Understanding hot-carrier cooling in semiconductors through ultrafast photoluminescence spectroscopy

This perspective explores the fundamentals of hot-carrier cooling using time-resolved photoluminescence (TRPL) spectroscopy and diverse analysis methods, including full lineshape (FLS) modeling, to reliably extract hot-carrier temperature dynamics, while highlighting strategies to tune cooling rates for advanced optoelectronics. TRPL spectroscopy stands out as a uniquely powerful tool for probing hot-carrier dynamics, directly accessing radiative recombination from thermalized carrier populations to uncover intrinsic carrier temperature and its temporal evolution. Unlike absorption-based techniques, which often target non-emissive states or demand indirect modeling of state filling and bleaching, TRPL delivers a physically intuitive perspective on carrier cooling via the emitted photon energy distribution. Yet, accurate extraction of hot-carrier parameters requires rigorous spectral modeling; common approaches like peak shifts or high-energy tail fitting, despite their popularity, suffer from ambiguities tied to spectral broadening, fitting-range selection, and overlapping relaxation processes. FLS modeling represents a pivotal advance, utilizing the entire spectral profile for self-consistent retrieval of carrier temperatures and cooling rates, rooted in solid physical foundations. This methodology proves crucial for setting reliable benchmarks as interest surges in hot-carrier materials and devices-especially hot-carrier solar cells (HCSCs)-facilitating robust comparisons across materials and experiments to inform next-generation optoelectronic innovations. Here, TRPL paired with advanced modeling offers a timely framework for deciphering hot-carrier behavior. Challenges persist for viable HCSCs: hot-phonon bottlenecks (HPB) hinder cooling through delayed LO-phonon decay, but achieved lifetimes still fall short for practical device extraction. Colloidal core/shell quantum dots (II-VI and heterostructures) exhibit slower dynamics owing to phonon confinement, diminished escape pathways, and interfacial scattering, though they often miss elite optoelectronic qualities. Metal-halide perovskites shine with superior absorption, extended diffusion lengths, defect resilience, and tunable bandgaps, yet cool more rapidly. Hybrid designs, such as perovskite heterostructures or core/shell mimics, hold promise by fusing these assets-amplifying HPB effects alongside efficient charge transport-to propel HCSCs toward practical deployment.