Influence of Free Surfaces on Polymorph Selection in Vapor-Deposited Glasses of Organic Semiconductors

Organic semiconductors are of interest for a broad range of optoelectronic and energy harvesting applications. In all these applications, a high degree of control over crystallization outcomes is of central importance. Here, we aim to understand the effect of free surfaces on polymorph selection during annealing of vapor-deposited glasses. To this end, we prepared vapor-deposited glasses of CBP (4,4′-bis(carbazol-9-yl)biphenyl) both with and without a capping layer. We demonstrate using polarized optical microscopy, grazing incidence diffraction, and Raman spectroscopy that the surface and bulk of CBP films select different crystal polymorphs. Our work demonstrates that elimination of free surfaces is crucial for achieving polymorphic purity in crystalline films of small-molecule organic semiconductors.