22%-efficient Cd-free Cu(In,Ga)(S,Se)2 solar cell by all-dry process using Zn0.8Mg0.2O and Zn0.9Mg0.1O:B as buffer and transparent conductive oxide layers

Cd-free Cu(In,Ga)(S,Se)(2) (CIGSSe)-based solar cell fabricated by all-dry process is developed to eliminate optical loss in CdS buffer and be readily applied into in-line process. (Zn,Mg)O:B (BZMO) films can be deposited with the increased band-gap energy (E-g) by metal organic chemical vapor deposition, and their optical and electrical properties are compared with those of (Zn,Mg)O:Al (AZMO) films prepared by sputtering method. It is shown that the AZMO and BZMO are suitable as transparent conductive oxide (TCO) layer to avoid the optical loss at short wavelength of the solar cell. The free-carrier absorption of the BZMO films is additionally lower than that of the AZMO, attributable to the lower carrier concentration. The low resistivity of the BZMO films with Mg content up to 0.1 in a range of 3 x 10(-2) to 6 x 10(-2) Omega.cm is observed. The CIGSSe solar cells are then fabricated with the Cs-treated CIGSSe layers as the absorbers. Consequently, the CIGSSe solar cell with the conventional CdS buffer layer possesses the conversion efficiency (eta) of 21.7%. In addition, Cd-free CIGSSe solar with a structure of glass/Mo/CIGSSe/Zn0.8Mg0.2O/Zn0.9Mg0.1O:B has the increased short-circuit current density to 39.8 mA/cm(2), thereby enhancing the eta to 22.0%. This is because there is the reduced free-carrier absorption and increased E-g (3.57 eV) of Zn0.9Mg0.1O:B layer, which is proved for the high-level technology. The recombination rates of the solar cells are moreover discussed.