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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.

In materials science, it is a well-accepted fact that metals are generally ductile, and ceramics are always brittle. Here, the authors observe high level of plasticity and excellent elasticity in perovskite oxide under compression and bending with diameter of 2.1 μm.

Abstract All-inorganic lead-free CsSnBr3 is attractive for applications in solar cells due to its nontoxicity and stability, but the device performance to date has been poor. Besides the intrinsic properties, impurities induced from electrodes may significantly influence the device performance. Here, we systematically studied the stability, transition energy levels, and diffusion of impurities from the most commonly used electrodes (Au, Ag, Cu, graphite, and graphene) in CsSnBr3 based on density functional theory calculations. Our results reveal that, whereas graphite and graphene electrodes exhibit negligible influence on CsSnBr3 due to the relatively high formation energies for carbon impurities in CsSnBr3, atoms from the metal electrodes can effectively diffuse into CsSnBr3 along interstice and form electrically active impurities in CsSnBr3. In this case, a significant amount of donor interstitial impurities, such as $$Ag_i^ +$$ A g i + , $$Cu_i^ +$$ C u i + , and $$Au_i^ +$$ A u i + , will be formed under p-type conditions, whereas the Sn-site substitutional acceptor impurities, namely $$Au_{Sn}^{2 - }$$ A u S n 2 − , $$Ag_{Sn}^{2 - }$$ A g S n 2 − , and $$Cu_{Sn}^{2 - }$$ C u S n 2 − , are the dominant impurities, especially under n-type conditions. In particular, except for $$Au_i^ +$$ A u i + , all these major impurities from the metal electrodes act as nonradiative recombination centers in CsSnBr3 and significantly degrade the device performance. Our work highlights the distinct behaviors of the electrode impurities in CsSnBr3 and their influence on the related devices and provides valuable information for identifying suitable electrodes for optoelectronic applications.