Lattice Dynamics\nof Cu<sub>2</sub>ZnSn(S<sub><i>x</i></sub>,Se<sub>1–<i>x</i></sub>)<sub>4</sub> Kesterite Thin-Film Solar Cells Studied\nby Nuclear Inelastic Scattering

Phonons play a crucial role in thermalization and non-radiative\nrecombination losses in semiconductors, impacting the power conversion\nefficiency of solar cells. To shed light on the lattice dynamics in\nCu<sub>2</sub>ZnSn(S<sub><i>x</i></sub>,Se<sub>1–<i>x</i></sub>)<sub>4</sub> (CZTSSe) thin-film solar cells and\nvalidate the extensive number of theoretical studies, we determine\nthe <sup>119</sup>Sn-partial phonon density of states (Sn-PDOS) by\nnuclear inelastic X-ray scattering. CZTSSe-based devices, one with\nnear-stoichiometric and two with off-stoichiometric compositions,\nare investigated, and the results are correlated with the corresponding\npower conversion efficiencies (PCEs) of 3.2, 7.6, and 10.6%, respectively.\nCompared to the near-stoichiometric cell, the main Sn-PDOS peak of\nthe off-stoichiometric cells broadens and slightly shifts to higher\nenergy; this effect is correlated with the type and concentration\nof the characteristic defects in the studied samples. Furthermore,\nthe Sn-PDOS of the 10.6% device is also obtained under <i>operando</i> (maximum power point) and open-circuit conditions. A comparison\nof the Sn-PDOS before and after the <i>operando</i> measurements\nsuggests structural changes, likely due to the formation of metastable\ndefects. In agreement with the theoretical studies, the Sn-PDOS of\nthe CZTSSe absorber shows additional peaks compared to CZTSe attributed\nto coupling of Sn to the vibrations of Se and S atoms. This work paves\nthe way for a further understanding of the lattice dynamics and subsequent\nenhancement of the PCEs of thin-film solar cells as well as other\napplied materials and devices containing elements that are Mössbauer-active\nand hence suitable for nuclear inelastic scattering.