Bandgap-graded Cu₂Zn(Sn_1-xGe_x)S ₄ thin-film solar cells derived from metal chalcogenide complex ligand capped nanocrystals

We demonstrate organic residue free, bandgap-graded Cu ₂Zn(Sn_1-xGe_x)S₄ (CZTGeS) thin-film solar cells based on metal chalcogenide complex (MCC) ligand capped nanocrystals (NCs). The bandgap of the CZTGeS films is tuned by varying the amount of Sn₂S₆^4- MCC ligand absorbed on the surface of the Cu₂ZnGeS₄ (CZGeS) NCs, without an undesirable postselenization process. Using CZGeS NCs inks with three different Sn/(Ge+Sn) ratios, bandgap-graded CZTGeS thin films are obtained via multicoating and annealing procedures. Compositional and spectroscopic analyses along the film thickness confirm that the band-graded CZTGeS absorber layer, with a gradually increasing bandgap from the back contact to the p-n junction, is successfully accomplished. Compared with an ungraded band structured CZTGeS cell, this normal grading structure facilitates both higher short circuit current and open-circuit voltage, facilitating a power conversion efficiency of 6.3%.