Solvent driven performance in thin floating-films of PBTTT for organic field effect transistor: Role of macroscopic orientation

Considering the advantages of floating film transfer method (FTM), we have investigated the optical and electronic characteristics of PBTTT-C14 thin-films prepared by the static and the dynamic casting on liquid substrate. It has been demonstrated that judicious selection of solvents during FTM switches the casting mode from the static casting (S-FTM) using high boiling point solvent to the dynamic casting (D-FTM) from low boiling point solvent. Although both of the methods provide the edge-on oriented structure of PBTTT-C14 by XRD, the structural and the optical analyses reveal relatively extended π-conjugation length in parallel D-FTM film as compared to that of S-FTM. A high field-effect mobility (μ) of 0.11 cm2/V.s was exhibited by OFETs fabricated by parallel D-FTM film even without any high temperature post-annealing up to the liquid crystalline phase transition. This observed value of μ for parallel D-FTM is 4.7 and 12.8 times higher than the isotropic S-FTM and the perpendicular D-FTM films, respectively

Ambipolar Transport in Bilayer Organic Field-Effect Transistor Based on Poly(3-hexylthiophene) and Fullerene Derivatives

Ambipolar characteristics in an organic field-effect transistor (FET) with a bilayer structure consisting of poly(3-hexylthiophene) (P3HT) and a fullerene derivative (PCBM) are reported. P3HT was deposited by a floating film transfer method (FTM) with toluene solution on spin-coated PCBM. The FTM-deposited film was found to show relatively high hole mobility even when cast using toluene solution. Even after coating P3HT on PCBM by FTM, a relatively high n-type transport was obtained. This indicates that FTM employed in this study is a mild way to coat an organic thin film on an organic semiconductor layer in terms of minimizing the effect of carrier transport in the underlayer. The transport characteristics have been discussed in comparison with those of ambipolar FETs prepared by other methods previously reported

Implication of Molecular Weight on Optical and Charge Transport Anisotropy in PQT-C12 Films Fabricated by Dynamic FTM

Large area (>20 cm × 2 cm)-oriented thin films of PQT-C12 with varying molecular weight and polydispersity index (PDI) were fabricated by the ribbon-shaped floating film transfer method aiming toward their application as an active semiconductor element of organic field effect transistors (OFETs). Investigation on the influence of the molecular weight and PDI upon the extent of molecular alignment and anisotropic charge transport was systematically carried out. It has been demonstrated that high molecular weight in combination with low PDI not only leads to a very high optical anisotropy >10 but also high charge carrier anisotropy with a hole mobility of about 0.07 cm2/V·s for OFETs using parallel-oriented PQT-C12 thin films. Such a structure–property correlation is highly beneficial for the development of high performance organic electronic devices by synergistic and amicable tuning of the optoelectronic anisotropies and polymer synthetic variables

大面積リボン状浮遊膜の分子配向制御による共役高分子系有機FETにおける異方性電荷輸送の検討

Conjugated polymers (CPs) have emerged as one of the potential candidates as active semiconducting elements in organic electronics owing to their low cost device fabrication in the area of organic field effect transistors, organic light emitting diodes and solar cells etc. The main feature of this class material lies in the preparation of the thin film via facile solution processing. CPs are susceptible to anisotropic charge transport owing to their inherent one-dimensional nature. In order to delineate optical anisotropy and anisotropic charge transport various techniques for molecular alignment of CPs have been attempted in the recent past. Existing problems like mechanical damage, solubility of under-layer and difficulty in multilayer film fabrication during molecular alignments needs the development of suitable methods. To circumvent these issues, floating film transfer method (FTM) having capability of anisotropic thin film fabrication have been proposed in the recent past. Although in the proposed FTM oriented films could be easily obtained, most commonly observed circular orientation hinders further upscaling of this method for the large area applications. In this thesis, a new improvisation for unidirectional film spreading during FTM have been made by implementing a newly designed PTFE slider leading to ribbon-shaped floating films and named as Ribbon-shaped FTM. A number of most widely used CPs such as PQT-C12, F8T2, non-regiocontrolled (NR) P3HT, PBTTT-C14, PTB7 and regioregular (RR) P3ATs etc. have been successfully oriented using ribbon-shaped FTM. These oriented films have been characterized by a number of techniques like polarized electronic absorption spectroscopy, atomic force microscopy, X-ray diffraction. Parametric optimization for film casting conditions such as viscosity/temperature of the liquid substrate, temperature and concentration polymer solution were amicably carried out. Influence of these casting conditions on the nature of ribbon-shaped FTM in terms of extent of macroscopic film formation, variation on the optical anisotropy and film thickness were investigated in detail. Amongst several CPs used for investigation, PQT-C12 exhibited not only the optical anisotropy but also the pronounced anisotropic charge transport with highest charge carrier mobility for OFETs based on oriented CPs. PQT-C12 was utilized for in-depth investigation pertaining to the implication molecular weight and its distribution on the optoelectronic anisotropies by synthetizing polymers with different molecular weight and polydispersity index (PDI). It has been found too high or low molecular weights or not favorable for promoting molecular orientation and relatively smaller PDI promotes the facile anisotropic charge transport. One of the batch of synthesized PQT-C12 and large area thin film by ribbon-shape FTM exhibited remarkably high optical anisotropy (DR?22) under at optimized casting condition. Microstructural investigation of these highly oriented films as probed by in plane GIXD exhibited edge-on orientation for the films fabricated under ambient conditions. A clear dependence of extent of molecular orientation on charge carrier mobility and anisotropic charge transport was demonstrated. Intractability of polythiophene led to development of RR-poly(3-alkylthiophene) derivatives but drastic decrease (4-5 orders) in mobility as function increasing alkyl chain length and enforced maximum research on hexyl substituted derivative (P3HT). Efforts were directed to prepare large area oriented thin films of RR-P3ATs by ribbon shaped FTM and influence of molecular orientation on alkyl chain length was investigated. A decease in DR with increasing alkyl chain length substitution was explained by increasing extent of interdigitating alkyl chains as confirmed by XRD results. Moreover, drastic hampering of charge carrier mobility as function of alkyl chain was not observed for FTM oriented films, which was explained by edge-on orientation as evidenced by in-plane GIXD investigations.九州工業大学博士学位論文 学位記番号：生工博甲第335号 学位授与年月日：平成31年3月25日1 Introduction|2 Experimental|3 Ribbon-shaped FTM: parametric optimizations|4 Charge transport in poly(quarterthiophene): Implication of optical anisotropy and molecular weight|5 Optoelectronic properties of conjugated polymers prepared by ribbon-shaped FTM|6 Orientation and anomalous charge transport in regioregular poly (3-alkylthiophenes)|7 General conclusion and future work九州工業大学平成30年

Semiconducting Polymer Design and Interface Engineering for Efficient Charge Transport

A major challenge to achieve macroscopic conjugated polymer (CP) alignment leading to efficient charge transport stems from the intrinsic disordered and entangled nature of CP chains. Liquid crystalline (LC) CP design principles for directed alignment and their application to build a better understanding of charge transport in plastic electronics are discussed in this dissertation. First, molecular design parameters affecting the CP alignment are thoroughly investigated. The identified parameters, correlating with alignment characteristics via LC properties, are (a) the planarity of polymer chains; (b) intramolecular interaction moieties for induced chain planarity; (c) the effective bulkiness of side chains; and (d) surface energy of CPs. Second, cleavable side chains were introduced to the LC CPs as another design factor to achieve solvent-resistant highly aligned polymer films. The alignment behavior of the resulting new LC CP was examined in detail by adopting the floating film transfer method (FTM). In this method, an optimum amount of a high boiling point solvent was found to be vital to provide enough time for CPs to align. A high mobility anisotropy of ~14 was obtained through well-aligned CPs under the optimized condition. The subsequent side chain removal led to the formation of solvent-resistant highly aligned CP films. Overall, the outcomes provide insights into the realization of anisotropic properties of CPs in the solid thin films and offer an opportunity to enable a wide range of applications in organic electronics. FTM was employed as an interface engineering tool to investigate the charge transport of CPs in organic field effect transistors (OFETs). Although the transistor performance has been known to be critically affected by the polymer film-dielectric interface, it has been very difficult to isolate the contribution of CP alignment near the interface from that of the bulk film to the device performance. FTM has the capability of modulating CP alignment directions discretely in multilayered films, providing an opportunity to solve the daunting task. The resulting CP films prepared by FTM consisted of a bottom layer close to the polymer-dielectric interface and a top layer in contact with the source-drain electrodes. When the bottom layer had a parallel CP orientation and the CPs in the top layer were oriented perpendicular to the source-drain direction, the average hole mobility was larger by a factor of 3.3 than that of the opposite case. Moreover, OFET devices with combinations of the various bottom and top layer CP orientation directions revealed that the CP orientation direction of the bottom layer governed the overall device performance with a much smaller contribution from that of the top layer. These findings support that the CP alignment near the polymer-dielectric interface is a decisive factor for the charge transport in OFETs. Possible device performance enhancement through interface engineering is also demonstrated by investigating how the work function of electrodes can be modulated. Changes in the work function were demonstrated by means of electrophoretic deposition of ionic polyelectrolytes, electrospraying of a neutral polymer, and even after inserting an insulating spacer layer between a work function modifying layer and an electrode. The results consistently show that the work function can be controlled via a combination of the surface interaction and the charge-based through-space interaction, which can lead to the precise work function modification of electrodes for effective charge injection and extraction in organic electronics.PHDMacromolecular Science & EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/155202/1/dasyang_1.pd

Molecular Orientation and Anisotropic Charge Transport in the Large Area Thin Films of Regioregular Poly (3-alkylthiopnes) Fabricated by Ribbon-Shaped FTM

Introduction of alkyl chains is a plausible approach to impart the facile solution processability in regioregular (poly-3-alkylthiophenes) (RR-P3ATS) but drastic fall in the conductivity as a function of increasing alkyl chain length restricted the versatile use of its hexyl substituted derivative (P3HT). In this work, fabrication of large area oriented thin films of RR-P3ATs with varying alkyl chain lengths was successfully demonstrated using ribbon shaped floating film transfer method (FTM). Observed decrease in the molecular orientation with increasing alkyl chain length was explained by enhanced alkyl chain inter-digitation as evidenced by trends of d-spacing estimated from the out-of-plane XRD results. In-plane GIXD revealed edge-on orientation in the thin films of all of the P3ATs prepared by ribbon-shaped FTM, which is highly desired for planer charge carrier transport. Parallel oriented thin film of longest alkyl chain (octadecyl) substituted P3AT poly (3-octadecylthiophene) exhibited a field effect transistor (FET) mobility of 1.9 × 10−2 cm2/V, which is about 4–5 orders of magnitude higher as compared its previously reported values estimated by time-of-flight and FET methods. Thus, fabrication of thin films by FTM leads to versatile choice of RR-P3ATs for solution processing for FETs using common organic solvents without having any detrimental impact of alkyl chain length on the charge carrier mobility

Abbreviations and acronyms

This booklet provides a partial list of acronyms, abbreviations, and other short word forms, including their definitions, used in documents at the Goddard Space Flight Center (GSFC). This list does not preclude the use of other short forms of less general usage, as long as these short forms are identified the first time they appear in a document and are defined in a glossary in the document in which they are used. This document supplements information in the GSFC Scientific and Technical Information Handbook (GHB 2200.2/April 1989). It is not intended to contain all short word forms used in GSFC documents; however, it was compiled of actual short forms used in recent GSFC documents. The entries are listed first, alphabetically by the short form, and then again alphabetically by definition

Investigation of graphene as electrode in n-type OFETs and its use in nanometric devices

This work aims to investigate the use of CVD-graphene as electrode material in nanometric channel n-type Organic Field Effect Transistors (OFETs) based on thermally evaporated thin films of the perylene-3,4,9,10-tetracarboxylic acid diimide derivatives (PDIF-CN2 and PDI8-CN2). We firstly explored the electrical response of nano devices with standard bottom-contact/ distributed bottom gate architecture. By a thorough comparison with short channel transistors made with gold electrodes, output characteristics of the graphene-based devices suggests that SCLC contribution is suppressed. Moreover, current on/off ratios independent of the channel length (L) and enhanced response for high longitudinal biases are demonstrated for (L) down to 140 nm. Further advances have been reached by the use of a proper device architecture for nano devices with patterned local gate tracks and an ultra-thin films (8nm) of Hafnium Dioxide as high-k gate dielectric. The largely improved gate modulation results in a proper output currents saturation for channel length down to 200nm, with supply biases of few volts. Through impedance spectroscopy, overlap capacitances and the overall AC response of CVD-graphene electrodes have been investigated as well. The cut-off frequency of the nanodevice has been indirectly evaluated considering the DC transconductance and the measured overlap capacitance of the graphene electrodes. Values of the order of 150 kHz has been obtained for channel lengths of 200nm. Lastly, the organic/graphene interfaces and their injection and extraction phenomena have been further investigated in micrometric architectures. In particular, the problem of contact resistances have been analyzed via Scanning Kelvin Probe Force Microscopy (SKPFM) and the energetics of the interfaces has been reconstructed by the analysis of UV Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS)