S. C. Tse

Electron transport in naphthylamine-based organic compounds

Two naphthylamine-based hole transporters, namely, N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′diamine (NPB) and 4,4′,4″-tris(n-(2-naphthyl)-n-phenyl-amino)-triphenylamine (2TNATA), were found to possess electron transporting (ET) abilities. From time-of-flight measurements, values of electron mobilities for NPB and 2TNATA are (6–9)×10−4 and (1–3)×10−4cm2∕Vs, respectively, under an applied electric field range of 0.04–0.8MV∕cm at 290K. An organic light-emitting diode that employed NPB as the ET material was demonstrated. The electron conducting mechanism of NPB and 2TNATA in relation to the Marcus theory [Rev. Mod. Phys. 65, 599 (1993)] from quantum chemistry will be discussed.

Electron transit time and reliable mobility measurements from thick film hydroxyquinoline-based organic light-emitting diode

The time delay (τd) in the transient electroluminescence (EL) signal of a bilayer organic light-emitting diode with a structure of indium-tin oxide /N,N′-diphenyl-N,N′-bis(3methylphenyl)-(1,1′-biphenyl)-4,4′-diamine /tris(8-hydroxyquinoline) aluminum (Alq3)/Al has been measured and analyzed as a function of the thickness (D) of the Alq3 layer. For a thin layer of Alq3 (D 200 nm), τd approaches the intrinsic electron transit time through Alq3. Electron mobility of Alq3 can be evaluated for the thick-film devices and the results are in excellent agreement with independent time-of-flight measurements. The application of transient EL in mobility measurement for C540-doped Alq3 is discussed.

Polymeric conducting anode for small organic transporting molecules in dark injection experiments

Poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) is used as a hole-injecting anode for small organic hole transporters in current-voltage (JV) and dark injection space-charge-limited current (DI-SCLC) experiments. The hole transporters under investigation are phenylamine-based 4,4′,4″-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (MTDATA), N,N′-diphenyl-N,N′-bis(1-naphthyl) (1,1′-biphenyl)-4,4′diamine (NPB), and N,N′-diphenyl-N,N′-bis(3-methylphenyl)(1,1′-biphenyl)-4,4′diamine (TPD). Clear DI-SCLC transient peaks were observed over a wide range of electric fields in all cases. For MTDATA and NPB, hole mobilities evaluated by DI experiments are in excellent agreement with mobilties deduced from independent time-of-flight technique. It can be concluded that, for the purpose of JV and DI experiments, PEDOT:PSS forms an Ohmic contact with MTDATA and a quasi-Ohmic contact with NPB despite the relatively low-lying highest occupied molecular orbital of the latter. In the cas...

Single-layer organic light-emitting diodes using naphthyl diamine

N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′diamine (NPB), a common hole transporter, was employed to fabricate single-layer organic light-emitting diodes (OLEDs). With a quasi-Ohmic anode, NPB device exhibited a bulk-limited hole current in the low-voltage region. Electron injection and light emission were clearly observed for applied voltages exceeding 4V. In order to confine the recombination zone, intentional doping was applied to the single-layer device. After doping with perylene, the luminance and current efficiency of NPB device increased dramatically. It is expected that more efficient single-layer OLEDs can be achieved by using the doping strategy.

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.

Experimental and Theoretical Demonstration on the Transport Properties of Fused Ring Host Materials for Organic Light-Emitting Diodes

The charge transport properties of three tertiary-butyl (t-Bu) substituted anthracene derivatives (ADN), critical blue host materials for organic light-emitting diodes (OLEDs), have been investigated experimentally and computationally. From time-of-flight (TOF) measurements, all ADN compounds exhibit ambipolar characters. The hole and electron mobilities are in the range (1–5)×10-7 cm2 V-1 s-1 under an external applied field of about 1 MV cm-1. Un-substituted ADN has the highest carrier mobilities while heavily t-Bu substituted ADN has the least. The electron and hole conducting properties of are consistent with ab initio calculation, which indicates that the frontier orbitals are localized mainly on the anthracene moiety. t-Bu substitutions in ADN increase the hopping path lengths among the molecules and hence reduce the electron and hole mobilities. The results demonstrate that t-Bu substitution is an effective means of engineering the conductivity of organic charge transporter for OLED applications.

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.