Non-equilibrium dynamics, materials and structures for hot carrier solar cells: A detailed review

Since their advent around the start of the millennium, hot carrier solar cells came into the focus of a broader research community as one of the so-called third generation photovoltaic concepts. As an exciting research field enthusiastically covered by an increasing number of publications, some aspects of hot carrier solar cell research, namely data interpretation and conclusions with respect to high efficiency devices, appear to show some discrepancies. It therefore appeared timely to provide a detailed review of current hot carrier solar cell research from the fundamentals of non-equilibrium carrier dynamics to complete solar cells to enable advancement with the knowledge of solid state and semiconductor physics being fully taken into account. In our work, we discuss the hot carrier non-equilibrium dynamics right from the process of hot carrier generation, going beyond the standard 1-dimensional approach, hence exploring phononic and other dynamic limits as occurring in real materials. Thermodynamic modelling of hot carrier solar cells in the literature presented conversion efficiencies from 0.04 to 84%. This situation called for an evaluation and a comparison against the Shockley-Queisser efficiency limit. The assessment of characterisation techniques used for dynamic and steady-state detection of hot carrier populations form another part of this review, including to what extent certain data can or should be used in regards to hot carrier solar cells. With this wealth of information, we work through III-V, IV-IV, II-VI, and non-trivial materials which were proposed for hot carrier absorbers in the literature. With the physics and materials considered, we then examine energy-selective contact designs which also have to fulfil the criterion of carrier selectivity. Finally, we look at the whole hot carrier solar cell, departing from the original concept to more feasible designs and qualitatively new approaches.