Regulation Of Bulk And Heterojunction Properties Of Cu₂ZnSn（S,Se）₄ Solar Cells

Kesterite Cu₂ZnSn（S,Se）₄（CZTSSe）semiconductor materials have high research prospects in the photovoltaic field due to their excellent photoelectric properties and earth-abundant constituent elements.At present,CZTSSe solar cells based on the dimethyl sulfoxide（DMSO）solution system have achieved the highest certified efficiency of 13.0%,but still far from the theoretical efficiency limit of 32%.On one hand,improving the crystal quality of CZTSSe active layer and interfacial contact properties is important way to achieve efficiency breakthroughs.On the other hand,in-depth understanding the charge loss mechanism within CZTSSe solar cells can guide the further device optimization.The research work of this thesis focuses on the potential mechanism of efficiency loss and the experimental methods for preparing high-quality CZTSSe absorbers and improving the heterojunction properties,and the following results have been achieved:Firstly,the mechanism of synergistic effect between different defects in CZTSSe materials inducing fast charge recombination has been proposed,which can help understand the causes of the extremely short minority lifetime in the absorber and the large open-voltage(V_OC)loss of the devices.Based on the systematic summary and analysis of the properties of atomic disorder and deep defects in CZTSSe materials,it is found that although the band tail states,caused by the atomic disorder,will reduce the effective bandgap,it has little effect to the charge loss,whereas the deep defects can act as effective non-radiative recombination centers to induce charge recombination.In the synergistic effect,the deep defects are the key to charge recombination,and the widespread band tail states will accelerate the capture of carriers by deep defects.In addition,we also listed four experimental methods for detecting defects,and systematically organized the defects information obtained by these methods,trying to locate the key defects that influence the minority lifetime of CZTSSe materials.Furthermore,the factors leading to the charge recombination at interfaces are also discussed.Comprehensive and dialectical understanding and effective control of the bulk defects and interfacial properties of the CZTSSe solar cells are important research directions for preparing high-efficiency devices.Secondly,a simple sodium-incorporation post-treatment method has been proposed,this is,the Cu₂ZnSn S₄（CZTS）precursor film is spin-coated with Na Cl ethanol/water solution.In the subsequent selenization process,the introduction of sodium atoms is conductive to the formation of Cu_2-xSe fluxing agent,thereby promoting the transport of metal elements,and obtaining compact and smooth CZTSSe films with large grains and less secondary phase Cu₂Sn（S,Se）₃.The secondary ion mass spectroscopy（SIMS）depth profiles have shown that the sodium atoms are distributed in a gradient in the absorber,and the photoluminescence spectra have shown that the introduction of appropriate sodium atoms can inhibit the band tail states.By optimizing the concentration of Na Cl solution,as high as 11.18%efficiency has been achieved,and the average efficiency of sodium-doped experimental group is 13%higher than that of the control group.Besides,the sodium-doped CZTSSe devices have wider depletion region and longer charge recombination lifetime.This work provides an easy sodium doping method for preparation of high-quality CZTSSe films and high-performance devices,in the meantime,it can also help to further understand the effect of sodium atoms in CZTSSe solar cells.Thirdly,a two-step annealing treatment for CZTSSe/CdS heterojunction to improve the interface performance has been developed.That is,a low-temperature（90℃）process is introduced before the high-temperature（210℃）treatment,and CZTSSe solar cell with efficiency of 12.33%has been achieved based on this method.Further investigation reveals that,the CZTSSe/CdS heterojunction band alignment with a smaller“spike”barrier can be realized by the two-step annealing treatment,which is beneficial to the carrier transportation and can reduce the charge recombination loss,thereby enhancing the open-circuit voltage(V_OC)and fill factor（FF）of the devices.Besides,the two-step annealing can effectively avoid the disadvantages of direct high-temperature treatment（such as the reduction of CdS film coverage and excess element diffusion at interface）,and improve the CdS crystallization,and decrease defect density within the device,especially the interfacial defects.This work provides an effective method to improve the CZTSSe/CdS heterojunction properties for efficient kesterite solar cells,and helps to understand the influence of different annealing temperature on the interface.