Linear Technology article discussing solar charging, using maximum power tracking - see extracts
The idea dates back to 2002 when I was working on my dissertation on methods of energy scavenging that could be integrated on silicon. On a footnote, I added simple calculations demonstrating that if the surface area of a 'candy bar' mobile phone is covered with solar cell of 5% efficiency (which is pretty rudimentary), including a DC/DC converter, the overall output power is sufficient to augment the battery, providing power for stand-by indefinitely.
And here it is! Or rather a simplified version that is equally effective if not better!
Specifically what you need:
The Motorola F3 - a very cheap (I suspect the cheapest mobile phone there is) phone that is widely available both in US and UK. It is this affordability and basic ruggedness that I had chosen this model. More on this later...
A solar panel - the one I had acquired is available online (more on this later) but any other suitable make can be used provided it is rated 4.5V and above, having the dimension not larger than 100mm x 40mm.
A Schottky diode, 1N5817 or equivalent - a basic component which is not hard to find, you can even use the SMT version as long as you can solder some wires on both ends.
Some wires, I use enameled wire, more on this later...
Some basic soldering + a sharp pen knife + aluminium tape + epoxy + a hand drill + super glue
That's it! Surprised it is so easy? Why not get your hands dirty and build one!
The finish product is shown below.