A more than 70% enhancement in the thermoelectric power factor of single-crystal silicon is demonstrated in silicon nano-films, a consequence of the introduction of networks of dislocation loops and extended crystallographic defects. Despite these defects causing reductions in electrical conductivity, carrier concentration, and carrier mobility, large corresponding increases in the Seebeck coefficient and reductions in thermal conductivity lead to a significant net enhancement in thermoelectric performance. Crystal damage is deliberately introduced in a sub-surface nano-layer within a silicon substrate, demonstrating the possibility to tune the thermoelectric properties at the nano-scale within such wafers in a repeatable, large-scale, and cost-effective way.