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Current density-voltage (J-V) characteristics of N,N′-di-[(1-naphthalenyl)-N,N′-diphenyl]-(1.1′ biphenyl)-4,4′-diamine (α-NPD) doped with 2,3,5,6-tetrafluoro-7,7′,8,8′-tetracyanoquinodimethane have been studied as a function of doping concentration (0–0.8 wt %) and temperature (105–300 K). The current density was found to increase with increase in doping concentration. In the doped samples as field increases above 3.3×104 V/cm the current abruptly starts increasing at a higher rate, which is ascribed as due to increased free charge carrier generation in the bulk. The enhanced free charge carrier generation is due to field assisted thermal dissociation of donor-acceptor pairs (Poole–Frenkel process) as well as charge injection at the interface. The released carriers increase the charge carrier density which brings the Fermi level near the highest occupied molecular orbital level of the α-NPD and reduces the space charge region near the interface favoring the tunneling of charge carrier across the interface, which is enough to support Ohmic conduction. The carrier generation has been found to be a thermally activated process. At higher fields (i.e., above 1.52×105 V/cm) the nonlinear J-V characteristics have been explained as due to field dependent mobility of holes. [ABSTRACT FROM AUTHOR]

The studies related to the current density-voltage (J-V) characteristics of pure N,N'-di-[(1-naphthalenyl)-N,N'-diphenyl]-(1.1- biphenyl)-4,4'-diamine ([alpha]-NPD) doped with 2,3,5,6-tetrafluoro-7,7',8,8'-tetracyanoquinodimethane ([F.sub.4]-TCNQ) hole only devices at various dopant concentrations and at various temperatures are presented. The increased free charge carrier generation is attributed to field assisted thermal dissociation of donor-acceptor pairs (Poole-Frenkel process) as well as charge injection at the interface.

Abstract: Bright white organic light-emitting diodes (WOLEDs) with single active layer has been demonstrated from blue emitting zinc complex bis(2-(2-hydroxyphenyl)benzoxazolate)zinc [Zn(hpb)2] doped with orange luminescent 4-(dicyanomethylene)-2-methyl-6-(p-dimethyl-aminostyryl)-4H-pyran (DCM) dye. White light electroluminescence (EL) spectrum from Zn(hpb)2 has been achieved by adjusting the concentration of DCM dye. WOLED with a structure of ITO/α-NPD/Zn(hpb)2:DCM (x%)/BCP/Alq3/LiF/Al has been fabricated. The EL spectra covering the whole visible spectra range of 400–700nm, with two peaks at 446 and 555nm has been measured. The device emits white light at 10V with Commission Internationale de I’ Eclairage (CIE) coordinates (0.27, 0.31) and brightness 1083Cd/m2. The maximum current efficiency of the device was 1.23Cd/A at 9.5V and maximum luminance reaches 2210Cd/m2 at 12V. [Copyright &y& Elsevier]

We report the synthesis, structure, and photophysical and electroluminescent (EL) properties of a series of heteroleptic bis(pyridylphenyl)iridium(III) complexes with various ancillary guanidinate ligands. The reaction of the bis(pyridylphenyl)iridium(III) chloride [(ppy)(2)Ir(μ-Cl)](2) with the lithium salt of various guanidine ligands Li{(N(i)Pr)(2)C(NR(1)R(2))} at 80 °C gave in 60-80% yield the corresponding heteroleptic bis(pyridylphenyl)/guanidinate iridium(III) complexes having a general formula of [(ppy)(2)Ir{(N(i)Pr)(2)C(NR(1)R(2))}], where NR(1)R(2) = NPh(2) (1), N(C(6)H(4)(t)Bu-4)(2) (2), carbazolyl (3), 3,6-bis(tert-butyl)carbazolyl (4), N(C(6)H(4))(2)S (5), N(C(6)H(4))(2)O (6), indolyl (7), NEt(2) (8), N(i)Pr(2) (9), N(i)Bu(2) (10), and N(SiMe(3))(2) (11). These heteroleptic cyclometalated (C^N) iridium(III) complexes showed intense absorption bands in the UV region assignable to π-π* transitions and weaker metal-to-ligand charge-transfer transitions extending to the visible region. These complexes also showed intense emissions at room temperature. Their photoluminescence spectra were influenced to some extent by the ancillary guanidinate ligands, giving λ(max) values in the range of 528-560 nm with quantum yields (Φ) of 0.16-0.37 and lifetimes of 0.61-1.43 μs. Organic light-emitting diodes were fabricated by the use of these complexes as dopants in various concentrations (5-100%) in a N,N'-dicarbazolylbiphenyl host. High current efficiency (η(c); up to 137.4 cd/A) and power efficiency (η(p); up to 45.7 lm/W) were observed under appropriate conditions. Their high EL efficiency may result from efficient trapping and radiative relaxation of the excitons formed in the EL process. Because of the steric hindrance of the guanidinate ligands, no significant intermolecular interaction was observed in these complexes, thus leading to the reduction of self-quenching and triplet-triplet annihilation at high currents. The EL emission color could be changed in the range of green to yellow by choosing appropriate guanidinate ligands.