Effects of Hydrogenation on Optoelectronic Properties of a-C:H Thin-Film White-Light-Emitting Diodes With Composition-Graded Carrier-Injection Layers

In this paper, thin-film white-light-emitting diodes (TFWLEDs) were fabricated with a hydrogenated intrinsic amorphous carbon (i-a-C:H) film as the luminescent layer and a composition-graded (CG) hydrogenated intrinsic amorphous silicon carbide (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 electroluminescence (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.