Optimization of the doping process and light scattering in CdS:Mn quantum dots sensitized solar cells for the efficiency enhancement

Abstract

In this work a double-layer photoanode composed of TiO2 nanocrystals (NCs) and hollow spheres (HSs) was applied in CdS:Mn sensitized solar cells. TiO2 NCs with dominant size of 25 nm were synthesized by a facile hydrothermal method. TiO2 HSs were also prepared through the liquid phase deposition (LPD) of TiO2 on carbon spheres followed by a calcination process. The double electron transport layer of quantum dot sensitized solar cells (QDSCs) was formed of a nanocrystalline TiO2 layer covered by a light scattering HSs film. The corresponding thicknesses were also controlled to be about 10 µm and 7 µm, respectively. The sensitization of photoanode with Mn doped CdS NCs were carried out by a successive ionic layer adsorption and reaction (SILAR) technique. The apparent $$\frac{\text{Mn}}{\text{Mn}+\text{Cd}}$$ molar ratio was altered in a wide range of 0–9%. The corresponding QDSCs were fabricated and the doping process was optimized for the improved power conversion efficiencies. According to the results, the QDSC with a double-layer photoanode sensitized with $$\frac{\text{Mn}}{\text{Mn}+\text{Cd}}$$ ratio of 7.0% showed the maximum efficiency of 3.26%. This value was increased about 61% and 18% compared to those of the reference cells with single nanocrystalline and double-layer photoanodes sensitized with un-doped CdS NCs. The reason was addressed due to the higher light scattering and Mn related electron energy states within the bandgap energy of CdS NCs and the improved electron transport property of the cell. A ZnS passivating layer was also utilized as the electron blocking layer on the surface of the optimized doped photoanode with different thicknesses. It was shown that the highest energy conversion efficiency of 3.55% was achieved for three cycles of ZnS SILAR deposition.