Chemical Defects And Energetic Disorder Impact The Energy-level Alignment Of Functionalized Hexaazatriphenylene Thin Films

In recent decades,with the rapid development of science and technology,the process of practical application on organic semiconductor materials and devices has been accelerated.Especially,with the deepening of research on organic solar cell(OSC)and organic light-emitting diode(OLED),the related organic optoelectronics has become a mature interdisciplinary emerging field.The energy level alignment(ELA)at organic-inorganic and organic-organic interface is fundamental for any organic-based devices,considering that the interface electronic structure controls the charge injection process in organic semiconductors.Consequently,studying the interface properties of devices is the premise and foundation to improve their performance fundamentally.Furthermore,it was found that the mystery of ELA mechanism must be related to the density of gap states(DOGS),which control the position of the Fermi-level(EF).However,despite considerable efforts,there is no consensus as to the origin of the ELA mechanism and charge transport,and thus the various device characteristics.In order to study the origin of gap states in organic molecular thin films,and the arrangement of energy levels at the interface,this thesis uses photoelectron spectroscopy technology.Thereby,ultraviolet photoelectron spectroscopy(UPS),X-ray photoelectron spectroscopy(XPS)and density functional theory(DFT)were applied to study the interface properties of mechanical semiconductor and electrode material constructed by 5,6,11,12,17,18-hexaazatrinaphthylene(HATNA)and 1,4,5,8,9,12-hexaazatriphenylene2,3,6,7,10,11-hexacarbonitrile(HAT-CN)molecules on indium-tin-oxide(ITO),Au(111)and MoO3 substrate materials by in-situ evaporation.The work mainly includes the following aspects:(1)The interfacial interaction,adsorption behavior,growth pattern and energy level alignment of HATNA molecules on three different substrates were studied.The results show that the growth pattern of HATNA molecules on the three substrates was island growth.By UPS study,it was found that HATNA was physically adsorbed on ITO and Au(111),while it was chemically adsorbed on MoO3 substrate.In terms of energy level arrangement,the alignment of energy levels at the interface between HATNA and substrate is determined by the energy disorder of organic molecules at the interface.It is found that vacuum levels in the three systems have significantly shifted down,and the direction of energy level bending is downward due to the unequal width of HOMO and LUMO of HATNA molecules.The reasons for the downward shift of vacuum levels are different:For ITO substrate,the downward shift of vacuum energy levels is caused by push-back effect and interfacial charge transfer.For Au(111)substrate,it is all caused by push-back effect.For MoO3 substrate,the decrease of vacuum energy level is mainly due to charge transfer induced by interfacial dipole.(2)HATNA and HAT-CN molecules were deposited layer-by-layer on three substrates by molecular beam epitaxy,and additional multilayers were prepared at a "low rate" of 2 A/min and a "high rate" of 20 A/min.while the possibility of various chemical defects produced by both molecules was simulated by DFT.The presence of gap states in HAT-CN films was confirmed by UPS and the absence of such gap states in HATNA films(normal deposition rate).DFT calculations also indicate that HATNA is indeed less prone to chemical defects than HAT-CN,and that the gap state is due to chemical defects,which occur during vacuum sublimation of the source material.In order to demonstrate the effect of chemical defects on the HAT-CN thin film ELA,in addition to UPS,the energylevel bending quantification of organic molecular thin films from monolayer to multilayer was also obtained by XPS.From the energy level diagram,it is found that the Fermi level pinning independent of the substrate appears in HAT-CN films,and resulting in high work functions on almost all substrates.(3)The effect of air exposure on the electronic structure of HATNA and HAT-CN organic molecular films was studied.It was found that air exposure did not change the island distribution pattern of organic molecules on the substrate,and the HATNA molecular films deposited at normal evaporation rate on MoO3 substrate showed a"dopant" defect state after air exposure.The reaction of some HAT-CN molecules with water oxygen and other components in the air led to the decrease of the work function.Meanwhile,it was found that air exposure would expand the gap state of the film,mainly because the impurities in the air further introduced the "defect" state in the film.From the XPS spectrum,both chemical shifts and changes in peak intensity are due to the introduction of related impurities in the air and the creation of new bonds.