Preparation Of Ti₃C₂T_x And Its Application In Interface Layers Of Polymer Solar Cells

Polymer solar cells(PSCs)have attracted extensive research attention due to their low cost,light weight,and large-area roll-to-roll fabrication,etc.In recent years,the power conversion efficiency(PCE)of single-junction solar cells has exceeded 18%by optimizing the device structures,synthesizing new donor/acceptor materials,precisely adjusting the micromorphology of active layers,and optimizing the interface layers.However,compared with the commercial inorganic silicon solar cells,PSCs still suffering from lower performance,which restricts their commercialization.It is well known that the fabrication of new donor/acceptor materials requires a long experimental period,and the study of device structures often requires complicated techniques.Hence,it will be a facile and effective method to further promote the PCE of PSCs by optimizing the interface layer.Ti₃C₂T_x,as a representative of early transition metal carbon/nitrides(MXene),has a wide range of applications in electrochemical energy storage,membrane separation,sensor and catalysts due to its ultra-high conductivity(4900 S cm^-1),carrier mobility(1.0 cm² v^-1 s^-1),and superior mechanical strength.In this paper,we first synthesized Ti₃C₂T_x with a mild method,and applied Ti₃C₂T_x as a transport layer material in PSCs.By modulating the contact characteristics between interface layer and active layer,the transfer and collection of charges in the interface layer was improved,thereby improving the PCE of devices,the main research contents are as follows:In the first part,we synthesized Ti₃C₂T_x with a mild method,and used Ti₃C₂T_x as the hole transport layer of PSCs to prepare PBDB-T:ITIC based devices.The device performances are improved by adjusting the thicknesses of Ti₃C₂T_x HTLs.Compared with the device without HTL,the PCE of Ti₃C₂T_x devices has increased from 4.21%to 10.53%.The introduction of Ti₃C₂T_x layer,with ultra-high conductivity and carrier mobility,significantly improves the hole transfer in device and facilitates the transfer of holes from active layer to ITO electrode.Water contact angle test indicates that the introduction of Ti₃C₂T_x HTLs increases the hydrophobicity of ITO electrode,improves the contact between ITO and active layer,and effectively improves short circuit current(J_sc)and fill factor(FF).Benefiting from the stronger hydrophobicity of Ti₃C₂T_x HTLs,which effectively slowed the corrosion of active layer by H₂O in the atmosphere via measuring the PCE changes of devices in air.Therefore,the lifetimes of devices based on Ti₃C₂T_x are significantly improved.In the second part,Ti₃C₂T_x was utilized to improve the uneven structure and low conductivityofthecommonlyusedanodeinterfacelayer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS).The results of X-ray photoelectron spectroscopy(XPS),Raman spectra(Raman),and electronic paramagnetic resonance(EPR)show that upon the incorporation of Ti₃C₂T_X,the abundant functional groups(–O–,–OH,and–F)on Ti₃C₂T_X form electrostatic interaction with-SO₃H of insulated PSS chains,screening the coulombic attraction between conductive PEDOT and PSS chains and induceing the conformational transition of PEDOT from a coil(benzoid)to a liner/expanded-coil structure(quinoid).The liner PEDOT chains have stronger inter-chain interactions compared with the coil conformation,leading to the closely packed and expanded PEDOT nanocrystals.Therefore,the inter-chain charge transport will be enhanced.Besides,Ti3C2TX crisscross the PSS regions and connect the discontinuity of the crystal clusters,constructing additional transfer highways for carriers.Therefore,these interconnected conductive networks make it possible to transfer carrier facilely from one PEDOT nanocrystals to another.These collectively contribute to the facilitated conductivities of PEDOT:PSS/Ti₃C₂T_x,reducing charge transfer resistance.Compared with PEDOT:PSS device,the PCE of PEDOT:PSS/Ti₃C₂T_x device has increased from 9.72%to 11.02%,also,the stability of PEDOT:PSS/Ti₃C₂T_x PSC increases.In the third part,Zn O/Ti₃C₂T_x composite ETLs was fabricated by in-situ method to address the drawbacks of the commonly used cathode interface layer zinc oxide(Zn O).The study found that,two-dimensional Ti₃C₂T_X nanosheets could passivate the surface traps of Zn O by the formation of the Zn-O-Ti bonding;Besides,Ti₃C₂T_x act as bridges and construct additional charge transfer paths,enhancing the transfer and collection of electrons.Meanwhile,Zn O/Ti₃C₂T_x composite ETLs were found more hydrophobic than pristine Zn O interface layer,which is beneficial to improve the contact between ITO electrode and organic active layer.The superior crystallinity and light absorption of the organic active layer(PBDB-T:ITIC),which promotes the light capture and exciton separation.Ultimately,efficiencies of Zn O/Ti₃C₂T_x composite ETLs are improved remarkably.In the fourth part,bio-inspired PDA modified Ti₃C₂T_x(PDA-Ti₃C₂T_x)as a multifunctional additive is added to Zn O for the first time with a view to addressing the mismatched energy level of Zn O with acceptor materials.Experimental results indicate that the abundant catechol groups of PDA-Ti3C2Tx coordinate with Zn²⁺of Zn O by sharing a lone electron pair of oxygen in catechol groups and form strong and stable chelate interactions,leading to the passivation of defect sites on Zn O surfaces and the promotion of electron transfer.On the other hand,the formation of chelate interactions induces the interface dipole pointing toward the polymer between the electron-accepting Zn O(Zn²⁺)and the electron-donating PDA(catechol),thus the reduced W_F is obtained by composite transport layer,promoting the collection of carriers.Ultimately,V_oc,J_sc,and FF of IPSCs are improved.Finally,Work functions of 2D Ti₃C₂T_x are tuned by ethanolamine rich in amine groups(-NH₂)and rhodium chloride(Rh Cl₃),making it match well with the organic active layer(PM6:Y6).Ethanolamine treated Ti₃C₂T_x(D-Ti₃C₂T_x)and rhodium chloride treated Ti₃C₂T_x(R-Ti₃C₂T_x)are utilized as ETLs and HTLs,respectively.-NH₂ groups were adsorbed on the surfaces of nanosheets by formatting hydrogen bond interactions with-O-,OH and-F groups of Ti₃C₂T_x.The formed dipole between nanosheets and ITO induces the W_F reduction of cathod interface layer,enhancing the collection of carriers.Rh³⁺in metal chlorides are spontaneously reduced to Rh particles by Ti₃C₂T_x.Therefore,increasing the W_F of Ti₃C₂T_x.Experiments have shown that tuning the W_F of Ti₃C₂T_x with appropriate means could expand the the application potential of Ti₃C₂T_x in PSCs.