We report the study of the thermoelectric properties of degenerate, boron-doped polycrystalline silicon on insulator structures. The occurrence of a regime where both the Seebeck coefficient and the conductivity increase is confirmed. This results in a power factor P of 13 mW K-2 m-1. We propose that such high values of P may be determined by adiabatic energy filtering occurring at grain boundaries decorated by segregated boron.