Probing Photostability of Tin-Containing Mixed Halide Perovskites—The Good and the Bad

Mixed halide compositions offer interesting avenues to tune the bandgap of perovskites to make them suitable as wide bandgap absorbers in perovskite-based multijunction solar cells and as emitter layers in red and blue perovskite light emitting diodes (LEDs). However, the instability of bromide/iodide (Br/I) mixtures under light illumination has been long reported, where low-bandgap iodide-rich phases tend to form for >20% Br contents, resulting in the funnelling of photogenerated carriers and red-shifting of photoluminescence (PL). Over the years, various mechanisms have been presented to account for such halide segregation effects – yet the exact mechanism remains elusive. Very recently, the model of halide oxidation has been found to be powerful in explaining the mechanism of excitation-induced halide segregation in lead-based mixed halide perovskite compositions and is consistent with most of the reported experimental observations.

On the other hand, several strategies related to compositional engineering and defect passivation have been devised to mitigate the issue of halide phase segregation under operational conditions. However, many of these approaches demonstrate the mitigation of halide segregation only under certain chosen operational conditions (e.g. light intensity, time under illumination, halide ratio etc.) and hence definitive conclusions regarding the applicability of those approaches under other (potentially harsher) conditions cannot always be guaranteed.

In this work, we systematically study the impact of Sn incorporation on the photostability of mixed halide perovskites. While a clear redshift in the PL peak and absorption onset is observed under light soaking for Pb-based compositions, which signifies photoinduced halide segregation, no such shifts are observed for Sn-containing compositions irrespective of the Pb/Sn mixing ratio. Furthermore, while halide segregation in Pb perovskites is often associated with an increase in luminescence under light, we observe an opposite effect of photodarkening for Sn-containing perovskites, possibly due to the formation of photoinduced defects. We also visualize the difference in the impact of light soaking between Pb and Sn-containing perovskites in the microscale using wide-field hyperspectral photoluminescence imaging. It is important to note that we explored a wide variety of processing and operational conditions to assess the extent of photostability of Sn-containing mixed halide perovskites, which suggests an intrinsic stability against halide segregation in these materials. Subsequently, we also investigated the impact of light soaking on the resulting charge carrier dynamics and charge transport in mixed halide perovskites with and without Sn in the composition, which again exhibit distinctive trends. Moreover, scan-rate dependent field effect transistor (FET) measurements and electrochemical impedance spectroscopy measurements on fabricated p-i-n solar cells, aided further by ab-initio simulations also reflect the trend of suppressed ionic migration in Sn-containing mixed halide perovskites as compared to their Pb-only analogues. Next, we rationalize the contrasting trends of photostability of Pb-only and mixed Pb-Sn mixed halide perovskites by suggesting a hypothesis of preferential Sn²⁺ oxidation over I^– oxidation in the presence of photogenerated holes, which seems to explain most of our experimental observations. Finally, we highlight important differences in the degradation pathways of Pb-only and mixed Pb-Sn based solar cells, thereby indicating that the seeming absence of halide segregation in mixed Pb-Sn devices does not alone guarantee an improvement in their operational stability.

Therefore, our work represents an important advance in the understanding of halide segregation behaviour in mixed halide perovskites, which can be further leveraged to demonstrate efficient and stable solar cells, LEDs and a plethora of other optoelectronic devices.