The problem with most phone-oriented gamepads is the bulk, but that problem might be solved. If Justice Frangipane’s team has their way, that chunkiness will be gone. Their proposed FlipSide case for iPhones centers on Bluetooth 4.0 gamepad controls that are clipped to the back when not using them.
They attach to the front when playing. As far as power, the combination of a sensitive solar cell and a thin film battery from Infinite Power Solutions should keep the case powered up with even indoor lighting. The challenge is getting the case produced and Frangipane is looking for crowdfunding to make the FlipSide a reality.
If they make the donation target, there could also be an Android version. Go help out and get in on this.