Process Optimization And Mechanism Study Of CZTS Thin-film Photovoltaic Devices Using Magnetron Sputtering And Post-sulfurization

Cu₂Zn Sn S₄(CZTS)has become a research hotspot in the field of new thin-film solar cells due to its earth-abundant,non-toxic and environmental friendly constituents,high absorption coefficient and appropriate band gap(?1.5 e V).However,at present,it is difficult to meet the needs of large-scale applications due to low photoelectric conversion efficiency(PCE).The method of post sulfurization of magnetron sputtering metallic stack precursors is a safe and non-toxic method,which is suitable for large area fabrication,and has potential industrial application value.However,it is difficult for the elements of precursor prepared by layered deposition of metals to diffuse homogeneously,which restricts the growth of high quality CZTS absorber layer.The technology of CZTS thin-film devices prepared by layered sputtering and post sulfurization is not mature,and there are few related research of high efficiency CZTS devices prepared by this method.In this paper,CZTS devices were fabricated by post sulfurization of magnetron sputtering metallic stack precursors(Sputtering and post sulfurization)method,the fabrication process and mechanism were studied.The key problems such as poor-crystallinity of absorber layer(double-layer grains distribution),inhomogeneous distribution of Sn elements,too thick Mo S₂ and many voids at back contact interface Mo/CZTS and serious interface recombination of heterojunction were solved.Finally,a CZTS thin-film solar cell with PCE of 8.90%(Total area:0.23 cm²,active area:0.21 cm²,PCE of active area:9.75%)was fabricated.The main achievements of this paper are as follows:1.By optimizing the sulfurization process,CZTS large grains running through the whole absorber layer were prepared.The scientific mechanism of the optimized process to promote grains growth was revealed.CZTS grains prepared by sputtering and post-sulfurization method are often grow insufficiently.And the phenomenon of double-layer grains distribution often occurs in the absorber layer.Three-step heating method was used to provide more thermal energy to promote the diffusion of elements.In addition,Sn S powder was introduced into the sulfurization process.It is found for the first time that Sn S can not only compensate for losses of Sn,but also react with S to form the intermediate Sn S₂,which is a fluxing agent to promote the grain growth under high S atmosphere and high temperature,and finally CZTS large grains throughout the thickness of the film were obtained.2.The problem of inhomogeneous distribution of Sn elements was solved by optimizing the heating rate of sulfurization process.The absorber layer with uniform distribution of elements was obtained,and the conversion efficiency of CZTS device was improved to 7.02%.In order to improve the crystallization quality of CZTS and restrain the losses of Sn element,Sn S was introduced during the sulfurization process.However,during the cooling process after sulfurization,Sn S_x that did not participate in the reaction would remain on the surface of the CZTS film,resulting in the Sn-rich on the local surface of CZTS.By slowing down the heating process of the whole sulfurization stage,that is,decreasing the heating rate,the homogeneous of Sn element distribution is promoted.The effects of heating rate on the element distribution and morphology of CZTS films were systematically studied.The optimum heating rate was determined and the problem of inhomogeneous distribution of Sn was solved.The CZTS device with efficiency of7.02%was fabricated.3.By introducing Al-doped Zn O(AZO)layer,the thickness of Mo S2 at the back interface of Mo/CZTS is effectively suppressed and the voids are eliminated.The physical mechanism of improving the back interface by AZO layer is clarified.The efficiency of CZTS photovoltaic device was further improved to 8.44%.Voids and thick Mo S₂are easy to appear at the back contact region of Mo/CZTS,which is due to the reactions of Mo with S,and Mo with CZTS during sulfurization process.AZO is used as the intermediate layer to improve the quality of back interface for the first time.The AZO layer with appropriate thickness can effectively inhibit the formation of Mo S₂.The thickness of Mo S₂ is only about 50 nm after the inserting of20 nm AZO layer at the back interface,and the voids are avoided.In addition,the specific mechanism of improving the back contact interface by AZO layer was deeply studied.Because the electronegativity of O element is higher than that of S element,AZO can be completely sulfurized into Zn S only at high temperature and high concentration of S atmosphere.Before that,the AZO intermediate layer can protect Mo.Finally,high quality Mo/CZTS back contact interface and CZTS solar cell with efficiency of 8.44%were obtained.4.The CZTS/Cd S interface was treated by post-heating treatment(PHT)process.The crystallization quality of n-type Cd S is improved.Owing to the inter-diffusion of elements,PHT process improved the heterojunction band alignment and reduced the interface recombination of heterojunction.Consequently,CZTS thin-film photovoltaic devices with an efficiency of 8.90%was fabricated.On the basis of the optimization of absorber layer and Mo/CZTS interface,the CZTS/Cd S interface was optimized by PHT process.It is found that PHT process can promote the diffusion of Cd elements to CZTS,while Zn and Cu elements diffuse to Cd S.The inter-diffusion of elements improves the band alignment of the heterojunction and the chemical composition of the CZTS surface.In addition,PHT process can promote recrystallization of Cd S layer,which greatly improves the crystallization quality of Cd S layer.High quality heterojunction reduces interface recombination.Finally,CZTS thin-film photovoltaic device with a total area efficiency of 8.90%was obtained under the optimum PHT process.Reducing the deep level defects(i.e.Sn_Znand its defect cluster)in CZTS bulk is expected to further improve the device performance.