Zn1-xMgxO:B (BZMO) films were deposited by metal organic chemical vapor deposition (MOCVD). Their optical and electrical properties were investigated for suitability as transparent conductive oxide (TCO) of the Cu(In,Ga) (S,Se)(2) (CIGSSe) solar cells. It is disclosed that with the increase in the [Mg]/([Mg] + [Zn]) of the BZMO up to 0.12 the optical band-gap energy (E-g) is increased to 3.52 eV, and the resistivity is reduced to about 3.1 x 10(-3) Omega cm owing to the increase in the Hall mobiltity. Moreover, the severe free-carrier absorption in the BZMO film is not observed. The BZMO is consequently appropriate as the TCO layer of the CIGSSe solar cells. The K-treated CIGSSe layers were applied as the absorbers of the solar cells. As a result, the 21.1%-efficient CIGSSe solar cell with a structure of Zn0.88Mg0.12O:B/CdS/CIGSSe/Mo/glass is obtained. In addition, the 20.2%-efficient Cd-free CIGSSe solar with a structure of Zn0.88Mg0.12O:B/Zn0.80Mg0.20O/CIGSSe/Mo/glass, fabricated by all-dry process, is realized with high short-circuit current density (J(sc)) of 39.7 mA/cm(2). The high Jsc is attributable to no optical loss in CdS buffer layer, no severe free carrier absorption, and the increased E-g of the BZMO layer.