Zn1−xMgxO second buffer layer of Cu2Sn1−xGexS3 thin-film solar cell for minimizing carrier recombination and open-circuit voltage deficit

Abstract

Zn1−xMgxO (ZMO) second buffer was applied to replace conventional ZnO second buffer layer of Cu2(Sn,Ge)S3 (CTGS) solar cells to reduce interface carrier recombination and open-circuit voltage deficit (VOC,def). Structure of the solar cell is glass/Mo/CTGS/CdS first buffer/ZMO second buffer/ZnO:Al (AZO)/Ni-Al. Bandgap energy (Eg) of the ZMO films is changed from 3.20 to 3.65 eV by varying Mg/(Mg + Zn) ratio from 0 (for pure ZnO) to 0.26. The increase in the Eg of ZMO films is attributable to the move of conduction band minimum (EC). It is determined that ZMO second buffer with increase in Eg from 3.20 to 3.55 eV yields the smoothing conduction band offset (CBO) at ZMO/CdS interface varying from −0.26 to +0.09 eV. Saturation current density (J0) is therefore reduced to the lowest value, and activation energy of recombination (EA) is the closet to the Eg of the CTGS absorber layer, implying the lowest interface carrier recombination (reduced VOC,def), and thereby increasing conversion efficiency (η) to 4.5% (Eg of ZMO: 3.55 eV). On the other hand, the ZMO second buffer with Eg larger than 3.55 eV results in the CBO at ZMO/CdS interface of +0.19 eV (Eg of ZMO: 3.65 eV), making the well of EC structure in ZMO/CdS/CTGS stacking layers, thus increasing interface carrier recombination, and severely decreasing the η to about 0.2%.