Defect calculations using a combined SCAN and hybrid functional in γ-CsPbI3

Abstract

γ-CsPbI₃ solar cells have achieved promising efficiencies, yet the quantitative understanding of their defect properties is limited due to the severe computational challenges when using hybrid functionals. We have discovered an algorithm to improve the convergence speed through a combination of structural relaxation with a strongly constrained and appropriately normed (SCAN) Meta-generalized-gradient approximation (Meta-GGA) functional and further ionic and electronic calculations with the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional. The static HSE calculations with SCAN results as inputs are qualitatively reliable in defect calculations, different from one-ionic step HSE calculations based on GGA inputs. Contradictory to previous GGA defect results, a suppressed bipolar conductivity by p-type V_Cs and V_Pb, and n-type Cs_I is found. Additionally, stable bipolar defects I_int and Cs_Pb, with features of strong bond orbital coupling or structural deformation, detrimentally serve as carrier-traps. This strengthened bond orbital coupling in γ-CsPbI₃ causes more defect charge states than organic perovskites with larger lattice constants.