有機/無機氧化鋅奈米複合薄膜光電及電子元件

Optoelectronics and Electronic Devices of Organic/Inorganic ZnO Nanocomposites

李俊育(Chun-Yu Lee)

指導教授 ： 林清富

國立臺灣大學/電機資訊學院/光電工程學研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.02589

摘要

在本論文中，我們研究了多種奈米結構氧化鋅/有機異質接面的光電元件及電子元件，在光電元件研究的部份，我們著重在氧化鋅發光元件，包括了以相分離技術製作氧化鋅奈米粒子/有機異質接面發光二極體、以乾式塗佈技術製作單層氧化鋅奈米粒子/有機異質接面發光二極體、氧化鋅柱陣列/poly(3-hexyl thiophene)發光二極體、氧化鋅柱陣列/polyfluorene白光發光二極體、二氧化鈦:氧化鋅共軸奈米線/poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate)發光二極體，在電子元件研究的部份，我們製做了軟性氧化鋅/polymethylmethacrylate透明電晶體。
A. 氧化鋅發光元件:
1. 以相分離技術製作氧化鋅奈米粒子/有機異質接面發光二極體
我們以相分離技術製作氧化鋅奈米粒子/有機異質接面紫外光發光二極體。實驗中以共焦顯微鏡觀察氧化鋅奈米粒子與有機材料(TPD:PMMA)相分離的情況，當氧化鋅奈米粒子與TPD:PMMA水溶液為一適當比例調製，在將此水溶液以旋轉塗佈的方式塗佈在基板上時，氧化鋅奈米粒子會與TPD:PMMA形成分層結構，此相分離技術會使得電子電洞對容易在氧化鋅奈米粒子中結合。實驗結果顯示，元件有穩定及良好的整流特性，另外，以90奈米粒徑的氧化鋅奈米粒子製作的發光元件，可測得對應氧化鋅能隙發光的頻譜，其波峰峰值為392奈米，並且無氧化鋅表面缺陷的發光波長。
2. 以乾式塗佈技術製作單層氧化鋅奈米粒子/有機異質接面發光二極體
我們利用乾式塗佈法(dry coating)將氧化鋅奈米粒子(ZnO nanoparticle)與有機電子/電洞傳輸材料結合，製作出單層氧化鋅奈米粒子的三明治結構發光元件-電洞傳輸層/氧化鋅奈米粒子/電子傳輸層，單層的氧化鋅奈米粒子可減少因為奈米粒子互相堆疊所造成的孔洞缺陷(pinhole defect)，而提高發光效率，我們並以掃描電子顯微鏡(Scanning electron microscope)觀察其氧化鋅奈米粒子單層結構，在實驗中我們各別使用三種不同的電洞傳輸層，均可觀察到對應氧化鋅的能隙發光(energy band-gap) 392 nm及結構中局部電洞傳輸層與電子傳輸層直接結合所造成的電洞傳輸層本身的螢光發光，在此研究中並討論其元件發光機制。
3. 氧化鋅柱陣列/poly(3-hexylthiophene)發光二極體
利用改良過的氧化鋅種子層，將氧化鋅柱以水熱法成長到有機電洞傳輸層上poly(3-hexylthiophene) (P3HT)，因此可形成氧化鋅柱陣列/P3HT異質接面發光二極體，在此結構中氧化鋅為n型材料，P3HT為p型材料，氧化鋅柱陣列可促進電子在元件中的傳導。實驗結果顯示，當元件有加入氧化鋅柱陣列，於相同電壓下，電流有3倍提高，另外，在電激發光頻譜中，其發光強度也有1.5倍提高，最後並探討P3HT的厚度對元件發光特性的影響。
4. 氧化鋅奈米-微米結構/polyfluorene白光發光二極體
此章節我們研究氧化鋅奈米-微米結構/polyfluorene (PF)異質接面白光發光二極體。在第一部份，我們製作ITO/PF/ZnO microrods:SOG/Al的白光發光元件，利用水熱法將氧化鋅柱陣列成長到有機電洞傳輸層PF上，在此結構中氧化鋅為n型材料，PF為p型材料。實驗結果顯示，此元件有一涵蓋可見光波長400-800奈米的頻譜，此一寬發光頻譜是由於氧化鋅缺陷發光(綠-黃光)與有機PF藍光結合所形成的，而氧化鋅缺陷發光的增強是由於氧化鋅在成長過程所形成的Zn(OH)2缺陷複合中心所導致，另外，我們並以快速傅利葉紅外線光譜儀檢測氧化鋅柱陣列/PF薄膜，並觀察到OH基對應Zn(OH)2的吸收波長在3441, 3502, 與 3574 cm-1，此實驗證實了於此結構有大量的Zn(OH)2，利用此項技術可達到低成本製作白光發光元件。在第二部份，我們製作ITO/ZnO nanowires:PF/Al的白光發光元件，此元件的電激發光波長涵蓋了400-700奈米，其結果與第一部份ITO/PF/ZnO rods:SOG/Al結構類似。
5. 二氧化鈦:氧化鋅共軸奈米線/poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate)發光二極體
利用二氧化鈦覆蓋修飾氧化鋅表面缺陷，製作二氧化鈦:氧化鋅共軸奈米線/poly(3,4-ethylenedioxythiophene)-poly(styrene-sulfonate)紫外光發光二極體，當元件經過二氧化鈦覆蓋修飾氧化鋅表面缺陷，其電激發光可明顯觀察到氧化鋅能隙發光大大被提升，並且氧化鋅表面缺陷發光大幅被抑止，此實驗利用1.表面修飾2.減少二氧化鈦在氧化鋅表面堆疊所形成的孔洞缺陷來提升元件紫外光的強度，對於最佳化的元件而言，氧化鋅能隙/表面缺陷電激發光比，可被提升約250倍，此製作方式有很大的潛力製作高效率氧化鋅電激發元件。
B. 氧化鋅電子元件:
6. 軟性氧化鋅/polymethylmethacrylate透明電晶體
我們已溶液製程方式製作透明氧化鋅薄膜電晶體於軟性基板，並以有機高分子polymethylmethacrylate (PMMA)為閘極絕緣材料，為了增進氧化鋅薄膜與PMMA之間的相容性，我們以氧電漿修飾PMMA表面，在實驗中並觀察了不同濃度氧化鋅溶液製作的氧化鋅對元件特性的影響，當氧化鋅溶液的濃度越大所形成的氧化鋅缺陷越少，最佳條件的氧化鋅電晶體可達到7.53 cm2/Vs的電晶體載子移動率。

關鍵字

氧化鋅 ； 有機 ； 奈米複合薄膜 ； 相分離 ； 乾式塗佈 ； 奈米粒子 ； 奈米柱

並列摘要

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.

並列關鍵字

ZnO ； organic ； nanocomposite ； phase-segregation ； dry-coating ； nanoparticle ； nanorod

參考文獻

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