CuS/CdS Quantum Dot Composite Sensitizer and Its Applications to Various TiO₂ Mesoporous Film-Based Solar Cell Devices

A nanoscale composite sensitizer composed of CuS and CdS quantum dots (QDs) was prepared by a simple but effective layer-by-layer reaction between a metal cation (Cu²⁺ or Cd²⁺) and a sulfide anion (S^2-). The as-prepared composite CuS/CdS QD sensitizer displayed an enhanced photon-to-current conversion over the sensitizing range of the visible spectrum compared to the counterpart of the pure CdS sensitizer. At the optimized ratio of the deposited amounts of CuS and CdS, the best CuS/CdS-sensitized mesoporous TiO₂ cell with a polysulfide electrolyte showed an overall power conversion efficiency of 3.60% with a short circuit current (J_sc) of 11.77 mA/cm², an open circuit voltage (V_oc) of 0.65 V, and a fill factor (FF) of 0.47. From the transmission electron microscopy images, the initially deposited CuS seemed to take a nucleation site to accumulate more CdS in the later deposition. The kinetic studies by impedance and V_oc decay measurements also revealed that the CuS/CdS and CdS QD sensitizers made a similar interface between TiO₂ and the electrolyte, but the former had a larger resistance of charge transfer with a longer lifetime of excitons after light absorption than the latter. To enhance the sensitizing power further, a multilayer QD sensitizer of CuS/CdS/CdSe was prepared by successive ionic layer adsorption and reaction (SILAR). This led to the best performance of 4.32% overall power conversion efficiency. Finally, a hybrid sensitizing system of inorganic QD (CuS/CdS) and organic dye (coded MK-2) was tested with a [Co(bpy)₃]^2+/3+ redox mediator. The CuS/CdS/MK-2 dye-sensitized cell showed over 3.0% efficiency under the standard illumination condition (1 sun).