Optoelectronics and Electronic Devices of Organic/Inorganic ZnO Nanocomposites
Date Issued
2010
Date
2010
Author(s)
Lee, Chun-Yu
Abstract
In this dissertation, we have reported the optoelectronic and electronic properties of the nanostructural ZnO/organic heterosturcture device. In the study of the optoelectronics, we focused on the ZnO-based light-emitting-diodes (LEDs). The devices including ZnO nanoparticles/organic LEDs using phase-segregation technique, monolayer ZnO nanoparticles/organic LEDs using dry-coating technique, ZnO nanorods (NRs)/poly(3-hexylthiophene) LEDs, and ZnO nanostructure/polyfluorene white LEDs, TiO2/ZnO coaxial nanowires/poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate) LEDs. In the study of the electronics, we investigated the flexible ZnO transparent thin film transistor with polymethylmethacrylate as a dielectric layer.
Part A. ZnO-based light-emitting-diodes
1. ZnO nanoparticles/organic LEDs using phase-segregation technique
We 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 TPD:PMMA 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 90 nm ZnO nanoparticles, it exhibits a very narrow spectrum with a peak at 392 nm and no defect-related emission. The emission peak well corresponds to the ZnO band-gap energy.
2. Monolayer ZnO nanoparticles/organic LEDs 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 (90 nm) 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 380 nm 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.
3. ZnO rods/poly(3-hexylthiophene) LEDs
We report that ZnO rod array is grown on organic layer of poly(3-hexylthiophene) (P3HT) using modified seeding layer. Thus ZnO rod array/P3HT heterojunction light-emitting diodes could be fabricated using the hydrothermal method, in which ZnO acts as an n-type material and P3HT as a p-type material. The ZnO rod array improves the electron transportation in the devices. Three fold enhancement of current density of the device is observed due to rod array formed on P3HT. The electroluminescence (EL) of the optimized ZnO-based device is 1.5 times larger than that without NRs. The influence of the P3HT thickness for the EL spectrum is also discussed.
4. ZnO nano-micro structure/polyfluorene white LEDs
In this chapter, we have reported the white-light electroluminescence from ZnO nano-micro structure/PF hybrid heterojunction. In part I, we report bright white-light electroluminescence (EL) from diode structure consisting of ZnO rods and a p-type conducting polymer of poly(fluorine) (PF) using hydrothermal method. The device structure is ITO/PF/ZnO microrod array:SOG/Al. ZnO microrod array is successfully grown on organic layer of PF using modified seeding layer. The EL spectrum shows a broad emission band covering the entire visible range from 400 to 800 nm. The white light emission is possible because the ZnO-defect related emission from the ZnO rod array/PF heterostructure is enhanced over thousand times stronger than that from the usual ZnO rod structure. This strong green-yellow emission associated with the ZnO defects, combined with the blue PF-related emission, result in the white light emission. The enhancement of the ZnO-defect emission is caused by the presence of Zn(OH)2 at the interface between ZnO rod array and PF. Fourier transform infrared spectroscopy reveals that the absorption peaks at 3441, 3502, and 3574 cm-1 corresponding to the OH group are formed at the ZnO rod array/PF heterostructure, which confirms the enhancement of defect emission from the ZnO rod array/PF heterostructure. The processing procedure revealed in this work shows a convenient and low-cost way to fabricate ZnO-based white-light emitting devices. In part II, the characteristics of a nanocomposite consisting of the blue-emitting polymer polyfluorene and ZnO nanowires are reported. The device structure is ITO/ZnO nanowires:PF/Al. The electroluminescence spectrum of the white light emission is from about 400 nm to 750 nm.
5. TiO2/ZnO coaxial nanowires/poly(3,4-ethylenedioxythiophene)- poly(styrene-sulfonate) LEDs
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 un-coated 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 of the pinhole traps on the surface of ZnO NWs. It is found that for the optimized device, the EL intensity ratio between the bandgap 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.
Part B. ZnO-based thin film transistors
6. Flexible ZnO transparent thin film transistor with polymethylmethacrylate as a dielectric layer
The authors report solution-processed ZnO thin film transistors (TFTs) on a flexible substrate, using polymethylmethacrylate (PMMA) as a dielectric layer. To improve the compatibility between the ZnO active layer and the PMMA dielectric, an O2-plasma treatment has been applied to the PMMA dielectric. The structural and electrical characteristics of ZnO-TFT, which have different channel morphologies produced by various concentrations of the ZnO solution, were investigated. The ZnO trap centers of the ZnO-TFTs were decreased as the concentration of the ZnO solution increased. The ZnO-TFT with the optimized channel morphology exhibited a high field-effect mobility of 7.53 cm2/Vs.
Subjects
ZnO
organic
nanocomposite
phase-segregation
dry-coating
nanoparticle
nanorod
Type
thesis
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