BOE is Boosting 8K TV Light Output
A couple of weeks ago, we reported on some developments aimed at 8K TV light at the annual SID Symposium. One of the previous papers was from BOE in China, now the biggest maker of LCDs in the world, from a team of researchers in Beijing. We have since found a second paper which was also developed by a team from BOE but based in Chengdu, China.
The critical point of the second paper (34.1 in the program) was developing a new method of aligning the liquid crystal (LC) molecules in an LCD. The new method gives better transmittance – a beneficial improvement in the case of 8K LCDs. Some explanation of the alignment issue may be helpful for those not deeply into the details of LCDs.
Line Up the Molecules
In the original twisted nematic (TN) LCDs, polarized light comes through the bottom layer of the glass sandwich and goes into the LC material. The polarized lght is twisted or not twisted by the liquid crystal material depending if there is a voltage applied or not. If the polarized light is twisted to match the exit polaizer, light passes through the LCD. Otherwise it is blocked and you get a dark pixel.
However, the LC crystals have to be aligned with the polariser. If the molecules are not correctly aligned, some of the light will not be affected in the desired way.
Traditionally, this alignment was achieved by putting a thin layer of polyimide on the inside of the bottom glass. The glass is then literally rubbed with a very fine cloth shaped like corduroy. This leaves a pattern of grooves in the polymer, and the molecules take their alignment from the pattern. Where the molecule comes into contact with the layer, it tends to be tilted slightly (known as the pre-tilt).
Rubbing worked but was prone to create non-uniformities, and particles from the rubbing process could cause quality defects. Furthermore, having all the liquid crystals tilted in one direction meant that light passing through the liquid crystal was affected differently according to the angle it entered relative to the tilt. That difference reduced the viewing angle performance.
To even things out, the idea of multi-domain LCDs was invented. In this idea, you sub-divide the pixel and tilt the molecules at different angles to get averaging of the tilt of the molecules, so light coming through sees close to the same LC material. This was made possible by switching to vertically aligned (VA) and IPS modes for nearly all LCDs.
In VA panels (the most common type used in 8K TVs), the pre-tilt of the molecules is random as the same alignment with the polarizer is not needed. That leads to unpredictable results, and two main methods were developed to solve this, MVA (using protrusions to ‘nudge’ the molecules to tilt a particular way) and PVA, developed by Samsung and using an electric field to pre-tilt the molecules. Both methods improved VA panels a lot in contrast and viewing angle.
From an initial concept of two domains per pixel, the technology was developed in PVA to give four LC domains per pixel. That brings viewing angles closer to those enjoyed by IPS panels. (The writer has seen references to the development of 8 domain per pixel architectures under development!).
These techniques could not be used with cloth rubbing, so companies developed a range of techniques to align the layers using UV light to affect how the polyimide was deposited. That allowed for multiple domains with different alignments in different parts of the pixel. Merck and others have further developed the techniques to avoid using polyimide by putting additives in the LC mixture with reactive mesogens (RM) to self-align the LCs.
(At the time of writing, Merck gave a detailed presentation on this topic here.)
Back to the BOE Future
Anyway, after all that preamble, back to the BOE paper. The Chengdu factory of BOE has found that it can expose the alignment area twice, first at 45 degrees azimuth angle and then at 135 degrees. With careful control of the conditions of the process, it can achieve a 90-degree final result. In this way, it doesn’t need to use the special materials that Merck’s system uses. That allows a wider choice of different materials, the researchers said.
The final result was that the double alignment, combined with a wire grid polariser, which is more efficient, could boost overall transmission. Transmission rises from 4.5% in the case of the SUVA1 technique to 6.08%, which is a 35% boost in the amount of light (the result is from a simulation). That increased efficiency can be used to boost the luminance of the panel in an 8K TV or could also be used to reduce the power needed in the backlight, which is a significant benefit. Work is continuing to further optimize the details of SUVA5 for 8K TV.
While BOE has not reported any results for 8K panels, the light transmittance is lower than with 4K panels so this work will be especially significant at this resolution.