Predicting the Mechanical Properties of Organic Semiconductors Using Coarse-Grained Molecular Dynamics Simulations

The ability to predict the mechanical properties of organic semiconductors is of critical importance for roll-to-roll production and thermomechanical reliability of organic electronic devices. Here, we describe the use of coarse-grained molecular dynamics simulations to predict the density, tensile modulus, Poisson ratio, and glass transition temperature for poly­(3-hexyl­thiophene) (P3HT) and its blend with C₆₀. In particular, we show that the resolution of the coarse-grained model has a strong effect on the predicted properties. We find that a one-site model, in which each 3-hexyl­thiophene unit is represented by one coarse-grained bead, predicts significantly inaccurate values of density and tensile modulus. In contrast, a three-site model, with one coarse-grained bead for the thiophene ring and two for the hexyl chain, predicts values that are very close to experimental measurements (density = 0.955 g cm^–3, tensile modulus = 1.23 GPa, Poisson ratio = 0.35, and glass transition temperature = 290 K). The model also correctly predicts the strain-induced alignment of chains as well as the vitrification of P3HT by C₆₀ and the corresponding increase in the tensile modulus (tensile modulus = 1.92 GPa, glass transition temperature = 310 K). We also observe a decrease in the radius of gyration and the density of entanglements of the P3HT chains with the addition C₆₀ which may contribute to the experimentally noted brittleness of the composite material. Although extension of the model to poly­(3-alkyl­thiophenes) (P3ATs) containing side chains longer than hexyl groupsnonyl (N) and dodecyl (DD) groupscorrectly predicts the trend of decreasing modulus with increasing length of the side chain measured experimentally, obtaining absolute agreement for P3NT and P3DDT could not be accomplished by a straightforward extension of the three-site coarse-grained model, indicating limited transferability of such models. Nevertheless, the accurate values obtained for P3HT and P3HT:C₆₀ blends suggest that coarse graining is a valuable approach for predicting the thermomechanical properties of organic semiconductors of similar or more complex architectures

<i>J-V</i> curves of P3HT:PCBM solar devices.

<p>(a) The average <i>J-V</i> curves for devices with transparent electrodes consisting of PEDOT:PSS (<i>N</i> = 7), PEDOT:PSS and grids patterned by polypropylene knife (<i>N</i> = 6), and PEDOT:PSS and grids patterned by steel razor (<i>N</i> = 4). (b) The <i>J-V</i> curves for the highest efficiency cells from the sample sets from (a). (c) The <i>J-V</i> curves for the highest efficiency larger cells (∼0.5 cm² compared to ∼0.1 cm² in (a) and (b)).</p

Measuring the Glass Transition Temperature of Conjugated Polymer Films with Ultraviolet–Visible Spectroscopy

The glass transition temperature (<i>T</i>_g) of a conjugated polymer can be used to predict its morphological stability and mechanical properties. Despite the importance of this parameter in applications from organic solar cells to wearable electronics, it is not easy to measure. The <i>T</i>_g is often too weak to detect using conventional differential scanning calorimetry (DSC). Alternative methodse.g., variable temperature ellipsometryrequire specialized equipment. This paper describes a technique for measuring the <i>T</i>_g of thin films of semicrystalline conjugated polymers using only a hot plate and an ultraviolet–visible (UV–vis) spectrometer. UV–vis spectroscopy is used to measure changes in the absorption spectrum due to molecular-scale rearrangement of polymers when heated past <i>T</i>_g, corresponding to the onset of the formation of photophysical aggregates. A deviation metric, defined as the sum of the squared deviation in absorbance between as-cast and annealed films, is used to quantify shifts in the absorption spectra. The glass transition is observed as a change in slope in a plot of the deviation metric versus temperature. To demonstrate the usefulness of this technique, a variety of semiconducting polymers are tested: P3BT, PBTTT-C14, F8BT, PDTSTPD, PTB7, PCDTBT, TQ1, and MEH-PPV. These polymers represent a range of solid-state morphologies, from highly ordered to predominantly amorphous. A successful measurement of <i>T</i>_g depends on the ability of the polymer to form photophysical aggregates. The results obtained using this method for P3BT, PBTTT-C14, F8BT, and PDTSTPD are in agreement with values of <i>T</i>_g that have been reported in the literature. Molecular dynamics simulations are used to show how the morphology evolves upon annealing: above the <i>T</i>_g, an initially kinetically trapped morphology undergoes structural rearrangement to assume a more thermodynamically preferred structure. The temperature at which onset of this rearrangement occurs in the simulation is concomitant with the spectroscopically determined value of <i>T</i>_g

Schematic diagrams summarizing the two implementations of abrasion lithography.

<p>(a) Method I uses mechanical abrasion with a sharp tool to pattern water-soluble thin films. (b) Method II produces patterns by direct abrasion of glass substrates.</p

Images of transparent electrodes fabricated by abrasion lithography.

<p>(a) Photograph showing the high transparency of grids produced by Method II. A reflection in the bottom left corner of the glass substrate shows the copper wires. (b) Optical micrograph showing the wires produced by Method II.</p

Efficient Characterization of Bulk Heterojunction Films by Mapping Gradients by Reversible Contact with Liquid Metal Top Electrodes

The ways in which organic solar cells (OSCs) are measured and characterized are inefficient: many substrates must be coated with expensive or otherwise precious materials to test the effects of a single variable in processing. This serial, sample-by-sample approach also takes significant amounts of time on the part of the researcher. Combinatorial approaches to research OSCs generally do not permit microstructural characterization on the actual films from which photovoltaic measurements were made, or they require specialized equipment that is not widely available. This paper describes the formation of one- and two-dimensional gradients in morphology and thickness. Gradients in morphology are formed using gradient annealing, and gradients in thickness are formed using asymmetric spin coating. Use of a liquid metal top electrode, eutectic gallium–indium (EGaIn), allows reversible contact with the organic semiconductor film. Reversibility of contact permits subsequent characterization of the specific areas of the semiconductor film from which the photovoltaic parameters are obtained. Microstructural data from UV–vis experiments extracted using the weakly interacting H-aggregate model, along with atomic force microscopy, are correlated to the photovoltaic performance. The technique is used first on the model bulk heterojunction system comprising regioregular poly­(3-hexylthiophene) (P3HT) and the soluble fullerene derivative [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM). To demonstrate that the process can be used to optimize the thickness and annealing temperature using only small (≤10 mg) amounts of polymer, the technique was then applied to a bulk heterojunction blend comprising a difficult-to-obtain low-bandgap polymer. The combination of the use of gradients and a nondamaging top electrode allows for significant reduction in the amount of materials and time required to understand the effects of processing parameters and morphology on the performance of OSCs

Dulling of cutting tools.

<p>(a) SEM micrographs showing the progression of the dulling of a razor used in Method II, from out of box to 50 cycles. (b) The dulling of the razor plateaus at around 50 cycles.</p

Optical micrographs of junctions of copper and copper/nickel microwires.

<p>(a) A microwire junction fabricated by Method I. Surface roughness caused by the razor inadvertently abrading the substrate is apparent. (b) A microwire junction fabricated by patterned a PAA film with a polypropylene picnic knife, which was too soft to abrade the glass substrate, and thus the microwires appear to have a smoother topography. (c) A microwire junction patterned by direct abrasion of glass by a steel razor. Significant roughness generated by the razor is clearly visible.</p

Overview of the gesture-decoding glove.

<p>(a) Photograph of the glove. (b) Photograph detailing the breakout boards for the MCU, accelerometer, and Bluetooth on the PCB. (c) A circuit diagram of a voltage divider. (d) Schematic drawing of wireless gesture recognition system and the flow of information. Step 1: a gesture was made and the strain sensors transduced the positions of the knuckles into variable resistance values. Step 2: the variable values of resistance were converted into voltages by the voltage dividers. Step 3: the MCU measured the nine voltages and, through a binary comparison process, used them to generate a nine-bit key. Step 4: the binary key was used to determine which letter was to be transmitted wirelessly.</p

The Language of Glove: Wireless gesture decoder with low-power and stretchable hybrid electronics

<div><p>This communication describes a glove capable of wirelessly translating the American Sign Language (ASL) alphabet into text displayable on a computer or smartphone. The key components of the device are strain sensors comprising a piezoresistive composite of carbon particles embedded in a fluoroelastomer. These sensors are integrated with a wearable electronic module consisting of digitizers, a microcontroller, and a Bluetooth radio. Finite-element analysis predicts a peak strain on the sensors of 5% when the knuckles are fully bent. Fatigue studies suggest that the sensors successfully detect the articulation of the knuckles even when bent to their maximal degree 1,000 times. In concert with an accelerometer and pressure sensors, the glove is able to translate all 26 letters of the ASL alphabet. Lastly, data taken from the glove are used to control a virtual hand; this application suggests new ways in which stretchable and wearable electronics can enable humans to interface with virtual environments. Critically, this system was constructed of components costing less than $100 and did not require chemical synthesis or access to a cleanroom. It can thus be used as a test bed for materials scientists to evaluate the performance of new materials and flexible and stretchable hybrid electronics.</p></div