Polarization-induced transport in ferroelelctric organic field-effect transistors

In this research we study the role of ferroelectric dielectrics in organic field-effect transistors (FETs) to understand the mechanism of charge transport in organic semiconductors. The ferroelectric nature of the polymer, poly(vinylidene fluoride) (PVDF)), has been known for over 45 years. However, its role in interfacial transport in organic/polymeric FETs is not that well understood. PVDF and its copolymer, polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE), as a dielectric in organic FETs is a perfect test-bed for conducting transport studies where a systematic tuning of the dielectric constant with temperature may be achieved. By choosing small molecule organic semiconductors -- pentacene and 6,13 bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) -- along with a copolymer PVDF-TrFE as the dielectric layer, the FET characteristics are monitored as a function of temperature. Pentacene FETs show a weak temperature dependence of the charge carrier mobility in the ferroelectric phase of PVDF-TrFE, which is attributed to polarization fluctuation driven transport resulting from a coupling of the charge carriers to the surface phonons of the dielectric layer. A negative coefficient of carrier mobility is observed in TIPS-pentacene upwards of 200 K with the ferroelectric dielectric, while an activated transport is observed with non-ferroelectric dielectrics. We show that this behavior is correlated with the nature of the trap states in TIPS-pentacene. We also developed the method of dipole engineering of the PVDF-TrFE films to enhance the properties of organic FETs. PVDF-TrFE, despite its applications in a vast range of work (including as a gate dielectric in organic FET and sensing applications) poses concerns such as higher conductivity compared to other polymer non-ferroelectric dielectrics. We have come up with new methods of electrical poling the dielectric layer to enhance FET performance as well as reduce gate leakage issues. We demonstrate the effect of polarization rotation in PVDF-TrFE on the performance of small-molecule-based organic FETs. The subthreshold swing and other transistor parameters in organic FETs can be controlled in a reversible fashion by switching the polarization direction in the PVDF-TrFE layer. We further demonstrate a novel method of selective poling of the dielectric layer. By using solution processed TIPS-pentacene as the organic semiconductor, it is shown that textured poling of the PVDF-TrFE layer dramatically improves FET properties compared to unpoled or uniformly poled ferroelectric films. The texturing is achieved by first vertically poling the PVDF-TrFE film and then laterally poling the dielectric layer close to the gate electrode. TIPS-pentacene FETs show on/off ratios of 105 and hole mobilities of 1 cm2/Vs under ambient conditions with operating voltages well below-4 V. This research opens prospects of achieving low-operating FETs without any expensive patterning techniques.Includes bibliographical reference

Correlating charge transport to structure in deconstructed diketopyrrolopyrrole oligomers: A case study of a monomer in field-effect transistors

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of n-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with pi-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with a = 20.89 angstrom, b = 13.02 angstrom, c = 5.85 angstrom, alpha = 101.4 degrees, beta = 90.6 degrees, and gamma = 94.7 degrees for one phase (TR1) or two monomers with a = 24.92 angstrom, b = 25.59 angstrom, c = 5.42 angstrom, alpha = 80.3 degrees, beta = 83.5 degrees, and gamma = 111.8 degrees for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm(2)/V s and an on/off ratio of 10(6)are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph

Magnetic and Electric Characteristic of Nickel-Nickel Oxide Nanocomposite Thin Films for Electronic Applications

Metal-metal oxide nanocomposites are very important for today\u27s industrial and research field. Nanocomposite of nickel (Ni)-nickel oxide (NiO) films have potential applications including chemical sensor, bio sensors, infrared detectors, smart windows, and solar cells. It is very important to understand how to synthesize and modify the process parameters to get optimum materials. It is also important to characterize and understand the physical properties of these materials. The purpose of this thesis is to synthesize Ni-NiO thin films and characterized them for electronic applications. Nanocomposite Ni-NiO thin films were grown using pulsed laser deposition technique using a nickel target at four different oxygen pressures via 10-4 mbar, 50x10-3 mbar, 10-3 mbar, and 10-2 mbar of oxygen pressure and at temperature of 500 C. Nickel thin film in absence of oxygen and at 500 C is also grown. High quality Ni-NiO nanocomposite thin films were obtained which were confirmed by X-Ray Diffraction and Raman spectroscopy. UV-visual absorption spectroscopy data shows that the nanocomposite films are transparent semiconductors with the same band gap but a different absorbance. Magnetization data collected using a superconducting quantum interference device (SQUID) magnetometer in conjunction with electronic data shows that these Ni-NiO nanocomposite thin films have ferromagnetic, paramagnetic, and super paramagnetic properties. Resistivity of such thin films is varied by a large range from 107Ωm to 10-5 Ωm. Nanocomposite thin films of Ni-NiO could be useful for future spintronic and optoelectronic devices

Hybrid ZnO-organic semiconductor interfaces in photodetectors: A comparison of two near-infrared donor-acceptor copolymers

Hybrid organic-inorganic photodiode interfaces have gained significant interest due to their unique physical properties such as mechanical flexibility and high photosensitivity. Two diketopyrrolopyrrole (DPP)-based donor-acceptor copolymers with different backbone conformations are characterized in an inverted non-fullerene photodiode architecture using ZnO nano-patterned films as the electron transport layer. The DPP copolymer with a thienothiophene unit (PBDT-TIDPP) is more planar and rigid compared to the DPP system with a thiophene unit connecting the donor and acceptor moieties within the monomer (PBDT-TDPP). The hybrid interfaces were optimized by using poly(3-hexylthiophene) (P3HT) as the p-type layer for monitoring the critical thickness and morphology of the ZnO layer. The maximum photoresponsivity from a P3HT:ZnO photodiode was found to be 56 mA/W. The photoresponsivity of PBDT-TTDPP:ZnO photodiodes were found to be more than two orders of magnitude higher than PBDT-TDPP:ZnO photodiodes, which is attributed to an enhanced transport of carriers due to the planar backbone conformation of the PBDT-TTDPP copolymer. Capacitance-voltage measurements from hybrid Schottky barrier interfaces further shed light into the nature of photocarriers and device parameters. First-principles time-dependent density-functional theoretical calculations yield a higher absorptivity for the PBDT-TTDPP dimer compared to PBDT-TDPP. (C) 2017 Elsevier B.V. All rights reserved

Bioinspired Peptide Nanostructures for Organic Field-Effect Transistors

Peptide-based nanostructures derived from natural amino acids are superior building blocks for biocompatible devices as they can be used in a bottom-up process without the need for expensive lithography. A dense nanostructured network of l,l<i>-</i>diphenylalanine (FF) was synthesized using the solid–vapor-phase technique. Formation of the nanostructures and structure–phase relationship were investigated by electron microscopy and Raman scattering. Thin films of l,l-diphenylalanine micro/nanostructures (FF-MNSs) were used as the dielectric layer in pentacene-based field-effect transistors (FETs) and metal–insulator–semiconductor diodes both in bottom-gate and in top-gate structures. Bias stress studies show that FF-MNS-based pentacene FETs are more resistant to degradation than pentacene FETs using FF thin film (without any nanostructures) as the dielectric layer when both are subjected to sustained electric fields. Furthermore, it is demonstrated that the FF-MNSs can be functionalized for detection of enzyme–analyte interactions. This work opens up a novel and facile route toward scalable organic electronics using peptide nanostructures as scaffolding and as a platform for biosensing

Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications

A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore <b>DPP-C8-Th-Py</b>. The polymers <b>Poly-DPP-Th-Pt</b> and <b>Poly-DPP-Ph-Pt</b> differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer <b>Poly-DPP-Th-Pt</b> has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as donors in bulk-heterojunction solar cells with a polymer:<b>PC₇₁BM</b> ratio of 1:7. The thiophene-containing polymer <b>Poly-DPP-Th-Pt</b> shows a respectable power conversion efficiency (PCE) of 1.66% with a high fill factor (FF) of ∼66%. Higher charge carrier mobility was observed for <b>Poly-DPP-Th-Pt</b> when used in field-effect transistors compared to <b>Poly-DPP-Ph-Pt</b>

Correlating Charge Transport with Structure in Deconstructed Diketopyrrolopyrrole Oligomers: A Case Study of a Monomer in Field-Effect Transistors

Copolymers based on diketopyrrolopyrrole (DPP) cores have attracted a lot of attention because of their high p-type as well as n-type carrier mobilities in organic field-effect transistors (FETs) and high power conversion efficiencies in solar cell structures. We report the structural and charge transport properties of <i>n</i>-dialkyl side-chain-substituted thiophene DPP end-capped with a phenyl group (Ph-TDPP-Ph) monomer in FETs which were fabricated by vacuum deposition and solvent coating. Grazing-incidence X-ray diffraction (GIXRD) from bottom-gate, bottom-contact FET architectures was measured with and without biasing. Ph-TDPP-Ph reveals a polymorphic structure with π-conjugated stacking direction oriented in-plane. The unit cell comprises either one monomer with <i>a</i> = 20.89 Å, <i>b</i> = 13.02 Å, <i>c</i> = 5.85 Å, α = 101.4°, β = 90.6°, and γ = 94.7° for one phase (TR1) or two monomers with <i>a</i> = 24.92 Å, <i>b</i> = 25.59 Å, <i>c</i> = 5.42 Å, α = 80.3°, β = 83.5°, and γ = 111.8° for the second phase (TR2). The TR2 phase thus signals a shift from a coplanar to herringbone orientation of the molecules. The device performance is sensitive to the ratio of the two triclinic phases found in the film. Some of the best FET performances with p-type carrier mobilities of 0.1 cm²/V s and an on/off ratio of 10⁶ are for films that comprise mainly the TR1 phase. GIXRD from in operando FETs demonstrates the crystalline stability of Ph-TDPP-Ph

Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications

A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore <b>DPP-C8-Th-Py</b>. The polymers <b>Poly-DPP-Th-Pt</b> and <b>Poly-DPP-Ph-Pt</b> differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer <b>Poly-DPP-Th-Pt</b> has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as donors in bulk-heterojunction solar cells with a polymer:<b>PC₇₁BM</b> ratio of 1:7. The thiophene-containing polymer <b>Poly-DPP-Th-Pt</b> shows a respectable power conversion efficiency (PCE) of 1.66% with a high fill factor (FF) of ∼66%. Higher charge carrier mobility was observed for <b>Poly-DPP-Th-Pt</b> when used in field-effect transistors compared to <b>Poly-DPP-Ph-Pt</b>