Enhancing the Performance of Dye-Sensitized Solar Cells by Nanoscale Interface and Structure Control

Abstract

Dye-sensitized solar cells (DSSCs) have become one of the most promising candidates for next generation of photovoltaic cells, due to low-cost process, flexibility, and ease of changing colors of cells. Despite their emergence with a relatively high efficiency of ~8%, the efficiency is still remained early ten percent over twenty years. To improve the performance of the DSSC, all components of the cell should be extensively investigated. The dye should be developed to have the smaller bandgap to increase the light absorption in visible region. The semiconductor should reduce the recombination loss at a metal oxide/electrolyte interface and be able to utilize a scattering effect efficiently. Lastly, the electrolyte should have lower redox potential to reduce a voltage loss and be non-volatile for long-term use. In chapter 1, the current state and recent improvements of three major components (dye, metal oxide, and electrolyte) are demonstrated. Especially, achievements in developing metal oxides are discussed in detail to emphasize the importance of nanoscale interface and structure control, which are main objective of this research. In chapter 2, the effect of TiO2-coating on the ZnO-based DSSCs performance is systematically investigated with various TiO2-coating thicknesses. The TiO2 layer plays an important role in the suppression of the formation of Zn2+/dye complexes and the recombination at ZnO/dye/electrolyte interfaces. As a result, both the open-circuit voltage (Voc) and short-circuit current (Jsc) are improved, and the power-conversion efficiency is consequently enhanced by a factor of three. In chapter 3, the scattering layer is optimized by varying mixture ratios of nanoporous ZnO spheres (NS) and ZnO nanoparticles (NP). Nanoporous ZnO spheres in the scattering layer provide not only the effective light scattering but also a large surface area. Furthermore, added ZnO nanoparticles to the scattering layer facilitate the charge transport and increase the surface area by filling-up large voids of nanoporous ZnO sphere composite. The bilayer structure using the optimized scattering layer improves the solar cell efficiency by enhancing both the short-circuit current (Jsc) and fill factor (FF), compared to the non-mixed scattering layer. In chapter 4, expected synergy effects of topic 1 (TiO2 coating) and topic 2 (mixed scattering layer) are theoretically demonstrated. When the optimized nanostructure is coated with TiO2, the open-circuit voltage (Voc) higher than that of topic 2 (~0.56 V) is expected from the increased photo-generated current (Jph) and reduced recombination rate (krec). The fill factor (FF) is expected to be enhanced more than that of topic 1 (~60%) due to the decreased series resistance (Rs) by improved electron transport. The short-circuit current (Jsc) is considered to be most enhanced, because all factors relevant to Jsc (light harvesting (ηLHE), electron injection (ηinj), and charge collection (ηcc) efficiency) are improved by adding nanoparticles and TiO2-coating layer. Therefore, it is concluded that the collaboration of two topics (TiO2 coating on optimized nanostructure) is essential for highly efficient DSSCs.