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High-mobility semiconducting polymers offer the opportunity to develop flexible and large-area electronics for several applications, including wearable, portable and distributed sensors, monitoring and actuating devices. An enabler of this technology is a scalable printing process achieving uniform electrical performances over large area. As opposed to the deposition of highly crystalline films, orientational alignment of polymer chains, albeit commonly achieved by non-scalable/slow bulk alignment schemes, is a more robust approach towards large-area electronics. By combining pre-aggregating solvents for formulating the semiconductor and by adopting a room temperature wired bar-coating technique, here we demonstrate the fast deposition of submonolayers and nanostructured films of a model electron-transporting polymer. Our approach enables directional self-assembling of polymer chains exhibiting large transport anisotropy and a mobility up to 6.4 cm 2  V −1  s −1 , allowing very simple device architectures to operate at 3.3 MHz. Thus, the proposed deposition strategy is exceptionally promising for mass manufacturing of high-performance polymer circuits. Semiconducting polymers with high mobility are essential for the development of high-frequency flexible electronics, whose fabrications rely on robust printing techniques. Bucella et al . report the fabrication of n -type polymer field-effect transistors, with mobility up to 6.4 cm 2  V −1 s operated at 3.3 MHz, by room temperature bar-coating technique.

Self-assembly of monolayers of functional molecules on dielectric surfaces is a promising approach for the development of molecular devices proposed in the 1970s. Substrate chemically bonded self-assembled monolayers of semiconducting conjugated molecules exhibit low mobility. And self-assembled monolayer molecular crystals are difficult to scale up and limited to growth on substrates terminated by hydroxyl groups, which makes it difficult to realize sophisticated device functions, particularly for those relying on n-type electron transport, as electrons suffer severe charge trapping on hydroxyl terminated surfaces. Here we report a gravity-assisted, two-dimensional spatial confinement method for bottom-up growth of high-quality n-type single-crystalline monolayers over large, centimeter-sized areas. We demonstrate that by this method, n-type monolayer molecular crystals with high field-effect mobility of 1.24 cm 2  V −1  s −1 and band-like transport characteristics can be grown on hydroxyl-free polymer surface. Furthermore, we used these monolayer molecular crystals to realize high-performance crystalline, gate-/light-tunable lateral organic p–n diodes. New methods for obtaining large-area monolayer molecular crystals (MMCs) on hydrophobic surfaces are needed to realize the full potential of MMCs for organic electronics. Here, the authors demonstrate bottom-up growth of high-grade n-type MMCs, which show superior performance in device applications.

Polymer chains are attached to nanoparticle surfaces for many purposes, including altering solubility, influencing aggregation, dispersion, and even tailoring immune responses in drug delivery. The most unique structural motif of polymer-grafted nanoparticles (PGNs) is the high-density region in the corona where polymer chains are stretched under significant confinement, but orientation of these chains has never been measured because conventional nanoscale-resolved measurements lack sensitivity to polymer orientation in amorphous regions. Here, we directly measure local chain orientation in polystyrene grafted gold nanoparticles using polarized resonant soft X-ray scattering (P-RSoXS). Using a computational scattering pattern simulation approach, we measure the thickness of the anisotropic region of the corona and extent of chain orientation within it. These results demonstrate the power of P-RSoXS to discover and quantify orientational aspects of structure in amorphous soft materials and provide a framework for applying this emerging technique to more complex, chemically heterogeneous systems in the future. The orientation of polymer chains in the corona of polymer-grafted nanoparticles has never been measured. Here, the authors use polarized resonant soft X-ray scattering to measure local chain orientation in polystyrene grafted gold nanoparticles and quantify the thickness of the anisotropic region of the corona as well as the extent of chain orientation within it.

Efficient injection of charge carriers from the contacts into the semiconductor layer is crucial for achieving high-performance organic devices. The potential drop necessary to accomplish this process yields a resistance associated with the contacts, namely the contact resistance. A large contact resistance can limit the operation of devices and even lead to inaccuracies in the extraction of the device parameters. Here, we demonstrate a simple and efficient strategy for reducing the contact resistance in organic thin-film transistors by more than an order of magnitude by creating high work function domains at the surface of the injecting electrodes to promote channels of enhanced injection. We find that the method is effective for both organic small molecule and polymer semiconductors, where we achieved a contact resistance as low as 200 Ωcm and device charge carrier mobilities as high as 20 cm 2 V −1 s −1 , independent of the applied gate voltage. Minimizing contact effects in organic semiconductor-based devices is a key step toward the development of a low-cost technology for next-generation electronics. Here, the authors reduce contact resistance in organic devices by engineering electrodes with high work function surface domains.

We show that glasses with aligned smectic liquid crystal-like order can be produced by physical vapor deposition of a molecule without any equilibrium liquid crystal phases. Smectic-like order in vapor-deposited films was characterized by wide-angle X-ray scattering. A surface equilibration mechanism predicts the highly smectic-like vapor-deposited structure to be a result of significant vertical anchoring at the surface of the equilibrium liquid, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy orientation analysis confirms this prediction. Understanding of the mechanism enables informed engineering of different levels of smectic order in vapor-deposited glasses to suit various applications. The preparation of a glass with orientational and translational order from a nonliquid crystal opens up an exciting paradigm for accessing extreme anisotropy in glassy solids.

Recent demonstrations of inverted thermal activation of charge mobility in polymer field-effect transistors have excited the interest in transport regimes not limited by thermal barriers. However, rationalization of the limiting factors to access such regimes is still lacking. An improved understanding in this area is critical for development of new materials, establishing processing guidelines, and broadening of the range of applications. Here we show that precise processing of a diketopyrrolopyrrole-tetrafluorobenzene-based electron transporting copolymer results in single crystal-like and voltage-independent mobility with vanishing activation energy above 280 K. Key factors are uniaxial chain alignment and thermal annealing at temperatures within the melting endotherm of films. Experimental and computational evidences converge toward a picture of electrons being delocalized within crystalline domains of increased size. Residual energy barriers introduced by disordered regions are bypassed in the direction of molecular alignment by a more efficient interconnection of the ordered domains following the annealing process. Though solution-processed conjugated polymers with inverted temperature activated transport have been reported, the origin of this behaviour is unclear. Here, the authors realize temperature-independent electron transport above 280 K in a donor-acceptor copolymer through microstructural engineering.

In bulk heterojunction organic photovoltaics, electron-donating and electron-accepting materials form a distributed network of heterointerfaces in the photoactive layer, where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces due to their complex three-dimensional arrangement and the lack of techniques to measure local order. Here, we report that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, bulk heterojunction solar cells. Using resonant soft X-ray scattering, we characterize the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on (−1) orientations relative to these heterointerfaces. By manipulating the degree of molecular orientation through the choice of molecular chemistry and the characteristics of the processing solvent, we are able to show the importance of this structural parameter on the performance of bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT–DTBT, PBnDT–DTBT or PBnDT–TAZ. X-ray scattering experiments indicate that the molecular orientation at the interfaces of bulk heterojunction organic solar cells influences the cells’ fill factor and short-circuit current.

We discuss the construction of a new SAXS/WAXS beamline at the Advanced Light Source at Lawrence Berkeley Laboratory. The beamline is equipped with a multilayer monochromator in order to obtain a high X-ray flux. The detrimental effects that the increased bandwidth transmitted by this monochromator could have on the data quality of the SAXS and WAXS patterns is shown to be negligible for the experimental program intended to be operated on this beamline.

This dissertation details the establishment and expansion of resonant soft X-ray scattering techniques to reveal the internal structure of organic thin films. These films are increasingly important in numerous electronic systems, including organic thin film transistors, organic photovoltaics, and organic light emitting diodes. These devices use the electrical properties of polymers to respectively turn on and off conduction, turn light into electricity, and create light. The performance of each of these systems depends critically on their physical structure but unfortunately, traditional techniques fail to adequately characterize that structure. This dissertation will explore the use of soft X-ray scattering to reveal the mesoscale structure or organic electronic devices. This begins with an overview of the field to make the case for soft X-rays being an appropriate and novel tool. Next, to explain how to collect accurate soft X-ray scattering, the development of a new and unique soft X-ray scattering facility will be presented. Having the tools, the next step is to develop scattering theories and models for understanding and correctly analyzing scattering from these complicated devices. This includes development and comparison of analysis techniques and theory to simulate scattering. This simulation system is then used in the development of a theory to understand the novel phenomenon of anisotropic X-ray scattering from isotropic organic samples. Finally, I will describe the development and first use of a method able to simultaneously measure size scales and chemical structure with depth sensitivity in thin films: Grazing Resonant Soft X-ray Scattering. This work provides valuable understanding and tools to the field of materials characterization, opening up new opportunities for principled design of organic electronics.

The depth dependence of crystalline structure within thin films is critical for many technological applications, but has been impossible to measure directly using common techniques. In this work, by monitoring diffraction peak intensity and location and utilizing the highly angle-dependent waveguiding effects of X-rays near grazing incidence we quantitatively measure the thickness, roughness and orientation of stratified crystalline layers within thin films of a high-performance semiconducting polymer. In particular, this diffractive X-ray waveguiding reveals a self-organized 5-nm-thick crystalline surface layer with crystalline orientation orthogonal to the underlying 65-nm-thick layer. While demonstrated for an organic semiconductor film, this approach is applicable to any thin film material system where stratified crystalline structure and orientation can influence important interfacial processes such as charge injection and field-effect transport.