Evan Welchman

The Influence of Isomer Purity on Trap States and Performance of Organic Thin‐Film Transistors

Organic field-effect transistor (OFET) performance is dictated by its composition and geometry, as well as the quality of the organic semiconductor (OSC) film, which strongly depends on purity and microstructure. When present, impurities and defects give rise to trap states in the bandgap of the OSC, lowering device performance. Here, 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene is used as a model system to study the mechanism responsible for performance degradation in OFETs due to isomer coexistence. The density of trapping states is evaluated through temperature dependent current-voltage measurements, and it is discovered that OFETs containing a mixture of syn- and anti-isomers exhibit a discrete trapping state detected as a peak located at ~ 0.4 eV above the valence-band edge, which is absent in the samples fabricated on single-isomer films. Ultraviolet photoelectron spectroscopy measurements and density functional theory calculations do not point to a significant difference in electronic band structure between individual isomers. Instead, it is proposed that the dipole moment of the syn-isomer present in the host crystal of the anti-isomer locally polarizes the neighboring molecules, inducing energetic disorder. The isomers can be separated by applying gentle mechanical vibrations during film crystallization, as confirmed by the suppression of the peak and improvement in device performance.

Decomposition mechanisms in metal borohydrides and their ammoniates

Ammoniation in metal borohydrides (MBs) with the form (BH4)x has been shown to lower their decomposition temperatures with of low electronegativity (χp ≲ 1.6), but raise it for high-χp MBs (χp ≳ 1.6). Although this behavior is just as desired, an understanding of the mechanisms that cause it is still lacking. Using ab initio methods, we elucidate those mechanisms and find that ammoniation always causes thermodynamic destabilization, explaining the observed lower decomposition temperatures for low-χp MBs. For high-χp MBs, we find that ammoniation blocks B2H6 formation—the preferred decomposition mechanism in these MBs—and thus kinetically stabilizes those phases. The shift in decomposition pathway that causes the distinct change from destabilization to stabilization around χp = 1.6 thus coincides with the onset of B2H6 formation in MBs. Furthermore, with our analysis we are also able to explain why these materials release either H2 or NH3 gas upon decomposition. We find that NH3 is much more strongly coordinated with higher-χp metals and direct H2 formation/release becomes more favorable in these materials. Our findings are of importance for unraveling the hydrogen release mechanisms in an important new and promising class of hydrogen storage materials, allowing for a guided tuning of their chemistry to further improve their properties.

Positional disorder in ammonia borane at ambient conditions

This papers resolves a long standing experimental paradox in one of the most promising hydrogen storage materials. The authors use an innovative simulation technique to show that rapid molecular rotations explain how a molecule with a three-fold rotational axis crystallizes at room temperature into a form with a four-fold symmetry about the same axis.

H4-alkanes: A new class of hydrogen storage material?

Abstract The methane-based material (H 2 ) 4 CH 4 , also called H4M for short, is in essence a methane molecule with 4 physisorbed H 2 molecules. While H4M has exceptionally high hydrogen storage densities when it forms a molecular solid, unfortunately, this solid is only stable at impractically high pressures and/or low temperatures. To overcome this limitation, we show through simulations that longer alkanes (methane is the shortest alkane) also form stable structures that still physisorb 4 H 2 molecules per carbon atom; we call those structures H4-alkanes. We further show via molecular dynamics simulations that the stability field of molecular solids formed from H4-alkanes increases remarkably with chain length compared to H4M, just as it does for regular alkanes. From our simulations of H4-alkanes with lengths 1, 4, 10, and 20, we see that e.g. for the 20-carbon the stability field is doubled at higher pressures. While even longer chains show only insignificant improvements, we discuss various other options to stabilize H4-alkanes more. Our proof-of-principle results lay the groundwork to show that H4-alkanes can become viable hydrogen storage materials.

The structure and unconventional dihydrogen bonding of a pressure-stabilized hydrogen-rich (NH3BH3)(H2)x (x = 1.5) compound

Combining X-ray diffraction, Raman spectroscopy, and ab initio simulations we characterize an extremely hydrogen-rich phase with the chemical formula (NH3BH3)(H2)x (x = 1.5). This phase was formed by compressing ammonia borane (AB, NH3BH3) in an environment with an excess of molecular hydrogen (H2). This compound can store a total of 26.8 wt% hydrogen, both as molecular hydrogen and chemically bonded hydrogen in AB, making it one of the most hydrogen-rich solids currently known. The new compound possesses a layered AB structure where additional H2 molecules reside in channels created through the weaving of AB layers. The unconventional dihydrogen bonding network of the new compound is significantly modified from its parent AB phase and contains H⋯H contacts between adjacent AB molecules and between AB and H2 molecules. H–H can be either a proton donor or a proton acceptor that forms new types of dihydrogen bonding with the host AB molecules, which are depicted as H–H⋯H–B or H–H⋯H–N, respectively. This study not only demonstrates the strategy and the promise of using pressure for new material synthesis, but also unleashes the power of combining experiments and ab initio calculations for elucidating novel structures and unusual bonding configurations in dense low-Z materials.