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 SiO₂ 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 cm²/(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