Performance Enhancement from Hybrid Optoelectronic Devices Comprising Zinc Oxide Nanorods and Tungsten Oxide Layers
|關鍵字:||氧化鋅奈米柱陣列;反結構發光二極體;高分子太陽能電池;Zinc oxide nanorod arrays;Inverted light emitting diode;Polymer solar cell|
|摘要:|| 本研究之目的為使用水熱法製備氧化鋅奈米柱陣列以建構混成式光電元件，並藉由氧化鎢薄膜進一步提升元件特性。本研究分為三個部分，第一部分首先製備厚度52 nm之氧化鋅晶種層以成長氧化鋅奈米柱陣列；氧化鋅奈米柱長度控制在300 nm，其直徑則為40–70 nm。本研究另以鍛燒溫度300 oC製作厚度52 nm之氧化鋅薄膜以作為對照組。所有樣品均藉由掃描式電子顯微鏡及原子力顯微鏡以確認其厚度、奈米結構型態及表面形貌。循環電位儀結果顯示氧化鋅奈米柱或薄膜之價帶及傳導帶分別位於-6.9及-3.7 eV。紫外–可見光吸收光譜顯示所有氧化鋅奈米型態於可見光波段之穿透率高於90%，適合應用於光電元件。傅立葉轉換紅外線光譜結果顯示鍛燒溫度提高，有助於氧化鋅鍵結形成及減少有機團基含量。
於第二部分中，製作結構為ITO/ZnO-rod/P2-PF6/MEH-PPV/ PEDOT/Au之反結構混成式高分子發光元件，以氧化鋅奈米柱陣列做為電子傳輸層、P2-PF6作為潤濕劑、MEH-PPV作為發光層。最佳元件之驅動電壓、最大亮度及最高效率分別為3.8 V、10,620 cd/m2及0.25 cd/A，其亮度遠高於以鈣/鋁作為陰極之正結構元件、或含氧化鋅薄膜之反結構元件。引入氧化鎢層於PEDOT與金電極之間作為陽極緩衝層，能繼續提升元件表現。元件之驅動電壓、最大亮度及最高效率分別達到6.9 V、21,881 cd/m2及0.43 cd/A。
於第三部分中，製作結構為ITO/ZnO-rod/P2-PF6/P3HT:PC61BM/ PEDOT/WO3/Au之反結構混成式高分子太陽能元件，以P3HT:PC61BM作為主動層。最佳元件效果為使用氧化鋅奈米柱搭配氧化鎢薄膜分別做為電子傳輸層與電洞萃取層；該元件之開路電壓、短路電流、填充因子及光電轉換效率分別為0.52 V、12.12 mA/cm2、42%及2.61%。相較於氧化鋅薄膜製作之元件，以氧化鋅奈米柱製作元件之短路電流及效率獲得改善。此部分亦利用含吡咯併吡咯二酮團基之聚噻吩衍生物作為主動層以製作太陽能元件，其開路電壓、短路電流、填充因子及光電轉換效率更達到0.58 V、16.21 mA/cm2、40%及3.74%。以上結果說明氧化鋅奈米柱陣列具有應用於光電元件之潛力，並朝向實用化邁進。|
The goal of this research is aimed to prepare zinc oxide (ZnO) nanorod arrays by hydrothermal method for construction of hybrid optoelectronic devices, and to further improve the device performance by inserting tungsten oxide (WO3) layer. This research is divided into three parts. In the first part, the ZnO seed layer with thickness of 52 nm was firstly prepared for growth of ZnO nanorod arrays; the length of ZnO nanorods was controlled to be 300 nm, and their diameters were in the range of 40–70 nm. In this study, the ZnO films with thickness of 52 nm were prepared with calcination temperature at 300 oC for comparison. All samples were characterized by scanning electron microscope and atomic force microscope to confirm their thickness, nanostructure, and surface morphology. The results of cyclic voltammetry showed that the valence and conduction band of ZnO nanorods and thin film were located at -6.9 and -3.7 eV, respectively. The UV-visible absorption spectra showed that the transmittance of all ZnO nanostructures are higher than 90% in the visible region, which is suitable for application in optoelectronic devices. The results of Fourier Transform Infrared Spectroscopy showed that the ZnO bonding was favored to form and the content of organic components was reduced with higher calcination temperature. In the second part, inverted hybrid polymer light emitting devices with configuration of ITO/ZnO-rod/P2-PF6/MEH-PPV/PEDOT/Au were fabricated, using ZnO nanorod arrays as electron transporting layer, P2-PF6 as wetting agent, and MEH-PPV as emissive layer. The threshold voltage, maximum brightness, and maximum current efficiency of the best device were 3.8 V, 10,620 cd/m2, and 0.25 cd/A, respectively, revealing much higher brightness compared with conventional devices using Ca/Al as cathode, or inverted devices based on ZnO thin film. By introducing a layer of WO3 between PEDOT and gold electrode as anode buffer layer, the device performance was further enhanced. The threshold voltage, maximum brightness, and maximum current efficiency of the device were promoted to 6.9 V, 21,881 cd/m2, and 0.43 cd/A, respectively. In the third part, inverted hybrid polymer solar devices with configuration of ITO/ZnO-rod/P2-PF6/P3HT:PC61BM/PEDOT/WO3/Au were fabricated, using P3HT:PC61BM as active layer. The best device performance was obtained by using ZnO nanorods and WO3 layer as electron transporting and hole extraction layer, respectively; the open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE) were 0.52 V, 12.12 mA/cm2, 42%, and 2.61%, respectively. Comparing with devices using ZnO thin film, the devices based on ZnO nanorod arrays showed higher Jsc and PCE values. In this part, the polythiophene derivatives containing 1,4-diketopyrrolopyrrole were also used as active layer for fabrication of solar devices, and the Voc, Jsc, FF and PCE reached 0.58 V, 16.21 mA/cm2, 40%, and 3.74%, respectively. All above results demonstrate the potential of ZnO nanorod arrays applied in optoelectronic devices, moving toward the practical use.