Multilayered semiconductor (CdS/CdSe/ZnS)-sensitized TiO₂ mesoporous solar cells: All prepared by successive ionic layer adsorption and reaction processes

A model semiconductor-sensitizer layer of CdSe with under- or overlayers of CdS or ZnS by pre- or postadsorption was prepared on the surface of mesoporous TiO₂ films by a series of successive ionic layer adsorption and reaction (SILAR) processes in solutions containing corresponding cations and anions. The growth of each semiconductor layer was monitored by taking UV-visible absorption spectra and high-resolution transmission electron microscopy (TEM) images. The all SILAR-prepared multicomponent sensitizer consisting of CdS/CdSe/ZnS layers was evaluated in a polysulfide electrolyte solution as a redox mediator in regenerative photoelectrochemical cells. The CdS and ZnS layers with the CdSe layer sandwiched in between were found to significantly enhance photocurrents. The best photovoltaic performance was obtained from the CdS/CdSe/ZnS-sensitizer with the ZnS layer on the top, yielding an overall power conversion efficiency of 3.44% with a mask around the active film and 3.90% with no mask. The effect of the mask on short-circuit current (J_sc) and overall efficiency (η) measurements was shown to be increasingly critical in semiconductor-sensitized solar cells as they exhibit high photocurrents. The polysulfide electrolyte, which acted as an effective electron transfer mediator for CdS and/or CdSe sensitizers, was not as effective for PbS-based sensitizers prepared by the same SILAR process.