Hybrid organic–inorganic ternary copper halides have emerged as promising lead-free
semiconductors for optoelectronic applications, particularly light-emitting devices and
scintillators. Their excellent environmental stability, high photoluminescence quantum yields
(PLQYs), large Stokes shifts, and negligible self-absorption are closely associated with self-
trapped exciton (STE) emission. Recent studies have demonstrated the potential of copper
halide thin films for charged-particle detection [1,2] and highlighted compositional and
structural engineering to tailor their optical properties [3]. Within this material family,
guanidinium copper iodide ((Gua)₃Cu₂I₅) offers an intriguing platform for investigating the
interplay between chemical composition, crystal structure, lattice dynamics, and excited-state
relaxation.
In this work, we investigate the structural, compositional, and photophysical properties of
guanidinium copper iodide thin films fabricated using a scalable spray-coating technique. By
controlling the synthesis conditions, two distinct crystalline phases can be obtained,
exhibiting markedly different emission characteristics: a blue-emitting α-phase and a yellow-
emitting β-phase, both displaying high PLQYs. A comparative investigation of their elemental
compositions, crystal structures, and morphologies provides insight into the origins of their
distinct optical responses and the possible contributions of compositional and structural
variations to exciton self-trapping.
The photophysical properties of both phases are systematically characterized using steady-
state, time-resolved, and temperature-dependent photoluminescence spectroscopy.
Particular attention is devoted to the temperature evolution of emission intensity, spectral
position, linewidth, and excited-state lifetimes to investigate thermal quenching and
exciton–phonon interactions. These investigations are complemented by cryogenic
temperature-dependent Raman spectroscopy, which provides insight into lattice dynamics
and possible structural transformations. Finally, their radioluminescent responses under
alpha-particle irradiation are investigated to assess their potential for scintillation
applications.
By combining compositional and structural characterization with temperature-dependent
optical spectroscopy, this study explores the fundamental factors governing STE emission in
guanidinium copper iodide. The results highlight the importance of understanding the
coupled influence of chemical composition, crystal structure, and lattice dynamics when
engineering the emission properties of hybrid copper halides for lead-free scintillation
applications.