Electroluminescent display devices
View Patent ↗In an active matrix electroluminescent display device a storage capacitor ( 24 ) is provided in each pixel for storing a voltage to be used for addressing a drive transistor ( 22 ) which controls the illumination of the electroluminescent display element ( 20 ). A discharge photosensitive element ( 34 ) is provided for discharging the charge storage capacitor in dependence on the display element's light output. A further photosensitive element ( 40 ) is provided so as to cancel photocurrents produced in the discharge photosensitive element ( 34 ) as a result of light input to the discharge photosensitive element other than from the pixel's display element ( 20 ), the further photosensitive element being shielded ( 44 ) from light emitted by the display element while being exposed to light from other directions. Irregularities in the display output due to the effects of the unwanted light inputs to the discharge photosensitive elements are then avoided.
1. An active matrix electroluminescent display device comprising an array of pixels, each pixel comprising:
an electroluminescent display element;
a drive transistor for driving a current through the display element;
a storage capacitor for storing a voltage to be used for addressing the drive transistor;
a discharge photosensitive element for discharging the storage capacitor in dependence on the light output of the display element; and
a further photosensitive element which is shielded from light emitted by the display element while being exposed to light from other directions, and which is connected so as to cancel photocurrents produced in the discharge photosensitive element by light from the other directions.
2. The display device according to claim 1 , wherein the drive transistor is connected between a power supply line and the display element.
3. The display device according to claim 2 , wherein the discharge photosensitive element is connected in parallel with the storage capacitor between the power supply line and the gate of the drive transistor, and wherein the further photosensitive element is connected in series with the discharge photosensitive element between the gate of the drive transistor and a reference potential.
4. The display device according to claim 3 , wherein the reference potential is provided by a reference potential line shared by other pixels.
5. The display device according to claim 4 , wherein the pixels are arranged in rows and columns with each row of pixels having a respective row address conductor via which the row of pixels is selected in a row address phase, and wherein the pixels of a row share a respective reference potential line, and wherein the reference potential line associated with a row of pixels comprises a row address conductor associated with an adjacent row of pixels.
6. The display device according to claim 1 , wherein the discharge photosensitive element and the further photosensitive element comprise photodiodes.
7. The display device according to claim 1 , wherein the discharge photosensitive element and the further photosensitive element comprise diode-connected transistors.
8. The display device according to claim 1 , wherein the pixel further includes an address transistor ( 26 ) connected between an input signal line and an input to the pixel coupled to a node between the storage capacitor and the gate of the drive transistor.
9. The display device according to claim 1 , wherein the device comprises a substrate, active matrix circuitry comprising the pixel drive transistors, storage capacitors and photosensitive elements overlying the substrate, and wherein the pixel display elements comprise an electroluminescent layer overlying the active matrix circuitry.
10. The display device according to claim 9 , wherein the discharge photosensitive element and the further photosensitive element in each pixel are arranged close together.
11. The display device according to claim 9 , wherein the pixel display elements include a transparent conductive electrode layer between the electroluminescent layer and the active matrix circuitry, wherein a light shield is arranged in each pixel between the further photosensitive element and the overlying electroluminescent layer to shield the further photosensitive element from light directly from the electroluminescent layer, and wherein the discharge photosensitive element is exposed to light generated in the overlying electroluminescent layer.
12. The display device according to claim 11 , wherein the pixel display elements include a light opaque electrode layer at the side of the electroluminescent layer remote from the active matrix circuitry.
13. The display device according to claim 11 , wherein the pixel display elements include a second transparent electrode layer at the side of the electroluminescent layer remote from the active matrix circuitry, and wherein a further light shield is arranged on the second transparent electrode layer and overlying the discharge photosensitive element of a pixel.