Fabrication and optical characterization of ultrathin III-V transferred heterostructures for hot-carrier absorbers

A hot-carrier solar cell (HCSC) is a high-efficiency photovoltaic concept where electrons and holes are at a higher temperature than the lattice, allowing an additional thermoelectric energy conversion. There are two requirements for a HCSC: establishing a hot-carrier population and converting the temperature into extra voltage through energy-selective contacts. We focus on the generation of hot carriers, and the design of absorbers that can make this generation easier. Fundamentally, this requires to increase the power density absorbed per volume unit, so the photocarriers cannot fully thermalize (phonon bottleneck). Beyond simply increasing the light intensity, the main control knobs to favor hot carriers include reducing the thickness of the absorber, increasing its absorptivity, and reducing its bandgap. In this proceeding, we report the fabrication of structures that aim at measuring the influence of these different parameters. We justify our choices for sample structure and fabrication method from the need for high thermal conductivity, in order to prevent lattice heating. We characterize our structures in order to determine precisely the final thickness of all layers, and the absorptivity of the absorber layer. These samples are to be used for an analysis of the temperature with many variable parameters, in order to better understand the thermalization mechanisms and design better absorbers. Ultimately, our objective is to implement all solutions together in order to evidence a hot carrier population under concentrated sunlight illumination.