The issue with cell phones today is that they consume more power than can be generated with energy harvesters. In my opinion the best option would solar. However, most users don't keep there phone in direct light. And indoor solar cells also generate not enough power yet.
I have a great idea that combines several hot topics: energy harvesting, wearables, alternative energy sources, and reduction of emissions contributing to climate change. It uses fuel cell technology to convert methane from natural sources into electricity. The tie-in to wearables is the use of the methane produced by the human digestive tract, so this would be embedded in undergarments of the appropriate type. Of course, like nearly all such "harvestable" alternative energy sources, it tends to be intermittently available and therefore would have to work with a more or less conventional rechargeable battery for supporting sustained loads. Suggestions for a suitable marketing name for this product/technology are actively solicited!