Lead halide perovskites are outstanding materials for next generation photovoltaics due to their excellent optoelectronic properties and the relatively high defect tolerance, leading to solar cell efficiencies reaching 25%.[1] The development of less toxic tin-based perovskites, on the other hand, is still limited by two apparently related phenomena, i.e. self p-doping and tin oxidation, reducing efficiencies to ~10%.[2]

Defects play a fundamental role in determining the stability and the optoelectronic quality of these materials. Deep defects are responsible of charge trapping that strongly impacts solar cell efficiency. Defects can also activate chemical degradation paths compromising the long term stability of the devices.  

Here we highlight the important role played by density functional theory (DFT) simulations in the study of defects and defect-related phenomena in lead / tin perovskites. Technical aspects of defect computational modelling are illustrated, with emphasis on the role of theory in the evaluation of defects properties, such as the role of spin-orbit coupling and self-interaction corrections.[3] Thus, a global picture of the defects chemistry in the prototype MAPbI₃, MAPb_0.5Sn_0.5I₃ and MASnI₃ perovskites is presented. Defect stability and the major source of charge trapping in these materials is discussed, by focusing on the role of the metal.

Our analysis reveals that the electronic structure modulation induced by the metal strongly impacts defect properties in lead and tin perovskites. The different band edges energies lead to a different charge trapping activity in these materials, dominated by iodine in MAPbI₃ and by tin in MASnI₃. The inherently low ionization potential of tin perovskites compared to lead is mainly responsible of the high stability of acceptor defects, such as tin vacancy and interstitial iodine, which are at the origin of the material p-doping.[4] The intrinsic mechanisms potentially limiting the long term stability of these perovskites are also discussed, particularly the possible internal processes activating the oxidation of Sn(II) to Sn(IV) at the tin perovskite surface.

References

Green et al. Nat. Photon. 2014, 8, 506-514

Noel et al. Energy Environ. Sci. 2014, 7, 3061-3068.

Meggiolaro et al. ACS Energy Lett. 2018, 3, 2206-2222.

Meggiolaro et al. J. Phys. Chem. Lett. 2020, 11, 3546-3556.