LightMeter Pro 2 I have it and have played with it. I have used it more for balancing light sources in a studio setting. Would be easy to check how accurate it is based on your camera settings.
https://itunes.apple.com/us/app/lightmeter-pro/id388881295?mt=8
Read the review below that link.
I suppose something like this might work in a pinch, but, call me a purist, I don't think I would ever trust any exposure measurement to a cell phone, much less one in a situation where you have a wide range of EV's.