Single crystal Mo thin films are successfully grown on single-crystal sapphire {0001} substrates using a newly developed two-step sputtering method. Cu(In 1-x ,Ga x )Se 2 (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}//MoSe 2 {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 (V OC ) of epi-CIGS solar cell is found to be higher than poly-CIGS solar cell. The net carrier concentration (N CV ) 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 V OC . This large difference in the N CV is discussed based on the results of low-temperature photoluminescence measurements.