Microstructure of Methylammonium Lead iodide Perovskite Thin Films: A Comprehensive Study of the Strain and Texture

Abstract

Controlling the microstructure of hybrid halide perovskite thin films is essential for optimizing their performance in optoelectronic devices. It is well established that the strain state of the perovskite layer affects its stability. Likewise, the orientation of the perovskite lattice is a determining parameter as these materials have shown pronounced anisotropies in their physical and mechanical properties. In this work, the authors focus on the understanding of the mechanisms that govern the strain and texture observed in MAPbI3 thin films deposited on various oxide substrates. A thorough study of the evolution of the strain of the perovskite layer upon cooling down to room temperature from the crystallization temperature (100 °C) shows an essentially relaxed behavior of the perovskite layers. This result contradicts the commonly accepted hypothesis according to which MAPbI3 layers synthesized above ambient temperature are strained due to the large mismatch in the thermal expansion coefficients of the perovskite and its substrate. The texture in MAPbI3 layers is investigated by means of synchrotron full‐field diffraction X‐ray microscopy. This technique allows the direct observation of the [hh0] and [00l]‐oriented domains at the origin of the observed textures, demonstrating both their twin and ferroelastic nature. The stability of the different domain orientations is investigated by DFT calculations, illustrating the determining role of the chemical environment at the film‐substrate interface. PbI2‐ terminated surfaces are found to favor the [hh0] orientations while for MAI‐terminated ones, both [hh0] and [00l] domains are equally stabilized. The different results constitute an important step of clarification and understanding from the perspective of controlling the microstructure of perovskite layers.

High Fill Factor and Reduced Hysteresis Perovskite Solar Cells Using Small-Molecule-Engineered Nickel Oxide as the Hole Transport Layer

“…Indeed, the nature of the first perovskite plane at the interface with the substrate has been shown to govern the texture and strain of the active layer. 52 A relatively stable, ultrathin PbI 2 layer can ensure the uniform growth of the perovskite layer in the following annealing step and at the same time reduce the enrichment of PbI 2 on the surface of the active layer, which has been shown to decrease the probability of charge recombination at the interface between the perovskite and top charge transport layer (ETL). 53 Based on the above analyses, the DMAP-modified Cu:NiO x layer is expected to be beneficial for enhancing the solar cell device performance, due to a better interfacial contact and improved crystalline quality of the perovskite layer.…”

“…After the introduction of the passivation layer, the coordination of DMAP molecules with PbI 2 is supposed to slow down the conversion of PbI 2 to the perovskite at the initial deposition process. Indeed, the nature of the first perovskite plane at the interface with the substrate has been shown to govern the texture and strain of the active layer . A relatively stable, ultrathin PbI 2 layer can ensure the uniform growth of the perovskite layer in the following annealing step and at the same time reduce the enrichment of PbI 2 on the surface of the active layer, which has been shown to decrease the probability of charge recombination at the interface between the perovskite and top charge transport layer (ETL) …”

Textured MAPbI3 Thin Films Achieved via Solvent Engineering in the Solution‐Shearing Process

“…Ferroelasticity, or the presence of subgrain twin domains of differing orientations like the (110)/(200) planes present here, is inherent in tetragonal MAPbI 3 perovskite films. [ 42 ] Kennard et al used GIWAXS to show that the {110} and {002} families of planes are the predominant twin domain types in MAPbI 3 . [ 43 ] At room temperature, MAPbI 3 perovskite films possess ferroelastic domains that cohabitate the same crystal grain because of the phase transition that the material goes through after synthesis.…”

In Situ and Operando Characterizations of Metal Halide Perovskite and Solar Cells: Insights from Lab-Sized Devices to Upscaling Processes

“…The intensity of the Bragg peaks decreased, and peaks from PbI 2 and PbBr 2 appeared similar to the XPS outcome. Strain and crystal tilt are also suggested to occur under X-ray beam illumination. , The chemical composition is the less affected property, but still suffers damage under prolonged exposure, such as a reduced intensity in the probed area . Besides these experiments providing essential information, it is important to note that they used different sample compositions, quality, experiment conditions, X-ray energy, and beam spot.…”