Performance Improvement Of Cu₂ZnSn?S,Se?₄ Thin Film Solar Cells Via Interface Engineering

Recently,kesterite Cu₂ZnSn?S,Se?₄?CZTSSe?has drawn much attention over the world for its potential to be an ideal absorber in solar cells due to its earth-abundant and nontoxic elements,high absorption coefficient(>10⁴ cm^-1)and tunable band gap?1.0¹.5 eV?.Although a world-record power conversion efficiency?PCE?of 12.6%has been reached for CZTSSe solar cell,the performance of CZTSSe solar cell still needs to be further improved compared with the Cu?In,Ga?Se2 and CdTe thin film solar cells whose PCE have exceeded 20%already.It has been demonstrated that the low open-circuit voltage(V_oc)or open-circuit voltage deficit(V_oc-def=E_g/q-V_oc)are currently the biggest hurdle preventing CZTSSe devices from achieving higher efficiency.It is reported that the V_oc-def of solar cells is basically above 60%,which is mainly caused by two aspects:?1?Too much defects and band tail states within crystal can result in a serious recombination of photogenerated carriers.2)Interfacial problems induced by secondary phases at Mo/CZTSSe and CZTSSe/CdS interface.Due to the extremely tiny region that allows the chemical composition of single-phase CZTSSe to change,some secondary phases are prone to form at interfaces relative to CZTSSe,which can lead to the increase of recombination and thus decrease the short-circuit current density(J_sc)and V_oc of solar cells.Therefore,increasing V_oc or cutdown of V_oc-def has become a critical scientific issue to improve the conversion efficiency of CZTSSe solar cells.There have been many reports on the out-of-order defects and band tail states of CZTSSe absorber,but the interfacial problem still remains unsolved effectively.In this present work,we did some research mainly about how the secondary phases at the surface of CZTSSe affect the performance of solar cell and how to decrease the interfacial recombination of photogenerated carriers by the surface modification and structure adjustment of CZTSSe.The results obtained by us are listed as follows:1.The surface of CZTSSe films were etched by solution method to remove secondary phase,and elucidated the effects of etching on surface structure and performance of solar cell.The PCE has a maximum increase of 26%by etching.It is demonstrated by XRD,XPS,EDS and Raman measurement that the CZTSSe film surface contains a small amount of Cu₂ZnSn₃Se₈ and ZnSe secondary phases besides kesterite CZTSSe.After etched by KMnO₄/H₂SO₄+Na₂S,the Cu₂ZnSn₃Se₈ is completely but the ZnSe partially removed from the surface.And then the solar cell with conventional structure was fabricated with etched CZTSSe film as absorber.The promotion of etching to the performance of solar cell is mainly attributed to the increase of R_sh which improves the V_oc of CZTSSe solar cell and thus leads to the increase of PCE.2.We design a surface Cd-doping of Cu-Zn-Sn-S precursor film method to fabricating CZTSSe thin films.By optimizing the content of Cd cation,a higher efficiency of 7.30%for Cu2Zn1-xCdxSn?S,Se?4?CZCTSSe?solar cells was obtained.CZCTSSe Semiconductor thin-film was prepared with Zn partially replaced by Cd,and the CZCTSSe film of Cd/?Cd+Zn?ratio was 0⁶.51%.With increasing Cd-doping,the ZnSe secondary phase was decreased or even disappeared on the CZCTSSe surface which measured by Raman.Additionally,the results shown that the Cd-doping can reduce the lattice mismatch between the absorber layer and CdS,and contributed to the CdS epitaxial growth on surface of CZCTSSe film.And Cd-doping can also broaden depletion region width?W_d?and then yielded a high photocurrent?J_L?which can improve the performance of solar cell.3.By optimizing the Zn/Sn cationic ratios,the CZTSSe solar cell with the highest PCE of 8.01%which is the highest value for CZTSSe solar cell prepared with the N,N-dimethyl formamide?DMF?-based solution approach was obtained at Cu-poor and Zn-rich condition.The single-phase kesterite CZTSSe films were prepared with optimizing nominal cationic ratios of Zn/Sn using DMF-based solution approach,and the corresponding CZTSSe solar cells were also fabricated.Our results revealed that the interfacial secondary phases and interfacial structure of CZTSSe/Mo can be optimized by changing the cationic ratios so as to decrease the reverse saturation current density?J₀?and increase the shunt resistance(R_sh).The depletion region width?W_d?of solar cells were broadened due to the decrease of antisite defect Cu_Zn and net hole concentrations?N_B?in CZTSSe induced by the decreasing Cu/?Zn+Sn?,leading to a high J_L.4.Using the low temperature measurement system,we have analyzed the dependence relationship between performance parameters of CZTSSe solar cells and temperature.The essential reason for limiting the performance of the devices is the back contact barrier heights.The J-V-T curves of CZTSSe solar cells were measured by low temperature measurement system at ranging from 50 K to 300 K.By research on performance and electrical parameters,found that R_s is the main reason for affecting the performance of solar cells.For the J-V-T curves roll-over which appear in the low temperature,we provide a detailed analysis and explanation for the formation mechanism of the double diode structure.The results show that the essential factor limiting the performance of CZTSSe solar cells is the back contact barrier heights,which can be indicated from that R_s rises sharply at low temperatures and hindering the transport of holes to the back electrode.