a diode and a step up, because one lithium cell is 3,6 V. Also you need a bit of logic to prevent lipo from damage. There are chip that already do everything for you, take a look here: Solar powered lithium-polymer battery charger
a diode and a step up, because one lithium cell is 3,6 V. Also you need a bit of logic to prevent lipo from damage. There are chip that already do everything for you, take a look here: Solar powered lithium-polymer battery charger