Tin-based halide perovskites are promising lead-free absorbers, but their susceptibility to Sn(II) oxidation make their optoelectronic properties strongly time dependent under realistic operating conditions. Rather than treating this chemically dynamic behavior solely as a stability problem, their soft lattice and defect chemistry can be leveraged to understand and ultimately control the reversible and irreversible transformations that govern device performance.

Building on this perspective, thiophene-2-ethylammonium halides (TEAX, X = I, Br, Cl) are introduced as sulfur-based additives that coordinate with Sn in the precursor solution, adjust crystallization dynamics, and inhibit Sn(II) oxidation in FASnI3 films. These interactions enhance crystallinity, increase the Sn2+/Sn4+ ratio, and suppress bulk and surface defect formation, enabling power conversion efficiencies up to about 12% and markedly improved operational stability under continuous illumination. In particular, TEABr-based devices maintain more than 95% of their initial efficiency for thousands of hours in inert atmosphere, while TEAX-treated films retain far higher Sn2+ content than control samples after ambient exposure.

Our study examines the evolution of TEAX-modified Sn-based perovskite solar cells under electrical and ambient stress, highlighting pathways of reversible performance loss and spontaneous recovery associated with defect reconfiguration, ion migration, and redox chemistry in the perovskite and interfacial layers. Incorporation of TEAI in unencapsulated FASnI3 devices enables a pronounced in operando self-healing effect, where initial degradation under 60% relative humidity is followed by performance recovery and even enhancement beyond the initial efficiency, in stark contrast to rapidly failing control cells. By combining photoluminescence with electrochemical impedance spectroscopy, the work tracks changes ative recombination and charge transport, correlating luminescence signatures with frequency-resolved resistive and capacitive responses that fingerprint distinct degradation mechanisms.

They also guide the design of additives, interlayers, and operational protocols that promote performance recovery while suppressing irreversible degradation, pointing toward robust, high-performance, lead-free perovskite devices in which chemical dynamics are monitored and actively managed rather than merely tolerated.