S. W. Tong

A bis-salicylaldiminato Schiff base and its zinc complex as new highly fluorescent red dopants for high performance organic electroluminescence devices

Schiff base 2,3-bis[(4-diethylamino-2-hydroxybenzylidene)amino]but-2-enedinitrile (BDPMB) and its zinc complex (BDPMB-Zn) with donor–acceptor–donor (D–A–D) type ICT properties in the neutral form were used as novel red-emitting dopants in OLEDs; bright saturated red-emitting EL devices with excellent colour chromaticity coordinates (x, y = 0.670, 0.325 for BDPMB; x, y = 0.655, 0.325 for BDPMB-Zn) and good efficiency (1.35 cd A−1 for BDPMB; 0.50 cd A−1 for BDPMB-Zn) were obtained.

Efficient CsF/Yb/Ag cathodes for organic light-emitting devices

A high-performance cathode consisting of an ultrathin CsF layer and a rare-earth ytterbium (Yb) metal is reported for application in organic electroluminescent devices. Standard tris-(8-hydroxyquinoline) aluminum/α-napthylphenylbiphenyl diamine devices with this bilayer cathode showed dramatically reduced operating voltage and a low turn-on voltage of 2.42 V as compared to 3.75 and 2.95 V in devices using, respectively, the Mg:Ag and single-layer Yb cathodes. At a current density of 200 mA/cm2, devices with the CsF/Yb cathode exhibited high luminance efficiency of 3.45 cd/A and power efficiency of 1.27 lm/W. Analysis by x-ray photoemission spectroscopy suggested that the performance improvement is related to the substantial reduction of electron injection barrier at the cathode/organic interface. It was found that upon Yb deposition, CsF dissociates to liberate low work function Cs metal atoms resulting in a cathode with a lower electron injection barrier and thus a better balance of carriers in the devic...

Improved performance and stability of organic light-emitting devices with silicon oxy-nitride buffer layer

The use of silicon oxy-nitride (SiOxNy) as an anode buffer layer in organic light-emitting devices (OLEDs) with a configuration of indium tin oxide (ITO)/SiOxNy/α-naphtylphenyliphenyl diamine (NPB)/8-hydroxyquinoline aluminum/Mg:Ag has been studied. With a SiOxNy buffer layer several angstroms thick, the device efficiency increased from 3.0 to 3.8 cd/A. The buffer layer also protected the ITO surface from contamination due to air exposure. Upon exposing the cleaned ITO substrate to air for one day before device fabrication, the device current efficiency and turn-on voltage degraded to 2.1 cd/A and 4.3 V, respectively, from 3 cd/A and 3.3 V for the device fabricated on an as-cleaned ITO surface. In contrast, devices prepared on air-exposed SiOxNy/ITO surface had almost the same current efficiency (3.85 cd/A) and turn on voltage (3.7 V) comparing to devices (3.8 cd/A and 3.7 V) fabricated on freshly prepared SiOxNy/ITO surface. The results suggested that SiOxNy is a promising anode buffer layer for OLEDs, f...

Anode modification of polyfluorene-based polymer light-emitting devices

A glycerol-modified poly(3,4-ethylene dioxythiophene) (PEDOT): poly(styrene sulfonate) (PSS) layer was used as an anode buffer layer in polymer light-emitting devices using poly(9,9-dioctylfluorene) (F8) as the emitter. Devices with a configuration of indium tin oxide/PEDOT:PSS (with or without glycerol)/F8/CsF/Al were fabricated. It was found that the glycerol-modified device showed a much larger current density than the unmodified device. At an operating voltage of 6 V, the glycerol-modified device showed a luminance of 1300 Cd/m2 and a current efficiency of 1.7 Cd/A compared to the corresponding values of 500 Cd/m2 and 1.3 Cd/A in the unmodified device. Analysis by ultraviolet spectroscopy suggests that the two devices have the same energy level structure and the performance improvement should not be due to change in the PEDOT/polymer interface. It was further found that incorporating a suitable amount of glycerol into the PEDOT:PSS layer can increase its conductivity by six times. This leads to a bett...

Experimental study of a chemical reaction between LiF and Al

The interaction between LiF and Al has been studied by x-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). At room temperature, reaction between LiF and Al already occurred in the presence of Alq3, as revealed by the 0.25 eV shift of the Li 1s peak relative to the F 1s peak. Upon heating the LiF-coated Al substrate above 70 °C, reaction between LiF and Al also occurred in the absence of Alq3, suggested by the emergence of a peak 0.85 eV below the original Li 1s peak. The relatively large shift of 0.85 eV indicated that the emergent Li state possessed a metallic character. The reaction between LiF and Al upon heating was also corroborated by HREELS measurements. Heating of the Al/LiF/polymer device enhanced its electroluminescence, and may be associated with free Li atoms.

Distinct interfaces of poly (9,9-dioctylfluorene-co-benzothiadiazole) with cesium and calcium as observed by photoemission spectroscopy

We report that the green-emitting polymer—poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), a copolymer of poly(9,9-dioctylfluorene) (F8), formed two distinct interfaces with calcium and cesium, respectively, as observed in photoemission measurements. Ultraviolet photoemission spectroscopy showed that Ca formed a stable interface with F8BT without significantly changing the electronic structure of F8BT, which is in contrast to the Cs/F8BT interface where bipolaron states occurred in the forbidden gap as a result of charge transfer processes. X-ray photoemission spectroscopy revealed that Ca covalently bonded with the sulfur atoms whereas Cs preferably interacted with the nitrogen in the F8BT. The results are useful to account for the undesirable device performance using CsF/Al as a cathode in the F8BT-based polymer light-emitting device. The exposure of Cs/F8BT interface to residual gases at a pressure of 2.0×10−9 mbar for 2 and 12 h, respectively, slightly and largely eliminated the gap states. Depos...

Role of ultrathin Alq3 and LiF layers in conjugated polymer light-emitting diodes

We report that an ultrathin layer of tris-(8-hydroxyquinoline) aluminum (Alq3) interposed between a bilayer cathode of LiF/Al and an emissive layer can significantly improve electroluminescence efficiency in a polymer light-emitting device (PLED). At a current density of 35 mA/cm2, a poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenyl-enevinylene] (MEH-PPV)-based PLED with a cathode of Alq3/LiF/Al has a quantum efficiency of 2.5%, considerably higher than that of a MEH-PPV-based PLED with a bare Al or a LiF/Al cathode. Interfacial characteristics of MEH-PPV/LiF/Al and MEH-PPV/Alq3/LiF/Al were investigated by photoelectron spectroscopy, and the results were used to explain the difference in device performance.We report that an ultrathin layer of tris-(8-hydroxyquinoline) aluminum (Alq3) interposed between a bilayer cathode of LiF/Al and an emissive layer can significantly improve electroluminescence efficiency in a polymer light-emitting device (PLED). At a current density of 35 mA/cm2, a poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenyl-enevinylene] (MEH-PPV)-based PLED with a cathode of Alq3/LiF/Al has a quantum efficiency of 2.5%, considerably higher than that of a MEH-PPV-based PLED with a bare Al or a LiF/Al cathode. Interfacial characteristics of MEH-PPV/LiF/Al and MEH-PPV/Alq3/LiF/Al were investigated by photoelectron spectroscopy, and the results were used to explain the difference in device performance.

Conducting fluorocarbon coatings for organic light-emitting diodes

Conducting fluorocarbon coatings (CFx) have recently been used as an anode buffer layer in organic light-emitting diode (OLEDs) for enhancement of stability and carrier injection. The effect of CFx on OLEDs was related to its conductivity. We found the resistivity of poorly conducting (about 1010 Ω cm) CFx coatings can be substantially decreased to 105 Ω cm by either near UV or Ar ion irradiation. OLEDs were prepared on untreated and treated CFx for performance comparison. The UV treatment of CFx improved the device performance, while the Ar ion treatment led to deterioration. X-ray photoelectron spectroscopy analysis showed the UV treatment created graphitic regions leading to the higher conductivity of the CFx layer but the underlying indium tin oxide (ITO) anode remained intact. In contrast, argon ion bombardment caused damage to the ITO anode which contributed to poor device performance, although the CFx conductivity was similarly increased.

Interfaces between 8-hydroxyquinoline aluminum and cesium as affected by their deposition sequences

Abstract The electronic structures of tris(8-hydroxyquinoline) aluminum (Alq3) deposited on clean or Cs pre-covered Ag substrates have been studied by ultraviolet photoelectron spectroscopy. Interface of Cs deposited on Alq3 has also been prepared for comparison. For a low coverage of Cs on Ag, deposition of Alq3 on top of the Cs cannot induce any new electronic features. The low work function of the Cs reduces the barrier height of electron injection at the Alq3/Cs/Ag contact to 0.3 eV, as compared to 1.6 eV for the Alq3/Ag contact. For high Cs coverage, the Cs may diffuse as neutral atoms and undergo oxidation into the Alq3 layer and form a new gap state at 0.9 eV above the Alq3 highest occupied state, which is the same as that of Cs deposited on the Alq3.

Role of ytterbium and ytterbium/cesium fluoride on the chemistry of poly(9,9-dioctylfluorene-co-benzothiadiazole) as investigated by photoemission spectroscopy

Since ytterbium (Yb) possesses a low work function of 2.6 eV and Yb fluoride generally has a high negative free energy of formation, it is conceivable to use Yb, either directly or parasitically, with a metal fluoride, as a cathode in organic light-emitting diodes (OLEDs). In this work, the electronic structure and chemistry at the interface of Yb/poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) were investigated by ultraviolet and x-ray photoemission spectroscopy (UPS and XPS, respectively). We found that the deposition of Yb on top of F8BT foremost formed organometallic complexes with the sulfur but interacted partly with the nitrogen of F8BT, and eventually formed a Yb–C complex at higher Yb coverages. In the UPS spectra, Yb deposition increased the relative peak intensities corresponding to the σ-bonds originated from the aliphatic side chains, implying that some of the π-conjugated carbons in the polymer backbone may be destroyed. These results agree well with the disappearance of the π-to-π* tra...