Single crystal Mo thin films are successfully grown on single‐crystal sapphire {0001} substrates using a newly developed two‐step sputtering method. Cu(In1‐x,Gax)Se2 (CIGS) thin films are then epitaxially grown on the Mo thin films by three‐stage co‐evaporation. Following that, a CdS buffer layer is deposited on the epitaxial (epi)‐CIGS thin films by a chemical bath deposition (CBD) method, leading to the overall epitaxial relations such as CdS{111}//CIGS{112}//MoSe2{0001}//Mo{110}//sapphire {0001}. The Cd‐diffusion into the CIGS occurs to approximately 30 nm from the CIGS/CdS interface boundary together with depleted Cu at the CIGS surface region. This suggests that Cd diffuses through Cu vacancies in the intra‐grain rather than at the grain boundaries. Two distinct solar cells are fabricated using epi‐ and polycrystalline (poly)‐CIGS thin films deposited on sapphire substrates in the same deposition run. The open‐circuit‐voltage (VOC) of epi‐CIGS solar cell is found to be higher than poly‐CIGS solar cell. The net carrier concentration (NCV) derived from the capacitance‐voltage (C‐V) measurements of epi‐CIGS solar cell is 3.5 times higher than poly‐CIGS solar cell, suggesting one of the causes of higher VOC. This large difference in the NCV is discussed based on the results of low‐temperature photoluminescence measurements.