Compensation technique for luminance degradation in electro-luminance devices

1. A method of operating a pixel circuit to compensate for shifts in characteristics of the pixel circuit, wherein the pixel circuit includes:

a light emitting device;

a storage capacitor for charging to a voltage that is a function of a programming voltage and the voltage of said light emitting device during a programming cycle;

a driving transistor for supplying, via a source terminal, a driving current to the light emitting device during a driving cycle, the driving transistor having a gate terminal connected to a first terminal of the storage capacitor, the driving transistor having a threshold voltage less than said initial voltage; and

a second transistor for providing a discharging connection between the first terminal of the storage capacitor and a drain terminal of the driving transistor during a programming cycle of the pixel circuit according to a second voltage signal supplied, via a second select line, to a gate terminal of the switching transistor, the discharging connection providing a path to partially discharge the storage capacitor through the driving transistor and the light emitting device during the programming cycle;

wherein the method of operating a pixel circuit to compensate for shifts comprises:

disconnecting the second terminal of the storage capacitor from the source terminal of the driving transistor by setting the first select line to a voltage below a threshold voltage of the switching transistor;

connecting the first terminal of the storage capacitor to the drain terminal of the driving transistor by setting the second select line to a voltage above a threshold voltage of the second transistor;

applying a voltage to the storage capacitor to charge the storage capacitor with said initial voltage;

allowing the storage capacitor to partially discharge via the discharging connection to compensate for shifts in the threshold voltage of the driving transistor and shifts in the on voltage of the light emitting device;

disconnecting the first terminal of the storage capacitor from the drain terminal of the driving transistor by setting the second select line to a voltage below the threshold voltage of the second transistor;

connecting the second terminal of the storage capacitor to the source terminal of the driving transistor by setting the first select line to a voltage above the threshold voltage of the switching transistor to define the gate-source voltage of the driving transistor by the voltage stored in the storage capacitor; and

sending a driving current through the light emitting device by connecting a first voltage supply to the drain terminal of the driving transistor.

2. The method of operating a pixel circuit to compensate for shifts of claim 1 , wherein the allowing the storage capacitor to partially discharge is carried out by discharging the storage capacitor until the storage capacitor is charged with a compensated voltage close to the sum of the threshold voltage of the driving capacitor, the on voltage of the light emitting device, and a programming voltage.

3. The method of operating a pixel circuit to compensate for shifts of claim 1 , wherein responsive to the connecting the second terminal of the storage capacitor to the light emitting device, the second terminal of the storage capacitor has a voltage equal to the on voltage of the light emitting device, and the first terminal of the storage capacitor has a compensated voltage close to the sum of the threshold voltage of the driving capacitor, the voltage drop of the light emitting device, and a programming voltage.

4. The method of operating a pixel circuit to compensate for shifts of claim 3 , wherein the driving current is sent through the light emitting device according to the compensation voltage in the first terminal of the storage capacitor.

5. The method of operating a pixel circuit to compensate for shifts of claim 1 , wherein the applying the voltage to the storage capacitor comprises:

applying a negative pre-charging voltage to the second terminal of the storage capacitor, the negative pre-charging voltage being determined according to a function including a programming voltage;

connecting the first voltage supply to the first terminal of the storage capacitor through the second transistor to charge the storage capacitor and thereby turn on the driving transistor.

6. The method of operating a pixel circuit to compensate for shifts of claim 5 , further comprising:

disconnecting the first voltage supply from the first terminal of the storage capacitor.

7. The method of operating a pixel circuit to compensate for shifts of claim 1 , wherein the storage capacitor is charged with said initial voltage by a second voltage supply different from the first voltage supply.