Result

In order to confirm the stability of the luminance and color performance in AC-PDP, the luminance variation of the conventional 46-inch panel was investigated. A full white 255 gray level image driving was applied on the panel after the power turns on.

As one can see in Fig. 5.13, the luminance decay as a function of time and tend to remain stable 70 minutes later. It is believed that this phenomenon is related with the influence of temperature[4,6]. As the temperature of the phosphor arises, the emission efficiency from phosphor goes down. This phenomenon is so called temperature quenching effect of the phosphor. The higher surface temperature of the phosphor results in the higher probability that luminance center will get through

non-luminescent process with its absorbing energy dissipated in the form of heat[13].

Fig. 5.13 Luminance degradation as a function of sustain time

Fig. 5.14 shows the luminance degradation as a function of time with different cells.

Patterns with full red, green and blue color are applied on the panel after the power turns on. The luminance of each color also decays with time but remain stable after 70 min. It is noticed that the degree of luminance degradation is different with color, which is 4.7%, 4.8%, 4.4% for red, green and blue cells, respectively. The disparity among phosphors may result in color variation for AC-PDP. For this reason, the color temperature is recorded as a function of sustain time, as shown in Fig. 5.14d. It reveals that the color temperature also varies as a function of time. This phenomenon consists with the experimental results as Fig. 5.14a, 5.14b and 5.14c shown above.

Since color performance corresponds to the properties of phosphors, it fits in with previous study that white image sticking phenomenon is to be caused by lower phosphor efficiency resulting from the higher temperature at phosphors[4].

0 20 40 60 80 100

Fig. 5.14(a) Luminance degradation of the red cells as a function of sustain time

0 20 40 60 80 100

Fig. 5.14 (b) Luminance degradation of the green cells as a function of sustain time

0 20 40 60 80 100

Fig. 5.14 (c) Luminance degradation of the blue cells as a function of sustain time

0 20 40 60 80 100 120

Fig. 5.14 (d) Color temperature variation as a function of full white display time

In order to investigate the degree of image sticking phenomenon, the recovery ability of the panels is needed to be introduced. Recovery ability is based on the time needed to recover from the image sticking condition to normal condition, which is, from the abnormal luminance or color performance to regular performance.

In this study, all data of recovery ability were measured after warming up the panels by full white displayed for 80 min to completely eliminate the effect of temperature

arising after power turns on, as previous section mentioned. Then a square-shaped white image was displayed for a period of time. After that, the pattern was changed to full area white displayed again to characterize the white image sticking phenomenon (WIS).

Fig. 5.15 shows the luminance of plasma display panel for different display times of square-shaped white image. The display pattern is the same as Fig. 5.12 (a) shows.

The X axis in this figure means the display time of the pattern. After a period of display time, the display pattern changes to full white background and luminance degradation was measure at the position of square pattern, as Fig 5.12 (b) shows. The luminance at 0 time indicates the initial luminance of the panel. It reveals that the luminance difference increases on the increase of display time of square-shaped pattern. However it is relatively stable when the display time extends more than 10 minutes. The result indicates that temporal white image sticking phenomenon tend to saturate after 10 minutes successive static image displayed. It may caused by static surface temperature of phosphorous layer after long time display of the pattern area.

Even though temporal white image sticking doesn’t get worse after more than 10 minutes display, much longer pattern sustain time may cause permanent damage to the MgO films and phosphorous layer due to the strong iterant sustain pulse. It causes severer degradation for the pattern area and makes us hard to identify the recovery ability of the temporal image sticking phenomenon.

The recovery ability of the panels with different gas concentration and total pressure is shown in Fig. 5.16. In this case, the white pattern was displayed for 30 sec, 1 min, 2 min, 5 min and 10 min in the center of the panels. After that, the pattern was changed to full white background again to characterize the white image sticking phenomenon (WIS). The luminance variation of the image sticking cells in the square pattern area was recorded as a function of time. In the center of the panel, the luminance at 80 minutes warming up is considered as the standard value to compare with the values of the image sticking cells after square pattern displayed. An area near but outside the square pattern was selected to be the reference point also. However, the luminance of the area outside the image sticking area almost didn’t change during the experiment. Each case was repeated multiple times to eliminate the experimental errors.

0 20 40 60 80 100 120

Fig. 5.15 Luminance degradation of the PDP after different pattern sustain time

As shown in Fig. 5.16, it proves that the PDP module with the conventional parameters (6% Xe gas ratio, 475 torr total inert gas pressure) exhibits superior recovery ability compared with the panels which include higher total pressure (No.2, 6% Xe gas ratio, 550 torr total pressure) or higher Xe partial pressure (No.3, 9% Xe gas ratio, 475 torr total pressure). For all cases in Fig. 5.16, the panel with conventional parameter (6% Xe, 475 torr total inert gas pressure) has less luminance difference than the other panels right after the pattern time, and recover to normal luminance in 2 to 5 min (slightly shorter than the others). For cases of 2min and 5min recovery time, the panel No.2 with 6% Xe gas concentration and 550 torr total gas pressure has higher luminance difference and longer recovery time than the panel No.3 with 9% Xe gas concentration and 475 torr total gas pressure. For case of 30 sec and 1min, the No.2 panel only has the longer recovery time than that of the No.3 panel. Furthermore, in all of cases the panel No.4 with higher Xe partial pressure (9%) and higher total pressure (6%) has the worst recovery ability, including the initial higher luminance difference and longer recovery time. Therefore, the higher Xe gas concentration and higher total gas pressure result in higher degree of white image sticking phenomenon. Furthermore, the influence of 550 torr is larger than that of 9%

Xe partial pressure. Compared with the advantages of higher luminance and luminous efficiency at higher Xe partial pressure and total pressure, it revealed worse white image sticking characteristic, slightly.

It is not the result what we expected. The possible reason is discussed here. The high Xe partial pressure and high total gas pressure result in higher Xe excitation energy and higher electron heating efficiency, which make stronger VUV (Vacuum Ultra Violet) generation. Strong VUV achieves high luminance and high luminous efficiency of PDP. However, strong VUV also leads to high cell temperature. For conventional ac-PDP, the phosphor absorbs VUV and converted the energy into visible light emission. The main peak of VUV absorbed by phosphor is at 147 nm, 150 nm and 173 nm while the visible light is about 400 nm ~ 700 nm according to the colors emitted by phosphors. Therefore, the phosphor conversion efficiency is about 25% due to the wavelength difference of the VUV and visible light. The 75% energy is dissipated in the phosphorous layer as heat and results in cell temperature increase.

Due to the temperature quenching effect of the phosphor, the phosphor efficiency decreases with the increased temperature. The temperature difference between the image sticking cell and non image sticking cell produces luminance difference between them. Therefore white image sticking occurs. That is why high Xe partial pressure and high total gas pressure make high luminance and high luminous efficiency but cause worse image sticking characteristic.

-2 0 2 4 6 8 10 12

Fig. 5.16 (a) Recovery ability of the panels with 30 sec pattern sustain time

-2 0 2 4 6 8 10 12

Fig. 5.16 (b) Recovery ability of the panels with 1 min pattern sustain time

-2 0 2 4 6 8 10 12

Fig. 5.16 (c) Recovery ability of the panels with 2 min pattern sustain time

-2 0 2 4 6 8 10 12

Fig. 5.16 (d) Recovery ability of the panels with 5 min pattern sustain time

-2 0 2 4 6 8 10 12

Fig. 5.16 (e) Recovery ability of the panels with 10 min pattern sustain time

5.3.3 Summary

In this work, an image sticking observation of 46 inch WVGA AC-PDP was performed. The luminance degradation related to white image sticking phenomenon was investigated. The white image sticking phenomenon become more serious as a

function of Xe concentration (6%, 9%), and total pressure(475 torr, 550 torr). The effect of high total pressure (550 torr) is slightly more critical than that of high Xe partial pressure (9%).

Chapter 6 Conclusion

In this study, the degradation of MgO thin films and image sticking phenomenon were investigated. For the first part, several lifetime tests were proposed for MgO thin films in an ac-PDP. After long time operation, the driving margin area of the panel decreased. This is due to the degradation of MgO thin film by the ion bombardment in plasma. From the result, we found the panels with red phosphor have an obvious degradation. However, the Panels with green and blue phosphor only varied slightly.

The driving margin is directly related to properties of MgO thin films. Therefore, we demonstrated that the degraded margin area is caused by contaminations in the MgO thin films by red phosphor.

In order to find out the contaminations in the MgO thin films, an accelerated lifetime test was performed for a conventional surface discharge type AC-PDP and a vertical discharge type one. The surface morphology and contaminations in the MgO thin films were investigated. By means of XPS, we confirmed that the element Yttrium, which is one of the main elements in the red phosphor, is contaminated in the MgO thin films of the vertical discharge type panel. This result consists with the result mentioned in previous section. We believe that the degradation of the MgO thin films in AC-PDP was partially caused by contaminations from phosphor. Even though the surface discharge type has stronger erosion on the MgO thin film because of long time discharge (16000 hr), there is no contaminations from the phosphor. It is difficult to detect the tiny amounts of the impurity and another method needs to be tried. The change of discharge characteristic in the MgO thin films can be an important property which needs to be studied.

In the second part, an image sticking observation of 46 inch WVGA AC-PDP was performed. The luminance degradation related to white image sticking phenomenon was investigated. The white image sticking phenomenon become more serious as a function of Xe concentration (6%, 9%), and total pressure(475 torr, 550 torr). The result may be due to the temperature difference between the image sticking cell and no image sticking cell caused by the strong VUV. For future work, it is essential to investigate the influence of higher temperature caused by inert gas setup with different driving voltage.

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