Surface Polarization Doping in Diketopyrrolopyrrole-Based Conjugated Copolymers Using Cross-Linkable Terpolymer Dielectric Layers Containing Fluorinated Functional Units

Abstract

It is essential to tune the electrical properties of inorganic semiconductors via a doping process in the fabrication of cutting-edge electronic devices; however, the doping in organic field-effect transistors (OFETs) is limited by the uncontrollable dopant diffusion and low doping efficiencies. This study proposes the use of a fluorinated functional group in a polymer dielectric layer as an effective p-type doping strategy for ambipolar diketopyrrolopyrrole (DPP)-based donor−acceptor (D−A)-type semiconducting copolymer films used in OFETs, without generating structural perturbations. To experimentally verify the surface polarization doping effect of the fluorinated group, two terpolymerspoly(pentafluorostyrene-co-3-azidopropyl-methacrylate-copropargyl-methacrylate) (5F-SAPMA), wherein fluorinated units are included, and poly(phenyl-methacrylate-co-3-azidopropylmethacrylate-co-propargyl-methacrylate) (PhAPMA), without fluorinated unitsare designed and synthesized for use in OFETs. The synthesized 5F-SAPMA and PhAPMA films were cross-linked through the click reaction between the alkyne and azide units in the terpolymers at 150 °C to provide chemical, thermal, and mechanical stabilities and solvent resistance. The electrical characterization of the OFETs with the newly synthesized terpolymer dielectrics reveals that the surface polarization induced by the fluorinated groups of the 5F-SAPMA dielectrics leads to the generation of additional hole charges and helps minimize the broadening of the extended tail states in the vicinity of the valence band (highest occupied molecular orbital (HOMO) level). This not only enables a transition from the ambipolar to p-type dominant characteristics but also helps increase the hole mobility from 0.023 to 0.305 cm 2 /(V•s).

Recent Progress in Fluorinated Dielectric‐Based Organic Field‐Effect Transistors and Applications

“…[72][73][74] Wang and co-workers utilized the diffusion of fluorinated poly(methyl methacrylate) (PMMA) to construct the fluorine-rich semiconductor/dielectric interface to realize the manufacturing of the bottom gate OFETs with solutionprocessed poly(3-hexylthiophene) (P3HT) active layer (Figure 3a). [75] More specifically, they constructed a bilayer polymer dielectric which contains fluorinated PMMA 51 -ec-FMA 1 as the [71] Copyright 2021, American Chemical Society.…”

“…proposed using fluorinated functional groups in the polymer dielectric layer as an effective p‐type doping strategy to improve the electrical performance of the device without causing structural perturbations. [ 71 ] In order to investigate the surface polarization doping effect of fluorinated groups, the device with bottom gate top contact was constructed, as shown in ( Figure a). The poly(pentafluorostyrene‐co‐3‐azidopropyl‐methacrylate‐co‐propargyl‐methacrylate) (5F‐SAPMA) with fluorinated units and poly(phenyl‐methacrylate‐co‐3‐azidopropyl‐methacrylate‐co‐propargyl‐methacrylate) (PhAPMA) without fluorinated units are designed and synthesized for dielectric layers in OFETs.…”

“…e) Output curves of the OFETs with the 5F-SAPMA dielectrics. Reproduced with permission [71]. Copyright 2021, American Chemical Society.…”

Corona Poling Induced Phase Transition to Highly Polar Phase in P(VDF-TrFE-CFE) Dielectric and Charge Transport of Organic Field-Effect Transistors

“…So, PVDF-based dielectrics can be favorable to hole injection into p-channels and hole mobility of OFETs. 19,20 Also, PVDFbased dielectrics have been applied to piezoelectric devices because of their large dipole characteristics and used in the manufacture of low-voltage operating electronic devices because of high capacitance; furthermore, they have been commonly applied to memory devices because they have ferroelectricity characteristics. Among these materials, P(VDF-TrFE-CFE), the high-k dielectric material, contains a chlorofluoroethylene (CFE) unit, a ferroelectric relaxor unit to alleviate the ferroelectricity of PVDF, lowering the operation energy barrier for on/off of electronic device, which is used as a dielectric material for organic memory or transistor devices.…”

“…Polyvinylidene fluoride (PVDF)-based polymer dielectrics are ferroelectric high-k polymer dielectrics because they contain regularly repeating fluorine-containing bonds with high electronegativity and possess a large dipole in their molecular structures. − C–F bonds are well-known as a group with a strong dipole by polar covalent bonding because fluorine is an element with the greatest electronegativity. So, PVDF-based dielectrics can be favorable to hole injection into p-channels and hole mobility of OFETs. , Also, PVDF-based dielectrics have been applied to piezoelectric devices because of their large dipole characteristics and used in the manufacture of low-voltage operating electronic devices because of high capacitance; furthermore, they have been commonly applied to memory devices because they have ferroelectricity characteristics. Among these materials, P­(VDF-TrFE-CFE), the high-k dielectric material, contains a chlorofluoroethylene (CFE) unit, a ferroelectric relaxor unit to alleviate the ferroelectricity of PVDF, lowering the operation energy barrier for on/off of electronic device, which is used as a dielectric material for organic memory or transistor devices. , However, in these high-k polymer dielectrics, the energetic disorder can occur for molecular structure energy stabilization, which can induce deep trap sites in dielectric films. − Therefore, when the dipole direction in the molecular structure is aligned in one direction, the capacitance of the dielectric film can be maximized, thereby substantially increasing the number of charge carriers passing through the OFET channel.…”