Aluminum-doped Zn1 - xMgxO as transparent conductive oxide of Cu(In, Ga)(S, Se)2-based solar cell for minimizing surface carrier recombination

A 19.5%-efficient Cu(In,Ga)(S,Se)(2) (CIGSSe)-based solar cell is obtained by replacing traditional CdS/ZnO buffer layers with Cd0.75Zn0.25S/Zn0.79Mg0.21O buffer layers for increasing short-circuit current density because band-gap energies of Cd0.75Zn0.25S and Zn0.79Mg0.21O are wider than those of CdS and ZnO, respectively. This yields the increase in external quantum efficiency in a short wavelength range of approximately 320 to 550 nm. Moreover, difference of conduction band minimum (E-C) between Zn1-xMgxO:Al (transparent conductive oxide, TCO) layer and CIGSSe absorber is optimized by varying [Mg]/([Mg]+[Zn]), x. It is revealed that Zn1-xMgxO:Al films with [Mg]/([Mg]+[Zn]) in a range of 0.10 to 0.12, enhancing E-g from 3.72 to 3.76eV, are appropriate as TCO because of their enhanced mobility and decreased carrier density. Addition of 12% Mg into ZnO:Al to form Zn0.88Mg0.12O:Al as TCO layer effectively decreases surface carrier recombination and improves photovoltaic parameters, especially open-circuit voltage and fill factor. This is the first experimental proof of the concept for optimizing E-C difference between TCO and absorber to minimize surface carrier recombination. Ultimately, conversion efficiency () of CIGSSe solar cell with alternative Cd0.75Zn0.25S/Zn0.79Mg0.21O/Zn0.88Mg0.12O:Al (TCO) layers is enhanced to 20.6%, owing to control of total E-C alignment, which is higher up to 12.6% relative as compared with the solar cell with traditional CdS/ZnO/ZnO:Al layers.