Compensation Pixel Circuit to Improve Image Quality for Mobile AMOLED Displays

• Published in: IEEE journal of solid-state circuits Vol. 54; no. 2; pp. 489 - 500
• Main Authors: Lin, Chih-Lung, Lai, Po-Chun, Shih, Li-Wei, Hung, Chia-Che, Lai, Po-Cheng, Lin, Tsu-Yuan, Liu, Kuang-Hsiang, Wang, Tsang-Hong
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.02.2019; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Active matrix displays; Active matrix organic light emitting diodes; Active matrix organic light-emitting diode (AMOLED); Circuits; Compensation; Diodes; Displays; Flicker; Image quality; low-temperature polycrystalline silicon (LTPS); Mobile handsets; Organic light emitting diodes; pixel circuit; Pixels; Programming; Semiconductor devices; Sensors; Silicon; Silicon films; Thin film transistors; thin-film transistor (TFT)
• ISSN: 0018-9200; 1558-173X
• DOI: 10.1109/JSSC.2018.2881922

This paper proposes a new pixel circuit using low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs) for use in mobile active matrix organic light-emitting diode (AMOLED) displays. The purpose of a pixel circuit is to supply stable and uniform currents to current-driving devices of OLEDs. The proposed pixel circuit uses overlapping compensation times to extend the period of precise sensing of <inline-formula> <tex-math notation="LaTeX">V_{\mathrm {TH}} </tex-math></inline-formula> variations of LTPS TFTs. Moreover, the proposed circuit prevents a current from flowing into an OLED when the OLED is not operated in the emission stage to avoid image flicker. An 1.3-in panel that uses the proposed pixel circuit is fabricated. Measurement results verify that the increased <inline-formula> <tex-math notation="LaTeX">T_{\mathrm {COMP}} </tex-math></inline-formula> of 33.3 <inline-formula> <tex-math notation="LaTeX">\mu \text{s} </tex-math></inline-formula> results in higher contrast ratio and lower required data voltages than those at <inline-formula> <tex-math notation="LaTeX">T_{\mathrm {COMP}} </tex-math></inline-formula> values of 4.3 and 2.2 <inline-formula> <tex-math notation="LaTeX">\mu \text{s} </tex-math></inline-formula>. Red, green, and blue images displayed by the fabricated panel with a <inline-formula> <tex-math notation="LaTeX">T_{\mathrm {COMP}} </tex-math></inline-formula> of 33.3 <inline-formula> <tex-math notation="LaTeX">\mu \text{s} </tex-math></inline-formula> with a 255 gray level are all much more uniform than with <inline-formula> <tex-math notation="LaTeX">T_{\mathrm {COMPS}} </tex-math></inline-formula> of 4.3 and 2.2 <inline-formula> <tex-math notation="LaTeX">\mu \text{s} </tex-math></inline-formula>. White images with a <inline-formula> <tex-math notation="LaTeX">T_{\mathrm {COMP}} </tex-math></inline-formula> of 33.3 <inline-formula> <tex-math notation="LaTeX">\mu \text{s} </tex-math></inline-formula> for 16 and 8 gray levels on the fabricated panel confirm that the serious line and spot defects in the images with <inline-formula> <tex-math notation="LaTeX">T_{\mathrm {COMP}} </tex-math></inline-formula> values of 4.3 and 2.2 <inline-formula> <tex-math notation="LaTeX">\mu \text{s} </tex-math></inline-formula> are effectively eliminated. Therefore, the proposed pixel circuit is suitable for use in mobile AMOLED displays.