P3HT Field-effect Transistors Research Articles

Solid‐State Electrolyte Dielectrics Based on Exceptional High‐k P(VDF‐TrFE‐CTFE) Terpolymer for High‐Performance Field‐Effect Transistors

AbstractHigh‐performance and low‐voltage organic and inorganic field‐effect transistors (FETs) with solid‐state electrolyte gate insulator that is composed of an exceptional high‐k fluorinated dielectric and an ion‐gel‐blend polymer matrix are reported. The structuring polymer is high‐k poly(vinylidenefluoride‐trifluoroethylene‐chlorotrifluoroethylene) (P(VDF‐TrFE‐CTFE)) terpolymer. The ion gel is made of poly(vinylidene fluoride‐co‐hexafluroropropylene) (P(VDF‐HFP)) and 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl) imide ([EMIM][TFSI]) ion liquid. The blend polymer matrix has high measured capacitance of ≈2 to 5.5 µF cm−2 at 100 Hz, which is attributed to formation of electrical double layers (EDLs) at the insulator/semiconductor interface. High effective carrier mobility (μeff) and low operating voltage ≤2 V with just 0.5 v% of P(VDF‐HFP)‐[EMIM][TFSI] solution in the bulk P(VDF‐TrFE‐CTFE) insulating layer are demonstrated, coupled with low gate‐leakage current levels. Excellent μeff = 1.30 ± 0.19 cm2 V−1 s−1 is achieved in P3HT FETs with SEGI, which is a remarkable boost from 0.48 ± 0.09 cm2 V−1 s−1 at 30 V when pure P(VDF‐TrFE‐CTFE) dielectric is used. Other semiconductors are also tested: IGZO has μeff = 11.09 ± 2.07 cm2 V−1 s−1, and PDFDT has μeff = 2.42 ± 0.46 cm2 V−1 s−1. These remarkable increases in mobility are attributed to the high concentration of induced carriers in the semiconducting channel.

Fabrication, electrical characterization and device simulation of vertical P3HT field-effect transistors

Abstract Vertical organic field-effect transistors (VOFETs) provide an advantage over lateral ones with respect to the possibility to conveniently reduce the channel length. This is beneficial for increasing both the cut-off frequency and current density in organic field-effect transistor devices. We prepared P3HT (poly[3-hexylthiophene-2,5-diyl]) VOFETs with a surrounding gate electrode and gate dielectric around the vertical P3HT pillar junction. Measured output and transfer characteristics do not show a distinct gate effect, in contrast to device simulations. By introducing in the simulations an edge layer with a strongly reduced charge mobility, the gate effect is significantly reduced. We therefore propose that a damaged layer at the P3HT/dielectric interface could be the reason for the strong suppression of the gate effect. We also simulated how the gate effect depends on the device parameters. A smaller pillar diameter and a larger gate electrode-dielectric overlap both lead to better gate control. Our findings thus provide important design parameters for future VOFETs.

The Approach for the Trade-off Study Between Field-effect Mobility and Current on/off Ratio in P3HT Field-effect Transistors

Presented herein are the results of the study that was conducted on the electrical characteristics of organic field-effect transistors based on poly(3-hexylthiophene), particularly the thickness and annealing temperature of their active layer is varied. The changes in field-effect mobility and current on/off ratio were explored. It was observed that both increasing annealing temperature from <TEX>$60^{\circ}C$</TEX> to <TEX>$100^{\circ}C$</TEX> and various concentrations influence the trade-off relations between the mobility and current on/off ratio. The surface morphology of the 2-<TEX>${\mu}m^2$</TEX> area with various thicknesses was scanned via atomic-forcemicroscopy(AFM) to verify the relationship between surface morphology, which is related to the thickness of the film, and device performance.

Temporal and thermal properties of optically induced instabilities in P3HT field-effect transistors

In this contribution we report on temporal and thermal properties of an optically induced instability in poly(3-hexylthiophene) (P3HT) based organic field-effect transistors (OFET) fabricated on a flexible polyethylene terephthalate (PET) substrate. By illuminating depleted p-type top-gate P3HT field-effect transistors with visible light a substantial shift of the threshold-voltage of up to +20V and an increase in the off-current by three orders of magnitude has been observed. Both phenomena, the threshold-voltage shift and the increase of the off-current, require the presence of oxygen and are persistent for days at room temperature. The effect is explained by the formation of a charge-transfer-complex (CTC) of P3HT and oxygen known from literature to act as an electron trap. Here, we discuss the temporal and thermal stability of the detrapping of photo-generated charge carriers from such traps and the loss of trapping sites by diffusion of oxygen out of the P3HT layer. It is demonstrated that the thermally activated detrapping of electrons from the oxygen induced CTC states is much faster than the removal of the oxygen induced traps.

Low-Voltage P3HT Field-Effect Transistors Fabricated Using High-k Gate Insulators

Hafnium silicate (HfSiO5) thin film (ε r = 7.7) and strontium titanate (SrTiO3) thin film (ε r = 12.1) were prepared on a heavily doped n-type silicon wafer by sputtering. Wet-processed P3HT field-effect transistors (FETs) fabricated using the self-assembled monolayer (SAM)-treated HfSiO5 or SAM-free SrTiO3 as a gate insulator showed saturated output characteristics at a driving voltage as low as −10 V for HfSiO5 and −3 V for SrTiO3. Hole mobilities of P3HT-FETs fabricated on the HfSiO5 and the SrTiO3 were 1.1 × 10−3 and 5.7 × 10−3 cm2/Vs, respectively.

Low-temperature printing of crystalline:crystalline polymer blend transistors

By blending poly(3-hexylthiophene) with poly(vinylidene fluoride) and with careful solvent selection, field-effect transistors are fabricated from solution with deposition temperatures suitable for use in low-cost roll-to-roll manufacturing techniques. A percolation threshold of only ∼3 wt.% P3HT is demonstrated, with field-effect mobilities of 0.04 cm 2/Vs obtained in such PVDF-rich blend devices being equal to neat P3HT field-effect transistors. We illustrate the potential of this material system by the fabrication of working field-effect transistor devices, wherein the semiconductor blend is deposited via flexo-printing. Flexible polyethylene naphthalate substrates and an organic gate dielectric were utilized for this purpose.

A comparative MD study of the local structure of polymer semiconductors P3HT and PBTTT

Atomistic molecular dynamics simulations of P3HT and PBTTT-C12 at finite temperatures are carried out to investigate the nanoscale structural properties that lead to higher measured hole mobility in PBTTT versus P3HT field-effect transistors. Simulations of the polymer melts show that the structural properties in PBTTT facilitate both intra- and inter-chain charge transport compared with P3HT due to a greater degree of planarity, closer and more parallel stacking of the thiophene and thienothiophene rings, and possible interdigitation of the dodecyl side chains. The crucial role played by the bulky dodecyl side chain and thienothiophene ring, respectively, in determining intra-chain and inter-chain structural order is clarified.

Enhancement of the field-effect mobility of poly(3-hexylthiophene)/functionalized carbon nanotube hybrid transistors

With the aim of enhancing the field-effect mobility of poly(3-hexylthiophene) (P3HT) field-effect transistors (FETs), we added functionalized multiwalled carbon nanotubes (CNTs) to the P3HT solution prior to film formation. The nanotubes were found to be homogeneously dispersed in the P3HT films because of their functional groups. We found that at the appropriate CNT concentration (up to 10wt% CNT), the P3HT FETs have a high field-effect mobility of 0.04cm2V−1s−1, which is an improvement by a factor of more than 10. This remarkable increase in the field-effect mobility over that of the pristine P3HT film is due to the high conductivity of the CNTs which act as conducting bridges between the crystalline regions of the P3HT film, and the reduction in the hole-injection barrier due to the low work function of CNTs, which results in more efficient carrier injection.

Ambipolar Field-Effect Transistors Based on Poly(3-hexylthiophene)/Fullerene Derivative Bilayer Films

Field-effect transistors (FETs) based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric methyl ester (PCBM) films have been studied for various structures with metal electrodes on a SiO2/Si substrate. The pressure dependence of transport characteristics in P3HT FET during evacuation was measured. It was found that hole mobility is independent of atmospheric pressure, whereas hole density decreases significantly with evacuation. FET characteristics in various configurations (top and bottom) of contact with Au and Al were examined in P3HT and PCBM films. The results indicate that bottom contact with Au gives better performance for bilayer FETs. Ambipolar transport was observed for the bilayer structure of PCBM (upper)/P3HT (lower)/Au (bottom contact).