| Summary: | 碩士 === 國立臺灣科技大學 === 光電工程研究所 === 102 === Recently, active matrix organic light-emitting diode (AMOLED) has attracted a much attention due to its extraordinary properties, such as wide viewing angle, fast response time, high contrast ratio, high color saturation and self-emissive ability. However, luminance of AMOLED decayed after long time operation to lead short lifetime of product. Recently, using the reverse bias to prolong OLED lifetime is investigated.
In the driving pixel circuit for AMOLED, low-temperature polycrystalline-silicon thin-film transistors (LTPS-TFTs) plays an important part due to its high current driving capability. Since the high current driving capability of LTPS-TFTs, the size of pixel circuit can be minimized to reach higher display quality. However, the various characteristic of LTPS-TFTs due to the excimer laser annealing process, which will further cause the non-uniform driving current under same gray-scale. Hence, many pixel circuit researches are proposed and this issue is effectively ameliorated. Thus, in this thesis, we want to design novel pixel circuits that not only compensate the non-uniformity of TFTs but also the OLED luminance degradation issue, and the high speed operation is also applied in the circuits.
Recently, Metal Oxide TFT (a-IGZO TFT) is paid much attention because a-IGZO TFT has the current driving capability between LTPS-TFT and a-Si TFT and has great uniformity to fabricate large size display on fabrication process. However, a-IGZO TFT has non-uniform of electrical characteristics under long time operation which will further cause the non-uniform driving current under same gray-scale. Hence, many pixel circuit researches are proposed and this issue is effectively ameliorated. Thus, in this thesis, we want to design novel pixel circuits that can compensate the non-uniformity of TFTs and the stereo 3D effect is also applied in the circuits. The proposed pixel circuit is also applied the reverse bias to ameliorate OLED lifetime.
Base on above reason, we proposed two compensating pixel circuits are driven by LTPS-TFTs and a-IGZO TFTs respectively and with high quality simultaneous emission (SE) driving scheme. The two proposed pixel circuits are 2.5T0.5C and 5T2C respectively. Through the parameter modulation, these two circuits achieved the performances what we expected and verified by AIM-SPICE.
The first proposed pixel circuit is composed of an upper of pixel and a lower of pixel. In the 2.5T0.5C pixel circuit, the high speed driving is verified by simulator. The current error rate under various threshold voltage of TFT(ΔVTH = ±0.33 V) is less than 4%. To verify by simulation results, the magnitude of reverse bias which is applied to OLED is determined by the magnitude of driving current in the 2.5T0.5C pixel circuit. Hence, the reverse bias application in the proposed pixel circuit can ameliorate the OLED lifetime effectively. Base on the mention above, the proposed pixel circuit not only offers a stable current to maintain the image uniformity under high speed driving but also prolongs the OLED lifetime effectively. Furthermore, we simulated an assumed 30 inch display with all resistance and capacitance of signal lines. Simulation results demonstrate that the current error rate of four corners of display is less than 1.2%.。
In the 5T2C pixel circuit, the high speed 3D driving is verified by simulator. The current error rate of this pixel circuit under various threshold voltage of TFT(ΔVTH = ±3 V) and OLED (ΔVTH = +0.33 V) is less than 11%. Furthermore, we utilize a signal line which controls the switch as a signal of reverse bias. Hence, the reverse bias application in the proposed pixel circuit can ameliorate the OLED lifetime effectively.
Due to the above simulation results, the capability of compensation is achieved as what we expected under high speed driving. Hence, we believed that the two proposed pixel circuits have excellent current driving capability to offer the stable current and prolong OLED lifetime effectively. It is suitable in large size and high resolution AMOLED displays.
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