Hole Trapping: The Critical Factor for Quantum Dot Sensitized Solar Cell Performance

The performance of the current quantum dot (QD) solar cells is limited by several deficiencies. One of them is the existence of surface traps, especially hole traps, which are blocking the hole injection into the electrolyte. The trapping can be efficiently suppressed by growing a shell of wider band gap material around the core dot. Optimum parameters of such a shell layer for photovoltaic applications are, however, not established. We study effects of the shell formation on the ultrafast carrier dynamics and the performance of QD-sensitized solar cells. We can disentangle electron and hole dynamics and demonstrate that the QD shell diminishes surface hole trapping. By combining the knowledge about the hole trapping and electron injection into metal oxide we can clearly correlate the electron and hole dynamics with the solar cell efficiency as a function of the shell thickness. We conclude that the optimal shell thickness is 1.3 nm for this system.