Interface Characteristics Studies On Organic Electronics

Organic electronics have been attracted intensive and increasing attention in thepast two decades due to their potential advantages of low cost, compatibility with largearea and flexible substrates，versatile chemical design and synthesis, light weight andease of fabrication. Fully functional devices including organic light-emitting diodes(OLEDs), organic photovoltaic cells (OPVs), organic field-effect transistors (OFETs),photodetectors and memory have all been demonstrated and show great prospects ininformation techonology application. For example，OLEDs can be used for flat paneldisplays and solid-state Lighting, OPVs as a renewable and clean energy can effectivelyalleviate the current energy needs of the community and OFETs can be used to fabricatethe display backplanes and smart cards. It is widely recognized that the interface play animportant role in determine the performance and lifetime of organic electronic devices.Despite impressive breakthroughs in organic electronics technology have been achieved,a thorough understanding of the underlying physics within organic devices is stilllacking because of the formation of interface dipole, electronic polarization andcharge-transfer excitons in the organic interface. It is not applicable to accuratelydescribe and predictthe interfacial electronic structures and energy level alignment withconventional models in inorganic semiconductors. Therefore, it is necessary toinvestigate the organic interface to get a deep understanding of the underlying physicalmechanisms.In this thesis, the interface electronic structure and energy level alignment of theinterconnector in tandem organic electronics devices and OPV devices with normal andinverted structures have been systematically investigated by ultraviolet and X-rayphotoemission spectroscopies (UPS and XPS). In addation, the effect of electrical doping, substrate modification and electron transport layer on the interfacial barrier andthe characteristics of the organic heteojunction films after thermal annealing also havebeen studied. More details are listed below:(1) Interfacial electronic structures and energy alignments of interconnectors in tandemorganic electronic devicesThe interfacial electronic structures and energy level alignments of intermediateconnectors using HATCN as p-type layer, including (a) MADN/NPB,(b)Li:Bphen/NPB,(c) HATCN/NPB,(d) Li:Bphen/HATCN/NPB were studied via UPSand XPS. The function of Li:Bphen and HATCN in the interconnectors was discussedby analysing the energy level alignment. Based on the above results, high efficient andstable white light emitting diode was fabricated using TBPI and Bphen as replacementof MADN to see the effect of electron injection on device performance. Finally, theeffect of interconnectors including (a) Bphen/SubPc,(b) HATCN/SubPc,(c)Li:Bphen/SubPc,(d) Li:Bphen/HATCN/SubPc on the tandem OPV devices werediscussed by studying the interfacial electronic structures and energy level alignments.(2) Effect of interfacial tuning on the injection barrierThe effect of electrical doping on the electron injection barrier was discussed bystudying the electronic structure and energy level alignment of CsF:Alq3films withdifferent doping concentration. Next, the electronic structures of different electrontransport layers (ETL) were studied via XPS and UPS. The effect of ETL on theinjection and device performance was investigated by combine the electronic structureand energy level alignment. The electronic structures of organic film on modified ITOsubstrates were aslo investigated via XPS and UPS.(3) Interfacial electronic structures and energy alignments of organic photovoltaic cellswith normal and inverted structuresThe Interfacial electronic structures and energy alignments between SubNc/C60,MoO3/SubNc/C60, Cs2CO3/SubNc/C60, and C60/SubNc, MoO3/C60/SubNc,Cs2CO3/C60/SubNc were studied via XPS and UPS. The differences between nomal andinverted structures with MoO3and Cs2CO3were discussed. The Interfacial electronic structure and energy alignment of SubPc intead of SubNc were also studied toinvestigate the impact of device strcture with MoO3and Cs2CO3on the interfacialelectronic structure and energy level alignment. The corresponding devices were alsofabricated to validate the above analysis.(4) Effect of thermal annealing on the organic filmThe electronic structures of CuPc/C60, TiOPc/C60, PTCDA/C60bilayers andCuPc:C60, TiOPc:C60, PTCDA:C60mixed films were studied by XPS and UPS. Adetailed analysis of phase separation between organic films with different molecularstructures was gived based on the XPS and UPS results. Then the electronic structuresof CuPc/PTCDA，CuPc:PTCDA and TiOPc/PTCDA, TiOPc:PTCDA were also studiedto prove the relations between molecular structures and phase separation.