Research On The Effect Of Defect States In Organic Materials On The Arrangement Of Interface Energy Levels

In the past decades,semiconductor industry,both in inorganic semiconductor materials and organic semiconductor materials has a fast development.Compared with the inorganic semiconductor,organic semiconductor has the superiority of nature,its production cost is cheap,and it has a lighter weight and more variety,available device for making process is straightforward compared with inorganic electronic devices.Due to these favorable characteristics,organic semiconductor materials have attracted the attention of many scientists and companies,and many companies and research institutions have invested a lot of research resources in them At present,organic semiconductor has many common products,such as OLED lighting,display,etc.Although organic devices have made great progress,the ideal"zero" injection barrier ohmic contact has not been achieved experimentally at the interface,so there is still a lot of room for development of organic devices.In the substrate-organic contact,the position of Fermi energy level has a great influence on the device performance,which determines the charge injection barrier.However,the relationship between the position of Fermi energy level and the distribution of density of states(DOS)and density of gap states(DOGS)needs quantitative analysis.In addition,doping is commonly used to improve the conductivity and carrier mobility of organic electronic devices.After doping,the conductivity of different organic material devices has different phenomena,which also needs further research.This paper focuses on the above points on the theoretical and experimental research.The research content includes the following parts:We report computational and experimental studies(1)on the Fermi level(EF)pinning phenomena in weakly interacting electrode-organic semiconductor systems with pentacene,N,N-Di(naphthalene-l-yl)-N,N'-diphenyl-benzidine(a-NPD)and Poly(9,9-dioctylfluorene-alt-benzothiadiazole)(F8BT),and(2)to elucidate a common reason of the pinning phenomena which have been widely observed for various organic materials on inert electrode surfaces.For(1),the computed results on the electrode-dependence of EF-HOMO and EF-LUMO distance agreed excellently with UPS and Kelvin-probe results.For(2)we found theoretically that the pinning phenomena occur at systems even without any electronic states in the HOMO-LUMO gap,and thereby indicate that this is a universal Fermi-level pinning-like phenomena for various band gap materials,even if specific interface states do not exist upon contact.We further obtained for Gaussian-distributed HOMO and LUMO that the minimal hole and electron injection barriers are quantitatively determined by degree of standard deviations of HOMO and LUMO bands,which are in excellent agreement with the experimental resultsThe mechanism of original n-and p-type "intrinsic”organic films 8-Hydroxyquinoline aluminum(Alq3)and N,N'-bis-1-naphthyl-N,N'-dipbhenyl-1,1'-biphenyl-4,4'-diamine(NPB)via using molybdenum trioxide(MoO3)as a p-type dopant has been studied.It is found that the conductivities extracted from "holes-only" devices both for Alq3 and NPB films are improved as a function of doping molar ratio(MR)of MoO3.However,the differences of change of conductivities in NPB films and Alq3 films after doping can be found,where(1)a rapid increase of conductivity in Alq3 with slope S>1 is shown in the entire MR range;(2)in NPB films,the conductivity changes with a slope of S>1 can be found in the low doping region(MR?0.04)and a slope of?1 can be found with MR?0.04.According to these observation,we proposed that different distributions of occupied and unoccupied DOGS in energy gap of organic films is the origin of "intrinsic" n-/p-type organic films,and therefore can be passivated differently via MoO3.These findings suggest the effective way for improving free carriers concentrations by reducing DOGS in organic films.