On the Stability of Potential Photovoltaic Absorber In5S4

Due to the violation of the valence electron counting (VEC) rule as well as the excess Sn introduced in the synthesis, cubic In5S4 has been recognized as In4SnS4. However, recent reports have indicated the presence of In5S4 via X-ray diffraction characterization, and no extra Sn participates in the synthesis. Through density functional theory calculation, we find that In5S4 obeys the VEC rule by partly forming S1-in the S-S dimer configuration. Even though In5S4 is found to be theoretically unstable against InS and In secondary phases, the phonon dispersion and ab initio molecular dynamics simulation show that In5S4 exhibits dynamic stability, indicating that the nonequilibrium conditions (high pressure and high temperature) could help stabilize In5S4. Benefiting from low-lying p states of S-S dimer as well as s and p states from [In5]7+ tetrahedron in conduction bands, In5S4 exhibits a small direct band gap of 1.33 eV and strong visible light absorption. Our results provide a new perspective of charge balance on the stability issue of In5S4 and prove that it has great potential to be applied in the ultimate optoelectrical application.