Researchers from Pohang University of Science and Technology, University of Electronic Science and Technology of China and Sungkyunkwan University have developed a volatile-assisted surface reconstruction strategy that addresses one of the most critical limitations of tin (Sn2+) halide perovskites: their intrinsic redox instability and rapid degradation under ambient conditions.
Sn2+-based halide perovskites are widely considered promising lead-free semiconductors due to their tunable bandgaps, low effective mass, and efficient charge transport. However, their practical deployment has been hindered by the presence of undercoordinated Sn2+ surface sites, which are highly susceptible to oxidation. This process induces uncontrolled self-p-doping and generates trap states, ultimately destabilizing device operation and severely limiting performance reproducibility. To overcome this, the researchers introduced a volatile-assisted coordination mechanism based on potassium acetate (KAc). When applied to CsSnI3 films, acetate ions transiently coordinate with undercoordinated Sn2+ sites, forming tin acetate (Sn(Ac)2), a volatile intermediate that is removed during mild annealing. This process effectively eliminates reactive surface tin species.
Simultaneously, the removal of Sn species triggers an in situ surface reconstruction: iodide ions combine with potassium to form a KI-rich layer that passivates the surface. This dual action both removes defect-prone sites and replaces them with a chemically stable, self-passivating interface. The result is a transformation of the initially SnI2-terminated, defect-rich surface into a chemically equilibrated and defect-mitigated structure.
This reconstructed interface suppresses trap states and stabilizes local stoichiometry, which directly impacts device physics. The reduction of self-p-doping enables precise control over carrier density and threshold voltage, a long-standing challenge in Sn-based semiconductors. As a result, the team demonstrated high-performance p-type thin-film transistors with near-zero threshold voltage, hole mobility exceeding 50 cm2/V·s, and on/off current ratios above 108.
Beyond performance improvements, the reconstructed KI-rich layer functions as a thermally and chemically robust barrier against environmental degradation. Devices maintained stable operation for over one month at 100 °C, while also showing dramatically improved air stability - surviving more than four hours in ambient conditions compared to minutes for untreated devices.
This approach addresses the fundamental trade-off in Sn2+ perovskites between electronic performance and chemical stability. By dynamically removing undercoordinated Sn sites and re-establishing surface equilibrium through a volatile coordination pathway, the method enables defect equilibration without compromising charge transport.
This work establishes volatile-assisted surface reconstruction as a general strategy for regulating defect chemistry and self-doping in metastable semiconductors. The ability to simultaneously enhance transport characteristics, environmental stability, and device reproducibility could enable the integration of Sn-based perovskites into practical electronic applications, including p-channel transistors for complementary circuits, vertically stacked memory architectures, and large-area electronics.
