Polymorph selection at the surface of organic semiconductor glasses

From pharmacology to organic electronics, polymorph selection plays a decisive role in the performance of molecular materials across a broad range of applications. Although polymorphism has been investigated from the time of Ostwald, studies have primarily focused on solid-form selection in the bulk. Polymorph selection at surfaces is critical for materials with high surface to volume ratio like thin films and nanoparticles. For crystallization from the melt or glassy state, there is currently no framework to predict which polymorph is promoted at the surface. Here we study polymorph selection at the surface of glasses of model polymorphic organic semiconductor CBP (4,4'-bis(N-carbazolyl)-1,1'-biphenyl). Using physical vapor deposition (PVD), we produce glasses of CBP with and without a capping layer and observe crystallization upon annealing using a combination of polarized optical microscopy (POM), grazing incidence wide angle X-ray scattering (GIWAXS), and Raman spectroscopy. We observe that films with free surfaces produce a mixture of α and γ polymorphs, while capped films crystallize exclusively into the thermodynamically favored α form. We hypothesize that the air-film interface creates a favorable environment for the nucleation of the metastable γ polymorph, while the interior of the glass promotes the selection of the α polymorph. Our work creates insights into leveraging thermodynamic ranks of solid forms to generate predictions on polymorph selection at free surfaces.