Ok-Keun Song

Hole mobility of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine investigated by using space-charge-limited currents

The hole mobility of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) at various thicknesses (50–1000nm) has been estimated by using space-charge-limited current measurements. A thin layer of buckminsterfullerene has been used for a quasi-Ohmic contact between NPB and indium tin oxide. The mobility at bulk property dominant thickness is in excellent agreement with the results from time-of-flight method. For the typical thickness of organic light-emitting devices, the hole mobility of NPB, 1.63×10−5cm2∕Vs, at 50nm is smaller than the value 7.64×10−4cm2∕Vs at 1000nm (electric field at 0.1MV∕cm). The authors suggest that the lower mobility is caused by the interfacial trap states.

Effective hole injection of organic light-emitting diodes by introducing buckminsterfullerene on the indium tin oxide anode

We demonstrate that dramatically improved hole injection can be achieved by inserting a very thin C60 film between the indium tin oxide (ITO) electrode and N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB) layer. This result is ascribed to the formation of an interfacial dipole layer of buckminsterfullerene (C60) on the ITO electrode. The dipole layer induces the surface potential shift that contributes to improve the charge injection efficiency. The chemical shift was downward to help lower the hole injection energy barrier from the ITO electrode to the NPB layer, consistent with the moderately strong electron accepting nature of C60. The enhanced-charge injection provides a simple way of reducing the power consumption of organic electronic devices for real applications.

Effects of interfacial stability between electron transporting layer and cathode on the degradation process of organic light-emitting diodes

The authors have demonstrated that the increase of electron injection barrier height between tris(8-hydroxyquinoline)aluminum (Alq3) and LiF∕Al cathode is one of the most critical parameters to determine the reliability of organic light-emitting diode with the typical structure of indium tin oxide/N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine/Alq3∕LiF∕Al. The electrical properties of several devices (hole only, electron only, and integrated double-layered devices) have been measured in the function of operating time to analyze the bulk and interface property changes. Bulk properties of trap energy and mobility in an organic layer have been estimated by using trap-charge-limited currents and transient electroluminescence measurements.

Ohmic contact probed by dark injection space-charge-limited current measurements

The authors demonstrate through dark injection space-charge-limited current (DI-SCLC) and trap-free SCLC measurements that an indium tin oxide (ITO)/buckminsterfullerene (C60) electrode can form a quasi-Ohmic contact with N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB). The DI-SCLC results show a clear peak current along with a shift of the peak position as the field intensity varies, implying an Ohmic (or quasi-Ohmic) contact. A theoretical simulation of the SCLC also shows that ITO/C60 forms an Ohmic contact with NPB when the electric field intensity is higher than 30 kV/cm. The Ohmic contact makes it possible to estimate the NPB hole mobility through the use of both DI-SCLC and trap-free SCLC analysis.

Apparent thickness dependence of mobility in organic thin films analyzed by Gaussian disorder model

The authors report the investigation of thickness dependence of mobility in N, N′-bis(naphthalen-1-yl)-N, N′-bis(phenyl) benzidine (NPB), and tris(8-hydroxyquinoline) aluminum (Alq3) thin films based on the Gaussian disorder model. The energetic disorder of 118 meV at thickness of 50 nm is larger than that of 88 meV at 300 nm in NPB. From the mobility prefactor and intersite distance, the carrier hopping distance between NPB molecules of thinner film is larger than that of thicker film. It suggests that the lower mobility at thinner thickness of NPB is affected by not only the interfacial trap states but also the molecular arrangement on the surface. Contrary to the results of NPB, the energetic disorder of Alq3 at various thicknesses was obtained to be 150±7 meV that is almost independent of the thickness of film. The small changes in hopping distance of Alq3 at different thicknesses have been observed, and this trend is clearly different from the case of NPB.

Enhanced performance of organic light-emitting diodes with an air-stable n-type hole-injection layer

An air-stable n-type organic semiconductor, N,N-bis(4-trifluoromethoxybenzyl)-1,4,5,8-naphthalene-tetracarboxylic di-imide (NTCDI-OCF3), can function as an excellent hole-injection layer to improve the hole injection from an indium tin oxide (ITO) anode to a hole-transporting layer (HTL). Significantly improved hole injection was achieved by introducing an ultrathin layer of NTCDI-OCF3 between ITO and HTL, leading to a lower operational voltage and relatively less power consumption. The results can be attributed to the reduced hole-injection energy barrier from ITO to HTL, as shown by x-ray photoelectron spectroscopy measurements, due to the surface dipole that is formed by the NTCDI-OCF3. The thickness dependence of NTCDI-OCF3 was also examined.

Thickness dependence of the trap states in organic thin film of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine

The authors have investigated the relationship between the trap states (exponential trap distribution in energy and density) and the thickness of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl) benzidine (NPB). The thickness dependent hole mobility of NPB can be attributed to the trap states. The origin of deep trap states at thinner film can be attributed to both surface dipole of buckminsterfullerene and the interaction between NPB and indium tin oxide at the interface. The influence of interfacial trap states on charge drift mobility is getting weaker as the thickness increases and is negligible when the thickness of NPB is thicker than 300nm.

Spontaneous charge transfer from indium tin oxide to organic molecules for effective hole injection

Naphthalene tetracaboxylic dianhydride (NTCDA) shows strong chemical interaction with metal atoms in an indium tin oxide (ITO) substrate to form charge transfer (CT) complexes. The CT complex at the ITO/NTCDA interface can lower the energy barrier height for hole injection from ITO into the hole transporting layer of N,N′-diphenyl-N, N′-bis(1-naphthyl)(1,1′-biphenyl)-4,4′ diamine (NPB). The operational voltage of an emissive device at a current density of 100 mA/cm2 was significantly reduced from 12.2 to 9.2 V by simply inserting a thin layer of NTCDA between the ITO and NPB. The results enable the achievement of organic light-emitting diodes that consume relatively less power.

New Zn Complex Derivatives for Red OLEDs Host Materials

New two metal complexes, di-(Phenyl dipyrrylmethene)zinc (Zn(PPM)2) and di-(Pentafluorophenyl dipyrrylmethene)zinc (Zn(PFPPM)2) as a host material instead of Alq3 were synthesized. To evaluate electroluminescent properties, multi-layered organic light-emitting devices were fabricated by using 4-(dicyanomethylene)-2-tert-buthyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as a dopant and Alq3 as an electron transporting layer. Alq3 and Zn(PPM)2 host EL devices exhibited DCJTB emission peak at around 617 nm due to energy transfer from Alq3 and Zn(PPM)2 to DCJTB. However Zn(PFPPM)2 host device shows no DCJTB emission peak because Zn(PFPPM)2 device emit EL light of 563 and 700 nm. The Zn(PPM)2 device showed same luminance efficiency as Alq3 device, but showed better power efficiency of 1.2 times than Alq3 device.

P‐147: Asymmetric High Performance Hybrid Tandem White OLEDs for Lighting Applications

Two efficient and color-stable asymmetric hybrid tandem white organic light emitting diode (WOLED) devices have been fabricated with a combination of fluorescent blue and phosphorescent green/red emissive layers. Two different color temperatures of 3000K and 5000K are easily achieved by simply changing the stack order of fluorescent blue and phosphorescent green/red layers. with the help of outcoupling film, high power efficiency of ∼40 lm/W and significantly improved reliability have been achieved.