Highly Contrasting Static Charging and Bias Stress Effects in Pentacene Transistors with Polystyrene Heterostructures Incorporating Oxidizable N,N′-Bis(4-methoxyphenyl)aniline Side Chains as Gate Dielectrics

Abstract

Charge storage and trapping properties of polymer dielectrics govern the charge densities of adjacent semiconductors and greatly influence the on−off switching voltage (threshold voltage, V th ) of organic field-effect transistors (OFETs) when the polymers are used as gate insulators. Intentional charging of polymer dielectrics in OFETs can change V th and affect the bias stress. We describe a chemical design and fabrication protocol to construct multilayer-stack dielectrics for pentacene-based OFETs using different polystyrene (PS)-based polymers in each layer, with oxidizable N,N-bis(4-methoxyphenyl)anilino (TPAOMe)-substituted styrene copolymers in arbitrary vertical positions in the stacks. Thermal, byproduct-free cross-linking of benzocyclobutene subunits provides integrity to the multilayer structure by preventing dissolution of the previous deposited layer. Neutron reflectivity data verified the multilayer morphology. We compared the V th shift before and after charging the stacks by application of ±100 V across 0.5−1 μm total film thicknesses. Bias stress was the dominant effect in bilayer devices with a TPAOMe layer in contact with the pentacene, indicated by the direction of V th shift associated with either polarity of external electric field. In structures with no TPAOMe subunit in contact with the pentacene, when charging with −100 V on top of the source and drain electrodes, electron injection from pentacene to dielectric was the major charging mechanism, again consistent with the bias stress direction. When charging with +100 V, bilayer devices without TPAOMe showed little change in V th , suggesting there was no bias stress effect or charge injection in these devices for this charging polarity. For the bilayer devices with the TPAOMe layer in the bottom, and the trilayer devices with TPOMe in the middle, when +100 V was applied, the V th shifts were opposite those expected from bias stress. Dipole formation or partial ionization of chargeable groups at the interface between the dielectric layers are likely polarization mechanisms in these cases. A simple analytical model supports the plausibility of these mechanisms. This work provides examples of both stabilization and shifting of V th , and therefore controlling charge carrier density, in semiconductors overlying the dielectric multilayers.

Maximized Hole Trapping in a Polystyrene Transistor Dielectric from a Highly Branched Iminobis(aminoarene) Side Chain

“…In our previous work, neutron reflectivity and scanning electron microscope (SEM) measurements showed the existence of distinct boundaries of multilayer devices created by cross-linked PS (XLPS). , There was also no structural change within the device during the charging process, according to the neutron reflectivity experiment. Therefore, Δ V th should not relate to any structural change-induced effects.…”

“…Compound 5 was synthesized according to the procedure reported for 4′-(bis(4-methoxyphenyl)amino)-4-hydroxybiphenyl in the reference except 5-bromo-1,2,3-trimethoxybenzene and 3,4,5-trimethoxyaniline (obtained from Sigma-Aldrich) were used as starting materials. 40 NMR of the t-butyldimethylsilyl protected and deprotected 5 are included in Figure S3. All of the cross-linkable or chargeable polymers used in this work are listed in Figure 2.…”

“…36−38 Applying selfassembled monolayers (SAMs) on top of silicon oxide gate dielectrics with amino or fluorine groups can result in the accumulation of electrons and holes at the interface and thus tune the threshold voltage. 39 In a previous study, 40 we demonstrated a series of crosslinkable and chargeable polystyrene multilayer-stack dielectrics as the sole gate dielectrics (with no oxide barrier layers) to shift the threshold voltage of OFET devices and therefore modulate the charge carrier density in the adjacent semiconductor layers. The chargeable polystyrene was substituted with "TPAOMe", which is a 4-methoxylated modification of the widely used electron donor TPA to increase its electron-donating capability.…”

“…We applied two methods to modify the electron-donating ability of the dendritic TPA unit: tetracyanoethylene (TCNE) was used to react with dendritic TPA polymer to form a charge-transfer complex or adduct and nominally n-type ZnO nanoparticles (NP) that can form coordinate covalent bonds with TPA units were embedded within the polymer. Analogous to the methods in previous work, 40 we created three types of vertical multilayer devices with the substituted PS layer as single-layer dielectric, on top of a cross-linked PS layer, and sandwiched between two PS layers. We compared the charging effects in two situations: the substituted PS layer in contact with the semiconductor and within the bulk of the dielectric.…”

The Electrical Properties of Fabricated Pentacene Based Phototransistor with Polystyrene Gate Insulator

“…The high-quality interface can be achieved in OFETs using polymer dielectrics [16]. There are not many studies in the literature about pentacene OFET with polystyrene polymer gate [17][18][19]. One of those researches is studied by Sung-woo lee et al [15].…”