Beyond single crystals: Imaging rubrene polymorphism across crystalline batches through lattice phonon Raman microscopy

Abstract

Polymorphism is an issue troubling numerous scientific fields. A phenomenon where molecules can arrange in different orientations in a crystal lattice, polymorphism in the field of organic photovoltaic materials can dramatically change electronic properties of these materials. Rubrene is a benchmark photovoltaic material showing high carrier mobility in only one of its three polymorphs. To use rubrene in devices, it is important to quantify the polymorph distribution arising from a particular crystal growth method. However, current methods for characterizing polymorphism are either destructive or inefficient for batch scale characterization. Lattice phonon Raman spectroscopy has the ability to distinguish between polymorphs based on low frequency intermolecular vibrations. We present here the addition of microscopy to lattice phonon Raman spectroscopy, which allows us to not only characterize polymorphs efficiently and nondestructively through Raman spectroscopy but also concurrently gain information on the size and morphology of the polymorphs. We provide examples for how this technique can be used to perform large, batch scale polymorph characterization for crystals grown from solution and physical vapor transport. We end with a case study showing how Raman microscopy can be used to efficiently optimize a green crystal growth method, selecting for large orthorhombic crystals desired for rubrene electronic device applications.

Structural Disorder at the Edges of Rubrene Crystals Enhances Singlet Fission

“…Raman spectra collected in the center of a rubrene crystal display three prominent low-frequency modes at 32, 73, and 102 cm –1 (Figure C) that have previously been assigned to lattice phonon modes of orthorhombic rubrene. − Raman maps collected near the edges of rubrene crystals show that both the 32 and 102 cm –1 modes are strongly suppressed within a region extending 1–2 μm from the crystal’s edge (Figures C and S5), indicating a disruption of rubrene’s orthorhombic crystal structure in this region. No new modes appear that are associated with rubrene’s other known crystalline polymorphs, such as its triclinic and monoclinic forms, , which allows us to rule out their formation at orthorhombic crystal edges. Our results are most consistent with a scenario wherein rubrene’s orthorhombic structure becomes sufficiently disordered that its lattice phonon spectrum is largely washed out.…”

“…48−50 Raman maps collected near the edges of rubrene crystals show that both the 32 and 102 cm −1 modes are strongly suppressed within a region extending 1−2 μm from the crystal's edge (Figures 3C and S5), indicating a disruption of rubrene's orthorhombic crystal structure in this region. No new modes appear that are associated with rubrene's other known crystalline polymorphs, such as its triclinic and monoclinic forms, 48,49 which allows us to rule out their formation at orthorhombic crystal edges. Our results are most consistent with a scenario wherein rubrene's orthorhombic structure becomes sufficiently disordered that its lattice phonon spectrum is largely washed out.…”

X‐Ray Techniques Dedicated to Materials Characterization in Cultural Heritage

“…PXRD is often used as a useful tool for distinguishing polymorphs, which are defined as materials with the same chemical composition and different crystalline structures. [61] Semi-quantitative PXRD measurement provides the weight percentage of the phase content using Rietveld analysis. [62] Having prior knowledge of the elemental composition of samples can notably enhance confidence in correct phase identification.…”

“…Williamson–Hall (W–H), Halder–Wagner (H–W), Warren–Averbach (W–A), and size–strain plot (SSP) [60] methods are used to determine the crystallite size and the intrinsic micro‐strain. PXRD is often used as a useful tool for distinguishing polymorphs, which are defined as materials with the same chemical composition and different crystalline structures [61] . Semi‐quantitative PXRD measurement provides the weight percentage of the phase content using Rietveld analysis [62] .…”

Mixed Rubrene Cocrystals Offer Insights into Intermolecular Interactions Influencing Crystal Packing

“…Both rubrene mixtures produced crystals of various morphologies. To determine if mixed cocrystals were present or weresimply crystals of the individual rubrene components, we used Raman spectroscopy as a fast screening method to analyze the crystalline batches . We expected signatures in the fingerprint region (800–1600 cm –1 ) for both molecules if a mixed cocrystal was formed, as well as signatures unique from the individual components in the lattice phonon region (0–500 cm –1 ), which could indicate a new crystal packing formation .…”