Well‐Balanced Ambipolar Conjugated Polymers Featuring Mild Glass Transition Temperatures Toward High‐Performance Flexible Field‐Effect Transistors

Xiang

et al. 2020

Adv Materials Inter

Self Cite

devices, namely ferromagnetic (FM) spin injectors and detectors and non-ferromagnetic organic medium. When evaluating OSV device performance, magnetoresistance (MR) behavior is an important criterion. [7-11] Undoubtedly, decent MR value is strongly expected for its close proof to long distance spin-conserved transport. Continuous efforts of seeking suitable spin polarizer and transport media fueled the development of OSVs during the past decade. [2-4,11-14] On the other hand, debate on MR signal has never ceased since the first report on organic giant magnetoresistance by Xiong. [7] In general, MR response in a prototype OSV device is considered to be positive, namely device resistance under magnetization antiparallel states is larger than that of parallel states. However, negative (abnormal) MR response in OSVs is increasingly discovered thus must not be fortuity. Consequently, distinguishing the working mechanisms of the negative MR signals becomes an important component of organic spintronics. To date, there are several viewpoints to explain this abnormal spin response: 1) Negative polarization is the most representative also the earliest explanation which is primarily extracted from Julliere tunneling formula. [15-17] That is when spin injector is positively (negatively) polarized, while spin detector is negatively (positively) polarized, the MR behavior is negative. However, such explanation is invalid when injecting Organic spin valves (OSVs) have become an essential building block of next-generation memory devices which focus on spin degree of transporting carriers. Meanwhile, negative magnetoresistance (MR) effect in the OSV devices is increasingly observed which deserves further exploration for the rich spin physics behind. In this work, the negative MR response in ferromagnetic (FM) metal-based OSVs using donor−acceptor (D−A) conjugated polymer based on the naphthalenediimide units as a spacer material is observed. The negative MR effect does not result from negative polarization at spin injection and detection interface as well as tunneling anisotropic magnetoresistance effect, but from the spin transport inside the D−A polymer spacer. Even the stacking sequence of the spin injection and detection electrodes is reversed or changed the polymer coating solvents, the D−A polymer contributed negative MR response still can be observed. For further identifying negative MR origin, the bottom FM metal polarizer is replaced into high-polarization La 0.7 Sr 0.3 MnO 3 thin film, negative MR feature is well reproduced. Based on these points, it is deduced that the spin-orientation reversal inside D−A polymer spacer is the ultimate reason for such negative MR response. The filtering-like spin reversal activity is also in positive proportion to intermolecular interaction.

Section: Resultssupporting

confidence: 69%

Section: Introductionmentioning

confidence: 99%

Negative Magnetoresistance Behavior in Polymer Spin Valves Based on Donor−Acceptor Conjugated Molecules

Xiang

et al. 2020

Adv Materials Inter

Self Cite

“…In this regard, semiconducting conjugated polymers have been extensively investigated due to their merits of flexibility and solution processability, in addition to their high charge mobilities . Various conjugated polymers, in particular conjugated donor (D)–acceptor (A) polymers, have been explored for high‐performance semiconductors with high hole or electron mobilities or balanced hole/electron mobilities …”

Section: Introductionmentioning

confidence: 99%

“…For this reason, significant attention has been paid to the design and synthesis of various conjugated moieties and the covalent linkage of them into conjugated polymers . Conjugated frameworks, such as diketopyrrolopyrrole (DPP), isoindigo (IID), naphthalenediimide (NDI), and benzodifurandione‐based oligo( p ‐phenylene vinylene) (BDOPV), have been widely used as building blocks for semiconducting conjugated polymers. Meanwhile, our group and others have shown that side alkyl chains not only endow the polymers with solubilities in organic solvents and solution processability but also affect the interchain packing at the microscopic level, and thin film crystallinity and morphology at the macroscopic level, resulting in the modulation of charge transporting property of the polymer thin films.…”

Section: Introductionmentioning

confidence: 99%

The Effects of Side Chains on the Charge Mobilities and Functionalities of Semiconducting Conjugated Polymers beyond Solubilities

et al. 2019

Recent decades have witnessed the rapid development of semiconducting polymers in terms of high charge mobilities and applications in transistors. Significant efforts have been made to develop various conjugated frameworks and linkers. However, studies are increasingly demonstrating that the side chains of semiconducting polymers can significantly affect interchain packing, thin film crystallinity, and thus semiconducting performance. Ways to modify the side alkyl chains to improve the interchain packing order and charge mobilities for conjugated polymers are first discussed. It is shown that modifying the branching chains by moving the branching points away from the backbones can boost the charge mobilities, which can also be improved through partially replacing branching chains with linear ones. Second, the effects of side chains with heteroatoms and functional groups are discussed. The siloxane‐terminated side chains are utilized to enhance the semiconducting properties. The fluorinated alkyl chains are beneficial for improving both charge mobility and air stability. Incorporating H bonding group side chains can improve thin film crystallinities and boost charge mobilities. Notably, incorporating functional groups (e.g., glycol, tetrathiafulvalene, and thymine) into side chains can improve the selectivity of field‐effect transistor (FET)‐based sensors, while photochromic group containing side chains in conjugated polymers result in photoresponsive semiconductors and optically tunable FETs.

“…Ambipolar conjugated polymers, which transport holes and electrons simultaneously, have great potential in simple and low cost fabrication of light emitting field‐effect transistors (FETs) and complementary‐like inverter through one‐step printing process . The most effective way to realize ambipolar polymers is copolymerization of an electron donor and acceptor unit (D–A) having the same charge transport capability.…”

Section: Introductionmentioning

confidence: 99%

Controlling Ambipolar Charge Transport in Isoindigo‐Based Conjugated Polymers by Altering Fluorine Substitution Position for High‐Performance Organic Field‐Effect Transistors

Kim

Park

et al. 2019

Adv Funct Materials

A molecular design strategy to achieve highly balanced ambipolar charge transport for donor–acceptor (D–A) isoindigo (IIG)‐based copolymer through systematic selection of fluorination positions is reported. To study fluorine substitution site effects on electronic and structural properties, two fluorinated IIG‐based copolymers (PIIG‐iFT2 and PIIG‐oFT2) are synthesized, which contain two fluorine atoms at the bithiophene (T2) inner and outer site and compare them with a nonfluorinated copolymer of IIG and T2 (PIIG‐T2) as the reference polymer. Fluorination at the outer site of T2 in PIIG‐oFT2 polymer effectively lowers molecular energy levels and increases molecular planarity more than fluorination at the T2 inner site. PIIG‐oFT2 organic field‐effect transistors show highly balanced ambipolar mobility, hole mobility (μh)/electron mobility (μe) = 1 by increasing electron mobility, whereas PIIG‐T2 (μh/μe = 9.0) and PIIG‐iFT2 (μh/μe = 2.4) exhibit unbalanced ambipolar transport. The ambipolar complementary‐like inverter is also demonstrated by simple one‐time coating of PIIG‐oFT2 with gain = 21.