F.L. Wong

Aluminum-doped zinc oxide films as transparent conductive electrode for organic light-emitting devices

Highly transparent conductive, aluminum-doped zinc oxide (ZnO:Al) films were deposited on glass substrates by midfrequency magnetron sputtering of metallic aluminum-doped zinc target. ZnO:Al films with surface work functions between 3.7 and 4.4 eV were obtained by varying the sputtering conditions. Organic light-emitting diodes (OLEDs) were fabricated on these ZnO:Al films. A current efficiency of higher than 3.7 cd/A, was achieved. For comparison, 3.9 cd/A was achieved by the reference OLEDs fabricated on commercial indium–tin–oxide substrates.

Efficient organic photovoltaic devices using a combination of exciton blocking layer and anodic buffer layer

By using bathophenanthroline (BPhen) as an exciton blocking layer (EBL) at the organic/cathode contact of a standard copper phthalocyanine/C60 organic photovoltaic (OPV) device, power conversion efficiency was substantially increased from 0.86% to 2.64%. The BPhen-based devices showed a 45% increase in power conversion efficiency over that of an equivalent device with an EBL of bathocuproine. The performance improvement was analyzed in terms of the electron energy levels, optical transparencies and electron mobilities of the two EBLs. Based on these results, the roles of and requirements for an effective EBL were discussed. Combining the use of BPhen and a WO3 anodic buffer layer further increased the power conversion efficiency of the OPV device to 3.33%.

Effective organic-based connection unit for stacked organic light-emitting devices

A bilayer connection unit of Mg-doped Alq3 and F4-TCNQ-doped m-MTDATA was investigated for application in stacked organic light-emitting device. This connection unit led to a stacked OLED with a luminous efficiency twice that of a single-unit OLED. Electronic structures, including relevant electron energy levels, of the various interfaces in the stacked OLED were studied by using ultraviolet photoemission spectroscopy and used to discuss the working mechanisms of the stacked OLED. The p-type dopant F4-TCNQ was shown to induce a large band bending of 1.36eV and facilitates efficient carrier injection from the connection units into the carrier-transporting layers.

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...

Efficiency enhancement and retarded dark-spots growth of organic light-emitting devices by high-temperature processing

Abstract Device stability and growth of dark spots are major concerns for the large-scale applications of organic light-emitting devices (OLEDs). OLEDs with crystalline hole-transporting layer have been fabricated by depositing organic layers at elevated substrate temperatures. Such devices showed significant improvement in electroluminescent efficiency, morphological stability, storage stability, and also retarded dark-spot growth. The lower hole mobility in crystalline NPB films is attributed to the improved device performance, leading to a better carrier balancing in the NPB/AlQ 3 interface. Such crystalline NPB films also demonstrate a smoother surface, even after high-temperature annealing, and gives beneficial advantages for improving the thermal durability of OLEDs.

Long-lifetime thin-film encapsulated organic light-emitting diodes

Multiple fluorocarbon (CFx) and silicon nitride (Si3N4) bilayers were applied as encapsulation cap on glass-based organic light-emitting diodes (OLEDs). When CFx/Si3N4 bilayers were deposited onto the OLED structure, the devices showed performance worse than one without any encapsulation. The adverse effects were attributed to the damage caused by reaction species during the thin-film deposition processes. To solve this problem, a CuPc interlayer was found to provide effective protection to the OLED structure. With a structure of CuPc/(CFx/Si3N4)×5, the encapsulated device showed an operation lifetime over 8000 h (higher than 80% of that achieved with a conventional metal encapsulation).

Operation stability enhancement in organic photovoltaic device by a metal doped organic exciton blocking layer

While metal diffusion in organic layers have been considered as causes for performance degradation in organic light-emitting devices, we show that suitable metal doping can instead improve physical stability of organic films. By using a metal doped organic exciton blocking layer (EBL), enhanced stability is demonstrated in unpackaged CuPc/C60 organic photovoltaic devices (OPV). While devices with a pure organic EBL of bathocuproine and tris(8-hydroxyquinolinato)aluminum (Alq3) show over ∼20% decreases in efficiency for first 150 min of operation, the device with magnesium-doped Alq3 EBL shows less than ∼5% variation in efficiency during the same period.

Substrate effects on the interface electronic properties of organic photovoltaic devices with an inverted C60/CuPc junction

Deposition sequence and substrate work function in controlling the interface energy level alignment in organic photovoltaic (OPV) devices with copper phthalocyanine (CuPc) as the donor and fullerene (C60) as the acceptor were studied using ultraviolet photoelectron spectroscopy. We found that the energy offset at the highest occupied molecular orbital of donor (HOMOD) and the lowest unoccupied orbital of acceptor (LUMOA), which limits the maximum open-circuit voltage of heterojunction OPV, can be changed from 0.64 (C60 on CuPc) to 0.86 eV (CuPc on C60) by reversing the deposition sequence. Furthermore, by controlling the substrate work function from 2.81 to 5.07 eV, the LUMOA-HOMOD offset can be effectively tuned from 0.86 to 1.27 eV. The results suggest that electrodes in OPV devices can have significant influences on the electronic structures and energy levels of the donor/acceptor interface, and thus provide a viable means for performance enhancement.

A new pyrene cored small organic molecule with a flexible alkyl spacer: a potential solution processable blue emitter with bright photoluminescence

A new pyrene cored small organic molecule viz. 1,3,6,8-tetrakis(4-((5-(9H-carbazol-9-yl)pentyl)oxy)phenyl)pyrene (PY-II) was designed and synthesized. The carbazole moiety with an alkyl spacer was introduced at 1, 3, 6 and 8 positions of the pyrene core to improve the charge transport properties and solution processability. PY-II exhibited excellent solubility in common organic solvents and high thermal stability up to 345 °C. The photoluminescence quantum yield (PLQY) of PY-II in solution was found to be 0.9 with bright blue emission near 450 nm which is just appropriate for the human eye. The solution processed non-doped OLED device fabricated using PY-II as an emissive layer afforded a pure blue emission with CIE coordinates of 0.16 and 0.16, a power efficiency of 0.17 lm W−1, a maximum current efficiency of 0.41 cd A−1 and a maximum brightness of 202 cd m−2.

New iridium derivatives with good electrophosphorescence properties

In this study, we synthesize a new iridium complex by introducing sterically bulky spacers into the framework of fac tris(2-phenylpyridine) iridium [Ir(ppy)3]. The main purpose is to reduce concentration quenching in Ir(ppy)3. The new complex exhibits a high (0.71) photoluminescence (PL) quantum yield in solution. The devices fabricated with the new Ir complex as an emitting dopant confirm that concentration quenching is almost negligible even at relatively high doping concentrations. For example, at a current density of 100 mA/cm2, the current efficiency for the devices with 7 and 26 wt% dopants are 8.9 and 10.2 cd/A respectively. These characteristics can be explained by a better energy transfer between the host and dopants upon introducing the sterically hindered spacers into the phosphorescent dyes.