Abstract

The p-doping of organic semiconductors, that is, conjugated organic molecules
(COMs) and polymers (COPs), is generally done using strong molecular acceptors as
dopants. In principle, high doping efficiency can be achieved with dopants of high
electron affinity (EA) to promote integer electron transfer between COP/COM and the
p-dopant. Common dopants of high EA (> 5 eV) are, however, often unstable, show
low solubility in common solvents with most COPs/COMs, and tend to diffuse through
the organic semiconductor owing to low molecular weight. Furthermore, their planarity
can promote the formation of ground-state charge transfer complexes (CPXs) with the
COPs/COMs, which is detrimental to doping efficiency due to fractional instead of integer
charge transfer. To address this issue, this thesis focuses on a new strategy towards
more efficient molecular p-dopants: The optimization of EA to promote integer electron
transfer is therein augmented by a novel strategy to inhibit CPX formation, which relies
on directed dopant design exploiting steric hindrance. First, to identify promising
alternative dopants with high EA, we systematically compared the interplay between
molecular EA and steric shielding of the core resulting from the peripheral substitution
of analogues molecules with cyclohexadiene and cyclopropane cores. To this end, we performed
a simple analysis based on Hammett parameters followed by density functional
theory (DFT) calculations on a library of modified doping agents. Second, based on the
outcome of the DFT pre-characterization we focused on cyclopropane core-based dopants
and synthesized and characterized 2,’,2”-(cyclopropane-1,2,3-triylidene)tris(2-(perfluoro
phenyl)-acetonitrile) (PFP3CN3-CP) as one of the most promising species. PFP3CN3-CP
has pendant functional groups that sterically shield its central core while still maintaining
a comparably high EA. By using various spectroscopy and electrical characterization
we demonstrate for the prototypical COP, poly(3-hexylthiophene) (P3HT), that, indeed,
CPX formation can be inhibited by exploiting steric hindrance brought by PFP3CN3-CP.
It outperforms a planar dopant with similar EA, showing tenfold higher conductivity and
superior stability in aging experiments. Overall, this thesis introduces a novel strategy
for improving p-doping efficiency in organic semiconductors by incorporating steric
hindrance to prevent CPX formation. It advances the development of more efficient
p-dopants, contributing valuable knowledge to the field.