Solution-processed ZnO Electron Transport Layer Defect Optimization Enabling Performance Enhancement Of CZTSSe Solar Cells

The environmentally friendly kesterite Cu₂ZnSn（S,Se）₄（CZTSSe）thin-film solar cells have drawn worldwide attention as one of the leading photovoltaic technologies due to its earth-abundant constituents,long-term tolerance stability and superior photovoltaic performance.Considerable research efforts have been recently devoted to interface engineering,defect passivation and crystallization control,and the latest power conversion efficiency（PCE）achieved by CZTSSe solar cells is 14.9%.Although the solution-processed absorber offers advantages in controlling the elemental composition at the molecular level and lowering the production cost,the challenge is to continue this progress in increasing efficiency,and its open-circuit voltage(V_oc)and fill factor（FF）are still far behind other leading PV technologies.It is believed that the insufficient p-to-n inversion at front contact leads to large interface recombination loss,thus limiting the device performance.In fact,the widely adopted intrinsic ZnO（i-ZnO）window layer also has significant influences on the overall performance.It can not only undertake the role of electrons transport/collection,but also combine with Cd S buffer layer to limit the V_oc loss through the electronic isolation of leakage shunts.However,for physically sputter-deposited i-ZnO,this material often accounts for nonstoichiometry under oxygen-poor atmosphere,which will result in self-doping via intrinsic defects such as oxygen vacancies（V_O）and zinc interstitials（Zn_i）.These dominant defects in ZnO mainly affect the electrical properties and can delay the electrons transport process and cause unnecessary carrier recombination before achieving the collection electrode.Therefore,further controlling the defect states within ZnO window layer under oxygen-rich atmosphere is of paramount importance for the realization of high quality ZnO/Cd S/CZTSSe heterojunction.To overcome the issues of the conventional sputtered i-ZnO,through organic molecular poly（ethylene oxide）（PEO）modification or Li doping to passivate the surface traps and bulk defects of ZnO,respectively,to improve electron transport and enhance interfacial electron extraction.The specific work is divided into the following parts:The first part,the surface traps of ZnO NPs are passivated by poly（ethylene oxide）（PEO）to improve the performance of CZTSSe solar cell.We present the extrinsic solution-processed ZnO nanoparticles（NPs）by PEO modification as window layer to improve the electrical properties of ZnO/Cd S/CZTSSe heterojunction interface.PEO is supposed to be coordinated to the surface of the ZnO nanoparticle by sharing the lone electron pair of oxygen in PEO with ZnO,which effectively passivates the surface traps of ZnO.PEO modifications are mainly introduced by spin-coating,and PEO concentration in chlorobenzene solution（0.5 mg/m L）and annealing temperature（85℃）of films are optimized.It is demonstrated that PEO modification to the ZnO nanoparticle surface can effectively passivate its surface traps,suppress the recombination loss of carriers,reduce the series resistance,and improve the carrier transmission performance at the interface.Consequently,both J_sc and FF of the CZTSSe solar cells are considerably improved,and the PCE of the device is increased to 12.23%.The modification of ZnO NPs with PEO provides an effective method for passivating ZnO traps and improving heterojunction interfaces.The electrical properties of ZnO have been found to depend on a multitude of factors,most significantly the surface termination and defect chemistry,both of which can contribute to changes in the surface carrier concentration and band-bending.However,PEO modified ZnO NPs can only reduce the surface traps of ZnO,and the intrinsic defects of ZnO are not sufficiently changed,resulting in insignificant V_oc increase.In the second part,the bulk defects of ZnO NPs are passivated by Li doping to improve the performance of CZTSSe solar cell.To further passivate ZnO defects and improve charge extraction and separation at the p-n junction interface by Li doping.It is turned out that the insertion of Li in host ZnO lattice is dominated by interstitial doping and the occupancy sites are V_Oand Zn_i.Li doping is mainly achieved by optimizing the Li/Znmolar ratio,with 2%Li doping ZnO for improved device performance.We use the solution-processed ZnO:Li NPs as window layer to replace the conventional sputtered i-ZnO for CZTSSe solar cells,and its electric benefits on structuring charge-extraction p-n junction interface are comprehensively investigated.Due to the electron doping characteristics,the introduced Li_i increases the carrier concentration,Fermi level and decreases deep level defects.These electric benefits effectively enhance quasi-Fermi level at ZnO:Li/Cd S/CZTSSe heterojunction and broaden the depletion region width at p-type absorber side,thereby accelerating the carrier transport process within ZnO and charge extraction process at p-n junction and depress the charge recombination process at this interface.The ZnO:Li NPs window layer contributes significant gains in FF and V_oc and impels the PCE up to 12.6%.These findings prove the ZnO:Li NPs window is an ideal alternative to sputtered i-ZnO,and meanwhile,lowers the manufacture cost of CZTSSe solar cells and offers the potential for future application in photovoltaics.