Compensation technique for luminance degradation in electro-luminance devices
A method and system for compensation for luminance degradation in electro-luminance devices is provided. The system includes a pixel circuit having a light emitting device, a storage capacitor, a plurality of transistors, and control signal lines to operate the pixel circuit. The storage capacitor is connected or disconnected to the transistor and a signal line(s) when programming and driving the pixel circuit.
1. A pixel circuit comprising:
a light emitting device having an initial ON voltage;
a power source for supplying a driving current to said light emitting device;
a driving transistor having a first terminal coupled to said power source and a second terminal coupled to said light emitting device, for supplying a driving current to the light emitting device during a driving cycle, the driving transistor also having a gate terminal, the driving transistor having a threshold voltage less than said initial ON voltage of said light emitting device;
a source of a programming voltage;
a storage capacitor having a first terminal coupled to said gate terminal of said driving transistor, and a second terminal coupled to said source of the programming voltage and to a node between said driving transistor and said light emitting device for charging to a voltage that is a function of said programming voltage and said initial ON voltage of said light emitting device during a programming cycle, so that the voltage between said gate terminal and said second terminal of said driving transistor is a function of said programming voltage and said initial ON voltage of said light emitting device, at said node between said driving transistor and said light emitting device, during a driving cycle.
2. The pixel circuit of claim 1 , further comprising:
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 the programming cycle according to a second voltage signal supplied, via a second select line, to a gate terminal of the second 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.
3. The pixel circuit of claim 1 , wherein the storage capacitor is configured to be charged with said initial voltage during a pre-charging cycle having a duration less than a duration of the programming cycle, said initial voltage exceeding a compensated voltage, the compensated voltage being substantially the same as the sum of a programming voltage, a threshold voltage of the driving transistor, and a voltage drop of the light emitting device.
4. The pixel circuit of claim 3 , wherein the storage capacitor is configured to partially discharge during a compensation cycle having a duration less than the duration of the programming cycle, until the storage capacitor is charged with the compensated voltage and the current through the driving transistor and the light emitting device is substantially zero.
5. The pixel circuit of claim 3 , wherein the compensated voltage is stored in the storage capacitor at the conclusion of the pre-charging cycle, and wherein the pre-charging cycle precedes the driving cycle of the pixel circuit.
6. The pixel circuit of claim 1 , wherein the light emitting device is configured to emit light responsive to the driving current flowing through the light emitting device, and wherein the driving current flowing through the light emitting device is controlled-according to the gate-source voltage of the driving transistor during the driving cycle.
7. The pixel circuit of claim 6 , wherein the pixel circuit is configured to compensate for a shift in an on voltage of the light emitting device by allowing the storage capacitor to partially discharge through the light emitting device during the programming cycle such that the gate-source voltage of the driving transistor during the driving cycle accounts for the on voltage of the light emitting device.
8. The pixel circuit of claim 6 , wherein the pixel circuit is configured to compensate for a shift in the threshold voltage of the driving transistor by allowing the storage capacitor to partially discharge through the driving transistor during the programming cycle such that the gate-source voltage of the driving transistor during the driving cycle accounts for the threshold voltage of the driving transistor.
9. The pixel circuit of claim 2 , wherein the light emitting device is configured to emit light responsive to the driving current flowing through the light emitting device, and wherein the driving current flowing through the light emitting device is controlled according to the gate-source voltage of the driving transistor during the driving cycle.
10. The pixel circuit of claim 9 , wherein the pixel circuit is configured to compensate for a shift in an on voltage of the light emitting device by allowing the storage capacitor to partially discharge via the discharging connection during the programming cycle such that the gate-source voltage of the driving transistor during the driving cycle accounts for the on voltage of the light emitting device.
11. The pixel circuit of claim 9 , wherein the pixel circuit is configured to compensate for a shift in the threshold voltage of the driving transistor by allowing the storage capacitor to partially discharge via the discharging connection during the programming cycle such that the gate-source voltage of the driving transistor during the driving cycle accounts for the threshold voltage of the driving transistor.
12. The pixel circuit of claim 1 , wherein the light emitting device is an organic light emitting diode.
13. The pixel circuit of claim 1 , wherein the pixel circuit is incorporated in an active matrix organic light emitting diode display.