You need a solar panel connected to a battery with sufficient capacity connected to a power supply that can provide the required voltage and current to the light string. There are a number of such devices on the market.

Thanks! So batteries are required as well? Maybe not efficient then? Do you happen to have a link to a device as an example?

It is not a matter of efficiency. Solar panels generate energy only when there is sufficient light.

Batteries are needed to store the energy harvested by the solar panels for use at a later point in time (night).

If the lights run on line voltage/ low voltage, you could conceivably use a GoalZero or similar system (Inergy, ...) of panels + battery.

Or you could get something like powerpractical's luminoodle and combine it with your solar panel of choice.

https://powerpractical.com