At a surface level, the idea is appealing, as manufacturers search for ways to prolong battery life in spite of the fact that advancements to battery capacity are nowhere to be seen. However, solar panels need to be relatively big and exposed the sun to work. This doesn’t really seem to gel with the form factor of a phone, which are inherently small and stowed in pockets when not in use.
Perhaps the technology will be more applicable as part of an iPad or MacBook, where the potential surface area of a solar panel is many orders of magnitude larger, but I just can’t see it for the iPhone.