OLED Driving TFT Threshold Voltage Compensation Circuit
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Solution Overview
Problem
Organic light emitting devices (OLEDs) with active matrix structures face reduced brightness and durability due to increased threshold voltage of driving TFTs, which affects the output current and overall display performance.
Innovation Solution
Incorporating a threshold voltage compensator and a threshold voltage restorer between the gate and drain of the driving TFT, connected to separate scan lines, to store and compensate for the threshold voltage, ensuring consistent output current and brightness regardless of TFT degradation, along with a storage capacitor to maintain data signals and gate voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a driving TFT is used to supply output current to the OLED, then the device can be driven with lower voltage and faster response, but the threshold voltage of the TFT increases over time causing brightness degradation
Solution Approach 1:
The storage capacitor stores the gate voltage of the driving TFT in advance before degradation occurs. By pre-storing the initial threshold voltage, the system can compensate for subsequent threshold voltage increases without requiring continuous monitoring or adjustment, thus maintaining brightness despite TFT degradation over time.
Solution Approach 2:
The circuit uses the stored gate voltage from the storage capacitor as feedback to compensate for threshold voltage changes in the driving TFT. By feeding back the initial threshold voltage condition, the system can maintain stable operation even as the TFT degrades, effectively counteracting the brightness reduction caused by threshold voltage increase.
2Reliability
If the threshold voltage of the driving TFT increases, then the output current decreases, but this reduces the brightness of the OLED
Solution Approach 1:
The storage capacitor stores the gate voltage in advance before the TFT degrades. This preliminary storage of the initial threshold voltage condition allows the system to compensate for subsequent degradation effects, maintaining the output current and brightness despite the TFT's threshold voltage increasing over time and reducing durability.
Solution Approach 2:
The circuit compensates for threshold voltage changes by using the stored gate voltage to adjust the operating conditions. By changing the gate voltage parameter based on the stored initial value, the system counteracts the threshold voltage increase and maintains stable output current and brightness, thereby preserving display quality despite TFT degradation.
3Illumination intensity
If a storage capacitor is added to maintain gate voltage, then threshold voltage compensation is achieved, but device complexity increases
Solution Approach 1:
The storage capacitor is merged with the existing gate voltage line of the driving TFT, sharing the same node and utilizing the existing circuit infrastructure. This integration approach allows threshold voltage compensation without adding separate independent circuits, thereby minimizing the increase in device complexity while achieving brightness stability.
Solution Approach 2:
The storage capacitor serves multiple functions: it stores the gate voltage for the driving TFT, provides threshold voltage compensation, and maintains brightness stability. By making this single component multi-functional, the patent reduces the need for additional separate circuits, thereby limiting the increase in device complexity while achieving the desired brightness stability.
Data Source
AI summary
Provided is a light emitting device. Particularly, the light emitting device comprises a threshold voltage compensator. The threshold voltage compensator is connected between a gate and a drain of the driving TFT and has a gate connected to a second scan line to temporarily store at the storage capacitor a gate voltage reflecting a threshold voltage of the driving TFT in response to a second scan signal supplied by a second scan line and to transmit the data signal regardless of variations in the threshold voltage of the driving TFT when the output current is supplied to the light emitting diode.