Exciton localization and trap-assisted recombination govern wavelength-selective photodetection in lead-free...

Layered two-dimensional halide double perovskites are often considered promising lead-free semiconductors because of their low bandgaps and strong optical absorption. Despite these properties, their use in high-performance broadband optoelectronic devices has remained limited, suggesting that absorption alone does not determine device efficiency. In this work, the photophysical behaviour and photodetector response of Cs₄CuSb₂Cl₁₂ (CCSC) nanocrystals are examined to identify the factors that restrict broadband operation. Temperature-dependent and time-resolved photoluminescence (PL and TRPL) measurements show strong coupling between electronic excitations and lattice vibrations, leading to recombination through high-energy surface trap states. Femtosecond transient absorption measurements reveal that photoexcited carriers relax rapidly into nonradiative deep trapped configurations, explaining why efficient charge extraction is difficult even under strong illumination. These effects indicate that exciton localization and fast carrier trapping dominate the photoresponse. Photodetectors fabricated with and without a CuI hole-transport layer exhibit distinct electrical behavior, including voltage offsets associated with ionic motion and interfacial charge accumulation. The inclusion of CuI reduces leakage currents and improves charge selectivity, resulting in enhanced photocurrent, responsivity, and detectivity at 365 nm illumination, along with stable operation over extended time periods (7200 seconds). Measurements at 365 nm and under broadband illumination further show that a strong photocurrent response occurs primarily under high-energy excitation, whereas photons absorbed at lower energies contribute weakly to charge collection. Overall, this study explains why CCSC, despite its favorable bandgap and absorption, shows limited broadband device performance and highlights the role of high-energy excitations, fast trapping, and ionic effects in governing its optoelectronic response.