Center for Advanced Soft Electronics

Abstract Two narrow bandgap non‐fullerene acceptors (NBG‐NFAs), namely, COTIC‐4F and SiOTIC‐4F, are designed and synthesized for the fabrication of efficient near‐infrared organic solar cells (OSCs). The chemical structures of the NBG‐NFAs contain a D′‐D‐D′ electron‐rich internal core based on a cyclopentadithiophene (or dithienosilole) (D) and alkoxythienyl (D′) core, end‐capped with the highly electron‐deficient unit 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile (A), ultimately providing a A‐D′‐D‐D′‐A molecular configuration that enhances the intramolecular charge transfer characteristics of the excited states. One can thereby reduce the optical bandgap ( E g opt ) to as low as ≈1.10 eV, one of the smallest values for NFAs reported to date. In bulk‐heterojunction (BHJ) OSCs, NBG‐NFA blends with the polymer donor PTB7‐Th yield power conversion efficiencies (PCE) of up to 9.0%, which is particularly high when compared against a range of NBG BHJ blends. Most significantly, it is found that, despite the small energy loss ( E g opt − e V OC ) of 0.52 eV, the PTB7‐Th/NBG‐NFA bulk heterojunction blends can yield short‐circuit current densities of up to 22.8 mA cm −2 , suggesting that the design and application of NBG‐NFA materials have substantial potential to further improve the PCE of OSCs.

Abstract A series of narrow bandgap electron acceptors is designed and synthesized for efficient near‐infrared (NIR) organic solar cells. Extending π‐conjugation of donor frameworks leads to an intense intramolecular charge transfer, resulting in broad absorption profiles with band edge reaching 950 nm. When blended with an electron donor polymer PTB7‐Th, IOTIC‐2F exhibits efficient charge transfer even with a small energetic offset, so as to achieve a large photogenerated current over 22 mA cm −2 with small energy losses (≈0.49 eV) in solar cell devices. With an intense NIR absorbance, PTB7‐Th:IOTIC‐2F‐based cells achieve a power conversion efficiency of 12.1% with good visible transparency (52% transmittance from 370 to 740 nm). Analysis of film morphology reveals that processing with solvent additives enhances crystalline features of acceptor components, while keeping an appropriate level of donor:acceptor intermixing in the binary blends. The incorporation of the third component, ITIC‐2F, into the PTB7‐Th:IOTIC‐2F blends increases the device efficiency up to 12.9%. The improvement is assigned to the cascaded energy‐level structure and desirable nanoscale phase separation of the ternary blends, which is beneficial to the photocurrent generation. This work provides an efficient molecular design strategy to optimize nonfullerene acceptor properties for efficient NIR organic photovoltaics.

Abstract In this progress report, recent advances in the development of organic transistors with superior bias stress stability and in the understanding of the charge traps that degrade device performance under prolonged bias stress are reviewed, with a particular focus on materials science and engineering methods. The phenomenological aspects of bias stress effects and the experimental methods for investigating charge traps are described. The recent progress in the bias stress stability of organic transistors is discussed in terms of those components that are the main focus of attempts to improve bias stress stability, i.e., organic semiconductor layers, gate dielectrics, and source/drain contacts. A brief summary of this progress is presented and the outlook for future research in this field is assessed. This report aims to summarize recent progress in this field and to provide some guidelines for studying bias stress–induced charge‐trapping phenomena.

Pseudo-regular alternating PDPP–TVS copolymers using an asymmetric monomer (thiophene–vinylene–selenophene (TVS)) are synthesized. Unlike regular alternating copolymers, these polymers are highly soluble in nonchlorinated solvents such as tetra­hydrofuran, toluene, xylene, and tetralin. The organic field-effect transistor devices fabricated using these polymers dissolved in nonchlorinated solvents exhibit a high hole mobility up to 8.2 cm2 V−1 s−1. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

The electrical properties of organic semiconductors (OSCs), whether they are conjugated small molecules or polymers, can be tailored by incorporating electrically insulating units (EIUs), which are organic moieties consisting of nonconjugated units. EIUs can be introduced to a thin film by synthetically connecting them to the otherwise conjugated OSC molecules or by blending them in as separate EIU molecules with the OSCs during the thin‐film fabrication process. The engineered EIUs are capable of imparting various additional functions to the OSC thin film and improving their electrical properties. In this review article, a comprehensive overview of various effects of EIUs on OSC thin films and their consequent electrical performance when used as active layers in organic field‐effect transistors (OFETs) is provided. A broad range of studies of the synthetic approaches of incorporating EIUs, such as those using side chains, block copolymers, and conjugation‐break spacers, and of the blending approaches with organic insulators is discussed. Finally, a brief summary and perspectives for future research in this field are presented.

Abstract Herein is demonstrated that the polymer chain ends of polymer gate‐ dielectrics (PGDs) in organic field‐effect transistors (OFETs) can trap charges; the bias‐stress stability is reduced without changes in the mobilities of the transistor devices as well as the morphologies of the organic semiconductors. The bias‐stress stabilities of OFETs using PGD with various molecular weights (MWs) are investigated. Under bias stress in ambient air, the drain current decay and the threshold voltage shift are found to increase as the MW of the PGD decreases (MW effect). This MW effect is caused by the variation in the density of polymer chain ends in the PGDs with MW: the free volumes at the polymer chain ends act as charge‐trap sites, resulting in drain current decay during bias stress. The free volumes at polymer chain ends are sufficiently large to allow the residence of water molecules, the presence of which significantly increases the density of charge‐trap sites. In contrast, polymer chain ends without trapped water molecules do not allow charge trapping and so bias‐stress stability is independent of the MW of the PGD. It is also found that the hydrophilicity/hydrophobicity of the chain ends of the PGD can affect bias‐stress stability; carboxyl‐terminated polystyrene exhibits a much higher trap density and lower bias‐stress stability than hydrogen‐terminated polystyrene when these devices are exposed to humid nitrogen.

A new narrow bandgap non-fullerene electron acceptor was designed, synthesized, and characterized for near-infrared organic photovoltaics.

A new donor–acceptor organic semiconducting co-polymer (PDPP-TAT) containing acetylene linkages based on dithienyl-diketopyrrolopyrrole (tDPP) has been synthesized and compared with a tDPP-based co-polymer (PDPP-TVT) containing vinylene linkages. The sp-hybridized carbons in the acetylene linkages result in favorable overlap of the electron wave functions of the tDPP units along the main chain. Further, the π-conjugation of PDPP-TAT was found to be highly insensitive to the chain conformation, in contrast to that of PDPP-TVT. As a result, PDPP-TAT provides favorable charge transport for electrons as well as holes, and enables facile charge transport in amorphous and tie-molecular regions connecting its crystalline domains. PDPP-TAT exhibits ambipolar characteristics with a high electron/hole mobility ratio (μe/μh) of ∼0.3 in field-effect transistors, whereas PDPP-TVT exhibits unipolar characteristics with a μe/μh value that is a factor of 30 lower. Our results demonstrate that the conformation sensitivity of charge transport is a vital factor in the electrical performances of actual organic transistor devices.

Abstract Highly crystalline thin films in organic semiconductors are important for applications in high‐performance organic optoelectronics. Here, the effect of grain boundaries on the Hall effect and charge transport properties of organic transistors based on two exemplary benchmark systems is elucidated: (1) solution‐processed blends of 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C 8 ‐BTBT) small molecule and indacenodithiophene‐benzothiadiazole (C 16 IDT‐BT) conjugated polymer, and (2) large‐area vacuum evaporated polycrystalline thin films of rubrene (C 42 H 28 ). It is discovered that, despite the high field‐effect mobilities of up to 6 cm 2 V −1 s −1 and the evidence of a delocalized band‐like charge transport, the Hall effect in polycrystalline organic transistors is systematically and significantly underdeveloped, with the carrier coherence factor α < 1 (i.e., yields an underestimated Hall mobility and an overestimated carrier density). A model based on capacitively charged grain boundaries explaining this unusual behavior is described. This work significantly advances the understanding of magneto‐transport properties of organic semiconductor thin films.

This article describes a novel method for the direct synthesis of patterned graphene on transition-metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) with chemical vapor deposition (CVD) that uses a UV/ozone-treated solid carbon source, 1,2,3,4-tetraphenylnaphthalene (TPN) as the graphene growth precursor. The UV/ozone treatment of the TPN film on the MoS2 layer improves the interfacial adhesion between the TPN and MoS2 layers. The surface-adhered TPN is directly converted to graphene on the MoS2 layer, which results in a sharp interface between graphene and MoS2. The graphene/MoS2 heterostructure with interfacial bonding yields excellent electrical and mechanical characteristics that facilitate charge injection by reducing contact resistance and improving bending stability. The excellent contact enhances the field-effect mobility of MoS2 field-effect transistors to values up to three times higher than that of the devices using source-drain electrodes prepared with the conventionally transferred CVD-grown graphene. The proposed method for the direct synthesis of graphene on TMDs is expected to have wide applications in nanoelectronics based on 2D materials.

Atomically thin two-dimensional transition metal dichalcogenides (2D TMDs) are of interest for neuromorphic computing due to their extraordinary properties such as low power consumption, robustness, flexibility, and layered anisotropic transport properties. Here, we present metal-ion assisted 2D MoS2 neuromorphic devices fabricated using a simple sputtering method. This method enables us to synthesize large-scale and uniform nanostructured polycrystalline MoS2 films on flexible substrates. We found that the small grain of the MoS2 film effectively enhances the ion transport through the grain boundaries or interfaces in the MoS2 film, which results in excellent neuromorphic characteristics such as bipolar electrical property, short-/long-term plasticity (STP/LTP) with a high ratio of ILRS/IHRS (∼105), paired-pulse facilitation (PPF), and stability. Furthermore, it was found that the memory performance parameters such as the SET/RESET voltage (VSET/VRESET) and the programming/erasing current ratio (Ion/Ioff) can be affected by the concentration of ions inserted into MoS2. This work provides insight for realizing practical neuromorphic devices and understanding ion-mediated synaptic behavior of nanocrystal structures, which can be tuned for high-efficiency neuromorphic devices.

Utilizing the intrinsic mobility-strain relationship in semiconductors is critical for enabling strain engineering applications in high-performance flexible electronics. Here, measurements of Hall effect and Raman spectra of an organic semiconductor as a function of uniaxial mechanical strain are reported. This study reveals a very strong, anisotropic, and reversible modulation of the intrinsic (trap-free) charge carrier mobility of single-crystal rubrene transistors with strain, showing that the effective mobility of organic circuits can be enhanced by up to 100% with only 1% of compressive strain. Consistently, Raman spectroscopy reveals a systematic shift of the low-frequency Raman modes of rubrene to higher (lower) frequencies with compressive (tensile) strain, which is indicative of a reduction (enhancement) of thermal molecular disorder in the crystal with strain. This study lays the foundation of the strain engineering in organic electronics and advances the knowledge of the relationship between the carrier mobility, low-frequency vibrational modes, strain, and molecular disorder in organic semiconductors.

Neuromorphic computing based on two-dimensional transition-metal dichalcogenides (2D TMDs) has attracted significant attention recently due to their extraordinary properties generated by the atomic-thick layered structure. This study presents sulfur-defect-assisted MoS2 artificial synaptic devices fabricated by a simple sputtering process, followed by a precise sulfur (S) vacancy-engineering process. While the as-sputtered MoS2 film does not show synaptic behavior, the S vacancy-controlled MoS2 film exhibits excellent synapse with remarkable nonvolatile memory characteristics such as a high switching ratio (∼103), a large memory window, and long retention time (∼104 s) in addition to synaptic functions such as paired-pulse facilitation (PPF) and long-term potentiation (LTP)/depression (LTD). The synaptic device working mechanism of Schottky barrier height modulation by redistributing S vacancies was systemically analyzed by electrical, physical, and microscopy characterizations. The presented MoS2 synaptic device, based on the precise defect engineering of sputtered MoS2, is a facile, low-cost, complementary metal-oxide semiconductor (CMOS)-compatible, and scalable method and provides a procedural guideline for the design of practical 2D TMD-based neuromorphic computing.

Tetracyanoquinodimethane leads to trap-enhanced photoconductive gain and highly efficient broadband sensing for bulk heterojunction organic photodetectors.

Abstract A novel strategy for analyzing bias‐stress effects in organic field‐effect transistors (OFETs) based on a four‐parameter double stretched‐exponential formula is reported. The formula is obtained by modifying a traditional single stretched‐exponential expression comprising two parameters (a characteristic time and a stretched‐exponential factor) that describe the bias‐stress effects. The expression yields two characteristic times and two stretched‐exponential factors, thereby separating out the contributions due to charge trapping events in the semiconductor layer‐side of the interface and the gate‐dielectric layer‐side of the interface. The validity of this method was tested by designing two model systems in which the physical properties of the semiconductor layer and the gate‐dielectric layer were varied systematically. It was found that the gate‐dielectric layer, in general, plays a more critical role than the semiconductor layer in the bias‐stress effects, possibly due to the wider distribution of the activation energy for charge trapping. Furthermore, the presence of a self‐assembled monolayer further widens the distribution of the activation energy for charge trapping in gate‐dielectric layer‐side of the interface and causes the channel current to decay rapidly in the early stages. The novel analysis method presented here enhances our understanding of charge trapping and provides rational guidelines for developing efficient OFETs with high performance.

Abstract Charge carrier mobility is an important characteristic of organic field‐effect transistors (OFETs) and other semiconductor devices. However, accurate mobility determination in FETs is frequently compromised by issues related to Schottky‐barrier contact resistance, that can be efficiently addressed by measurements in 4‐probe/Hall‐bar contact geometry. Here, it is shown that this technique, widely used in materials science, can still lead to significant mobility overestimation due to longitudinal channel shunting caused by voltage probes in 4‐probe structures. This effect is investigated numerically and experimentally in specially designed multiterminal OFETs based on optimized novel organic‐semiconductor blends and bulk single crystals. Numerical simulations reveal that 4‐probe FETs with long but narrow channels and wide voltage probes are especially prone to channel shunting, that can lead to mobilities overestimated by as much as 350%. In addition, the first Hall effect measurements in blended OFETs are reported and how Hall mobility can be affected by channel shunting is shown. As a solution to this problem, a numerical correction factor is introduced that can be used to obtain much more accurate experimental mobilities. This methodology is relevant to characterization of a variety of materials, including organic semiconductors, inorganic oxides, monolayer materials, as well as carbon nanotube and semiconductor nanocrystal arrays.

Oligo(ethylene glycol)-incorporated hybrid linear alkyl side chains, serving as solubilizing groups, are designed and introduced into naphthalene-diimide-based n-channel copolymers. The synthesized polymers exhibit unipolar n-type operation with an electron mobility of up to 1.64 cm(2) V(-1) s(-1), which demonstrates the usefulness of the hybrid side chains in polymer electronics applications.

The effects of SAMs on the electrical stabilities of graphene FETs were examined by using various SAMs.

Spin-casting a polymer semiconductor solution over a short period of only a few seconds dramatically improved the molecular ordering and charge transport properties of the resulting semiconductor thin films. In this process, it was quite important to halt spinning before the drying line propagation had begun. Here, we elucidated the effects of the substrate surface characteristics on the drying kinetics during spin-coating, systematically investigated the microstructural evolution during semiconducting polymer solidification, and evaluated the performances of the resulting polymer field-effect transistors. We demonstrated that the spin time required to enhance the molecular ordering and electrical properties of the polythiophene thin films was strongly correlated with the solidification onset time, which was altered by surface treatments introduced onto the substrate surfaces.

Novel propeller-shaped non-fullerene small molecule acceptors with imide-linked perylene diimides on a 9,9′-spirobi[9<italic>H</italic>-fluorene] core, namely SF-iPDI2 and SF-iPDI4, were designed and synthesized for organic solar cells.