Fig. 2: Ultrafast transient absorption spectra and kinetics of singlets and free charges in neat Y6, and optical pump terahertz probe conductivity.

a Transient absorption spectra, normalised to the absolute area integral, of a neat film of Y6 at different time delays. 700 nm, 200 fs pump pulse, excitation density ~ 10¹⁶ excitations/cm³. The spectrum evolves from the exciton, blue line, with a negative peak at 915 nm, to the red-shifted charge/CT state spectrum, orange line. b Confirmation of charge and exciton transient absorption spectra by comparison of neat species (extracted using genetic algorithm) with species resolved from blends of Y6 with hole accepting materials (PTB7-Th, polyTPD). The green dashed line shows the hole spectra of PTB7-Th blended with PCBM. c Singlet component from GA of transient absorption (dark blue line), at an excitation density of ~8 × 10¹⁶cm⁻³, compared with up-converted PL signal (cyan), at an excitation density of ~2 × 10¹⁶cm⁻³. d Initial kinetics of the exciton (blue) versus charge/CT (orange) TA kinetics in the first 2.5 ps after photoexcitation. The charge species shows a rise concomitant with the fast initial decay of the singlet species. 550 nm, 200 fs pump pulse, excitation density ~ 10¹⁸ excitations/cm³.