Recombination control in high-performance quantum dot-sensitized solar cells with a novel TiO2/ZnS/CdS/ZnS heterostructure

Charge recombination occurring at the TiO2/QDs/electrolyte interface is a crucial factor that limits the power conversion efficiency (eta) of quantum dot-sensitized solar cells (QDSSCs). This paper presents a new approach by inserting a ZnS layer between the TiO2 and CdS/ZnS to prepare a TiO2/ZnS/CdS/ZnS sensitized photoelectrode for QDSSC applications. The CdS QDs and ZnS passivation layers were deposited using a reproducible and controlled successive ionic layer adsorption and reaction method. The TiO2/ZnS/CdS/ZnS based QDSSCs exhibited a power conversion efficiency (eta) value of 3.69%, which is significantly higher than the 3.02% and 2.09% observed for solar cells with a TiO2/CdS/ZnS device and without a passivation layer (TiO2/CdS), respectively. The elevated performance of the TiO2/ZnS/CdS/ZnS-based QDSSCs was attributed to the pre-assembled ZnS layer enhancing the light harvesting and acting as a blocking layer to shield the TiO2 core from the outer QDs and the electrolyte, thereby retarding the interfacial recombination of electrons from the TiO2 with the electrolyte or with the QDs. Electrochemical impedance spectroscopy and open circuit voltage decay measurements showed that the TiO2/ZnS/CdS/ZnS-based QDSSCs inhibit charge recombination remarkably at the photoanode/electrolyte interface and prolong the electron lifetime.