Grain Boundaries And Interface Passivation Of Graphene And Its Derivatives In CZTSSe Solar Cells

The copper zinc tin sulfur selenium（Cu₂ZnSn（S,Se）₄,CZTSSe）solar cells derived from copper indium gallium selenium（Cu（In,Ga）Se₂,CIGS）solar cells have attracted worldwide research interest due to its high absorption coefficient,rich elemental composition,and non-toxicity.However,at present,its maximum certification efficiency is only 14.9%,which is far below the Shockley-Queisser（S-Q）theoretical limit efficiency（over 33%）and CIGS（23.6%）.A large open-circuit voltage deficit(V_OC,def,defined as V_OC,def=（Eg/q）-V_OC,E_gis the bandgap,q is the electronic charge)is the main obstacle to further improving the efficiency of CZTSSe photovoltaic devices.It’s found that the carrier recombination loss caused by the grain boundaries（GBs）and surface dangling bonds in the polycrystalline absorber is one of the major factors leading to V_OC,def.CZTSSe thin film solar cells use polycrystalline thin films as p-type absorbers and follow a heterojunction structure.Unsaturated cationic dangling bond defects exist on the CZTSSe thin films surface,while anionic dangling bond defects exist at GBs.These defects become the recombination centers of charge carriers,limiting their mobility and lifetime,thereby deteriorating device performance.Hence,passivating the GBs and the CZTSSe absorber surface to improve the power conversion efficiency（PCE）of CZTSSe thin film solar cells for early commercialization is an urgent issue for researchers to overcome.As a two-dimensional material,graphene has high conductivity and mobility characteristics.Researchers usually introduce it as a carrier transport material into solar cells to improve carrier mobility and accelerate the transfer of electrons through GBs,thereby improving charge transfer;On the other hand,the oxygen-containing functional groups in graphene oxide（GO）,a derivative of graphene,can be used to saturate the cationic dangling bonds on the CZTSSe absorber surface,reduce the interfacial defects density,and inhibit interfacial carrier recombination.Therefore,in order to address the issue of carrier recombination caused by dangling bond defects on the surface and GBs of CZTSSe polycrystalline thin films,the following two aspects of work are mainly carried out in this paper:In the first work,in order to solve the problem of unsaturated cationic dangling bond defects on the CZTSSe absorber surface,we propose to use oxygen-containing functional groups（C-OH,C-O,etc.）in GO as Lewis bases to passivate the unsaturated cationic dangling bond defects of the CZTSSe surface.The GO dispersions were spin-coated onto the selenized CZTSSe films,and the dispersion concentration,spin-coating speed,hot plate temperature and annealing time were optimized to achieve the best performance of CZTSSe devices.Scanning electron microscopy（SEM）,Kelvin probe force microscopy（KPFM）,X-ray photoelectron spectroscopy（XPS）and Fourier transform infrared spectroscopy（FTIR）results showed that GO was successfully introduced into the CZTSSe surface and the oxygen-containing functional groups in GO could combine with the undercoordinated Cu,Zn and Sn cations on the CZTSSe surface,resulting in the reduction of interfacial defect density.Electrical characterization further showed that the introduction of the GO passivation layer increases the depletion region width and the conduction band offset（CBO）,but decreases the valence band offset（VBO）of the CZTSSe absorber,thus enhancing the carrier transport process at the heterojunction interface and significantly suppressing the nonradiative recombination at the interface.Finally,based on the passivation effect of the GO passivation layer on the CZTSSe absorber surface defects,the device performance of CZTSSe increased from 10.50%to 12.79%,mainly attributed to the improvement of open-circuit voltage(V_OC)and fill factor（FF.）In the second work,to address the problem that the GBs of CZTSSe absorber limit the carrier transport,we propose to use the graphene of high conductivity and high mobility to introduce it as an active additive into the CZTSSe precursor solution and distribute it in the GBs of polycrystalline films after high temperature selenization,so as to passivate the GBs and improve the charge transport.The graphene dispersion was firstly investigated and ethylenediamine,which is required for the preparation of the precursor solution,was finally selected as the dispersant and found to be well dispersed and exhibited the Tyndall effect;Then,the optimal amount of graphene was determined by investigating the amount of graphene introduced into the CZTSSe precursor solution.The X-ray diffraction（XRD）,Raman and SEM results showed that graphene was successfully introduced into the CZTSSe films and its introduction does not change the crystal structure and morphology of CZTSSe absorber,and the graphene distributed in the GBs can effectively improve the current and potential at the GBs,as well as enhance carrier mobility,which is conducive to inhibiting GBs recombination,extending minority carrier lifetime,mediate the separation,transport and collection of photogenerated carriers.Finally,based on the significant passivation effect of GBs,the use of 0.05wt%graphene additives increased the PCE from 10.40%to 12.90%,the short circuit current density(J_SC)from 33.67 m A/cm²to 38.22 mA/cm²,and V_OCfrom 471.48 mV to 494.24 mV.