flexible and cheap solar cells? howto transform the energy?
You can do all you want by shopping.

You can try ebay www.ebay.com
 and amazon www.amazon.com

both of which will have a number of sellers of this sort of thing.

What country are you in?
 Doing it your self makes little sense in most cases.
 if you do want to make your own solar to iPhone charger then having a switching power supply in the middle may be a good idea.

Here's quite a good way to make one yourself. Solar Minty Boost





Basic Minty Boost here.





Instructables Minty Boost