DFT/CAM-B3LYP analysis of intermolecular interactions and morphological stability of binary PTB7-Th/Y6 blends in organic solar cells

Abstract

Y6 and PTB7-Th are widely used acceptor and donor materials in high-efficiency organic solar cells; however, their binary blends are known to suffer from poor long-term morphological stability. Here, we provide a molecular-level explanation for this instability using a systematic density functional theory (DFT) conformational analysis of heterogeneous (PTB7-Th/Y6) and homogeneous (Y6/Y6 and PTB7-Th/PTB7-Th) molecular pairings. Geometry optimizations and binding energy calculations were performed using B3LYP and long-range corrected CAM-B3LYP methods with the 6-31G(d) basis set, with explicit consideration of side-chain effects. A stepwise optimization strategy, progressing from individual sub-units to half-molecules and full molecular assemblies was employed. The results reveal that PTB7-Th homo-pairs exhibit the strongest intermolecular binding, followed by PTB7-Th/Y6 hetero-pairs, while Y6/Y6 interactions are comparatively weak. This hierarchy of binding energies per atom (with typical values of ∼2.2 eV for hetero-pairs and ∼1.9 eV for the average of homo-pairs) indicates that PTB7-Th/Y6 binary blends are intrinsically prone to excessive miscibility and morphological instability, consistent with experimental observations that favor ternary blend architectures. Importantly, we demonstrate that sub-unit interaction analysis reliably predicts the stability and preferred geometry of full molecular systems, establishing a transferable computational framework for assessing donor–acceptor compatibility in organic solar cells.