| Abstract: | 本論文的第一部份探討利用組成梯度能隙非晶質碳化矽薄膜作為載子注入層以及氧氣電漿處理氧化銦錫透明電極來提升高分子發光二極體的光電特性。首先以組成梯度取代固定能隙的非晶質碳化矽薄膜作為高分子發光二極體的電子注入層時，元件的臨界電壓與亮度分別由7.3 V、3162 cd/m2提升至6.3 V、5829 cd/m2，元件特性的提升是由於利用組成梯度技術可形成能帶較平順的電子注入層而提升載子的注入效率，也減少了載子在電子注入層與發光層間的復合。此外，利用氧氣電漿處理氧化銦錫透明電極也可以提升元件臨界電壓與亮度分別至5.1 V、6250 cd/m2。再者，結合組成梯度能隙非晶質碳化矽薄膜作為高分子發光二極體的電子與電洞注入層以及氧氣電漿處理氧化銦錫透明電極可進一步提升元件的亮度至9350 cd/m2。 第二部份探討氫化製程對具有組成梯度能隙載子注入層的非晶質碳薄膜白光發光二極體的影響，以及利用附加組成梯度能隙非晶質矽鍺薄膜作為載子注入層來提升薄膜白光發光二極體的光電特性。首先，我們製作出一個以非晶質碳薄膜為發光層以及組成梯度能隙非晶質碳化矽薄膜為載子注入層的薄膜白光發光二極體，此薄膜白光發光二極體可操作在直流與交流的模式下。實驗中發現發光層與載子注入層的氫化製程處理可大幅提昇元件的光電特性，元件在順向與逆向偏壓的直流條件下可達到最高的亮度分別為813與507 cd/m2，元件特性的提升是由於氫化製程可填補非晶質薄膜表面的斷鍵。此外，我們也探討了非晶質碳薄膜白光發光二極體的電傳導機制，在低偏壓的範圍時，傳導機制由歐姆電流所主導；在高偏壓的範圍時，則是由普爾-法蘭克放射電流所主導。再者，我們發現到當外加交流訊號頻率超過1 KHz時，元件的電致發光頻譜有明顯紅移的現象，這可歸因於非晶質材料較低的載子遷移率所導致。最後，利用附加組成梯度能隙非晶質矽鍺薄膜作為載子注入層可進一步提升薄膜白光發光二極體的光電特性，這是因為組成梯度能隙非晶質矽鍺薄膜可形成較低的位障與在鋁電極與非晶質鍺薄膜間形成部分的多晶質鍺薄膜層，因而提升載子的注入效率與降低接面間的接觸電阻。 In this dissertation, first, the optoelectronic characteristics of poly(2-methoxy-5-(2’ethyl-hexoxy)-1, 4-phenylene-vinylene) (MEH-PPV) polymer light-emitting diodes (PLEDs) with thin, doped composition-graded (CG) amorphous silicon-carbide (a-SiC:H) carrier-injection layers have been investigated. The optoelectronic characteristics of MEH-PPV PLEDs have been improved by employing thin doped CG a-SiC:H films as carrier-injection layers and O2-plasma treatment on indium-tin-oxide (ITO) transparent electrode, as compared with previously reported ones having doped constant-optical-gap a-SiC:H carrier-injection layers. For PLEDs with an n-type a-SiC:H electron injection layer (EIL) only, the electroluminescence (EL) threshold voltage and brightness were improved from 7.3 V, 3162 cd/m2 to 6.3 V, 5829 cd/m2 (at a current density J = 0.6 A/cm2), respectively, by using the CG technique. The enhancement of EL performance of the CG technique was due to the increased electron injection efficiency resulting from a smoother barrier and reduced recombination of charge carriers at the EIL and MEH-PPV interface. Also, surface modification of the ITO transparent electrode by O2-plasma treatment was used to further improve the EL threshold voltage and brightness of this PLED to 5.1 V, 6250 cd/m2 (at a J = 0.6 A/cm2). Furthermore, by employing the CG n[p]-a-SiC:H film as EIL [hole injection layer (HIL)] and O2-plasma treatment on the ITO electrode, the brightness of PLEDs could be enhanced to 9350 cd/m2 (at a J = 0.3 A/cm2), as compared with the 6450 cd/m2 obtained from a previously reported PLED with a constant-optical-gap n-a-SiCGe:H EIL and p-a-Si:H HIL. In addition, the effects of hydrogenation on optoelectronic properties of intrinsic amorphous carbon (i-a-C:H) thin-film-white light-emitting diodes (TFWLEDs) with CG carrier-injection layers and the improved optoelectronic properties of TFWLEDs by additionally CG intrinsic amorphous silicon germanium (i-a-SiGe:H) as the carrier-injection layer have been investigated. The TFWLEDs were fabricated with i-a-C:H film as the luminescent layer and CG intrinsic a-SiC:H (i-a-SiC:H) film as the carrier-injection layers. The demonstrated TFWLEDs could be operated under direct-current (dc) forward or reverse bias or sinusoidal alternating-current (ac) voltage. The hydrogenation process for the luminescent or CG carrier-injection layer has been investigated to greatly enhance the optoelectronic properties of the obtained TFWLEDs. For the hydrogenated TFWLEDs, the highest obtainable brightnesses were 813 and 507 cd/m2 at an injection current density of 0.6 A/cm2 and the lowest EL threshold voltages were 9.1 and 8.9 V, under dc forward and reverse biases, respectively. These enhanced optoelectronic properties were attributed to the passivation of dangling bonds and the forming of more H2-compensated amorphous film by the employed hydrogenation process. In addition, the electrical transport mechanisms of the TFWLEDs were studied. In the low applied-bias range, the ohmic current was the dominated one. In the high applied-bias range, a Poole-Frenkel emission current resulted from the field-assisted hopping along the traps in amorphous film was observed. Moreover, a significant red-shift in EL spectra has been observed while the applied ac frequencies were higher than 1 kHz and its origin has been attributed to the lower mobilities of charge carriers. Furthermore, the optoelectronic characteristics of i-a-C:H TFWLEDs with CG i-a-SiC:H layers had been obviously improved with additionally incorporated CG i-a-SiGe:H (CG Ge) carrier-injection layers. The enhancement of EL performance with CG Ge carrier-injection layers was due to the increased carrier-injection efficiency and reduced contact resistance resulting from the lower barrier and partially formed polycrystalline Ge layer between the Al (electrode)/Ge interface. |
