The Influence Of Electron Injection And Transport On The Performance Of Organic Optoelectronic Devices

As the development of organic semiconductor materials and thin-filmoptoelectronics, organic optoelectronic devices have received considerable attention inthe past twenty years, which is called "plastic electronics". Organic optoelectronicdevices have many advantages, such as availability of production with large area, lowcost, easy to preparation, and large selectable range of materials. Therefore, it has wideapplication prospect in the area of flat panel lighting, radio frequency tag and electronicpaper. Organic light-emitting devices （OLEDs）, organic photovoltaic cells （OPVs） andorganic field effect transistor （OFET）, as the main parts of organic photoelectric devices,have got significant development. The carrier injection and transport in organicphotoelectric devices play important roles in the performance of the devices. However,the low carrier concentration and low mobility of organic semiconductor materialsseriously obstruct the improvement of device performance. Focusing on organiclight-emitting device, organic solar cells and organic field effect transistor, the problemswhich lie in the electron injection and transport of organic photoelectric devices wereinvestigated. By optimizing the electron injection layer, the performance and stability ofthe devices are improved.1. The influence of different electron transfer materials on the performance oforganic light-emitting devices was studied. Blue phosphorescence dyebis[（4,6-difluorophenyl）-pyridinato-N,C2’]（FIrpic） and yellow phosphorescence dyebis[2-（4-tertbutylphenyl）benzothiazolato-N,C2’] iridium （acetylacetonate）[（t-bt）2Ir（acac）] were used as dopant. Donor material1,1-bis[（di-4-tolylamino）phenyl]cyclohexane （TAPC） was used as the host material ofFIrpic and acceptor material tri[3-（3-pyridyl）-mesityl]borane （3TPYMB） was used asthe host material of （t-bt）₂Ir（acac）. Different kinds of electron transfer materialsbathophenanthroline （Bphen） and1,3,5-tris（2-N-phenylbenzimidazo-lyl）benzene （TPBi）were used. Four White OLED devices with dule emitting layers of different structurewere fabricated. The result showed that by using Bphen with higher electron affinityand electron mobility as the electron transfer layer, the devices achieved higherperformance. Furthermore, a thin layer of electron transfer and hole transfer materialwas inserted between the double emitting layers, acting as a spacer layer, and its influence on carrier transport and devices performance was studied. The result showedthat the spacer layer could decrease the majority charge carrier （electron or hole）, thusenhanced the electron-hole balance and increased the device performance. In addition,the emitting mechanism and carrier transport property were analyzed, with the help ofstudies on the energy level of the devices.2. The influence of doping DCJTB into CuPc on the performance of OPV deviceswas studied. The light absorption, exciton generation, and carrier transport of thedevices were investigated. To study the influence of doping rate on the performance ofdevice, seven kinds of devices with different doping density were prepared. The devicestructure is ITO/CuPc:DCJTB（Y%）（20nm）/C₆₀（40nm）/Bphen（2.5nm）/Ag（100nm）,wherein Y is the density of DCJTB being doped into the donor layer of CuPc. The resultshows that DCJTB can broaden the absorption of device. However, as the carriermobility of DCJTB is relatively low, the device performance is lower than before. But itdoes not deny the feasibility of the method that the light absorption efficiency can beimproved by doping and then the device performance would be improved. If othermaterials with higher mobility used as the doping material, the carrier transport will notbe impacted, and the light absorption efficiency will be improved. Then thephotoelectric conversion efficiency will be improved.3. The influence of different cathode modification materials Cs₂CO₃, mixedgraphene:Cs₂CO₃and ZnO nano-particles on organic photovoltaic cells with invertedstructure based on SubPc/C₆₀was studied. The result shows that introducing adequatecathode modification layer can improve the photovoltaic performance and stability ofthe devices. The devices with mixed cathode modification layer based on Cs₂CO₃andgraphene: Cs₂CO₃, can increase the power conversion efficiency （PCE） by twice of thedevice without the modification layer, while the devices with ZnO nano-particles ascathode modification layer can increase the power conversion efficiency by four times,with open circuit voltage （VOC） up to0.89V. In addition, the introducing of cathodemodification layer and inverted structure can effectively decrease the series resistance.And the stability of the devices can be improved.4. The influence of adopting three kinds of organic electron transfer material TPBi,bathocuproine （BCP） and Bphen as buffer layer on the performance of N-type organicfield-effect transistor based on C₆₀was investigated. Specifically, the influence ofelectron transport buffer layer on the device performance e.g. saturation current, carrier mobility, threshold voltage and switching current was studied. Meanwhile, the thicknessof different buffer layers was optimized. The result shows that the optimized depth ofTPBi is5nm, and that of BCP and Bphen are3nm. When Bphen is used as buffer layer,the device shows the best performance, and its carrier mobility gets as high as0.65cm2/Vs. Therefore, we designed electron transporting OFET device with multilayerstructure with ultra thin pentacene as the inducing layer of the C₆₀organicsemiconductor crystallize, electron transporting material Bphen as the buffer layerbetween the organic semiconductor layer and metal layer. The carrier mobility can getas much as5.2cm2/Vs.