Contact Formation Between Organic Semiconductor Thin Films And Crystalline Metal Surfaces

In recent years organic electronics have become more and more the focus of interdisciplinary research and many organic devices are superior to their inorganic counterparts in terms of high efficiency,flexibility,and low cost production.Although a great deal of research has been conducted on the electrical properties of organic devices,numerous crucial problems still remain.Of these,the study of charge transport properties in organic semiconductor systems has been one of the most important subjects that have remained a puzzle for many years.Often,electrode work functions are modified to tune hole(electron)injection barriers.One successful approach to lower hole injections barriers is pre-covering the metal substrate with a monolayer of a strong electron accepting molecule,which undergo a charge transfer with the metal and thus increases the effective metal work function.On the research of plenty of organic electronic devices,the electronic properties at interface between two different materials are the most important which include charge transfer,which kind of adsorption and molecular exchange.In order to learn different electronic properties and molecular structures between organic semiconductor materials and noble metal crystals,we mainly used ultraviolet photoelectron spectroscopy(UPS),X-ray photoelectron spectroscopy(XPS)and low energy electron diffraction(LEED)to carry out three related experiments.The first experiment focused on perylene-3,4,9,10-tetracarboxylic diimide(PTCDI)thin films on Au-(111),Ag-(111)and Cu-(111).By means of the experiment,PTCDI showed different behaviors on different substrates.It was physisorbed on Au-(111)according to UPS spectra.Meanwhile,we could deduce that the interaction between PTCDI and the more active substrate Cu-(111)was chemisorption as it was obviously to observe filled former lowest unoccupied molecular orbitals(F-LUMO).The second part concentrated on picene which was deposited on Ag-(111),Ag-(110),Cu-(111)and Cu-(110).It is found that picene monolayers on the(111)surfaces and on Ag-(110)comprise a flat lying and a tilted molecule in the unit cell,and on Cu(110)no picene order could be evidenced by LEED.The last group of experiment concentrated on two kinds of heterostructures.We chose 6,-13-pentacenequinone(P20)and 5,-7,-12,-14-pentacenetetrone(P40)which have similar chemical structures to combine with copper-phthalocyanine(CuPc),and Ag(111)was used as the substrate.In the first bilayer structure,CuPc was gradually deposited on the substrate pre-covered by monolayer P40.According to our measurement,CuPc was lying on the P40 layer and there was no extra interaction between CuPc and Ag(111).However,in the second bilayer,we found that CuPc pushed away P20 molecules as deposition,and CuPc was directly contact with the substrate.By using UPS,XPS and LEED,it not only makes us to study the electronic properties of organic semiconductor materials deeper but also provides theoretical foundation for organic electronic research.Eventually,it is our best wishes that organic electronics could develop faster.