Photoinduced Charge Separation in Single‐Component Squaraine Films: The Key Role of Electrostatic Disorder

Neat films of organic dyes are an interesting technological platform for solar cells and photodetectors. Recently, the conventional biphasic bulk‐heterojunction paradigm for photoinduced charge separation was challenged by the observation that charge carriers can be photogenerated in single‐component materials. A comprehensive experimental and theoretical study is presented here on neat films of a near‐infrared squaraine dye, recently exploited to build an efficient photodetector relying on a single‐component active material. Experimental data collected on pristine and annealed films lead to the conclusion that field‐assisted photoinduced charge generation mainly occurs in the amorphous phase. This interpretation is fully supported by a theoretical model that, relying on crystallographic data and validated against optical spectra, adopts an essential state description of the squaraine dye and accounts for electrostatic and charge‐transfer intermolecular interactions. In the amorphous domain charge generation is made possible by the large electrostatic disorder induced at molecular sites by the randomly distributed surrounding molecules. Weak coupling between exciton and charge‐transfer states favors the formation of charge‐separated states. The present charge‐generation mechanism is applicable to the squaraine dye family and may also extend to other multibranched donor‐acceptor molecular systems. As such, it provides general guidelines for the rational design of single‐component active materials.