Solar energy is plentiful and efficient, but is has major downsides with reliability. The planet's rotation, clouds, and a myriad of other factors limit the area where it can be obtained.
Solar Cells can be gathered from chests in aerial environments which will routinely charge after they are opened. Keep an eye out for the charging animation and when it is done, simply visit the chest again to collect more cells.
Each chest can only charge a limited amount of Solar Cells which increases the higher the chest is located.
Can be found at:Edit
Aerial Planet: 140-250 altitude
Solar Cell only can be obtained from chest.
When you find a chest that gives you some solar cells, just stand over the chest and it will slowly fill up your cargo with solar cells.