An increase or modification of structural order in the vicinity of a solid substrate is known for a wide range of materials. For molecular materials crystallizing on a solid surface it has been observed that new polymorphic forms may exist near the interface with the substrate, which have structures different to those observed in the bulk. Such phases are termed as substrate‐induced phases (SIPs). The presence of an SIP in a compound or a class of materials can be of crucial significance in terms of their physical properties. However, the factors that drive such a process are not clearly understood or studied in depth. In this feature article, we review the current state of understanding concerning SIPs, giving examples of systems where SIPs have been observed, discussing their origins, and which questions remain to be answered. The role of the substrate in controlling the growth and subsequent structural order has been discussed in detail and the impact of polymorphism on organic electronic device properties has been addressed. Finally, the origin of SIPs has been correlated with their crystal structures and the differences with respect to the bulk structure are highlighted.

Combined experimental and theoretical investigations on thin films of pentacene are performed in order to determine the structure of the pentacene thin film phase. Grazing incidence x-ray diffraction is used for studying a pentacene thin film with a nominal thickness of $180\mathrm{nm}$. The crystal structure is found to exhibit the lattice parameters $a=0.592\mathrm{nm}$, $b=0.754\mathrm{nm}$, $c=1.563\mathrm{nm}$, $\alpha =81.5°$, $\beta =87.2°$, and $\gamma =89.9°$. These crystallographic unit cell dimensions are used as the only input parameters for ab initio total-energy calculations within the framework of density functional theory revealing the molecular packing within the crystal structure. Moreover, we calculate the electronic band structure of the thin film phase and compare it to that of the bulk phase. We find the intermolecular bandwidths of the thin film phase to be significantly larger compared to the bulk structure, e.g., the valence bandwidth is twice as large. This remarkable effect is traced back to an enhanced intermolecular $\pi \text{−}\pi$ overlap due to the upright standing molecules in the thin film phase.