Abstract
The relaxation of hot-carriers (HCs), generated by high-energy spectral region of sunlight, constitutes a major loss channel for photovoltaic devices. An effective harvest of HC excess energy enables a feasible pathway to break the Shockley-Queisser limit. Nevertheless, such approach is challenged by traditional semiconductors, whose rapid energy dissipation competes with charge extraction. Lead halide perovskites (LHPs) and group III-V/II-VI semiconductor quantum dots (QDs) have recently attracted increasing attention for the slow HC cooling properties, demonstrating potential as next-generation efficient HC solar cells. However, the intricate HC relaxation mechanisms and how to manipulate the HC properties in these materials are still shrouded in the unknown.
Part of this thesis concentrates on deciphering the HC-related spectral features of bulk MAPbI2Br in transient absorption measurements. The below-bandgap photoinduced absorption is revealed to originate from hot-biexciton Coulombic interaction. Additionally, the initial red shift in the ground state bleaching is proved to be dominated by HC cooling, rather than a transient bandgap change.
Another part of this thesis deals with the intrinsically slow HC cooling properties and mechanisms in HgTe QDs. The retarded relaxation stems from the robust intrinsic and hot-phonon bottleneck effects, further enhanced by biexciton Auger recombination at high excitations. QDs capped with shorter ligands possess enhanced Auger efficiency due to tunneling-mediated interparticle excitonic coupling enabled by wavefunction penetration.
The remainder of this thesis focuses on how to engineer the HC relaxation kinetics in mixed 3D/quasi-2D CsPbI3 perovskite nanocrystals. We observed an abnormally slow carrier thermalization in 3D domain due to energy cascade transfer from quasi-2D phases. Through modulation of quasi-2D phase concentrations, we demonstrate precise control over the energy cascade manifold, achieving a 40% reduction in funneling efficiency.
The presented findings and device-oriented approaches could facilitate the development of emerging material systems for advanced energy conversion, photonic devices, and nanoelectronic applications.