1. Field of the Invention
The invention generally relates to a pixel structure. More particularly, the invention relates to the pixel structure of an active matrix organic electro-luminescent display.
2. Description of Related Art
Information technology has become a mainstream industry in our society, thanks to the development of various portable communication and display products. Because the display panel is an indispensable communication interface for man to acquire information, its development is particularly important. Among the display panels, organic electro-luminescent display (OELD) has the greatest potential to become the major display product in the next generation, with the advantages including self-illuminating screen, wide viewing angle, low power consumption, simple manufacturing process, low cost, a wide operating temperature range, a high response speed and full-color display.
The organic electro-luminescent display (OELD) utilizes the self-illuminating property of the organic light-emitting material to display an image. According to the molecular weight of the organic light-emitting material, the OELD panel can be classified into small molecule organic electro-luminescent display (SM-OELD) and polymer electro-luminescent display (PELD). The light-emitting structure of both types of OELD comprises a pair of electrodes and an organic material layer sandwiched between the two electrodes. When a DC voltage is applied to the electrodes, holes are injected from the anode into the organic light-emitting material layer while electrons are injected from the cathode into the organic light-emitting material layer. Due to the electric potential difference produced by an external electric field, hole and electron carriers moving inside the organic light-emitting material layer may collide and trigger radiative recombination. A portion of the energy released by the recombination of the electron and hole may excite the organic light-emitting molecules into an excited state. When the excited molecule releases its energy and returns to a ground state, a definite portion of the energy is released as photons to emit light. Accordingly, the organic electro-luminescent display (OELD) panel emits light following this principle.
FIG. 1A is a cross-sectional view showing a pixel structure of a conventional active matrix-type OLED. FIG. 1B shows a relationship between the wavelength and transmittance of the light-emitting layer shown in FIG. 1A. Please refer to FIG. 1A, the conventional pixel structure 100 is controlled by a scan line and a data line arranged on a substrate 110. The pixel structure 100 comprises a plurality of amorphous silicon thin film transistors (a-Si TFT) 120a and 120b, a plurality of dielectric layers 130 and 140, an organic electro-luminescent unit 150 and a pixel define layer 160. The a-Si TFT 120a and 120b are arranged on the substrate 110 and are electrically connected with each other, in order to control the organic electro-luminescent unit 150. The dielectric layer 140 made of silicon nitride is arranged over the a-Si TFT 120a and 120b. Besides, the organic electro-luminescent unit 150 and the pixel define layer 160 are arranged on the dielectric layer 140 respectively.
More specifically, the a-Si TFT 120a and 120b comprise gate electrodes 122a and 122b, channel layers 124a and 124b, source/drain electrodes 126a and 126b, respectively. The gate electrodes 122a and 122b are arranged on the substrate 110. The dielectric layer 130 arranged on the substrate 110 covers the gate electrodes 122a and 122b. The channel layers 124a and 124b are disposed on the dielectric layer 130. The source/drain electrodes 126a and 126b are arranged on the channel layers 124a and 124b, respectively. In addition, the organic electro-luminescent unit 150 comprises a transparent electrode 152, a light-emitting layer 154 and a metal electrode 156. The transparent electrode 152 is electrically connected to the a-Si TFT 120b. The light-emitting layer 154 and the metal electrode 156 are sequentially arranged on the transparent electrode 152.
As shown in FIGS. 1A and 1B, a light emitted from the light-emitting layer 154 travels through the transparent electrode 152, the dielectric layer 130 and 140 and the substrate 110 toward the outside of the OLED. As shown in FIG. 1B, the average transmittance of red light (640 nm), green light (515 nm) and blue light (470 nm) in the conventional pixel structure is about 97%. So, the dielectric layers 130 and 140 have little influence on the light emitted from the organic electro-luminescent unit 150. In other words, the display quality of the active matrix-type OLED having the conventional pixel structure 100 is restricted by the characteristics of the organic electro-luminescent unit 150. Compared with the a-Si TFT 120a and 120b, a low temperature poly silicon (LTPS) TFT has the advantages of higher carrier mobility and lower operation voltage, thus another pixel structure applied in the active matrix-type OLED has been developed.
FIG. 2A is a cross-sectional view showing another pixel structure applied in a conventional active matrix-type OLED. FIG. 2B shows the relationship between the wavelength and transmittance of the emitting layer shown in FIG. 2A. Please refer to FIG. 2A, the pixel structure 200 is controlled by a scan line and a data line arranged on a substrate 210. The conventional pixel structure 200 comprises a plurality of LTPS thin film transistors 230a and 230b, a plurality of dielectric layers 220, 240, 250 and 260, an organic electro-luminescent unit 150 and a pixel define layer 160. The LTPS thin film transistors 230a and 230b are electrically connected with each other. The organic electro-luminescent unit 150 is controlled by the LTPS thin film transistors 230a and 230b. The dielectric layer 220 is arranged on the substrate 210. Then, the LTPS thin film transistors 230a and 230b are arranged on the dielectric layer 220. Besides, the dielectric layer 260 is over the LTPS thin film transistors 230a and 230b. The organic electro-luminescent unit 150 and the pixel define layer 160 are arranged on the dielectric layer 260 respectively.
More specifically, the LTPS thin film transistors 230a and 230b comprise gate electrodes 232a and 232b, channel layers 234a and 234b, source/drain electrodes 236a and 236b respectively. The channel layers 234a and 234b are disposed on the dielectric layer 220. The dielectric layer 220 could prevent the metal ions inside the substrate 210 from diffusing to the channel layers 234a and 234b. The dielectric layer 240 is disposed over the channel layers 234a and 234b, and the source/drain electrodes 236a and 236b are arranged above the channel layers 234a and 234b respectively. It should be noted that the gate electrodes 232a and 232b are arranged on the dielectric layer 240, and the LTPS thin film transistors 230a and 230b are both dual-gate thin film transistor structures. But the LTPS thin film transistors 230a and 230b can also be single gate thin film transistor structures. The dielectric layer 250 is arranged over the gate electrodes 232a and 232b. The dielectric layer 260 is arranged on the dielectric layer 250.
As shown in FIGS. 2A and 2B, a light emitted from the light-emitting layer 154 travels through the transparent electrode 152, the dielectric layer 260, 250, 240 and 220, and the substrate 210 toward the outside of the OLED. The dielectric layers 260 and 250 are made of silicon nitride, and the dielectric layer 240 is made of silicon dioxide. The material of dielectric layer 240 can be silicon dioxide/silicon nitride. As shown in FIG. 2B, the transmittance of a specific light (500 nm) and a red light (640 nm) with respect to the conventional pixel structure 200 are about 96% and 83% respectively. That is, the luminance characteristics of the organic electro-luminescent unit 150 will be affected by the dielectric layers 240 and 220.