Co-application of TiO2 nanoparticles and randomly directed TiO2 nanorods in the photoelectrode of the CdS:Mn quantum dots sensitized solar cells and optimization of the doping for the efficiency improvement

In this research double-layer photoelectrodes including TiO2 nanocrystals (NCs) and nanorods (NRs) were fabricated and applied in quantum dot sensitized solar cells (QDSCs). TiO2 mesoporous sub-layer was formed of hydrothermally grown TiO2 NCs with dominant size of 25 nm. The randomly directed inclined TiO2 NRs were also grown on the surface of the mesoporous layer through a second hydrotermal process. These could simultaneously create high surface area, enhanced light scattering and better electron transport for the double-layer photoelectrode. Mn doped CdS NCs were applied for light sensitization of the photoanode with apparent Mn/Mn + Cd molar ratio of 0-9%. Different photoelectrodes were fabricated and effect of Mn doping on sensitization and corresponding photovoltaic performance was investigated. According to the results, the optimized photoanode with Mn/Mn + Cd molar ratio of 7% represented the maximum power conversion efficiency of 3.04%. This efficiency was increased about 31% compared to the reference cell which was sensitized with undoped CdS NCs. The reason was attributed to the tunable mid-gap electron energy states created by Mn doping inside the band-gap energy of the CdS NCs and better electron transfer inside the cell.