Optimization Of Back Interface And Defect Control Of High Efficiency Cu₂ZnSn(S,Se)₄ Thin Film Solar Cells

Cu₂ZnSn(S,Se)₄(CZTSSe)semiconductor has the advantages of earth abundant component elements and non-toxic,tunable band gap,and high light absorption coefficient.It is considered to be the promising absorber material for the new generation of inorganic compound thin-film solar cells.Bad back contact interface and large open-circuit voltage deficit(V_OC-deficit,E_g/q-V_OC)are two key factors limiting CZTSSe thin film solar cells.In this paper,a safe and environmentally friendly dimethyl sulfoxide(DMSO)solution method is used to prepare CZTSSe solar cells.Aiming at the issue of the back contact interface of CZTSSe solar cells:By introducing SnS selenization additives and MoO₂ sacrificial layer at the Mo/CZTSSe interface,systematically optimized the back contact interface of the device.By manipulating the phase structure of the precursor film and introducing Se&SnS co-selenization process to reduce the V_OC-deficit of the device,the effects of growth conditions on the band-tailing effects,the formation of defects,the micro-area secondary phase and the heterojunction interface recombination are discussed.Based on the optimization of the back contact interface and the control of defects,a CZTSSe thin-film solar cell with a photoelectric conversion efficiency(PCE)of 12.03%was prepared.The main contents and results are presented as follows:1.By optimized the ratio of Zn/Sn in the precursor solution,the secondary phase in CZTSSe is eliminated,the grain size is increased,and the crystallinity of the film is improved.CZTSSe films with different Zn/Sn ratios were prepared based on precursor solutions with different Zn/Sn ratios.When Zn/Sn=1.10,the secondary phase disappears,the grain size is larger,the film crystallinity is the best,and the photovoltaic performance of the CZTSSe solar cell is also the best.On this basis,the thickness of the absorber layer was increased by increasing the times of spin-coating,and the selenization temperature was reduced to reduce the thickness of Mo(S,Se)₂,which further improved the photovoltaic performance of CZTSSe solar cells and achieved a PCE of 7.31%.2.Introduced SnS selenization additive in the selenization process can improve the quality of CZTSSe absorber layer and optimize the Mo/CZTSSe interface contact.By introducing SnSe or SnS selenization additives in the selenization process,the effects of selenization atmosphere on the CZTSSe film and the Mo/CZTSSe back contact interface was studied.The Sn-atmosphere can effectively suppress the loss of Sn element and the formation of voids in the CZTSSe film.The S-atmosphere can not only increase the S/(S+Se)ratio in CZTSSe,but also effectively reduce the thickness of the Mo(S,Se)₂ layer at the back contact interface.Finally,by adding SnS selenization additives,we successfully prepared a CZTSSe thin-film solar cell with a PCE of 8.53%.3.The technology of preparing MoO₂ sacrificial layer on Mo electrode was obtained.MoO₂ sacrificial layer of can improve the back interface of CZTSSe solar cell and greatly reduce the back contact barrier.The van't Hoff equation was used to reveal the chemical mechanism of the transformation of MoO₂ into Mo Se₂.The Mo electrode was modified by air-annealing process.After annealing,the surface of the Mo electrode is transformed into MoO₂.After selenization,MoO₂ can be transformed into Mo Se₂,and the Van't Hoff equation can describe the chemical mechanism of this phenomenon well.Therefore,we put forward the concept of MoO₂sacrificial layer for the first time.The introduction of the MoO₂ sacrificial layer into the Mo/CZTSSe interface can fully optimize the back contact of the CZTSSe device and greatly reduce the back contact barrier.Without the Mg F₂ anti-reflection layer,by introducing a MoO₂ sacrificial layer at the Mo/CZTSSe interface,the PCE of the CZTSSe device is significantly improved.In addition,by improving the preparation environment of the precursor film,the voids at the back contact interface disappeared,and the PCE of CZTSSe cell was also greatly increased to 10.07%.4.The preparation process of the precursor film was optimized,and the pure-phase precursor film was obtained.The pure-phase precursor film can reduce the band-tailing effects of CZTSSe and reduce the interfacial recombination of heterojunction.Finally,CZTSSe solar cells with an efficiency of 11.51%were prepared.The influence of the phase structure of the precursor film on the defects and performance of CZTSSe solar cells was studied.Due to the heterogeneous distribution of composition and phase of the mixed-phase precursor film,there are more detrimental intrinsic defects in the CZTSSe.The composition and phase distribution of the pure phase precursor film are uniform,the chemical potential is stable,and the resulting absorber layer has fewer detrimental defects.Thus,use of pure-phase precursor films can effectively reduce the band-tailing effects of CZTSSe,broaden the depletion width of the heterojunction,and reduce the CZTSSe/Cd S interface recombination,thereby significantly improving the photovoltaic performance of CZTSSe solar cells(PCE=11.51%).5.The Se&SnS co-selenization process can inhibit the generation of micro-area secondary phase and effectively reduce the detrimental defect of CZTSSe.The total area efficiency of CZTSSe solar cells is improved to 12.03%.The effects of Se&SnS co-selenization process on micro-area secondary phase of CZTSSe,intrinsic defects and device performance were studied.The Se&SnS co-selenization process can effectively suppress the Cu₂(S,Se)and Zn(S,Se)micro-area secondary phase formed by the decomposition of CZTSSe.This process can also reduce the concentration of Cu _Zn antisite defects,thereby reducing the pinning-effect at the interface and enhancing band-bending.On the other hand,reducing the concentration of Cu _Zn antisite defects can also reduce the[2Cu _Zn+Sn _Zn]defect clusters in CZTSSe,thereby reducing the band-tailing effects of the absorber layer.Moreover,the Se&SnS co-selenization process also reduces the recombination of CZTSSe devices in the space-charge region,broadens the depletion width,and reduces the non-radiative recombination of CZTSSe.Based on the Se&SnS co-selenization process,the V_OC-deficit of the CZTSSe device is effectively suppressed,and the PCE is also greatly increased to 12.03%.