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Emerging photovoltaic concepts (hot-carrier, multi-exciton generation and intermediate-band solar cells) have so far not been able to reach their promised efficiencies. This is partly because they require power densities that are difficult to achieve, even with concentrated sunlight illumination. Besides the need for novel materials, it is therefore critical to develop strategies to maximize the power absorbed per unit volume. In this presentation, I emphasize the key role light trapping can play in this regard and discuss theoretical and practical limitations of broadband absorption enhancement. Focusing on hot-carrier solar cells, I show how thermalization can be strongly suppressed in thin, quantum layers, and the challenges in implementing such absorbers. I also suggest alternative designs that can partly alleviate the thermalization constraints.
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