Interface Modification And Study On Charge Transfer Behavior Of CdS/cdSe Quantum Dots Sensitized Solar Cells

Quantum dot sensitized solar cells（QDSCs）represent the third generation of solar cell technology,which have focused on widespread concern with their low cost,simple synthesis,and broad application prospects.During the exploration process of QDSCs,Ⅱ-Ⅵ semiconductor compounds,cadmium chalcogenides（CdX,X=S,Se,Te）have the advantages of relatively small bandgap and chemical stability to capture photons in the visible and near-infrared regions.Thus,QDSCs exhibit excellent photoelectric properties.Currently,there have been numerous reports on QDSCs assembled using CdS and CdSe as sensitizers,but the efficiency of cells obtained using a single sensitizer is often unsatisfactory.Therefore,CdS/CdSe co-sensitized quantum dot system with good band alignment and photo-stability has become a hot research topic in the field of QDSCs.Although major development has taken place in the photoelectric conversion efficiency of QDSCs assembled from CdS/CdSe co-sensitized systems,there are some issues need to be resolved,including（1）the recombination of photo-generated electrons on CdS and CdSe quantum dots and the oxidation state of the electrolyte,and（2）the recombination of photo-generated electrons on CdS quantum dots and holes trapped in CdSe.These unfavorable recombination processes are key factors that limit the CdS/CdSe-based cells,and how to effectively suppress charge recombination in QDSCs has become a current research focus.Based on this,a photoanode model consisting of CdS/CdSe modified with ZnS was utilized in this study.The modified photoanodes were then assembled into a cell to thoroughly investigate the effect of ZnS on the behavior of photoinduced charge separation and transfer at the interface between the photoanode and electrolyte.The results showed that the weakly conducting ZnS leads to the holes to be trapped at the CdS/CdSe interface and triggered a series of recombination processes.To address the charge recombination problem at the CdS/CdSe interface,a CdS/CuInS₂/CdSe system was constructed to improve the photoinduced electron transfer at the CdS/CdSe interface and further suppress interface charge recombination.There are two specific parts of the research content,which are as follows:（1）In this study,CdS/CdSe photoanodes were constructed first,and then ZnS was deposited on the surface of CdS/CdSe photoanodes using the SILAR method to construct CdS/CdSe/ZnS photoanodes.The charge transfer behavior at the interface between the above photoanodes and the electrolyte were deeply analyzed.The surface photovoltage technique was used to monitor the separation,transfer and recombination behavior of photo-generated charges at the interface of CdS/CdSe photoanodes,CdS/CdSe/ZnS photoanodes,and electrolytes under conditions with and without the injection of electrolyte.The results showed that ZnS can effectively inhibit the transfer of electrons to oxidized electrolyte at the interface,but it also hinders the charge transfer between holes and reduced electrolyte,leading to severe recombination reactions at the interface of CdS/CdSe.Therefore,it is necessary to find new methods to effectively inhibit charge recombination without affecting charge separation and transfer.（2）To address the problem of charge recombination at the interface of CdS/CdSe,this study chose CuInS₂（CIS）synthesized in aqueous phase to modify the solid-solid interface of CdS/CdSe and successfully constructed CdS/CIS/CdSe photoanodes.The results showed that the intermediate CIS layer effectively passivated the charge recombination process at the CdS/CdSe interface,which in turn improved the photovoltaic performance of solar cells.The integrated results of the transient photocurrent showed that there were higher surface charges in CdS/CIS/CdSe photoanode(0.0608μC·cm^-2)compared to CdS/CdSe photoanode(0.0133μC·cm^-2).This further supporting that the addition of the passivation layer CIS effectively passivated the CdS/CdSe interface,which in turn promoted the effective separation and transport of photogenerated electrons at the CdS/CdSe interface.Finally,the photovoltaic performance of solar cells assembled with CdS/CIS/CdSe photoanodes was increased to 4.52%.