Doubling the power conversion efficiency in CdS/CdSe quantum dot sensitized solar cells with a ZnSe passivation layer

The surface passivation layer in quantum dot sensitized solar cells (QDSSCs) plays a very important role in preventing surface charge recombination and, thus, improving the power conversion efficiency. The present study demonstrated the introduction of a ZnSe passivation layer prepared with a successive ionic layer absorption and reaction (SILAR) method in CdS/CdSe co-sensitized solar cells, though not likely in the ideal form of a conformal overlayer, have significantly enhanced the power conversion efficiency, which was found to be far more efficient than the most widely used ZnS passivation layer. Not only can the ZnSe passivation layer reduce surface charge recombination, but can also enhance the light harvesting. The short-circuit current density, open-circuit voltage, fill factor, and the corresponding photovoltaic conversion efficiency were all significantly improved with the introduction of a ZnSe passivation layer but varied appreciably with the layer thickness. When three SILAR cycle layer was applied, the power conversion efficiency is as high as 6.4%, which is almost doubled the efficiency of 3.4% for the solar cell without ZnSe passivation layer. For the comparison, the CdS/CdSe co-sensitized solar cells with optimum ZnS passivation layer was also fabricated, which generated a power conversion efficiency of 4.9%, much lower than 6.4% of ZnSe passivated QDSSCs. This work demonstrated that ZnSe would be a good alternative to ZnS as a passivation material.