Nanostructured materials and their charge transport properties in photoanodes of dye sensitized solar cells

Since the big progress of dye sensitized solar cells (DSCs) by adopting TiO2 nanoparticles for a photoanode in 1991, DSCs have been intensively studied as an alternative to conventional Si-based solar cells. As a main component of DSCs, a photoanode composed of a nanostructured semiconducting oxide network plays a significant role in determining performances of DSCs in terms of light harvesting efficiency (LHE) and charge collection efficiency related to charge transport and recombination. Nanomaterials with various morphologies, such as particles, rods and tubes have been fabricated and investigated to improve performances of DSCs. Among them, submicrometer-sized aggregates of nanocrystallites have demonstrated to be promising as a photoanode of DSCs for higher power conversion efficiency. Such hierarchical structures make it possible to have both high specific surface area for dye molecule adsorption and internal light scattering within the photoanode, leading to a much enhanced LHE. This work focused on the surface modification and charge transport characterization of such hierarchically structured photoanodes. First, a core-shell configuration was fabricated by atomic layer deposition (ALD) process, which was achieved by depositing ultrathin TiO2 layer on inner surface of ZnO aggregate film in which the TiO2 shell was anticipated to act as a chemical and energy barrier. Although the ALD-TiO2 coating failed to improve chemical stability of the ZnO aggregate against to an acidic dye solution due to the ultrathin thickness (< 1 nm), the ALD-TiO2 shell layer effectively suppressed charge recombination at the interface. As a result of the reduced charge recombination, Voc, and FF of DSCs were increased, leading to 20 % enhancement of power conversion efficiency. Second, effects of annealing temperatures on ALD-TiO2 coated aggregates of ZnO nanocrystallites were investigated in terms of sintering behavior and charge transport. 350 °C as the maximum temperature was typically used to preserve the specific surface area of ZnO aggregates. Nitrogen sorption analyses revealed that the ALD-TiO2 layer improved thermal stability of ZnO aggregates at high temperature, ALD-TiO2 coated ZnO aggregates retained the same specific surface area even annealed at 450 °C. The higher annealing temperature resulted in an improved crystallinity, resulting in the highest charge transfer resistance when annealed at 450 °C. As a result, the DSC with photoanode made of ALD-TiO2 coated ZnO aggregates annealed at 450 °C showed the highest Voc and FF, with a little reduced Jsc and thus, the highest power conversion efficiency. Third, charge transport properties such as electron lifetime, chemical diffusion coefficient and diffusion length of ZnO nanorod aggregates were investigated. Electrochemical impedance spectroscopy (EIS) was used to characterize charge transport properties, and it was found that increased crystal sizes and widened necks with higher annealing temperature reduced charge diffusion resistance (Rd), and increased diffusion length from 50 microm to 140 microm. As a result, the power conversion efficiency increased 25 %. Lastly, TiO2 nanoparticles were added into TiO2 aggregates with different ratios; 10 wt% and 20 wt% nanoparticle. As a result, diffusion resistance was found to be reduced and the corresponding diffusion length was increased by filling the bottlenecks between adjacent aggregates with nanoparticles, while additional reduction of diffusion resistance was not observed when the amount of the added TiO 2 nanoparticles increased from 10 to 20 wt%, indicating that there is a saturation point for charge transport. Even though there was no significant impact of the added nanoparticles on electron lifetime despite of the increase of surface area, admixing TiO2 aggregates with TiO2 nanoparticles improved Jsc and eventually leaded to the enhanced efficiency by 30 % as a result of the improved diffusion length and increased surface area.