Annealing effect on Cu2ZnSn(S,Se)4 solar cell with Zn1-xMgxO buffer layer

(Zn,Mg)O buffer layer has attracted attention with the potential to control the conduction-band offset (CBO) at the interface between the buffer layer and the Cu2ZnSn(S,Se)(4) (CZTSSe) absorber, where the (Zn, Mg) O buffer layer is deposited by sputtering. The bandgap energy (E-g) of (Zn, Mg) O layers is tunable from 3.3 to 7.7 eV by changing the Mg/(Zn+Mg) ratio. Thus, short-wavelength response of solar cells with a (Zn,Mg)O buffer layer is higher than that with a CdS buffer layer. Nevertheless, the solar-cell performances with a (Zn,Mg)O buffer layer is significantly lower than that with a CdS buffer layer. The low performances are attributed to the sputtering damage on the CZTSSe absorber. To recover sputtering damage, the annealing treatment was conducted on CZTSSe solar cells. However, after the annealing treatment, the performances of CZTSSe solar cells decreased. Photoluminescence (PL) spectra of the CZTSSe absorbers without and with the annealing treatment were measured to evaluate the (Zn, Mg) O/CZTSSe junction quality. From the results, the PL intensity presents a slight variation, while the Eg of the CZTSSe absorbers decreases. Thus, the defects near the space-charge region activate. Consequently, the decrease of the sputtering damage was not confirmed by the annealing treatment. Therefore, a damage-free method for the deposition of (Zn,Mg) O layer is necessary. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim