AbstractPhotovoltaic (PV)‐powered vehicle applications are very attractive for reducing CO2 emission and creation of new market. Development of Si tandem solar cells is very promising for high‐efficiency and low‐cost solar cells. This paper presents our approaches for III–V/Si 3‐junction tandem solar cells. In this paper, 24.9% efficiency Si heterojunction solar cells and mechanically stacked four‐terminal 35.8% InGaP/GaAs/Si three‐junction tandem solar cell are shown. Roadmap for realizing high‐efficacy three‐junction tandem solar cells of more than 40% is discussed in this paper. Because efficiencies of record cell of 36.1% with III–V/Si three‐junction tandem cells and 33.9% with perovskite/Si two‐junction cells are lower compared to 39.5% with III–V three‐junction solar cells, it is necessary to clarify and reduce several losses of Si tandem solar cells. This paper presents high efficiency potential of III–V/Si three‐junction and perovskite/Si two‐junction solar cells analyzed by using our analytical procedure and discusses about non‐radiative recombination, optical, and resistance losses in those Si tandem cells. Current status of various solar cell module efficiencies including our new record efficiency Si tandem solar cell module with an efficiency of 33.66% is also analyzed.