Pixel Circuit Threshold Voltage Compensation for OLED Displays
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Solution Overview
Problem
In organic EL display devices, variations in the I-V characteristic of the organic EL element and the threshold voltage or mobility of the drive transistor over time lead to non-uniform emission luminance and accelerated reduction in emission current, degrading reliability due to lengthened write times caused by variations in the sampling transistor's threshold voltage.
Innovation Solution
A display device with a pixel circuit comprising a first transistor, a second transistor, and a holding capacitor, where the ON-period of the first transistor is controlled to counteract variations in threshold voltage, using specific voltage levels and timing to maintain the photo-emission element in an extinction state, thereby suppressing the acceleration of temporal reduction in light emission current.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a sampling transistor is provided in the pixel circuit to compensate for threshold voltage variations, then emission luminance uniformity is improved, but write time is lengthened due to minus bias voltage application, accelerating temporal reduction in emission current
Solution Approach 1:
The patent applies a minus bias voltage to the sampling transistor in advance during non-display periods to pre-compensate for threshold voltage variations. This preliminary action stabilizes the transistor's characteristics before the actual display operation, ensuring uniform emission luminance without affecting write time during active display periods.
Solution Approach 2:
The patent implements periodic application of minus bias voltage to the sampling transistor during specific non-display periods (such as horizontal blanking intervals). This periodic action allows the transistor characteristics to be stabilized at regular intervals, maintaining emission luminance uniformity while minimizing impact on write time.
2Stability of the object's composition
If the ON-period of the first transistor is extended to counteract threshold voltage variations, then emission luminance stability is improved, but temporal reduction in light emission current is accelerated
Solution Approach 1:
The patent carefully controls the ON-period duration of the first transistor as a critical parameter. By optimizing this time parameter, the patent achieves sufficient charge transfer to counteract threshold voltage variations and stabilize emission luminance, while preventing excessive ON-period that would accelerate temporal reduction in light emission current.
Solution Approach 2:
The patent incorporates compensation mechanisms that respond to threshold voltage variations in the sampling transistor. By monitoring and compensating for these variations, the system maintains emission luminance stability without requiring excessive extension of the first transistor's ON-period, thereby preserving long-term reliability.
3Manufacturing precision
If compensation functions are added to the pixel circuit for I-V characteristic variations, then emission luminance uniformity is improved, but device complexity increases
Solution Approach 1:
The patent designs the sampling transistor to serve multiple functions: it acts as a switch for data signal writing, a compensation element for threshold voltage variations, and a control element for emission luminance uniformity. This multi-functionality reduces the need for separate compensation circuits, thereby limiting the increase in device complexity.
Solution Approach 2:
The patent combines the compensation function with the existing sampling transistor structure rather than adding separate compensation circuits. By merging the threshold voltage compensation function into the sampling transistor's operation, the patent achieves emission luminance uniformity while minimizing additional circuit complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach suppresses the acceleration of temporal reduction in light emission current and improves reliability by stabilizing the threshold voltage of the transistor, ensuring consistent emission luminance and extending the lifespan of the display device.
Implementation Method 1
a pixel circuit having a first transistor, a second transistor and a holding capacitor
Implementation Method 2
a current-drive optical element, for example, an organic EL (Electro Luminescence) element, as a photo emission element of a pixel, emission luminance of the optical element varying depending on a current value
Data Source
AI summary
A display device is provided having improved reliability compared with the related art. The display device includes, for each pixel: a photo-emission element and a first MOS transistor connected in series between a first power source line and a second power source line; a capacitor connected to be inserted between a gate and a source of the first MOS transistor; and a second MOS transistor connected to be inserted between a signal line to be applied with a image signal voltage and the gate of the first MOS transistor, the second MOS transistor being controlled by a scan signal to change between ON-state and OFF-state, wherein ON-period of the first transistor is established within a period in which the photo-emission element is maintained to an extinction state and the signal line is applied with a voltage having a fixed level independent from the image signal voltage.