Traps in the spotlight: How traps affect the charge carrier dynamics in Cs2AgBiBr6 perovskite

Abstract

Suitable optoelectronic properties of lead halide perovskites make these materials interesting semiconductors for many applications. Toxic lead can be substituted by combining monovalent and trivalent cations, such as in Cs 2 AgBiBr 6 . However, efficiencies of Cs 2 AgBiBr 6 -based photovoltaics are still modest. To elucidate the loss mechanisms, in this report, we investigate charge dynamics in Cs 2 AgBiBr 6 films by double-pulse excitation time-resolved microwave conductivity (DPE-TRMC). By exciting the sample with two laser pulses with identical wavelengths, we found a clear photoconductance enhancement induced by the second pulse even 30 μs after the first laser pulse. Modeling the DPE-TRMC results, complemented by photoluminescence and transient absorption, we reveal the presence of deep emissive electron traps, while shallow hole trapping is responsible for the long-lived transient absorption signals. These long-lived carriers offer interesting possibilities for X-ray detectors or photocatalysis. The DPE-TRMC methodology offers unique insight into the times involved in charge trapping and depopulation in Cs 2 AgBiBr 6 . • Double-pulse excitation TRMC is developed to study trapping processes • Cs 2 AgBiBr 6 shows high photoconductance up to 30 μs after the first excitation • A high yield of free charges results from populated trap states by the first pulse • Emission of holes residing in shallow states extends over many microseconds Trap-assisted electron recombination in Cs 2 AgBiBr 6 thin films, combined with low hole mobility and shallow trapping, can be a major drawback of Cs 2 AgBiBr 6 in photovoltaic devices. Caselli et al. reveal that the long-lived charge carriers in Cs 2 AgBiBr 6 can offer a suitable, lead-free alternative for other applications, such as X-ray detectors or photocatalysis.