| dc.description.abstract | 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.
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