Bimolecular Recombination in Lead-Halide Perovskites

Transient photoluminescence is a frequently used method in the field of halide perovskite photovoltaics to quantify recombination by determining the characteristic decay time of an exponential decay. This decay time is often considered to be a single value for a certain perovskite film. However, there are many mechanisms that lead to non-exponential decays. Here, it is shown that photoluminescence decays in many lead-halide perovskites are non-exponential and follow a power-law relation between PL intensity and time that is caused by shallow defects. Decay times therefore vary continuously as a function of time and injection level. In situations where recombination is bimolecular and decays follow a power law, the differential decay time equals the time delay after the laser pulse for long time delays and therefore completely lacks quantitative information about the recombination rate. Quantifying recombination using transient PL measurements, therefore, requires analyzing the lifetime as a function of injection level rather than time. As an alternative to the continuously varying decay time, a bimolecular recombination coefficient can also be determined, which correlates with the photoluminescence quantum efficiency. Finally, the influence of the repetition rate and the background subtraction method on the analysis of power-law type PL decays is discussed.