# Photochemical Control of Perovskite Crystal Formation at Room Temperature

## TL;DR

This paper introduces a new method using UV light to control perovskite crystal formation at room temperature, enabling easier production of stable, blue-emitting perovskites.

## Contribution

The study introduces a photochemically-assisted crystallization control technique (PACCT) for perovskite fabrication at room temperature.

## Key findings

- UV light can promote the growth of 2D perovskites that are difficult to synthesize using thermal methods.
- The PACCT method enables blue emission and stable thin films with photoluminescence lasting over 1000 hours at high humidity.
- UV-induced deprotonation of organic cations is identified as a key mechanism in the crystallization process.

## Abstract

Lead halide perovskites combine outstanding optoelectronic performance with low‐cost and scalable manufacturing routes. However, their commercial success remains limited by a still‐evolving understanding of crystallization dynamics, a lack of sustainable, material‐informed processing techniques, and persistent challenges in fabricating blue‐emitting and 2D compositions with sterically‐impeded cations. Here, the interaction of light with organic–inorganic perovskite precursor solutions is uncovered by presenting a photochemically‐assisted crystallization control technique (PACCT). This low‐energy method leverages UV illumination to modulate reaction kinetics, allowing controlled crystal formation in both thin films and flexible perovskite‐polymer composites under ambient atmosphere at room temperature. The treatment's influence on the crystallization process either impedes (3D perovskites) or promotes (2D perovskites) crystal growth. In the latter case, this facilitates the growth of perovskites that can otherwise be challenging using thermal approaches, enabling a facile pathway toward blue emission and thin films with a photoluminescence stability exceeding 1000 h at 80 % relative humidity. For both reaction pathways, mechanistic descriptions are identified based on the UV‐induced deprotonation of the organic cation. Together, these results introduce a novel strategy for perovskite fabrication and provide new insights into their crystallization dynamics, offering a versatile and scalable route for advancing next‐generation optoelectronic materials with a reduced energy footprint.

The authors demonstrate how ultraviolet (UV) light can enable the crystallization of two‐dimensional (2D) perovskite crystals that are otherwise difficult to grow using conventional thermal methods. These findings are supplemented with a detailed mechanistic description for the interaction of the UV irradiation with the perovskite precursor components.