Annealing Behavior with Thickness Hindered Nucleation in Small-Molecule Organic Semiconductor Thin Films

We introduce a comprehensive overview of spontaneous and thermal induced structure changes in thin films of the high performance small-molecular organic semiconductor 5,11-bis(triethyl silylethynyl) anthradithiophene (TES-ADT). Most importantly, we find that nucleation of the TES-ADT high temperature β phase can be induced only for films thicker than a critical thickness, while it is hindered for thinner films. The described limited crystal phase nucleation, the circumstances of which are unique in the realm of small-molecule organic semiconductors, is explained in terms of a two step nucleation mechanism while taking into account the anisotropic surface energy of crystal grains. Furthermore, we identify changes in the optical absorption spectra observed during aging of as deposited films with a spontaneous transformation from the amorphous phase to the α₂ phase via the α₁ phase of TES-ADT. The transformation takes place on the time scale of several days. Finally, we determine temperature dependence of lattice parameters of the α₁ and α₂ phases and the lattice parameters of the β phase. Thermal expansion of the α₁ phase is found to be much lower than that of the α₂ phase.