The idea is simple.
Why not take advantage of concentrated solar pv cells for space rockets. The flame from those rockets contain a lot of energy. Then some of that energy could be recaptured by solar cells on the rocket.
Thermophotovoltaic cells (TPV) with a high efficiency (>50%) can maybe be used. Since the temperatures could reach over ?3000 °C. The cells need to be protected. A TPV could perhaps then be built in.
If then the rocket fuel mass could be reduced it should save a lot of money since you then could lift more cargo.