Crystal Morphology and Growth in Annealed Rubrene Thin Films

While controlled crystallization of organic thin films holds great potential for enhancing the performance of electronic devices, quantitative understanding of the processes involved is limited. Here, we characterize the thin film crystal growth of the organic semiconductor rubrene during annealing using polarized optical microscopy with a heated stage for in situ measurements, followed by atomic force microscopy and X-ray diffraction. During annealing, the film undergoes transitions from predominant growth of a polycrystalline triclinic crystal structure to single crystal orthorhombic, followed by polycrystalline growth of the orthorhombic polymorph. Observation of crystal morphology with time allows determination of the crystal orientation, which is used in conjunction with crystal size measurements to determine the crystallization activation energies for the observed growth phases and crystal planes.