Crack engineering for the construction of arbitrary hierarchical architectures.

Three-dimensional hierarchical morphologies widely exist in natural and biomimetic materials, which impart preferential functions including liquid and mass transport, energy conversion, and signal transmission for various applications. While notable progress has been made in the design and manufacturing of various hierarchical materials, the state-of-the-art approaches suffer from limited materials selection, high costs, as well as low processing throughput. Herein, by harnessing the configurable elastic crack engineering-controlled formation and configuration of cracks in elastic materials-an effect normally avoided in various industrial processes, we report the development of a facile and powerful technique that enables the faithful transfer of arbitrary hierarchical structures with broad material compatibility and structural and functional integrity. Our work paves the way for the cost-effective, large-scale production of a variety of flexible, inexpensive, and transparent 3D hierarchical and biomimetic materials.

Synthesis of a novel low-bandgap polymer based on a ladder-type Heptacyclic arene consisting of outer thieno[3,2-b]thiophene units for efficient photovoltaic application.

A novel conjugated polymer PIDTT-quinoxaline (Qx) based on the coplanar thieno[3,2-b]thiophene-phenylene-thieno[3,2-b]thiophene structure is synthesized and evaluated as an electron-donor material for bulk-heterojunction polymer solar cells (BHJ PSCs). The absorption spectra, electrochemical, charge transport, and film morphology properties as well as theoretical modeling of PIDTT-Qx are investigated to understand its intrinsic structure-property relationship. As expected, this polymer with an extended π-conjugated backbone exhibits a narrow-bandgap and board absorption spectrum for enhanced light harvesting. BHJ PSCs (ITO/PEDOT:PSS/polymer:PC71 BM/interlayer/Al) afford a maximum power conversion efficiency of 5.05% with an open-circuit voltage of 0.84 V, a short-circuit current density of 11.26 mA cm(-2) , and a fill factor of 53.4%. These results demonstrate the potential of PIDTT-Qx as an efficient electron-donor material for BHJ PSCs.

Atomic-level chemical reaction promoting external quantum efficiency of organic photomultiplication photodetector exceeding 108% for weak-light detection.

Low-cost, solution-processed photomultiplication organic photodetectors (PM-OPDs) with external quantum efficiency (EQE) above unity have attracted enormous attention. However, their weak-light detection is unpleasant because the anode Ohmic contact causes exacerbation in dark current. Here, we introduce atomic-level chemical reaction in PM-OPDs which can simultaneously suppress dark current and increase EQE via depositing a 0.8 nm thick Al2O3 by the atomic layer deposition. Suppression in dark current mainly originates from the built-in anode Schottky junction as a result of work function decrease of hole-transporting layer of which the chemical groups can react chemically with the bottom surface of Al2O3 layer at the atomic-level. Such strategy of suppressing dark current is not adverse to charge injection under illumination; instead, responsivity enhancement is realized because charge injection can shift from cathode to anode, of which the neighborhood possesses increased photogenerated carriers. Consequently, weak-light detection limit of the forwardly-biased PM-OPD with Al2O3 treatment reaches a remarkable level of 2.5 nW cm-2, while that of the reversely-biased control is 25 times inferior. Meanwhile, the PM-OPD yields a record high EQE and responsivity of 4.31 × 108% and 1.85 × 106 A W-1, respectively, outperforming all other polymer-based PM-OPDs.

Influence of intermolecular N-H...pi interactions on molecular packing and field-effect performance of organic semiconductors.

Charge transport in organic semiconductors is strongly dependent on their molecular packing modes in the solid state. Therefore, understanding the relationship between molecular packing and charge transport is imperative, both experimentally and theoretically. However, so far, the fundamental effects of solid-state packing and molecular interactions (e.g. N-H...pi) on charge transport need further elucidation. Herein, indolo[3,2-b]carbazole (ICZ) and a derivative thereof are used as examples to approach this scientific target. An interesting insight obtained thereby is that N-H...pi interactions among ICZ molecules facilitate charge transport for higher mobility. Subtle changes in the of N-H...pi interactions can significantly influence both the molecular packing and the charge-transport properties. Therefore, a method for exploiting intermolecular N-H...pi interactions would yield novel molecular systems with designable characteristics.

Novel Dimethylmethylene-Bridged Triphenylamine-PDI Acceptor for Bulk-Heterojunction Organic Solar Cells.

A novel, star-shaped electron acceptor, DMTPA-PDI3, derived from a planar dimethylmethylene-bridged triphenylamine core with three acetylene-linked perylene diimide (PDI) units is developed as a nonfullerene acceptor for organic solar cells (OSCs). DMTPA-PDI3 manifests significantly reduced intramolecular twisting, enabling sufficient system-wide π-electron delocalization leading to broadened spectral absorption and raised lowest unoccupied molecular orbital level. As a result, higher and more balanced hole and electron transport properties are observed. Active layers for OSCs comprising DMTPA-PDI3 acceptor and PBT7-Th donor exhibit suppressed intermolecular aggregation, giving rise to uniform nanophase network formation. These OSC devices have afforded respectably high power-conversion efficiency of about 5%.

Naphthalene diimide-difluorobenzene-based polymer acceptors for all-polymer solar cells.

Regio-random (P1) and -regular (P2) difluorobenzene-naphthalene-containing polymer acceptors were developed for bulk-heterojunction all-polymer solar cells (all-PSCs). P2 exhibited significantly higher crystallinity in thin films, providing high spectral absorptivity and electron mobility than P1. When used in all-PSC devices, P2 afforded a respectably higher power conversion efficiency of over 5%.

Enhancing Crystalline Structural Orders of Polymer Semiconductors for Efficient Charge Transport via Polymer-Matrix-Mediated Molecular Self-Assembly.

A facile polymer-matrix-mediated molecular self-assembly of polymer semiconductors into highly crystalline orders for efficient charge transport in organic thin-film transistors is demonstrated. Phenomenal enhancements in field-effect mobility of about one order of magnitude and current on/off ratio of two to three orders of magnitude are realized with polyacrylonitrile-incorporated polymer semiconductor compositions via solution deposition.

Solution-Processed Donor-Acceptor Polymer Nanowire Network Semiconductors For High-Performance Field-Effect Transistors.

Organic field-effect transistors (OFETs) represent a low-cost transistor technology for creating next-generation large-area, flexible and ultra-low-cost electronics. Conjugated electron donor-acceptor (D-A) polymers have surfaced as ideal channel semiconductor candidates for OFETs. However, high-molecular weight (MW) D-A polymer semiconductors, which offer high field-effect mobility, generally suffer from processing complications due to limited solubility. Conversely, the readily soluble, low-MW D-A polymers give low mobility. We report herein a facile solution process which transformed a lower-MW, low-mobility diketopyrrolopyrrole-dithienylthieno[3,2-b]thiophene (I) into a high crystalline order and high-mobility semiconductor for OFETs applications. The process involved solution fabrication of a channel semiconductor film from a lower-MW (I) and polystyrene blends. With the help of cooperative shifting motion of polystyrene chain segments, (I) readily self-assembled and crystallized out in the polystyrene matrix as an interpenetrating, nanowire semiconductor network, providing significantly enhanced mobility (over 8 cm(2)V(-1)s(-1)), on/off ratio (10(7)), and other desirable field-effect properties that meet impactful OFET application requirements.

Rod-like ß-FeOOH@poly(dopamine)-Au-poly(dopamine) nanocatalysts with improved recyclable activities.

A novel rod-like ß-FeOOH@poly(dopamine)-Au-poly(dopamine) nanocomposite is developed for recyclable catalysis. Firstly, the rod-like ß-FeOOH template was coated in situ by a layer of poly(dopamine) (PDA) to form a core/shell nanostructure. Then the negatively charged Au nanocatalysts were well-immobilized onto the periphery of the ß-FeOOH@PDA nanorod. To protect the Au nanocrystals from leaching during the catalytic reactions, another PDA layer was coated onto the above particles to form a sandwich-like PDA-Au-PDA shell on the ß-FeOOH rod core. The reduction of Rhodamine B (RhB) was introduced as a model reaction to evaluate the catalytic activity of the as-prepared nanocomposites. It was found that the catalytic rate sharply increased with an increasing amount of the nanocatalyst. Benefitting from the thin outer layer of PDA, the recyclability of the nanocatalyst dramatically increased. After five times of catalytic reaction, the activity was maintained as high as 98.3%, while the ß-FeOOH@PDA-Au showed it to be retained at only 73.4%.

Molecular packing and electronic processes in amorphous-like polymer bulk heterojunction solar cells with fullerene intercalation.

The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c] thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

A high-sensitivity near-infrared phototransistor based on an organic bulk heterojunction.

High-gain photodetectors with near-infrared (NIR) sensitivity are critical for biomedical applications such as photoplethysmography and optical coherence tomography where detected optical signals are relatively weak. Current photodetection technologies rely on avalanche photodiodes and photomultipliers to achieve high sensitivity. These devices, however, require a high operation voltage and are not compatible with CMOS based read-out circuits (ROCs). In this work we demonstrate a solution-proceeded NIR phototransistor structure based on a bulk heterojunction (BHJ) of a narrow bandgap polymer, poly(N-alkyl diketopyrrolo-pyrrole dithienylthieno[3,2-b]thiophene) (DPP-DTT), and [6,6]-phenyl-C61-butyric acid methylester (PCBM). The device exhibits ultrahigh responsivity (∼5 × 10(5) A W(-1)) as well as wide tunability (>1 × 10(4)) of photoconductive gain. Using the current-voltage and transient photocurrent measurements we show that the high responsivity is due to the combined effects of fast transport of holes in the polymer matrix and slow detrapping of electrons from the isolated PCBM domains. The wide gain tunability and the efficient suppression of noise current are achieved through the use of the optically tunable gate terminal. We demonstrate that our phototransistor can be used as the detection unit in a photoplethysmography sensor for non-invasive, continuous finger pulse wave monitoring. The high-sensitivity of the phototransistor allows the use of a low-power light source, thus reducing the overall power consumption of the sensor. This, together with the solution processibility and the simple device configuration (which is compatible with conventional ROCs), make the phototransistor a very promising component for the next generation low-cost, mobile biomedical devices for health monitoring and remote diagnostics.

A stable solution-processed polymer semiconductor with record high-mobility for printed transistors.

Microelectronic circuits/arrays produced via high-speed printing instead of traditional photolithographic processes offer an appealing approach to creating the long-sought after, low-cost, large-area flexible electronics. Foremost among critical enablers to propel this paradigm shift in manufacturing is a stable, solution-processable, high-performance semiconductor for printing functionally capable thin-film transistors - fundamental building blocks of microelectronics. We report herein the processing and optimisation of solution-processable polymer semiconductors for thin-film transistors, demonstrating very high field-effect mobility, high on/off ratio, and excellent shelf-life and operating stabilities under ambient conditions. Exceptionally high-gain inverters and functional ring oscillator devices on flexible substrates have been demonstrated. This optimised polymer semiconductor represents a significant progress in semiconductor development, dispelling prevalent skepticism surrounding practical usability of organic semiconductors for high-performance microelectronic devices, opening up application opportunities hitherto functionally or economically inaccessible with silicon technologies, and providing an excellent structural framework for fundamental studies of charge transport in organic systems.

Thiophene polymer semiconductors for organic thin-film transistors.

Printed organic thin-film transistors (OTFTs) have received great interests as potentially low-cost alternative to silicon technology for application in large-area, flexible, and ultra-low-cost electronics. One of the critical materials for TFTs is semiconductor, which has a dominant impact on the transistor properties. We review here the structural studies and design of thiophene-based polymer semiconductors with respect to solution processability, ambient stability, molecular self-organization, and field-effect transistor properties for OTFT applications. We show that through judicial monomer design, delicately controlled pi-conjugation, and strategically positioned pendant side-chain distribution, novel solution-processable thiophene polymer semiconductors with excellent self-organization ability to form extended lamellar pi-stacking orders can be developed. OTFTs using semiconductors of this nature processed in ambient conditions have provided excellent field-effect transistor properties.

Bottom-contact poly(3,3'''-didodecylquaterthiophene) thin-film transistors with gold source-drain electrodes modified by alkanethiol monolayers.

A series of alkanethiol monolayers (CH 3(CH 2) n-1 SH, n = 4, 6, 8, 10, 12, 14, 16) were used to modify gold source-drain electrode surfaces for bottom-contact poly(3,3'''-didodecylquaterthiophene) (PQT-12) thin-film transistors (TFTs). The device mobilities of TFTs were significantly increased from approximately 0.015 cm (2) V (-1) s (-1) for bare electrode TFTs to a maximum of approximately 0.1 cm (2) V (-1) s (-1) for the n = 8 monolayer devices. The mobilities of devices with alkanethiol-modified Au electrodes varied parabolically with alkyl length with values of 0.06, 0.1, and 0.04 cm (2) V (-1) s (-1) at n = 4, 8, and 16, respectively. Atomic force microscopy investigations reveal that alkanethiol electrode surface modifications promote polycrystalline PQT-12 morphologies at electrode/PQT-12 contacts, which probably increase the density of states of the PQT-12 semiconductor at the interfaces. The contact resistance of TFTs is strongly modulated by the surface modification and strongly varies with the alkanethiol chain length. The surface modifications of electrodes appear to significantly improve the charge injection, with consequent substantial improvement in device performance.

Printed silver ohmic contacts for high-mobility organic thin-film transistors.

Oleic acid-stabilized silver nanoparticles prepared by a facile synthesis afforded highly conductive elements upon proper annealing. Regioregular polythiophene-based thin-film transistors (OTFTs) using source/drain electrodes prepared from these silver nanoparticles provided excellent field-effect characteristics, despite a significant difference between the work function of silver and the HOMO of polythiophene semiconductor. This was attributable to conductive doping of the semiconductor interface by residual oleic acid or its thermally transformed derivative from the silver electrodes, thus enabling ohmic contact formation. This is in sharp contrast to the OTFTs with silver electrodes fabricated from both vacuum deposition and oleylamine-stabilized silver nanoparticles, which exhibited markedly lower mobility and current on/off ratio, a consequence of energetic mismatch of the electrode/semiconductor pairs.

Enabling gate dielectric design for all solution-processed, high-performance, flexible organic thin-film transistors.

A novel solution-processed, compositionally and structurally stable dual-layer gate dielectric composed of a UV-cured poly(4-vinyl phenol)-co-poly(methyl methacrylate) bottom layer and a thermally cross-linked poly(methyl silsesquioxane) top layer for organic thin-film transistors is described. This gate dielectric design, coupled with compatible solution-processable semiconductor and conductor materials, has enabled fabrication of all solution-processed, high-performance organic thin-film transistors on flexible substrates. High field-effect mobility and current on/off ratio, together with other desirable transistor properties, are demonstrated.

Facile synthesis of silver nanoparticles useful for fabrication of high-conductivity elements for printed electronics.

A facile synthesis of stable silver nanoparticles having a particle size of <10 nm is described. The synthesis involved reduction of silver acetate with a substituted hydrazine, such as PhNHNH2, in the presence of a 1-alkylamine, such as C16H33NH2, in toluene at 25-60 degrees C. Spin-coated thin films or printed electronic features of alkylamine-stabilized silver nanoparticles could be easily converted at 120-160 degrees C into highly conductive films or elements with conductivity of 2-4 x 104 S cm-1. Organic thin-film transistors with printed silver source/drain electrodes of this nature exhibited field-effect transistor properties which are similar to those of the devices using vacuum-deposited silver electrodes.

Indolo[3,2-b]carbazole-based thin-film transistors with high mobility and stability.

Proper functionalization of indolo[3,2-b]carbazole led to a new class of high-performance organic semiconductors suitable for organic thin-film transistor (OTFT) applications. While 5,11-diaryl-substituted indolo[3,2-b]carbazoles without long alkyl side chains provided amorphous thin films upon vacuum deposition, those with sufficiently long alkyl side chains such as 5,11-bis(4-octylphenyl)indolo[3,2-b]carbazole self-organized readily into highly crystalline layered structures under similar conditions. OTFTs using channel semiconductors of this nature exhibited excellent field-effect properties, with mobility up to 0.12 cm(2) V(-1) s(-1) and current on/off ratio to 10(7). As this class of organic semiconductors has relatively low HOMO levels and large band gaps, they also displayed good environmental stability even with prolonged exposure to amber light, an appealing characteristic for OTFT applications.

High-performance semiconducting polythiophenes for organic thin-film transistors.

Conjugated polymers have been widely studied as potential semiconductor materials for organic thin-film transistors (TFTs). However, they have provided functionally poor transistor properties when the TFTs are fabricated in air. We have developed a class of liquid crystalline regioregular polythiophenes, PQTs, that possess sufficient air stability to enable achievement of excellent TFT properties under ambient conditions. These polythiophenes exhibit unique self-assembly ability and form highly structured thin films when deposited from solution under appropriate conditions. TFTs fabricated in air with PQT channel layers have provided high field-effect mobility to 0.14 cm2 V-1 s-1 and high current modulation to over 107, together with other desirable transistor properties. These high-performance polythiophenes will therefore help bring the long-standing concept of low-cost organic/polymer transistor circuits closer to commercial reality.

Lamination method for the study of interfaces in polymeric thin film transistors.

A method for the fabrication of polymeric thin-film transistors (TFTs) by lamination is described. Poly(dimethylsiloxane) stamps were used to delaminate thin films of semiconducting polymers from silicon wafers coated with a self-assembled monolayer (SAM) formed from octyltrichlorosilane. These supported films were laminated onto electrode structures to form coplanar TFTs. The fabrication process was used to make TFTs with poly(3-hexylthiophene), P3HT, and poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene], PQT-12. TFTs, where these polymers were laminated onto gate dielectrics coated with SAMs from octyltrichlorosilane, had effective field-effect mobilities of 0.03 and 0.005 cm2/(V s), respectively. TFTs where PQT-12 was laminated onto gate dielectrics that were not coated with a SAM also had mobility of 0.03 cm2/(V s). In contrast, TFTs fabricated by spin-coating PQT-12 onto the same structure had mobilities ranging from 10-3 to 10-4 cm2/(V s). These results suggest that the lower mobilities of polymer TFTs made with hydrophilic gate dielectrics are caused by molecular ordering in the semiconducting film rather than electronic effects of dipolar groups at the interface.