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Abstract
The design and operating principles of the optical hot carrier solar cell (OHCSC) have been investigated in this thesis. An OHCSC integrates a hot carrier absorber and a conventional solar cell using a photonic structure called the optical contact, which enhances the carrier hot-luminescence transfer rate at the bandgap frequency of the solar cell to reduce the thermalization losses during photovoltaic conversion. Such an optically-integrated system has the physical features of both a hot carrier solar cell and a spectrum up/down converter. Therefore, it is a valuable general method to explore spectrum management for solar cells.
One goal of this study was to establish a comprehensive understanding of the device characteristics in both thermal equilibrium (detailedbalance) and non-equilibrium (relaxation-time-approximation) steady states. These demonstrate the general efficiency enhancement conditions for the optically-integrated system and particular dependencies of its operating voltage on the luminescence yield.
The device performance relies on the slow carrier cooling rate in the absorber and the large enough luminescence enhancement ratio of the optical contact, which is on the order of-1 ns and -1000 x for the up-converter while -500 ps and -100 x for the down-converter.
The second aspect was to develop a theoretical framework for the analysis of the optical contacts. Using the electromagnetic Green's functions constructed by the quasi-normal modes, the photon transfer spectrum function can quantify the coupling rate more accurately, especially for the near-field coupling. The hot-luminescence transfer has been investigated in detail using a plasmonic core-shell nanowire illustration, which shows about 4800 times transfer enhancement and an improved device performance in up-conversions.
This study also explored hot carrier absorber candidates in the lead-halide perovskite family (APbX3) using ultrafast optical characterization and first-principle calculations. A stronger phonon bottleneck effect was shown in hybrid perovskites than in their inorganic counterparts to prolong the carrier cooling period. At least a 10 times slower carrier-phonon relaxation rate was observed in
FAPbb comparing to the cesium-based inorganic system. The up-conversion of low-energy phonons has been proposed to be responsible for the bottleneck effect, which also suggests a general way for achieving long-lived hot carriers in materials.