I would like to know the technology behind e-Ink and why is it used mostly in e-readers? Why can't it be used in mobiles?
Donald McLean's answer is on the money. You might be interested in the Yota Phone though; it's got an eInk display on the back and a normal display on the front: cnet.com/products/yotaphone-2014– TomJun 17, 2014 at 21:52
e-Ink is, generally, a synonym of electronic paper. The form used in most e-readers is E Ink (proprietary and trademarked), produced by the E Ink Corporation. The technology basically uses microcapsules filled with white and black charged particles. The charged particles are manipulated by electrodes at the top and bottom of the capsule. When the top contains only white particles, the pixel is white. As more black particles are added, the pixel becomes darker until it becomes completely black.
The main advantage of eInk displays are that they offer superb contrast even in bright light (try reading a cell phone on the beach while wearing sunglasses). The primary disadvantage is that these displays are very slow relative to LCD, which makes them unsuitable for high frame rate applications such as video and many games.
e-readers are one application where high contrast is useful and high frame rates are not necessary, and so they have become the primary use of electronic paper technologies.