I bought a very cheap (£4) small USB solar panel to mount on my rear bicycle rack, does the job.

https://www.aliexpress.com/item/1005008036267839.html

It was very basic and fragile so I modded it with taped cardboard on the back with cutouts for the cable to go through and duct taped some velcro straps on. It's doing about 4W charging in bright sunlight.

Would be interesting to know what other solar solutions people are using on their bicycles....


USB Solar Panel mounted on bicycle rack, cable going into pannier to charge phone

USB Solar Panel mounted on bicycle rack

USB Solar Panel, modded with straps.