Compensation Driving Methods of AMOLED Displays for Image Quality Improvement and Lifetime Extension

Abstract

능동매트릭스 유기발광다이오드 (AMOLED) 디스플레이는 최신과 미래디스플레이를 위해 전도 유망한 장치로서 연구되어 왔다. 발광다이오드 (OLED)를 구동하기 위한 능동 소자로서 주로 수소 처리된 비정질실리콘 (a-Si:H) 박막트랜지스터 (TFT), 산화물 TFT, 다결정실리콘 (poly-Si) TFT가 사용된다. 그러나 기존의 TFT들은 공간적인 문턱전압 편차가 발생하고 시간적인 문턴전압 변화가 생기는 문제가 있다. OLED는 색을 표현하는 광원으로서, 높은 색재현율, 얇게 제작 가능한 형태, 빠른 응답 속도 및 유연한 특성의 장점을 가지기 때문에 AMOLED 디스플레이에 사용된다. 하지만 사용 시간에 따라 OLED의 휘도가 감소하는 문제가 있다. 이 문제들은 고화질 장수명 AMOLED 디스플레이를 구현을 어렵게 만든다. 본 논문에서는 a-Si:H TFT를 이용하는 AMOLED 디스플레이의 수명을 연장하는 방법이 포함된 실시간 외부보상 방법을 제안한다. 이 방법의 주된 아이디어는 TFT의 문턱전압 편차를 외부적으로 센싱하고 저장한 후, 저장된 정보를 사용하여 입력 이미지를 변조하는 방식으로 보상을 수행한다. 또한 음의 바이어스를 절반의 프레임 시간 동안 TFT에 인가하여 TFT의 사용시간을 늘린다. 이를 검증하기 위하여 30인치 1920x1080 해상도의 AMOLED 디스플레이의 테스트 패턴을 사용하였고, 제안된 방식을 사용하여 휘도 오차를 -14.3 ~ 21.5% 로부터 -1.1 ~ 1.7% 로 저감하였다. Poly-Si TFT를 사용하는 AMOLED 디스플레이에 있어서 화소의 전기광학적 특성을 보상하기 위하여, 광학계를 사용하되 최적의 메모리을 사용하는 휘도 보상 방식을 제안한다. 계조 단계와 휘도의 관계와 연관되는 새로운 모델을 제안하고, 제안된 모델을 바탕으로 최소한의 메모리를 사용할 수 있는 보상 알고리즘을 제안한다. 제안된 방식의 아이디어는 계조 단계에 따른 휘도를 광학계를 이용하여 측정하고, 측정된 휘도로부터 특징 파라미터를 추출하는 것이다. 추출된 파라미터를 이용하여 데이터를 변조 함으로서 휘도 균일도는 향상 될 수 있다. 제안된 방식을 검증하기 위하여 350 cd/m2를 최대 휘도로 갖는 40인치 1920×1080 AMOLED 디스플레이 패널을 사용하였다. 제안된 방식은 적색, 녹색, 청색의 휘도 오차를 각각 ± 38.64%, ± 36.32%, ± 43.12% 에서 ± 2.68%, ± 2.64%, ± 2.76%로 감소 시켰다. 이와 같은 높은 균일도를 달성하기 위한 최적화된 파라미터 요구 비트 수는 6-bit이고, 필요로하는 전제 메모리 양은 74.6 Mbit 이다. Poly-Si TFT 백플레인을 사용하는 AMOLED 디스플레이에 있어서 OLED 열화를 보상하는 방법을 제안하였다. 시간에 대한 OLED 열화에 의한 휘도 감소를 보상하기 위하여 수정된 stretched exponential 모델을 제안하여, 그 결과 OLED 패널의 수명을 연장 할 수 있다. AMOLED 패널은 poly-Si TFT를 사용하여 제작하고, 제안된 방식을 검증하기 위해 측정을 진행 하였다. 초기 350 cd/m2의 휘도를 사용하여 40시간동안 구동하여 측정 하였으며, 제안된 방식을 사용하지 않은 패널과 제안된 방식을 사용한 패널의 적색, 녹색, 청색의 각각의 열화는 각각 0.3% 와 6.0%, 4.0% 와 17.8%, 7.4% 와 30.4%로 측정 되었다. 이에 따라 제안된 보상 방식은 기존의 방식에 대비하여 적색, 녹색, 청색의 각각의 색깔 별로 수명을 72.5, 15.5, 20.75 배 향상 시켰다. 이에 따라 제안된 방식들은 여러 AMOLED 디스플레이 응용분야에 적용 가능하고, 고화질 상 수명의 AMOLED 디스플레이에 적용 가능 할 것으로 기대 된다. |Active matrix organic light-emitting diode (AMOLED) displays have been spotlighted as a promising device for the high-end and future display applications. As the active devices, thin film transistors (TFTs) such as hydrogenated amorphous silicon (a-Si:H) TFTs, oxide TFTs, and poly-Si TFTs have been widely used to drive an organic light-emitting diode (OLED). However, the developed TFTs have several problems of spatial threshold voltage variations and temporal threshold voltage shifts. The OLED is used as a colored light source due to its high color reproducibility, thin form factor, fast response time, and high flexibility for AMOLED displays. However, the OLED luminance is degraded as the operation time goes by, thereby having difficulties in achieving high image quality and long lifetime AMOLED displays. This dissertation proposes various compensation methods for high image quality and long lifetime AMOLED displays. First, a real-time external compensation method with lifetime extension is proposed for AMOLED displays using a-Si:H TFTs. The proposed method compensates for threshold voltage (Vth) shifts by externally sensing and storing the Vth of TFTs, and then modulating input images using the stored Vth. Also, a negative bias is applied to TFTs for a half frame time to extend the lifetime of TFTs. To verify the proposed compensation method, a test pattern of the AMOLED pixels for 30-inch 1920×1080 AMOLED panel using a-Si:H TFT is fabricated. The measurement results show that the luminance error range is reduced from -14.3 ~ 21.5% to -1.1 ~ 1.7% with the use of the proposed compensation method. Second, a luminance compensation method using optical sensors with the optimized memory size is proposed to compensate for the optoelectrical characteristics of the pixel for AMOLED displays using poly-Si TFTs. A new luminance model of the pixel having a relationship between the gray level and luminance of the pixel is proposed, and in addition, a compensation algorithm using a minimized memory is proposed. The proposed luminance compensation method measures the luminance according to gray level using optical sensors and then extracts the characterization parameters from the measured luminance. The luminance uniformity is improved by modulating input image using the extracted parameters. To verify the proposed compensation method, a 40-inch 1920×1080 AMOLED panel with a maximum luminance of 350 cd/m2 is used. The measurement results show that the proposed compensation method reduces a luminance error from ± 38.64%, ± 36.32%, and ± 43.12% to ± 2.68%, ± 2.64%, and ± 2.76% for red, green, and blue colors, respectively. The optimized bit depth of each parameter is 6-bit and the total required memory to achieve high luminance uniformity is 74.6 Mbits. Finally, a compensation method for OLED degradation in AMOLED displays using poly-Si TFT backplane is proposed. A modified stretched exponential decay (SED) model, which is employed to characterize the OLED degradation according to time, compensates for the associated luminance decrease; thereby the lifetime of an OLED panel can be extended. The AMOLED panel is fabricated using poly-Si TFTs and measured to verify the proposed compensation method. The measurement results show that the luminance degradation with and without using the proposed method is 0.3% and 6.0%, 4.0% and 17.8%, and 7.4% and 30.4%, for red, green, and blue OLEDs, respectively, after 40 hours of operation under a 350 cd/m2 initial luminance. Thus, the proposed compensation method extends the lifetime of the AMOLED panel up to 72.5, 15.5, and 20.75 times longer for the red, green, and blue OLEDs, respectively, compared to the conventional methods. Therefore, it is expected that the proposed methods can be applied to various AMOLED display applications that require high image quality and long lifetime.; Active matrix organic light-emitting diode (AMOLED) displays have been spotlighted as a promising device for the high-end and future display applications. As the active devices, thin film transistors (TFTs) such as hydrogenated amorphous silicon (a-Si:H) TFTs, oxide TFTs, and poly-Si TFTs have been widely used to drive an organic light-emitting diode (OLED). However, the developed TFTs have several problems of spatial threshold voltage variations and temporal threshold voltage shifts. The OLED is used as a colored light source due to its high color reproducibility, thin form factor, fast response time, and high flexibility for AMOLED displays. However, the OLED luminance is degraded as the operation time goes by, thereby having difficulties in achieving high image quality and long lifetime AMOLED displays. This dissertation proposes various compensation methods for high image quality and long lifetime AMOLED displays. First, a real-time external compensation method with lifetime extension is proposed for AMOLED displays using a-Si:H TFTs. The proposed method compensates for threshold voltage (Vth) shifts by externally sensing and storing the Vth of TFTs, and then modulating input images using the stored Vth. Also, a negative bias is applied to TFTs for a half frame time to extend the lifetime of TFTs. To verify the proposed compensation method, a test pattern of the AMOLED pixels for 30-inch 1920×1080 AMOLED panel using a-Si:H TFT is fabricated. The measurement results show that the luminance error range is reduced from -14.3 ~ 21.5% to -1.1 ~ 1.7% with the use of the proposed compensation method. Second, a luminance compensation method using optical sensors with the optimized memory size is proposed to compensate for the optoelectrical characteristics of the pixel for AMOLED displays using poly-Si TFTs. A new luminance model of the pixel having a relationship between the gray level and luminance of the pixel is proposed, and in addition, a compensation algorithm using a minimized memory is proposed. The proposed luminance compensation method measures the luminance according to gray level using optical sensors and then extracts the characterization parameters from the measured luminance. The luminance uniformity is improved by modulating input image using the extracted parameters. To verify the proposed compensation method, a 40-inch 1920×1080 AMOLED panel with a maximum luminance of 350 cd/m2 is used. The measurement results show that the proposed compensation method reduces a luminance error from ± 38.64%, ± 36.32%, and ± 43.12% to ± 2.68%, ± 2.64%, and ± 2.76% for red, green, and blue colors, respectively. The optimized bit depth of each parameter is 6-bit and the total required memory to achieve high luminance uniformity is 74.6 Mbits. Finally, a compensation method for OLED degradation in AMOLED displays using poly-Si TFT backplane is proposed. A modified stretched exponential decay (SED) model, which is employed to characterize the OLED degradation according to time, compensates for the associated luminance decrease