Surface Functionalization Of Ti₃C₂T_x MXene And Its Application In Perovskite Solar Cells

Perovskite solar cells（PVSCs）have become a promising photovoltaic technology due to the advantages of low cost,light weight and high power conversion efficiency（PCE）.At present,the PCE of PVSCs still can be further enhanced compared to the theoretical limit efficiency（45%）,and the poor stability of PVSCs is severely unbeneficial to the large-scale application.Therefore,it is very important to further improve the efficiency and stability of PVSCs for the application.The aim of this work is to develop surface-functionalized Ti₃C₂T_xas interface layer to improve the efficiency and stability of PVSCs.First,tetrabutylammonium bromide（TBAB）is applied in the surface functionalization of Ti₃C₂T_x,denoted as TBAB-Ti₃C₂T_x,which is used as cathode buffer layer（CBL）to construct efficient and stable inverted PVSCs.Electron paramagnetic resonance measurement and density functional theory calculations demonstrate that the electrons of the bromide ion in TBAB is transferred to Ti₃C₂T_x,and thus TBAB-Ti₃C₂T_x achieves higher conductivity and significantly reduced work function of 3.9 e V,which can improve energy level matching and enhance charge extraction.In addition,the TBAB-Ti₃C₂T_x CBL can effectively inhibit the migration of iodide ions from the perovskite layer to the Ag cathode,thereby synergistically suppressing the defect state and reducing charge combination.Using MAPb I₃ perovskite layer without post-treatment,the control device possesses an efficiency of 16.25%,while the TBAB-Ti₃C₂T_x-based device obtains a significantly higher efficiency of 21.65%,which is one of the best efficiencies reported for the devices based on MAPb I₃/PC₆₁BM with different CBLs.These results indicate that TBAB-Ti₃C₂T_x should be a promising CBL for high-performance inverted PVSCs and inspire the further applications of quaternary ammonium functionalized MXenes in PVSCs.Second,（3-aminopropyl）trimethoxysilane（APS）is applied in the surface functionalization of Ti₃C₂T_x,denoted as APS-Ti₃C₂T_x,and the APS-Ti₃C₂T_x is further doped in Sn O₂ to construct hybrid electron transport layer（ETL,Sn O₂:APS-Ti₃C₂T_x）for highly efficient and stable carbon-based perovskite solar cells.The-OH generated by APS hydrolysis can combine with the-OH on the Ti₃C₂T_x surface to form the Ti-O-Si bonds through dehydration and condensation.Due to the large molecular dipole of APS,the work function of Sn O₂:APS-Ti₃C₂T_x is significantly reduced to 3.88 e V compares to Sn O₂:p-Ti₃C₂T_x（4.83 e V）,which can improve energy level matching between the ETL and the perovskite layer.In addition,the Sn O₂:APS-Ti₃C₂T_x ETL can passivate the perovskite defects and reduce the interfacial charge recombination between the ETL and the perovskite layer.As a result,the Sn O₂:p-Ti₃C₂T_x-based device obtains a slightly improved efficiency（9.83%）compared to the Sn O₂-based device efficiency（8.35%）,while the Sn O₂:APS-Ti₃C₂T_x-based device shows a significantly improved efficiency of 11.15%.After storage in air for 120 h,the Sn O₂-based and Sn O₂:p-Ti₃C₂T_x-based PVSCs maintain 70.92%and 74.93%of their initial PCEs,respectively,while the Sn O₂:APS-Ti₃C₂T_x-based PVSCs still retain 80%of the initial PCE,showing superior air stability.