Solar thermal propulsion is the idea of using solar energy to heat a propellant and using that energy for thrust. If it worked as hoped it could be very useful in LEO and beyond. Portal Space Systems, the start-up here, points out there are already plenty of use cases for it now moving satellites into different orbits. This idea has been around since the 1950s, and could even be described as low-tech (it could work with mirrors heating water), but the logistics and infrastructure to support it might not be so simple, or economically viable.

I have a hard time seeing how this could possibly be more efficient than the existing propulsion methods.