Preparation And Electrochemical Performances Of Counter Electrodes For QDSSCs

Quantum dot sensitized solar cells（QDSSCs）have attracted much attention because of their high theoretical efficiency,simple preparation process and low cost.QDSSCs are mainly composed of a photoanode,an electrolyte and a counter electrode,named as"sandwich"structure.As an important part of QDSSCs,the counter electrode plays a key role in the catalytic reduction reaction and charge transport of solar cells.In this work,the promoting mechanisms of the electrode on charge collection and charge transfer process were investigated.Firstly,CH₄N₂S was used as nitrogen and sulfur source,and the N/S co-doped graphene counter electrodes were prepared by hydrothermal method.The effects of doping amount and hydrothermal temperature on the electrochemical properties of graphene were investigated by XRD,SEM,XPS and electrochemical analyses.The results showed that N/S co-doping helped to form a 3D mesh-like sheet structure,which can effectively enhance the wettability of the electrode in the electrolyte.N and S atoms were embedded in the sheet structure to make the surface spin state and charge density of the graphene asymmetric,and the surrounding carbon atoms become active,thereby increasing the reduction activity of the counter electrode to S_n^2-.What’s more,when the reaction temperature was 200°C,the N/S co-doped graphene expressed a large specific surface area.This electrode material can provide more active sites for the catalytic reduction reaction with the lowest R_ct.The conversion efficiency of 2.3%was achieved by the prepared N/S co-doped graphene counter electrode and CdS/CdSe co-sensitized photoanode,which was increased by 147%compared to the pure graphene counter electrode.Secondly,Co_0.85Se films were prepared by hydrothermal and solvothermal method,respectively.The growth mechanisms and electrochemical properties of Co_0.85Se were analyzed by XRD,SEM,EDS and electrochemical tests.The results showed that Se impurities existed in the prepared film which hindered the charge transfer at the electrode/electrolyte interface.When the Co_0.85Se thin film was prepared by solvothermal method,the initial concentration of the reactant had a great influence on the morphology of Co_0.85Se thin films.When the Co²⁺concentration of the reactant was 8 mM,a uniform Co_0.85Se thin film could be prepared with the lowest R_ct,indicating that the electrode has good electrocatalytic activity for S_n^2-in the electrolyte.The results suggested that the charge can be rapidly transferred improving the electrochemical performances of the electrode.The PCE values of QDSSCs prepared by using the Co_0.85Se films electrodes were 14%higher than those reported in previous literatures.Finally,the Co_0.85Se/graphene composite counter electrodes were prepared by solvothermal method.The results of XRD,SEM and EDS analyses showed that the Co_0.85Se particles could be uniformly grown in the graphene sheet structure.Electrochemical tests confirmed that graphene can quickly transfer the external circuit electrons to Co_0.85Se particles,and reduce the S_n^2-in the electrolyte through Co_0.85Se.Compared with single material counter electrode materials,the conductivity and catalytic activity of Co_0.85Se/graphene composite counter electrode materials were effectively enhanced.The QDSSCs prepared by using Co_0.85Se/graphene composite as the counter electrode and CdS/CdSe co-sensitized TiO₂ as photoanode got the photoelectric properties with J_sc=9.69 mA/cm²,V_oc=0.56 V,FF=47.94%,PCE=2.6%.