Study On The Mechanism Of Open Circuit Voltage Deficit In Cu₂ZnSn（S,Se）₄ Solar Cells:Grain Growth And CZTSSe/CdS Interface

Developing effective,low-cost,stable solar absorbers composed of nontoxic,earth-abundant elements has long been the holy grail of next-generation photovoltaics（PV）technology.Kesterite structured copper zinc tin sulfoselenides（CZTSSe）absorbers with a theoretical efficiency of 33.2%basically meet the above criteria.However,the efficiency of CZTSSe solar cells had stagnated at12.6%for a long time due to the severe open circuit voltage deficit(V_OC,def).Despite the similarity between CZTSSe and CIGS in terms of structure and electrical properties,the record efficiency CZTSSe solar cell has a V_OC,def of 0.373 V,while the record efficiency（23.4%）CIGS solar cell has a V_OC,def of only 0.105 V.The key to developing CZTSSe solar cells is to reduce the V_OC,def.In this context,this dissertation prepares CZTSSe solar cells by DMSO solution process and investigates the precursor solution chemistry,grain growth mechanism,intrinsic defect control,and CZTSSe/Cd S interface optimization to achieve CZTSSe controlled synthesis,harmful defect suppression,and CZTSSe/Cd S interface recombination elimination.Finally,CZTSSe solar cells with a V_OC,def as low as 0.295 V and a maximum power conversion efficiency（PCE）of 13%are achieved,and the main contents of this thesis are as follows:（1）Study of the effect of annealing temperature of precursor films on the quality of CZTSSe film and the PV performance of the CZTSSe solar cells.Based on the characterization of precursor films and CZTSSe absorbers prepared at different annealing temperatures,it was revealed that the effective control of precursor film crystallinity is important for the preparation of high-quality CZTSSe absorber,and the mechanism of grain growth of precursor films with different crystallinity in the selenization reaction is elaborated.It is demonstrated that annealing temperatures below 450°C produce low-crystallinity precursor films,which can be selenized into high-quality CZTSSe absorber.This work obtained the optimal annealing parameters for the precursor film,which provided an experimental fundamental for the subsequent research work in this dissertation.（2）The study of solution chemistry,solution-to-absorber chemical reaction paths and their effects on the PV performance of the CZTSSe solar cells.In this dissertation,we prepared precursor solutions using Sn Cl₂ and Sn Cl₄ as Sn precursors and Cu Cl,Zn Cl₂,Zn（OAc）₂ and SC（NH₂）₂（Tu）as Cu,Zn and S precursors,respectively,and elucidated the chemical reaction paths from the two solutions to the films by studying the chemical reactions in the two solutions,revealing the basic principles of the solution chemistry determining the reaction paths and the optoelectronic characteristics of the absorber films.We found that Sn Cl₂ is coordinated with thiourea to form Sn（SC（NH₂）₂）_xCl₂ in the precursor solution with Sn Cl₂ as the precursor,and its solid Sn-S bond causes the solution to generate a Sn S phase after thermal annealing,which in turn leads to a precursor film consisting of a mixed phase(Sn S,Zn S,Cu_2-xS,CZTS).The precursor film transforms into CZTSSe absorber by a multiphase fusion reaction path,resulting in the CZTSSe film having poor optoelectronic properties and the PCE of the device efficiency being only 8.8%.In the percursor solution with Sn Cl₄ as the precursor,Sn Cl₄ only coordinated with DMSO to form Sn（DMSO）₂Cl₄,and the weaker Sn-O coordination bond broke during annealing allowing Sn⁴⁺to bond with the neighboring S atoms,resulting in a prepared precursor film consisting of only the CZTS phase.It is converted to CZTSSe absorber through the single-phase conversion reaction path,which has good morphology and excellent optoelectronic properties,and the PCE of CZTSSe solar cells fabricated from it reaches12.2%.We also found that the heterojunction heat treatment（JHT）significantly enhanced the V_OCand FF of the devices prepared from the precursor solution from Sn Cl₄,with little effect on the devices prepared from the precursor solution from Sn Cl₂.With JHT process,the CZTSSe solar cell prepared from the precursor solution from Sn Cl₄ has a V_OC,defas low as 0.297 V and a PCE of 12.4%.（3）Grain growth mechanism of multi-phase chemistry and single-phase conversion and its intrinsic connection with the V_OC,def of the CZTSSe solar cells.On the basis of the aforementioned study（2）,the grain growth mechanism of mixed-phase(Sn S,Zn S,Cu_2-xS,CZTS)and single-phase precursor films（CZTS）transformed into CZTSSe absorber is further investigated,and the root cause of the grain mechanism affecting the V_OC,def of CZTSSe solar cells is elucidated.We found that for mixed-phase precursor films,the sulfide is first converted to selenide,followed by the multiphase selenide fusion to form CZTSSe with complex multi-step grain growth mechanism,during which the presence of a large number of secondary phases,especially the Sn Se₂ phase,causes drastic compositional fluctuations,leading to the formation of deep level defects on the surface of CZTSSe films,resulting in severe recombination in the device space charge region.The single-phase CZTS precursor film undergoes single-phase conversion grain growth mechanism to form CZTSSe film,the grain growth without the participation of any secondary phases,this grain growth mechanism can realize the controlled synthesis of high-quality CZTSSe film and high-efficiency CZTSSe solar cells.（4）Ag alloying strategy to suppress Cu _Znantisite defects and band-tailing for CZTSSe thin films.Based on the understanding of the aforementioned solution chemistry and grain growth mechanism,Ag alloyed CZTSSe thin films（ACZTSSe）were prepared by a single-phase conversion reaction path,which effectively suppressed the band-tailing and reduced the V_OC,def of device.We found that Ag⁺and Cu⁺have similar chemical reactions in solution and both coordinate with thiourea.The precursor solutions are annealed to form single-phase ACZTS precursor films,followed by single-phase conversion grain growth to form high-quality ACZTSSe absorbers.Ag alloying improves the film morphology,reduces the concentration of Cu _Zn,effectively suppresses the formation of[2Cu _Zn+Sn _Zn]clusters,significantly reduces the band tailing.With 5%Ag alloying,the PCE of CZTSSe solar cells increased from 11.2%to 12.5%and the V_OC,def decreased from 0.319 V to 0.295 V.（5）Study of the underlying mechanism of heterojunction heat treatment（JHT）to reduce the recombination at the ACZTSSe/Cd S interface.The previous study found that JHT process significantly enhanced the V_OC and FF of the CZTSSe solar cells prepared from Sn Cl₄ precursor solutions,but the mechanism was unknown.By systematically studying the changes of the composition on the surface of CZTSSe during the deposition of the Cd S buffer layer,the recombination paths before and after the JHT process,and the element diffusion and micro-nano structure in the ACZTSSe/Cd S interface region,we reveal the primary sources of interface recombination of ACZTSSe/Cd S solar cells and elucidate the underlying mechanism of JHT to reduce interface recombination.We found that,during the Cd S deposition process,the Zn portion of the ACZTSSe surface is etched by NH₄OH,forming a defect-rich ACZTSSe/Cd S interface,which is an important cause of the interface recombination of CZTSSe solar cells.JHT enables the migration and rearrangement of elements near the ACZTSSe/Cd S interface,forming an ACZTSSe/Cd S coherent interface,which effectively reduces the interface defect concentration and significantly inhibits interface recombination,thus enhancing V_OC and FF of the solar cells.As a result of these efforts,a record efficiency ACZTSSe solar cell was achieved in this dissertation with a certified efficiency of13.0%and V_OC,J_SC and FF of 0.529 V,33.5 m A cm^-2 and 72.9%,respectively.In summary,in order to address the V_OC,def challenges of the CZTSSe solar cell,the dissertation has systematically conducted some exploratory studies on the controlled synthesis of high-quality CZTSSe thin films,the bulk defect control,and the CZTSSe/Cd S interface improvment.The research ideas proposed and the scientific principles revealed in this dissertation provide several valuable techniques and enlightening knowledge for the development of CZTSSe and other compound semiconductors.