Interface Modification Of Cu₂ZnSn（S,Se）₄ Thin Film Solar Cell And Its Effect On Photovoltaic Performance

The compound of kesterite(Cu₂ZnSn(S_x,Se_1-x)₄,CZTSSe)is a P-type direct band gap semiconductor,whose structure is similar to CIGS.The band gap is continuously adjustable between 1.0-1.5 e V,and the absorption coefficient can reach more than 10⁴ cm^-1.It’s an ideal absorption material for thin film solar cells,which has many advantages such as simple film preparation process,abundant composition elements and non-toxic.At present,the photoelectric conversion efficiency of CZTSSe thin film solar cells has reached 13.6%,which is close to the commercial level,but still far from the theoretical efficiency of 32.2%.Exploring effective ways to improve the conversion efficiency of CZTSSe solar cells is a research hot spot in this field.In this paper,with the title of"Interface Modification of Cu₂ZnSn（S,Se）₄ Thin Film Solar Cells and Its Effect on Photovoltaic Performance,"the author systematically summarizes the research conducted during master’s degree study.It mainly includes the following three aspects of research content and achievements:（1）The effect of Mo electrode storage and back contact treatment on CZTSSe battery performance was researched.Firstly,the Mo electrode of different storage environments and surfaces was used as the back electrode.Then,CZTS precursor film was prepared on the back electrode by spinning coating method,and CZTSSe photosensitive layer was obtained after selenization.Finally,CdS buffer layer,ZnO window layer,ITO window layer transparent conductive layer and Al grid electrode were deposited on the absorption layer by chemical bath deposition,magnetron sputtering and thermal evaporation respectively.Then we’ve completed the whole manufacturing progress of solar cell device.CZTSSe thin film solar cell assembled based on Ar storage Mo electrode increased FF by 14.4%and PCE by 7.22%to 8.01%compared with low vacuum（10 kPa）storage Mo.Compared with the Mo electrode before polishing,the FF and PCE of the Mo electrode after polishing are increased by 12.1%and 7.77%respectively.The Voc of Mo electrode stored in Ar environment increased by 11mV,FF by 5%and PCE by 8.30%after polishing.The results indicates that the Mo electrode stored in a low vacuum（10kPa）will produce a thicker oxide layer due to the presence of oxygen.While the surface resistance is larger than before which hinds the contact of Mo/CZTSSe interface,and the series resistance is larger after device assembly.The device performance of Mo electrode stored in Ar environment is better.At the same time,increasing the roughness of Mo surface,eliminating the surface oxide layer,descending the surface resistance and the series resistance of the device can be accompolish through proper mechanical polishing,and eventually,increasing the photoelectric conversion efficiency.（2）The interface quality between the photosensitive layer and buffer layer is one of the important factors affecting the conversion efficiency of CZTSSe thin film solar cells.In this work,an efficient and simple method for CZTSSe absorption layer treated with（NH₄）₂S solution at room temperature was proposed to improve the surface wettability,improving the quality of the heterojunction interface and improve the photovoltaic performance of the device.First,the effects of CZTSSe films treated with 2.78 mol/L（NH₄）₂S solution（pH 9.7）for 0 min,10 min,20 min,30 min,40 min,50 min,60 min and90 min were investigated.Then,the variation rule of device performance was studied by assembling solar cells with samples treated under different conditions.After 50 min treatment,the surface wettability can be significantly changed to super hydrophilic,and high quality CdS/CZTSSe heterojunction can be formed in the chemical bath.Finally,the device efficiency of 9.88%,51%higher than that of the untreated battery（6.54%）,the Voc of 452 mV increased by 37 mV,FF increased by 8%and up to 60.56%.The results shows that（NH₄）₂S solution can not only increase the built-in electric field and depletion layer width,but also inhibit carrier recombination and prolong carrier lifetime,so as improving the photovoltaic performance of the device.In this study,the effect of（NH₄）₂S solution treatment on the quality of heterojunction was analyzed from the perspective of interface wetting.The research results will be helpful to further understanding the effects of interface treatment on the improvement of equipment performance.（3）Aiming at the PN junction quality of CZTSSe solar cells,a new NH₄Br solution post-treatment technology which can effectively reduce the open-circuit voltage loss was proposed in this work.First,CZTSSe film was immersed in 0.1 mol/mL,0.01 mol/mL and 0.001 mol/mL NH₄Br solution for 5min,and the effect of CZTSSe film on the surface morphology of the absorption layer was studied.Then,the solar cells were assembled and the device performance parameters were analyzed.After treatment with 0.01 mol/mL NH₄Br solution,the 424 mV Voc increased by 50 mV,Jsc increased from 25.35 mA/cm²to 29.38 mA/cm²,FF increased by 26.32%,PCE increased from 5.24%to 7.67%.It was found that the surface of CZTSSe was treated with appropriate concentration of NH₄Br solution with appropriate concentration turned into a hydrophobic state,and the grain size was uniform,which reduced the bandtail effect and the open-circuit voltage loss.