Chun-Yu Lee

Double side electroluminescence from p-NiO/n-ZnO nanowire heterojunctions

Double side light emission devices based on p-NiO/n-ZnO nanowire heterojunctions have been fabricated on indium tin oxide substrate by radio frequency magnetron sputtering combined with hydrothermal process. According to the energy band alignment, the detected broad visible and narrow ultraviolet electroluminescence arise from defect and band edge transitions in ZnO nanowires, respectively. The unique property of the double side emission is due to the nature of the large band gap of NiO film. It provides a good opportunity for the emission of a light emitting device with different colors on the top and back sides, simultaneously.

Electroluminescence from ZnO nanoparticles/organic nanocomposites

The authors report ultraviolet electroluminescence from ZnO nanoparticle-based devices prepared by the phase-segregation technique. The conditions for phase segregation are investigated using confocal microscopy. With proper parameters for phase segregation, the ZnO nanoparticles and N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine: poly(methyl methacrylate) can be separated into two layers upon spin-coating process. The method allows electrons and holes to recombine in the ZnO nanoparticles. The I-V curve shows stable and excellent rectification. For the device with 90nm ZnO nanoparticles, it exhibits a very narrow spectrum with a peak at 392nm and no defect-related emission. The emission peak well corresponds to the ZnO band-gap energy.

Electroluminescence from monolayer ZnO nanoparticles using dry coating technique

We report ultraviolet electroluminescence from ZnO nanoparticle-based devices prepared by the dry-coating technique. With dry-coating process, the structure of the ZnO nanoparticle monolayer (90nm) in the device can be easily achieved. The method reduces the density of pinhole defects in the ZnO nanoparticles. The confirmation for dry coating is investigated using field-emission scanning electron microscopy. The devices show the ZnO band-gap emission peak at 380nm and the background emission from the interface between the host matrix and Aluminum tris-8-hydroxyquinoline. The origins of the ZnO band-gap emission and background emission are also discussed.

Enhanced ultraviolet electroluminescence from ZnO nanowires in TiO2/ZnO coaxial nanowires/poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate) heterojunction

The ultraviolet (UV) electroluminescence (EL) from the TiO2/ZnO coaxial nanowires (NWs)/poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate) inorganic/organic heterostructure devices is greatly enhanced and the defect emission is significantly suppressed compared with the uncoated ZnO NW device at room temperature. The origins of the great changes in EL of ZnO NW devices are attributed to the surface modification of the sputtered TiO2 coating and the reduction in the pinhole traps on the surface of ZnO NWs. It is found that for the optimized device, the EL intensity ratio between the band gap and defect emission can be greatly enhanced by up to about 250 times its prior level. Such ZnO NW devices with enhanced UV emission have potential applications in the highly efficient solid state emitters.

High Efficiency Flexible Polymer Solar Cells Based on PET Substrates with a Nonannealing Active Layer

The inverted bulk-heterojunction solar cell on the polyester (PET) substrate with a nonannealing active layer is investigated. The atomic force microscope images show that the morphology of the nonannealing active layer of the inverted plastic solar cell evolves with time, which improves the performance of the solar cell. Our investigations show that the grain size of the active layer increases with time, resulting in improvements in the fill factor (from 34.8 to 62.8%) and shunt resistance (from 107 to 505 Ω cm 2 ) as well as a reduction in the series resistance (from 4.82 to 0.96 Ω cm 2 ). The easily processed inverted device with a nonannealing active layer on the indium tin oxide-coated PET substrate exhibits a high power conversion efficiency of ~ 3.66%.

Nano-Structured and Micro-Structured Semiconductors for Higher Efficiency Solar Cells

Hybrid thin-film solar cells are fabricated on the glass substrate using ZnO nanorods, well-aligned single-crystalline Si nanowires (SiNWs), and poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as well as micro-structured III-V semiconductors . The effect of introducing a solution-processed fullerene as an electron transport layer on the polymer/ZnO nanorod array composite solar cells is investigated . Devices with the fullerene layer exhibit a larger short circuit current density (10.16 mA/cm2) than those without this layer (9.37 mA/cm2). For the Si NWs with P3HT:PCBM, the well-aligned SiNWs are fabricated from Si wafer and transferred onto the glass substrate with the P3HT:PCBM. Such SiNWs provide uninterrupted conduction paths for electron transport, enhancing the optical absorption to serve as an absorber, and increase the surface area for exciton dissociation . Our investigations show that Si NWs are promising for hybrid organic photovoltaic cells with improved performance by increasing the short-circuit current density from 7.17 to 11.61 mA/cm2. III-V semiconductors are also used for thin-film solar cells. InGaP/GaAs two-junction square-based (25 mum times25 mum) cuboid arrays with a height of 6.52 mum were released from GaAs substrates by the epitaxial lift-off process. These InGaP/GaAs cuboid arrays were transplanted to the P3HT film spun on ITO glass substrate. In addition to the significant cost reduction, our method shows the rapid transplantation and the potential for high-efficiency large-area devices fabrication.

Polarization Raman spectroscopy of GaN nanorod bundles

We performed polarization Raman spectroscopy on single wurtzite GaN nanorod bundles grown by plasma-assisted molecular beam epitaxy. The obtained Raman spectra were compared with those of GaN epilayer. The spectral difference between the GaN nanorod bundles and epilayer reveals the relaxation of Raman selection rules in these GaN nanorod bundles. The deviation of polarization-dependent Raman spectroscopy from the prediction of Raman selection rules is attributed to both the orientation of the crystal axis with respect to the polarization vectors of incident and scattered light and the structural defects in the merging boundary of GaN nanorods. The presence of high defect density induced by local strain at the merging boundary was further confirmed by transmission electron microscopy. The averaged defect interspacing was estimated to be around 3 nm based on the spatial correlation model.

2 /spl mu/m emission from Si/Ge heterojunction LED and up to 1.55 /spl mu/m detection by GOI detectors with strain-enhanced features

The Ge/Si heterojunction MOS LED can emit ~2 mum light due to the radiative recombination at type II heterojunction. The fully depleted 800 nm germanium on insulator (GOI) can increase the responsivity and the speed of the MOS detector due to the large absorption of Ge and the electrical field in the active layer, respectively. Moreover, the external strain can tune the emission energy (~11 meV) by modifying the band structure. The responsivity of GOI MOS detector is also improved by ~10% using the external strain. Both heterojunction and strain (internal or external) can enhance the performance of Si-based optoelectronics

All-solution-processed-inverted polymer solar cells on PET substrates with CuOx thin film as an anode interlayer

Low-cost solution process becomes the strategic principle for the fabrication of polymer solar cell. However, the cathode or anode interlayer are usually deposited by using vacuum evaporation due to the hydrophobicity problem and the temperature restriction of PET substrates. Here, we have successfully fabricated copper oxide (CuOx) thin film as an anode interlayer in the inverted P3HT:PC61BM solar cells on the PET substrates at low temperature via the sol-gel method. The CuOx thin film can module the Schottky barrier and form an ohmic contact at the organic/metal interface, which make it a great hole transportation layer. In addition, we post- anneal the devices at 110°C to compensate the defect state at the organic/inorganic interface. Thus, the fill factor of the devices has been increased from 53.3% to 63.5% and the power conversion efficiency of devices has been improved from 3.04% to 3.95%

Synthesis and characterization of ZnO nanorod arrays and their integration into polymer solar cells

ZnO nanorod arrays (ZNAs), grown from aqueous solutions, were integrated into polymer solar cells. Morphological, crystalline and optical properties of ZNAs were studied. Infiltration-improved polymer/ZNAs solar cells by adding fullerene were assembled and characterized.