Single Crystal-Like Performance in Solution-Coated Thin-Film Organic Field-Effect Transistors

In electronics, the fi eld-effect transistor (FET) is a crucial cornerstone and successful integration of this semiconductor device into circuit applications requires stable and ideal electrical characteristics over a wide range of temperatures and environments. Solution processing, using printing or coating techniques, has been explored to manufacture organic fi eld-effect transistors (OFET) on fl exible carriers, enabling radically novel electronics applications. Ideal electrical characteristics, in organic materials, are typically only found in single crystals. Tiresome growth and manipulation of these hamper practical production of fl exible OFETs circuits. To date, neither devices nor any circuits, based on solution-processed OFETs, has exhibited an ideal set of characteristics similar or better than today’s FET technology based on amorphous silicon. Here, bar-assisted meniscus shearing of dibenzo-tetrathiafulvalene to coat-process self-organized crystalline organic semiconducting domains with high reproducibility is reported. Including these coatings as the channel in OFETs, electric fi eld and temperature-independent charge carrier mobility and no bias stress effects are observed. Furthermore, record-high gain in OFET inverters and exceptional operational stability in both air and water are measured.The authors thank the ERC StG 2012-306826 e-GAMES project, the Networking Research Center on Bioengineering, Biomaterials, and Nanomedicine (CIBER-BBN), the DGI (Spain) project BE-WELL CTQ2013-40480-R, and the Generalitat de Catalunya (2014-SGR- 17). Research in Sweden was fi nancially supported by the Advanced Functional Materials Center at Linköping University, the Önnesjö Foundation, the Knut and Alice Wallenberg Foundation (Power Paper project, scholars), the Swedish Foundation for Strategic Research (SSF, Synergi project). F.G.D.P. thanks Universidad Técnica de Ambato and Secretaría de Educación Superior, Ciencia, Tecnología e Innovación for funding through a doctoral scholarship “Convocatoria abierta 2010.” The authors also thank Witold Tatkiewicz for his help with the ImageJ software.Peer reviewe

Improving the Robustness of Organic Semiconductors through Hydrogen Bonding

Molecular organization plays an essential role in organic semiconductors since it determines the extent of intermolecular interactions that govern the charge transport present in all electronic applications. The benefits of hydrogen bond-directed self-assembly on charge transport properties are demonstrated by comparing two analogous pyrrole-based, fused heptacyclic molecules. The rationally designed synthesis of these materials allows for inducing or preventing hydrogen bonding. Strategically located hydrogen bond donor and acceptor sites control the solid-state arrangement, favoring the supramolecular expansion of the π-conjugated surface and the subsequent π-stacking as proved by X-ray diffraction and computational calculations. The consistency observed for the performance of organic field-effect transistors and the morphology of the organic thin films corroborate that higher stability and thermal robustness are achieved in the hydrogen-bonded material.The authors acknowledge the MCIU of Spain (projects RTI2018-101092-B-I00, PGC2018-099568-B-I00, RED2018- 102815-T, and Unidad de Excelencia María de Maeztu CEX2019-000919-M), the Fundación Séneca - Agencia de Ciencia y Tecnología de la Región de Murcia (20959/PI/18), the Generalitat Valenciana (PROMETEO/2020/077 and SEJI/2018/035), and European feder Funds (PGC2018- 099568-B-I00) for financial support. M.M.-M. acknowledges Fundación Séneca for a postdoctoral Saavedra Fajardo contract (20406/SF/17). J.C. and J.A. are, respectively, grateful to MCIU for their predoctoral FPI and “Ramón-y-Cajal” Fellowships (RyC-2017-23500)

Polymer Field-Effect Transistors.

High Ionisation Potential (IP) amorphous conjugated polymers are very practical semiconductors and promising candidates for printing applications as they exhibit 1) high air-stability due to the high IP, and 2) reproducible electrical performance due to the uniformity of amorphous morphology. However they generally exhibit low mobilities on the order of 10-3 cm2/Vs and below. This work is based mainly on two high-IP amorphous conjugated polymers poly(indenofluorene-triarylamine) (PIFTAA) and poly(indenofluorene-phenanthrene) (PIFPA). The long term ambient stability of PIFTAA and PIFPA with IPs of 5. 45 eV and 5. 79 eV respectively is characterised in Field-Effect Transistors (FETs) over a period of 4 and 2 months respectively. FET parameters such as the turn-on voltage and subthreshold slope are found to be generally stable, and the charge carrier mobility is found to degrade at an approximate rate of 10% per month, which is amongst the lowest reported values for organic semiconductors. PIFTAA and particularly PIFPA exhibit high field-effect saturation mobilities of 0. 03 - 0. 04 cm2/Vs and 0. 2 - 0. 3 cm2/Vs respectively, which are unusually high for amorphous conjugated polymers. The morphologies are examined by atomic force microscopy, grazing incidence wide angle x-ray scattering, and differential scanning calorimetry, and no evidence of crystallinity is detected, suggesting that the conjugated polymers are indeed amorphous. To investigate charge transport in PIFTAA and PIFPA, FETs of multiple channel lengths are fabricated, providing mobility data for multiple electric fields, and measured over a range of temperatures. In addition to PIFTAA and PIFPA, the measurements are performed on typical amorphous conjugated polymers poly(triarylamine) (PTAA) and poly(indenofluorene-triarylamine-triarylamine) (PIFTAATAA), with mobilities of 0. 003 cm2/Vs and 0. 004 cm2/Vs respectively. The gate voltage dependence of the mobility extracted from FET measurements, as well as the 1/T2 fit of the mobility with temperature is consistent with a Gaussian Density of States. The indenofluorene copolymers PIFTAA, PIFTAATAA, and PIFPA exhibit clear negative electric field dependence of the mobility, signature of high spatial disorder in the polymer films. The temperature dependence of the mobility is fed into the Gaussian Disorder Model, which indicates that the source of the high mobility for PIFPA is mainly strong intermolecular coupling indicated by the high pre-factor mobility as well as low energetic disorder along the path of charge flow. These results challenge the widely accepted concept that high crystallinity is a requirement for mobility exceeding 0. 1 cm2/Vs in organic semiconductors. Finally, a new type of contact-limited transistor is demonstrated with conjugated polymers. Source-Gated Transistors (SGTs) have a similar structure to FETs, and the main difference is the Schottky source/drain-semiconductor contact, which results in depletion of the near-source region of the semiconductor. Consequently, the behaviour of the transistor changes significantly as compared to FETs. SGTs are demonstrated with several electrodeconjugated polymer combinations. SGTs saturate at significantly lower voltages than FETs, and saturation is not lost for short channels and thick insulators. Also, evidence of independence of the current from channel length is observed, which is consistent with contact-based modulation as opposed to FET channel-based modulation. These advantages come at a cost of output current of at least one order of magnitude, while the intrinsic voltage gain is mostly maintained

Polymer field-effect transistors

High Ionisation Potential (IP) amorphous conjugated polymers are very practical semiconductors and promising candidates for printing applications as they exhibit 1) high air-stability due to the high IP, and 2) reproducible electrical performance due to the uniformity of amorphous morphology. However they generally exhibit low mobilities on the order of 10-3 cm2Ns and below. This work is based mainly on two high-IP amorphous conjugated polymers poly(indenofluorene-triarylamine) (PIFTAA) and poly(indenofluorene- phenanthrene) (PIFPA). The long term ambient stability of PIFTAA and PIFPA with IPs of 5.45 eV and 5.79 eV respectively is characterised in Field-Effect Transistors (FETs) over a period of 4 and 2 months respectively. FET parameters such as the turn-on voltage and subthreshold slope are found to be generally stable, and the charge carrier mobility is found to degrade at an approximate rate of 10% per month, which is amongst the lowest reported values for organic semiconductors. PIFT AA and particularly PIFPA exhibit high field-effect saturation mobilities of 0.03 - 0.04 cm2Ns and 0.2 - 0.3 cm2Ns respectively, which are unusually high for amorphous conjugated polymers. The morphologies are examined by atomic force microscopy, grazing incidence wide angle x-ray scattering, and differential scanning calorimetry, and no evidence of crystallinity is detected, suggesting that the conjugated polymers are indeed amorphous. To investigate charge transport in PIFTAA and PIFPA, FETs of multiple channel lengths are fabricated, providing mobility data for multiple electric fields, and measured over a range of temperatures. In addition to PIFT AA and PIFP A, the measurements are performed on typical amorphous conjugated polymers poly(triarylamine) (PTAA) and poly(indenofluorene-triarylamine-triarylamine) (PIFTAATAA), with mobilities of 0.003 cm2/Vs and 0.004 cm2Ns respectively. The gate voltage dependence of the mobility extracted from FET measurements, as well as the lIT2 fit of the mobility with temperature is consistent with a Gaussian Density of States. The indenofluorene copolymers PIFTAA, PIFTAATAA, and PIFPA exhibit clear negative electric field dependence of the mobility, signature of high spatial disorder in the polymer films. The temperature dependence of the mobility is fed into the Gaussian Disorder Model, which indicates that the source of the high mobility for PIFPA is mainly strong intermolecular coupling indicated by the high pre-factor mobility as well as low energetic disorder along the path of charge flow. These results challenge the widely accepted concept that high crystallinity is a requirement for mobility exceeding 0.1 cm2/Vs in organic semiconductors.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Precise Characterisation of Molecular Orientation in a Single Crystal Field-Effect Transistor Using Polarised Raman Spectroscopy

Charge transport in organic semiconductors is strongly dependent on the molecular orientation and packing, such that manipulation of this molecular packing is a proven technique for enhancing the charge mobility in organic transistors. However, quantitative measurements of molecular orientation in micrometre-scale structures are experimentally challenging. Several research groups have suggested polarised Raman spectroscopy as a suitable technique for these measurements and have been able to partially characterise molecular orientations using one or two orientation parameters. Here we demonstrate a new approach that allows quantitative measurements of molecular orientations in terms of three parameters, offering the complete characterisation of a three-dimensional orientation. We apply this new method to organic semiconductor molecules in a single crystal field-effect transistor in order to correlate the measured orientation with charge carrier mobility measurements. This approach offers the opportunity for micrometre resolution (diffraction limited) spatial mapping of molecular orientation using bench-top apparatus, enabling a rational approach towards controlling this orientation to achieve optimum device performance.This work was funded by the Department for Business, Innovation & Skills through the National Measurement System as part of the Innovation, Research and Development programme. M.S. acknowledges equipment funding from EPSRC UK Grant EP/I017569/1. S.G. and M.T would like to acknowledge the financial support from ERC StG 2012-306826 e-GAMES project and CIBER-BBN. Thanks are due to Prof. J.E. Anthony and his group from the University of Kentucky, for synthesising the functionalised pentacene derivative used in this work, and also, to J. Wade at Imperial College London for the DFT calculations.Peer reviewe

Laser Patterned Polymer/Carbon Nanotubes Composite Electrodes for Flexible Silicon Nanowire Transistors

Fabrication techniques such as laser patterning offer excellent potential for low cost and large area device fabrication. Conductive polymers can be used to replace expensive metallic inks such as silver and gold nanoparticles for printing technology. Electrical conductivity of the polymers can be improved by blending with carbon nanotubes. In this work, formulations of acid functionalized multiwalled carbon nanotubes (f-MWCNTs) and poly(ethylenedioxythiophene) [PEDOT]:polystyrene sulphonate [PSS] were processed, and thin films were prepared on plastic substrates. Conductivity of PEDOT:PSS increased almost four orders of magnitude after adding f-MWCNTs. Work function of PEDOT:PSS/f-MWCNTs films was ∼0.5 eV higher as compared to the work function of pure PEDOT:PSS films, determined by Kelvin probe method. Field-effect transistors source–drain electrodes were prepared on PET plastic substrates where PEDOT:PSS/f-MWCNTs were patterned using laser ablation at 44 mJ/pulse energy to define 36 μm electrode separation. Silicon nanowires were deposited using dielectrophoresis alignment technique to bridge laser patterned electrodes. Top-gated nanowire field effect transistors were completed by depositing parylene C as polymer gate dielectric and gold as the top-gate electrode. Transistor characteristics showed p-type conduction with excellent gate electrode coupling, with an ON/OFF ratio of ∼200. Thereby, we demonstrate the feasibility of using high workfunction, printable PEDOT:PSS/f-MWCNTs composite inks for laser patterned source/drain electrodes for nanowire transistors on flexible substrates

Structure−property correlation behind the high mobility of carbazolocarbazole

A comparative study of carbazolocarbazole isomers and their respective N-alkyl derivatives confirms the good performance of carbazolo[2,1-a]carbazole as hole transporting material in organic field effect transistors. The azaphenacene structure of this molecule forms a dense packing promoted by particularly short longitudinal shifts between molecules establishing face-to-face and edge-to-face interactions. Computational calculations have determined an almost isotropic 2D transport environment within a lamellar structure. This favorable solid state arrangement, in combination with appropriate interfacial layers, has led to a high mobility (1.3 cm2V-1s-1) that validates the aptitude of this molecular material as organic semiconductor