Current solar panel efficiency rates are around 17%. This means that 17% of all light that meets the panel can be converted into energy. In order to soak up enough energy to charge effectively, the device would need to be face down with the device under some sort of natural or artificial light. This would also mean that the device does not charge when in a pocket or bag.

The idea and concept sounds good but in reality it is flawed.

I suggest look at other ways to charge the device using solar such as solar bag. This way, youcan charge whilst you are on-the-move, your device is safe in your bag and it will also geneate enough power to really deliver the charge to your device it requires.