Wednesday, November 26, 2014

OLEDS and Why Your Old CRT TV still Works

Two stories about TV, not directly about OLED, that I will tie together down below. This note is about OLEDs. Although you can not buy a LCD TV that was made in the US, the active matrix LCD was actually invented in the US, in Pittsburgh by Westinghouse. Westinghouse was a prominent TV brand that was developing new technology. Westinghouse did not survive in the TV business long enough to capitalize on their invention because they had a problem at their factory. It seems a worker replaced a steel mesh filter in their water reclaim system with a copper coated steel mesh filter. That put just enough copper into the wash water and just enough wash water residue was left on the TV screen to serve as a phosphor poison. The TV tubes that the Westinghouse factory was making would go dark after about 6 months of use. The resultant recall of half a year’s production put Westinghouse out of the TV business. Parts per billion of copper put them out of business.

Another major brand of TV in those times was General Electric (GE). Although GE made glass for things such as light bulbs and such, GE bought their glass for CRT tubes from others. In addition to making glass, GE had an array of materials technologies including silicones and polycarbonate. At one point, in order to put downward pressure on glass prices, GE threatened to develop a polycarbonate CRT bulb. The glassmakers looked at the threat and determined that although such a product was possible and did have some advantages relative to glass, the product would only last about a year before there was enough inward migration of atmospheric gas to render the polycarbonate CRT non-functional.

The point of both stories is that emissive technologies tend to be sensitive to the most minute amount of contaminants. That is why some of the companies developing OLED technology concentrate on an ultra-clean manufacturing environment. It is also why the other key to OLED’s future is hermaticity. In a CRT, glass provided an absolute hermetic environment. The CRT was made in a clean environment, the inside of the tube, where the phosphors were, was maintained in high vacuum. Further a sacrificial barium “getter” was deposited on the inside of the tube to bind any stray oxygen that was left over from manufacture.

So, the phosphors did their thing in an absolutely pristine environment that was maintained as long as the tube continued to hold its vacuum, which is tantamount to forever for a consumer product. In terms of product chemistry, the environment virtually eliminated any alternative pathways that could be formed between the phosphor in its elevated state and when it drops back down to its base state by emitting a photon. The industry employed other tricks, such as moving to higher and higher voltage phosphors. This brought the product to the point where the phosphor aging was no longer the primary aging limitation but metallization of the glass from decades of electron bombardment.

The high voltage architecture may have some relevance to OLED design as well. But certainly, cleanliness and hermaticity are the key to making OLED technology work.

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