High-Performance, Flexible NO2 Chemiresistors Achieved by Design of Imine-Incorporated n-Type Conjugated Polymers.

Flexible and mechanically robust gas sensors are the key technologies for wearable and implantable electronics. Herein, the authors demonstrate the high-performance, flexible nitrogen dioxide (NO2 ) chemiresistors using a series of n-type conjugated polymers (CPs: PNDIT2/IM-x) and a polymer dopant (poly(ethyleneimine), PEI). Imine double bonds (C = N) are incorporated into the backbones of the CPs with different imine contents (x) to facilitate strong and selective interactions with NO2 . The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO2 sensors with PNDIT2/IM-0.1 and PEI (1:1 by weight ratio) exhibit outstanding sensing performances, such as excellent sensitivity (ΔR/Rb = 240% @ 1 ppm), ultralow detection limit (0.1 ppm), high selectivity (ΔR/Rb < 8% @ 1 ppm of interfering analytes), and high stability, thereby outperforming other state-of-the-art CP-based chemiresistors. Furthermore, the thin film of PNDIT2/IM-0.1 and PEI blend is stretchable and mechanically robust, providing excellent flexibility to the NO2 sensors. Our study contributes to the rational design of high-performance flexible gas sensors.

Effects of the Selective Alkoxy Side Chain Position in Quinoxaline-Based Polymer Acceptors on the Performance of All-Polymer Solar Cells.

The effects of the position of alkoxy side chains in quinoxaline (Qx)-based polymer acceptors (PAs) on the characteristics of materials and the device parameters of all-polymer solar cells (all-PSCs) are investigated. The alkoxy side chains are selectively located at the meta, para, and both positions in pendant benzenes of Qx units, constructing PAs denoted as P(QxCN-T2)-m, P(QxCN-T2)-p, and P(QxCN-T2), respectively. Among them, P(QxCN-T2)-m exhibits the deepest energy levels owing to the enhanced electron-withdrawing effect of meta-positioned alkoxy chains, which is in contrast to P(QxCN-T2)-p where para-positioned alkoxy chains have an electron-donating property. In addition, the meta-positioned alkoxy chains induce good electron-conducting pathways, while the para-positioned ones significantly interrupt crystallization and intermolecular interactions between the conjugated backbones. Thus, when the PAs are applied to all-PSCs, a power conversion efficiency (PCE) of 5.07% is attained in the device using P(QxCN-T2)-m with efficient exciton dissociation and good electron-transporting ability. On the contrary, the P(QxCN-T2)-p-based counterpart has a PCE of only 1.62%. These results demonstrate that introducing alkoxy side chains at a proper location in the Qx-based PAs is crucial for their application to all-PSCs.

Sigmoidal Dependence of Electrical Conductivity of Thin PEDOT:PSS Films on Concentration of Linear Glycols as a Processing Additive.

We investigate the sigmoidal concentration dependence of electrical conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) processed with linear glycol-based additives such as ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), hexaethylene glycol (HEG), and ethylene glycol monomethyl ether (EGME). We observe that a sharp transition of conductivity occurs at the additive concentration of ~0.6 wt.%. EG, DEG, and TEG are effective in conductivity enhancement, showing the saturation conductivities of 271.8, 325.4, and 326.2 S/cm, respectively. Optical transmittance and photoelectron spectroscopic features are rather invariant when the glycols are used as an additive. Two different figures of merit, calculated from both sheet resistance and optical transmittance to describe the performance of the transparent electrodes, indicate that both DEG and TEG are two most effective additives among the series in fabrication of transparent electrodes based on PEDOT:PSS films with a thickness of ~50-60 nm.

Effect of Threshold Voltage Window and Variation of Organic Synaptic Transistor for Neuromorphic System.

Synaptic devices, which are considered as one of the most important components of neuromorphic system, require a memory effect to store weight values, a high integrity for compact system, and a wide window to guarantee an accurate programming between each weight level. In this regard, memristive devices such as resistive random access memory (RRAM) and phase change memory (PCM) have been intensely studied; however, these devices have quite high current-level despite their state, which would be an issue if a deep and massive neural network is implemented with these devices since a large amount of current-sum needs to flow through a single electrode line. Organic transistor is one of the potential candidates as synaptic device owing to flexibility and a low current drivability for low power consumption during inference. In this paper, we investigate the performance and power consumption of neuromorphic system composed of organic synaptic transistors conducting a pattern recognition simulation with MNIST handwritten digit data set. It is analyzed according to threshold voltage (VT) window, device variation, and the number of available states. The classification accuracy is not affected by VT window if the device variation is not considered, but the current sum ratio between answer node and the rest 9 nodes varies. In contrast, the accuracy is significantly degraded as increasing the device variation; however, the classification rate is less affected when the number of device states is fewer.

Cyano-Functionalized Quinoxaline-Based Polymer Acceptors for All-Polymer Solar Cells and Organic Transistors.

Quinoxaline (Qx) derivatives are promising building units for efficient photovoltaic polymers owing to their strong light absorption and high charge-transport abilities, but they have been used exclusively in the construction of polymer donors. Herein, for the first time, Qx-based polymer acceptors (PA s) were developed by introducing electron-withdrawing cyano (CN) groups into the Qx moiety (QxCN). A series of QxCN-based PA s, P(QxCN-T2), P(QxCN-TVT), and P(QxCN-T3), were synthesized by copolymerizing the QxCN unit with bithiophene, (E)-1,2-di(thiophene-2-yl)ethene, and terthiophene, respectively. All of the PA s exhibited unipolar n-type characteristics with organic field-effect transistor (OFET) mobilities of around 10-2  cm2 V-1 s-1 . In space-charge-limited current devices, P(QxCN-T2) and P(QxCN-TVT) exhibited electron mobilities greater than 1.0×10-4  cm2 V-1 s-1 , due to the well-ordered structure with tight π-π stacking. When the PA s were applied in all-polymer solar cells (all-PSCs), the highest performance of 5.32 % was achieved in the P(QxCN-T2)-based device. These results demonstrate the significant potential of Qx-based PA s for high-performance all-PSCs and OFETs.

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 6767 (2020)OPLEDP0146-959210.1364/OL.409743.

Solid-State Organic Electrolyte-Gated Transistors Based on Doping-Controlled Polymer Composites with a Confined Two-Dimensional Channel in Dry Conditions.

We report comprehensive and comparative studies on chemical and electrochemical controls of doping characteristics of various poly(3,4-ethylenedioxythiophene) (PEDOT) composites complexed with sulfonates. Chemical treatment of PEDOT composites was conducted with a dedoping agent, tetrakis(dimethylamino)ethylene (TDAE), resulting in the changes in conformation and bulk charge-carrier density. Electrochemical control of doping states was done with a solid-state ionogel based on an ionic liquid dispersed in a polymer matrix. With this approach, we can fabricate solid-state organic electrolyte-gated transistors (OEGTs) with a large current modulation, a high mobility of holes, and a low driving voltage. Our OEGTs are operational in a dry environment and, surprisingly, form the two-dimensional channel of the interfacial charge carriers modulating the conductance under gate bias, unlike conventional liquid-based OEGTs. The charge-carrier mobility and the on-to-off current ratio reach up to ∼7 cm2 V-1 s-1 and over 104, respectively, from the chemically dedoped PEDOT composites. The ionogel-based gating of the layer of TDAE-treated PEDOT composites induces a reversible transition between a highly doped bipolaronic state and neutral/polaronic states, as revealed by the absorption profiles under gate bias. We also demonstrate in-plane OEGTs, in which the dedoped channel and the conductive source/drain electrodes are made of a single PEDOT composite layer.

Enhancement of the response time in organic photorefractive composites using alkoxy-substituted PDCST as a nonlinear optical chromophore.

The ease of the molecular orientation of a chromophore has an important effect on the electro-optical (EO) properties of polymeric photorefractive (PR) composites. A derivative of 4-piperidinobenzylidene-malononitrile (PDCST) with an alkoxy group added as a side branch was synthesized to improve the molecular orientation characteristics. Electrophoresis was performed on the polymeric PR composite to which the improved PDCST had been added. The optical properties and response times were examined to evaluate the effects of the substitution of the alkoxy group. PDCST substituted with the alkoxy group showed enhanced EO properties and a PR grating formation rate.

Impact of Chlorination Patterns of Naphthalenediimide-Based Polymers on Aggregated Structure, Crystallinity, and Device Performance of All-Polymer Solar Cells and Organic Transistors.

The aggregation properties of conjugated polymers can play a crucial role in their thin film structures and performance of electronic devices. Control of these aggregated structures is particularly important in producing efficient all-polymer solar cells (all-PSCs), considering that strong demixing of the polymer donor and polymer acceptor typically occurs during film formation because of the low entropic contribution to the thermodynamics of the system. Here, three naphthalenediimide (NDI)-based polymer acceptors with different backbone chlorination patterns are developed to investigate the effect of the chlorination patterns on the aggregation tendencies of the polymer acceptors, which greatly influence their crystalline structures, electrical properties, and device performances of the resultant all-PSCs and organic field-effect transistors (OFETs). The counterparts of NDI units, dichlorinated bithiophene (Cl2T2), monochlorinated bithiophene (ClT2), and dichlorinated thienylene-vinylene-thienylene (Cl2TVT), are employed to synthesize a series of P(NDIOD-Cl2T2), P(NDIOD-ClT2), and P(NDIOD-Cl2TVT) polymers. The P(NDIOD-Cl2T2) polymer takes advantage of strong noncovalent bonding induced by its chlorine substituents, resulting in the formation of optimal face-on oriented crystalline structures which are suitable for efficient all-PSC devices. In comparison, the P(NDIOD-Cl2TVT) polymer forms bimodal crystalline structures in thin films to yield optimal performances in the resultant OFETs. When the three chlorinated polymers are applied to all-PSCs with the PBDTTTPD polymer donor, P(NDIOD-Cl2T2) achieves a maximum power conversion efficiency (PCE) of 7.22% with an appropriate blend morphology and high fill factor, outperforming P(NDIOD-ClT2) (PCE = 4.80%) and P(NDIOD-Cl2TVT) (PCE = 5.78%). Our observations highlight the effectiveness of the chlorination strategy for developing efficient polymer acceptors and demonstrate the important role of polymer aggregation in modulating the blend morphology and all-PSC performance.

Selective Ammonia-Sensing Platforms Based on a Solution-Processed Film of Poly(3-Hexylthiophene) and p-Doping Tris(Pentafluorophenyl)Borane.

Here, we fabricate ammonia sensors based on organic transistors by using poly(3-hexylthiophene) (P3HT) blended with tris(pentafluorophenyl)borane (TPFB) as an active layer. As TPFB is an efficient p-type dopant for P3HT, the current level of the blend films can be easily modulated by controlling the blend ratio. The devices exhibit significantly increased on-state and off-state current levels owing to the ohmic current originated from the large number of charge carriers when the active polymer layer contains TPFB with concentrations up to 20 wt % (P3HT:TPFB = 8:2). The current is decreased at 40 wt % of TPFB (P3HT:TPFB = 6:4). The P3HT:TPFB blend with a weight ratio of 9:1 exhibits the highest sensing performances for various concentrations of ammonia. The device exhibits an increased percentage current response compared to that of a pristine P3HT device. The current response of the P3HT:TPFB (9:1) device at 100 ppm of ammonia is as high as 65.8%, 3.2 times that of the pristine P3HT (20.3%). Furthermore, the sensor based on the blend exhibits a remarkable selectivity to ammonia with respect to acetone, methanol, and dichloromethane, owing to the strong interaction between the Lewis acid (TPFB) and Lewis base (ammonia).

Nanostructural Manipulation of Poly(3-hexylthiophene) Aggregates for Organic Electrolyte-Gated Transistors.

In this study, we combine solubility-driven formation of poly(3-hexylthiophene) (P3HT) nanoaggregates and ion-gel-based organic electrolyte-gated transistors (OEGTs), to develop high-performance low-voltage switching devices. By in-situ solution blending of a good solvent (chloroform) and a poor solvent (acetone), we obtain dispersions of P3HT nanoaggregates. The aggregation and molecular ordering of P3HT are analyzed by UV-Vis absorption spectroscopy, atomic force microscopy imaging, and X-ray diffraction. The resulting P3HT aggregates are used as an active component of high-capacitance ion-gel dielectric based on P(VDF-HFP)/[EMIM][TFSI]. Wellconnected conductive channels in thin films of P3HT aggregates allow the effective modulation of current in ion gel-gated transistors with an on-state current above 10-3 A at an operational voltage less than -1 V. In searching for the optimal ratio of solvents, the highest mobility of 1.36 cm² V-1 s-1 in the tested OEGTs was observed when 5 vol% of acetone was incorporated into the stock solution of P3HT. This observation suggests that the nanostructural manipulation of polythiophene-based semiconductor is an effective method to produce efficient pathways for charge transport in OEGTs.

Selective Wet-Etching of Polymer/Fullerene Blend Films for Surface- and Nanoscale Morphology-Controlled Organic Transistors and Sensitivity-Enhanced Gas Sensors.

Surface and nanoscale morphology of thin poly(3-hexylthiophene) (P3HT) films are effectively controlled by blending the polymer with a soluble derivative of fullerene, and then selectively dissolving out the fullerene from the blend films. A combination of the polymer blending with fullerene and a use of diiodooctane (DIO) as a processing additive enhances the molecular ordering of P3HT through nanoscale phase separation, compared to the pristine P3HT. In organic thin-film transistors, such morphological changes in the blend induce a positive effect on the field-effect mobility, as the mobility is ~5-7 times higher than in the pristine P3HT. Simple dipping of the blend films in butyl acetate (BA) causes a selective dissolution of the small molecular component, resulting in a rough surface with nanoscale features of P3HT films. Chemical sensors utilizing these morphological features show an enhanced sensitivity in detection of gas-phase ammonia, water, and ethanol.

Synergistic Effects of Processing Additives and Thermal Annealing on Nanomorphology and Hole Mobility of Poly(3-hexylthiophene) Thin Films.

Control of the nanoscale molecular ordering and charge-carrier mobility of poly(3-hexylthiophene-2,5-diyl) (P3HT) was achieved by the combined use of processing additives and thermal annealing. Evaluation of four processing additives (1,8-octanedithiol (ODT), diphenyl ether (DPE), 1-chloronaphthalene (CN), and 1,8-diiodooctane (DIO), which are commonly used for the fabrication of organic solar cells, revealed that the nanoscale molecular ordering and, therefore, the charge-carrier mobility, are largely affected by the additives, as demonstrated by spectral absorption, X-ray diffraction, and atomic force microscopy. Thermal annealing selectively influenced the morphological changes, depending on the solubility of P3HT in the additive at high temperature. In the case of CN, in which P3HT can be dissolved at moderate temperature, significant molecular ordering was observed even without thermal annealing. For DIO, in which P3HT is only soluble at elevated temperature, the mobility reached 1.14 × 10-1 cm² V-1 s-1 only after annealing. ODT and DPE were not effective as processing additives in a single-component P3HT. This study provides insight for designing the processing conditions to control the morphology and charge-transport properties of polymers.

Fast photorefractive response in polymeric composites enabled by the control of chromophore free volume.

The molecular orientation of a chromophore importantly affects the electro-optic characteristics of polymeric photorefractive composites. We designed methyl, ethyl, and isopropyl derivatives of 4-piperidinobenzylidene-malononitrile (PDCST) with the aim of enhancing molecular orientation properties, and investigated the effects of alkyl substitution on the electro-optic properties and response times of polymeric photorefractive composites. The three alkyl-substituted PDCSTs showed enhanced electro-optic responses and photorefractive grating buildup rates.

Enhanced photorefractive performance of polymeric composites through surface plasmon effects of gold nanoparticles.

We investigated the photorefractivity enhancement of polymeric composites by introducing gold nanoparticles (NPs). The gold NPs enhance the photocharge generation rate of sensitizers through plasmon resonance coupling achieved between NPs and sensitizers. Systematic studies show that the presence of gold NPs has increased photocharge generation efficiency, photoconductivity, diffraction efficiency, refractive index modulation, and photorefractive (PR) grating formation rate. The gold-NP-doped PR sample exhibits 2 times larger photocharge generation efficiency and photoconductivity, and 2.5 times faster PR grating formation rate compared to the control sample without the NPs.

High-mobility n-type conjugated polymers based on electron-deficient tetraazabenzodifluoranthene diimide for organic electronics.

High-mobility p-type and ambipolar conjugated polymers have been widely reported. However, high-mobility n-type conjugated polymers are still rare. Herein we present poly(tetraazabenzodifluoranthene diimide)s, PBFI-T and PBFI-BT, which exhibit a novel two-dimensional (2D) π-conjugation along the main chain and in the lateral direction, leading to high-mobility unipolar n-channel transport in field-effect transistors. The n-type polymers exhibit electron mobilities of up to 0.30 cm(2)/(V s), which is among the highest values for unipolar n-type conjugated polymers. Complementary inverters incorporating n-channel PBFI-T transistors produced nearly perfect switching characteristics with a high gain of 107.

Air-stable ambipolar field-effect transistors and complementary logic circuits from solution-processed n/p polymer heterojunctions.

We demonstrate the use of n/p polymer/polymer heterojunctions deposited by sequential solution processing to fabricate ambipolar field-effect transistors and complementary logic circuits. Electron and hole mobilities in the transistors were ∼0.001-0.01 cm(2)/(V s) in air without encapsulation. Complementary circuits integrating multiple ambipolar transistors into NOT, NAND, and NOR gates were fabricated and shown to exhibit sharp signal switching with a high voltage gain.

Polymer nanowire/fullerene bulk heterojunction solar cells: how nanostructure determines photovoltaic properties.

We report studies of bulk heterojunction solar cells composed of self-assembled poly(3-butylthiophene) nanowires (P3BT-nw) as the donor component with a fullerene acceptor. We show that the nanostructure of these devices is the single most important variable determining their performance, and we use a combination of solvent and thermal annealing to control it. A combination of conductive and photoconductive atomic force microscopy provides direct connections between local nanostructure and overall device performance. Films with a dense random web of nanowires cause the fullerene to aggregate in the interstices, giving a quasi-ordered interpenetrating heterojunction with high short-circuit current density (10.58 mA/cm(2)), but relatively low open circuit voltage (520 mV). Films with a low density of nanowires result in a random bulk heterojunction composed of small crystalline PCBM and P3BT phases. Fewer nanowires result in higher open circuit voltage (650 mV) but lower current density (6.02 mA/cm(2)). An average power conversion efficiency of 3.35% was achieved in a structure which balances these factors, with intermediate nanowire density. The best photovoltaic performance would be realized in a material structure which maintains the interpenetrating network of nanowires and fullerene phases (high current density), but avoids the device bridging we observe, and the recombination and shunt losses associated with it (high open-circuit voltage).