Luminance uniformity enhancement methods for an OLED light source
View Patent ↗An electroluminescent device utilizes a plurality of embodiments to minimize the variation in luminance due to changes in the voltage across the device. These include a highly conductive hole injection layer in a bi-layer device, a thick hoe transporting interlayer in a tri-layer device and the addition of an external resistor in series with the device.
1. A system comprising:
an organic light emitting diode device having an emitting area of 3 cm 2 ; and
an additional external resistor in series with said organic light emitting diode device, said additional external resistor having a resistance R higher than the dynamic resistance V OLED /I of said organic light emitting diode device, R>V OLED /I, wherein the external resistor causes the organic light emitting diode device to have a luminance non-uniformity across the emitting area that is less than 10% and a luminance uniformity across the emitting area of the organic light emitting diode to be improved in comparison to an organic light emitting diode without an external resistor.
2. The system of claim 1 wherein said additional external resistor has a resistance value in the range of 1 to 10 times of the dynamic resistance of said organic light emitting diode device.
3. The system of claim 1 wherein the dL/dV of the organic light emitting diode device is reduced by the additional external resistor compared to a device without an external resistor, wherein dL is a luminance change across the emitting area and dV is a voltage change across the emitting area.
4. The system of claim 3 wherein the anode comprises indium-tin-oxide (ITO) formed as an area and the voltage change is along the area.
5. The system of claim 1 wherein the resistance R of the additional external resistor is during operating conditions higher than the dynamic resistance V OLED /I of the organic light emitting device.
6. The system of claim 1 wherein the organic light emitting diode device has an anode and a cathode and the anode has a higher resistivity than the cathode.
7. The system of claim 6 wherein the anode comprises indium-tin-oxide (ITO).
8. The system of claim 6 wherein the anode has a resistivity of 10 to 20 Ohm/cm.
9. The system of claim 1 , wherein the organic light emitting diode device has an emitting area of at least 3 cm 2 .
10. The system of claim 1 , wherein the organic light emitting diode comprises a highly conductive hole injection layer.
11. The system of claim 10 , wherein the conductivity of said highly conductive hole injection layer is greater than 500 S/cm.
12. The system of claim 11 , wherein the conductivity of said highly conductive hole injection layer is greater than 1000 S/cm.
13. The system of claim 10 , wherein the thickness of the highly conductive hole injection layer is such that the total resistance of the anode layer and the highly conductive hole injection layer is a factor of at least 1.5 less than the resistance of the anode layer alone.
14. The system of claim 10 , wherein the thickness of the highly conductive hole injection layer is greater than about 300 nm.
15. The system of claim 14 , wherein the thickness of the highly conductive hole injection layer is greater than about 600 nm.