Organic light emitting device pixel circuit and driving method therefor

What is this patent about?

’599 is related to the field of active matrix organic light emitting displays (AMOLEDs) and specifically addresses the challenge of maintaining uniform brightness across a panel. In these displays, individual pixels rely on driving transistors to regulate the current flowing through electroluminescent elements. However, manufacturing variations often lead to inconsistent threshold voltages across the transistor array, which results in non-uniform light emission and poor image quality even when identical data signals are applied to different pixels.

The underlying idea behind ’599 is to eliminate the influence of transistor variability by forcing the driving transistor to act as its own reference during the data loading phase. Rather than relying on a separate matched transistor to mirror current, the circuit uses a feedback mechanism to temporarily configure the driving transistor as a diode-connected load. This allows the circuit to capture the specific threshold voltage of that individual transistor and store a compensated voltage level in a capacitor, ensuring the final output current is determined solely by the data signal and the supply voltage.

The claims of ’599 focus on a multi-transistor pixel architecture that utilizes a dedicated compensation transistor to bridge the gate and drain of a driving transistor. This arrangement is activated by a scan signal to facilitate self-compensation of the driving transistor's threshold voltage. The independent claims further define a switching network that isolates the electroluminescent element during the programming phase and subsequently connects it to the driving current path only when a separate light-emitting signal is triggered.

In practice, the invention operates through a sequence of initialization, programming, and emission. During initialization, a previous scan signal clears the storage capacitor to a baseline voltage. When the current scan line activates, the data voltage is fed to the driving transistor while the compensation transistor closes the loop to create the diode-connected state. This causes the gate to settle at a potential equal to the data voltage minus the transistor's unique threshold voltage, effectively 'baking' the error correction into the stored charge.

This approach differs from prior art by moving away from current-mirror designs, which are notoriously difficult to implement due to the near-impossible task of perfectly matching two adjacent transistors. By employing a voltage-driving manner that incorporates self-detection, the system avoids the slow charging times associated with high-parasitic data lines in current-driven displays. The result is a robust pixel circuit that achieves high gradation and uniform luminance regardless of the inherent physical deviations in the thin-film transistor backplane.