If we consider nuclear-rocket technology not feasible (whether due to technological or political limitations), the second best options for future transportation in solar system are perhaps solar-electric ships... there are many concepts to use ion-thrusters in combination with photovoltaics in large scale - both historical and more recent.
With respect to thin-film solar cells I have some idea what are the ultimate limits of thrust-to-weight ratio - currently something like 6W/g ... in future it can be in principle developed may be up to 100-400 W/g (10-40% conversion efficiency in 1 micron foil at Earth orbit)
Now what about Ion thrusters? What makes them heavy? I guess current experimental realizations do not try to push thrust-to-weight up hard enough, simply because they don't have enough power to feed them. But in combination with these thin-film solar cell, the bottleneck starts to be the weight of thrusters.
Intuitively I assume classical ion thrusters with grid electrodes can be made lighter than Hall thrusters. While the grids can be made of thin metallic foils or wires, the Hall thrusters need quite heavy electromagnets. But I don't really know the engineering challenges which have to be solved in order to make them lighter.
What I really like with this respect is this concept where the thrusters are distributed over the solar array