OLED Advances

August 14, 2012, Emerging Display Technology Conference, Santa Clara, CA—G. Rajeswaran form Moser Baer Technologies presented the advances in OLED technology for displays and lighting. The latest technologies have evolved from work starting in the mid ’80s.

Kodak started their efforts to make solar cells, but the materials didn’t absorb energy, they emitted light. Along the way, they introduced dopants to tune the colors. The dyes improved efficiency and stability and led them to some key requirements for the emitters and dyes.

The cathode materials have to have a low work function. The electron transport layer, emissive layer, hole transport layer, hole injection layer and ITO anode were vacuum thermally deposited on the substrate. The backplane started a passive matrix with off-board row and column drivers. The physical structure was that of a pillar on a base.

The LEDs were only on during a row time, and were off the rest of the time. The cathode materials were more conductive than the anodes, so it carried all of the current for the pixels in a row. This structure requires cathode patterning and high-pulsed drive currents proportional to row count. One problem is that a shorted pixels results in cross talk or line defects, and significant capacitance issues that must be managed. The good part is that this is simple structure with easy fabrication.

Active matrix with integrated drivers in low temperature poly silicon could light the LEDs for the full screen cycle, and therefore were much brighter. The biggest problem is that the drivers need to have good current sharing. The backplane constitutes most of the cost and the process needed an excimer laser for the anneal steps. The displays showed mora due to the variations in threshold voltages

As a result of the problems, they changed the process and driving circuits to get better threshold compensation. The color filters are integrated with the white LEDs. Now, manufacturers are using multiple layers and tandem LEDs for better uniformity. The manufacturing process and optimization calls for more careful selection of deposition sources and compatible materials. The white light is getting better and R&D is starting to get oxide TFTs working for the active matrix materials.

Manufacturing needs to have good control of the encapsulation process for better reliability, smaller size pixels, and the creation of fewer pin holes to make better displays. The industry has shown a number of prototypes and demonstrations of the latest technologies.

For lighting, the needs are in getting users to accept the specifications and start the transition to volume. Now, they are looking for realistic market opportunities and the voice of the consumer. Costs are an inhibiter of faster adoption as the variety of form factors show the possible new uses for the lights.

Current lights can produce about 70 lumens per watt, and 2000 cd/m2. lifetimes are in the 20k hours range. The biggest issue is the costs of over $250/m2. Production roadmaps show a rapid decrease in costs over time, and the simple process can be in microfabs and not in big semiconductor fabs.