Steve Park

Selective dispersion of high purity semiconducting single-walled carbon nanotubes with regioregular poly(3-alkylthiophene)s

Conjugated polymers, such as polyfluorene and poly(phenylene vinylene), have been used to selectively disperse semiconducting single-walled carbon nanotubes (sc-SWNTs), but these polymers have limited applications in transistors and solar cells. Regioregular poly(3-alkylthiophene)s (rr-P3ATs) are the most widely used materials for organic electronics and have been observed to wrap around SWNTs. However, no sorting of sc-SWNTs has been achieved before. Here we report the application of rr-P3ATs to sort sc-SWNTs. Through rational selection of polymers, solvent and temperature, we achieved highly selective dispersion of sc-SWNTs. Our approach enables direct film preparation after a simple centrifugation step. Using the sorted sc-SWNTs, we fabricate high-performance SWNT network transistors with observed charge-carrier mobility as high as 12 cm(2) V(-1) s(-1) and on/off ratio of >10(6). Our method offers a facile and a scalable route for separating sc-SWNTs and fabrication of electronic devices.

Stretchable Energy‐Harvesting Tactile Electronic Skin Capable of Differentiating Multiple Mechanical Stimuli Modes

The first stretchable energy-harvesting electronic-skin device capable of differentiating and generating energy from various mechanical stimuli, such as normal pressure, lateral strain, bending, and vibration, is presented. A pressure sensitivity of 0.7 kPa(-1) is achieved in the pressure region <1 kPa with power generation of tens of μW cm(-2) from a gentle finger touch.

Flow-enhanced solution printing of all-polymer solar cells.

Morphology control of solution coated solar cell materials presents a key challenge limiting their device performance and commercial viability. Here we present a new concept for controlling phase separation during solution printing using an all-polymer bulk heterojunction solar cell as a model system. The key aspect of our method lies in the design of fluid flow using a microstructured printing blade, on the basis of the hypothesis of flow-induced polymer crystallization. Our flow design resulted in a ∼90% increase in the donor thin film crystallinity and reduced microphase separated donor and acceptor domain sizes. The improved morphology enhanced all metrics of solar cell device performance across various printing conditions, specifically leading to higher short-circuit current, fill factor, open circuit voltage and significantly reduced device-to-device variation. We expect our design concept to have broad applications beyond all-polymer solar cells because of its simplicity and versatility.

Understanding Polymorphism in Organic Semiconductor Thin Films through Nanoconfinement

Understanding crystal polymorphism is a long-standing challenge relevant to many fields, such as pharmaceuticals, organic semiconductors, pigments, food, and explosives. Controlling polymorphism of organic semiconductors (OSCs) in thin films is particularly important given that such films form the active layer in most organic electronics devices and that dramatic changes in the electronic properties can be induced even by small changes in the molecular packing. However, there are very few polymorphic OSCs for which the structure-property relationships have been elucidated so far. The major challenges lie in the transient nature of metastable forms and the preparation of phase-pure, highly crystalline thin films for resolving the crystal structures and evaluating the charge transport properties. Here we demonstrate that the nanoconfinement effect combined with the flow-enhanced crystal engineering technique is a powerful and likely material-agnostic method to identify existing polymorphs in OSC materials and to prepare the individual pure forms in thin films at ambient conditions. With this method we prepared high quality crystal polymorphs and resolved crystal structures of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), including a new polymorph discovered via in situ grazing incidence X-ray diffraction and confirmed by molecular mechanic simulations. We further correlated molecular packing with charge transport properties using quantum chemical calculations and charge carrier mobility measurements. In addition, we applied our methodology to a [1]benzothieno[3,2-b][1]1benzothiophene (BTBT) derivative and successfully stabilized its metastable form.

Large‐Area Assembly of Densely Aligned Single‐Walled Carbon Nanotubes Using Solution Shearing and Their Application to Field‐Effect Transistors

Dense alignment of single-walled carbon nanotubes over a large area is demonstrated using a novel solution-shearing technique. A density of 150-200 single-walled carbon nanotubes per micro-meter is achieved with a current density of 10.08 μA μm(-1) at VDS = -1 V. The on-current density is improved by a factor of 45 over that of random-network single-walled carbon nanotubes.

Ultrahigh electrical conductivity in solution-sheared polymeric transparent films

Significance Many applications, including solar cells and touch screens, require coatings that are both optically transparent and electrically conductive. Most device structures use indium tin oxide to serve as this transparent conductor (TC), even though it accounts for a disproportionally large amount of device cost. Alternatives, such as polymer-based materials, may not only provide additional cost benefits, but may also allow for added functionality, such as flexibility. In this paper, we examine conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) deposited by solution shearing as a TC film. Morphological and chemical changes caused by solution shearing deposition lead to record-high conductivities and overall excellent TC performance. With consumer electronics transitioning toward flexible products, there is a growing need for high-performance, mechanically robust, and inexpensive transparent conductors (TCs) for optoelectronic device integration. Herein, we report the scalable fabrication of highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin films via solution shearing. Specific control over deposition conditions allows for tunable phase separation and preferential PEDOT backbone alignment, resulting in record-high electrical conductivities of 4,600 ± 100 S/cm while maintaining high optical transparency. High-performance solution-sheared TC PEDOT:PSS films were used as patterned electrodes in capacitive touch sensors and organic photovoltaics to demonstrate practical viability in optoelectronic applications.

High-Yield Sorting of Small-Diameter Carbon Nanotubes for Solar Cells and Transistors

We describe herein a high-yield method to selectively disperse semiconducting CoMoCAT (CO disproportionation on Co-Mo catalysts) single-walled carbon nanotubes (SWNTs) with regioregular poly(3-alkylthiophenes) polymers. We observed that the dispersion yield was directly related to the length of the polymers alkyl side chains. Molecular dynamics simulations in explicit toluene (real toluene molecules) indicate that polythiophenes with longer alkyl side chains bind strongly to SWNTs, due to the increased overall surface contact area with the nanotube. Furthermore, the sorting process selectively enriches smaller-diameter CoMoCAT SWNTs with larger bandgaps, which is ideal for solar cell applications. Compared to the larger diameter sorted HiPco (High-Pressure CO) SWNTs, solar cells fabricated using our sorted CoMoCAT SWNTs demonstrated higher open-circuit voltage (Voc) and infrared external quantum efficiency (EQE). The Voc achieved is the highest reported for solar cells based on SWNT absorbers under simulated AM1.5 solar illumination. Additionally, we employed the sorted CoMoCAT SWNTs to fabricate thin film transistors with excellent uniformity and device performance.

Highly effective separation of semiconducting carbon nanotubes verified via short-channel devices fabricated using dip-pen nanolithography.

We have verified a highly effective separation of semiconducting single-walled carbon nanotubes (sc-SWNTs) via statistical analysis of short-channel devices fabricated using multipen dip-pen nanolithography. Our SWNT separation technique utilizes a polymer (rr-P3DDT) that selectively interacts with and disperses sc-SWNTs. Our devices had channel lengths on the order of 300-500 nm, with an average of about 3 SWNTs that directly connected the source-drain electrodes. A total of 140 SWNTs were characterized, through which we have observed that all of the SWNTs exhibited semiconducting behavior with an average on/off current ratio of ~10(6). Additionally, we have characterized 50 SWNTs after the removal of rr-P3DDT, through which we have again observed semiconducting behavior for all of the SWNTs with similar electrical characteristics. The relatively low average on-conductance of 0.0796 μS was attributed to the distribution of small diameter SWNTs in our system and due to the non-ohmic Au contacts on SWNTs. The largely positive threshold voltages were shifted toward zero after vacuum annealing, indicating that the SWNTs were doped in air. To the best of our knowledge, this is the first time numerous SWNTs were electrically characterized using short-channel devices, through which all of the measured SWNTs were determined to be semiconducting. Hence, our semiconducting single-walled carbon nanotube sorting system holds a great deal of promise in bringing forth a variety of practical applications in SWNT electronics.

Significant Enhancement of Infrared Photodetector Sensitivity Using a Semiconducting Single‐Walled Carbon Nanotube/C60 Phototransistor

A highly sensitive single-walled carbon nanotube/C60 -based infrared photo-transistor is fabricated with a responsivity of 97.5 A W(-1) and detectivity of 1.17 × 10(9) Jones at 1 kHz under a source/drain bias of -0.5 V. The much improved performance is enabled by this unique device architecture that enables a high photoconductive gain of ≈10(4) with a response time of several milliseconds.

Wafer-scale fabrication and characterization of thin-film transistors with polythiophene-sorted semiconducting carbon nanotube networks.

Semiconducting single-walled carbon nanotubes (SWCNTs) have great potential of becoming the channel material for future thin-film transistor technology. However, an effective sorting technique is needed to obtain high-quality semiconducting SWCNTs for optimal device performance. In our previous work, we reported a dispersion technique for semiconducting SWCNTs that relies on regioregular poly(3-dodecylthiophene) (rr-P3DDT) to form hybrid nanostructures. In this study, we demonstrate the scalability of those sorted CNT composite structures to form arrays of TFTs using standard lithographic techniques. The robustness of these CNT nanostructures was tested with Raman spectroscopy and atomic force microscope images. Important trends in device properties were extracted by means of electrical measurements for different CNT concentrations and channel lengths (L(c)). A statistical study provided an average mobility of 1 cm(2)/V·s and I(on)/I(off) as high as 10(6) for short channel lengths (L(c) = 1.5 μm) with 100% yield. This highlights the effectiveness of this sorting technique and its scalability for large-scale, flexible, and transparent display applications.