The Preparation And Research Of Cu₂Sn_1-xGe_xS₃（CTGS）materials And Thin Films And Solar Cells

Cu₂SnS₃（CTS）thin film materials are considered an absorber layer suitable for thin film solar cells due to its non-toxic composition,abundant element content,higher absorption coefficient more than 10⁴cm^-1 and the element ratio is easy to control.At present,the highest conversion efficiency of CTS thin film solar cel with 4.63%was repoted,which is much less than theoretical efficiency of 30%.The main problem is that the structure of CTS film suitable for device fabrication is monoclinic and CTS has a low value of band gap less than 1eV,which limits the open circuit voltage of the devices.On the other hand,CTS films usually have a high carrier concentration,which also limits the conversion efficiency of the device to some extent.In this paper,Ge element was doped to replace a part of the Sn element in CTS films and to obtain Cu₂Sn_xGe_1-xS₃（CTGS）thin film by cation substitution method.Two kinds of different CuSnGe（CTG）precursors were prepared by RF magnetron sputtering metal elemental target（Sn,Ge,Cu）and elemental target Cu combined with alloy target SnGe,respectively.Then,CTGS absorber layer films were formed by sulfurizing the precursors at high temperature.In this work,the effects of different sulfurization processes on the quality of CTGS film and the effect of Ge content on the CTGS film prepared by sputtering pure elemental target were studied.The CTGS films with good crystal quality,superior composition distribution and uniform surface were obtained by optimizing the process.Finally,the complete CTGS thin film solar cells with a certain photoelectric conversion efficiency were prepared by two kinds of CTGS absorber layers based on different precursors.The main research contents of this paper are as follows:（1）The CTG precursors were prepared by magnetron sputtering metal elemental target Ge,Sn and Cu,and then the precursors were sulfurized at high temperature to obtain CTGS films.The effects of different Ge contents and sulfurization processes（sulfurization temperature and sulfurization time）on the properties of CTGS films were investigated.Finally,the effects of different positions of Ge doping in the precursors on the quality of the CTGS films were also investigated under the conditions of optimum Ge content and sulfurization process.（2）The CTG precursors were prepared by RF sputtering of SnGe alloy target and Cu target,and then the CTGS absorber layers films were formed by high temperature sulfurization process.The effects of different annealing temperature and time on properties of CTGS flms were studied.（3）The CTGS films prepared by the above wo kinds of precursors were compared and analyzed,especially the differences in crystal quality,surface and cross-sectional characteristics.（4）The CTGS films prepared by the above two kinds of precursors were applied to thin film solar cells,and the CTGS thin film solar cells with a complete structure of Mo/CTGS/CdS/i-ZnO/AZO/Al were developed.Meanwhile,the the effects of different CTGS absorber films on the performance of the devices were investigated.The corresponding research results in this paper are as follows:（1）It is determined that the optimal Ge content is about 10%,and then the CTG precursors formed by sputtering metal were subjected to sulfurization process at 530℃,550℃and 570℃for 15 and 20min,respectively.The comparative study found that sulfurization was carried out at 550℃for 15min,the CTGS films has good crystallinity,flat surface with larger grain size,and the moderate elements ratio.If the sulfurization temperature was lower than 550℃,the surfaces of the films were rough and uneven grain size.When the sulfurization temperature was higher than 550℃,a large number of cavities were found on the surface and cross-section of the film due to serious loss of Sn element.At the same time,the incorporation of Ge at the top demonstrated that element ratio was bad off stoichiometry and secondary phases existed.The crystal quality has been significantly improved As the Ge in the middle of precursors and the incorporation of Ge at the bottom greatly improved the surface morphology and electrical properties of the films.（2）The CTG precursors prepared by combining the alloy target（SnGe）and the elemental target（Cu）was subjected to sulfurization at530℃,550℃and 570℃for 15 and 20 min,respectively.The results demonstrated that the CTGS sample obtained using sulfurization temperature of 550℃for 20min had better crystal quality and the film surfaces with large grains were flat and dense.Meanwhile,it was found that the obtained CTGS film under the sulfurization temperature of 530℃and sulfurization temperature of 550℃for 15min was insufficiently sulfurized leading to secondary phase Cu₂S,and the surface of the film was rough and the grain boundary was obvious.When the sulfurization temperature was higher than 550℃,the Sn loss in the film was severe,and as sulfurization time was over 20min,the film underwent a thermal decomposition reaction,and the secondary phases were regenerated.（3）By comparing the best quality CTGS films made from two kinds of different precursors,it was found that the CTGS films prepared by the elemental target had a larger grain size than the CTGS film prepared by the SnGe/Cu precursors,and the electrical properties were also better,as well as the concentrations of carriers were also reduced.In addition,EDS analys is showed that the loss of Sn element in the CTGS films prepared from the alloy target was more serious in the process of high temperature sulfurization.（4）The highest photoelectric conversion efficiency（PCE）of the CTGS thin film solar cell prepared by the metal elemental target precursors is 2.82%,which is very close to the efficiency of the CTGS thin film solar cell prepared by the SnGe/Cu precursor of 2.97%..