The tin iodide-based hybrid perovskite (C6H5C2H4NH3)2SnI4 [(PEA)2SnI4] is promising as the semiconductor in field-effect transistors (FETs) because of its easy film processability and high carrier mobility. However, the stability of (PEA)2SnI4 FETs in air remains a significant issue. In this study, we show that the source of this degradation is oxygen. We observed that the structure of (PEA)2SnI4 degraded in the presence of oxygen, along with the formation of gaps between grains. With the aim of suppressing the oxygen-induced degradation, we optimized (PEA)2SnI4 spin-coating conditions to increase the grain size and simply encapsulated a (PEA)2SnI4 semiconductor with the fluorine-based polymer CYTOP. Adopting these methods led to the greatly improved stability of FET performance in air. We propose that oxygen had reduced penetration into (PEA)2SnI4 films with larger grains. The drain current of optimized FETs remained almost unchanged over 5 h of operation, which is in contrast to the control device that decayed within 1 h.
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