Optical Absorption Properties in Pentacene/Tetracene Solid Solutions

Modifying the optical and electronic properties of crystalline
organic thin films is of great interest for improving the performance
of modern organic semiconductor devices. Therein, the statistical
mixing of molecules to form a solid solution provides an opportunity
to fine-tune optical and electronic properties. Unfortunately, the
diversity of intermolecular interactions renders mixed organic crystals
highly complex, and a holistic picture is still lacking. Here, we
report a study of the optical absorption properties in solid solutions
of pentacene and tetracene, two prototypical organic semiconductors.
In the mixtures, the optical properties can be continuously modified
by statistical mixing at the molecular level. Comparison with time-dependent
density functional theory calculations on occupationally disordered
clusters unravels the electronic origin of the low energy optical
transitions. The disorder partially relaxes the selection rules, leading
to additional optical transitions that manifest as optical broadening.
Furthermore, the contribution of diabatic charge-transfer states is
modified in the mixtures, reducing the observed splitting in the 0–0
vibronic transition. Additional comparisons with other blended systems
generalize our results and indicate that changes in the polarizability
of the molecular environment in organic thin-film blends induce shifts
in the absorption spectrum.