Engineering Carrier Thermalization, Relaxation, and Funneling in Mixed 3D/Quasi-2D CsPbI₃ Perovskite Nanocrystals

Harvesting the excess energy from hot carriers (HCs) represents a viable pathway to surpass the Shockley-Queisser limit in photovoltaic devices. However, such an approach faces challenges in bulk materials, where rapid energy dissipation competes with charge extraction. Promisingly, low-dimensional nanostructures, and in particular quasi-2D metal halide perovskite phases, can prolong HC cooling assisted by cascade energy transfer, whereas the physical mechanisms remain largely unknown. Here, we engineer HC thermalization, relaxation, and funneling dynamics in mixed 3D/quasi-2D CsPbI₃ nanocrystals. We found a slow carrier thermalization of up to 0.9 ps in these materials due to the cascade energy transfer from the quasi-2D component. Both hot-phonon and funneling bottleneck effects augment the thermalization and hinder the energy cascade at higher carrier densities. Moreover, we tailor the energy cascade manifold by tuning the amount of the quasi-2D component, further retarding the funneling efficiency by ∼40% and thus preserving the excess energy from dissipation. This study reveals the intricate role of carrier funneling in HC relaxation kinetics, underscoring the prospect of low-dimensional perovskites for next-generation solar cell development.