Photoexcited carrier dynamics of double-layered CdS/CdSe quantum dot sensitized solar cells measured by heterodyne transient grating and transient absorption methods.

The charge dynamics in the double-layered quantum dot sensitized solar cell (QDSSC) was studied to clarify the reason why the cell performance was much improved by a double-layer coating, by using the heterodyne transient grating (HD-TG) and transient absorption methods, based on a previous study for a conventional QDSSC (N. Maeda et al., Phys. Chem. Chem. Phys., 2013, 15, 11006.) In the double-layered QDSSC, the layer order of CdS and CdSe affected the cell performance. When CdS is in between TiO2 and CdSe, the conversion efficiency was enhanced by 70%, while it was lowered by 50% in the opposite order. From the information on charge dynamics, it was found that electrons were efficiently injected to TiO2 by appropriate band alignment of CdS and CdSe, while only a part of the electrons were transferred to the TiO2 when the layer order was opposite. Furthermore, the reverse electron transfer does not matter for the conversion efficiency, because the process increased even for the appropriate layer order.