Transfer of sub-bandgap photons between the thermal emitter and the cell is an important factor in the efficiency of thermophotovoltaic (TPV) power generation. Development of thin-film cells promises to enhance the reflection of sub-bandgap photons back to the emitter by mitigating undesired, sub-bandgap absorption that occurs in conventional TPVs. Here, we experimentally compare the sub-bandgap reflectance of an InGaAs-based cell structure with and without the growth substrate (InP). The thin-film structure exhibits higher sub-bandgap reflectance relative to the substrate-based structure. This increase in reflectance translates into significantly higher TPV efficiency based on an energy conversion simulation.