Using 4D STEM to Probe Mesoscale Order in Molecular Glass Films Prepared by Physical Vapor Deposition.

Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor-deposited and liquid-cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface-diffusion-mediated ordering during film deposition. These results demonstrate the ability of low-dose 4D STEM to characterize a mesoscale structure in a molecular glass system which may be relevant to organic electronics.

Ultrathin and Ultrasensitive Direct X-ray Detector Based on Heterojunction Phototransistors.

Most contemporary X-ray detectors adopt device structures with non/low-gain energy conversion, such that a fairly thick X-ray photoconductor or scintillator is required to generate sufficient X-ray-induced charges, and thus numerous merits for thin devices, such as mechanical flexibility and high spatial resolution, have to be compromised. This dilemma is overcome by adopting a new high-gain device concept of a heterojunction X-ray phototransistor. In contrast to conventional detectors, X-ray phototransistors allow both electrical gating and photodoping for effective carrier-density modulation, leading to high photoconductive gain and low noise. As a result, ultrahigh sensitivities of over 105  µC Gyair -1  cm-2 with low detection limit are achieved by just using an ≈50 nm thin photoconductor. The employment of ultrathin photoconductors also endows the detectors with superior flexibility and high imaging resolution. This concept offers great promise in realizing well-balanced detection performance, mechanical flexibility, integration, and cost for next-generation X-ray detectors.

Role of Postdeposition Thermal Annealing on Intracrystallite and Intercrystallite Structuring and Charge Transport in Poly(3-hexylthiophene).

The performance of electronic devices comprising conjugated polymers as the active layer depends not only on the intrinsic characteristics of the materials but also on the details of the extrinsic processing conditions. In this study, we examine the effect of postdeposition thermal treatments on the microstructure of poly(3-hexylthiophene) (P3HT) thin films and its impact on their electrical properties. Unsurprisingly, we find thermal annealing of P3HT thin films to generally increase their crystallinity and crystallite coherence length while retaining the same crystal structure. Despite such favorable structural improvements of the polymer active layers, however, thermal annealing at high temperatures can lead to a net reduction in the mobility of transistors, implicating structural changes in the intercrystallite amorphous regions of these semicrystalline active layers take place on annealing, and the simplistic picture that crystallinity governs charge transport is not always valid. Our results instead suggest tie-chain pullout, which occurs during crystal growth and perfection upon thermal annealing to govern charge transport, particularly in low-molecular-weight systems in which the tie-chain fraction is low. By demonstrating the interplay between intracrystallite and intercrystallite structuring in determining the macroscopic charge transport, we shed light on how structural evolution and charge-transport properties of nominally the same polymer can vary depending on the details of processing.

Tunable Properties of MAPLE-Deposited Thin Films in the Presence of Suppressed Segmental Dynamics.

Processing polymer thin films by physical vapor deposition has been a major challenge due to material degradation. This challenge has limited our understanding of morphological control by top-down approaches that can be crucial for many applications. Recently, matrix-assisted pulsed laser evaporation (MAPLE) has emerged as an alternative route to fabricate polymer thin films from near-gas phase growth conditions. In this Letter, we investigate how this approach can result in a stable two-phase film structure of semicrystalline polymers via a unique combination of MAPLE and flash calorimetry. In the case of MAPLE-deposited poly(ethylene oxide) (PEO) thin films, we find a 35 °C enhancement in the glass transition temperature relative to melt-crystallized films, which is associated with irreversible chain adsorption in the amorphous region of the film. Remarkably, by varying substrate temperature during deposition, we reveal the ability to significantly tune the crystal orientation, extent of crystallinity, and lamellar thickness of MAPLE-deposited PEO thin films.

Computationally aided design of a high-performance organic semiconductor: the development of a universal crystal engineering core.

Herein, we describe the design and synthesis of a suite of molecules based on a benzodithiophene "universal crystal engineering core". After computationally screening derivatives, a trialkylsilylethyne-based crystal engineering strategy was employed to tailor the crystal packing for use as the active material in an organic field-effect transistor. Electronic structure calculations were undertaken to reveal derivatives that exhibit exceptional potential for high-efficiency hole transport. The promising theoretical properties are reflected in the preliminary device results, with the computationally optimized material showing simple solution processing, enhanced stability, and a maximum hole mobility of 1.6 cm2 V-1 s-1.

Assessing the Huang-Brown Description of Tie Chains for Charge Transport in Conjugated Polymers.

Intercrystallite molecular connections are widely recognized to tremendously impact the macroscopic properties of semicrystalline polymers. Because it is challenging to directly probe such connections, theoretical frameworks have been developed to quantify their concentrations and predict the mechanical properties that result from these connections. Tie-chain connectivity similarly impacts the electrical properties in semicrystalline conjugated polymers. Yet, its quantitative impact has eluded the community. Here, we assess the Huang-Brown model, a framework commonly used to describe the structural origins of mechanical properties in polyolefins, to quantitatively elucidate the effect of tie chains on the electrical properties of a model conjugated polymer. We found that a critical tie-chain fraction of 10-3 is needed to support macroscopic charge transport, below which intercrystallite connectivity limits charge transport, and above which intracrystallite disorder is the bottleneck. Extending the Huang-Brown framework to conjugated polymers enables the prediction of macroscopic electrical properties based on experimentally accessible morphological parameters. Our study implicates the importance of long and rigid polymer chains for efficient charge transport over device length scales.