Crystal Chemistry and Electronic Structure of the Photovoltaic Buffer Layer, (In<sub>1‑<i>x</i></sub>Al<sub><i>x</i></sub>)<sub>2</sub>S<sub>3</sub>

We have investigated the effects of the aluminum substitution in In<sub>2</sub>S<sub>3</sub> using an approach that combines X-ray diffraction, nuclear magnetic resonance (NMR) experiments, and density functional theory (DFT) calculations. For the very first time, an understanding of the evolution of the local structure in this family with Al-content is reached based on the synergy of these three techniques. In particular, the comparison of the measured and simulated <sup>27</sup>Al NMR quadrupolar constants allows an unambiguous attribution of the Al-sites in the investigated (In<sub>1‑<i>x</i></sub>Al<sub><i>x</i></sub>)<sub>2</sub>S<sub>3</sub> compounds (0 ≤ <i>x</i> ≤ 0.185). It shows that for high Al-contents a new tetrahedral site (Td(4a)) is occupied, leading to a tetragonal to cubic modification. The present paper provides a definite explanation of the band gap reduction upon the aluminum substitution in In<sub>2</sub>S<sub>3</sub>, which is directly related to the occupation of the Td(4a) site. The present results could help to improve the performances of solar cells using In<sub>2</sub>S<sub>3</sub>-based compounds as alternative buffer layers.