Preparation And Performance Characterization Of Copper Cobalt Tin And Sulfur(selenium)(CCTS(Se))thin Film

Since the 20th century,the modern economic society has experienced unprecedented rapid development,but the ensuing excessive energy consumption and global environmental problems have also intensified.At present,countries around the world have recognized the importance of coordinating social and economic growth with sustainable human development,and are constantly striving to find ideal new energy sources to replace traditional non-renewable energy,such as coal,oil and natural gas.Among them,solar energy has attracted wide attention from scientists because of its renewability,safety and cleanliness.Solar cells have a wide range of applications and research prospects owing to its efficient use of solar energy.Among the current mainstream solar cells,copper-based poly chalcogenide Cu₂XSn（S,Se）₄（X=Mn,Fe,Co,Ni,Zn and Cd）（CXTSSe）thin film solar cells have received more and more concern.They not only have a similar crystal structure to Cu（In,Ga）（Se,S）₂（CIGS）thin film solar cells which have been put into commercial use,but also be composed of earth abundant and environmental friendly elements.Meanwhile,they also the hold suitable optical band gap values（1.2-1.6 eV）and an ideal light absorption coefficient(>10⁴ cm^-1).Although the highest conversion efficiency of Cu₂ZnSn（S,Se）₄ thin film solar cell has reached 12.6%up to now,it is still far from the theoretical conversion efficiency of 32.4%and the Cu（In,Ga）（Se,S）₂ solar cells of 21.7%.There are many factors affecting the improvement of photoelectric conversion efficiency of solar cells.In fact,the similar ionic radii of Cu⁺（0.91?）and Zn²⁺（0.88?）can easily result in antisite defects,which seriously affects the efficiency of the battery.In order to improve this problem,Co²⁺（0.68?）with smaller ionic radius was used instead of Zn²⁺,and the preparation process and properties of Cu₂CoSn（S,Se）₄systematic films were studied detailedly in this paper.Firstly,Cu₂CoSnS₄ thin films were successfully fabricated on glass substrates via magnetron sputtering followed by high temperature vulcanization annealing.In the early days of our work,the optimal Co/Sn molar ratio and deposition mode of the precursors were locked respectively（the Co/Sn molar ratio is 1 and the precursor deposition method is single deposition）,and the appropriate annealing time was also selected（30 min）.Then,on the basis of above,the effects of metal precursor deposition order and vulcanization temperature on the growth and properties of Cu₂CoSnS₄ films were studied in detail to find the best preparation conditions.The results show that the crystallinity of all films increases with increasing annealing temperature.However,the deposition order of metal precursors and the annealing temperature have a great influence on the growth and optical properties of the films.For the Cu/Sn/Co deposition order,a pure phase CCTS film is obtained when the annealing temperature is higher than 520°C.When the deposition order is Cu/Co/Sn,it is confirmed by XRD characterization that a small amount of impurity phases always exist in the samples as the annealing temperature is raised from 450°C to 600°C.However,it can been seen from Raman spectroscopy that the Cu₂CoSnS₄ film annealed at 600°C has the lowest bond-stretching force constant and perfect crystal quality.Moreover,the Mapping,XPS and UV-Vis-NIR absorption spectroscopy confirmed that each element is evenly distributed in this sample with corresponds chemical valence state to Cu₂CoSnS₄,and the optical performance of the sample is also ideal.In order to further improve the photoelectric properties of the films to meet the requirement for use as an absorber layer of thin film solar cells.In the subsequent work,the precursor obtained under the Cu/Co/Sn deposition sequence was subjected to high temperature vulcanization/selenium co-treatment,Cu₂CoSn（S,Se）₄ thin films were successfully prepared after attempts of different Se/S mass ratios and annealing temperatures.Researches show that the obtained CCTSSe films are all stannite structure;the morphology of the film changes greatly as the Se/S mass ratio increases.At the same time,the study on the optical properties of each sample found that the addition of Se generally contributes to the improvement of the visible light absorption capacity of the films,and the band gap values of all samples are between 1.1 eV and1.6 eV.