If you put a large cap (~10,000 uF) across the solar panel it will work much better for charging. Otherwise the current will porpoise up and down and the charger never really gets to go through it’s cycle without interruption. Adding the cap smooths out the current and stabilizes the whole system, making it much more efficient.
Note – just use a standard electrolytic cap. Supercaps do not seem to work well for this.