The thickness and porosity of TiO 2 mesoporous film were optimized for better distribution of quantum dots to enhance the performance of CdS/CdSe quantum dot cosensitized solar cells. The CdS and CdSe quantum dots were prepared on TiO 2 mesoporous film through a successive ion layer absorption and reaction (SILAR) method and a chemical bath deposition (CBD) method, respectively. It was found that the distribution of quantum dots was inhomogeneous from the surface to the interior of the TiO 2 film, being mainly concentrated at the upper layer of the TiO 2 film. As a result, simply increasing film thickness did not make significant contribution to improving solar cell efficiency since only a small portion of quantum dots might access the interior of the film, leading to an exposure of TiO 2 nanoparticles in electrolyte and thus reducing the electron lifetime due to increased charge recombination rate. Our study revealed that the efficiency could reach its maximum, ∼4.62%, with the TiO 2 film, the thickness of which was around 11 μm, and porosity was optimized by adding 12 wt % ethyl cellulose into the paste for making the TiO 2 film.

Light scattering is a method that has been employed in dye-sensitized solar cells for optical absorption enhancement. In conventional dye-sensitized solar cells, large TiO(2) particles with sizes comparable to the wavelength of visible light are used as scatterers by either being mixed into the nanocrystalline film to generate light scattering or forming a scattering layer on the top of the nanocrystalline film to reflect the incident light, with the aim to extend the traveling distance of incident light within the photoelectrode film. Recently, hierarchical nanostructures, for example nanocrystallite aggregates (among others), have been applied to dye-sensitized solar cells. When used to form a photoelectrode film, these hierarchical nanostructures have demonstrated a dual function: providing large specific surface area; and generating light scattering. Some other merits, such as the capability to enhance electron transport, have been also observed on the hierarchically structured photoelectrode films. Hierarchical nanostructures possessing an architecture that may provide sufficient internal surface area for dye adsorption and meanwhile may generate highly effective light scattering, make them able to create photoelectrode films with optical absorption significantly more efficient than the dispersed nanoparticles used in conventional dye-sensitized solar cells. This allows reduction of the thickness of the photoelectrode film and thus lowering of the charge recombination in dye-sensitized solar cells, making it possible to increase further the efficiency of existing dye-sensitized solar cells.

Mesoporous TiO 2 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 TiO 2 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 TiO 2 beads prepared with a two-step method, in which an optimal amount of ammonia was adopted to etch TiO 2 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 TiO 2 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.

This paper presents a systematic investigation on the effects of iodine content in the electrolyte on electron recombination, charge, and ion transportation to the power conversion performance of dye-sensitized solar cells under lower light intensities. By analyzing the current–voltage behavior and electrochemical impedance spectroscopy results, the effects of iodine content on charge recombination, ion transportation, and light intensity are found to be the major factors. The power conversion efficiency of DSSC under low light intensity can be significantly improved when electrolyte composition is optimized, which results in an exclusive application for indoor use.