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by: David Barnes
Emerging Technology - High moisture barriers have been one of the toughest nuts to crack for flexible, organic electronics. At the recent Flextech Alliance conference, I heard news of new processes for very moisture proof (high barrier) films that could enable very long lived organic displays and electronics.
Materials used in OLED or OPV devices degrade rapidly when they contact oxygen or moisture, so they must be sealed tightly. In contrast, inorganic devices such as conventional TFT LCD or TFT-EPD survive the presence of humid air about a thousand times longer. In fact, EPD, such as electrophoretic E Ink displays, need some moisture so its encapsulation is meant to keep water in, rather than out. Back in 2010–2011, I studied barrier requirements for several clients and we hoped that producers of organic or thin-film solar cells would generate enough demand for high-barrier films that suppliers would develop cost-effective solutions for next-generation displays. The subsequent shake-out among solar cell producers delayed developments, however. Producers of OLED or other advanced products are still searching for solutions to the problem.
As I hinted above, moisture barriers are rated in orders of magnitude (powers of ten). The term WVTR, for water-vapor transmission rate, is specified in grams per square meter per day. Many conventional devices such as TFT LCD survive transmission rates of 10^-2 (0.01) or more while engineers assume transmission rates of 10^-6 or less are necessary for a long-lived OLED TV. Given that we are talking about a leakage rate, there is a trade-off between the barrier strength and product life. A utility-grade solar cell or premium OLED TV may need to operate flawlessly for a decade or more while a flexible OPV cell for charging a smartphone may need to survive only a few years. For display makers today, however, OLED must connote quality so they seek high barriers to moisture. There aren’t any… yet.
3M took an important step in 2010 when it reinvested in a factory and created R2R capacity for polymer (PET) barrier
films with WVTR in the 10^-3 regime. Such films are far better than food or pharmaceutical packaging films, which is what producers had been using.
Konica Minolta also developed mid-barrier technology and supplied it to Konarka (for solar cells). We’ll never know how that film would have worked but it was a positive indicator.
The quest for high barriers (aka ultra-high, less than 10^-3) has taken three paths since the early days of thin-film capacitor development by firms such as GE. As depicted in this simplified diagram, the three paths include:
The former Vitex technology Samsung is working with is a version of oxide/polymer technology pioneered at Pacific Northwest National Laboratory (PNNL). The Holst Centre is working on a prototype R2R line for nitride/organic dyads based on early Philips work and Universal Display is developing CVD chamber methods for cross-inked films. Progress has seemed slow but steady.
There were two surprises at last week’s FlexTech Alliance conference that gave me hope for commercial solutions sooner rather than later, however. Ravi Prasad, CTO of VITRIfLEX, presented work on an oxide-based process using a capping layer. Metrology labs measured WVTR in the 10^-5 to 10^-6 range in some of his samples, so the company plans to ramp a prototype R2R line later this year.
Perhaps even more promising news came from Brad Aitchison of BENEQ. He said the firm has delivered prototype R2R equipment for atomic layer deposition (ALD). That technology has been considered ideal because ALD can create pin-hole-free oxide layers, but it has also been considered difficult to implement.
I hear that BENEQ is working on a dual-layer AlO/TiO system for polymer substrates that could create really nice layers for subsequent processing steps. If so, we may have more than one way to make high-barrier substrates or laminates in the near future. I can hardly wait to hear what comes at FlexTech’s conference next year in Phoenix. – David Barnes