Mechanical Properties of Organic Electronic Polymers on the Nanoscale

Abstract

electronic polymers, have played a pivotal role in the development of flexible and printed electronics over the last two decades. [1][2][3] Composed from rings and chains of carbon atoms, these materials sport a low mass density, as well as electronic, optical, and mechanical properties that are tailored through the chemical design of their constituent molecular units. Weak inter-chain van der Waals bonding within thin films of stacked conjugated organic polymers renders them soft, with intrinsically low Young's moduli several orders of magnitude smaller than conventional inorganic semiconductors such as silicon. [4] These mechanical properties, coupled with a conjugated organic polymer's ability to transport both charges and ions through their matrix, have expanded their use in new research areas such as organic bioelectronics and neural recording. [5][6][7][8][9][10] Organic polymers such as poly(3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) are now a routine choice for flexible microelectrode array implants, [11] and their incorporation is known to improve electrical measurements through lower electrode impedances and higher signal-to-noise ratios. However, a recent nanoscale mechanical characterization of the PEDOT:PSS film elements used in such microelectrode arrays showed local variations over an individual probe, on a length scale of a couple of square micrometers. [12,13] This variation was Organic semiconducting polymers have attractive electronic, optical, and mechanical properties that make them materials of choice for large area flexible electronic devices. In these devices, the electronically active polymer components are micrometers in size, and sport negligible performance degradation upon bending the centimeter-scale flexible substrate onto which they are integrated. A closer look at the mechanical properties of the polymers, on the grain-scale and smaller, is not necessary in large area electronic applications. In emerging micromechanical and electromechanical applicationswhere the organic polymer elements are flexed on length scales spanning their own micron-sized active areas, it becomes important to characterize the uniformity of their mechanical properties on the nanoscale. In this work, the authors use two precision nanomechanical characterization techniques, namely, atomic force microscope based PeakForce quantitative nanomechanical mapping (PF-QNM) and nanoindentation-based dynamical mechanical analysis (nano-DMA), to compare the modulus and the viscoelastic properties of organic polymers used routinely in organic electronics. They quantitatively demonstrate that the semiconducting near-amorphous organic polymer indacenodithiopheneco-benzothiadiazole (C16-IDTBT) has a higher carrier mobility, lower modulus, and greater nanoscale modulus areal uniformity compared to the semiconducting semicrystalline organic polymer poly[2,5-bis(3-tetradecylthiophen-2-yl) thieno[3,2-b]thiophene] (C14-PBTTT). Modulus homogeneity appears intrinsic to C16-IDTBT but can be improved in C14-PBTTT u...

Dynamic self-stabilization in the electronic and nanomechanical properties of an organic polymer semiconductor

“…Over the scan of 5 m × 5 m as shown in Fig. 4(a) , the modulus appears highly homogeneous confirming our earlier report 63 . Figure 4(b) plots the nanoscale topography, stiffness, adhesion, and modulus of the stabilized C16-IDTBT film but on a length scale of a few hundred square nanometers.…”

“…4(e) . After a month, the films achieve a nominal value in the adhesion of ~27 nN, which compares well with measurements carried out on fully stabilized thin films after several weeks 63 . Clearly, at this stage the solvent content has decreased to such low levels that capillary condensates do not form and bridging of polymer chains no longer occurs due to decreased polymer chain mobility at the surface.…”

“…The modulus line scan remains uncorrelated with the topography which is an indication that the probed texture is not an artefact of the tip used in the measurements. In addition, the nanomechanical modulus probed using the sharper 10 nm tip shows significant features on the scale of 50 nm or less, whereas the same films probed with a 30 nm tip radius charted a far more uniform modulus 63 . As shown in Fig.…”

“…c Line scan of the modulus and topography along the dotted white lines shown in (b) measured using a tip radius of 10 nm. A scan of the modulus with a tip radius of 30 nm is shown in purple for comparison 63 . d Comparison of modulus histograms measured with two different tip radii; a 30 nm tip radius (larger than ordered film features) and a 10 nm tip radius (equivalent or less than ordered film features).…”

Improving OFF‐State Bias‐Stress Stability in High‐Mobility Conjugated Polymer Transistors with an Antisolvent Treatment

“…This modulus value is in good agreement with a previously measured value of 1.2 GPa for a time-stabilized IDT-BT sample. [43] In comparison, we find a bimodal distribution with softer and stiffer areas for the acetonitrile-treated films, as clearly observed in Figure 4f,i for 1 µm × 1 µm and 0.5 µm × 0.5 µm areas. By fitting the distribution to two Gaussians, we extracted average modulus values for the stiffer and softer areas of 1.6 ± 0.7 GPa and 0.63 ± 0.31 GPa for 1 µm × 1 µm scans, and 1.5 ± 0.6 GPa and 0.7 ± 0.2 GPa for 0.5 µm × 0.5 µm scans, respectively.…”