Coexisting Glassy Phases with Different Compositions in NFA-Based Bulk Heterojunctions

Organic solar cell (OSC) bulk heterojunctions (BHJ) typically feature a rich phase morphology with the phase composition and distribution significantly affecting processes such as charge generation, recombination and extraction, and in turn, device performance. While fullerene-based BHJs are relatively well understood structurally, especially when blends with a flexible-chain donor are employed, donor: non-fullerene acceptor (NFA) blends are more challenging to elucidate. The reason is that NFAs often display different polymorphs; moreover, their glassy states can be complex. Focusing on blends of the widely investigated donor polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT), and the prototype 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), we reveal here the coexistence of two glassy phases: a molecularly intermixed and an ITIC-rich one. In P3HT-rich blends, both glassy phases are present as nanosized domains, evenly distributed in the BHJ, as visualized via vapor phase infiltration (VPI) "staining". In contrast, the 1:1 (by weight) and NFA-rich blends show clear, lateral phase separation between large (>500 nm) domains of the glassy phases and thinner polymer-rich domains that are unaffected by annealing. Our observations help to explain earlier P3HT: ITIC device studies; and also highlight the complexity of NFA-based BHJs, emphasizing the need for a deeper understanding of the phase behavior of such systems.