PhD Oral Exam - Somaiyeh Charoughchi, Chemistry

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

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 corebased 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. PFP3CN3CP 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.