Displays can be created from an array of organic light emitting devices (“OLEDs”) each controlled by individual circuits (i.e., pixel circuits) having transistors for selectively controlling the circuits to be programmed with display information and to emit light according to the display information. Thin film transistors (“TFTs”) fabricated on a substrate can be incorporated into such pixel circuits.
Generally, such pixel circuits include a drive transistor that conveys current through an organic emissive layer. Light is generated within the emissive layer due to recombination of holes and electrons passing through the layer in opposite directions. Accordingly, the intensity of emitted light is controlled by the amount of current flowing through the emissive layer, and the color of the emitted light is determined by the energy transitions allowed during recombination events, which is a function of the particular organic material selected as the emissive layer. Furthermore, current through the emissive layer is controlled by the voltage(s) applied to the drive transistor, which adjusts the conductivity of the drive transistor's channel region to control the current levels through the emissive layers (and the light emission).
Color displays are created by arranging a display with roughly a third of pixels emitting red light, a third emitting green light, and a third emitting blue light, with each group of three pixels forming an RGB pixel group composed of three sub-pixels that are independently programmed. Color content is displayed by programming each RGB pixel group to emit light according to a desired color for each position created by adding the RGB content together. Providing RGB pixels can be achieved by using emissive layers that generate light in red, green, and blue, respectively and patterning the appropriate emissive layers in the emissive regions of desired pixel circuits to create a desired RGB pattern.
Manufacturing such displays thus requires precisely patterning the respective emissive layers such that the appropriate emissive material is positioned in the proper emission region for each pixel. Shadow masks formed of thin metal are employed to screen regions where the emissive material is not desired while the emissive material is deposited on the display through the shadow mask. A shadow mask thus has a pattern of holes corresponding to the emission regions of all the red pixels and is aligned such that the holes match with the red pixels and the red emissive material is deposited. The shadow mask can then be moved (or another shadow mask positioned) such that holes align with the green pixels, and the green emissive material is deposited, etc. At very high pixel resolution (i.e., small pixel size) the holes in the shadow mask must become very small and the accuracy of the procedure suffers from non-uniformities in the emissive layers and difficulties in accurately aligning small holes.
In other examples, color displays can be formed from white emissive layers that are filtered to allow particular colors to be transmitted. That is, emissive layers that generate white light (which is itself a combination of red, green, and blue) can be provided in the emissive layers of all pixels. Color filters are arranged on the display substrate associated with particular pixels such that red, green, and blue light are transmitted from the display according to a desired RGB pattern. Filtering each color wastes a lot of power as a lot of generated light energy is simply filtered out with only a fraction being transmitted to contribute to the displayed color content.
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