Investigation On The Interface Passivation Modification For The Efficient Cu₂Zn Sn?S,Se?₄ Solar Cells

Kesterite Cu₂ZnSn?S,Se?₄?CZTSSe?cell is one of the most competitive thin-film solar cells due to its abundant,nontoxic and low-cost compositions,then,much attentions have been attracted from the photovoltaic industry over the past decades.As for the kesterite cell history,power conversion efficiency?PCE?of the first CZTS cell is merely 0.66%fabricating with the structure of SLG/Mo/CZTS/CdS/AZO/Al?SLG and AZO denote soda lime glass substrate and ZnO:Al?in 1996,however,the champion PCE of CZTSSe cell has rapidly broken to 12.62%?Certified by Newport Corporation?by 2019,which reveals its huge potential.Noting that there still has been a relatively large performance gap between CZTSSe and the counterpart Cu?In,Ga?Se₂?CIGS??PCE=23.4%?solar cells,let alone for the comparison with Shockley-Queisser?SQ?simulation value?PCE=32.8%?.Therefore,it is crucial to improve PCE for the sake of accelerating the CZTSSe cell industrialization process.The factor impeding the PCE improvement of CZTSSe solar cell mainly includes two aspects.On the one hand,the fabrication problem of high-quality absorber involving second-phases-free,low defect density,outstanding crystal quality and compactness without voids characteristics.On the other hand,the interface carriers recombination problem,such as the CZTSSe/CdS,Mo/CZTSSe interfaces recombination.Targeting the aforementioned two problems,by introducing the buffer layer with high resistance and back surface field?BSF?into back interface of CZTSSe cell,we have improved the absorber crystal quality and realized the chemistry and field passivation for interfacial defects,thus,reducing effectively the interface recombination.The detailed results are displayed as below:1.The absorber crystal quality was improved and also Na diffusion from SLG substrate to absorber wasn't influenced after the incorporation of appropriate thick CuAlO₂ buffer layer into the back interface of CZTSSe cell.Furthermore,this compact and smooth buffer layer application can stabilize the Mo/CZTSSe interface during high temperature selenization,then,the interfacial layer MoSe₂ formation is suppressed effectively.It is found that the MoSe₂ thickness has been reduced from463 nm to 244 nm without and with the 10.6 nm thick CuAlO₂ layer incorporation,respectively,as a result,the series resistance?R_S?also decreased accordingly.Besides,as far as the band alignment is concerned,the adoption of CuAlO₂ layer causes a negligible impact on the holes transportation from absorber to back electrode,and simultaneously decreases the electrons recombination rate with large extent.After experiment optimization,the PCE of CZTSSe cell has been improved from 4.12%to 5.10%when the 10.6 nm thick CuAlO₂ layer was incorporated,which is mainly attributed to the increase in short-circuit current density(J_SC)and fill factor?FF?.2.It is demonstrated that Nb atom can dope into MoSe₂ lattices to substitute the Mo atom(Nb_Mo)and form MoSe₂:Nb solid solution via XRD and Raman spectrum characterization.Moreover,the interfacial layer n-MoSe₂ has realized p-type transition after Nb-doping by the analysis of Hall-effect and field effect transistors?FETs?measurement results,the carrier concentration of p-MoSe₂:Nb is3.54×10¹⁸ cm^-3.Based on these results,we further obtained the fermi level?E_F?of MoSe₂,MoSe₂:Nb and CZTSSe through XPS and UPS measurements,and it is found that E_F of MoSe₂:Nb is lower than that of CZTSSe.Consequently,BSF formed in back interface,which inhibits the interface carrier recombination and thus enables the photovoltaic performance improvement.On the one hand,as a result of the BSF generated locally passivated to the back electrode,shunt resistance(R_Sh)increases from 82.11 to 497.51?·cm² after introducing BSF.Also,increase in R_Sh is ascribed to the reduced secondary phases at the back interface induced by the blocking effect of the NbO_x layer for the reaction between absorber and back electrode.On the other hand,BSF incorporation leads to the decrease in interface layer MoSe₂ thickness and resistivity from 173 to 80 nm and from 63.7 to 1.86?·cm,respectively,thus results in the decrease of R_S from 4.39 to 1.75?·cm².In addition,the smaller ideal factor?A?,reversion saturation current density?J₀?and?E_gA obtained from temperature dependent J-V result also validate the depressed interface recombination as BSF application.The corresponding PCE was improved from 4.72%to 7.15%because of the enhancement of open circuit voltage(V_OC)and FF,primarily stemming from the doubling aspects of increased R_Sh,decreased R_S,and alleviative recombination velocity of back interface induced by the BSF.3.Cu_Zn acceptor defect is the main reason for the p-type conduction in CZTSSe.By analyzing the XRD and Raman results,it is concluded that Cu_Zn concentration increases with increasing Cu/?Zn+Sn?in CZTS precursor solution,then carrier concentration of CZTSSe becomes higher.Based on the p-n junction capacitance effect,BSF width distribution in absorber can be tuned via modulating the carrier concentration of absorber.Smaller carrier concentration creates wider BSF distribution,and then smaller interface recombination is obtained,leading to the decrease in J₀ from 2.50×10^-1 to 3.71×10^-2mA·cm^-2.Also,the response of J_SC towards to the long-wavelength photon is enhanced,reducing the electric loss.Additionally,MoSe₂:Nb possesses more compact surface without voids as compared to that of MoSe₂,which induces larger R_Sh?from 143.27 to 413.22?·cm²?further considering the BSF generated locally passivated to the back electrode.As a result,an accompanying rise in V_OC and J_SCC are achieved,which increases from 0.317 to 0.371 V and from 30.35 to 34.09 mA·cm^-2 as compared to the BSF-free case,respectively.As for the J_SC improvement,stronger light-trapping effect originated from pyramidal morphology in Mo:Nb is the other important factor.Therefore,the PCE is improved from 5.01%to 7.35%,which demonstrates the importance of BSF width distribution optimization for the progress of CZTSSe solar cells.