Epitaxial Cu(In,Ga)Se2 Thin Films on Mo Back Contact for Solar Cells

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,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}//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 (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.