S. K. So

Application of admittance spectroscopy to evaluate carrier mobility in organic charge transport materials

We examine the feasibility of admittance spectroscopy (AS) and susceptance analysis in the determination of the charge-carrier mobility in an organic material. The complex admittance of the material is analyzed as a function of frequency in AS. We found that the susceptance, which is the imaginary part of the complex admittance, is related to the carrier transport properties of the materials. A plot of the computer-simulated negative differential susceptance versus 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=0.56τr. Thus, knowledge of τr can be used to determine the carrier mobility in the material. Devices with the structure ITO/4,4′,4″ -tris[N, -(3-methylphenyl)-N-phenylamino] triphenylamine/Ag have been designed to investigate the validity of the susceptance analysis in the hole mobility determination. The hole mobilities were measured both as functions of the electric field and the temperature. The hole mobi...

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.

Photodesorption of NO from Ag(111) and Cu(111)

The adsorption, thermal reactions, and photoreactions of NO on Ag(111) and Cu(111) at 80–85 K have been studied by thermal‐desorption spectroscopy (TDS), high‐resolution electron‐energy‐loss spectroscopy (HREELS), and photon‐induced desorption. Adsorption of NO on both surfaces is quite complicated. At saturation coverage, a number of chemical species are present, including atop and bridge‐bonded NO, atomic N and O, and N2O. Photodesorption of NO, N2, and N2O is observed simultaneously under low‐power photon irradiation in the wavelength range for 260–600 nm. From TD and HREEL spectra before and after photon irradiation, it is established that on both surfaces the atop NO is photoactive. Photon polarization, power‐, and wavelength‐dependences studies indicate that the mechanisms for photodesorption are nonthermal. A substrate‐mediated mechanism involving photogenerated carriers at low photon energies (<3 eV) and a direct excitation mechanism of the adsorbate‐surface complex at high photon energies are use...

Hole transports in molecularly doped triphenylamine derivative

Hole mobilities of N 0 ; N 0 -bis(3-methylphenyl)-(1; 1 0 -biphenyl)-4; 4 0 -diamine (TPD), TPD doped with 5, 6, 11, 12-tetraphenylnathacene (rubrene), and TPD doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryle) 4H-pyran (DCM1) were measured by a time-of-flight (TOF) technique between 200 and 300 K. Pristine TPD has a hole mobility of about 2 � 10 � 3 cm 2 V � 1 s � 1 at room temperature. The corresponding TOF time transients were non-dispersive. Upon doping, the hole mobility decreased slightly for rubrene-doped TPD, and sharply for DCM1-doped TPD. The TOF time transients remain non-dispersive for both kind of doped samples. The dependence of the mobility on electric field and temperature for undoped and doped TPD was investigated. 2002 Elsevier Science B.V. All rights reserved.

Highly efficient deep blue organic electroluminescent device based on 1-methyl-9,10-di(1-naphthyl)anthracene

The author have developed 2-methyl-9,10-di(1-naphthyl)anthracene (α,α-MADN) as an effective wide band gap host material for Forster energy transfer to the unsymmetrical mono(styryl)amine deep blue fluorescent dopant (BD-1). This guest/host emitting system, at the optimal doping concentration of 3%, can also increase the probability of carrier recombination near the hole-transport/emitting layer interface for the blue organic light emitting device which produces electroluminescence efficiencies of 3.3cd∕A and 1.3lm∕W and a deep blue CIEx,y color coordinates of (0.15, 0.13) that are 50% better than those of the traditional β,β-isomeric host (MADN) with the same dopant.

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.

The adsorption and reactions of NO on Ag(111) at 80 K

The adsorption of NO on Ag(111) at 80 K has been studied by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). At low exposures (≤0.05 L) NO is adsorbed in part dissociatively and in part molecularly in two different threefold bridge states (in upright and bent or tilted orientation with respect to the surface normal). The NO molecules in the threefold bridge position are desorbed at 100 K. With increasing exposure the desorption temperature shifts gradually to 110 K. At medium exposures (∼0.13 L) additional NO is adsorbed molecularly in an atop position with an upright orientation in admixture with atomic N and O and molecular NO adsorbed in threefold bridge states. The NO molecules adsorbed in atop position are weakly bonded on the surface and are desorbed at about 90 K. Simultaneously, N2O is formed and adsorbed on the surface. At saturation a new bent or tilted NO species in atop position appears on the surface and the amount of N2O significantly incr...

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...

Carrier trapping and scattering in amorphous organic hole transporter

The effects of dopants on the hole transporting properties of N,N′-diphenyl-N,N′-bis(1-naphthyl)(1,1’-biphenyl)-4,4’diamine (NPB) have been studied by time of flight. Five dopants: copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyrle)-4H-pyran (DCM1), 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran (DCM2), 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) are used in this study. The dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbital relative to that of NPB. Generally, traps are found to induce significant reduction in hole mobility while there is a slight reduction for scattering. Two different underlying charge transport mechanisms are proposed.

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.