Xiao-Tao Hao

Thickness dependence of structural, optical and electrical properties of ZnO:Al films prepared on flexible substrates

The thickness dependence of structural, optical and electrical properties of the ZnO:Al films on polypropylene adipate (PPA) substrates have been studied. It was observed that with an increase in film thickness, the crystallite sizes and the density of the films were increased, the resistivity was decreased, and the average transmittance in the wavelength range of the visible spectrum was also slightly decreased.

Violet luminescence emitted from ZnO films deposited on Si substrate by rf magnetron sputtering

Highly orientated polycrystalline ZnO films have been deposited on Si substrate at room temperature (RT) by rf magnetron sputtering. A strong violet photoluminescence (PL) located at 402 nm and a weak ultraviolet (UV), PL located at 384 nm are observed when excited with 300 nm light. The former PL originated from the electron transition from conduction band tail states to valence band tail states and the latter is produced due to electron transition from conduction band to valence band. With an increase in intensity of the excitation light, the violet emission peak increases super-linearly and the UV emission increases linearly. After high temperature annealing in air, the crystallinity of obtained films is improved, the violet emission becomes weak and the UV emission gets strong.

Flexible top-emitting electroluminescent devices on polyethylene terephthalate substrates

An aluminum-laminated polyethylene terephthalate (Al-PET) is used as the substrate for flexible organic light-emitting devices (OLEDs). The efficient flexible electroluminescent devices have a top-emitting OLED architecture. An acrylic layer is formed on the Al-PET surface to improve the surface morphology and also the adhesion between the substrate and the anode. Poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) was used as hole transporting layer. The light-emitting polymer used is a phenyl-substituted poly(p-phenylenevinylene). Bilayer anodes of Ag∕CFX and Ag/indium-tin oxide and a semitransparent top cathode were used for the flexible polymer OLEDs. For a flexible polymer OLED with a 110-nm-thick light-emitting polymer, it exhibited superior electrical and optical characteristics with a luminous efficiency of 4.56cd∕A at an operating voltage of 7.5V.

Hydrogenated aluminium-doped zinc oxide semiconductor thin films for polymeric light-emitting diodes

Highly conducting transparent thin films of aluminium(Al)-doped zinc oxide (ZnO:Al) were deposited by a radio frequency magnetron-sputtering technique using an argon and hydrogen gas mixture at room temperature. Hydrogen serves as a shallow donor and plays a critical role in improving the Al doping efficiency to enhance the conductivity of thin films. The effect of hydrogen partial pressure on the properties of ZnO:Al films was investigated in detail. Polycrystalline ZnO:Al films with a surface roughness of about 2 nm, conductivity of 1.97 ? 103 S cm?1, transmittance of over 83% in the visible wavelength region and an optical band gap of 3.93 eV were achieved at a hydrogen partial pressure of 7.5 ? 10?4 Pa. A ZnO:Al film with the desired properties was used as an anode contact in a bi-layer polymeric light-emitting diode. A polyethylene dioxythiophene?polystyrene sulfonate doped with poly(styrenesulfonic acid) (PEDOT: PSS) and phenyl-substituted poly(p-phenylene vinylene) (Ph-PPV) were employed as a hole transport layer and a light-emitting layer, respectively. The electro-luminescence performance of the aforementioned diode was studied and compared to a control device with an indium tin oxide anode.

Bias voltage dependence of properties for ZnO:Al films deposited on flexible substrate

Abstract Highly transparent conducting Al-doped zinc oxide (ZnO:Al) films with good adherence were deposited on water-cooled polypropylene adipate (PPA) substrate using bias r.f. magnetron sputtering. Films with resistivities as low as 4.6×10 −4 Ω cm and transmittance over 80% have been obtained by adjusting the negative bias voltage of the substrate. The dependence of the structural, electrical and optical properties of these films on the substrate bias voltage was studied in detail.

Comparison of the properties for ZnO:Al films deposited on polyimide and glass substrates

Transparent conducting aluminum–doped zinc oxide (ZnO:Al) films have been prepared on polyimide (PI) and Corning 7059 substrates by r.f. magnetron sputtering technique at low substrate temperature (25–210 °C). Polycrystalline ZnO:Al films having a preferred orientation with the c-axis perpendicular to the substrate were deposited with resistivity as low as 8.5 × 10 −4 cm on PI substrates and 7.1 ×10 − 4 cm on glass substrates. The average transmittance exceeded 74 and 85% in the visible spectrum for 360 and 390 nm thick films deposited on PI and glass, respectively. A comparison of the properties of the films deposited on glass and organic substrates was performed.

Electronic density tailing outside π-conjugated polymer surface

Penning ionization electron spectroscopy (PIES) was adopted to examine surface electronic property of conjugated poly(3-hexylthiophene) (P3HT) aiming to detect the electronic density tailing outside a polymer surface. The electronic wave function of the highest occupied molecular orbital (HOMO) state is shielded by side hexyl chain for a P3HT film with edge-on conformation and was not detected by PIES, while it is tailing outside the polymer surface for face-on conformation and was observed clearly by PIES. The presence of HOMO electronic wave function outside the polymer surface makes it possible to form HOMO-HOMO overlapping with overlayer materials, and therefore more efficient charge transfer is expected in a heterojunction structure for device application.

Dual Förster resonance energy transfer effects in non-fullerene ternary organic solar cells with the third component embedded in the donor and acceptor

Non-fullerene ternary organic solar cells (OSCs) are new promising candidates for future applications in the area of organic photovoltaics. However, their low short-circuit current (JSC) values impede efforts at increasing their power conversion efficiency (PCE) levels. Maximizing the JSC is one of the critical elements for enabling high performances of OSCs. To improve the JSC of the non-fullerene ternary OSCs based on poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b:3,3-b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (PTB7-Th:ITIC), a polymer of poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) was added to the binary system. The PCDTBT was shown to embed in both PTB7-Th and ITIC, and introduced dual Forster resonance energy transfer (FRET) effects in the resulting ternary system. In addition, the PCDTBT may have decreased the molecular coherence lengths of PTB7-Th and ITIC and influenced the charge transport. Furthermore, the PCDTBT apparently also decreased the d-spacing of the π–π stacking of PTB7-Th and ITIC, which likely increased the intermolecular charge hopping efficiency. Doping an appropriate amount of PCDTBT in the system yielded an increase in the JSC from 13.89 to 16.71 mA cm−2. The increased JSC resulted in a 15% enhancement of the PCE, which indicated that introducing dual FRET effects is an effective way to enhance the JSC and thus the performance of the OSCs.

Surfactant-mediated formation of polymeric microlenses from interfacial microdroplets

Nano- and micro-scale lenses have a range of potential applications, such as in antireflective layers in photovoltaic or light emission devices, and in super resolution imaging in the near field modes. One of the protocols to mass produce polymeric microlenses is through the polymerization of microdroplets of a monomer precursor that are produced at solid–liquid interfaces by a solvent exchange technique. In this work, we have advanced this protocol by using surfactants. A cationic surfactant was added to the liquid phase for the control over the formation and morphology of polymerisable microdroplets and their resultant microlenses (i.e. the polymerized microdroplets). The results demonstrate that the surfactant could enable the production of polymerizable microdroplets on hydrophilic substrates by the solvent exchange technique, and eliminate the restriction by the substrate wettability on the microlens fabrication. Furthermore, the size distribution and aspect ratio of microlenses could be tuned by the surfactant concentration.

Three-dimensional femtosecond laser fabrication of waveguide beam splitters in LiNbO 3 crystal

We report on the fabrication of waveguide beam splitters in z-cut LiNbO3 crystal by direct femtosecond laser writing. The guidance is valid only for TM polarization due to the Type I modification of extraordinary refractive index (ne) induced by femtosecond laser pulses. With single scan of femtosecond laser beams over the crystal bulk, the structures with channel geometry have been produced. In this work, such waveguide configurations were created as one-dimensional (1D) straight waveguide, two-dimensional (2D) 1 × 2 and three-dimensional (3D) 1 × 4 waveguide beam splitters. The waveguide beam splitters are characterized at the wavelength of 632.8 nm and 1064 nm both experimentally and numerically. This work opens the way for laser-written 3D LiNbO3 waveguide beam splitters as novel 3D nonlinear photonic devices.