Dye-sensitized solar cells with high efficiency: from interface control to solidification

Some items to increase photovoltaic performances of dye sensitized solar cells (DSC) are reported. Focus is put on the fabrication of the electron collection path and ionic path. In order to increase the surface coverage of TiO2 nano-particles with dye molecules, the dye adsorption on TiO2 layers was carried out under a pressurized CO2 atmosphere (CO2 process). The CO2 process promoted the dye adsorption and shortened the dye adsorption time to 1/10 - 1/100. In addition, solar cells prepared by the CO2 process had higher Voc and Jsc than those prepared by the conventional dipping process. The increase in the photovoltaic performance was explained by the large electron diffusion coefficient in TiO2 layers and by longer electron life time in TiO2 layers. Thermally stimulated current measurement (TSC) implied that the surface electron trap on the TiO2 nano-particle was passivated by the sufficient dye adsorptions on the TiO2 surfaces. In addition, it was found that the dye aggregation was prevented by the CO2 process, which increased the photovoltaic performances. Hybrid dye sensitized solar cells having two-dye-layer-structures were fabricated by the CO2 process for the first time in order to absorb the light having wide range of wavelength. In addition, three-dimensional W electrodes were fabricated on the thick TiO2 layer in order to collect electrons in the TiO2 layer effectively. Solid type DSCs are reported. High performance quasi-solid DSCs were fabricated by preparing ionic paths in the quasi-solid electrolyte layers. The ionic path was fabricated by the surface modification of the straight nano-pore walls in a porous Al2O3 membrane, where, I-/I3 - ion species were concentrated and were expected to diffuse by Grotthuss mechanism.