| Solving the problems of global warming and reducing carbon emissions requires large-scale application of solar energy.Cu(In,Ga)Se2(CIGS)thin film solar cells have achieved high conversion efficiency and have been commercially produced,but rare elements such as indium and gallium limit their large-scale application.Due to the extraordinary properties of Cu2Zn Sn S4(CZTS),such as earth-abundant and non-toxic elements,optimal and direct bandgap of 1.5 e V,p-type conductivity,high stability,compatible with the production process of CIGS thin film solar cells,kesterite CZTS solar cells are considered to be the successor of CIGS solar cells.Since the first preparation of CZTS thin film solar cells with an efficiency of 0.66%in 1996,the world record efficiency of pure sulphide CZTS thin film solar cell has achieved 10%efficiency.CZTSSe solar cells prepared by the hydrazine solution approach demonstrated 12.6%record efficiency are not suitable for commercial production due to the high toxicity.Compared with commercial solar cells such as CIGS,monocrystalline silicon and Cd Te,the device efficiency of CZTS is still low,because of the separation of secondary phases,crystallization of CZTS after sulfurization,various defects in CZTS and the buffer layer interface contact.Based on the above analysis,the CZTS thin films and CdS thin films were prepared by the same vacuum preparation method as the CIGS production process.The sulfurization process of the CZTS films and the characteristics of the CdS films prepared by the vacuum method were studied in depth.In this article,the magnetron co-sputtering method was used to prepare CZTS films,and the effects of sulfurization process conditions on the crystallization characteristics,surface morphology and elemental composition of CZTS films were studied.By improving the sulfurization process,high quality CZTS thin films were obtained,and thus the efficiency of the device was improved.The Mo electrode layer,CZTS absorber layer,CdS buffer layer,i-ZnO/ITO window layer and aluminum electrode were separately deposited by vacuum methods such as magnetron sputtering method to achieve the all-vacuum-processed CZTS thin film solar cells.The preparation of CZTS devices by the all-vacuum method can avoid the production of toxic waste liquid caused by the solution method,and is conducive to the low-cost,large-scale production of CZTS solar cells.As a result,the work results of this article are as follows:(1)The CZTS precursors were prepared by co-sputtering Cu2S/SnS2/Zn S targets and sulfurized in sulfur atmosphere.The effect of sulfurization temperature on the CZTS film was studied.The CZTS thin films prepared by the co-sputtering method showed uniform surface without voids and a kesterite structure with preferred orientation along the(112)plane.With the increase of the sulfurization temperature,the crystal grains aggregated and the secondary phase appeared.The band gap of all samples is higher than 1.5 e V.The Mo electrode layer,CdS buffer layer and i-ZnO/ITO window layer were prepared by vacuum method,and the structural,optical and electrical properties were characterized.The performance parameters of the films were in accordance with the requirements of preparing solar cells,which verified the feasibility of CZTS thin film solar cells prepared by the all-vacuum method.The reason for limiting the efficiency of the device was explored,and it was found that the sulfurization condition shows great influence on the crystallization quality of the CZTS films.The experimental results showed that at low sulfur partial pressure,the CZTS grain size is small,and will lead to VS defect,thus the device prepared is less efficient.The crystallization quality of the films was improved by increasing the partial pressure of sulfur,but the too high partial pressure of sulfur will cause Sn loss and appearance of SnS2 on the CZTS films,which is not conducive to device efficiency.The method of cracking sulfur molecules at high temperature can also improve the crystallization quality of CZTS films and improve the efficiency of the devices.Finally,by improving the sulfurization conditions,the efficiency of the cell was increased to 2.4%.(2)The reasons that affect the consistency of CZTS devices were investigated.The influence of sulfur vapor distribution on the phase structure and surface morphology of CZTS thin films,as well as the influence on the performance of the cell were investigated.The results showed that the distribution of sulfur vapor in the sulfurization chamber was not uniform.The main reasons were the diffusion process of sulfur vapor in space and the sulfurization environment in the double temperature zone.The experiment showed that the sulfur vapor consists of S2,S3,S4,S6,S8,S10 and other molecules,and different sulfur vapor molecules have different molecular weights,meaning that the diffusion rate of different sulfur vapor molecules is different according to Graham's Law.The lighter sulfur vapor molecules have higher activity and faster diffusion rate.The higher concentration of light sulfur vapor molecules is beneficial to the growth of CZTS grains.The results showed that there is an optimal position in the spatial distribution of sulfurization,which is conducive to improving the crystallization quality of CZTS films and avoiding the generation of secondary phases such as SnS2 and Zn S.The device efficiency at the corresponding position reached the optimal efficiency of 3.49%.(3)The effects of growth rate on crystal structure,optical properties,morphology and electrical properties of CdS thin films deposited by electron beam evaporation method were studied.The CdS films deposited at low growth rate show larger grains,higher transmittance and lower majority carrier concentration,implying the CdS films show higher crystallinity at lower growth rate.The CdS thin films deposited by this method was used to prepare CZTS solar cells.The statistical data of 150 devices showed that the average efficiency of the cell was 3.25%,and the optimal efficiency of the solar cell reached 4.87%,which proved the feasibility of this method.(4)CdS thin films were prepared by magnetron sputtering method.The structural,optical and morphological properties of the thin films prepared by sputtering were studied.The results showed that the average transmittance of CdS films at 50 nm thickness was91%,the band gap was 2.4 e V,and the surface was uniform and compact.The CdS films prepared by sputtering method were used to fabricate CZTS solar cells.The effects of different thickness of CdS films and different sulfurization time on the performance of CZTS solar cells were investigated.The statistical data for dozens of devices showed that the average open-circuit voltage was 0.56 V and the average conversion efficiency was relatively low.Further experiments showed that the device appeared self-healing effect after storage,which may be due to the reduction of defects.C-V and DLCP were used to measure the defect density of solar cells using the buffer layers deposited by different methods,confirming that the sputtering CdS process will introduce more deep defect stats,the produced defect states with a high density of 4.49×1017 cm-3 will increase the recombination centers in the CZTS films,reduce the depletion layer width,and deteriorate the device performance.The results showed that the high density defects produced by sputtering CdS and the high resistivity of CdS thin films limit the performance of the solar cells.The highest efficiency of CZTS thin film solar cells prepared by sputtering CdS was 3.37%. |
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