Improvement photovoltaic performance of quantum dot-sensitized solar cells using deposition of metal-doped ZnS passivation layer on the TiO2 photoanode

Abstract

The thin film of metal ions (Mn2+ and Cu2+) doped‑zinc sulfide (ZnS) as a passivation layer have been constructed on the TiO2 layer using the successive ionic layer adsorption and reaction (SILAR) method in quantum dot-sensitized solar cell (QDSSC). In the QDSSCs with Cu2S counter electrodes (CE), by doping photoanodes with Mn2+- and Cu2+-doped ZnS, about 40% and 22% improvements in efficiency have been observed compared to the cell based on the bare TiO2 film. Also, these photoanodes have been applied with CoS CE and their photovoltaic performance have been investigated. For both of the CEs, by incorporating Mn2+ ions in ZnS layer further increase have been observed in photocurrent compared to the cell based on the Cu2+-doped-ZnS. Electrochemical impedance spectroscopy (EIS) shows that ZnS layer decreases the recombination of electrons from the conduction band of TiO2 to the quantum dots or electrolyte, which is a beneficial effect on the photocurrent. The higher enhancement in photocurrent is attributed to the midgap states that the metal ions can create in the host material. The midgap states facilitate the charge transfer at the electrolyte/QD interface. By comparing the SEM images of the photoanods, it is found that the surface morphology of Mn2+ and Cu2+-doped ZnS photoanodes are differ from bare TiO2. The results suggest that the metal ions-doped-ZnS layer can be as an efficient passivation layer in the TiO2 photoanode. The above results prove that this optimization of the photoanode is as a simple method for enhancing photocurrent and power conversion efficiency in the QDSSCs.