ISM 2022 (Microscopy)

INSIGHTS INTO THE CHEMISTRY OF VAPOR PHASE INFILTRATION FOR IMAGING NON-FULLERENE ACCEPTORS

Oded Nahor 1 Anthony Cohen 2 Gitti L. Frey 1
1Department of Material Science and Engineering, Technion - Israel Institute of Technology, Haifa, Haifa, Israel
2Schulich Faculty of Chemistry, Technion - Israel Institute of Technology, Haifa, Haifa, Israel

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 entrapment is generally classified as either physical or chemical, with a weak (or none) or strong chemical reaction between the precursor and the organic host, respectively. 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 bulk heterojunction (BHJ) morphology. Currently, OSCs based on non-fullerene acceptors (NFA) exhibit high efficiencies making them the focal point and most promising for future technology development. The basic operational principles of NFA-based OSCs are similar to those realized for fullerene-based OSCs. Yet, the film morphologies` phase behavior and intricate details are not fully understood. In this work, we study the effect of VPI on NFA films to 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-b’]dithiophene (ITIC) and apply different diethylzinc VPI protocols and 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, which offers a new methodology for imaging not only OSCs but also the microstructure of complex thin-film organic blends through the judicious selection of reactive/non-reactive precursors to obtain chemical/physical staining for all types of electron microscopy.