Optoelectronic Properties Of Cu₂ZnSn?Se,S?₄ Thin Films Fabricated By Co-electrodeposition Technique

With the rapid development of social economy,the demand for global energy is continuouslly growing.With the mass exploitation and utilization of fossil energy,resource exhaustion and serious environmental deterioration have become one of constrains of sustainable development in human society.Recently,more and more attentions are focused on the clean and pollution-free solar power generation.Among all the solar cells,the solar cells based on silicon are well developed and its efficiency has reached 25%.However,the low absorption efficiency and high materilas cost restrict its long-term development and large-scale utilization.Nowadays,thin-film solar cells have attracted the attention of researchers due to their high material utilization,simple production process and flexbility,which has been regard as the developing direction of next-generation solar cells.Cu₂ZnSn?S,Se?₄?CZTSSe?is a promising material for the absorber layer of thin film solar cells.CZTSSe compound is the crystal-structural descendant from CIGS by substitution of Zn,Sn for In,Ga.All constituents in this CZTSSe film are abundant in the crust of the earth.CZTSSe film has high absorption coefficients(10⁴ cm^-110⁵ cm^-1),an optimal direct band gap,and theoretical maximum efficiency of 32.4%.To date,the reported highest power conversion efficiency for the mixed sulfo-selenide Cu₂ZnSn?S,Se?₄have reached 12.7%,which was fabricated by chemical solution-based method,and remains huge gaps between the current reported highest conversion efficiency and that of CIGS.Due to its sensitive element ratio,it is difficult to fabricate phase pure CZTSSe thin film with high crystalinity.Any component deviation will cause the formation of secondary phases,such as Cu₂S,Cu₂Se,ZnS,ZnSe,SnS,SnSe.Although much efforts have been devoted to reolve such problems,developing new techniques to avoid the formation of secondary phases is still highly desired.In the current research work,CZTSSe thin film photovoltaic material was prepared by co-electrodeposition.Electrodeposition method for preparing CZTSSe thin films has a lot of advantages,such as simple equipments,low-cost raw materials,large-area preparation and so on.Hence,our researches have devoted to investigate the correlation between its preparation methods and the resulting properties in structure,morphology and device performances.Three main research works are performed,which listed as follows:1.The one-step electrodeposition method for preparation of CZTS.The uniform,compact and smooth of the CZTS thin films were prepared by co-electrodeposition technique on Mo glass substrates in the galvanostatic mode from aqueous solution.And then,the influence of deposition parameters for CZTS films morphology such as deposition potential,the deposition time and the concentration of Zn²⁺ion were explored.The results showed that high-quality CZTS layer can be prepared successfully when the deposition potential is-1.15 V,the deposition time is 20 min and the concentration of Zn²⁺ion is 15 mM.2.The post-selenization of CZTS films to fabricate of CZTSSe films.CZTS precursor films were co-electrodeposited onto molybdenum?Mo?-coated soda-lime glass?SLG?substrates.The CZTS precursor layers were deposited at-1.15 V?vs Ag/AgCl?for 20 min at room temperature without stirring.Finally,the CZTS films were pre-heat treatment at 280oC for 30 min,and then the selenization temperatures of 550oC was performed.The influences of sulfurization temperature and sulfurization period on the morphologies,structural,component and optoelectronic properties of the CZTSSe films were explored.The results showed that the absorber layer thin-film has good absorption characteristics in the visible range.XRD and Raman measurements proved that phase pure Cu₂ZnSn(S_xSe_1-x)₄?CZTSSe?were fabricated and no obvious copper selenide secondary phases were detected.3.Applied the CZTSSe absorbing layer to thin film solar cell deviceOn the basis of preliminary research analysis,thin film solar cells based on the CZTSSe absorbing layer prepared by co-electrodeposition technique and post-selenization method.A 60 nm thick CdS buffer layer was deposited on the CZTSSe absorber by chemical bath deposition?CBD?.A 50 nm thick intrinsic zinc oxide?i-ZnO?and 300 nm thick indium tin oxide?ITO?films were sputtered in sequence by radio-frequency?RF?and direct current?DC?magnetron sputtering,respectively.Finally,the Ag grid electrodes were thermally evaporated through a metal shadow mask,which were used as the current collector.Efficiency of the of the solar cell device reached 2.81%with the open circuit voltage of 311 mV,the current density of 25.19 mA/cm²,and the fill factor of 35.94%.