Enhancement Of CZTSSe Photovoltaic Device Performance By Passivation Of Interface Defects Based On Surface Post-treatment Of Absorber

With the goal to achieve healthy and green sustainable development,the development of clean and renewable energy is imperative.As a device that can directly convert solar energy into electric energy,solar cells are of great significance to the development and utilization of solar energy.Among the many types of solar cells,Cu₂Zn Sn（S,Se）₄（CZTSSe）thin-film solar cells have shown remarkable potential in thin-film photovoltaics due to their high element abundance,low cost,and non-toxicity.However,the highest certified efficiency currently achieved by CZTSSe solar cells is only 13%,which is still far from the theoretical efficiency calculated by Shockley-Quiser（SQ）.Studies have made it clear that the main reasons for the current stagnation of CZTSSe solar cells efficiency are the larger open-circuit voltage(V_oc)loss and relatively lower fill factor（FF）,which are mainly caused by the recombination at the heterojunction interface.In CZTSSe solar cells,although the CZTSSe/Cd S heterojunction interface has exhibited a relatively benign band structure,there is still a serious interfacial recombination problem caused by high concentration of Cu_Zn antisite defects and[2Cu_Zn+Sn_Zn]defect clusters.A large number of Cu_Zn anti-main defects at the interface will not only lead to more carrier recombination centers,but also cause severe Fermi level pinning,resulting in severe losses of V_oc and FF of the device.[2Cu_Zn+Sn_Zn]defect clusters can induce severe band tail states via band gap disturbances or electrostatic potential fluctuations,which further leading to the deterioration of device performance.Therefore,in order to further fabricate efficient CZTSSe solar cells,it is particularly important to effectively passivate the high concentration of Cu_Zn antisite defects and[2Cu_Zn+Sn_Zn]defect clusters at the heterojunction interface.In view of this problem,the work of this paper mainly includes the following two parts:In the first part,we proposed the silver element post deposition treatment（PDT）method to passivate Cu_Zn defects at the heterojunction interface.A layer of silver element was thermally deposited on the surface of selenized absorber,and then a secondary selenization treatment was carried out.In this experiment,the optimal experimental conditions are determined by adjusting the thickness of Ag nanolayers,the secondary selenization temperature,the secondary selenization time.XPS and EDX et.al.characterization results showed that the Ag element successfully entered the CZTSSe lattice and formed a relative high concentration Ag substitution at the upper surface of the absorber after Ag-PDT.The results of C-V curve and Urbach energy（E_u）fitting of EQE tail state confirmed that the depletion region width of Ag-PDT sample was increased and the band tail state was reduced,which are beneficial to the separation and collection of photogenerated carriers.At the same time,C-DLTS and dark state J-V-T measurements demonstrated that Cu_Zn antisite defects at the upper interface of the absorber was obviously passivated,the interface recombination was significantly improved.Finally,under the optimal Ag-PDT treatment conditions,the efficiency of CZTSSe solar cells increased from 10.21%to 11.96%,largely attributed to the V_oc and FF increment.In the second part,we proposed Ge Se₂-PDT passivates heterojunction interface defects.Drawing on the PDT experimental process of the first job,Ge Se₂-PDT is adopted to try to introduce a relative high concentration of Ge on the upper surface of the absorption layer to replace the passivated heterojunction interface[2Cu_Zn+Sn_Zn]defect clusters.Optimal treatment conditions are determined by adjusting the thickness of Ge Se₂ nanolayers,the secondary selenization temperature,the secondary selenization time.XPS and SIMS et.al.characterization results proved that Ge elements successfully entered the CZTSSe lattice after Ge Se₂-PDT treatment and a relative high concentration Ge substitution was formed at the upper surface of the absorber.The results of Urbach energy（E_u）fitting of EQE tail state and C-V curve confirmed that the band tail state of Ge Se₂-PDT sample was obviously reduced and the carrier concentration was increased,which contribute to the improvement of carrier transport performance.In the meantime,C-DLTS and dark state J-V-T measurements demonstrated that[2Cu_Zn+Sn_Zn]defect clusters at the upper interface of the absorber was notably passivated,and the recombination of the interface was significantly improved.Ultimately,the Ge Se₂-PDT process increased the device’s V_oc from 437 m V to 470 m V,the FF from 65.88%to 69.95%,and the photoelectric conversion efficiency from 10.36%to 12.22%.