  Advances in hybrid organic/inorganic architectures for optoelectronics can be
achieved by understanding how the atomic and electronic degrees of freedom
cooperate or compete to yield the desired functional properties. Here we show
how work-function changes are modulated by the structure of the organic
components in model hybrid systems. We consider two cyano-quinodimethane
derivatives (F4-TCNQ and F6-TCNNQ), which are strong electron-acceptor
molecules, adsorbed on H-Si(111). From systematic structure searches employing
range-separated hybrid HSE06 functional including many body van der Waals
contributions, we predict that despite their similar composition, these
molecules adsorb with significantly different densely-packed geometries in the
first layer, due to strong intermolecular interaction. F6-TCNNQ shows a much
stronger intralayer interaction (primarily due to van der Waals contributions)
than F4-TCNQ in multilayered structures. The densely-packed geometries induce a
large interface-charge rearrangement that result in a work-function increase of
1.11 and 1.76 eV for F4-TCNQ and F6-TCNNQ, respectively. Nuclear fluctuations
at room temperature produce a wide distribution of work-function values, well
modeled by a normal distribution with {\sigma}=0.17 eV. We corroborate our
findings with experimental evidence of pronounced island formation for F6-TCNNQ
on H-Si(111) and with the agreement of trends between predicted and measured
work-function changes.
