K.L. Tong

Hole transport in molecularly doped naphthyl diamine

The effects of dopants on the hole-transporting properties of NPB, i.e., (N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′ diamine), were studied by time-of-flight technique and admittance spectroscopy. Three dopants were chosen in this study. They were 4-dicyanmethylene-2-methyl-6-4H-pyran (DCM1), rubrene (RB), and tris-(8-hydroxyquinoline) aluminum (Alq3). It can be shown that DCM1 behaves as hole traps whereas Alq3 behaves as hole scatterers in NPB. Generally, both trapping and scattering lower hole mobilities in NPB. The hole mobilities decrease when DCM1 and Alq3 are introduced into NPB whereas the hole mobility remains nearly unchanged when RB is doped into NPB. The effect of doping on carrier dispersion is also studied.

Direct determination of carrier mobilities of OLED materials by admittance spectroscopy

We show that admittance spectroscopy (AS) can be used to determine charge carrier mobilities and transport parameters in materials relevant to organic light-emitting diodes (OLEDs). Via computer simulation, we found that a plot of the negative differential susceptance vs frequency yields a maximum at a frequency τr-1. The position of the maximum τr-1 is related to the average carrier transit time τdc by τdc = 0.56 τr. Thus knowledge of τr can be used to determine the carrier mobility in the material. Devices with the structure anode/phenylamines/Ag have been designed to evaluate their mobilities. The extracted hole mobility data from AS in pristine and doped material systems are in excellent agreement with those independently extracted from time-of-flight (TOF) technique. In addition, materials with different energy levels of highest occupied molecular orbital (HOMO), are further examined in order to study the effects of injection barrier on the extracted mobility by AS. In the case of an Ohmic hole contact (e.g. ITO or Au /m-MTDATA), the mobility data is good agreement with TOF results. However, for a non-Ohmic contact, the extracted mobility appears to be smaller. Thus AS can be used a means of evaluating the quality of electric contact between the injection electrode and the organic material.

Oxadiazole-Triphenylamine Derivatives for OLEDs

Electroluminescent bipolar small molecules have been attracted with great interests recently. They are found to exhibit many interesting features such as (i) reducing the structural complexity of organic light emitting diodes (OLEDs) from multilayer heterojunction to monolayer homojunction devices; (ii) offering molecular p/n junction, and (iii) minimizing the formation of exciplexes. In this paper, the optical and electrical properties of novel oxadiazole-triphenylamine derivatives will be investigated. The derivatives are N-phenyl-N-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)phenylamine (POT) and N-phenyl-N-(4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)phenyl)phenylamine (m-POT). The absolute absorption coefficient and refractive index have been investigated by ellipsometry and modeling. The electron mobility of POT at room temperature has been studied. The results show that the derivatives have bipolar characteristics. The electron-transporting properties of POT is better than that of m-POT. The EL emission peaks of POT and m-POT are the same at 435nm which match with their photoluminescent (PL) peaks.

Blue emitting OLED with star-shaped multifunctional oligomer

A fluorescent star-shaped oligomer with a nitrogen atom as a core and both a hole transporting arylamine and an electron transporting 1,3,4-oxadiazole moiety, tri(4-(5-phenyl-1, 3, 4-oxadiazol-2-yl)phenyl)amine (TPOPA), has been designed and synthesized using a 5-step reaction procedure. The synthesized compound was characterized by elemental analysis, 1H-NMR and mass spectroscopy. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis show that TPOPA exhibits high thermal stability (Td, 373°C) and high glass-transition temperature (Tg, 116 °C). Photoluminescence measurements indicate that the star-shaped oligomer shows intense blue emission peaked at 445 nm with a high quantum yield of 0.68 under near UV light excitation. The HOMO value of TPOPA is -5.64 eV and the LUMO is -2.58 eV based on the electro-chemical determinations. Reversible anodic oxidation results suggest that the hole-transporting is predominant for TPOPA. Two single layered devices were fabricated by vacuum evaporation with configurations of ITO / CuPC (15 nm) / TPOPA (95 nm) / Ca(30 nm) / Al(100 nm) (device 1) and ITO / CuPC(15 nm) / TPOPA (175 nm) / Ca(30 nm) / Al(100 nm) (device 2), where TPOPA was used both as emitter and carrier - transporting material, CuPC as a hole-injection and electron block material. The devices show blue wide-band emission peaked at 438 nm with a maximum luminance of 650 cd/m2 and 512 cd/m2 under an operating voltage of 12 V, respectively.