3 research outputs found
Impact of Energetically Engineered Dielectrics on Charge Transport in Vacuum-Deposited Bis(triisopropylsilylethynyl)pentacene
The surface functionality of the
gate dielectrics is one of the
important variables to have a huge impact on the electrical performance
of organic field-effect transistors (OFETs). Here, we describe the
impact of energetically engineered dielectrics on charge transport
in vacuum-deposited 6,13-bis(triisopropylsilylethynyl)pentacene
(TIPS-pentacene) thin films for eventually realizing high-performance
OFETs. A variety of self-assembled monolayers (SAMs) bearing amino,
methyl, phenyl (PTS), or fluoro end groups were introduced onto the
SiO<sub>2</sub> dielectric surfaces to design energetically engineered
surfaces that can be used to explore the impact of surface functionalities
at a TIPS-pentacene/gate dielectric interface. The solvent-free vacuum
deposition of TIPS-pentacene was used to exclude solution-processing
effects resulting from fluid flows and solvent drying processes. The
TIPS-pentacene layer on the PTS-SAM yielded the best morphological
and crystalline structures, which directly enhanced the electrical
properties, exhibiting field-effect mobilities as high as 0.18 cm<sup>2</sup>/(V s). Furthermore, the hysteresis, turn-on voltage, and
threshold voltage were correlated with the surface potentials of various
SAM-dielectrics. We believe that systematic investigation of the energetically
engineered dielectrics presented here can provide a meaningful step
toward optimizing the organic semiconductor/dielectric interface,
thereby implementing flexible and high-performance OFETs
Gate-Bias Stability Behavior Tailored by Dielectric Polymer Stereostructure in Organic Transistors
Understanding
charge trapping in a polymer dielectric is critical to the design
of high-performance organic field-effect transistors (OFETs). We investigated
the OFET stability as a function of the dielectric polymer stereostructure
under a gate bias stress and during long-term operation. To this end, <i>iso</i>-, <i>syn</i>-, and atactic poly(methyl methacrylate)
(PMMA) polymers with identical molecular weights and polydispersity
indices were selected. The PMMA stereostructure was found to significantly
influence the charge trapping behavior and trap formation in the polymer
dielectrics. This influence was especially strong in the bulk region
rather than in the surface region. The regular configurational arrangements
(isotactic > syntactic > atactic) of the pendant groups on the
PMMA backbone chain facilitated closer packing between the polymer
interchains and led to a higher crystallinity of the polymer dielectric,
which caused a reduction in the free volumes that act as sites for
charge trapping and air molecule absorption. The PMMA dielectrics
with regular stereostructures (<i>iso</i>- and <i>syn</i>-stereoisomers) exhibited more stable OFET operation under bias stress
compared to devices prepared using irregular <i>a</i>-PMMA
in both vacuum and air
Branched Segments in Polymer Gate Dielectric as Intrinsic Charge Trap Sites in Organic Transistors
Charge
traps in polymer gate dielectrics determine the electrical
stability of organic field-effect transistors (OFETs), and polar alkoxy
groups are well-known extrinsic charge traps. However, the actual
location of intrinsic charge traps in nonpolar polymer gate dielectrics
has been poorly understood yet. Here, we demonstrate that the skeletal
structure of polymer chain plays an important role in determining
the electrical stability. To verify it, we prepared linear and branched
polystyrene (<i>l</i>-PS and <i>b</i>-PS) and
blended them, in which branched segments provide much larger free
volume than the other segments. The current-insulating performance
and field-effect mobility increased with decease of <i>b</i>-PS portion. In particular, the bias-stress stability was remarkably
varied according to the change of <i>b</i>-PS portion even
though all measurements excluded reactive components such as oxygen
and water; the increase of <i>b</i>-PS resulted in time-dependent
decay of mobility and threshold voltage under bias stress. This indicates
that the branched segments in <i>b</i>-PS provide intrinsic
and metastable charge trap sites. Our result suggests that the skeletal
structure of polymeric chains in gate dielectric is one of the important
factors affecting intrinsic long-term operational stability of OFET
devices