Study On Materials And Devices Of Cu₂ZnSnS₄ Thin Film Solar Cells

Due to the elemental abundance and non-toxicity,Cu₂ZnSnS₄（CZTS）solar cell has attracted much attention in the past decade,and the world record efficiency of pure CZTS solar cell has achieved to 11%by Chang Yan et al.in 2018.The best solar cells based on CZTSSe have already shown efficiency of almost 12.6%.In this paper,the preparation process and performance of the absorption layer and Cu₂ZnSnS₄ solar cell were studied.The absorber layer was prepared by co-sputtering and annealing,and its crystal structure,surface morphology and photoelectric properties were measured and analyzed.By optimizing the composition and CBD-CdS/CZTS interface,the conversion efficiency of 7.7%（full area）and 8.2%（effective area）was obtained.This thesis mainly got the following results:First of all,the effects of four key sputtering parameters,deposition power,pressure,temperature and time,on the resistance and light transmittance of ZnO films prepared by RF magnetron sputtering were studied.It is found that the film resistance is very sensitive to the above deposition parameters,while the optical transmittance is almost unchanged.Based on these properties,high-grade AZO thin films（sheet resistance of 4.54Ω/sq,transmittance more than 84%,1500nm thickness）and a CZTS thin films solar cell with conversion efficiency of 5.1%with AZO as electrode was grown by optimizing deposition parameters to 0.3Pa,200℃,120minutes,and 180W,using AZO ceramic target（2wt%Al₂O₃）.We also firstly modifying the traditional only solutions cleaning（OSC）method to burning treatment method（BTM）,which is not only time-saving,but also avoiding the stripping of Mo layer and the usage of toxic and organic reagent of acetone.Second,the mechanism of V_oc deficit in CZTS solar cell has been systematically investigated by depositing the CdS thin layer with sputtering（SP）and chemical bath deposition（CBD）,respectively.It is found that SP-CdS film exhibits larger crystallite size and more densely packed morphology than CBD-CdS film.However,CZTS solar cell with CBD-CdS shows less V_oc deficit and higher conversion efficiency.By virtue of Hall and TEM measurements,the mechanisms are further revealed:i)CdS can diffuse into CZTS layer easily and form a shallow n-type layer by CBD method.ii)CBD-CdS possesses high resistivity and tends to form colloidal,which can block pin-holes and type I voids between the grain boundaries.All of them improve the CZTS/CdS interface and raise the value of R_sh,and reduce the V_oc deficit and boost the conversion efficiency.By optimizing the stoichiometric ratio of Cu/Sn and Zn/Sn,the best conversion efficiency of CZTS solar cells with CBD-CdS is beyond 8.2%（active area）without MgF₂ anti-reflection coating.Third,the low temperature efficiency collapse and carrier recombination mechanism of CZTS thin film solar cells were studied by using temperature dependent J-V,C-f and PL spectra.The temperature dependent J-V shows that when the temperature decreases below 160K,the series resistance R_s increases sharply,resulting in the rapid collapse of the solar cell efficiency.The C-f spectrum of CZTS cell shows that there is a significant carrier freezing effect at low temperature,and a deep defect energy level with an activation energy of about 40 meV is distinguished;the C-f spectrum of CZTS monolayer shows similar results,indicating that the deep level defect in CZTS absorption layer is the main cause of carrier freezing.The PL spectra further shows that both shallow localized state and deep level luminescence peaks increase sharply while the temperature is lower than 160K.Further analysis indicates that the efficiency collapse of CZTS solar cells at low temperature should be closely related to the freezing out of carriers in[2Cu_Zn+Sn_Zn]cluster,[Cu_Zn+Sn_Zn]cluster,Sn_Zn defects and Cu_Zn defects.These defects can emit fluorescence at 1.27eV,0.92eV and 0.82eV and their intensity will be enhanced with decreasing the temperature.In the meanwhile,AMPS-1D simulation also shows blocking phenomenon of CZTS/CdS interface at low temperature.To summarise,both the freezing effect of defects and the blocking phenomenon of CZTS/CdS interface will lead to the collapse of efficiency.Finally,the effects of different annealing process and component ratio on the preparation and defect properties of CZTS absorption layer were studied.According to the melting point（118℃）and the boiling point（444.6℃）of sulfur,the affects of holding time at 280℃（S2 stage）and heating rate from 280℃to 610℃（S3 stage）are explored.By prolonging the holding time of S2 stage and the heating time of S3 stage,the flatness of the crystal surface and grain size are greatly improved,meanwhile the fine crystal layer is eliminated.Based on the FTIR（Bruker 80V）,the PL spectra,absorption and reflection spectra of CZTS films with different components were measured.The evolution of the deep and shallow defect states with the composition ratio is revealed.And the originations of the deep level fluorescence peaks of 0.78eV,0.82eV,0.92eV and 1.1eV,as well as the near band edge tail state fluorescence peaks of 1.27eV are also discussed.