Aiming at a large-scale production of perovskite solar cells, replacement of Pb2+ with a non-toxic element is highly desirable. Bimetal perovskites are a viable non-toxic alternative to lead perovskites. Recently, attention has been given to Cs2AgBiBr6, due to the presence of a long-lived contribution to its photoluminescence decay, the relatively low carrier effective masses and the excellent stability against temperature and moisture. Furthermore, it has been shown that alloying Cs2AgBiBr6 with a small percentage of Tl reduces the bandgap, bringing it to values close to that of state-of-the-art lead perovskites.
Here, we studied the charge carrier lifetimes in pristine and Tl-alloyed Cs2AgBiBr6 bimetal perovskite crystals by means of the time-resolved microwave conductance (TRMC) technique. This technique probes the generation and decay of mobile charges generated upon optical excitation by a nanosecond laser pulse. We performed TRMC measurements at different temperatures, varying both the wavelength and the intensity of the laser pulse. By varying the wavelength, the penetration depth of light can be tuned, thus controlling the location where charges are generated. For crystals with a thickness exceeding the charge carrier diffusion length, wavelength-dependent measurements give the possibility of disentangling surface and bulk effects. When charges are generated close to the surface (i. e. for excitation energies well above the bandgap), recombination is fast and occurs on a nanosecond timescale, which we assign to strong surface recombination. Excitation close to the bandgap results in a rapid initial decay followed by a long-lived tail, implying the presence of mobile carriers with a lifetime of many microseconds in the bulk of the material. We discuss the temperature and intensity dependence of the long-lived TRMC tails in terms of shallow trap states for electrons and holes.
For the bimetal perovskite alloyed with Tl we observed similar dynamics as in the pristine material; this finding suggests that recombination at the surface and in the bulk is similar in both pristine and Tl-alloyed Cs2AgBiBr6. Our results highlight the possibility of generating long-lived charge carriers in the bulk of pristine and Tl-alloyed Cs2AgBiBr6, prompting surface passivation as the main aim of future research on these materials.