The fit of the temperature dependence of the dephasing rate of M2 with the cubic overtone model is shown in Fig. ). It yields an anharmonic constant and the phonon-defect scattering of γ 0 = 0.030 ± 0.005 ps -1 and Γ 0 = 0.09 ± 0.02 ps -1 , respectively. We highlight that the model cannot reproduce the experimental trend without the inclusion of the defect-scattering contribution Γ 0 even if the two decay phonons are set as fitting parameters (with energy conservation as a condition) allowing for different decay processes. This implies that the phonon decoherence at low temperature is dominated by scattering by defects . This is not entirely surprising for a solution-processed polycrystalline material, but it is intriguing to observe the relevance of defects in the phonon dynamics and not just in the electronic processes. The microscopic nature of the defects is, however, out of scope of the current work.