Polymers for all-organic field-effect
transistors are under development
to cope with the increasing demand for novel materials for organic
electronics. Besides the semiconductor, the dielectric layer determines
the efficiency of the final device. Poly(methyl methacrylate) (PMMA)
is a frequently used dielectric. In this work, the chemical structure
of this material was stepwise altered by incorporation of cross-linkable
and/or self-organizing comonomers to improve the chemical stability
and the dielectric properties. Different types of cross-linking methods
were used to prevent dissolution, swelling or intermixing of the dielectric
e.g. during formation processes of top electrodes or semiconducting
layers. Self-organizing comonomers were expected to influence the
dielectric/semiconductor interface, and moreover, to enhance the chemical
resistance of the dielectric. Random copolymers were obtained by free
radical and reversible addition–fragmentation chain transfer
(RAFT) polymerization. With 6-[4-(4′-cyanophenyl)phenoxy]alkyl
side chains having hexyl or octyl spacer, thermotropic liquid crystalline
(LC) behavior and nanophase separation into smectic layers was observed,
while copolymerization with methyl methacrylate induced molecular
disorder. In addition to chemical, thermal and structural properties,
electrical characteristics like breakdown field strength (<i>E</i><sub>BD</sub>) and relative permittivity (<i>k</i>) were determined. The dielectric films were studied in metal–insulator–metal
setups. <i>E</i><sub>BD</sub> appeared to be strongly dependent
on the type of electrode used and especially the ink formulation.
Cross-linking of PMMA yielded an increase in <i>E</i><sub>BD</sub> up to 4.0 MV/cm with Ag and 5.7 MV/cm with PEDOT:PSS electrodes
because of the increased solvent resistance. The LC side chains reduce
the ability for cross-linking resulting in decreased breakdown field
strengths