Organic semiconductors (OSCs) have been extensively studied in a variety of fields such as device engineering, and solid-state physics for their optical and electronic properties. Their potential for flexible device fabrication as well as their lightweight, low-temperature, and solution-processable characteristics make them ideal for the next generation of photovoltaics and electronic devices. Along with their desirable processability, OSCs undergo a process called singlet fission. Singlet fission is an exciton multiplication process in which one incident photon is absorbed into multiple triplet excitons. For solar cell applications, utilizing singlet fission materials could result in quantum efficiencies greater than 100%, leading to higher maximum solar cell efficiencies. However, small molecule OSCs are van der Waals bonded, limiting OSC charge carrier transport capabilities. Moreover, OSCs can pack in a variety of polymorphic structures, giving orders of magnitude differences in charge transport capabilities. Therefore, control over OSC structural properties including morphology and molecular packing is vital for their commercialization and usability.
In this study, physical vapor deposition and dynamic spin coating was used to fabricate C7-perylene diimide (C7-PDI) and 2D perovskite bilayer thin films. Using grazing incidence x-ray diffraction (GIXRD), Raman, UV-Vis, and photoluminescence spectroscopy, it was determined that the d-spacing between the π-π stacking plane of the C7-PDI molecules is influenced through post-deposition thermal annealing. Discrete bandgap transition energies and relative differences in steady-state photoluminescence and peak shifting were observed, indicative of variation in intermolecular interactions. Furthermore, the addition of post-deposition thermal annealing results in increased photoluminescence intensity and promotes further changes in the π-π stacking distance, implicating changes in the singlet fission rate for C7-PDI. Additional studies are thereby required to determine the extent of influence thermal annealing has on the resulting structural and optical properties, including singlet fission for C7-PDI molecules.
