# Room-Temperature Perovskite Phase Transition of CsPbI3 for PV Manufacturing on Flexible Substrates

## TL;DR

Researchers found a way to create perovskite films at room temperature using an IPA bath, enabling flexible solar devices without high heat.

## Contribution

A room-temperature phase transition method for CsPbI3 using IPA bath, avoiding high-temperature annealing.

## Key findings

- IPA bath treatment enables direct formation of optically active black-phase CsPbI3 at room temperature.
- The process involves hydrogen from HPbI3 bonding with oxygen in DMF, aiding perovskite formation.
- Films produced have small grains and pinholes due to fast nucleation and slow grain growth.

## Abstract

Printing perovskite films typically involves a high-temperature
treatment exceeding 150 °C, which limits the manufacturing of
flexible devices. All inorganic CsPbI3 perovskite is particularly
promising for commercialization due to its high thermal stability.
Herein, we discovered that when using DMF precursors containing CsI
and HPbI3 for fabricating CsPbI3 films, an isopropanol
(IPA) antisolvent bath immersion treatment of the wet films can enable
a direct and rapid formation of optically active perovskite black
phases at room temperature without annealing. In situ photoluminescence
and in situ transmission techniques were employed to monitor and characterize
the transition from the wet film to the final perovskite phase. It
can be concluded that the relatively fast nucleation and slow grain
growth during the IPA-bath treatment result in films with small grains
and pronounced pinholes on the surface. Furthermore, FTIR, Raman,
and NMR techniques were used to investigate changes in the chemical
bonds. The characterization results revealed that the hydrogen in
HPbI3 can form a chemical bond with the oxygen in DMF,
resulting in mutual attraction. As DMF is extracted by IPA, the DMF
molecule simultaneously induces the hydrogen to leave its original
position, and then free cesium easily fills the vacancy left by hydrogen,
forming the black-phase CsPbI3 perovskite. This finding
reveals the mechanism of the room-temperature phase transition of
CsPbI3 facilitated by IPA post-treatment, and it explains
why the use of HPbI3 instead of PbI2 in the
precursor solution effectively lowers the reaction energy barrier
for CsPbI3 in previous works.