Design and characterization of photoelectrodes for high efficiency of dye-sensitized solar cells

Abstract

Since O’Regan and $Gr\ddot{a}tzel$ firstly reported dye-sensitized solar cells (DSSCs) in 1991, they have attracted considerable interest within the solar cell community as potential candidates to replace conventional Si-based solar cells in specialized applications due to their low production cost, easy fabrication, and flexibility. Generally, a DSSC has a sandwich type structure, composed of a transparent conducting substrate, photoelectrode, dye molecules, a redox electrolyte, and counter electrode. Accordingly, the interconnection between these components as well as the property of each component is important factor to determine the overall performance of DSSCs. In particularly, the photoelectrode, which plays a role to provide the sites for dye-loading and transfer the photo-injected electrons from the dye to external load, is significant part. The photoelectrode commonly has a mesoporous structure composed of the semiconductor oxide nanoparticles, which has an advantage of a high internal surface for adsorbing a large amount of dye. However, this mesoporous structure has some limiting factors that inhibit to realize the high performance of DSSCs. First one is the formation of interface between the transparent conducting substrate and electrolyte. It has been reported that the charge recombination occurs predominantly between the transparent conducting substrate and electrolyte. Second one is a relatively poor charge transport process. Many research results indicated that the electron transport in the mesoporous photoelectrode proceeds by a trap-limited diffusion process. This means that the photo-injected electrons repeatedly interact with a distribution of traps as they undertake a random walk through the photoelectrode, and their collection can be reduced. The last one is a negligible light harvesting efficiency (LHE), which is caused by a high transparency of mesoporous photoelectrode composed of the nanoparticles with size of about 20 nm. Thus, the d...