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Low-lying excited states in crystalline perylene

Significance Molecular solids are an important class of highly tunable, chemically diverse, cheap-to-process materials with promise for next-generation organic optoelectronics. Bound largely by noncovalent interactions, these materials harbor unique charge carrier generation and transport phenomena distinct from conventional semiconductors, an understanding of which requires a detailed description of the excited-state properties of molecular solids. Recent advances in synthetic techniques, ab initio theory, and spectroscopic measurements have enabled such a description, and here we perform quantitative theoretical calculations and high-resolution measurements on a high-quality single-crystal thin film of perylene, an exemplar molecular solid. We obtain excellent agreement between theory and experiment, and a clear picture of the low-lying excitations in perylene responsible for its distinctive optical properties. Organic materials are promising candidates for advanced optoelectronics and are used in light-emitting diodes and photovoltaics. However, the underlying mechanisms allowing the formation of excited states responsible for device functionality, such as exciton generation and charge separation, are insufficiently understood. This is partly due to the wide range of existing crystalline polymorphs depending on sample preparation conditions. Here, we determine the linear optical response of thin-film single-crystal perylene samples of distinct polymorphs in transmission and reflection geometries. The sample quality allows for unprecedented high-resolution spectroscopy, which offers an ideal opportunity for judicious comparison between theory and experiment. Excellent agreement with first-principles calculations for the absorption based on the GW plus Bethe–Salpeter equation (GW-BSE) approach of many-body perturbation theory (MBPT) is obtained, from which a clear picture of the low-lying excitations in perylene emerges, including evidence of an exciton–polariton stopband, as well as an assessment of the commonly used Tamm–Dancoff approximation to the GW-BSE approach. Our findings on this well-controlled system can guide understanding and development of advanced molecular solids and functionalization for applications.