Solar Impulse proved to be capable of flying day and night without a single drop of fuel while carrying a pilot. Wouldn't it be easier to design an unmanned solar powered aircraft? Are there any? What about helicopters? Are they more difficult to make? What about automated landing at night time? I suppose there should be a lot of people interested in fuel-free aircraft, but why the only one I've heard of is Solar Impulse?
The raison d'être of Solar Impulse is to carry a single human being around for a long time. Of course an unmanned version would have better performance (I doubt it would be easier to design), but what would be the purpose of such a craft?
Solar Impulse is so fragile that it can only fly in unusually clement weather, so any scheduled service is out of the question. This would need a conventional backup, and if one is around anyway, why not use the conventional backup also on the few days of the year which would allow Solar Impulse to fly? It would also save the staff of meteorologists and ground support that Solar Impulse needs.
For high-altitude observation solar-powered drones have been tested (and found lacking) in the past. Helicopters have such high power demands that solar propulsion is not a viable option.