Understanding Fragmentation of Organic Small Molecules in Atom Probe Tomography

In atom probe tomography of molecular organic materials, field ionization of either entire molecules or molecular fragments can occur, but the mechanism governing this behavior was not previously understood. This work explains when a doubly ionized small molecule organic material is expected to undergo fragmentation. We find that multiple detection events arising from post-ionization fragmentation of a parent molecular dication into two daughter ions is well explained by the free energy and geometries of the molecules computed using density functional theory. Of the systems studied, exergonic free energies for formation of the daughter ions, smaller activation energies for dissociation, and increases in bond length are all found to be quantitative predictors for ion fragmentation. This work expands the applicability of atom probe tomography to organic materials by increasing the fundamental understanding of processes occurring during this analysis technique

Effect of Diels–Alder Reaction in C₆₀-Tetracene Photovoltaic Devices

Developing organic photovoltaic materials systems requires a detailed understanding of the heterojunction interface, as it is the foundation for photovoltaic device performance. The bilayer fullerene/acene system is one of the most studied models for testing our understanding of this interface. We demonstrate that the fullerene and acene molecules chemically react at the heterojunction interface, creating a partial monolayer of a Diels–Alder cycloadduct species. Furthermore, we show that the reaction occurs during standard deposition conditions and that thermal annealing increases the concentration of the cycloadduct. The cycloaddition reaction reduces the number of sites available at the interface for charge transfer exciton recombination and decreases the charge transfer state reorganization energy, increasing the open circuit voltage. The submonolayer quantity of the cycloadduct renders it difficult to identify with conventional characterization techniques; we use atom probe tomography to overcome this limitation while also measuring the spatial distribution of each chemical species

Broad Spectral Response Using Carbon Nanotube/Organic Semiconductor/C₆₀ Photodetectors

We demonstrate that photogenerated excitons in semiconducting carbon nanotubes (CNTs) can be efficiently dissociated by forming a planar heterojunction between CNTs wrapped in semiconducting polymers and the electon acceptor, C60. Illumination of the CNTs at their near-infrared optical band gap results in the generation of a short-circuit photocurrent with peak external and internal quantum efficiencies of 2.3% and 44%, respectively. Using soft CNT-hybrid materials systems combining semiconducting small molecules and polymers, we have fabricated broad-band photodetectors with a specific detectivity >1010 cm Hz1/2 W1− from λ = 400 to 1450 nm and a response time of τ = 7.2 ± 0.2 ns

Porphyrins Fused with Unactivated Polycyclic Aromatic Hydrocarbons

A systematic study of the preparation of porphyrins with extended conjugation by <i>meso</i>,β-fusion with polycyclic aromatic hydrocarbons (PAHs) is reported. The <i>meso</i>-positions of 5,15-unsubstituted porphyrins were readily functionalized with PAHs. Ring fusion using standard Scholl reaction conditions (FeCl₃, dichloromethane) occurs for perylene-substituted porphyrins to give a porphyrin β,<i>meso</i> annulated with perylene rings (0.7:1 ratio of <i>syn</i> and <i>anti</i> isomers). The naphthalene, pyrene, and coronene derivatives do not react under Scholl conditions but are fused using thermal cyclodehydrogenation at high temperatures, giving mixtures of <i>syn</i> and <i>anti</i> isomers of the <i>meso</i>,β-fused porphyrins. For pyrenyl-substituted porphyrins, a thermal method gives synthetically acceptable yields (>30%). Absorption spectra of the fused porphyrins undergo a progressive bathochromic shift in a series of naphthyl (λ_max = 730 nm), coronenyl (λ_max = 780 nm), pyrenyl (λ_max = 815 nm), and perylenyl (λ_max = 900 nm) annulated porphyrins. Despite being conjugated with unsubstituted fused PAHs, the β,<i>meso</i>-fused porphyrins are more soluble and processable than the parent nonfused precursors. Pyrenyl-fused porphyrins exhibit strong fluorescence in the near-infrared (NIR) spectral region, with a progressive improvement in luminescent efficiency (up to 13% with λ_max = 829 nm) with increasing degree of fusion. Fused pyrenyl-porphyrins have been used as broadband absorption donor materials in photovoltaic cells, leading to devices that show comparatively high photovoltaic efficiencies

Coalescence of GaP on V‑Groove Si Substrates

Here, we study the morphology and dislocation dynamics of metalorganic vapor phase epitaxy (MOVPE)-grown GaP on a V-groove Si substrate. We show that Si from the substrate stabilizes the (0 0 1) GaP facet, which is critical for achieving coalescence. The SiNx caps covering the (0 0 1) tops of the V-grooves must be sufficiently small for the 3 × 1 GaP surface reconstruction caused by Si to continue to influence the GaP coalescence while the V-grooved sidewalls are covered. If the SiNx caps are too large, (1 1 1) diamond faceting develops in the GaP, and coalescence does not occur. On samples where coalescence is successful, we measure a root-mean-square roughness of 0.2 nm and a threading dislocation density of 5 × 107 cm–2. Dislocation glide was found to begin during coalescence through transmission electron microscopy. With further TDD reduction, these GaP on V-groove templates will be suitable for III-V optoelectronic device growth

Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics

It has been proposed that interface morphology affects the recombination rate for electrons and holes at donor–acceptor heterojunctions in thin film organic photovoltaic cells. The optimal morphology is one where there is disorder at the heterointerface and order in the bulk of the thin films, maximizing both the short circuit current and open circuit voltage. We show that an amorphous, buried functionalized molecular squaraine donor layer can undergo an “inverted” quasi-epitaxial growth during postdeposition processing, whereby crystallization is seeded by a subsequently deposited self-assembled nanocrystalline acceptor C₆₀ cap layer. We call this apparently unprecedented growth process from a buried interface “inverse quasi-epitaxy” where the crystallites of these “soft” van der Waals bonded materials are only approximately aligned to those of the cap. The resulting crystalline interface hastens charge recombination, thereby reducing the open circuit voltage in an organic photovoltaic cell. The lattice registration also facilitates interdiffusion of the squaraine donor and C₆₀ acceptor, which dramatically improves the short circuit current. By controlling the extent to which this crystallization occurs, the voltage losses can be minimized, resulting in power conversion efficiencies of η_P = 5.4 ± 0.3% for single-junction and η_P = 8.3 ± 0.4% for tandem small-molecule photovoltaics. This is a general phenomenon with implications for all organic donor–acceptor junctions. That is, epitaxial relationships typically result in a reduction in open circuit voltage that must be avoided in both bilayer and bulk heterojunction organic photovoltaic cells