  The precise energy level alignment between a metal electrode and an organic
semiconductor is required to reduce contact resistance and enhance the
efficiency of organic-semiconductor-based devices. One of the ways is to
include interlayers that mediate the energy level alignment, i.e., charge
injection layers (CILs). Here we introduce the monolayer thick CILs based on
the aromatic carboxylic acids that can induce the energy level shift in the
subsequent layers by up to 0.8 eV. By gradual chemical transformation of the
as-deposited molecules, we achieve a highly tunable energy level shift in the
range of 0.5 eV. We reveal that the position of both the work function and
energy-level position in the CIL increases linearly with the density of induced
dipoles. The energy level position of subsequent layers changes in the same way
as the CIL. Our results thus connect the energy alignment quantities, i.e.,
energy level positions of both CIL and subsequent layers and sample work
function. The high tunability would allow precise tuning of the active layers
deposited on the CIL, which marks the path towards efficient charge injection
layers on metal electrodes.
