| Phosphorescent iridium complexes doped fluorescent films are commonly used for the emission layer of high performance organic light-emitting diodes.Studying the charge carrier transport in these films is of great importance for designing OLED with balanced electron and hole transport,which is crucial for improving the performance.In present dissertation,hole transport in phosphorescent dyes,bis?2-?9,9-diethyl-9H-fluoren-2-yl?-1-phenyl-1 Hbenzoim idazol-N,C3?iridium?acetylacetonate?[?fbi?2Ir?acac?],tris?2-phenylpyrid ine?iridium[Ir?ppy?3],tris[1-phenylisoqu inolin ato-C2,N]irid ium[Ir?piq?3],bis?4,6-difluorophenyl?-pyrid ina-to-N,C20]-picolinate?FIrpic?and bis?4',6'-difluorophenylpyridinato?tetrakis?1-pyrazolyl?borate?FIr6?]doped host molecule films of[N,N-dipheny l-N,N-bis?1-naphthy lphenyl?-1,1-biphenyl-4,4-diamine?NPB?,4,4,4-tri?Ncarbazolyl?triphenylami?TCTA?and 4,4'-N,N'-dicarbazole-biphenyl?mCP?]are systematically investigated by using admittance spectroscopy?AS?method and important conclusions are drown.1.Hole mobility of phosphorescent iridium complexes,bis?2-?9,9-diethyl-9 H-fluoren-2-yl?-I-phenyl-1Hbenzoimidazol-N,C3?iridium?acetylacetonate?[?fbi?2Ir?acac?]?Tris?2-phenylpyridine?iridium[Ir?ppy?3]and tris[1-phenylisoquinolinato-C2,N]iridium[Ir?piq?3]doped fluorescent host NPB and TCTAfilms,as well as FIrpic and FIr6 doped mCP film,were investigated.Important results are obtained.?1?For thicker films?>300 nm?,the electric field dependence of hole mobility is,as expected,positive,i.e.,the mobility increases exponentially with the squareroot of electric field.However,for thin films?<300 nm?,the electric field dependence of hole mobility is negative,i.e.the hole mobility decreases exponentially with the electric field.?2?For?fbi?2Ir?acac?doped NPB film,the dopant molecule function as hole traps,and lower the hole mobility.For thickness at the order 100 nm,the hole mobility decreases exponentially with the square root of the electric field.?3?For Ir?piq?3 doped NPB film around 100 nm thick,the mobility decreases exponentially with the square root of the electric field,.In general,the variation of mobility with the doping concentration is insignificant.For an applied bias of 9 V,the mobility varies in the range of 1.310-52.410-5 cm2/Vs for a wide doping concentration range of from 1.0 wt.%to 9.0 wt.%.In addition,the dependence of mobility on the temperature is weak.For a doping concentration of 3.0 wt/.%and an electric field of 5×105 V/cm,the mobility varies in the small range of 1.8×10-5?2.1×10-5 cm2/Vs in a large temperature range of from 78K to 300K.?4?For?fbi?2Ir?acac?and Ir?ppy?3 doped TCTAfilms,in the case of light doping,the mobility decreases with the square root of the electric field,while for the case of heavy doping,the mobility first increase and reaches a maximum at a doping concentration at xth and then decrease.The value of xth,are 2.0 wt.%and 7.0 wt.%for TCTA:fbi)2lr?acac?and TCTA:Ir?ppy?3 films,respectively.?5?For Ir?pig?3 doped TCTA film,the hole mobility decreases with the electric field.The hole mobility are in the range of 1.4×10-5?7.0×10-5cm2/Vs the doping concentration from 1.0 wt.%to 9.0 wt.%.?6?For FIr6 and FIrpic doped mCP films,the hole mobility decreases with the electric field.At F=5×105 V/cm,the hole mobility are in the range of 2×10-5-5×10-5cm2/Vs for the doping concentration varies from 1.0 wt.%to 9.0 wt.%.2.The Capacitance-Voltage?C-V?characteristics of hole-only devices based on phosphorescent dyes doped fluorescent host molecule films are investigated.The results show that:?1?The hole only devices based on NPB:Ir?piq?3,TCTA:?fbi?2Ir?acac?,TCTA:Ir?ppy?3,TCTA:Ir?piq?3,mCP:FIr6 and mCP:FIrpic films have the similar C-V characteristics;?2?At low frequencies??1kHz?,the capacitance increases with the applied bias,and reaches a maxuimum of Cmax at a voltage of Vmax,and then decreases with the further increase of the bias voltage.The physical cause is the quantum tunnel injection of electrons from the cathode;?3?At medium frequencies around 100 kHz,the capacitance decreases with the applied bias increasing.3.Conductance-Voltage characteristics of hole-only device based on phosphorescent dye doped fluorescent host molecule films were investigated.The results show that:?1?For hole-only devices based on Ir?ppy?3 and Ir?piq?3 doped NPB films,light doping enhances the conductivity,while heavy doping declines the conductivity;?2?For mCP:FIrpic devices,doping enhances the conductivity,while for mCP:FIr6 devices,doping decreases the conductivity;?3?For TCTA:?fbi?2Ir?acac?hole-only devices,the conductivity decreases with the in versing doping concentrations;?4?For TCTA:Ir?ppy?3 hole-only devices,the conductivity first increase and then decrease with the doping concentration.4.It was found that in some devices with the structure of ITO/Mo03/fluorescent host:phosphorescent dye/Mo03/Al,the-?B-f spectroscopy exhibit double characteristic peaks.We suggest that a Schottky energy barrier is formed at organic/cathode contact,and the two charachteristic peaks in-?B-f spectroscopy originate from the hole transit through two adjacent regions:one is the organic bulk region and the other is the deplection region forming near the organic/cathode contact.5.In our research,the Mott-Schottky method is extended for determining the cathode workfunction in organic devices.As the bandbending resulted from interface dipole is avoided by the built-in voltage measurement at high frequency,the built-in voltage measured by this way equals exactly the workfunction difference between the anode and the cathode.Therefore the cathode workfunction can be determined from the measured built-in voltage and the known anode workfunction.6.A numerical AS model is developed for bipolar drift transport in single-layer OLEDs under the approximation of uniformly distributed recombination-rate density in the recombination region.Based on this model,a concise empirical formula is obtained,from which the sum of the electron mobility and the hole mobility can be determined from the measured AS data. |
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