The Influence Of Sn Oxidation State And Lithium Doping On The Grain Growth And Device Performance Of CZTSSe Thin Films Prepared By DMSO Solution Method

Energy crisis and environmental pollution urge the development of new energy sources.As clean energy,solar energy has been widely researched because of the advantage of inexhaustible,and one of the ways to utilize solar energy is solar cell.Among various kinds of solar cells,kesterite structured copper zinc tin sulfur selenium（Cu₂Zn Sn（S,Se）₄,CZTSSe）is a kind of potential green photovoltaic materials,enjoying the merits of abundant element component,large light absorption coefficient(>10⁴ cm^-1),adjustable band gap（1.0～1.5 e V）,and theoretical power conversion efficiency as high as32.2%.However,time has witnessed the slow development of this material.In 2013,Mitzi et al.fabricated the CZTSSe device with a world efficiency record of 12.6%using hydrazine solution method.In 2019,through vacauum sputtering,Kim et al.also successfully fabricated the champion CZTSSe device with a certification efficiency of 12.62%.It is both a challenge and an opportunity that the efficiency of the CZTSSe solar cell has still pinned at 12.6%after 6 years.CZTSSe and copper indium gallium selenium（Cu（In,Ga）Se₂,CIGS）have similar crystal structures and band gaps,but the CIGS record efficiency is 23.35%,much higher than 12.6%of CZTSSe.From the perspective of device photovoltaic parameters,the primary difference between CZTSSe and CIGS is the low open circuit voltage(V_oc).However,the low open circuit voltage of CZTSSe is a phenomenon behind the material property.Only from the microscopic perspective of grain growth can we understand this poor performance,and then improve device performance in a more direct and effective manner.The multi-element compositon of CZTSSe causes a large number of intrinsic defects in this material.What’s more,deep-level defects associated with Sn are extremely detrimental to device performance.In addition,Sn has two oxidation states in compounds.Therefore,it is necessary to investigate the grain growth of CZTSSe and conduct research on the effect of Sn valence state.Based on such advantages that different metal salts and dopants can be easily introduced,and valence state of elements and content of compounds can be pricisely controlled by simply weighing the precursors,solution method is chosed to carry out the experiments in this thesis.Combined with the concept of green development,dimethyl sulfoxide（DMSO）is utilized as solvent.In this thesis,the influence of Sn valence state(Sn²⁺and Sn⁴⁺)in precursor compounds on CZTSSe grain growth and device performance has been systematically investigated,and the effect of lithium doping has been studied,too.The research content is as follows:（1）The influcence of Sn valence state(Sn²⁺and Sn⁴⁺)in precursor compounds in DMSO solution on the property of CZTSSe thin film,progress of grain growth and performance of device was investigated,respectively.The influence of Sn valence state is reflected from the chemical reaction in the precursor solution first.Sn Cl₄ coordinates with the solvent DMSO to form Sn（DMSO）₄Cl₄,while Sn Cl₂ coordinates with thiourea（Tu）to form Sn（Tu）₂Cl₂.Sn⁴⁺precursor film is an amorphous CZTS phase,and its grain growth process is a substitution reaction of Se to S in CZTS.The advantage of Sn⁴⁺film is that the selenization reaction is easy to control,the device efficiency can generally reach 9%,and the best device efficiency achieves 9.72%;the disadvantage is that the double layer structure CZTSSe and a thick Mo Se₂ layer growing under the influence of Mo/CZTS interface limit the short circuit current density(J_sc)of the Sn⁴⁺device.The Sn²⁺precursor film is a mixed phase of crystalline CZTS and various sulfides,and its grain growth is a multiphase fusion reaction.The Sn²⁺film has a single layer structure CZTSSe as well as a thin Mo Se₂ layer,and the J_sc of Sn²⁺device is generally higher than that of Sn⁴⁺counterpart;the disadvantage is that numerous secondary phases in the growth process of Sn²⁺type make the film uniformity worse,resulting in the poor device efficiency,which is often not as good as Sn⁴⁺device,and the best device efficiency is only 8.87%.（2）The effects of lithium doping（2.5 at.%）on the properties of Sn²⁺and Sn⁴⁺films and device performance were studied.It was found that Li⁺cannot enter the CZTS lattice during the preparation of precursor films,and can only enter the CZTSSe lattice after high temperature selenization.The introduction of lithium improved the uniformity of Sn²⁺devices,and the fill factor（FF）has been significantly improved.However,there was no significant improvement on Sn⁴⁺device parameters with the same concentration of lithium doping.At the same time,it was verified that the multiphase fusion grain growth of Sn²⁺film requires more stringent control of selenization than the substitution reaction grain growth of Sn⁴⁺film through comparing the selenization experiments of different selenization furnaces.（3）The effect of lithium doping at different positions of Sn²⁺films on the device performance was investigated.Lithium doping could not improve the morphology of Sn²⁺film,but the closer the doping position to the Cd S buffer layer,the more pronounced J_sc increase in Sn²⁺device,in which J_scwas increased by nearly 2 m A/cm² when lithium doping on the top of the precursor film.Of course,it is the FF that was affected most significantly:the bottom doping of lithium destroyed the interface contact between the absorption layer and the Mo layer,the FF was seriously reduced,and the efficiency was worse than the standard one（device with no lithium doping）;the middle doping improved the quality of CZTSSe absorption layer,whose device enjoyed the largest V_oc and the efficiency was slightly higher than the standard;the top doping modified the interface between the absorber layer and the buffer layer.Each parameter of the top-doped device has been notably improved,and the best Sn²⁺device with an efficiency of 9.55%was obtained.In this thesis,the influence of Sn valence state(Sn²⁺and Sn⁴⁺)in precursor compounds in DMSO solution on the grain growth of CZTSSe was systematically investigated and two types of grain growth mechanism were proposed;the conjecture that the multiphase fusion grain growth of Sn²⁺film requires more stringent control of selenization than the substitution reaction grain growth of Sn⁴⁺film was verified;the efficiency of Sn²⁺device was improved to 9.55%by doping lithium deliberately and at the same time,the specific impact of lithium on the performance of Sn²⁺devices was studied.The optimization of device performance requires different strategies as there are Sn²⁺and Sn⁴⁺two distinct types of grain growth mechanisms,which provides new ideas for further improving the efficiency of CZTSSe thin film solar cells.