Today, household appliances with colour displays are a reality, at least for high-end models: they provide information, carry out controls and communicate with users.
With the Internet of Things (IoT), the vision for the future expects that the use of touchscreens expands to a higher and higher number of household appliances, low-cost ones included. Boosted by the growing connectivity and by the enabled functions for users’ apps, the touchscreen interface solves various operational problems and provides appreciated aesthetical solutions. Regardless of the application, household appliances are becoming very complex and they more and more frequently need a user interface able to communicate with immediacy.
On one hand, they must accept the various controls with a single touch, on the other hand they must communicate manifold information, from use choices in a specific condition to the current operation state. The technology of these systems is implemented through three components: touch sensor and controller, LCD display and management software.
Liquid crystal displays
The liquid crystal term refers to the intermediate state assumed by a substance, or better it indicates an intermediate material status between the one of a crystalline solid and the one of an isotropic liquid.
Liquid crystals own many of the mechanical properties of a liquid, such as for instance high fluidity, incapability of withstanding cutting efforts, formation and coalescence of drops. At the same time, they are very similar to crystals due to the anisotropy of their elastic, optical, electric and magnetic properties.
When crystals with a high level of order in the molecular sequence are fused, they turn into liquids, they gain fluidity but they lose their initial order.
The thin organic molecules have linear shape, when they are fused, they maintain their order in the molecular direction, even if they lose it in position. In the state in which molecules maintain a uniform direction, they also have refraction indexes, dielectric constants and other physical characteristics resembling those of crystals, depending on their direction, even if they are liquid. For this reason, they are called liquid crystals.
A LCD display has liquid-crystal material inserted between two glass plates. With no voltage applied between electrodes (which are transparent), liquid crystal molecules are aligned parallelly to the glass surface. When voltage is applied, they change direction and they rotate vertically as to the glass surface. This behaviour causes variations in optical characteristics. Therefore, the quantity of transmitted light can be controlled by combining the movement of liquid-crystal molecules with the polarization given by two polarized planes fixed on both external sides of glass plates.
A LCD is composed by many pixels, in their turn consisting of three sub-pixels (red-green-blue). In the case of Full-HD resolution, broadly used in numerous electronic devices, there are more than six million sub-pixels. To activate these millions of sub-pixels, a TFT (thin film transistor) is necessary in each sub-pixel. A TFT is a semiconductor device, it is used as control to provide suitable voltage to liquid crystals for each single sub-pixel.
A LCD TFT is obtained by stacking a glass substrate formed by TFT and transparent electrodes, a layer of liquid crystals, another glass substrate with a coloured filter (RGB) and transparent electrodes. Polarizers are placed on the outer side of each glass substrate and a backlighting source is positioned outside the sandwich on the rear side.
A voltage variation applied to liquid crystals modifies the panel transmittance, and then it changes the quantity of light that passes from the backlighting to the front display surface. This principle allows the LCD TFT display to produce coloured images.
Innovative solutions for displays
Japan Display develops, designs and produces displays for those applications that need many instantaneous visual information. The company has developed the Low Temperature Polysilicon (LTPS) technology.
The definition of a display is expressed by the density of pixels (pixels per inch). On a high-density display, the pixels that constitute the display are not recognizable and images appear clear and fluid.
Thin-film transistors used in a LCD TFT are implemented by using silicon as semiconductor. Nevertheless, it is very difficult to manufacture crystalline silicon on glass, and it was possible to use only the amorphous silicon (a-Si), but the mobility of electrons in this semiconductor is low to guide TFT quickly, limiting the pixel density on the display.
On the contrary, a low-temperature poli-silicon (LTPS) TFT LCD, prepared by forming polycrystalline silicon on a glass substrate at relatively low temperatures, reaches a high mobility of charges. Therefore, turning to this technology, it is possible to implement high-resolution and high-density displays, not achievable with a-Si.
The image quality
Conventional modalities used in liquid crystal displays like the twisted nematic (TN) have problems of brightness that changes colour according to the vision angle. In TN modality, liquid crystal molecules rise when a voltage is applied. When liquid crystals are in vertical position, their characteristics significantly change according to the vision angle. This results in a variation of colour and brightness related to different vision angles. In the In-Plane Switching (IPS) modality, liquid crystal molecules rotate in parallel, and not perpendicularly, to the panel, this allows vision angles close to the flat angle, with consistent accurate chromatic rendering. Therefore, IPS LCD offers limited variations of brightness and colour, produces high-quality images irrespective of the vision angle.
IPS-NEO, an evolution of IPS implemented by Japan Display, focuses on the black colour look. Conventional IPS displays featured a slight loss of light if seen from a bent angle, this resulted in a certain difficulty in expressing a strong black. IPS-NEO adopts a technology of photographic alignment to prevent the light loss during the black visualization.
On a LCD it is necessary to align liquid crystal molecules uniformly, in a specific direction. With the resolution increase, the subtle irregularities on the substrate surface can hinder the uniform alignment of liquid crystal molecules, finally causing a loss of light on the black display. IPS-NEO technology irradiates a special ultra-violet radius (UV) to align correctly liquid crystals, in sufficiently uniform way to eliminate the light loss on a high-definition display, too.
