Hot carrier solar cells: Controlling thermalization in ultra thin devices

Summary form only given. From present day 40% conversion efficiencies to the thermodynamic limit (>; 85%), there is still a lot of room for improvement. Hot carrier solar cells provide an attractive solution to fill this gap, by converting more efficiently the part of the incident power that is usually lost as heat. In those devices, the photogenerated carriers are not thermally equilibrated with the lattice. This occurs if the carrier thermalization pathways are saturated, either by reducing the electron-lattice interaction, or by increasing the carrier density. Antimony-based quantum well structures with 50 nm thick active material were synthesized for investigating their thermalization properties. The carrier temperature is determined as a function of the incident power density. Results indicate potential efficiency above 50% provided the incident power can be absorbed in a 50 nm thick absorber. Without specific care, reducing the absorber thickness would result in a reduced absorption and limited efficiency. Here, we propose a nanoscale structuration of the cell surface that enables strong absorption enhancement. 70 to 80 % of the incident power can be absorbed in a 50 nm thick GaSb layer. Going for high carrier density enables to lower the requirement on the cooling rate reduction. We show that using the structure described here and the thermalization rate measured on test samples, the potential efficiency is above 50%.