Investigation On Charge Mobility Of Organic Material By Space Charge Limited Currents

The charge mobility is one of the most important electrical characteristics of the materials. The charge-carrier mobility of an organic semiconducting material is a key selection criteria for solid-state devices such as organic light-emitting diodes (OLEDs), organic photovoltaic diodes (OPVs), and organic field-effect transistors (OTFTs). The measurement of charge mobility is one of the basic work on the investigation of organic materials. The mobility of a material can be determined in many different technologies and we use space charge limited currents (SCLC) measurement, which is simple and high precise. Based on the SCLC method, the following research was accomplished.We employed the SCLC method to evaluate the electron mobility of the controlled devices, based on tris(8-hydroxyquinolinato) aluminum (Alq3). The zero-field mobilities and field-dependent factors of the four devices have been fitted respectively. Compared with other measurement, we get that tne scale of Alq3zero-field mobility of electron and field-dependent factor are10-7(cm2·V-1·s-1) and10-3(cm/V)1/2, respectively. The scale of field-dependent (~l05-106(V·cm-1)) electron mobilities of Alq3is106(cm2·V-1·s-1). We also found that the deposition in the sequence of Alq3/LiF/Al makes a good ohmic contact between Alq3and Al, and in this case the carrier mobilities is determined by the property of material, rather than the injection. Among the electron transport materials, only Alq3/LiF/Al can form a good ohmic electron injection, and the SCLC method could be used to determine the electron mobilities.Secondly, we used SCLC measurements to evaluate the hole mobilities of the candidate hosts including N, N’-bis-(1-naphthyl)-N, N’-diphenyl-1,1’-bipheny1-4,4’-diamine (NPB),4,4’,4"-tris (3-methylphenylphenylamino)-triphenylamine (m-MTDATA),4,4’-bis(carbazol-9-yl)biphenyl (CBP) and bis(2-methyl-8-quinolinato)-4-phenylphenolate (Balq), through a study of "hole only" devices. Based on a careful analysis of the current density-voltage (J-V) device characteristics, it was found that hole mobilities to be2.23×103,2.06X104,2.57×104and1.43×10-10cm2·V-1·s-1,respectively. The field-dependent hole mobilities of the four organic materials were fitted respectively. When doping red phosphorescent dyes Bis(1-phenyl-isoquinoline)(Acetylacetonato)iridium(III)(Ir (piq)2(acac)) in these four hosts, the doping concentrations make different impact on the hole mobilities. Changing dopant-concentration confirms the limited effect on the current densities of NPB and m-MTDATA based devices. However, it reveals highly dopant concentration dependent characteristics to that of CBP and Balq based devices. According to the analysis, we found the guest/host energy level matching plays an important role on the carrier transportation of emissive layer.Based on the experiments above, we finally study the performance of CBP and NPB being the host of Ir (piq)2(acac) respectively in white OLEDs (WOLEDs). It was found that the WOLEDs based on NPB:Ir (piq)2(acac) have a better color stability, larger external quantum efficiencies (EQEs) and lower EQEs roll-off then the WOLEDs based on CBP:Ir (piq)2(acac). This phenomenon is of the factor that a higher hole mobility through the emissive layer results in a wider carrier distribution and a better carrier balancing. The better balancing spreads the recombination zone, reduced pileup of excitons at single EML. In addition, it also alleviate the free carriers around the emitters and restrains the triplet-triplet quenching effect.