Solvent‐Controlled Supramolecular Polymerization and Morphology‐Depended Photoconductivity Modulation in a Squaraine‐Naphthalene Diimide‐Squaraine Bulk p/n Heterojunction

Abstract We designed a donor–acceptor–donor (D–A–D) molecule, TISQ , consisting of squaraine as the electron donor and naphthalene diimide as the electron acceptor, connected via amide linkages that promote hydrogen bonding. This molecule undergoes self‐assembly with spatial segregation of donor and acceptor moieties depending on the nature of the solvent. In polar solvents, J‐type aggregates are formed via a cooperative nucleation–elongation mechanism, whereas in low‐polarity solvents, H‐type aggregates are formed through an isodesmic pathway. Microscopic analyses showed that J‐type aggregates adopt nanoparticle‐like morphologies, whereas H‐type aggregates assemble into higher‐order fibrous structures. Photoconductivity measurements demonstrated that J‐type aggregates exhibit enhanced charge transport relative to H‐type aggregates. The maximum transient photoconductivity ( φ Σ μ ) of the J‐type aggregate thin film reached 6.5 × 10 −5 cm 2 V −1 s −1 , exhibiting a photoconductivity 2.0 times greater than that of the H‐type aggregate thin film (3.2 × 10 −5 cm 2 V −1 s −1 ). Although bulk heterojunction solar cells with photoactive layers of the D–A–D molecule face challenges in forming continuous macroscopic p/n heterojunctions, these findings underscore that controlled self‐assembly of D–A–D molecules facilitates the formation of nanoscale p/n heterojunctions and the aggregate morphology plays a critical role in the charge transport.