Pixel Driving Circuit for OLED Brightness Uniformity
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Solution Overview
Problem
OLED displays face issues with uneven brightness due to non-uniformities in threshold voltage and mobility of low temperature polysilicon transistors, leading to the mura phenomenon and IR Drop, which affect display quality.
Innovation Solution
A pixel driving circuit with a driving sub-circuit, data writing sub-circuit, and light emitting controlling sub-circuit, utilizing transistors and a storage capacitor to control the light emitting element, where the light emitting current is determined by the threshold voltage of the OLED and the data signal, independent of the driving transistor's threshold voltage and power supply voltage drop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If low temperature polysilicon transistors are used in OLED display pixel circuits, then the display can be manufactured with lower cost and simpler process, but non-uniformities in threshold voltage and mobility occur leading to mura phenomenon and IR Drop
Solution Approach 1:
The pixel circuit is divided into multiple functional sub-circuits: a driving sub-circuit containing the low temperature polysilicon transistor for current driving, a data writing sub-circuit for voltage writing, and a light emitting controlling sub-circuit for independent light emitting control. This segmentation allows different parts of the circuit to have optimized functions, with the data writing sub-circuit handling threshold voltage compensation and the light emitting controlling sub-circuit ensuring uniform light output despite transistor variations.
Solution Approach 2:
A storage capacitor is introduced as an intermediary element between the data writing sub-circuit and the driving sub-circuit. This capacitor stores the threshold voltage of the low temperature polysilicon transistor and compensates for its non-uniformities. By using this intermediary storage element, the circuit decouples the direct relationship between transistor parameter variations and light output, thereby reducing the mura phenomenon while maintaining the simplicity of low temperature polysilicon manufacturing.
2Device complexity
If conventional pixel circuits are used where light emitting current depends on driving transistor threshold voltage, then circuit design is simpler, but IR Drop and uneven brightness occur affecting display quality
Solution Approach 1:
The light emitting controlling sub-circuit implements a feedback mechanism that monitors and adjusts the light emitting process independently of the driving transistor's threshold voltage. By using the storage capacitor to store threshold voltage information and the light emitting controlling sub-circuit to regulate current based on stored reference voltages rather than real-time transistor characteristics, the system creates a feedback loop that compensates for threshold voltage drift and IR Drop, ensuring uniform brightness across the display.
Solution Approach 2:
The data writing sub-circuit performs preliminary action by writing the threshold voltage of the low temperature polysilicon transistor to the storage capacitor before the light emitting phase. This preliminary threshold voltage compensation ensures that subsequent light emitting operations are based on compensated values rather than raw, varying transistor characteristics. The storage capacitor is pre-charged with compensating voltage, so when light emitting occurs, the current is determined by stable reference voltages rather than unstable transistor parameters, thereby preventing IR Drop and brightness non-uniformity.
Data Source
AI summary
Embodiments of the present disclosure provide a pixel driving circuit configured to drive a light emitting element to emit light. The pixel driving circuit may comprise a driving sub-circuit, coupled to the light emitting element; a data writing sub-circuit, coupled to the driving sub-circuit and configured to receive a scanning signal, a reference voltage signal, and a data signal, and supply the reference voltage signal and the data signal to the driving sub-circuit successively under a control of the scanning signal; and a light emitting controlling sub-circuit, coupled to the data writing sub-circuit and the driving sub-circuit, and configured to receive a first controlling signal and a second controlling signal, and to control the driving sub-circuit to drive the light emitting element to emit light under a control of the first controlling signal and the second controlling signal.