Dye-sensitized solar cells constructed with mesoporous TiO2 beads

Working electrodes or photoanodes of dye-sensitized solar cells (DSCs) play a vital role in dye loading, light harvesting, electron transport and thus the overall photovoltaic performance. With high surface area, superior light scattering and controllable pore sizes and diameters, mesoporous TiO2 beads have shown promising applications in DSCs and were employed as the major material for the studies in this thesis. Due to the unique hierarchical structure of TiO2 beads, a 'building block' concept has been developed by Cheng's group. These 'building blocks' could be pretreated and thus enhance light harvesting and electron transport properties. In the first part of this work, TiO2 beads were subject to a pre-sintering process to enhance inter-particle contacts within each bead and photoanodes constructed with the beads were compressed in a cold isostatic press (CIP) to enhance the contacts between beads in the films. Detailed studies were carried out to investigate the effect of presintering temperature and CIP pressure on properties such as bead morphology, diffuse reflectance, electron transport, and thus the photovoltaic performance of flexible DSCs. The 100 MPa pressed film made of 650°C pre-sintered beads was found to achieve the highest conversion efficiency (6.7 %), attributing to strong interparticle and inter-bead connections. Secondly, a novel pre-sensitizing technique was designed to prepare both bilayered and mix type photoanodes on flexible substrates using TiO2 beads. The pre-sensitizing technique was first time developed to prepare multilayered photoanodes for flexible DSCs in order to enhance the light harvesting and thus the photovoltaic performance. An overall energy conversion efficiency of 6.1 % was achieved with multilayered photoanodes consisting of a N719 sensitized bottom layer and a N749 pre-sensitized top layer. Besides, the effect of bead pore sizes and diameters on the diffusion of cobalt-complexes redox couple and the photovoltaic performance of glass based DSCs were investigated. The mass transport of the cobalt redox electrolyte was greatly improved by increasing pore size of the beads and an overall conversion efficiency of 8.5 % was achieved. In the final chapter, a simple compression process was employed in the preparation of photoanodes on glass. Enhancements in device performance were observed with compressed photoanodes using both P25 and highly crystalline TiO2 aggregates, which was mainly because of densely packed particles with well-developed interparticle connections that allow faster charge transports. Overall, an efficiency of 9.5 % was achieved for the device with compressed photoanodes using TiO2 aggregates.