The OLPC was a small device -- the usable space on the back was roughly 5" x 8". Even nowadays, this would be a very small solar panel; with early-2000s technology, it'd be completely useless. The project experimented with larger external solar panels costing $50 or more (roughly the same cost as the entire laptop!) and found that they were minimally effective for charging: https://wiki.laptop.org/go/XO_Solar