Influence Of Alloying On Optoelectronic Properties Of Copper-Based Polychalcogenide Thin Films

Cu₂ZnSn?S,Se?₄?CZTSSe?,as a member of copper-based polychalcogenide,has been considered as a promising absorber material for thin film solar cells due to its favorable optoelectronic properties with a high absorption coefficient and a suitable band gap,as well as earth-abundant constituents.So far,highest efficiency of 12.6%has been obtained for CZTSSe solar cells,which is much less than that of 32.8%for the theory prediction.One of the main reasons for the low efficiency is high-concentration Cu substituting Zn(Cu_Zn)antisite defects in CZTSSe absorber which will lead to a large voltage(V_oc)deficit and limit the enhancing of the V_oc.It is found that the CZTSSe always show p-type conductivity and cannot have an effective p-to-n type inversion near the CdS/CZTSSe interface because the high concentration of Cu_Znn at the interface pins the Fermi level at the middle of the band gap.As a result,only a small band bending can be induced in the absorber layer,causing a large deficit of V_oc.Therefore,suppressing the formation of the Cu _Zn defect is promising way to increase V_oc.It has been reported that the Cu_Zn antisite defects are facility formation due to the semblable ionic radius of Cu and Zn cation,which leads to low formation energy of the Cu_Znn antisite defects.One of the feasible methods to restrain the formation of Cu_Zn antisite defects is to replace Cu or Zn in CZTSSe by atoms with imparity sizes,which can increase the V_oc and power conversion efficiency?PCE?.In addition,the band alignment of the CZTS?e?/CdS heterojunction is not the ideal type,which is another factors limit the further enhancement of the PCE.Therefore,reasonable change the crystal structure and tune the band gap of CZTSSe thin film are effective strategy to further enhancement of the efficiency for the CZTSSe thin film solar cells.In view of these,we fabricated the(Cu_1-xAg_x)₂ZnSn?S,Se?₄?CAZTSSe?alloy thin films by introducing Ag to CZTSSe absorber to change the crystal structure and tune the band gap of CZTSSe thin film and then restrain the formation of Cu_Zn antisite defects and enhancement of the efficiency for the CZTSSe thin film solar cells in this thesis.In addition,we also explore suitable innovative materials to replace CZTSSe absorber.The results are as follows:1.We fabricated the(Cu_1-xAg_x)₂ZnSn?S,Se?₄?CAZTSSe?x???0?x?0.25?alloy thin films by a simple solution approach combined with a post-selenization technique,and investigated the influence of Ag contents on structure,morphology,band gap as well as electrical property of CZTSSe thin films.The results showed that Cu⁺cation in Cu₂ZnSn?S,Se?₄?CZTSSe?films was replaced by Ag⁺cation,forming homogeneous CAZTSSe?x??0?x?0.25?alloy thin films.The incorporation of Ag is found to accelerate the grain growth and increase the grain size.The optical band gap of CAZTSSe alloy thin films can be continuously tuned by change the content of Ag.The power conversion efficiency of the CAZTSSe solar cell with suitable Ag content is 6.04%,which demonstrated a promotion compared with the pure CZTSSe device?PCE of 3.39%?without Ag addition,and related mechanisms are suggested.2.Ag₂ZnSn?S,Se?₄?AZTSSe?thin films were synthesized at low temperatures using sol-gel method.A single phase of AZTSSe film with kesterite structure,high crystal quality and composition of Zn-poor,Sn-rich and Se-rich was prepared by optimized selenization contitions.It is found that the AZTSSe film shows an optical band gap of 1.4 eV and an n-type conduction with an electron concentration of5.7×10¹⁵ cm^-3 and a mobility of 9.1 cm²V^-1s^-1.A Cu₂ZnSnSe₄?CZTSe?/AZTSe-based solar cell was designed and prepared by inserting AZTSe layer between CZTSe and CdS of the traditional CZTSe-based solar cell.Compared with the CZTSe-based solar cell,we observed an increment in open circuit voltage(V_oc)by 95 mV and an accompanying rise in power-conversion efficiency?PCE?from 3.40%to 4.72%.3.To well understand the PCE improvement of the CZTSe/AZTSe-based solar cell,we calculated the band alignments of CZTSe/AZTSe and CZTSe/CdS heterojunctions using first-principles calculations,demonstrating that the CZTSe/AZTSe and CZTSe/CdS interfaces have type-II and type-I band alignments,respectively,and the AZTSe/CdS has larger conduction-band offset than CZTSe/CdS.Therefore,the increase in the V_oc is suggested to due to enlarged band bending and increased interfacial quality induced by the AZTSe.Combined with the calculation results,the mechanism of influence of the AZTSe on the PCE improvement is discussed in detail.Our results suggest that the addition of the AZTSe layer is a potentially applicable method to obtain CZTSe-based solar cells with higher V_oc and PCE.4.We fabricated solid solution of Cd in the Cu₂SnS₃?CTS:Cd?and Cu₂CdSnS₄?CCTS?thin film by Cd doping in the Cu₂SnS₃?CTS?with a simple solution approach and sulfurization process,and investigated the effects of Cd content and sulfurization on crystalline structure and quality as well as bandgap?Eg?.It was found that a face-centered cubic CTS?C-CTS?film with Eg of 0.82 eV can be prepared by the solution approach.The E_g can be tuned from 0.82-1.26 eV by changing the Cd doping content for the as-fabricated samples.The sulfurization process made the C-CTS dissolve into C-CTS and T-CTS,and the T-CTS:Cd transformed into the CCTS in the Cd content of 7.30-13.54 at%.A single phase of CCTS film with Eg of 1.37 eV was obtained in the content of 10.18-13.54 at%.They are considered as promising absorption layer materials for preparation of solar cell.The sulfurization process can change crystalline structure,improve crystalline quality and tune band gap of the CTS and CCTS films at the same Cd doping content compared with the as-prepared films.It is believed that the single phase of C-CTS,T-CTS:Cd and CCTS films will be promising absorber materials of solar cells.