Interfacial Electronic Structure Between Organic Semiconductor Thin Films And Electrode Materials

Since the 21st century,organic electronics has been developed rapidly.Optoelectronic devices,such as organic light-emitting diodes and solar cells,have made great breakthroughs.There are many aspects on studying organic electronics,while the interface properties is one of the most basic but rather important field in these devices,as fundamental understanding on the interface properties is considered to be the premise and basis to improve their performance.Although plenty of research has been carried out to optimize and improve the carrier transport and separation at the interface,there is still insufficient understanding for some complex interfacial systems.Base on the current studies,especially the study on the interfacial coupling mechanism of organic/inorganic heterojunctions,further work is necessary to be carried out.In this project,on the one hand,we introduced a novel macrocyclic conjugated organic semiconductor material(Mabiq),and have studied the interfacial properties between Mabiq molecules and four electrodes,Au(111),Ag(111),Cu(111)and ITO via X-ray Photoelectron Spectroscopy,Ultraviolet Photoelectron Spectroscopy methods and Density Functional Theory calculation.On the other hand,the different interfacial energy level arrangement and growth orientation of α-6T on clean Si(100)substrate and SiOx-covered Si(100)substrate were studied by XPS and UPS.This project includes the following four aspects:(1)The effect of Mabiq modification on the work function of metal single crystal and ITO.In this part,we use Mabiq monolayer as a modification material,and slowly deposit it on the surface of Au(111),Ag(111),Cu(111)and ITO by molecular beam epitaxy,and then study the effect of Mabiq on these functional substrates.The results show that the work function of these substrates can be significantly reduced after the modification of Mabiq monolayer.On the Au(111)substrate,the work function of which has been reduced by 1.34 eV;on the Ag(111)and Cu(111),the reduction is 1.01 eV and 1.04 eV,respectively;on the ITO substrate,the reduction is smaller but still reaches 0.56 eV.Further study has found that the drastic changes on work function is due to the combination of the push-back effect of the surface and the unique molecular dipole moment of the Mabiq molecule,which represents the advantages and potential of this material on modifying the electrode materials(2)Interfacial interaction between Mabiq and metal single crystals and ITO.These results show that Mabiq molecules have different adsorption behaviors on four substrates.At room temperature,Mabiq shows physisorption behavior on Au(111),Ag(111)and ITO which is weak coupling.However,Mabiq molecules on Cu(111)shows the chemsorption behavior,which is due to the chemical reactions that some copper atoms from the substrate undergo a metalation reaction with Mabiq molecules at room temperature,and finally form the Cu-Mabiq metal complex.Moreover,we also found that the growth modes of Mabiq molecules on the four substrates are different,which is related to the different dipole effects in the four interface systems.(3)Metalation of Mabiq molecules on the metal single crystal substrates.In order to explore the origin of metalation and the effect by the temperature,we have performed the XPS experiment at low-temperature maintained by liquid nitrogen.During the measurement,we deposited the molecules at 78K and increase the temperature step by step,finally we have found the onset of metalation reaction at 275K.Meanwhile,in another experiment,we found the metalation reaction site was transferred from the central bonding site to the second bonding site when the temperature has been reached to 480K.For the weakly coupled Mabiq/Ag(111)system,the metalation reaction was also found when the sample is annealed to nearly 500K.However,the transfer of the metalation reaction site was not found on Ag(111),and the reaction occurred at the second binding site,which was related to the coupling strength of the two different interface systems.(4)Energy level alignment of α-6T on Si(100)surface.The alignment of α-6T molecules on the clean Si(100)substrate and the Si(100)substrate covered with SiOx was investigated.We found that on the clean Si(100)substrate,the growth of α-6T molecules was ordered,the HOMO peak of the molecule is narrow,and there is no band bending on the molecular side in terms of energy level alignment.However,the disordered growth of α-6T molecules on the SiOx-covered Si(100)substrate resulted in a broader molecular HOMO peak and an obvious band-bending on the molecular side.Through XPS,UPS and DFT methods,we have carried out the study on the interface properties of Mabiq molecules on the four different electrode substrates.In the optoelectronic devices,the unique properties and interface mechanism of this new material have provided important theoretical support for its application.However,there are still many unclear properties of this material that need to be explored and studied.We hope that,in the future,Mabiq and the series of similar metal complexes can provide a broad application for organic electronic devices.For organic materials with mature applications such as α-6T molecules,a series of molecular structures can be designed and modified to have more properties.At the same time,the research on organic-inorganic hybrid heterojunctions will have more and more possibilities with the emergence of new materials.The research on the hybrid interface between α-6T molecule and its derivatives and various inorganic materials will be a new research hotspot in the future.