Effect of ZnS coatings on the enhancement of the photovoltaic properties of PbS quantum dot-sensitized solar cells

We have studied the effect of ZnS coatings on the photovoltaic properties of PbS quantum dot-sensitized solar cells (QDSSCs). PbS QDs are suitable materials for QDSSCs because of their wide tight absorption range; however, their high photochemical reactivity is a problem that needs to be overcome for solar cell applications. In this paper, a ZnS passivation layer was applied to PbS QDs adsorbed onto electrodes using a successive ionic layer adsorption and reaction (SILAR) method for different numbers of cycles in both aqueous and methanol solutions. It was demonstrated that, as a consequence of the low viscosity of methanol, the performance of the PbS QDSSCs was greatly enhanced when a methanol, rather than aqueous, solution was used in the SILAR method. The optical absorption properties of PbS QDs adsorbed on TiO2 electrodes with or without ZnS surface passivation were studied. An increase in size of the PbS QDs after coating with ZnS was confirmed by a red shift in the optical absorption spectra measured using photoacoustic spectroscopy. The photovoltaic properties of a sandwich structure of PbS QDSSCs were studied under "one sun" illumination using a polysulfide (S/S2-) redox system as the electrolyte and Cu2S on brass as the counter electrode. The short-circuit current density dramatically increased from 0.50 mA/cm(2) to 11.2 mA/cm(2) after the electrodes were modified with the ZnS coating. The maximum conversion efficiency that was achieved for PbS QDSSCs with a ZnS coating under one sun illumination was 1.92%. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4720468]