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Although the mixed-host organic light emitting diode (OLED) has been known for improved performance, the charge injection and mobility may not be uniform in the light emitting layer (EML) hence affecting the exciton formation and recombination. In this work, we investigate the effects of concentration profiles of electron- and hole-transport materials in EML on the charge injection and confinement as a result of in-situ formation of transport layers by thermal evaporation of a pre-mixed source. Experimental results show that the OLED performance depends on the continuously graded concentration profiles, and are correlated to numerical simulations which reveal charge confinement, spatial shift of recombination zone and lowering of hole mobility. [ABSTRACT FROM AUTHOR]

In this work, we separately investigated the dependence of electron injection and transport processes on the doping concentration by comparing the current density-voltage ( J–V ) characteristics of a series of electron-only devices with tris(8-hydroxyquinoline) aluminum (Alq 3 ) doped into N,N ′-diphenyl- N,N ′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) as transport layer. Here, Alq 3 molecules function as electron injection sensitive centers. Experimental results revealed that 0.2 wt% is the optimal doping concentration to facilitate electron injection from hole-block layer into host molecules. With increasing doping concentration, more and more electrons inject directly onto Alq 3 molecules while the average distance of Alq 3 molecules decreases gradually, thus which accelerate electron hopping between Alq 3 molecules. With the help of electroluminescent (EL) spectra, recombination centers of the 0.2 wt% and 0.6 wt% doped EL devices were probed to shift toward cathode and anode, respectively. These results suggested that electron hopping between Alq 3 molecules becomes the dominant electron transport mechanism when the doping concentration is higher than 0.6 wt%. [ABSTRACT FROM AUTHOR]

Lithium hydroxide doped tris(8-hydroxyquinoline) aluminum (Alq 3 :LiOH) is used as an effective interfacial layer for the fabrication of efficient inverted bottom-emission organic light-emitting diodes (IBOLEDs). When 15% LiOH was doped into an Alq 3 electron-transporting layer, the device properties such as the turn-on voltage, maximum luminance, and device efficiency improved, becoming better than those obtained with LiF doping and comparable to those of β-naphthylphenylbiphenyl diamine (NPB)/Alq 3 -based OLEDs with conventional geometry. Electrical analysis reveals that LiOH-doped Alq 3 layers have an enhanced electron injection and transport ability. X-ray and ultraviolet photoelectron spectroscopy results clearly show that the dipole layer formed at the indium tin oxide (ITO)/organic interface contributed to the reduction of the ITO work function, resulting in a decrease of the electron injection barrier. The enhanced electron injection and transport efficiency improves the charge carrier balance in IBOLEDs and leads to better device efficiency. Furthermore, improved morphology of the organic layer can be obtained by doping LiOH into Alq 3 , which improves device operational stability under thermal stress. [ABSTRACT FROM AUTHOR]

Abstract: Na2CO3 is used as an n-type dopant to improve the electro-optical properties of tris(8-hydroxyquinoline)aluminum (Alq3)-based organic light-emitting diodes (OLEDs). Device properties such as the turn-on voltage, maximum luminance, and device efficiency were improved as Na2CO3 was doped. The higher mobility of Na2CO3-doped samples has been derived using space-charge-limited current measurements. Photoelectron spectroscopy results show that some electrons transfer from Na2CO3 into Alq3, which moves the Fermi level close to the lowest unoccupied molecular orbital of Alq3. Therefore, both the electron-transporting ability and electron-injection efficiency were enhanced, which improves the charge carrier balance in OLEDs and leads to better device efficiency. [Copyright &y& Elsevier]

Abstract: The effects of buffer layers, including LiF, LiCl, NaF, NaCl, NaI, KI, RbF, RbCl, CsF, CsCl, MgF2, CaF2, BaF2, and BaCl2 on electron injection and device performance in organic light-emitting diodes based on tris-(8-hydroxyquinoline) aluminum, were investigated systematically. The insertion of the buffer layers at the organic/cathode interface not only reduced the operating voltage, but also enhanced the luminance and efficiency, which is attributed to the improvement of electron injection efficiency. It was found that the efficiency of the electron injection was closely related to the inherent properties of the buffer layer, such as its melting point (MP) and dielectric constant (ε), as well as with the buffer layer''s interface with the metallic electrode through the effective work function (WF). Low MP, low ε and low WF values result in an effective improvement in the injection of the electrons, and thus to the device performance. The electroluminescent performance was further improved by the introduction of calcium between the buffer layer and the aluminum electrode. [Copyright &y& Elsevier]

Abstract: Charge-transfer process from photoexcited gold nanoparticles to TiO2 is significant due to the very strong surface plasmon absorption band of gold nanoparticles and the very high photocatalytic activity of TiO2. Bioapplication of gold–TiO2 systems utilizing near-infrared (NIR) light based on the two photon excitation of the surface plasmon band will be interesting. In this study, TiO2 particles with a 30nm average diameter loaded with 10nm gold spherical nanoparticles were used to explore the electron injection kinetics by observing electrons in TiO2 using femtosecond IR-probe transient absorption (TA) spectroscopy. Optical excitation of the gold–TiO2 system using ~150fs light at 1200nm led to very fast (<240fs) electron generation in the conduction band of TiO2. Excitation intensity dependence of the initial TA signals gave a second order nonlinear relation, confirming the occurrence of two photon induced electron injection. Two photon absorption cross section of gold nanoparticle at 1200nm was estimated to be very high, around 108 GM (1GM=10−50 cm4 s photon−1 molecule−1). [Copyright &y& Elsevier]

Abstract: Extraction of electrons and holes photogenerated in CdSe quantum dots (QD) of 2.3 nm diameter, is monitored by Surface Photovoltage Spectroscopy. The extraction of electrons into a thin TiO2 layer increases five-fold by absorption of N3 dye molecules on top of the QD layer. This process is facilitated by efficient hole extraction from the valence band of the QDs to the ground state of the N3 dye. Our results represent a direct measurement of charge separation in the N3/QD/TiO2 system. [Copyright &y& Elsevier]

Abstract: The pentacene field effect transistors (FETs)'' operation for the injection carrier was revealed by means of the drain current–elapsed time (I ds–t) and optical second harmonic generation (SHG) measurements. The charge carriers forming the conducting channel of pentacene FETs were mainly holes injected from the Au source electrode. Carrier injection from source and drain electrodes was followed by the carrier trapping, and the SHG signal modulated by the change in the electric field distribution between Au the source and drain electrodes was shown. In particular, at the off state of the FET, electron injection and succeeding trapping were suggested. Furthermore, hole injection assisted by trapped electrons was also suggested. [Copyright &y& Elsevier]

Abstract: Polymer light-emitting diodes (PLEDs) were prepared using an environment-friendly alcohol-soluble quaternized aminoalkyl-substituted conjugated polyfluorene co-polymerizing 2,1,3-benzothiadiazole monomer (PFNBr-BTDZ05) as an emitter, a high work-function Au metal as a cathode. The device structure used here is indium–tin–oxide (ITO)/poly(N-vinylcarbazole) (PVK)/PFNBr-BTDZ05/Au, where PVK is an anode buffer layer acting as the hole injection. The maximal external quantum and luminance efficiencies of 0.73% and 0.86 cd/A are obtained, and the maximal luminance is nearly 700 cd/m2 at 13 V, which was the highest brightness value reported in PLEDs with the Au metal cathode. This conjugated polyelectrolyte has such excellent electron injection ability that the device with the structure of ITO/poly(ethylene dioxythiophene)/poly(2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylenevinylene)/PFNBr-BTDZ05/Al exhibits a high external quantum efficiency of 7.0%. The electron-injecting mechanism of PFNBr-BTDZ05 was discussed. [Copyright &y& Elsevier]