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Abstract In this work, the optical transition of self‐trapped excitons (STEs) and the emergent green emission in β‐Ga2O3 samples with/without Sn impurities at various doping levels have been investigated via temperature‐ and power‐dependent photoluminescence. The ultraviolet (UV) emissions ≈ 3.40 eV unanimously exhibit an excitonic nature related to STEs and typical negative thermal quenching (NTQ) characters. The NTQ activation energy decreases from 103.56 to 42.37 meV with the increased electron concentration from 2.1 × 1016 to 6.7 × 1018 cm−3, indicative of the reduced energy barrier that electrons should overcome to form stable STEs due to the lift‐up of Fermi level. In comparison, the green emissions ≈ 2.35 eV with two quenching channels are observed only in samples with Sn impurities at cryogenic temperatures. One channel is the nsnp‐ns2 transition of Sn2+, the other is donor‐acceptor pair recombination via (2VGa‐Sni)2− complex, which is energetically favorable as evidenced by density functional theory calculations. The semi‐classical quantum theory models fitting proves the transition from green to UV emissions with elevated temperature. The enhanced STEs emission with distinguished NTQ effect strengthens evidence that the stable polarons inherently limit the transport of holes in Ga2O3, and also support the potential of Ga2O3 materials for the development of UV optoelectronics.

Abstract Phase instability poses a serious challenge to the commercialization of formamidinium lead iodide (FAPbI3)-based solar cells and optoelectronic devices. Here, we combine density functional theory and machine learning molecular dynamics simulations, to investigate the mechanism driving the undesired α-δ phase transition of FAPbI3. Prevalent iodine vacancies and interstitials can significantly expedite the structural transition kinetics by inducing robust covalency during transition states. Extrinsically, the detrimental roles of atmospheric moisture and oxygen in degrading the FAPbI3 perovskite phase are also rationalized. Significantly, we discover the compositional design principles by categorizing that A-site engineering primarily governs thermodynamics, whereas B-site doping can effectively manipulate the kinetics of the phase transition in FAPbI3, highlighting lanthanide ions as promising B-site substitutes. A-B mixed doping emerges as an efficient strategy to synergistically stabilize α-FAPbI3, as experimentally demonstrated by substantially higher initial optoelectronic characteristics and significantly enhanced phase stability in Cs-Eu doped FAPbI3 as compared to its Cs-doped counterpart. This study provides scientific guidance for the design and optimization of long-term stable FAPbI3-based solar cells and other optoelectronic devices through defect control and synergetic composition engineering.