I'm not sure a DIY phone charger makes a lot of sense because the components would cost you way more than buying a readily available one. Searched a site that I used to buy my bike lights for "solar charger" and came up with 41 results. Price depends upon the capacity of the lithium polymer battery built into the charger. Starts at $10 including shipping.
solar phone chargers - Buy Cheap solar phone chargers - From Banggood