Small Molecular Organic Light-emitting Devices Research Articles

Direct Comparison of Solution- and Vacuum-Processed Small Molecular Organic Light-Emitting Devices with a Mixed Single Layer

It will be interesting and valuable information can be achieved if a direct comparison between organic light emitting devices (OLEDs) fabricated by vacuum evaporated method (vac) and solution-based manufacturing processes (sol) was realized. Small molecular OLEDs with a mixed organic layer structure (MOLOLEDs) make it possible for direct comparison between devices with the same materials but fabricated by the two processing methods. This article shows a direct comparison of the luminescence characteristics, charge conduction, and device physics between MOLOLEDs fabricated by vac- and sol-processing techniques. It gives an elementary explain how the organic/metal interfaces influence the charge conduction and device performance.

Solution-Processed Small Molecular Organic Light-Emitting Devices with a Mixed Single Layer

We investigated the characteristics of solution-processed mixed-single-layer organic light-emitting devices (OLEDs) by mixing an electron injection material, a hole transport material, and a dopant material based on 5,6,11,12-tetraphenylnaphthacene (rubrene). The mixed-single-layer OLEDs showed better performance by optimizing the solution concentration and mixing ratio of organic materials. The performance was further improved by mixing chloroform (95 wt %) and toluene (5 wt %) as a solvent. The maximum luminance and power efficiency obtained were 12,400 cd/m2 and 1.1 lm/W, respectively. The mixed-single-layer OLEDs by solution process can be expected as an alternative route to the fabrication of small-molecular OLEDs with reduced cost of devices and avoiding the complexities of the co-evaporation of multiple organic materials in the vacuum deposition process.

Solution-Processed Small Molecular Organic Light-Emitting Devices with a Mixed Single Layer

We investigated the characteristics of solution-processed mixed-single-layer organic light-emitting devices (OLEDs) by mixing an electron injection material, a hole transport material, and a dopant material based on 5,6,11,12-tetraphenylnaphthacene (rubrene). The mixed-single-layer OLEDs showed better performance by optimizing the solution concentration and mixing ratio of organic materials. The performance was further improved by mixing chloroform (95 wt %) and toluene (5 wt %) as a solvent. The maximum luminance and power efficiency obtained were 12,400 cd/m2 and 1.1 lm/W, respectively. The mixed-single-layer OLEDs by solution process can be expected as an alternative route to the fabrication of small-molecular OLEDs with reduced cost of devices and avoiding the complexities of the co-evaporation of multiple organic materials in the vacuum deposition process.

Surface modification and characterization of indium–tin oxide for organic light-emitting devices

We used various treatment methods such as ethanol treatment, sodium hydroxide solution treatment, sulfur acid treatment, and oxygen plasma treatment to modify the surface of indium–tin oxide (ITO) substrates for organic light-emitting devices (OLEDs). The surface properties of the treated ITO substrates were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle, surface energy measurements, four-point probe, X-ray Diffraction (XRD) and ultraviolet-visible spectrophotometer. The results showed that the ITO surface properties were closely related to the treatment methods, and the oxygen plasma is more efficient than other treatments as it leads to smoother surface, better surface stoichiometry, higher work function and surface energy, lower sheet resistance, and higher transmission of the ITO substrates. Moreover, small molecular organic light-emitting devices (SMOLEDs) using different treated ITO substrates as anodes were fabricated and investigated. It was found that surface treatment of ITO substrates has influence upon the injection current, the turn-on voltages of light emission, luminance, efficiency and lifetime. Oxygen plasma treatment on the ITO substrate yields the highest performance of SMOLEDs due to the improvement of interface formation and electrical contact of the ITO substrate with the small molecular material blend in the SMOLEDs.

Electroluminescence spikes, turn-off dynamics, and charge traps in organic light-emitting devices

The strong electroluminescence (EL) spikes and long tails occurring at the turn-off of a bias pulse in high-quality guest-host small molecular organic light-emitting devices are described and analyzed. The observed behavior of the spikes is found to be in excellent agreement with a model based on correlated charge pairs, with average charge pair separation of 20\char21{}50 \AA{}. Immediately following the spike is a long EL tail which decays as $1/t.$ This tail is due to the recombination of initially independent charges. Hence the EL decay is determined by the dynamics of trapped charges in the recombination zone.

Combinatorial fabrication and study of doped-layer-thickness-dependent color evolution in bright small molecular organic light-emitting devices

The behavior of [N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD)]/[5±0.6 wt % [(2-methyl-6-{2-[2,3,6,7-tetrahydro-1H,5H-benzo(i,j)quinolizin-9-yl] ethenyl}-4H-pyran-4-ylidene)propane-dinitrile (DCM2)-doped α-NPD]/[4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl] organic light-emitting devices is described. The color of the devices shifts from blue to red as the thickness of the doped layer increases from 0 to 35 Å. The 2-Å-thick doped layer device exhibited the highest brightness L∼120 Cd/m2 and external quantum efficiency ηext∼4.4% at a current density of 1 mA/cm2.

Bright white small molecular organic light-emitting devices based on a red-emitting guest–host layer and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl

Bright multilayer white organic light-emitting devices (OLEDs) based on red-emitting [2-methyl-6-[2-∼(2,3,6,7-tetrahydro-1H, 5H-benzo [ij] quinolizin-9-yl)-ethenyl]-4H-pyran4-ylidene] propane-dinitrile (DCM2)-doped N,N′-diphenyl-N,N′-bis(1-naphthylphenyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) and blue-emitting 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) are described. The blue emission increased relative to the red emission with increasing bias, but the color coordinates of the total emission were well within the white region at V>10 V. The most lightly doped device, which contained a 5 nm thick layer of 0.5 wt. % of the dye dopant, exhibited the highest power efficiency, 4.1 lm/W, external quantum efficiency of 3.0%, and white brightness Lmax>50 000 Cd/m2 (at ∼1100 mA/cm2).