Clapham Margaret L, Leighton Ryan E, Douglas Christopher J, Frontiera Renee R
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Chem Phys. 2021 Dec 21;155(23):234703. doi: 10.1063/5.0065496.
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.
多晶型现象是困扰众多科学领域的一个问题。在有机光伏材料领域，多晶型现象指分子在晶格中能够以不同取向排列，这种现象会极大地改变这些材料的电子特性。红荧烯是一种基准光伏材料，仅在其三种多晶型之一中表现出高载流子迁移率。要在器件中使用红荧烯，量化特定晶体生长方法产生的多晶型分布很重要。然而，当前用于表征多晶型的方法要么具有破坏性，要么在批量规模表征方面效率低下。晶格声子拉曼光谱能够基于低频分子间振动区分多晶型。我们在此展示了将显微镜技术与晶格声子拉曼光谱相结合，这使我们不仅能够通过拉曼光谱高效、无损地表征多晶型，还能同时获取多晶型的尺寸和形态信息。我们提供了一些示例，说明该技术如何用于对通过溶液生长和物理气相传输生长的晶体进行大规模、批量规模的多晶型表征。最后我们通过一个案例研究展示了拉曼显微镜技术如何用于高效优化绿色晶体生长方法，筛选出红荧烯电子器件应用所需的大型正交晶体。
