Tuning the Crystallinity of the Herringbone Packing Structure in Phthalocyanine Nanoribbons Using in Situ Spectroscopies.

Understanding and controlling the interplay between crystallinity and exciton dynamics is critical for developing more efficient organic thin films for semiconducting devices. In this work, in situ linear absorption and single-shot transient absorption measurements are used to characterize the molecular aggregation of copper phthalocyanine (CuPc) during film deposition, revealing an intermediate stage during deposition when the molecules have formed H-aggregates but have yet to agglomerate into a herringbone structure. Experiments with supersaturated solutions of CuPc showed that the steps of aggregation can be separated, with H-aggregates first forming in solution followed by columns of H-aggregates, of unknown length, coming together to form nanoribbons. Films cast from solutions of supersaturated CuPc have a more crystalline structure and fits to transient absorption spectra showed that exciton-exciton annihilation becomes more rapid compared to regular films. In addition to presenting a method to control exciton diffusion lengths in films that tend to adopt a herringbone packing structure, this study also provides a framework for how in situ spectroscopies can be used to design new aggregate structures and control exciton dynamics.