High-Throughput Automated Exploration of Phase Growth Kinetics in Quasi-2D Formamidinium Metal Halide Perovskites

Abstract

Quasi-two-dimensional (2D) metal halide perovskites (MHPs) are an emerging material platform for sustainable functional optoelectronics, however the uncontrollable, broad phase distribution remains a critical challenge for applications. This is because the basic principles for controlling multiple phases in quasi-2D MHPs remain poorly understood, due to the rapid crystallization kinetics during the conventional thin-film fabrication process. Herein, a high-throughput automated synthesis-characterization-analysis workflow is implemented to accelerate material exploration in formamidinium (FA)-based quasi-2D MHP compositional space, revealing the early-stage phase growth kinetics which fundamentally determines the phase distributions. We observe that the prominent n=2 2D phase restricts the growth kinetics of 3D-like phases – α-FAPbI3 MHPs where the spacers are coordinated to the surface. δ-FAPbI3 phase further inhibits the growth of 3D-like phases. It is found that thermal annealing is a critical step for proper phase growth, although it can lead to the emergence of unwanted local PbI2 crystallites. Additionally, fundamental insights into the precursor chemistry associated with spacer-solvent interaction that determines the morphologies and microstructures of quasi-2D MHP films are demonstrated. Our high-throughput study provides comprehensive insights into the fundamental principles in quasi-2D MHP phase control and enables new control of the functionalities of complex materials systems for sustainable device applications.

Accelerating materials discovery by high-throughput GIWAXS characterization of quasi-2D formamidinium metal halide perovskites

“…19,[21][22][23][24][25][26] Among them, the most intuitive determinant controlling the final phase distributions of quasi-2D MHPs is the ratio of 2D:3D compositions. 20 Nevertheless, there is not a united, generalized principles on how to design the quasi-2D MHP system with an optimal 2D:3D ratio for high-performance MHP optoelectronics, even though many publications have already demonstrated excellent device performances. In fact, the exploration of the optimal 2D:3D concentration of MHP from the massive quasi-2D compositional space through the classical batch-type experiments requires intensive efforts at the expense of tremendous time and energy.…”

“…16,17 In some specific cases, the phase heterogeneity in quasi-2D MHPs can be evolved along the vertical direction of the films -3D-rich at the top, 2D-rich at the bottom of the MHP films, 18,19 attributed to the imbalanced crystallization kinetics of 3D and 2D MHP components. 20 In fact, the crystallization of quasi-2D MHPsa chemically complex materials systemis largely dependent on multiple physico-chemical factors. 19,[21][22][23][24][25][26] Particularly, the ratio of 2D:3D compositions in the MHP precursors primarily influences the final phase constitution of the quasi-2D MHP films.…”

“…19,[21][22][23][24][25][26] Particularly, the ratio of 2D:3D compositions in the MHP precursors primarily influences the final phase constitution of the quasi-2D MHP films. 20 Notwithstanding, so far, there is no consensus or fundamental guidelines for designing an optimal 2D:3D composition ratio within a wide range of the MHP compositional space demonstrating best optoelectronic performances. In fact, most publications demonstrating high-performance quasi-2D MHP optoelectronics have utilized various 2D:3D composition ratios, each of which strongly stems from the preliminary optimization process of each research group, but with a handful of basis sets.…”

“…Herein, we implement a high-throughput automated synthesis-characterization workflow to systematically explore the phase heterogeneity of quasi-2D MHPs as a function of 2D:3D compositional ratios. 20,30 By using a robotic pipetting platform, an array of 96 quasi-2D MHP filmss are quickly fabricated, and the phase distributions and their time-evolutions in each film along the vertical directions and compositional changesare autonomously characterized via photoluminescence (PL) spectroscopy. A machine learning (ML)-based analysis effectively reveals the compositional and geometric constitutions of the associated phases in the system from the massive PL dataset, the amount of which is impossible for human to manually analyze.…”

“…We further explore the evolutions of crystallographic structures and orientations of the MHP phases in the quasi-2D compositional space via high-throughput grazing-incidence wide-angle X-ray scattering (GIWAXS) experiments, allowing comprehensive structural understandings of the functional materials system. Here, an automated peak analysis function is implemented for analyzing 285 individual GIWAXS patterns, 20 not only accelerating the overall materials discovery sequence but also uncovering the global trends of phase constitutions, vertical distributions and, orientations in the complex system. Our analysis revealed that vertically aligned α-FAPbI3 MHPs emerge at the compositional range exactly where the PL of the stable 3D-like phases is observed.…”