Mxene-based Perovskite Solar Cells

The development and utilization of renewable energy is the key to solve the current energy shortage and the environmental problems caused by the excessive consumption of fossil energy.In recent years,perovskite solar cells(PSCs),as a representative of the new generation of solar cells,have gained rapid development and widespread attention due to the advantages of low cost and high power conversion efficiency(PCE).However,many challenges are still existed to achieve its commercialization,such as PCEs,stability,toxicity of lead(Pb),large area preparation,electron/hole transport layer performances and cost.Two-dimensional(2D)MXene materials generally have the advantages of high conductivity,flexibility,tunable surface functional groups,tunable surface terminations and hydrophilicity,which have great potential to enhance the charge extraction ability,carrier mobility and improve the interface issues of PSCs.Herein,we try to construct new device system based on MXenes and investigate its working mechanism by continuously optimizing Ti₃C₂ Tx through a series of methods of surface terminations modulation,oxidation,and construction of heterostructures to finally enhance the performances of PSCs and fabricated excellent photovoltaic devices.The main research contents are divided into the following five sections:(1)Firstly,we introduced Ti₃C₂ Tx MXene nanosheets into the Sn O2 electron transport layer(ETL)as a conductive additive for CH3NH3 Pb I3 planar PSCs.Ti₃C₂ Tx nanosheets provided superior charge transfer paths,which enhanced electron extraction and enhanced carrier mobility while reducing the transfer resistance at the interface between the ETL and the perovskite layer,thus enhancing the short-circuit current density and fill factor of the device.After optimization,the enhanced performances from 17.23% to 18.34% of PSCs were achieved,while discovering the potential of Ti₃C₂ Tx as an ETL alone.This work proposes a new application area for MXene materials and provides a new path to enhance the photovoltaic performances of PSCs.(2)Owing to the unique optoelectronic properties and tunable surface termination of Ti₃C₂ Tx MXene,the Ti₃C₂ Tx films were modified by UV-ozone treatment and applied as an ETL in low-temperature processed planar-structured PSCs.Simple UV-ozone treatment of the metallic Ti₃C₂ Tx modulates its surface termination,electron transport mobility and interfacial properties of device,thus improving its suitability as an electron transport material.The finally achieved PCE of 17.17% and device stability were substantially enhanced to a comparable degree of traditional ETLs.As the first report on the use of pure MXene layer as an ETL in PSCs,this work shows great potential of MXenes to be used in PSCs and displays their promises for applications in photovoltaic technology in general.(3)In this work,oxidation of Ti₃C₂ Tx hydrocolloid was carried out to tune its properties desirable for an ETL in low-temperature processed PSCs.The calculations of the energy levels were carried out by the Vienna ab initio Simulation Package(VASP)code based on density functional theory(DFT).Oxidation of Ti₃C₂ Tx can generate TiO bonds and effectively reduce the defects of the film fabricated by spin-coating,while a transition from metallic material to semiconductor occurred after heavy oxidation.A better matching of energy levels between perovskite and ETL layer in the case of a hybrid of oxidized and pristine Ti₃C₂ Tx renders a champion power conversion efficiency(PCE)of 18.29 %.The improvement in PCE can be attributed to the increased electron mobility in the ETL,which promotes electron transport and reduces the electron-hole recombination.Hence,by presenting a simple method for high performance in PSCs by MXene-derived materials,the work demonstrates the great potential of these materials for applications in low-temperature processed PSCs and other photovoltaic technologies.(4)Here,in-situ oxidized Ti₃C₂T_x(O-Ti₃C₂T_x)MXene was introduced to form a nanoscale heterojunction with Sn O2 that was applied as an ETL in a MAPb I3-based PSC.Taking the advantage of transition from metallicity to semiconductivity during oxidation process of Ti₃C₂ Tx,the properties of Ti₃C₂ Tx could be tuned to have its energy levels well matched with the perovskite.Besides,the insertion of O-Ti₃C₂ Tx improved the electron mobility of Sn O2 ETL.Hence,O-Ti₃C₂T_x/Sn O2 heterojunction offered better electron extraction and reduced recombination between ETL and perovskite,thus achieving a champion PCE over 20% and long-term stability of 82% in air for 1000 hours based on CH3NH3 Pb I3 PSCs.(5)Cs₂AgBiBr₆ was applied as the active layer for inorganic lead-free double PSCs.Here,we employed D149 indoline dye to sensitize the Ti O2 ETL and introduced Ti₃C₂ Tx MXene nanosheets into Cs2 Ag Bi Br6 to enhance its crystallization,the absorption spectrum and intensity are both enhanced.Moreover,the calculations of the interface properties and electron structure were carried out based on DFT.Weak van der Waals forces between the interfaces of Cs2 Ag Bi Br6 and Ti₃C₂ Tx allow the perovskite to retain its semiconductor properties,while effectively weaken the Fermi level pinning(FLP).The high work function of Ti₃C₂ Tx adjusts the vanlence band of Cs2 Ag Bi Br6 through the Fermi level to improve carrier mobility.As a result,PCE of 4.47 % under1 sun illumination and 7.23 % under 200 lux indoor light illumination with enhanced long-term stability were achieved.Hence,this work further expands the application of Ti₃C₂ Tx for PSCs.The performances enhancement and mechanism investigation of MXene-based PSCs make us understand and recognize the potential of MXene materials in the field of photovoltaics,which also expand the application scope of MXene materials,thus finally contributing to the further development and application of low-cost renewable energy.