Recombination in SnO2-Based Quantum Dots Sensitized Solar Cells: The Role of Surface States

Tin oxide (SnO2) is one of the most promising electron transporters to further enhance the performance of quantum dots sensitized solar cells (QDSCs). Unfortunately, the performance of SnO2-based QDSCs is still poor. It was observed that surface modification toward a SnO2 photoelectrode such as a TiCl4 treatment is crucial to dramatically increase the performance of the devices. However, the mechanism of the TiCl4 treatment remains poorly understood. Here, systematic studies on the photoelectrochemical properties of SnO2-based QDSCs were performed in order to clarify the mechanism by which the TiCl4 treatment improves the performance of solar cells. Impendence spectroscopy results reveal that the photogenerated electrons transport in the porous SnO2 network rather than the TiO2 coating. Furthermore, a physical model considering the existence of monoenergetic surface states at the SnO2 surface was used to simulate the behavior of chemical capacitance at various forward biases. The accordance between the decrease of the surface states and the recombination reduction clearly indicates that the surface states act as the recombination centers to influence the device performance, which can be well described by Marcus-Gerischer theory. These combined findings provide new understanding of the recombination mechanism of SnO2-based sensitized solar cells and guidelines for further improving the performance of this system.