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We studied systematically the influence of in situ postannealing treatment of ultrahigh vacuum grown polycrystalline pentacene thin film transistor. The gradual grain growth with the elimination of defects and misoriented crystallites is confirmed in x-ray diffraction (XRD) data and the atomic force microscopy image as the annealing temperature increases. The XRD data reveal that the pentacene molecules are packed parallel to each other in an upright position with a tilting angle of 15.5°. The postannealing results in the enhanced field effect mobility of pentacene organic thin film transistors increases from 0.19±0.04 to 0.49±0.05 cm2/V s after annealing at 90 °C. We suggest that the abnormally small on/off current ratio (∼103) due to the large leakage current is attributed to the conduction via impurity levels originated from the structural isomers of pentacene.

We report on the fabrication of organic light-emitting devices (OLEDs) using a thin NaCl interlayer as an electron-injection medium. The results show that the device containing the NaCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. We also fabricated similar-structured comparable devices, which were prepared with a LiF layer as a different electron-injection medium. The maximum electroluminescent efficiency of the NaCl (1 nm)/Al cathode device was 2.85 cd/A, which is higher than the 2.25 cd/A of the LiF (1 nm)/Al cathode device. The ultrathin NaCl layer modified the carrier injection properties. In conclusion, the NaCl layer between a cathode and an emitting layer of OLEDs can be used as the carrier injection layer to improve the EL properties.

The photocurrent of germanium-doped indium-gallium oxide (GIGO) thin film transistors (TFTs) can be observed when the device is exposed to a ultra-violet light because GIGO is a wide band gap semiconducting material. Therefore, we decorated cadmium selenide (CdSe) quantum-dots (QDs) on the surface of GIGO to increase the photocurrent for low-energy light, i.e., visible light. A 10 nm GIGO film was deposited on the SiO2/Si substrate by a radio frequency sputter system. Also, we prepared CdSe QDs with sizes of ∼6.3 nm, which can absorb red visible light. QDs were spin-coated onto the GIGO film, and post-annealing was done to provide cross-linking between QDs. The prepared devices showed a 231% increase in photocurrent when exposed to 650 nm light due to the QDs on the GIGO surface. Measurements to construct an energy level diagram were made using ultraviolet photoelectron spectroscopy to determine the origin of the photocurrent, and we found that the small band gap of CdSe QDs enables the increase in photoc...

We report on transparent Ti-In-Sn-O (TITO) multicomponent anodes prepared by co-sputtering anatase TiO2−x and ITO targets to produce highly efficient phosphorescent organic light emitting diodes (OLEDs). In spite of the incorporation of low cost TiO2, the crystalline TITO electrode annealed at temperature of 600 °C showed a sheet resistance of 18.06 Ω/sq, an optical transmittance of 87.96% at a wavelength of 550 nm, and a work function of 4.71 eV comparable to conventional ITO electrode. Both the quantum (21.69%) and power efficiencies (90.92 lm/W) of the phosphorescent OLED fabricated on the TITO anode were higher than those of the OLED with the reference ITO anode due to the high transparency of the TITO electrodes. This indicates that the TITO electrode is a promising indium-saving electrode that can replace high-cost ITO electrodes in the manufacture of low-cost, highly efficient phosphorescent OLEDs.

C60 has been used as an electron accepting and transporting material in various organic electronic devices. In such devices, Al and Ag have been adopted as a common cathode in combination with electron injection layers (EIL), e.g., LiF. We found that the initial interface formations of C60/LiF/Al and C60/LiF/Ag are quite different in terms of interfacial electronic structures. We measured the interfacial electronic structures with photoemission spectroscopy and found that LiF works well as an EIL on Al but performs poorly on Ag. The origin of this difference could be attributed to the larger interface dipole on Al, highlighting the importance of the choice of cathode materials.

A simple method for controlling the effective work function (WF) of conductive multilayer graphene (MLG) film, synthesized by using chemical vapor deposition and transferred to a dielectric substrate, was developed. The WFs of the MLG during the step-by-step deposition of aluminum (Al) were measured using in situ ultraviolet photoelectron spectroscopy. Core-level spectra were also collected to investigate the chemical reaction that occurred when a small amount of Al was deposited onto MLG in a stepwise manner. The measurements revealed that the effective WF of the conductive MLG film could be controlled from 3.77 to 4.40 eV by the deposition of an Al layer less than 0.6 nm thick.

The electronic structures of tris(8-hydroquinoline) aluminum (Alq3)∕8-hydroquinolatolithium (Liq)/Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopy. We constructed complete energy level diagrams and analyzed chemical interactions at the interface. When Liq was inserted between Al and Alq3, the electron injection barrier was reduced by 0.56eV compared to the structure without Liq. Additionally, a gap state was observed in the gap of Liq, which is related to an interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Liq between them.

The electronic structure of tris(8-hydroquinoline) aluminum (Alq3)/rubrene/poly(ethylene glycol) dimethyl ether (PEGDE)/Al interfaces was studied using in situ ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. When a PEGDE/rubrene double layer was inserted between Al and Alq3, the electron injection barrier height was greatly reduced compared to the interface without PEGDE/rubrene or with a single insertion layer of either PEGDE or rubrene.

The electronic structures of tris-(8-hydroquinoline) aluminum (Alq3)∕Li2O∕Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2O between Al and Alq3 led to the highest occupied molecular orbital (HOMO) of Alq3 shifting to a higher binding energy compared to that without Li2O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Alq3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by...

The origin of the lowered electron injection barrier height of C60∕Cu was investigated by in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy (XPS). The onset of the highest occupied molecular orbital level was shifted by 0.2eV toward high binding energy upon the heat treatment, resulting in the improved injection characteristics of the device. Moreover, an unexpected gap state has been observed at 1.2eV below the Fermi level. The XPS core-level spectra revealed that the chemical reaction between C60 and Cu at the interface induced the gap state after heat treatment. The gap state pinned the Fermi level close to the lowest unoccupied molecular orbital of C60. We obtained the complete energy level diagram of C60∕Cu before and after the heat treatment.

The luminance-voltage characteristics and stability were highly improved by replacing LiF with CsCl in organic light-emitting devices. To investigate the origin of these improvements, ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy were used. The additional shifts of the vacuum, highest occupied molecular orbital, and lowest unoccupied molecular orbital levels due to the CsCl layer reduce the width and height of the electron injection barrier, resulting in the improved luminance-voltage characteristics of the devices. Additionally, the intervening CsCl layer between Al and Alq3 prevents N–Al reactions among Alq3 molecules and Al, which reduces distortion or breakdown of Alq3 molecules and slows the degradation of the device.

Electronic structures of Alq3∕NaCl∕Al and Alq3∕Al were studied by UV and x-ray photoelectron spectroscopy (XPS). The initial energy level of the highest occupied molecular orbital (HOMO) of Alq3∕Al was shifted when the ultrathin NaCl layer was inserted between them, although the vacuum level was not changed. The measured interface dipole was 1.1 eV, identical for both Alq3∕NaCl∕Al and Alq3∕Al. Our experiment shows that the dipole is formed in very short range (less than 0.1 nm) from the interface. The onset of the HOMO level of Alq3 was shifted 0.2 eV toward high binding energy for the additional NaCl layer, which lowered the barrier height and improved injection characteristics of the device. Moreover, a gap state was observed at 1.1 eV below the Fermi level when the NaCl was inserted between Alq3 and Al. The XPS core-level spectra revealed that the interaction on Alq3 and NaCl was very weak, which generated an unusual gap state without breaking or forming chemical bonds. We suggest that the weak interac...