Study On High-Performance Kesterite Thin-Film Solar Cells Based On Bulk Regulation

Kesterite Cu₂Zn Sn（S,Se）₄（CZTSSe）thin-film solar cell is one of the emerging solar cells.It has attracted extensive attention for its environmentally friendly,high-reserve,and low-price constituent elements,as well as its high light absorption coefficient,good response to weak light,and excellent stability.After years of development,the power convention efficiency（PCE）of kesterite solar cells has exceeded 13%,and the preparation method of precursor absorber has also changed from the initial vacuum-based method,through toxic hydrazine solution-based method,to the environmentally friendly solution-based method.At this stage,there is still a large gap between kesterite solar cell and its Shockley-Queisser limited（S-Q）PCE（33%）,and the most important factor is the crystal quality of the absorber.Kesterite has narrow phase diagram,high phase formation temperature,and complex phase evolution pathways.During the crystal growth process,secondary phases and multiple types of defects are prone to appear and concentrate in grain boundaries or surface of the absorber,which will cause large open-circuit voltage deficit(V_OC,def).Therefore,it is necessary to regulate the phase evolution and crystal growth of kesterite absorber.This paper focuses on the crystal quality of the absorber and conducts in-depth research on the phase evolution and crystal growth process of the kesterite absorber.The research investigates element doping,element alloying,phase evolution process,and crystal growth during selenization,resulting the following findings:（1）The crystal growth of CZTSSe is regulated by the highly soluble organic lithium salt lithium bis（trifluoromethanesulfonyl）imide（Li TFSI）.Results have shown that Li TFSI remains in its original form in the precursor film even after annealing process.During the phase evolution process in the first 300 seconds of selenization reaction,Li TFSI decomposes in situ in the film,producing Li F which forms a low-melting eutectic fluxing agent with selenium vapor.The fluxing agent facilitates the mass transfer of elements and grain fusion,resulting in larger single-layer grains,a smoother surface,and fewer pinholes inside and on the surface of CZTSSe absorber.This leads to a significant improvement in the crystal quality of CZTSSe absorber.Additionally,Li TFSI improves the electrical properties of kesterite solar cells by achieving better band alignment in the front interface,reducing surface defects,improving carrier lifetime,and reducing recombination.The resulting kesterite solar cell achieves a PCE of 13.2%in an active area（12.7%certified）and a PCE of 12.0%in a total area（11.5%certified）,which is the highest Li-doped PCE reported to date.（2）AgCl and Li TFSI are added to the precursor solution to implement a synergistic regulation strategy for Ag-Li co-doping.Various Li doping concentrations and Ag contents are systematically optimized through high-throughput experiments.The kesterite solar cell achieves a PCE of 13.62%（with antireflection treatment）and a V_OC/V_OC^SQ value(the ratio of V_OC and S-Q limited V_OC)of 60.71%under the condition of Ag/（Ag+Cu）=5%and 0.135 mol·L^-1 Li TFSI.The electrical properties of kesterite solar cells are significantly improved by the synergistic effect and complementary advantages of Ag-Li co-doping.Specifically,the energy band fluctuations related to band tail states are reduced,leading to a decrease in V_OC,def.Moreover,in terms of carrier dynamics process,carrier lifetime is prolonged,carrier recombination during transportation is suppressed,the defect concentration is reduced,and the contact properties of the back interface are enhanced.Additionally,in terms of heterojunction,the properties of depletion width and carrier concentration are enhanced,and lower interface defect concentration is achieved.（3）From the perspective of selenization kinetics,the rate of selenization reaction in a semi-closed graphite box can be adjusted by modifying the chamber pressure,which in turn regulates the phase evolution process of kesterite.By increasing the chamber pressure,in-situ real-time monitoring of selenium partial pressure reveals that the selenium partial pressure is suppressed during the early stages of selenization.This reduces the probability of collisions between the precursor film and the gaseous selenium vapor during the heating-up stage（200-400℃）.Additionally,positive chamber pressure can suppress the non-uniform diffusion of metal elements.The combined effect of the above two factors causes the phase evolution process to start at a relatively higher temperature（>400℃）,which suppresses the appearance of intermediate phases such as Cu_xSe and Cu₂Sn Se₃ that would normally form on the precursor surface.The actual phase evolution undergoes a direct pathway.Consequently,the resulting Ag-alloyed CZTSSe（ACZTSSe）absorber has high crystal quality,few pinholes,and a significantly reduced surface defect concentration.The bulk defect concentration of the kesterite solar cells is reduced by about one order of magnitude(from 5.31×10¹⁵ to 8.42×10¹⁴ cm^-3),and the electrical performance is significantly improved.Finally,a kesterite solar cell with a total area PCE of 14.1%（certified 13.8%）is achieved,which is the highest PCE reported to date.This work provides a kinetic control strategy for further understanding and regulating the phase evolution process of kesterite,particularly in optimizing the phase evolution pathway to realize high efficiency kesterite solar cells.It also serves as a reference for the growth and preparation process of other polycrystalline thin films.（4）From the perspective of solvent coordination engineering,we propose a strategy using a mixed solvent of 2-methoxyethanol-formamide（MOE-FA）to improve the crystal quality and uniformity of ACZTSSe absorbers by regulating the coordinating environment of Sn（IV）.The small-molecule FA in the mixed solution can coordinate with Sn（IV）to form a large-steric-hindrance configuration,which splits the framework structure formed by oxygen bridges of MOE into independent small structures.This,in turn,suppresses the thickness of organic residues in the precursor film.During the subsequent selenization process,the initial nucleated grains can fuse and grow,filling the pinholes left by the volatilization of organic residues inside the absorber.The morphology of the resulting ACZTSSe absorber consists of flat large grains on the top layer and dense grains on the bottom layer,improving absorber quality and uniformity significantly.Finally,kesterite solar cells with a PCE of 13.6%on a small area（total area,0.28 cm²）and a PCE of 12.7%（certified 12.4%）on centimeter-scale area（total area,1.0875 cm²）have been realized,which is the highest large-area（>1 cm²）PCE reported to date.