Probing Facet-Dependent Surface Defects in MAPbI₃ Perovskite Single Crystals

Abstract

Halide perovskites such as methylammonium lead iodide (MAPbI 3 ) currently attract considerable attention because of their excellent optoelectronic properties and performance in solar cell devices. Despite tremendous research efforts to elucidate their fundamental properties, ion migration with the presence of ionic defects is still not fully understood. Here, various types of ionic defects for specific (100) and ( 112) lattice facets in single-crystal MAPbI 3 have been investigated systematically. Our measurements reveal significant anisotropic properties. Photoluminescence (PL) and electrical transport measurements show that the (100) facet has higher PL intensity and over 1 order lower trap density compared to that of the (112) facet. We find that the facet-dependent variations of contact potential difference measured with Kelvin probe force microscopy under different bias voltages and light illuminations provide insights into different types of ionic defects on the surface of MAPbI 3 single crystals. We also observe a completely different ion migration behavior on specific crystal facets through nanoscale scanning probe microscopy investigations. Our results indicate that the (100) facet exhibits an n-type behavior dominated with I − vacancies, whereas the (112) facet exhibits a p-type behavior with MA + or Pb 2+ vacancies. The findings on the facet-dependent configuration of ionic defects provide deeper understanding on facet-dependent optoelectronic properties in single-crystal MAPbI 3 .

Shining Light on the Photoluminescence Properties of Metal Halide Perovskites

“…[ 216 ] Facet‐dependent photoluminescence of MAPbI 3 has been investigated by Kim et al by exciting the (100) and (112) crystal facets of large MAPbI 3 single crystals. [ 182 ] As described in more detail in Section 3.3, the authors observed differences in both the PL intensity and peak position between the different crystal facets. These differences were ascribed to facet‐dependent defect types, which leads to variations in nonradiative recombination and marginal changes in the bandgap.…”

“…Kim et al measured the facet‐dependent emission (λ exc = 409 nm, one‐photon excitation) on MAPbI 3 crystals grown by the inverse temperature crystallization process, showing a strong anisotropy of properties, which they attributed to native point defects. [ 181,182 ] They found that the emission intensity from the (112) crystallographic facet is reduced and red shifted when compared to the (100) facet; these occur at 784 and 776 nm, respectively. In order to understand how defects play a role in this effect they measured the contact potential difference (CPD) of the two facets by scanning kelvin probe force microscopy.…”

Managing grains and interfaces via ligand anchoring enables 22.3%-efficiency inverted perovskite solar cells

“…We acquired temperature-dependent capacitance versus frequency (C-f) plots to probe trap density and the energy depth of trap states (see supplementary materials). AALs have the potential to suppress trap density directly by filling in A-site vacancies and modifying grains and interfaces; additionally, they have the potential to provide a similar benefit by promoting the (100) orientation of grains in perovskite films 60 , which may have a different susceptibility to surface-defect formation than other random orientations in the pristine perovskite films 61,62 . To further investigate this possibility, we studied the dependence of trap density on the surface crystallographic facets using density functional theory (DFT) calculations based on a cubic FAPbI3 model 63,64 .…”

Halide Perovskite Single Crystals: Optoelectronic Applications and Strategical Approaches

“…This is called the anisotropy phenomenon. It was reported that the (100) and (112) facets of the MAPbI 3 single crystal exhibited different ion migration behaviour and defect status [42]. The I − vacancy dominated (100) facet followed n-type behaviour, while the (112) facet displayed p-type behaviour with MA + and Pb 2+ dominance.…”