We present a methodology for applying silicon pho-nonic crystals (Si PnCs) to photovoltaics (PV). One-dimensional PnCs made from ultrathin Si, indium tin oxide, and graphene are designed, as well as two-dimensional Si-only PnCs. The general elastic wave equations are employed to solve the frequency band structures. The obtained bandgap can effectively suppress the carrier relaxation. The potential of Si PnCs for PV applications is theoretically assessed within our model. The calculated upper limit of the thermodynamic efficiency is approximately 58%, which is well beyond the Shockley–Queisser limit of either a Si solar cell or an all-Si tandem solar cell. Very importantly, our calculations show that it is not necessary to fully suppress the carrier relaxation to achieve ultrahigh efficiency. This work offers a strategy to develop ultrahigh-efficiency single-junction Si solar cells using the Si PnCs with ultrathin absorbers at extremely material cost.