The mini-LED is an evolutionary technology for LCD TVs

Measuring around 0.1 millimeter, mini-LEDS are less intricate than micro-LEDs (0.01 millimeter) but are much more affordable and easier to manufacture in bulk. As a transitional technology for manufacturers of LCD TVs, mini-LEDs have already seen traction in the market.

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FALD and Challenges

Full Array Local Dimming (FALD) is a process of locally controlling LED light levels in a segmented backlight unit to enhance contrast in images, while decreasing the ‘halo’ effect around bright areas on black backgrounds. FALD delivers perfect contrast ratio making the image appear sharper for the same pixel resolution. It requires thousands of mini-LEDs to deliver the intended picture quality and consequently this increases the manufacturing cost of the backlight unit.

Fortunately, this is where OTFT technology from SmartKem can make a difference. SmartKem’s performance OTFTs can be used to generate an active matrix backplane driving arrays of thousands of individual mini-LEDs on a glass substrate, thereby delivering impressive high resolution images with perfect black levels. The ability to locally dim the backlight to only the areas that require it can also help to save energy, making FALD LED TVs more efficient in operation.

The Backplane Driving Matrix Scheme

 

As the IC connections are only required at the edge of the display, the number of connections scales up to 2x the square root of the number of LEDs. So, for example:

90,000 mini-LEDs = 650 connections (250 rows x 450 columns for 4:9 ratio display)

Driver ICs

If you were to use a direct wiring PCB based approach, the numbers of tracks required to connect to the LEDs are vastly different to the active matrix backplane. For instance, for 90,000 mini-LEDs, 180,000 connections plus a large number of ICs would be required. It is clear that using a TFT array for the backlight will significantly lower the cost of production for manufacturers. Coupled with this are advantages of lower cost substrates (sodalime glass), high flatness of glass, and higher resolution achievable with photolithography processes. The transparency of glass would also enable transparent displays with LED backlights to be used.

SmartKem Mask Design

  • Initial proof of concept with 3 separate designs, 10mm, 5mm and 2mm pitch
  • Transistors are capable of driving enough current for 10,000 nits brightness in the backlight

This figure demonstrates the overall design of the backplane. Consisting three different types of array, it includes:

 4x4 @10mm pitch
 5x5 @5mm pitch
 10x10 @2mm pitch

The backlight is driven by a simple row/column matrix driver circuit with a range of waveforms to demonstrate the LEDs turning on/off in several pattern combinations. These can be instructed to run in a full-on pattern, column by column, row by row or via a checkerboard approach. By using the OTFT backplane approach, mini-LEDs run much more efficiently and deliver improved quality that can enable LCD displays to rival the contrast ratio of OLED technology whilst at a much lower cost and higher reliability.

Benefits

Parameters SmartKem OTFT a-Si IGZO LTPS
Current usage In development
(demonstrated in e-paper,
LCD and OLED		 LCD and rigid
e-paper		 OLED TV Mobile phone (OLED
and some LCD)
Typical charge
mobility in display
pixel		 3 cm2/Vs 0.5 cm/Vs 10 cm2/Vs 50 cm2/Vs
Process temperature <150 C 300o C 320o C 350o C
Substrate
compatibility		 Wide range of plastics and
glass		 Glass Pl/Glass Pl/glass
Current driving
stability		 Very Good Average Very Good Excellent
Impact resistance Excellent Poor Poor Poor
Bend radius 0.5mm 4mm 2mm 4mm
Manufacturing
maturity		 Prototype Excellent Fair Good
Process cost Low Low Medium/High Medium/High