M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

5 4 H I G H L I G H T S 2 0 2 5 I

X-ray spectroscopy elucidates the stabilisation of higher-symmetry perovskites

• Inorganic perovskites such as caesium lead iodide are promising for high-efficiency optoelectronic devices, but their practical use is limited by structural phase instability and thin- film defects. • X-ray scattering and spectroscopy, and hard X-ray photoelectron spectroscopy at beamline BM25 were used to investigate how controlled dimethylammonium iodide doping modifies the structure, bonding, and symmetry of caesium lead iodide thin films. • The results reveal how dimethylammonium iodide incorporation reduces lattice distortion and stabilises a higher-symmetry room-temperature phase, offering a route to more durable and efficient perovskite optoelectronic devices.

The challenge

Inorganic metal halide perovskites have emerged as leading candidates for next-generation photovoltaic and photodetector applications due to their chemical

stability, high thermal tolerance (~370°C), cost- effective solution processability, and rapidly improving photoconversion efficiencies [1,2]. Among them, caesium lead iodide (CsPbI3) is especially attractive because its fully inorganic composition confers excellent thermal and chemical stability. Despite these advantages, its real-world deployment remains limited by a fundamental structural challenge: the photoactive black-phase polymorphs (α, β, γ; ~1.7 eV) are only stable at elevated temperatures and rapidly transform under ambient conditions into the yellow, non-perovskite d-phase (~2.82 eV), which is optically inactive. This phase instability, together with defects arising during thin-film formation, continues to restrict the durability and efficiency of CsPbI3-based devices.

Several strategies have been explored to stabilise the black phase, including mixed-dimensional 0D/3D heterostructures, halide substitution, and incorporation of dimethylammonium iodide (DMAI). Among these, DMAI has shown the greatest promise for simultaneously improving stability and optoelectronic performance, although its exact mechanism of action remained unclear. Reports have alternately proposed that DMA+ may either substitute Cs+ or act as a surface additive, with outcomes depending strongly on DMAI concentration, annealing

Fig. 41: In situ GIWAXS time-temperature (t-T) profiles (qx,y,z) showing the high-

temperature yellow-to- black phase transition

and subsequent thermal cycling under

different atmospheres. a) CsPbI3 without DMAI; (b,c) DMAI-

CsPbI3 thin films; (d,e) γ- and γ*-phases of

CsPbI3 and DMAI- CsPbI3, respectively;

(f) Thermal phase relationships of CsPbI3

thin films with and without DMAI and the stability of γ- and γ*-

phases under open air.