  Heterostructures composed of pentacene (PEN) molecules and transition metal
dichalchogenides (TMDs) are promising materials for small, flexible and
lightweight photovoltaic devices and various other optoelectronic applications.
The effects of changing concentration and orientation of adsorbed pentacene
molecules on two-dimensional monolayer substrates of TMDs, namely MoS$_2$,
MoSe$_2$, WS$_2$ and WSe$_2$, were investigated using first-principles
calculations based on density functional theory. We examined the structural and
electronic properties of the corresponding PEN/TMD heterostructures and
compared these between differing pentacene concentrations and the orientations
of pentacene with respect to the underlying substrate crystal structure. We
analyse the band alignment of the heterostructures and demonstrate a
concentration-dependent staggered-to-straddling (typeII-I) band gap transition
in PEN/MoSe$_2$
