A Clear-eyed Look at Obstacles to OLED and Oxide TFT Success

The Business Conference is a mainstay of the Display Week tradition. Analysts and industry representatives gather to give attendees a top-level overview of key aspects of the display industry, from global market forecasts to previews of innovative technology advances. This naturally involves a lot of predictions and promotions, often viewed through rose-colored glasses. And while there were plenty of those in evidence at this year’s conference, there were a number of refreshing insights presented. Perhaps the most refreshing of these was from SID Fellow Jun Souk of Hanyang University, formerly EVP of Samsung Electric. Some of the most compelling current technologies in the display industry are touch, oxide TFT, flexible OLED, and OED TV. Rather than focus on the compelling aspects of these advances, Professor Souk took some time to point out some of the obstacles that stand in the way of success.

For example, the ultimate solution for touch screens is to eliminate the separate touch module and integrate it into the display panel. But as Prof. Souk pointed out, there are plenty of barriers to adoption of this strategy. Bringing touch technology in-house can result in additional supply chain complications and reduced yields by adding complexity to the production processes. At the current time, it can make more sense to outsource the touch component, and let suppliers take on the risk and compete to provide the modules.

As for oxide TFTs, the technology offers some attractive potential in terms of improved electron mobility compared with the incumbent amorphous silicon (aSi), at a lower cost than the laser-annealed low-temperature polysilicon (LTPS). It has the potential to transform not only the large-format LCD industry, but may make the large OLED TV products feasible. Unfortunately, several problems remain before oxide TFT is truly ready for prime time, according to Prof. Souk. For example, as a semiconductor backplane, the material still has problems with stability in terms of the voltage threshold (Vth) and Vth uniformity across large areas. In addition, the planarization of the metal oxide surface is not as smooth as it needs to be; thin film devices such as OLEDs are adversely affected by relative small variations in thickness which in turn can affect performance.

Prof. Souk also addressed the OLED TV market. In addition to the oxide TFT problems described above, there are significant challenges involved in fabricating these large display panels. Small mask scanning (SMS) is the traditional approach to depositing the OLED emissive materials, but it is difficult to use successfully for large panels. Not only is the process very slow, it also has low yield ratios and there are problems with the materials mixing along their boundaries. Other deposition methods are under development, such as a vertical linear source, laser-induced thermal imaging using transfer from a carrier film, and nozzle printing of solution-based materials, but these have limitations of their own.

Some of these problems for large format OLED panels for TVs can be addressed by using color filters over a white-emitting OLED material. One problem with this approach is that the white OLED “backlight” relies on a tandem device that emits yellow and blue light, which mixes to produce white light. The red, green, and blue (RGB) filters then extract the desired light to create a full-color image. Since the backlight has spikes in the blue and yellow parts of the spectrum, however, it is not able to produce the required red and green parts of the image adequately.

None of this is to say that these technologies won’t eventually come to market at competitive price points with high quality performance. However, it was refreshing to hear about some of the shortcomings and bottlenecks standing between these technologies and commercial production. It is good to hear about future answers to our display problems, but it is also important to hear from knowledgeable sources who can describe the realistic obstacles that stand in the way of such success.--Alfred Poor