Ji Whan Kim

Low driving voltage and high stability organic light-emitting diodes with rhenium oxide-doped hole transporting layer

The authors report a promising metal oxide-doped hole transporting layer (HTL) of rhenium oxide (ReO3)-doped N,N′-diphenyl-N,N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB). The tris(8-hydroxyquinoline) aluminum-based organic light-emitting diodes with ReO3-doped NPB HTL exhibit driving voltage of 5.2–5.4V and power efficiency of 2.2–2.3lm∕W at 20mA∕cm2, which is significantly improved compared to those (7.1V and 2.0lm∕W, respectively) obtained from the devices with undoped NPB. Furthermore, the device with ReO3-doped NPB layer reveals the prolonged lifetime than that with undoped NPB. Details of ReO3 doping effects are described based on the UV-Vis absorption spectra and characteristics of hole-only devices.

Silane- and triazine-containing hole and exciton blocking material for high-efficiency phosphorescent organic light emitting diodes

One of the important factors for high efficiency phosphorescent organic light-emitting devices is to confine triplet excitons within the emitting layer. We synthesized and characterized a new hole blocking material containing silane and triazine moieties, 2,4-diphenyl-6-(4′-triphenylsilanyl-biphenyl-4-yl)-1,3,5-triazine (DTBT). Electrophosphorescent devices fabricated using the material as the hole-blocking layer and N,N′-dicarbazolyl-4,4′-biphenyl (CBP) doped with fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] as the emitting layer showed a maximum external quantum efficiency (ηext) of 17.5% with a maximum power efficiency (ηp) of 47.8 lm W−1, which are much higher than those of devices using bathcuproine (BCP) (ηext = 14.5%, ηp = 40.0 lm W−1) and 4-biphenyloxolate aluminium(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq) (ηext = 8.1%, ηp = 14.2 lm W−1) as hole-blocking layers.

Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells

Enhancing the short circuit current (JSC) by extending the absorption to the near-infrared (NIR) spectrum, which has 50% of the total solar photon flux, is a remaining task in small molecular solar cells. Here, high efficiency NIR absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is explained by the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an X-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.

Highly efficient orange organic light-emitting diodes using a novel iridium complex with imide group-containing ligands

We report the synthesis and device characterization of a novel iridium complex using a phenylpyridine derivative containing an imide functional group as a bis-cyclometallized ligand ((impy)2Ir(acac)) for orange organic light-emitting diodes (OLEDs). The photoluminescence of the dopant emits orange light with the main peak of 560 nm and a shoulder of 595 nm. The OLEDs made with the synthesized dopant also produce intense orange electroluminescence at 568–572 nm wavelengths depending on the device geometry. Very high peak external quantum efficiencies of 13.8–14.4%, peak power efficiencies of 29.2–32.7 lm/W, and low operation voltages of 5.3–5.4 V at 1000 cd/m2 have been demonstrated for the orange OLEDs.

Self-assembled perpendicular growth of organic nanoneedles via simple vapor-phase deposition: one-step fabrication of a superhydrophobic surface

We demonstrate a simple vapor-phase fabrication of self-assembled perpendicular organic nanoneedles on various substrates to generate superhydrophobic surfaces on them.

CuI interlayers in lead phthalocyanine thin films enhance near-infrared light absorption

The insertion of a CuI interlayer improved the preferential alignment of lead phthalocyanine (PbPc) molecules in the monoclinic phase, as demonstrated by x-ray diffraction analysis of ultra-thin (5 nm) and thin (30 nm) PbPc films. CuI triclinic phases were not observed. The improved preferential alignment and crystallinity in the monoclinic phase enhanced near-infrared photon absorption. The power conversion efficiency of a PbPc/C60 planar heterojunction organic photovoltaic device fabricated using the CuI templating layer structure could be improved from 1.3% to 2.5%.

Rapid Patterning of Single‐Wall Carbon Nanotubes by Interlayer Lithography

High resolution patterning of single-wall carbon nanotubes is achieved using interlayer lithography. Minimum feature sizes of 6 and 0.7 μm are demonstrated on glass and silicon, using the negative photoresist SU-8 and the positive photoresist AZ7220, respectively. Films patterned using SU-8 have a good balance of transparency and sheet-resistance, and are shown to be effective electrodes for organic solar cells. Copyright

An efficient interconnection unit composed of electron-transporting layer/metal/p-doped hole-transporting layer for tandem organic photovoltaics

We report an efficient interconnection unit (ICU) consisting of an electron transporting layer/metal/p-doped hole transporting layer (p-HTL) structure for tandem organic photovoltaic (TOPV) cells. The ICU satisfies all the requirements of optical transparency and low voltage loss and for functioning as an optical spacer. The variation of the short circuit current and open circuit voltage (VOC) of the TOPV cells with increasing thickness of the p-HTL in the ICU followed the theoretical predictions, proving that the ICU does not disturb the electrical characteristics of the TOPV cells up to a p-HTL thickness of 100 nm with minimal VOC loss (∼3%).

High efficiency and high photo-stability zinc-phthalocyanine based planar heterojunction solar cells with a double interfacial layer

The use of CuI and MoO3 as a double interfacial layer between indium tin oxide (ITO) and a zinc phthalocyanine (ZnPc) layer improves the power conversion efficiency (ηp) and the photo-stability at the same time in ZnPc based solar cells. Insertion of CuI without MoO3 increased ηp more than 2 times to 3.3%. However, the photo-stability is lowered even further due to diffusion of Cu. Insertion of the MoO3 layer between the ITO and CuI prevents the diffusion of Cu under UV illumination to achieve the improved photo-stability and ηp.

Efficient and colour-stable hybrid white organic light-emitting diodes utilizing electron―hole balanced spacers

High-efficiency two-colour white organic light-emitting diodes (WOLEDs) comprising a newly synthesized iridium complex orange phosphor ((impy)2Ir(acac)) and a blue fluorophor (BD012) have been realized by placing several kinds of thin spacers between two emitters. Hybrid WOLEDs with a spacer composed of a hole-transporting N,N-dicarbazolyl-3,5-benzene (mCP) and an electron-transporting 4,7-diphenyl-1,10-phenanthroline (Bphen) exhibit a high external quantum efficiency (EQE) of up to 8.4% and a negligible colour change (the colour coordinate of (0.39, 0.41) at 1000 cd m−2) with increasing brightness, whereas the device using a hole-transporting mCP spacer shows a relatively low EQE of 6.2% and a large shift of emitting colour with increasing brightness. Device performance is further characterized based on the charge transport behaviour of the spacers inserted between the two emitters.