The AM-OLED "Backplane Problem"

Amorphous silicon thin film transistor (a-Si TFT) backplane technology has proven to be highly successful for active matrix LCD displays—the juggernaut of the display industry. Active matrix OLED backplanes are a different story and require TFT materials that can hold up to their more strenuous demands. Finding a good solution has proven difficult and has come to be known as the OLED backplane problem.

The minimal conventional active matrix (AM) backplane circuit needed to independently light each pixel of an AMOLED display requires a switching transistor, a drive transistor, and a capacitor in the so called 2T+1C circuit (for details of how AM works see here). The requirements imposed on the semiconducting channel materials of these two thin film transistors (TFTs) are vastly different. The switching TFT (Sw-TFT) is normally off, is only switched on for microseconds once per display refresh cycle, and must source little current in its on-state. Accordingly amorphous silicon (a-Si) works well for the channel of the Sw-TFTs. The drive TFT (D-TFT), in contrast, must source a high current when a pixel needs to be brightly lit and, depending on the unpredictable needs of the storyline, it can be called upon to do so continuously, for long periods of time. The challenge of stably meeting these two requirements in the D-TFTs while also making the D-TFTs uniform over the large areas of TV displays has come to be called the OLED backplane problem. This constitutes the key problem that has prevented AMOLED, despite all its visual and (potential) manufacturing advantages from dominating the TV display market like the trend already begun in the hand held display market.

LCD and oled pixel structure. Source Cowen, 2016.

LCD and oled pixel structure. Source Cowen, 2016.

Attempts to overcome this challenge have been largely limited to engineering of the semiconducting channel materials. a-Si can be uniformly and inexpensively deposited over large areas, but lacks the carrier mobility to drive the currents necessary for the D-TFTs. Higher mobilities (100) can be achieved by excimer laser annealing (ELA) to crystallize the a-Si thin films to low temperature polycrystalline Si (LTPS), however at present this cannot be cost-effectively scaled to substrate sizes greater than Gen 5. LTPS also suffers from a more fundamental limitation: variation in the size, number, and orientation of the polycrystalline grains leads to pixel-to-pixel inhomogeneity. This can be compensated by additional transistors but that has still not proven adequate to permit cost effective production of larger displays. Indium-gallium-zinc-oxide (IGZO), a metal oxide semiconductor, can be sputtered uniformly over large areas, while providing mobilities intermediate to a-Si and poly-Si, but is highly sensitive to processing conditions which has limited production yields. Furthermore IGZO lacks the necessary stability under bias-stress conditions, requiring complex compensation circuitry to be incorporated into each pixel. Organic semiconductor materials are attractive for their homogeneity, low cost, and the variety of means by which they can be deposited, however, the best mobilities achieved to date are similar to a-Si and too low for OLED backplanes.

nVerpix has a different approach to solving the backplane problem: A reinvention of the drive TFT architecture.

 
 
 
 

LCD vs. OLED

Dispite continued progress in high-end LCD panels, OLED displays continue to match or exceed them in nearly every image quality metric. OLED panels exhibit higher brightness and contrast ratios, truer color across a wider range of viewing angle, and a fast response rate that virtually eliminates the motion blur associated with LCDs. They are also thinner and lighter, and can be fabricated on flexible substrates. However, despite these intrinsic performance advantages, high costs and yield issues have caused OLED to fall short of market expectations.

LCD versus OLED Market Projections.  Source Cowen, 2016.

LCD versus OLED Market Projections.  Source Cowen, 2016.