Efficient single layer RGB phosphorescent organic light-emitting diodes

An ultrathin layer of nickel oxide (NiO) was deposited on the indium tin oxide (ITO) anode to enhance the hole injections in organic light-emitting diode (OLED) devices. A very low turn-on voltage (3 V) was actually observed for the device with NiO on ITO. The enhancement of hole injections by depositing NiO on the ITO anode was further verified by the hole-only devices. The excellent hole-injection ability of NiO was also demonstrated by devising a device with patterned NiO on the ITO anode. Our results suggest that the NiO/ITO anode is an excellent choice to enhance hole injections of OLED devices.

Hole injection enhancement has been reported for organic thin-film transistors and light-emitting diodes at the indium tin oxide (ITO) anode side by introducing a LiF layer, which is usually used as an electron injection layer at the cathode side to reduce the electron injection barrier. We report a revised mechanism for the hole injection enhancement by studying a prototype interface of pentacene/LiF/ITO anode. Upon deposition of LiF on ITO, the work function of ITO decreases, and energy level realignment occurs between the pentacene and ITO. The hole injection barrier from the ITO to the pentacene highest occupied molecular orbital increases significantly with LiF insertion. Thus, the reduction in the hole injection barrier is not a critical factor for the hole injection enhancement. We suggest that a LiF insulating buffer layer enhances both injection barriers and tunneling through the barrier when a bias is applied.

Highly efficient two component phosphorescent organic light-emitting diodes based on direct hole injection into dopant and gradient doping

Enhanced performance of a poly(3-hexylthiophene):(6,6)-phenyl C61 butyric acid methyl ester bulk heterojunction polymer photovoltaic cell is reported by modifying the indium tin oxide (ITO) anode with chloroform solution. Instead of the traditional UV-ozone treatment, the optimized chloroform modification on ITO anode can result in an enhancement in the power conversion efficiency of an identical device, originating from an increase in the photocurrent with negligible change in the open-circuit voltage. The performance enhancement is attributed to the work function modification of the ITO substrate through the surface incorporation of the chlorine, and thus improved charge collection efficiency.

Standard organic light emitting diodes (OLEDs) are usually bottom-emitting, i.e. they emit light through a transparent and electrically conductive substrate. Usually, indium tin oxide (ITO) is used for this purpose. However, as indium is a very expensive metal, replacing it is of vital interest for cheap OLED mass production, especially when it comes to lighting applications. We suggest the use of a polymer instead of ITO, carrying out both hole transport and injection. In contrast to conventional approaches, which use a conductive polymer on top of ITO as smoothening and hole injection layer, we employ solely a highly conductive polymer in combination with an OLED comprising doped charge transport layers. This allows us to renounce the ITO layer underneath. We use a new, highly conductive formulation of PEDOT:PSS, called Baytron® PH 500, with a conductivity of typically 500 S/cm, providing a smooth and electrically well-conductive substrate for the OLED stack. The use of such a polymeric injection layer and of a doped small-molecule OLED stack results in a low operating voltage of the devices. The charge transport layers of the OLED consist of MeO-TPD (N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine) doped with a low percentage of F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane) for the hole transport layer and of Bphen (4,7-diphenyl-1,10-phenanthroline) co-evaporated with Caesium for the electron transport layer. We demonstrate both fluorescent and phosphorescent monochromic OLEDs based on Baytron® PH 500 which achieve good efficiencies. The OLEDs made on Baytron® PH 500 are compared with devices made on an ITO anode. Although the polymer possesses a somewhat lower conductivity than ITO, efficient devices can be fabricated. For example, using the blue emitter Spiro-DPVBi (2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene), we achieve an efficiency of up to 5.1 cd/A. As another example, we discuss green OLEDs based on the triplet emitter Ir(ppy)3 (fac tris(2-phenylpyridine) iridium) doped in a wide gap material. In this case, even a higher efficiency than on ITO is reached: 62 cd/A at a luminance of 100 cd/m², corresponding to an increase in external quantum efficiency by 15% as compared to ITO.

To improve the performance of blue fluorescent and green phosphorescent organic light-emitting diode devices, Merck developed novel green phosphorescent host and electron-transporting materials. The newly developed electron-transporting material improves the external quantum efficiency of blue fluorescent devices up to 8.7%, with an excellent lifetime. In combination with the newly developed host materials, the efficiency of green phosphorescent devices can be improved by a factor of 1.7, and the lifetime by a factor of 7.

Improved performance of the single-layer and double-layer organic light emitting diodes by nickel oxide coated indium tin oxide anode

Enhancement of efficiencies for tandem green phosphorescent organic light-emitting devices with a p-type charge generation layer

Green and low-cost approach to modify the indium tin oxide anodes in organic light-emitting diodes by electrochemical treatment in NaCl aqueous solution

In this work, the effect of replacing traditional indium tin oxide (ITO) anode with different anode materials is reported. The detailed simulation is conducted on organic light-emitting diode (OLED) consisting of commonly used emissive or organic layers with tris(8-hydroxyquinoline) aluminum (Alq3) at an operating wavelength of 540 nm. Different device simulations using aluminum zinc oxide (AZO), zinc mono-oxide (ZnO), graphene (7 nm/12 nm), and silver (Ag) anode materials with different work functions are modelled and simulated. For measuring OLED's optical results, the finite difference time domain (FDTD) approach was used. The angular distribution of OLEDs with reference to viewing angle with different anodes is reported. The results show the optical effects of the anode material with Graphene has better enhancement in light output compared to other materials. This work shows the far field contours of all the anode materials that improve light production and that optimum emission from an OLED to check the alternative to indium tin oxide anode material.

Reducing efficiency roll-off of phosphorescent organic light emitting diodes by using phosphor assisted energy funneling

Effect of change in pulsed DC frequency on indium–tin oxide anode on J– V– L performance of built-up organic light emitting diode during facing-target sputtering

In this study, we dissolved hole transport layer (HTL) material NPB in THF (tetrahydrofuran) solvant, and spin-coated the N,N'-Bis(naphthalene-l-yl)-N,N'-bis(phenyl)-benzidine (NPB) solution on the surface of Indium Tin Oxide (ITO) anode to enhance the luminance efficiency and lifetime of flexible phosphorescent organic light emitting diodes (POLEDs), where the 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was employed as hole blocking layer (HBL) and its thickness was optimized. Such an improvement in the device performance was attributed to the improved hole-electron balance. Finally, we employed 2,9-Dime-thyl-4,7-dphenyl-1,10-phenanhroline (BCP) or TPBi as hole blocking layer. The maximum luminance efficiency reaching 24.4 cd/A can be obtained.

We report the correlation between charge injection and charge balance using lithium fluoride/aluminum cathode and iridium oxide -coated indium tin oxide (ITO) anode in organic light emitting diodes (OLEDs). The turn-on voltage decreased from and luminous efficiency at increased from due to the presence of LiF and the high work function of . When was used in the anode region, the luminous efficiency increment rate decreased from 40% in Al cathode OLED to 7% in cathode OLED. The value of the carrier density suggested that the majority carrier changed from electrons in ITO- OLEDs to holes in ITO/ OLEDs, reducing the rate of increase. Therefore, it has been considered that charge injection should be increased and balanced in order to maximize the luminous efficiency.

Three bicarbazole based host materials with different interconnect positions between carbazole units, 3,3'-(9,9'-[3,3'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN33BCz), 3,3'-(9,9'-[3,4'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN34BCz), and 3,3'-(9,9'-[4,4'-bicarbazole]-9,9'-diyl)dibenzonitrile (3CN44BCz), were developed and their synthesis, material characterization, and device characterization were reported. Two carbazole units were connected via 3,3'-, 3,4'-, and 4,4'-positions to correlate the interconnect positions with photophysical properties and device performances. The linkage via 4,4'-position increased triplet energy and thermal stability of the host materials, while the linkage via 3,3'-position enhanced current density. All bicarbazole host materials showed good device performances and high quantum efficiency above 25% was attained using the biscarbazole derivatives for green and blue phosphorescent organic light-emitting diodes. In particular, the bicarbazole host materials with a linkage via 4-position showed high quantum efficiency above 30% in the green devices.