Preparation Of High Efficiency CZTSSe Thin Film Solar Cells Based On CdS Buffer Layer Regulation

Cu₂Zn Sn（S,Se）₄（CZTSSe）solar cells,as a lucrative candidate of Cu₂（In,Ga）Se₂（CIGSe）,have received great attention due to its high optical absorption coefficient,earth-abundant and benign constituents,and simple fabrication process.Recently,a certified 13%PCE of CZTSSe solar cells was reported,bringing more promise for the future commercialization of CZTSSe solar cells.However,the device performance of CZTSSe solar cells is still significantly lower than its counterpart CIGSe solar cells and Shockley-Queisser limit（33.2%）.The major barrier limiting the development of CZTSSe solar cell is the large open-circuit voltage deficit(V_OC-def).The buffer layer of the CZTSSe device is also immense importance for high performance devices because it plays a crucial role for effective extraction/transport of electrons generated by the active layer.Preparation of Cd S by chemical bath deposited（CBD）is the most frequently applied buffer layer in CZTSSe solar cells.However,the Cd S buffer layer prepared from CBD process has some inherent problems which inhibit the photovoltaic device performance.（1）The Cd S fabricated by CBD method is commonly S deficient,which resulting the existence of S vacancies（V_S）.Meanwhile,there are impurity phases formed during CBD process and present in the Cd S film.These S vacancies and impurity phases in Cd S film are likely to act as recombination centers,which hazards charge separation and result poor device performance.（2）The in-appropriate band alignment at the CZTSSe/Cd S interface,which cause extensive carrier interface recombination,is an important factor that deteriorates the V_OC.In addition,the CTZSSe solar cells suffer spectral response losses in the short wavelengths because of the low bandgap of Cd S（2.4 e V）.Therefore,it is necessary to solve the inherent problems of CBD prepared Cd S and modify the band alignment.The preparation of high-quality Cd S thin films and the optimization of its band alignment with CZTSSe are the keys to improve the quality of heterojunctions.For the above two problems,the major works for this thesis are as follows:（1）Ammonium sulfide treatment of Cd S buffer layer improves the interfacial recombination of the heterojunction:To modify the Cd S films,ammonium sulfide（AS）solution was spin-coated on the Cd S layers,followed by thermal annealing the film in air.The thermal annealing temperature was optimized to 90℃according to the device performance of the corresponding solar cells.The result indicated that O-containing compounds were formed on the Cd S and it could be removed from the Cd S thin film by AS treat process.The S loss in Cd S film can be compensated by AS treatment.The W/O AS device displayed a J_SC of 32.93m A/cm^-2,V_OC of 453.08 m V,and FF of 65.11%,thus resulting in a PCE of 9.72%.After AS treatment,the champion PCE of 11.98%with J_SC,V_OC,and FF of 35.70 m A/cm²,490.15 m V,and 68.48%is achieved.The larger W_d and higher V_bi in W/AS device are conducive to the light absorption and carrier extraction,corresponding to its much superior device performance.The W/AS device showed prolongedτ_TPV,demonstrates less charge recombination,which is beneficial for the carrier collection and performance of solar cells.（2）Constructing a Zn Cd S buffer layer with a gradient band structure improves the interfacial recombination of the heterojunction:In this part,we utilized the CBD method to prepare ternary gradient composition Zn _XCd_1-XS buffer layer for CZTSSe solar cells.The graded composition Zn _XCd_1-XS was realized by sequentially adding the Zn source in the reaction solution during the Cd S deposition process.By varying the concentration of Zn²⁺precursor solution,we successfully obtained a series of Zn_xCd_1-xS films with variable band gap values.XRD and Raman results indicate that Zn was doped into the lattice of the Cd S.Doping of Zn could enlarge the bandgap of Cd S.The in-depth UPS measurement confirmed that the Zn _XCd_1-XS buffer have gradual increased CBM structure.The introduction of the gradient band splits the large CBO at the heterojunction interface into many smaller offsets,thereby reducing the electron transport barrier;meanwhile,the introduction of the gradient band structure on the CZTSSe side with low Zn content（narrow band gap）passivates the Zn Cd S Electrical properties of the buffer layer metastable state;the introduction of high Zn content（wide bandgap）on the Zn O side increases the absorption of CZTSSe in the short-wave band.Benefit from the gradual band structure,the champion PCE of 12.35%with J_SC,V_OC,and FF of 36.90m A/cm²,504.81 m V,and 66.28%is achieved.