Replying to Electronics/Circuits Question

Hello NS'ers. I want to make a solar charger for my smartphone. Is there any reason I can just splice a mini USB cable directly to a solar cell and do it that way? Most online tutorials recommend using the solar cell to charge a Li/Ion battery and then using that indirectly to power up the phone. This basically doubles the cost of the setup and I'd like to avoid it. I know its a super random question but there's gotta be some people with experience on here.