Smartphones and electronic ink: a difficult symbiosis



    We talk a lot about one of the main advantages of YotaPhone, the second electronic ink display. But we usually did not go into details: why we chose this technology, what problems and difficulties we got in our way, what are the advantages and disadvantages of electronic ink displays, especially with regard to smartphones - we want to talk about all this.




    Electronic ink technology has a number of fundamental differences from traditional LCD displays. Let's look at what they are.

    In liquid crystal displays, the image is formed as follows: each pixel consists of three subpixels, red, blue and green. Behind the pixel matrix is ​​a backlight. Subpixels themselves are cells in which there are liquid crystals. Under the influence of electricity, they change their configuration, which allows you to adjust the color of the entire pixel. When the power is turned off, the liquid crystals return to their original shape, the backlight turns off and the image disappears.



    In "electronic paper" there are no liquid crystals at all. Instead, each pixel contains a capsule with solid pigment particles, and there is no backlight behind the pixel matrix. Electrophoresis is used to form the image: when electricity is applied, the capsules turn on one side or the other. To maintain this state, they do not need energy, the capsules remain in the same position until the next electrical impulse.





    Electricity is not wasted on backlighting and maintaining the state of pixels, only on changing the image. As a result, such a display consumes significantly less electricity compared to liquid crystal displays.

    The second advantage of electronic ink is its excellent readability in bright light, like conventional printed text. This is due to the use of solid particles for image formation. But any coin has two sides, and when working in the dark you will need external lighting. However, as is the case with a paper book.



    The main drawback of electronic ink is the monochrome image, the highest quality samples display 16 shades of gray. To date, there is no high-quality technology of color electronic ink. The weaknesses of the existing designs are:

    • Low color saturation, especially in low ambient light.
    • On average, half the resolution compared to monochrome displays.

    When choosing technology for the second YotaPhone display, the very low power consumption of electronic ink was a decisive factor. And their ability to save images even with a fully discharged battery expands the range of smartphone use cases. The electronic ink display does not need to be turned off to save energy, it always works. Having pre-selected the information that you want to receive on the second display, you will not even need to pick up a smartphone in order to immediately be aware of various reminders, updates and notifications. Just look at the second display. We wrote more about this here .



    Difficulties in implementing a second display


    People often ask why it is impossible to make a second display of the same resolution as the first? This is due to serious technological barriers. For example, a second display with a resolution of 1280x720 will require the introduction of a highly integrated control chip, which simply does not exist yet. And its development will require many months of work. But even if it existed, a large number of conclusions and a considerable physical size would not allow us to fit the microcircuit into the case of a modern convenient smartphone.

    I must say that the control chip has become the main source of difficulties when working on both generations of YotaPhone. It is a silicon array and cannot be bent without breaking. This greatly limits our ability to create a curved body. In particular, the second-generation YotaPhone case has only a slight bend on the sides.

    The second reason that prevents the increase in resolution of the second display is the increase in the size of the contact pads around the perimeter of the matrix. Such an array of conclusions is very difficult to hide under a narrow frame.

    A difficult task when assembling a smartphone was the lamination of a panel with a second display, especially on bends: this process requires very high accuracy. Otherwise, sticking-up, air bubble formation, cracking and so on are possible. The situation is complicated by the fact that the second-generation YotaPhone was the first smartphone to use Gorilla Glass curved glass, which further complicates the assembly process.

    The features of electronic ink displays require a very careful approach to interface design. Each element must be created taking into account the monochrome image, lack of backlight and low refresh rate. For the same reasons, it is necessary to optimize the display of various information - text, images, animation and video.

    About the second display in the second generation YotaPhone



    This fall, the new generation YotaPhone comes out. He will receive a second display on electronic ink with a resolution of 960x540. It will be completely touch-sensitive, which will not only increase the comfort of work, but also expand the possibilities of controlling a smartphone when working with the main display.

    Some users fear that the touch panel will degrade the sharpness and contrast of the second display. A few years ago, this could affect image quality. But today, touch panels have already reached such a level of “invisibility” that they have virtually no noticeable effect.



    A few years ago, backlit electronic ink devices appeared on the market. We are considering the possibility of introducing backlighting into the second generation YotaPhone, however, this idea has several “buts”:

    • High power consumption. The backlight would significantly reduce the main advantage of the second display - energy efficiency.
    • The difficulty of quality installation around the perimeter of a thin curved display.
    • Thickening of the assembled display unit.
    • Increase the cost of the device.

    Future plans


    Now a number of manufacturers are working on the creation of color displays on electronic ink, as well as on increasing the image refresh rate. We carefully monitor the progress of developers and, of course, are considering the possibility of using new products in future generations of our smartphones. In particular, we decided to periodically bring to your court the most interesting and promising ideas that can be implemented in our smartphones. Stay tuned for new posts.

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