Electroluminescent Devices Based On Polycrystalline Silicon Films For Large-Area Applications

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Measurements The current-voltage (I-V) characteristics, EL intensity and spectra, and the dynamic response of EL, including their temperature dependence in the range of 300 to 10 K, were measured in relation to the PL characteristics. The experimental results are compared with those obtained from the PS diodes fabricated on single crystalline silicon substrates. Regarding the PL characteristics, the decay time under pulsed photoexcitation by an N 2 laser, retention degree of linear polarization of excitation light, and electric-field PL quenching were investigated. As a preliminary experiment, combination of PPS diodes and poly-Si thin film transistor (TFT) drivers is also examined. RESULTS AND DISCUSSION Fundamental EL characteristics As shown in Fig. 2, PS diodes exhibit a definite rectifying behavior at room temperature, and

uniform visible EL emission was observed at forward and reverse bias voltages of about 5 V and 40 V, respectively. The forward biased condition in this case corresponds to the situation that a negative voltage is applied to the top contact with respect to the substrate, in which electrons and holes are injected into the PPS layer from the top contact and the substrate, respectively. At a forward bias voltage of 15 V, the EL emission was discernible even in the daylight. The corresponding EL efficiency is about 0.01%. This is comparable to the efficiency of our conventional devices with a single PS layer [2]. The EL intensity at sufficient injection level increases in proportion to the driving current. A typical photograph of light emission is shown in Fig. 3. Stability of the PPS devices was considerably higher than that of the PS devices. This is presumably due to stabilized electrical conduction in the active PPS layer. The EL stability was significantly enhanced in the ITO-contacted devices rather than in the Au-contacted ones.

5

5

42 3 c z

2

0 Z 1 H

22zZ 1-Z

zr

Il

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-1

-40

-20 0 VOLTAGE (V)

20

Fig.2. Current-voltage curve of a PPS diode and the corresponding EL intensity.

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Fig.3. Photograph of uniform light emission from a PPS-LED. Forward bias voltage applied in this case is 15 V.

Figure 4 shows the observed EL spectrum in comnparison to the corresponding PL spectrum. The fundamental features of these spectra coincide with each other. Also, the EL spectrum of PPS diodes are similar to those of our conventional PS diodes [2]. It appears that the light emission originates from the common recombination process. According to the results of the dynamic response, the PPS-LED operates up to frequencies of about 50 kHz. It is comparable to the maximum frequency of our conventional PS-LEDs.

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0.8 0.6

CO) z

0.4

z

0.2

w

0 400

500 600 700 800 WAVELENGTH (nm)

900

Fig.4. Emission spectrum (solid curve) of a PPS-LED. The PL spectrum of the PPS layer is also shown by the dashed curve for comparison.

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To investigate the EL emission characteristics in more detail, the temperature dependence of the I-V curve and the corresponding EL intensity were measured. The