Design of quaternary chalcogenide photovoltaic absorbers through cation mutation

Design of chalcogenide photovoltaic absorbers is carried out systematically through sequential cation mutation, from binary to ternary to quaternary compounds, using first-principles electronic structure calculations. Several universal trends are identified for two classes of quaternary chalcogenides (I₂-II-IV-VI₄ and I-III-II₂-VI₄ systems). For example, the lowest-energy structure always has larger lattice constant a, smaller tetragonal distortion parameter ¿ = c/2a, and larger band gap than the metastable structures for common-row cation mutations. The band structure changes on mutation illustrate that although the band gap decreases from binary II-VI to ternary I-III-VI₂ are mostly due to the p-d repulsion in the valence band, the decreases from ternary I-III-VI₂ to quaternary I₂-II-IV-VI₄ chalcogenides are due to the downshift in the conduction band caused by the wavefunction localization on the group IV cation site. We find that I₂-II-IV-VI₄ compounds are more stable in the kesterite structure, whereas the widely-assumed stannite structure reported in the literature is most likely due to partial disorder in the I-II (001) layer of the kesterite phase. Ten compounds are predicted have band gaps close to the 1 to 2 eV energy window suitable for photovoltaics.