Fig. 3: The photophysical characterization of the dry film and the mechanism for the light-induced charge generation.

a In situ AM-KPFM scan of the film surface during exposure to 60 s long light pulses (5 mW/cm²). Top: height data, middle: corresponding surface potential, bottom: potential profile along the dotted line of the middle panel. Light pulse intervals are highlighted in red. The substrate was ITO. b TA spectra of intrinsic p(C₆NDI-T) film (red) and the ITO/ p(C₆NDI-T) bilayer (blue) at different excitation irradiances at a pump-probe time delay of 0.3–1 ps. Overlaid are the DIA anion spectra (orange), steady-state absorption (gray), and time-resolved fluorescence spectra integrated over the first 100 ps (green). Note that the region around 750–850 nm in the TA spectra is impacted by the 800 nm white light seed scattering (Supplementary Fig. 9). c TA spectra of the ITO/p(C₆NDI-T) bilayer at different pump-probe delay times for irradiance of 36 mW/cm² (i.e., fluence of 24 μJ/cm²). d Picosecond-nanosecond kinetics of the ITO/p(C₆NDI-T) interface, compared with the time-resolved fluorescence kinetics. Overlaid are double exponential decay fits (Supplementary Table 1). e Energy diagram describing the formation of an ICT complex at ultrafast timescales and the ICT dissociation process into fully charge-separated (CS) states. Vertical upward transitions represent the pump and probe wavelengths of the different photo-excited species. f The proposed mechanistic view of the light-induced surface photovoltage. Energy band representation of the metal and the OMIEC before being in contact (left). S₁, S_0, and E_F stand for the first excited state, the ground state, and the Fermi level, respectively. Fermi level pinning before illumination and the induced band flattening upon illumination (right). Depletion layer and surface photovoltage change are denoted by L_d and Δ_OCP, respectively.