The relatively weak bond of metal-halide perovskites (MHPs) gives rise to an inherently soft crystal lattice which is naturally prone to disorder, [1] associated to formation of defects. Defects introducing levels in the material’s band-gap may act as traps and recombination centers for photogenerated charge carriers, limiting the device performance and possibly impacting the device temporal stability. Defects may also introduce ionic mobility channels in MHPs. Ion migration is boosted by the presence of vacancies and interstitial defects, acting as shuttles for ion hopping.[2] If the migrating defects are also charge traps, as it occurs for iodine defects in MAPbI3, one has migrating traps which can respond to the action of an electric field [3] and to the presence of photogenerated carriers.[4, 5] Some of the traps may also undergo photochemical reactions, such as the reported release of molecular iodine under light irradiation[6, 7]. Defects may also lay behind the reported material transformation under light exposure, followed by very slow relaxation to initial conditions.[8]

While bulk defects have been extensively investigated in MHPs,[9] there is increasing awareness that surfaces and grain boundaries may actually represent preferential sites for defect formation. Surfaces are also involved in defining the material work function, thus the interfacial energy level alignment with selective contacts.

We present ab initio modeling results of surfaces, defects and surface defects modeling in MHPs with selected examples of applications related to the effect of electric fields and charge carriers on the structural and electronic properties of perovskites relevant to stability and solar cell operation. A model to account for the surface-related ion migration is also presented.

References:

[1] Conings, B. et al. Adv. Energy Mater. 2015, 5, 1500477.

[2] Mosconi, E.; De Angelis, F. ACS Energy Lett. 2016, 1, 182-188.

[3] Chen, B. et al. Nat. Mater., 2018, in press.

[4] Birkhold, S.T. et al. ACS Energy Lett. 2018, 3, 1279−1286

[5] Meggiolaro, D. et al. Energy Environ. Sci. 2018, 11, 702-713.

[6] Meggiolaro, D. et al. ACS Energy Lett., 2018, 3, 447–451.

[7] Kim, G.Y. et al. Nat Mater 2018, 17, 445-449.

[8] Gottesman, R. et al. J. Phys. Chem. Lett. 2014, 5, 2662-2669.

[9] Meggiolaro, D.; De Angelis, F. ACS Energy Lett. 2018, 2206-2222.