Limits from Charge Carrier Doping, Mobility and Lifetime on the Potential Performance of Sn and Pb-Based Halide Perovskites

Measuring the quantum yield of photoluminescence is a powerful and widely used technique to characterize photovoltaic materials. It can reveal an upper limit for the potential solar cell performance from measurements on neat thin films and can even predict the implied current-voltage (<i>JV</i>) characteristics. However, the contributions from charge carrier doping and lifetimes to the implied voltage are usually not distinguished, and ideal charge carrier transport is assumed for the <i>JV</i> prediction. To gain more insights into the performance limiting factors, we use time-resolved photoluminescence, which allows us to determine the charge carrier lifetimes, doping, and mobilities for lead and tin halide perovskites.<br/>Carrier lifetimes are probed under solar AM1.5-equivalent illumination employing sinusoidal modulation as well as under solar AM1.5-equivalent illumination superimposed with a weak pulsed photoexcitation. Such illumination conditions directly yield the relevant lifetime for photovoltaic application and avoid photoluminescence decays that are not connected to charge recombination. The charge carrier lifetimes of a few microseconds measured for lead-based are much larger than the few ns obtained on tin-based perovskites, which translate into larger voltage losses for the tin perovskite solar cells.<br/>The doping is accessed from the initial photoluminescence amplitude and increases from ~10¹³cm^-3 for lead perovskites to &gt;10¹⁷cm^-3for tin perovskites. This doping of the lead perovskite is much smaller than the photogenerated carrier concentration under solar illumination at open-circuit conditions and also smaller than the charge needed to affect the built-in potential under short circuit conditions. Hence, it has a negligible impact on the solar cell performance and lead-perovskite can be considered intrinsic. In contrast, the doping of the tin-based perovskites is much larger than the photogenerated carrier concentration under solar illumination and partially compensates for the low charge carrier lifetime, and significantly contributes to the implied Voc.<br/>The charge carrier mobility is obtained from the initial decay component of photoluminescence after pulsed photogeneration. For the Pb-based perovskites, the charge carrier diffusion through the thin film takes ~100 ns and is described with a mobility of 0.4 cm²/Vs, which is in line with complementary Hall measurements. This mobility is significantly lower than the terahertz-derived mobility of ~32 cm²/Vs for the nm-scale intragrain transport and indicates strong transport barriers at grain boundaries which leads to fill-factor losses in the solar cells.