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

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 (φΣμ) of the J-type aggregate thin film reached 6.5 × 10^-5 cm² V^-1 s^-1, exhibiting a photoconductivity 2.0 times greater than that of the H-type aggregate thin film (3.2 × 10^-5 cm² 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.