Polycrystalline organic semiconductor films play a central role in organic electronics because their inherent order, relative to amorphous films, facilitates more efficient charge transport. Carrier mobilities in crystalline organic semiconductors are generally at least a factor of one hundred greater than in their amorphous counterparts, which is attractive for certain device applications, such as organic field effect transistors (OFETs), where higher charge mobilities result in better performance. [1][2][3][4][5] In analogy with conventional semiconductors (e.g., poly-Si), the electrical performance of polycrystalline organic semiconductor layers is sensitive to grain morphology and alignment, as well as to defects. [6][7][8][9][10][11] Indeed, recognition of the importance of microstructure has lead to extensive structural characterization of organic semiconductor films by X-ray diffraction, [12,13] and optical, [14] electron, [15][16][17][18] and scanning probe microscopy. [19,20] Yet there are still many aspects of organic semiconductor microstructure that are not well understood and detailed correlations with transport are rare. One surprising bottleneck to understanding microstructureproperty relationships has been the difficulty of producing clear images of grains in extremely thin, coalesced layers of organic semiconductors on technologically relevant substrates, such as gate dielectrics, which are critical components of OFETs.Here, we demonstrate that a novel scanning probe microscopy method, which we term Transverse Shear Microscopy (TSM), produces striking, high contrast images of grain size, shape, and orientation in films of polycrystalline organic materials. The ability to image grain orientation is a key feature of TSM and the resulting Grain Orientation Maps substantially enhance the possibilities for quantitative analysis of microstructure. For the ultrathin (1-2 nm) organic films we describe here, the grain orientation and shape recorded in the TSM images are difficult to visualize by any other microscopy method. Furthermore, by combining shear deformation experiments with theoretical analysis, we show that the mechanism of TSM orientation contrast originates from the intrinsic elastic anisotropy within individual grains. Thus, TSM has intriguing potential as a broadly applicable method for quantitative microstructure analysis, not only for organic semiconductors, but also for any suitably soft, crystalline material with a tensor modulus in the image plane. Our results substantially expand on an earlier report of TSM imaging, [19] in which we demonstrated orientation dependent contrast but did not analyze the film microstructure nor identify the imaging mechanism. In TSM, depicted in Figure 1A, the scanning direction of a force microscope probe tip is parallel to the cantilever axis, and the lateral deflection or twist of the cantilever is recorded. This mode of operation differs from the better-known lateral force microscopy (LFM) technique in one respect only, namely that in LFM the scanning ...
Semicrystalline poly(3-hexyl-2,5-thienylene vinylene) (P3HTV) with a low band gap of 1.65 eV has been synthesized by acyclic diene metathesis polymerization and incorporated into bulk heterojunction (BHJ) organic solar cells. The polymer was thermally characterized by differential scanning calorimetry and thermogravimetric analysis and was blended with the electron acceptor methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) to make a light-harvesting charge-transfer thin film. The properties of P3HTV/PCBM blends were studied as a function of PCBM composition by wide-angle X-ray scattering, atomic force microscopy, transmission electron microscopy, UV−vis absorption spectroscopy, and charge-transport and photovoltaic measurements. The PCBM solubility limit, that is, the phase separation point, was estimated to be 50 wt % PCBM. The phase behavior of the blend was directly correlated with electrical transport behavior in a field-effect transistor testbed. At the phase separation point, charge carrier transport switches from hole only to ambipolar (both electron and hole) due to the formation of an electron-transporting percolating network of PCBM domains. BHJ solar cells were constructed with P3HTV films blended with varying weight fractions of PCBM. In these cells, spun-cast films of P3HTV/PCBM mixtures were sandwiched between poly(3,4-ethylene dioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS)-coated ITO and Al electrodes. The best performance of polymer solar cells was observed at 50−60% PCBM, near the phase separation point at which power conversion efficiencies of 0.80−0.92% were measured under AM 1.5, 100 mW/cm2 illumination.
The authors report the fabrication and characterization of tetracene single-crystal field-effect transistors (FETs) utilizing an air or vacuum gap as the gate dielectric. The linear mobility of the device can be as high as 1.6cm2∕Vs in air, with a subthreshold slope lower than 0.5VnF∕decadecm2. By changing the orientation of the same crystal on the air-gap substrate, surface charge transport along different crystallographic directions was measured. There is pronounced anisotropy in the mobility; temperature dependent measurements show the mobility is activated (in contrast to air-gap FETs based on rubrene) and that the activation energy is independent of transport direction. Gate electrode displacement current was also recorded for these devices, allowing accurate determination of the gate induced surface charge and the fraction of trapped charge.
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