The Role of Grain Boundaries on Ionic Defect Migration in Metal Halide Perovskites

Abstract

Halide perovskites are emerging as revolutionary materials for optoelectronics. Their ionic nature and the presence of mobile ionic defects within the crystal structure have a dramatic influence on the operation of thin‐film devices such as solar cells, light‐emitting diodes, and transistors. Thin films are often polycrystalline and it is still under debate how grain boundaries affect the migration of ions and corresponding ionic defects. Laser excitation during photoluminescence (PL) microscopy experiments leads to formation and subsequent migration of ionic defects, which affects the dynamics of charge carrier recombination. From the microscopic observation of lateral PL distribution, the change in the distribution of ionic defects over time can be inferred. Resolving the PL dynamics in time and space of single crystals and thin films with different grain sizes thus, provides crucial information about the influence of grain boundaries on the ionic defect movement. In conjunction with experimental observations, atomistic simulations show that defects are trapped at the grain boundaries, thus inhibiting their diffusion. Hence, with this study, a comprehensive picture highlighting a fundamental property of the material is provided while also setting a theoretical framework in which the interaction between grain boundaries and ionic defect migration can be understood.

Defect Dynamics in MAPbI3 Polycrystalline Films: The Trapping Effect of Grain Boundaries

“…These values of energetics in G1 and G2 , the relative energies of the various grain domains and migration energy barriers, are well above the limit of accuracy of the MYP model, which is estimated to be ∼0.1–0.2 eV (compare, for example, vacancy formation and migration energies calculated using the present force field to the corresponding ab initio data). This makes us confident that the present results are qualitatively accurate and will be used in the following to interpret experimental results …”

“…Present results allow us to formulate a possible in‐depth explanation for recent results from Pungh et al . beyond the one proposed in the original article.…”

“…Pungh et al . have shown that iodide and, possibly, methylammonium defects formed upon intense illumination migrate out of the focus of a laser . Upon cessation of illumination, defects recombine but recombination is faster in monocrystals or in polycrystalline samples with larger grains with respect to films with smaller crystallites.…”

“…Photoluminescence (PL) and PL microscopy experiments have shown that light can induce the formation of defects, mainly iodide Frenkel pairs, though some evidence of formation of analogous MA defects has also been found. These defects migrate toward grain boundaries . Upon cessation of illumination, the film tends to restore its initial structure, with complementary defects recombining.…”

Flash Formation of I-Rich Clusters during Multistage Halide Segregation Studied in MAPbI1.5Br1.5

“…41,[45][46][47] On the other hand it has been shown that grain boundaries impede ion migration and act as defect trap sites. [48][49][50] Also the defect concentration within the perovskite film has also been reported to greatly affect phase segregation, in particular its kinetics. 21,35 By combining theory and experiment a linear dependence between halide vacancy density and halide segregation rate was shown.…”