Abstract
The work presented in this thesis manuscript focuses on the study of the behaviour of photovoltaic (PV) cells under thermal stress. Usually, the evaluation of the performance of PV cells is carried out under standard test conditions (STC) defined as: the cell temperature is set at 25 °C, the irradiance is set at 1,000 W/m2, and the solar spectrum corresponds to an air mass of AM 1.5. However, these temperature and illumination values are not representative of those encountered in real operating conditions. Indeed, space missions close to the sun and the hybridisation of photovoltaics with solar-thermal (PVT) or thermoelectric (PVTEG) conversion lead to operating temperatures that can exceed 100 °C. However, the photovoltaic conversion efficiency decreases with increasing temperature. Furthermore, it is found that a cell that performs best at 25 °C does not necessarily perform optimally at a much higher temperature (>100 °C). The thesis therefore proposes to provide a detailed analysis of the physics governing the behaviour of conventional gallium arsenide (GaAs) cells tested under thermal stress conditions (150–200 °C) which correspond to operating temperatures envisaged for systems hybridising PV conversion with solar-thermal and thermoelectric conversions. The first part of the thesis is dedicated to a presentation of the state of the art of solar cells that were tested at high temperatures. The second part is dedicated to the design of a test vehicle with conversion efficiency optimised at 25 °C. It provides the fabrication steps of the cell and describes the methods for characterising its performance (spectral response and current-voltage characteristics) and specific measurements (of electrical resistance related to the presence of metallic contacts, using the Hall effect, ellipsometry, and photoluminescence). The third part is dedicated to the presentation of the results of these characterisations for temperatures ranging from 150 to 200 °C. It provides an analysis of the optical (complex refractive index) and electrical (specific contact resistance, mobility, lifetime) properties deduced from the specific measurements and which will be useful in understanding the causes of the degradation of the cell performance at high temperature. The last part of the thesis is devoted to the analysis of the physics governing the behaviour of the cell tested from 150 to 200 °C. It reveals that at higher temperatures, the performance of the photovoltaic cells deviates from the theoretical (Shockley-Queisser) limit of PV conversion efficiency. In particular, a detailed assessment and analysis of the evolution of the conversion losses are provided in an original graphical representation. It is shown that the optical behaviour of the cell does not degrade at high temperatures and the impact of series resistance does not require any special grid optimisation work. The analysis of the cell behaviour reveals that the deviation from the Shockley-Queisser limit with increasing temperature is due to non-radiative recombination modelled at the interface between the emitter and the window layer. These results serve as a starting point for developing solar cells that operate optimally at high temperatures. They should subsequently enable the proposal of material and cell structure modifications to reduce photovoltaic efficiency losses at high temperatures.