Fig. 1. (Color online) (a) Bright-field cryo-TEM images of intermixed PM6:Y6 7 : 3 NPs and (b) phase-separated core-shell PM6:PCBM 2 : 8 NPs. (c) Energy level diagram of PM6, Y6, and PCBM measured by UPS and IPES. The dashed lines correspond to the proton reduction potential (H⁺/H₂), water oxidation potential (O₂/H₂O), and the calculated potential of the two-hole oxidation of ascorbic acid to dehydroascorbic acid in solution (DHA/AA) at pH 2 (the experimentally measured pH of 0.2 mol/L ascorbic acid).

Fig. 2. (Color online) Ultrafast TAS characterization for neat PM6, PM6:Y6 7 : 3, and PM6:PCBM 2 : 8 nanoparticles in aqueous suspensions. (a) Transient absorption spectra of PM6 NPs at different time delays following excitation at 550 nm (fluence: 7.5μJ/cm²). (b) Comparison of transient absorption decay dynamics for neat PM6, PM6:PCBM 2 : 8, and PM6:Y6 7 : 3 NPs excited at 550 nm and probed at 1150 nm, assigned to PM6 exciton decay, with the long-lived residual signal assigned to PM6 polaron decay in the heterojunction NPs. (c, d) Transient absorption spectra of PM6:PCBM 2 : 8 NPs (c) and PM6:Y6 7 : 3 NPs (d) at different time delays, also excited at 550 nm (fluence: 7.5μJ/cm²). The disconnect in the transient absorption spectra axis corresponds to the pump laser scattering (550 nm) and the switch from visible to NIR detector (800–850 nm). (e) Schematic representation of exciton decay and electron/energy transfer processes in these NPs.