Lattice Dynamics of Cu₂ZnSn(S _x ,Se_{1- x} )₄ Kesterite Thin-Film Solar Cells Studied by Nuclear Inelastic Scattering

Phonons play a crucial role in thermalization and non-radiative recombination losses in semiconductors, impacting the power conversion efficiency of solar cells. To shed light on the lattice dynamics in Cu₂ZnSn(S _x ,Se_1-x )₄ (CZTSSe) thin-film solar cells and validate the extensive number of theoretical studies, we determine the ¹¹⁹Sn-partial phonon density of states (Sn-PDOS) by nuclear inelastic X-ray scattering. CZTSSe-based devices, one with near-stoichiometric and two with off-stoichiometric compositions, are investigated, and the results are correlated with the corresponding power conversion efficiencies (PCEs) of 3.2, 7.6, and 10.6%, respectively. Compared to the near-stoichiometric cell, the main Sn-PDOS peak of the off-stoichiometric cells broadens and slightly shifts to higher energy; this effect is correlated with the type and concentration of the characteristic defects in the studied samples. Furthermore, the Sn-PDOS of the 10.6% device is also obtained under operando (maximum power point) and open-circuit conditions. A comparison of the Sn-PDOS before and after the operando measurements suggests structural changes, likely due to the formation of metastable defects. In agreement with the theoretical studies, the Sn-PDOS of the CZTSSe absorber shows additional peaks compared to CZTSe attributed to coupling of Sn to the vibrations of Se and S atoms. This work paves the way for a further understanding of the lattice dynamics and subsequent enhancement of the PCEs of thin-film solar cells as well as other applied materials and devices containing elements that are Mössbauer-active and hence suitable for nuclear inelastic scattering.