They in fact ARE being used in applications where it makes sense to do so. There are many examples of solar powered consumer electronics (e.g., pocket calculators, lighting, signs, etc).
There are number of technical economical and pragmatic reasons for not using PV in smart phones. let me elaborate few,
1) the spacial density of harvestable solar power is comparably low (less than100 W per square meter), therefore we need sufficiently large solar panel.
Even if we assume that we have sufficiently large PV panel to operate the smartphone..
2) Since today's solar panels absorb visible light as well, hence the solar panel could not be added in the front side of the phone. So only option is to add the PV panel to back cover. (at least until the researches on transparent solar panels produce some panels with sufficient power rating for smartphones )
3) In most of the time smart phones are kept out of sunlight ; Even when they are in use the back cover is mostly not directed toward the sun.
Hence there is very little benefit in adding a solar panel to a smartphone (with today's technology). Perhaps, a solar charger is the way !
Apple has had a patent approved related to the integration of solar charging in a device's touchscreen. Lets seen whether they are going to integrate it: