Using a-Si TFTs for AMOLED displays: Progress from Ignis

For LCDs, TFTs are only charging and discharging a capacitor (that is effectively what an LCD is electrically). Once the capacitor is charged the TFT can be switched off and the display continues to function. This has 2 key advantages for TFTs:

OLEDs are a current driven device, rather than voltage driven, as LCDs are, and at least one TFT has to be switched hard ON while the pixel is ON. That means if you have a blue sky or TV channel text the TFT will be on continuously, giving aging at least 1000 times as fast as for an LCD, but in reality much more because TFTs relax while OFF. And another problem is that you also get strong differential aging resulting in "burn-in" because TFTs in dark pixels will be switched OFF, therefore having no strong voltage stressing.

There are two approaches to making AMOLEDs; extremely stable TFTs or sophisticated correction circuits with unstable TFTs.

Early AMOLEDs used low temperature poly silicon (LTPS) because initially it was the only viable type of stable TFT and developers claimed high mobility as an advantage because you can make smaller circuits. This is true, but it is somewhat offset by non-uniformity issues.

 

 

Ordinarily OLED circuits need good uniformity and high stability. At first high mobility was seen as an advantage, but later many came to think that mobility in the range 1 to 20 cm^2/Vs gave the best combination of manufacturability and performance.

 

A particular problem with LTPS is that it has extremely high mobility, but suffers from non-uniformity due to shot-to-shot variations in the laser process and sensitivity to small changes in feature size.

 

Both of these can be mitigated by process and circuit techniques, but these are expensive and short-range uniformity remains the main problem with AMOLED products today, limiting their screen size.

 

Both Sony and Samsung have made AMOLED demonstrators with microcrystalline Si TFTs with excellent stability and uniformity and mobilities close to 2 cm^2/Vs.

 

Samsung used plasma deposition and Sony a simplified laser process that gives good uniformity, but lower mobility. Samsung have now stopped the microcrystalline Si work to concentrate on oxide TFTs with a slightly higher mobility of 14, but with good uniformity and stable in a process that is claimed to be very low cost to make with a-Si TFT equipment. Sony is reported to giving microcrystalline silicon top priority as they see it as an economic way to make AMOLEDs.

The alternative is to use sophisticated correction circuits with unstable TFTs. Ignis of Canada use a-Si TFTs and sophisticated measuring and correction circuits to make AMOLEDs on a-Si TFTs.

 

 

At the S.I.D. 2009 event earlier this year, they demonstrated stable AMOLED TV demonstrators made with Kodak OLED material and PVI a-Si TFTs.

 

If it can be proven that this works reliably and is scalable, it could make a-Si TFTs viable for OLEDs, leveraging a huge infrastructure.

 

For more information see www.ignis.ca .

 

Also attend Printed Electronics Europe 2010.