Mesoporous TiO2 beads for high efficiency CdS/CdSe quantum dot co-sensitized solar cells

Mesoporous TiO2 beads with a combined effective light scattering effect and large surface area were prepared and studied for quantum dot-sensitized solar cell (QDSC) application. The photoanode films were composed of submicrometer-sized beads consisting of packed TiO2 nanocrystallites. A power conversion efficiency up to 4.05% has been achieved for a CdS/CdSe quantum dot (QD) co-sensitized solar cell, which was constructed with the mesoporous TiO2 beads prepared with a two-step method, in which an optimal amount of ammonia was adopted to etch TiO2 spheres and achieve the desired porosity of the beads for QD adsorption. The high conversion efficiency was ascribed to a combined effect of the mesoporous structure, light scattering ability and good electrical conduction capability of the beads. It has been found that larger pores can be created by adding more ammonia during the solvothermal treatment, leading to easy penetration of the QDs into the inner pores of the mesoporous beads. An excessive amount of ammonia would lead to a low specific surface area and decrease of light scattering capability of the films. Electrochemical impedance spectroscopy analysis revealed a retarded charge recombination for the mesoporous TiO2 beads treated with ammonia in view of a decreased contact area of the beads with the electrolyte, reflected in the increase of both open circuit voltage and fill factor of the solar cells.