Seunguk Noh

Carrier conduction mechanism for phosphorescent material doped organic semiconductor

The mobility of charge carriers has been investigated in the pristine and phosphorescent material doped 4,4′,4″-tris(N-carbazolyl) triphenylamine (TCTA) using time-of-flight photoconductivity technique. Doping phosphorescent material fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] increases the electron mobility whereas the hole mobility decreases to the order of 10−4–10−6 cm2/V s measured at room temperature with different bias voltages. The analysis of field and temperature dependences of the mobility agrees well with the Gaussian disorder model. The calculated positional disorders (Σ) for TCTA, Ir(ppy)3-doped TCTA, and tris(1-phenylisoquinoline) iridium [Ir(piq)3]-doped TCTA are 0.12, 2.05, and 1.62 for hole, respectively; 3.89 for electron in only Ir(ppy)3-doped TCTA. The ambipolar transport for holes and electrons is possible by doping TCTA with Ir(ppy)3 (green dopant) whereas only hole transport with reduced mobility is achieved for Ir(piq)3 (red dopant).

Graphene-nanowire hybrid structures for high-performance photoconductive devices†

Graphene–CdS nanowire (NW) hybrid structures with high-speed photoconductivity were developed. The hybrid structure was comprised of CdS NWs which were selectively grown in specific regions on a single-layer graphene sheet. The photoconductive channels based on graphene–CdS NW hybrid structures exhibited much larger photocurrents than graphene-based channels and much faster recovery speed than CdS NW network-based ones. Our graphene–CdS NW structures can be useful because they were much faster than commercial CdS film-based photodetectors and had photocurrents large enough for practical applications.

The energy level spacing from InAs/GAas quantum dots : Its relation to the emission wavelength, carrier lifetime, and zero dimensionality

We have found that the level spacing between the ground and first excited states of InAs∕GaAs quantum dots (QDs) increases as the photoluminescence peak energy decreases, that is, as the QD increases in size. By means of simple numerical calculations, we confirm that this seemingly unusual level-spacing behavior originates from the low aspect ratio of typical QDs with a finite potential barrier. Carrier lifetime measurements show that QDs with a lower photoluminescence peak energy tend to have a shorter decay time, which can be attributed to better confinement of the electron wave function and the resultant increase in electron-hole wave function overlap.

Pyrene end-capped oligothiophene derivatives for organic thin-film transistors and organic solar cells

We have demonstrated an efficient one-step synthesis of pyrene end-capped thiophene oligomers as active materials for organic thin-film transistors (OTFTs) and donor materials for organic solar cells (OSCs). In particular, the OTFT device based on 5,5′′-di(pyren-1-yl)-2,2′:5′,2′′-terthiophene (3s) exhibited a reasonable field effect mobility of 0.11 cm2 V−1 s−1, and a power conversion efficiency of 1.20% was achieved when 3s was used as active materials in OSCs. These pyrene end-capped thiophene oligomers are considered to be good organic semiconducting materials for OTFTs and OSCs.

Solution processable donor materials based on thiophene and triphenylamine for bulk heterojunction solar cells

Solution processable and thermally stable donor materials 2,5-bis[4-(N,N-diphenylamino)styryl] thiophene (1) and 2,5-bis[4-(N,N-diphenylamino)styryl]-2,2′-bithiophene (2) were synthesized for bulk heterojunction solar cells, which consist of a thiophene, or bithiophene and triphenylamine unit linked with conjugated bonds. The best device performance, optimized at 1:PCBM = 1:3 and 2:PCBM = 1:4, exhibited the maximum open circuit voltages of 0.56 and 0.51 V, short circuit currents of 1.33 and 1.88 mA cm−2, and power conversion efficiencies of 0.23 and 0.34%, respectively, under simulated AM 1.5 solar irradiation at 100 mW cm−2.

Surface Coatings Based on Polysilsesquioxanes: Solution-Processible Smooth Hole-Injection Layers for Optoelectronic Applications

Optoelectronic devices usually consist of a transparent conductive oxide (TCO) as one electrode. Interfacial engineering between the TCO electrode and the overlying organic layers is an important method for tuning device performance. We introduce poly(methylsilsesquioxane)-poly(N,N-di-4-methylphenylamino styrene) (PMSSQ-PTPA) as a potential hole-injection layer forming material. Spin-coating and thermally induced crosslinking resulted in an effective planarization of the anode interface. HOMO level (-5.6 eV) and hole mobility (1 × 10(-6)  cm(2)  · Vs(-1) ) of the film on ITO substrates were measured by cyclovoltammetry and time-of-flight measurement demonstrating the hole injection capability of the layer. Adhesion and stability for further multilayer built-up could be demonstrated. Contact angle measurements and tape tests after several solvent treatments proved the outstanding film stability.

Study of the Cesium Carbonate (Cs2CO3) Inter Layer Fabricated by Solution Process on P3HT:PCBM Solar Cells

In this paper, we studied the effect of the electron injection layer, Cesium carbonate (Cs2CO3), thickness on the performance of organic solar cell (OSC) based on blends of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester fullerene derivative (PCBM). The polymer solar cell consists of molybdenum-oxide (MoO3) as a hole injection layer, P3HT and PCBM bulk hetero junction as an active layer, and Cesium carbonate (Cs2CO3) as an electron injection layer. We measured each device by current-voltage measurement and impedance spectroscopy which is widely used for equivalent circuit analysis of solid state structures. The device with the Cs2CO3 layer showed about 8–10% higher JSC and about 6–8% higher power conversion efficiency compared with the devices without the Cs2CO3 layer.

Effect of nanoscale SubPc interfacial layer on the performance of inverted polymer solar cells based on P3HT/PC71BM.

The effect of a nanoscale boron subphthalocyanine chloride (SubPc) interfacial layer on the performance of inverted polymer solar cells based on poly (3-hexyl thiophene) (P3HT) and [6,6]-phenyl-C(71)-butyric acid methyl ester (PC(71)BM) was studied. When a 1 nm SubPc layer was introduced between the active layer (P3HT:PC(71)BM) and MoO(x) in the device with ITO/ZnO/P3HT:PC(71)BM/SubPc/MoO(x)/Al configuration, the power conversion efficiency (PCE) was increased from 3.42 (without SubPc) to 3.59%. This improvement is mainly attributed to the enhanced open-circuit voltage from 0.62 to 0.64 V. When the Flory-Huggins interaction parameters were estimated from the solubility parameters through the contact angle measurement, it revealed that the interaction between SubPc and PC(71)BM is more attractive than that between SubPc and P3HT at the interface of P3HT:PC(71)BM/SubPc, through which charges are well transported from the active layer to the anode. This is supported by a decrease of the contact resistance from 5.49 (SubPc 0 nm) to 0.94 MΩ cm (SubPc 1 nm). The photoelectron spectra provide another evidence for the enhanced PCE, exhibiting that the 1 nm thick SubPc layer extracts more photoelectrons from the active layer than other thicknesses.

High‐Performance Photoconductive Channels Based on (Carbon Nanotube)–(CdS Nanowire) Hybrid Nanostructures

A photoconductive channel based on hybrid nanostructures comprising carbon nanotubes (CNTs) and CdS nanowires is fabricated by a directed assembly strategy and catalyst-assisted chemical vapor deposition (CVD). The photoconductive channels simultaneously exhibit large photocurrent and fast response speed. Furthermore, it can be easily applied to surfaces that are not flat, such as a glass tube. This is a simple but efficient strategy for various optoelectronic applications.

Surface treatment of molybdenum oxide for performance improvement of organic light emitting diodes

The molybdenum oxide (MoO3) thin layers are used as the hole injection layer in the organic light emitting diodes (OLEDs). The surface treatment of MoO3 with ultraviolet ozone (UVO), nitrogen (N2) and argon (Ar) gas plasma have been studied. The turn on voltage is lowered in UVO treated device whereas for plasma treated device the turn on voltage increases. The surface roughness as well as the interface resistance increases with all the treatments. The UVO treated MoO3 device performance and stability enhances; the luminous efficiency by 16% and power efficiency by 14% compared with the reference device. The performance improvement can be explained by the change in work function of MoO3 with UVO treatment.