Highly Efficient Light-Emitting Diodes Based on an Organic-Soluble Poly(p-phenylenevinylene) Derivative Carrying the Electron-Transporting PBD Moiety

Abstract

barrier between the anode and the I p of the HTL is the primary criterion for efficient hole injection. This points to a more complicated relationship between carrier injection than is predicted simply from energy level offset considerations. Other explanations for efficient hole injection are based on the importance of good adhesion between the organic layers and ITO. Defect states at the interface between the organic layer and the electrode have also been shown to be useful for enhancing injection over energy barriers. [14,15] Conversely varying the work function of ITO and hence the injection barrier into the HTL, was not found to lead to significant change in Z. [16] Thus we observe that injection of carriers can not be fully controlled by simply varying the energy offset barrier at the electrode. It should also be noted that no correlation between device efficiency and the size of the electron-blocking barrier (the difference between the LUMO levels of Alq 3 and the HTL) at the HTL/Alq 3 interface was observed.The most useful of the novel HTLs are ISB and PPD, [7] which maintain the device performance of TPD and a-NPD while significantly increasing the T g . HTLs with a T g equivalent to ISB but lower than PPD have been published previously. [12,13] Adachi et al. [12] reported a series of HTLs with a maximum luminance of 790 cd/m 2 for T g = 110 C at a current density of 30 mA/cm 2 for devices comparable to structure 2. At an equivalent current density, PPD and ISB have luminances of >1000 cd/m 2 and 880 cd/m 2 , respectively. Okutsu et al., for hole transport materials with triphenylamine and fluorene structures, reported a T g between 100 C and 112 C and luminance efficiencies of »2.0 cd/A, [13] for devices also similar in design to structure 2. For ISB and PPD, we find an almost equivalent efficiency at 5 mA of »1.8 cd/A. However Okutsu's devices have a significantly lower power efficiency, with a brightness of only 1 cd/m 2 at 6.0±9.3 V, in comparison to 3.8 V and 5.8 V for PPD and ISB, respectively. Thelakkat et al. reported other triphenylamine derivative HTLs having T g £ 123 C. However, OLEDs based on these materials have a twenty times smaller maximum luminance output than reported here. [17] An HTL with T g = 165 C, which has a quantum efficiency of 1.3 % when incorporated into a device architecture comparable to structure 3, has been patented. [18] Complete information regarding the device characteristics have, to our knowledge, not yet been reported. Other work has used a starburst amine molecule with a T g » 200 C [19] in a double-layer HTL structure. However, device performance comparable to that reported in this work was only achieved by using a second lower T g HTL in the device.In conclusion we have fabricated a series of devices utilizing high glass transition temperature HTLs. Two materials, PPD and ISB, have excellent device characteristics coupled to a high T g , demonstrating that high-T g HTLs can be used to fabricate efficient and stable OLEDs. We have found no relationship...

Synthesis and electroluminescent property of poly(p-phenylenevinylene)s bearing triarylamine pendants

“…There have been several reports on the improvement of p-conjugated polymers such as PPV and polyfluorene by introducing hole-transporting and/or electron-transporting groups to realize highly efficient electroluminescence [9][10][11][12][13][14][15][16][17]. For example, Jin et al reported PPV-based copolymers containing an oxadiazole group with an electron injecting and transporting ability [9].…”

Novel Luminescent Multilayer Films Containing π-Conjugated Anionic Polymer with Electronic Microenvironment

“…The first reason for the so low QY values is the imbalance of hole/electron injection and transport because the emissive polymer conduct holes are used in preference to electrons [21,22]. Due to the easily trapped excitons and constant nonradiative decay, there is the presence of defects in the polymer, so this is the second reason [23,24].…”

Synthesis of polythiophene containing heterocycle on the side chain: A review

“…[16] Their another possible application field is water-soluble biological sensors for the recognition of DNA, proteins, metal ions and small bioanalyses on the foundation of modifications in photoluminescence and UV-Vis spectroscopy. [17][18][19][20][21][22] At the first time, the functions of nonsubstituted polythiophene were restricted owing to its insolubility in water and water miscible solvents, as well as medium electrical conductivity. Solubility in water-miscible solvents and aqueous environment supports the formulation of the composites of polymers with noble-metal nanoparticles, which display the influence of local enhancement of electromagnetic field and the ability to make thin films.…”