Fabrication Of High-efficiency Cu₂ZnSn（S,Se）₄ Solar Cells Based On Improving Grain Growth And Front Interface

Cu₂Zn Sn（S,Se）₄（CZTSSe）thin-film solar cells developed from Cu（In,Ga）Se₂（CIGS）are regarded as one of the new generation of photovoltaic energy devices with application potential,which have the advantages of adjustable band gap,high abundance of constituent elements and high theoretical conversion efficiency.Compared with vacuum-based deposition,the preparation of CZTSSe solar cells based on the non-hydrazine-based solution method can further reduce the cost.Currently,the highest certified conversion efficiency of CZTSSe solar cells is 13.0%,which is much lower than 23.35%of CIGS solar cells.And improving the photoelectric conversion efficiency of CZTSSe solar cells prepared by non-hydrazine-based solution methods is a major scientific research problem that needs to be solved urgently in this field.As we all know,high-efficiency CIGS solar cells usually have good crystallinity with large-grain spanning monolayer structure,which is the key to obtaining high conversion efficiency and beneficial to the absorption of light by the absorber,the carrier transport and the reduction of carrier recombination.The CZTSSe solar cell with the highest certified efficiency is fabricated from DMSO molecular solutions by using tin（IV）salt rather than tin（II）,and the grain growth of Sn⁴⁺precursor film takes a direct transformation reaction path by the lattice of S is replaced by Se directly.However,selenium vapor penetrates the film to the back interface at the early stage of selenization,which causes large-grain double-layer structure with a top down and bottom up bi-direction grain growth,and the layered interface inside the absorber will cause carrier recombination,which is not conducive to carrier transport.In addition,the heterojunction interface,also known as the CZTSSe/Cd S interface,is an important region for the generation and separation of photogenerated carrier,in which the surface state has a trapping effect on carrier and can easily cause carrier recombination.In order to improve the grain quality and the surface property of CZTSSe absorber,and reduce carrier recombination,this thesis mainly carried out the following two aspects:（1）Solving the problem of grain structure delamination of the absorber and reducing the carrier recombination in the bulk:We adjusted the initial mass of selenium particles during selenization to regulate the degree of selenium vapor entering the film at high temperature,so that the direction of grain growth was regulated,which makes grains grow into CZTSSe absorber with large-grain spanning single-layer structure,and the negative influence of secondary phase is also avoided.Compared with the large-grain double-layer structure,the large-grain single-layer structure makes the bulk element distribution more uniform and decreases the band edge fluctuation caused by the uneven composition of the material elements,which causes that the carrier collection ability of the aligned neutral region is enhanced and the internal recombination of the bulk is reduced.And 11.05%efficiency of CZTSSe solar cell prepared based on this method has been obtained.This work provides a practical solution for fabricating high-quality CZTSSe absorbers and high-efficiency devices.（2）On the basis of the large-grain single-layer structure,improving the surface of CZTSSe absorber and reducing interface recombination:Polymethyl methacrylate（PMMA）organic passivation layer was prepared on the surface of absorber to reduce surface defects.At the same time,this way can avoid the problem that the inorganic interface layer is etched by ammonia during CBD,which decreases the passivation effect.And the passivation mechanism of PMMA is that the oxygen atoms in the carbon-oxygen double bonds in PMMA act as Lewis bases and interact with cation defects（Lewis acid） such as Cu,Zn and Sn that are not fully coordinated on the surface of absorber,so as to reduce the carrier trapping by defects.At the same time,the PMMA treatment not only increases the width of the depletion region,but also decreases the valence band maximum on the surface of absorber,resulting in the increase of the bending of the energy band,the enhancement of the built-in electric field strength and the reduction of interface recombination.However,PMMA also affects the absorption of light by absorber slightly, which is manifested as a slight decrease of the short-circuit current density.But compared with basic device,the open circuit voltage of the PMMA-treated device has increased from 472 m V to 510 m V,and the efficiency has increased from 11.10%to 12.06%.The strategy of PMMA organic passivation layer provides another idea for improving the efficiency of CZTSSe solar cells.