Fig. 4: Physical mechanism behind the unusual charge-carrier kinetics in NaBiS₂.

a Amplitude of the two principal components (A – blue, and B – red) extracted from the singular-value decomposition (SVD) analysis of the short-time TA spectrum. Black arrow indicates the wavelength corresponding to the optical bandgap of NaBiS₂. b Normalised kinetics of the two principal SVD components (colour legend same as in a), along with a monoexponential fit to component B (black dashed line). c Charge density isosurfaces of a relaxed electron-hole pair in a disordered NaBiS₂ supercell with sodium, bismuth, and sulphur atoms in pink, blue and yellow respectively. The translucent yellow and blue surfaces represent electron and hole densities, respectively. d Normalised absorption spectra of pristine and annealed (30 and 60 min) NaBiS₂ NC films. The annealing temperature was 100 ºC. Normalised signal kinetics of pristine and annealed (30 and 60 min) NaBiS₂ NC films acquired from (e) OPTP and (f) long-time TA measurements. The OPTP measurements had 400 nm wavelength pump excitations at a fluence of 23 μJ cm⁻² pulse⁻¹, and the long-time TA measurements had 355 nm wavelength pump excitations at a fluence of 32 μJ cm⁻² pulse⁻¹. In (d–f), the pristine sample is represented in green, 30 min annealed sample in red, and 60 min annealed sample in purple lines or circles. The black dashed lines in (e and f) represent the fitted curve for the pristine film.