Slow Hot Carrier Cooling and Quasi-Fermi Shift Dynamics in Few-Layer α-In₂Se₃ via Transient Absorption Spectroscopy

The slow cooling of hot carriers (HCs) is essential for realizing HC-based solar cells, transistors, and photodetectors, as it enhances the extraction efficiency of HCs’ excess energy. Recently, perovskites have gathered attention due to their long HC cooling times, but face challenges such as toxicity and low air stability. Here, ultrafast transient absorption (TA) spectroscopy is utilized to investigate HC dynamics in few-layer α-In₂Se₃, a nontoxic and air-stable 2D material with high carrier mobility. The lineshape analysis of TA spectra reveals the dynamics of HC temperature, density, quasi-Fermi energy, and bandgap, as well as their complex interplay. Notably, a slow carrier cooling time of ≈35 ps is observed, which is significantly longer than those of typical 2D materials and comparable to perovskites. This extended HC cooling, with the observed slow decay of the HC quasi-Fermi energy, provides significant advantages for HC devices. Furthermore, the key dynamic processes such as many-body interactions, bandgap renormalization, and lattice heating, are resolved from which critical parameters impacting HC device performance, including the defect-assisted Auger coefficient and interfacial thermal conductance, are extracted. This study not only highlights the potential of α-In₂Se₃ for HC applications but also provides a comprehensive understanding of its ultrafast photoresponse.