Insights into the chemistry of vapor phase infiltration for imaging non-fullerene acceptors

Vapor phase infiltration (VPI) processes offer a unique method to form hybrid materials by growing inorganic species inside polymer matrices from gaseous precursors. The VPI process follows three main stages: sorption of the gas phase precursors to the polymer matrix, their diffusion and entrapment in the matrix, and the chemical reaction. The distinct entrapment character of the precursors in different matrices can be utilized for imaging organic blends by selectively "staining" one component of the blend and imaging a cross-section in electron microscopy. Organic solar cell (OSC) films are an excellent platform to study VPI "staining" because they are composed of organic blends, a polymeric electron donor, and a small molecule electron acceptor (SMA), in a bulk heterojunction (BHJ) morphology. Currently, OSCs based on nonfullerene acceptors (NFA) exhibit high efficiencies making them the focal point and most promising for future technology development. Yet, the phase behavior and intricate details of the film morphologies are not fully understood. In this study we develop a method for direct imaging of NFA-based OSC blend morphology. We focus on the most investigated NFA, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-bi]dithiophene(ITIC) and apply different diethylzinc VPI protocols that reveal a strong chemical reaction between diethylzinc (DEZ), the gaseous precursor, and ITIC. This reaction leads to a unique staining mechanism with distinct feature characteristics. A similar reaction is also demonstrated for other NFAs that are currently popular in high-efficiency OSCs, confirming the suitability of this new methodology for imaging OSCs. These results give insights not only for imaging OSCs but also for developing VPI-based staining and imaging of other functional organic blends.