From sputtered metal precursors towards Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells with shallow back grading

Bandgap grading is often employed in thin film solar cell absorbers for creating the back surface field that can reduce interface recombination at the back contact. Here, we investigate different pathways to obtain back graded Cu2Zn(Sn1-x,Gex)Se4 thin film solar cells based on a co-sputtered metal precursor and rapid thermal annealing route. The absorber bandgap can be precisely tuned for the whole compositional range of x = 0…1. While Ge does not accumulate towards the back in absorbers fabricated from uniform precursor, Ge-back graded absorbers can be obtained from stacked metal precursors. A linear back grading with a bandgap energy difference of up to 40 meV has been achieved. However, no significant improvement in open-circuit voltage and near-infrared response could be observed for the kesterite devices. This indicates that even steeper gradients are required to obtain an effective back surface field.