Study On Composition Control For Cu₂ZnSn（S,Se）₄ Thin Films And Solar Cells

Quaternary semiconductor Cu2ZnSnS4(CZTS)has attracted much intention due to its outstanding properties as photovoltaic materials,such as its high absorption coefficient and readily available elements.In this work,we have investigated the preparing process of CZTS thin films solar cell and fabricated CZTS thin films with different Cu or Zn content.CZTSSe thin films and solar cells with different S/Se ratio are also produced.Furthermore,the electronic structure of CZTS is also studied through PL measurement.The results are summarized as follows:1.The influence of CZTS thin films with different Cu composition on the properties of CZTS films and performance of CZTS solar cells has been systematically investigated and results indicate that the increased Cu composition has a positive effect on grain growth.However,the low band gap impurity Cu2SnS3 and voids appeared in Cu-rich samples leaded to a dramatic decrease of Voc and extremely poor photoelectric performance.Cu content variation is realized by adjusting the sputtering time of Cu target.The results of XRD patterns,Raman spectra and far-IR spectra show that Cu composition has a significance effect on secondary phases formation in CZTS thin films.In Cu-poor case,SnS tends to form and when Cu composition increases to Cu-rich situation,Cu2SnS3 take the place while the formation of SnS is suppressed.The grain growth is enhanced as Cu content increased,but in Cu-rich some voids are observed near the Mo/CZTS substrates interface,caused by the incompleted reaction of ZnS and Cu2SnS3.J-V characteristics for all solar cells indicate that solar cell with Cu-rich absorber layer exhibits relatively poor conversion efficiency due to the low band gap secondary phase Cu2SnS3 in Cu-rich sample induces significant cell leakage and leads to low shunt resistance(RSh).The best conversion efficiency of 3.46%is achieved with Cu-poor sample.2.The influence of Zn composition on the characteristics of CZTS films and performance of CZTS solar cells has been studied.Resluts show that CZTS thin film with slightly Zn-rich composition suited to be absorber film of solar cell.CZTS thin films with different Zn composition are achieved by controlling the sputtering time of Zn target.From the result of XRD pattern and Raman spectra,we can see the increase of Zn content enhanced the formation of high resistance impurity ZnS.SEM result shows that as Zn content increase,the grain growth of CZTS thin film tends to be strengthened and Zn-poor CZTS thin film is very thin.Furthermore,through the result of IV curve,the Voc and Rsh of Zn-poor CZTS solar cell is lower than Zn-rich samples.That is,the increase of Zn content will lead to a dramatic promotion of Voc and solar cell efficiency,while Rs may also increased by ZnS.Conclusion can be drawn that CZTS thin film with slightly Zn-rich composition suited to be absorber film of solar cell.3.CZTSSe thin films and solar cells with controllable S/Se ratio are fabricated by sulfo-selenization sputtered precursors.The S/Se ratio of CZTSSe thin films is adjusted by tuning the consumption of S,Se powder during the sulfo-selenization process and single phase CZTSSe thin films have been fabricated.The result of morphology characterization shows that the S and Se vapour mixture has higher reactivity and the mix atmosphere promotes the grain growth of CZTSSe crystals,IV characteristics results of CZTSSe solar cells with various S/Se ratio reveal that CZTSSe solar cell with appropriate S/Se ratio has higher Jsc,fill factor and efficiency than the pure CZTS and CZTSe solar cell.4.The electronic structure and carriers recombination mechanisms of CZTS semiconductor has been studied through the analysis of PL spectra.Result shows that Cu-poor CZTS solar cell is compatible to the fluctuating potential model.The PL peak of CZTS thin film at 4K is asymmetric and broad.And the PL peak energy at-1.17 eV is much smaller than the literature reported band gap 1.5 eV.So,this PL peak can not correspond to the radiation from conduction band to balance band and donor-accepter pair model can not be used to describe CZTS.Temperature dependence of PL spectra for CZTS thin-film solar cell with Cu-poor composition shows that the intensity of PL peak decrease obviously as temperature increases.Through the calculation of thermal actibation energies of 2 nonradiative channels,the existence of acceptor level Cuzn is confirmed.Based on these analysis,we assume the potential fluctuation model is appropriate to describe CZTS material and use excitation power depentend PL spectra to verify this theory.The observed large peak shift(12 meV/decade)confirms that the fluctuating potential model is applicable to CZTS.