A light concentrator could be useful here. Gathering the light from several dozen square meters to the allocated space can deliver 300 watts. But that's not practical for a robot...
Tandem solar cells have been measured with efficiencies
around 37% for a one-sun irradiation. So 8 square feet of panel could yield 300 Watts - but that's a very bleeding-edge proposition, likely to cost as much as a new house.