Blended organic thin films have been studied during the last decades due to their applicability in organic solar cells. Although their optical and electronic features have been examined intensively, there is still a lack of detailed knowledge about their growth processes and resulting morphologies, which play a key role in the efficiency of optoelectronic devices such as organic solar cells. In this study, pure and blended thin films of copper phthalocyanine (CuPc) and the Buckminster fullerene $\left({\mathrm{C}}_{60}\right)$ were grown by vacuum deposition onto a native silicon oxide substrate at two different substrate temperatures, 310 and 400 K. The evolution of roughness was followed by in situ real-time x-ray reflectivity. Crystal orientation, island densities, and morphology were examined after the growth by x-ray diffraction experiments and microscopy techniques. The formation of a smooth wetting layer followed by rapid roughening was found in pure CuPc thin films, whereas ${\mathrm{C}}_{60}$ shows a fast formation of distinct islands at a very early stage of growth. The growth of needlelike CuPc crystals losing their alignment with the substrate was identified in co-deposited thin films. Furthermore, the data demonstrate that structural features become larger and more pronounced and that the island density decreases by a factor of four when going from 310 to 400 K. Finally, the key parameters roughness and island density were well reproduced on a smaller scale by kinetic Monte Carlo simulations of a generic, binary lattice model with simple nearest-neighbor interaction energies. A weak molecule-substrate interaction caused a fast island formation and weak interaction between molecules of different species was able to reproduce the observed phase separation. The introduction of different same-species and cross-species Ehrlich-Schwoebel barriers for interlayer hopping was necessary to reproduce the roughness evolution in the blend and showed the growth of CuPc crystals on top of the thin film in agreement with the experiment.

• Received 26 November 2020
• Revised 8 March 2021
• Accepted 16 March 2021

DOI:https://doi.org/10.1103/PhysRevMaterials.5.045601