Long-Range Binding of Defect Clusters Leads to Suppressed Ion Mobility in Cs-Doped Methylammonium Lead Iodide

Abstract

Partial substitution of CH3NH3 + with Cs+ ions has been reported to suppress the iodide vacancy diffusion in methylammonium lead iodide (MAPI) and enhance its structural stability. However, the defect chemistry of partial cation-substituted MAPI, i.e., Cs x (CH3NH3)1–x PbI3, and the effect of stoichiometry on the halide ion mobility have not been systematically investigated in the literature. Herein, classical force field-based atomistic simulations were performed to calculate the defect cluster binding energies and halide ion migration barriers in Cs x (CH3NH3)1–x PbI3 for different Cs-doping concentrations, exhibiting a weak dependence on stoichiometry. Higher diffusion activation energy due to Cs doping results mainly from the long-range binding between I– vacancy clusters and Cs+, which reduces the concentration of mobile vacancies. Clusters of Cs+ with up to 3 I– vacancies were found to be energetically stable with a binding energy of ∼0.2 eV at 6 Å from the Cs+ defect center. Tilting and rotation of neighboring PbI6 octahedra were also observed in doped MAPI, which causes a nonlocal strain in the crystal. Experimental characterization of Cs-doped MAPI solar cells confirmed the reduced hysteresis and its weaker dependence on Cs concentration along with higher activation energy of halide migration in Cs-doped MAPI. Time-resolved photoluminescence and thermal admittance spectroscopy tests also showed reduced defect density in the Cs-doped perovskite because of suppressed halide migration that leads to lower loss of halides. Weaker dependence of diffusion activation energy on the dopant concentration opens up the possibility for multication doping to further maximize the strain effect.

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“…Indeed, there is a consensus that I À vacancies (cations) are the most mobile species in halide perovskites [38,39,40,41] and that large Cs þ ions (cations) hinder the migration of I À vacancies. [48,49] To corroborate this hypothesis, we have performed 1D transient-mode electro-optical simulations of the two fabricated mC-PSCs (matching layer thicknesses, incident spectrum, and illuminated area) while including the effect of both anion and cation migration within the active regions of the stacks (i.e., mp-TiO 2 and mp-ZrO 2 incorporating the perovskites). We note that cations (anions) in the simulations are defined as generic mobile positive (negative) charges.…”

“…Indeed, there is a consensus that I − vacancies (cations) are the most mobile species in halide perovskites [ 38,39,40,41 ] and that large Cs + ions (cations) hinder the migration of I − vacancies. [ 48,49 ]…”

“…We also find that the anion mobility in both perovskites is much lower than the cation mobility, in agreement with the literature. [48,49,50,51] It was reported that, in such cases, including both species or just cations as mobile species in simulations yields identical electrical results. [50] It is worth mentioning that charge trapping and recombination were also modelled in the simulations by setting the same parameters for the two perovskites (see Table S2, Supporting Information).…”