A solar-powered cellphone sure sounds like a great idea: use the power of the sun to never have to charge your phone again, woo! For the last six months, Nokia has had some prototype solar phones out in the wild to see how well this actually works, and the results are in: the sun kinda sucks for charging phones.
The Nokia Lokki (the phone in the above pic) is a dumbphone with a solar panel stuck to the back. It uses more or less the minimum amount of power that a phone can use and still work, so if a solar panel is going to be able to power any sort of mobile phone, the Lokki is a good starting point. Nokia sent Lokki phones to Sweden, Kenya, the Arctic and the middle of the Baltic Sea, and tracked how well users were able to take advantage of solar charging over a period of several months.
So how well did it go? The short answer is that solar charging is a disappointing way to power a phone. The best results came from Kenya, where a security guard (who was able to leave his phone out in the sun most of the time) managed 20 hours of talk time over a period of 59 days. This is only 41 days of standby time, though, meaning that the phone was not continuously usable.
Up in Northern Europe, things were significantly worse, and users were only able to achieve about three days of standby (or 1.4 hours of talk time) for every six days of charging. And that was during the summer, when the sun is stronger and up longer. During the winter, solar phones in the northern hemisphere would only get about half that much energy.
Yeah, it's not looking good for solar charging, even if you can live without a smartphone. Nokia says that for people who live in the middle of nowhere, a solar phone still might make sense since it's grid-independent, but you'd have to make a concerted effort to keep it charged, you wouldn't be able to use it much. That, and there would be times (probably every day) where it wouldn't work at all. For the rest of us, the only real way to take advantage of solar charging for mobile electronics is to use a solar panel that's significantly larger than the device you're powering, and you'll still need to pay attention to the weather and not use your phone that much.
Until technological breakthroughs allow us to start manufacturing solar panels with enough conversion efficiency that they don't need 24/7 sunlight (which is, you know, sort of impossible in most of the world), it looks like we'll have to rely on other portable energy sources for the foreseeable future. Fuel cells, anyone?