The top layer is the cover glass, primarily composed of silicon dioxide (SiO2). Glass has a Mohs hardness of 6.5 and protects the phone's internal structure. However, this layer is the most easily scratched, causing heartache for many phone enthusiasts. Therefore, many users like to apply a protective film on top. These films are made of plastic film, a polymer material. Currently, there are four main types of screen protectors: PP, PVC, PET, and ARM. The most common cover glass on the market is Corning Gorilla Glass, produced by Corning. Lens Technology, which started solely with glass processing, has achieved phenomenal market success.
Currently, sapphire has emerged as a replacement for glass. The Apple Watch uses sapphire. Sapphire's main component is aluminum oxide (Al2O3), a single-crystal material with a Mohs hardness of 9, making it the hardest material besides diamond. It offers superior scratch resistance compared to glass. However, sapphire still has technological disadvantages, such as its relatively poor toughness. Toughness, unlike hardness, refers to a material's ability to resist crack propagation.
Indeed, sapphire is not a perfect substitute for glass at present. However, in the past two or three years, the cost of sapphire has decreased significantly. At the same time, more mature manufacturing processes have allowed sapphire screens to meet certain mass production requirements, contrary to rumors of "extremely low yield rates and difficulty in mass production." Perhaps applying a CVD or PVD layer of sapphire film to the glass surface could combine the advantages of both glass and sapphire, simultaneously solving the "hardness and brittleness" issues.
The second layer is the touch sensor layer, mainly divided into resistive and capacitive types, whose primary function is to detect touch operations. Currently, the touch sensing layer used is mainly made by depositing a layer of ITO (indium tin oxide, or tin-doped indium oxide) onto glass using magnetron sputtering technology. ITO is a mixture of indium (Group III) oxide (In₂O₃) and tin (Group IV) oxide (SnO₂), typically with a mass ratio of 90% In₂O₃ and 10% SnO₂.
Graphene is currently the most likely candidate to replace ITO and become the mainstream material for touchscreens. Graphene is the thinnest and strongest known nanomaterial in the world. It is almost completely transparent, with a light transmittance of 97.7%, and a thermal conductivity as high as 5300 W/m·K, higher than carbon nanotubes and diamond. At room temperature, its electron mobility exceeds 15000 cm²/vs, higher than carbon nanotubes or silicon crystals, while its resistivity is only about 1 Ω·m, lower than copper or silver, making it the material with the lowest resistivity in the world. Due to its extremely low resistivity and extremely fast electron migration, it is expected to be used to develop thinner, faster-conducting next-generation electronic components or transistors.
Its advantages are mainly manifested in the following ways:
(1) The screen image is more realistic. The graphene touchscreen, supported by a graphene film, has a light transmittance of up to 97.7%, resulting in better transparency and more realistic and pure colors. Traditional mobile phone screens have a light transmittance of around 95%, which makes the image appear yellowish under sunlight. However, graphene is almost completely transparent, so the screen does not have color distortion, resulting in a higher definition image.
(2) Graphene has high conductivity, which is very useful for touchscreen phones. Graphene phones have high sensitivity for multi-touch.
(3) Graphene has high flexibility, enabling future curved displays. It is not only ultra-thin and ultra-light, but can also be bent to nearly 180° in the hand. Phones assembled with such screens will be lighter and more durable, with shockproof and drop-resistant functions.
The third layer is the front panel, mainly used to install filters and generate images.
The bottom layer is the back panel, used to process millions of thin-film transistors.