Ultraviolet photoelectron spectroscopy (UPS) and Low-energy inverse photoemission spectroscopy (LEIPS) results show that the surface of PBDB-T:ITIC with 0.5% DIO features highly ordered crystalline ITIC with energy level (HOMO and LUMO) rising as much as 0.42 eV, leading to poor charge transport property. For DIO:ITIC binary surface, if the donor is absent, the spinodal demixing produces highly ordered crystals comparing to the crystals inside the film, while for CN:Y6 surface, the binodal demixing occurs as the liquid film bulk. Consequently, the Flory-Huggins interaction parameters are calculated.

It is found that during the solvent evaporation, a solvent-free liquid film surface (additive-enriched) is formed owing to volatility difference between chlorobenzene/chloroform (CB/CF) and DIO/CN. Here the phase evolution of the representative systems PBDB-T:ITIC (including ITIC’s derivates) and PM6:Y6 with DIO/CN additives is investigated by a combination of position-resolved and time-resolved spectroscopies, namely film-depth-dependent light absorption spectroscopy (FLAS) and time-dependent light reflection/scattering/fluorescence spectra. Although non-volatile additives like 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) are beneficial to improve power conversion efficiency (PCE) of organic solar cells (OSCs), the effects of such additives on phase evolution are now still ambiguous.

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