Surface crystallinity enhancement in organic solar cells induced by spinodal demixing of acceptors and additives

Although non-volatile additives like 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) are beneficial for improving the power conversion efficiency (PCE) of organic solar cells (OSCs), the effects of such additives on phase evolution are still ambiguous. Here the phase evolution of the representative systems PBDB-T:ITIC (including ITIC's derivatives) and PM6:Y6 with DIO/CN additives is investigated by a combination of position-resolved and time-resolved spectroscopy techniques, namely film-depth-dependent light absorption spectroscopy (FLAS) and time-dependent light reflection/scattering/fluorescence spectroscopy. It is found that during solvent evaporation, a solvent-free liquid film surface (additive-enriched) is formed owing to the volatility difference between chlorobenzene/chloroform (CB/CF) and DIO/CN. Consequently, the Flory-Huggins interaction parameters are calculated. For the DIO:ITIC binary surface, if the donor is absent, spinodal demixing produces highly ordered crystals compared to the crystals inside the film, while for the CN:Y6 surface, binodal demixing occurs as in the liquid film bulk. 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 the energy levels (HOMO and LUMO) rising as much as 0.42 eV, leading to poor charge transport properties. After thermal annealing (TA), the partial ITIC on the surface transforms to an edge-on orientation owing to Ostwald ripening, leading to a decrease of ITIC's energy level by 0.81 eV, which matches that of PBDB-T. In contrast, the surface absorption spectra of PM6:Y6 blend films deposited with 0.5% CN are invariable after TA treatment, contributing to less PCE variation. The performances of additive devices with different TA conditions are associated with the optical absorption of the film surface, proving that surface phase segregation significantly determines the photovoltaic performance.