  Blending of semiconductors for controlling the energy levels (band structure
engineering) is an important technique, in particular, for optoelectronic
applications. The underlying physics is the delocalized Bloch states, which
average over the potential landscape of the blend. For organic semiconductors,
it has been shown that two quite different effects, the dielectric constant and
electrostatic interaction between molecules, can be used to tune the energy gap
and ionization energy of disordered and weakly crystalline organic
semiconductor blends. It is so far not known whether the electronic
delocalization in organic crystals with large bandwidths can contribute to the
energy structure engineering of the blend in a way similar to that in inorganic
semiconductors. Here, we investigate the growth of highly ordered organic
thin-film blends with a similar chemical structure and show the effect of band
structure engineering by spectroscopic methods. We rationalize the experimental
results with comprehensive theoretical simulations, showing that the
delocalization is a significant effect. Our work paves the way for engineering
the band structure of highly ordered organic semiconductor thin films that can
be tailored for the desired optoelectronic device application.
