Effect of the molecular weight of the polymer gate dielectric on the performances of solution-processed ambipolar OTFTs (original) (raw)
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Journal of Electronic Materials, 2020
In this study, we present two regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT)-based top-gate bottom-contact configured organic thin-film transistors (OTFTs) using poly(a-methyl acrylate) (PMA) and poly(methyl methacrylate) (PMMA) polymers separately as gate insulators for comparison. In order to compare only the performance of the dielectrics, the other parts of the devices were kept qualitatively and quantitatively identical. Unlike PMMA, PMA is flexible, and flexibility is a desirable property for an OTFT. Thus, utilizing PMA can be advantageous if it supports higher performance of the transistor. In this respect, the electronic parameters of the fabricated devices were extracted from transfer and output characteristics to determine the performance of PMA in OTFT applications. Results showed that the mobility of the OTFT with PMA (PMA-OTFT) was nearly three times greater than that of the OTFT with PMMA (PMMA-OTFT), while the PMA-OTFT threshold voltage (V TH) was slightly less than that of the PMMA-OTFT, which was likely because of the greater effective capacitance (C EFF) of the PMA layer compared to that of the PMMA layer. This is the main advantage of the PMA. On the other hand, the major downside is found in the reduced onto off current (I ON /I OFF) and increased subthreshold swing originating from a huge off-current (I OFF), implying the existence of a large gate leakage current. Increasing the thickness of the PMA layer could reduce such large gate leakage current. However, this would lead to additional increase in the OTFT operating voltage. Therefore, further studies are required to improve the insulating property of the PMA polymer in order to substitute it for the PMMA.
Materials, 2010
We investigated the electrical stabilities of two types of pentacene-based organic thin-film transistors (OTFTs) with two different polymeric dielectrics: polystyrene (PS) and poly(4-vinyl phenol) (PVP), in terms of the interfacial charge depletion. Under a short-term bias stress condition, the OTFT with the PVP layer showed a substantial increase in the drain current and a positive shift of the threshold voltage, while the PS layer case exhibited no change. Furthermore, a significant increase in the off-state current was observed in the OTFT with the PVP layer which has a hydroxyl group. In the presence of the interfacial hydroxyl group in PVP, the holes are not fully depleted during repetitive operation of the OTFT with the PVP layer and a large positive gate voltage in the off-state regime is needed to effectively refresh the electrical characteristics. It is suggested that the depletion-limited holes at the interface, i.e., interfacial charge depletion, between the PVP layer and the pentacene layer play a critical role on the electrical stability during operation of the OTFT.
Analysis of the hysteresis in organic thin-film transistors with polymeric gate dielectric
Organic Electronics, 2011
Controlling threshold voltage (V TH ) and field-effect mobility (l FET ) in organic thin-film transistors (OTFTs) is of primary importance to attain reliable devices that can be harnessed in more complicated circuits and eventually commercialized. In particular hysteresis in OTFT transfer curves is an issue that has to be better understood and analyzed. In this regard, even if the interface between organic dielectric and organic semiconductor seems to play an important role, our study shows that a further and relevant factor is played by the transport of charges across the bulk of the dielectric layer. Here, an analytical approach is applied to identify and understand the different components that give rise to the hysteresis in the transfer curves of pentacene-based OTFTs using poly(vinyl alcohol) (PVA) and poly(vinyl alcohol) cross-linked with ammonium dichromate (PVAad) as the (polymeric) gate dielectric. Transfer curves simulations which include charge transport in the PVA show a hysteretic behavior in good agreement with the experimental data. Moreover, the hysteresis measured in OTFTs can be reduced by the insertion of an underlying dielectric layer of SiO 2 that blocks the above-mentioned effect. The residual contribution to the hysteresis is then analyzed by investigating the chemistry nature of both PVA and PVAad, which show different electron trapping efficiency at the interface with the pentacene layer. Therefore, a consistent explanation of how the polymeric dielectric is able to impact the transfer curves requires considering all mentioned factors.
Materials Science in Semiconductor Processing, 2017
In this work, we present a method to increase the performance in solution processed organic field effect transistors (OFET) by using gel as dielectric and molecular doping to the active organic semiconductor. In order to compare the performance improvement, Poly (methylmethacrylate) (PMMA) and Poly (3-hexylthiophene-2,5diyl) P3HT material system were used as a reference. Propylene carbonate (PC) is introduced into PMMA to form the gel for using as gate dielectric. The mobility increases from 5.72×10 −3 to 0.26 cm 2 V s-1 and operation voltage decreases from −60 to −0.8 with gel dielectric. Then, the molecular dopant 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F4-TCNQ) is introduced into P3HT via co-solution. The mobility increases up to 1.1 cm 2 V s-1 and the threshold voltage downs to −0.09 V with doping. The increase in performance is discussed in terms of better charge inducing by high dielectric properties of gel and trap filling due to the increased carrier density in active semiconductor by molecular doping.
Organic thin-film transistors based on a high mobility n-type semiconductor poly{[n,n9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)} P(NDI2OD-T2) and different polymer gate dielectrics are fabricated. The average electron mobility decreases from 0.76 to 0.08 cm2/Vs with the increase of the gate dielectric constant from 2.6 to 7.8. The P(NDI2OD-T2) film shows unconventional face-on molecular packing, which results in short distances and pronounced interactions between electrons and gate dielectric. Therefore, the decrease of the electron mobility with the increasing dielectric constant is attributed to the Fröhlich polaron effect for the interaction between electrons in the channel and ionic polarization cloud in the gate dielectric.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 2011
The electrical characteristics of pentacene organic field effect transistors (OFETs) based on crosslinked acrylic insulator as the gate dielectric are reported. Vacuum deposited thin films of crosslinked tripropyleneglycol diacrylate could be obtained by ultrahigh flash evaporation rate and subsequent irradiation using an electron-beam source. The characteristics of common gate OFETs, on highly conductive Si substrate, were tuned through the ease of control of the acrylic dielectric thickness achieving, without surface modification of the dielectric layer, a field effect mobility value of 0.09 cm 2 V À1 s À1 , a threshold voltage of 10 V, and an on/off current ratio of 1.3 Â 10 3 . This work could provide an alternative route to low cost and large area organic electronics manufacturing.
Correlation between morphology and ambipolar transport in organic field-effect transistors
Journal of Applied Physics, 2005
Attaining ambipolar charge transport in organic field-effect transistors (OFET) is highly desirable from both fundamental understanding and application points of view. We present the results of an approach to obtain ambipolar OFET with an active layer of organic semiconductor blends using semiconducting polymers in composite with fullerene derivatives. Clear features of forming the superposition of both hole and electron-enhanced channels for an applied gate field are observed. The present studies suggest a strong correlation of thin-film nanomorphology and ambipolar transport in field-effect devices.
Solution processable bilayered gate dielectric towards flexible organic thin film transistors
Organic Electronics, 2015
In this study, we have successfully explored the potential of a new bilayer gate dielectric material, composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) composite thin film and Polyacrylonitrile (PAN) through fabrication of metal insulator metal (MIM) capacitor devices and organic thin film transistors (OTFTs). The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material are optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques are applied to deposit PAN and PS-P123 as a bilayer dielectric layer on Polyimide (PI) substrates. Contact angle study is further performed to explore the surface property of this bilayer polymer gate dielectric material. This new bilayer dielectric having a k value of 3.7 intermediate to that of PS-P123 composite thin film dielectric (k $ 2.8) and PAN dielectric (k $ 5.5) has successfully acted as a buffer layer by preventing the direct contact between the organic semiconducting layer and high k PAN dielectric. The OTFT devices based on a,x-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37 Â 10 À2 and on/off current ratio of 10 3 which is one of the good values as reported before. Several bending conditions are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which are exposed to open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90°C.
Cross-Linked Polymer Gate Dielectric Films for Low-Voltage Organic Transistors
Chemistry of Materials, 2009
Cross-linked polymer films were investigated as new gate dielectric materials for low-voltage thinfilm transistors. Poly(4-vinylphenol) (PVP) was cross-linked through esterification reactions with commercially available bifunctional anhydrides, acyl chlorides, and carboxylic acids. The polymer dielectric films were evaluated based on surface morphology, capacitance, leakage current, and their compatibility with organic semiconductors. Thin insulating PVP films cross-linked with dianhydrides yielded a capacitance as high as 400 nF/cm 2 with leakage currents below 10 -8 A/cm 2 . Organic thin-film transistors (OTFTs) fabricated on these gate dielectric layers exhibited charge carrier mobilities as high as 3 cm 2 / (V s) for p-channel pentacene on octadecyltriethoxylsilane (OTS)-modified PVP and 0.045 cm 2 /(V s) for n-channel perfluorinated copper phthalocyanine (FCuPc).