Effect of doping of 8-hydroxyquinolinatolithium on electron transport in tris (8-hydroxyquinolinato) aluminum (original) (raw)
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Cheap p- and n-doping for highly efficient organic devices
Journal of Photonics for Energy, 2011
Electrically doped, organic transport layers are important for today's high efficiency organic (opto-)electronic devices. Doped organic layers have a strongly increased free charge carrier density compared to their undoped counterparts and also improve the charge carrier injection from adjacent electrodes into the organics. For practical applications, especially in optoelectronics, these layers have to have low absorption in the wavelength range of interest. The two nearly colorless p-and n-doping materials, rhenium heptoxide and cesium carbonate, are investigated focusing on their conductivity enhancement, injection improvement, and voltage drop over doped transport layers in organic light emitting diodes. They show very good doping properties already at moderate doping concentrations and prove that they can be used in variable thicknesses without a significant voltage increase. This makes them cheap, low absorbing alternatives to today's, well-established doping systems.
Alkali metal doping and energy level shift in organic semiconductors
Applied Surface Science, 2006
We have investigated Cs and Na doping in copper phthalocyanine (CuPc) and tris(8-hydroxyquinoline) aluminum (Alq) using photoemission spectroscopy. We observed valence and core level spectra changes at different doping levels, and found that the doping induces an energy level shift that can be seen in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by a significant modification of organic energy levels, such as the introduction of a new gap state, new core level components and a change of binding energies. Furthermore, we observed that the energy level shift in the first stage depends in a semi-logarithmic fashion on the doping concentration, whose slope cannot be explained by the conventional model used in inorganic semiconductors. These results indicate that the molecular nature and strong correlation must be considered for doping in organic semiconductors.
IEICE Trans. Electron, 2015
This paper presents 2-(hydroxyl) quinoline lithium (Liq) used as an n-type dopant to improve white hybrid organic light-emitting diode (WHOLEDs) performance. The Liq doped tris(8-hydroxyquinolinato) aluminum (Alq 3) layer possessed enhanced electron injection, efficient hole and electron balance in the emitting layer, as one of the most essential issues for device applications. This work investigates the optimum recipe (Liq concentration and thickness) of Alq 3 :Liq n-type doped electron injection layer (EIL) for WHOLED devices by comparing the current density and efficiency results with conventional Alq 3 /LiF technique. A blocking layer or interlayer is inserted between emitting layer and EIL to avoid excitons quenched. In this work suitable material and optimum thickness for blocking layer are studied, a white small-molecular organic light-emitting diode (SM-OLEDs) based on a 1,3,5-tris (N-phenylbenzimidazol-2-yl) benzene (TPBi) stamping transfer process is investigated. The proposed stamping transfer process can avoid the complexity of the vacuum deposition process.
High efficiency p–i–n organic light-emitting diodes with a novel n-doping layer
Microelectronics Reliability, 2010
This study demonstrated p-i-n organic light-emitting diodes (OLEDs) incorporating a novel n-doping transport layer which is comprised of cesium iodide (CsI) doped into tris-(8-hydroxyquinoline) aluminum (Alq 3 ) as n-doping electron transport layer (n-ETL) and a p-doping hole transport layer (p-HTL) which includes molybdenum oxide (MoO 3 ) doped into 4,4 0 ,4 00 -tris[2-naphthyl(phenyl)amino] triphenylamine (2-TNATA). The device with a 15 wt.% CsI-doped Alq 3 layer shows a turn on voltage of 2.4 V and achieves a maximum power efficiency of to 4.67 lm/W as well, which is significantly improved compared to these (3.6 V and 3.21 lm/W, respectively) obtained from the device with un-doped Alq 3 . This improvement is attributed to an increase in the number of electron carriers in the transportation layer leading to an efficient charge balance in the emission zone. A possible mechanism behind the improvement is discussed based on X-ray photoelectron spectroscopy (XPS).
2011
The array of organic conductivity dopants used for organic light-emitting devices (OLEDs) to reduce the operating voltage and improve power efficiency is extremely limited. Here we report a comparative theoretical study between newly proposed analogues and the standard state-of-the-art conductivity dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We used density functional theory to determine the bond lengths, bond angles, and electronic properties, such as the energy of the highest occupied molecular orbital (E HOMO) and the lowest unoccupied molecular orbital (E LUMO) states. The ground state structures of the proposed molecules were optimized at the B3LYP/6-31G* level. The results show that substitution of one or two fluorine groups in the F4-TCNQ core with a substituted phenyl ring or other electron-withdrawing moieties, will not substantially affect the geometry of the molecule or its electronic ability to accept electrons. The most significant finding was that the phenyl substitutions onto the TCNQ core are nearly perpendicular to the TCNQ plane, and thus there is no electronic communication between the two rings. This is extremely important, as such extension of the π conjugated system would negatively affect the E LUMO and thus the electron affinity of the molecule.
Organic semiconductors: fundamentals and applications
The first organic light emitting diodes (OLEDs) were demonstrated in 1987. To date they have been brought into marketable commodity. OLED matrix displays offer high contrast, wide viewing angle and a broad temperature range at low power consumption. Due to the sensitivity of organic thin films, new structuring techniques had do be developed. In recent years, in addition to advanced OLED activities, increasing effort has been put into the realization of organic integrated circuits for low-cost applications based on organic field-effect transistors. First transistors have shown the necessity to decrease operating voltage and to improve carrier mobility. Electrical current in organic devices is limited by the low conductivity of organic semiconductors and by energy barriers at the metal-organic semiconductor interface. Photoelectric measurements facilitate the determination of barrier height differences between various electrode setups. Further insight in the energy band alignment at organic heterointerfaces are gained by ultraviolet photoelectron spectroscopy. Energy transfer in a doped OLED emitting layer can be investigated by time-resolved photoluminescence measurements.