Shih-Yung Lo

Optoelectronic Characteristics of Direct-Current and Alternating-Current White Thin-Film Light-Emitting Diodes Based on Hydrogenated Amorphous Silicon Nitride Film

Direct-current and alternating-current white thin- film light-emitting diodes (DCW and ACW TFLEDs) have been fabricated and demonstrated with the intrinsic hydrogenated amorphous silicon nitride (i-a-SiN:H) film as the luminescent layer. The achievable brightness of the representative DCW and ACW TFLEDs were 200 and 170 cd/m2 at an injection-current density of 600 and 100 mA/cm2, respectively. The electroluminescence (EL) threshold voltage of the DCW TFLED was 10.9 V, and its peak wavelength and full-width at half-maximum (FWHM) of the EL spectrum were about 455 and 230 nm, respectively. For the ACW TFLED, the EL threshold voltage was 8.4 V, and its peak wavelength and FWHM of the EL spectrum were about 535 and 260 nm, respectively. In addition, their current-conduction mechanism was also investigated. Within the lower applied-voltage region, they showed an ohmic current, while for the higher applied-voltage region, the Frenkel-Poole emission was the main mechanism. It was also found that the H2 -plasma treatment of luminescent i-a-SiN:H layer of an ACW TFLED played an important role in improving device performances, such as decreased EL threshold voltage, increased brightness, and broadened and blue-shifted EL spectrum. The EL spectra of an ACW TFLED under either DC forward or reverse bias or the sinusoidal AC voltage were qualitatively very similar, with a peak wavelength at about 535 nm and a broad FWHM about 260 nm. Moreover, the EL intensity of an ACW TFLED increased with an AC frequency of up to 180 kHz and, then, decreased rapidly and became very weak as the frequency was up to about 500 kHz.

Alternating-Current White Thin-Film Light-Emitting Diodes Based on Hydrogenated Amorphous Carbon Layer

Alternating-current white thin-film light-emitting diodes (ACW-TFLEDs) have been fabricated and demonstrated with composition-graded hydrogenated intrinsic amorphous silicon carbide (i-a-SiC : H) layers. It was found that H2-plasma treatment of luminescent i-a-C : H layer played an important role in decreasing the ACW-TFLED electroluminescence (EL) threshold voltage, increasing the brightness, and broadening the EL spectrum. The EL spectra of the ACW-TFLED under either dc forward or reverse bias, or the sinusoidal alternating-current voltage were qualitatively very similar, with a peak wavelength at about 505 nm and a broad full-width at half maximum (FWHM) about 240 nm. This device revealed a brightness about 800(500)cd/m2 under dc forward (reverse) bias at an injection current density of 600 mA/cm2

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.

Hydrogenated Amorphous Silicon Carbide Alternating-Current Thin-Film Light-Emitting Diodes

Hydrogenated amorphous silicon carbide alternating-current thin-film light-emitting diodes (a-SiC:H ACTFLEDs) have been successfully fabricated on indium-tin-oxide-coated glass substrates and characterized. The electroluminescence (EL) spectra of an obtained ACTFLED under either a dc (positive or negative) bias or a sinusoidal ac voltage were qualitatively very similar, with a peak wavelength at about 655 nm and a shoulder at about 620 nm. The EL threshold voltage and brightness of the ACTFLEDs were sensitive to the contact behavior between the metal electrode employed and amorphous film. Moreover, the EL intensity of an ACTFLED increased with the frequency up to 500 kHz and then decreased rapidly and became very weak as the frequency increased to about 1 MHz. A model based on the equivalent circuit has also been proposed to explain this frequency-dependent EL behavior.