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Abstract: Organic molecular crystals encompass a vast range of materials from pharmaceuticals to organic optoelectronics, proteins and waxes in biological and industrial settings. Crystal defects from grain boundaries to dislocations are known to play key roles in mechanisms of growth1,2 and in the functional properties of molecular crystals3,4,5. In contrast to the precise analysis of individual defects in metals, ceramics and inorganic semiconductors enabled by electron microscopy, substantially greater ambiguity remains in the experimental determination of individual dislocation character and slip systems in molecular materials3. In large part, nanoscale dislocation analysis in molecular crystals has been hindered by the low electron doses required to avoid irreversibly degrading these crystals6. Here we present a low-dose, single-exposure approach enabling nanometre-resolved analysis of individual dislocations in molecular crystals. We demonstrate the approach for a range of crystal types to reveal dislocation character and operative slip systems unambiguously.

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Abstract: Understanding efficiency–durability relationships and related mitigation strategies is an important step toward the commercialization of organic photovoltaics (OPVs). Here, we report that a photoactivated 6-bridged azide cross-linker (6Bx) improves the morphological stability by suppressing the thermally activated diffusion of (Y6) acceptor molecules in PM6:Y6 bulk-heterojunction (BHJ)-based OPVs. Cross-linked PM6:Y6 (0.05 wt % 6Bx) BHJ OPVs retain 93.4% of the initial power conversion efficiency upon thermal aging at 85 °C for 1680 h (T80 = 3290 h). Molecular origins of enhanced thermal stability are corroborated by optical spectroscopy, surface imaging, 2D solid-state nuclear magnetic resonance (ssNMR), Raman spectroscopy, scanning electron diffraction (SED) measurements, and analysis of the BHJ thin films. The facile single-step cross-linking strategy in conjugation with advanced characterization methods presented in the study paves the way toward developing durable OPVs based on non-fullerene acceptors (NFAs).

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Abstract: The functional group-directed structures of coordination polymers (CPs) and metal–organic frameworks (MOFs) have made them key candidates for proton exchange membranes in fuel cell technologies. Sulfonate group chemistry is well established in proton conducting polymers but has seen less exploration in CPs. Here, we report solvent-directed crystal structures of Cu2+ and Ca2+ CPs constructed with naphthalenedisulfonate (NDS) and anthraquinone-1,5-disulfonate (ADS) ligands, and we correlate single crystal structures across this set with proton conductivities determined by electrochemical impedance spectroscopy. Starting from the Cu2+-based NDS and aminotriazolate MOF designated Cu-SAT and the aqueous synthesis of the known Ca2+-NDS structure incorporating water ligands, we now report a further five sulfonate CP structures. These syntheses include a direct synthesis of the primary degradation product of Cu-SAT in water, solvent-substituted Ca-NDS structures prepared using dimethylformamide and dimethylsulfoxide solvents, and ADS variants of Cu-SAT and Ca-NDS. We demonstrate a consistent 2D layer motif in the NDS CPs, while structural modifications introduced by the ADS ligand result in a 2D hydrogen bonding network with Cu2+ and aminotriazolate ligands and a 1D CP with Ca2+ in water. Proton conductivities across the set span 10−4 to >10−3 S cm−1 at 80 °C and 95% RH. These findings reveal an experimental structure–function relationship between proton conductivity and the tortuosity of the hydrogen bonding network and establish a general, cross-structure descriptor for tuning the sulfonate CP unit cell to systematically modulate proton conductivity.

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Abstract: Recently, increased attention has been focused on amorphous metal-organic frameworks (MOFs) and, more specifically, MOF glasses, the first new glass category discovered since the 1970s. In this work, we explore the fabrication of a compositional series of hybrid blends, the first example of blending a MOF and inorganic glass. We combine ZIF-62(Zn) glass and an inorganic glass, 30Na2O-70P2O5, in an effort to combine the chemical versatility of the MOF glass with the mechanical properties of the inorganic glass. We investigate the interfacial interactions between the two components using pair distribution function analysis and solid state NMR spectroscopy, and suggest potential interactions between the two phases. Thermal analysis of the blend samples indicated that they were less thermally stable than the starting materials, and had a Tg shifted relative to the pristine materials. Annular dark field scanning transmission electron microscopy tomography, X-ray energy dispersive spectroscopy (EDS), nanoindentation and 31P NMR all indicated close mixing of the two phases, suggesting that immiscible blends had formed.

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Abstract: Disorder in organic semiconductors (OSCs) plays a determining role in energy transport properties underpinning optoelectronic device performance [1]. Both energetic disorder native to perfect crystals as well as deviations from crystalline order control transport properties [2,3]. Alongside crystallographic defects like stacking faults and grain boundaries, dislocations that distort molecular packing can introduce exciton- or charge-carrier traps that significantly hamper intermolecular energy transport [4]. Electron microscopy has been a mainstay for probing these crystallographic defects in inorganic semiconductors. Recent progress in atomically resolved electron microscopy has enabled imaging of individual defects in hybrid perovskites [5]. But while hybrid perovskites show structural degradation on the order of <200 e–Å-2 before [6], small molecule OSCs may undergo comparable loss of structure under exposures <30 e–Å-2 [7,8]. Methods that enable the crystallographic analysis of dislocations in beam-sensitive OSCs are therefore a necessary first step to establish their performance effects. A dislocation is described crystallographically in terms of a displacement vector in the lattice, termed a Burgers vector b. Most attempts to characterize dislocations in organic molecular crystals have relied on techniques at low spatial resolution, including etch pit imaging [9] and scanning probe techniques [10]. These approaches are unable to directly record the crystallography of dislocations or access the nanometre spatial resolution required to isolate individual defects. In contrast, electron microscopy combines the necessary spatial resolution to image the dislocation line as well as the crystallographic detail from electron diffraction to retrieve the Burgers vector. Typically, such an analysis is carried out by many repeated electron beam exposures and sample rotations aimed at identifying the crystal planes associated with a diffraction vector g that are not distorted by the dislocation, a so-called ‘invisibility criterion’ at g.b = 0. Here, we advance this approach for OSCs to carry out unambiguous analysis of the dislocation Burgers vector and type (screw, edge, or mixed) using four-dimensional scanning transmission electron microscopy (4D-STEM) now in a single exposure at a fluence of ~10 e–Å-2. Thin films of p-terphenyl and anthracene were prepared by solution crystallization as a set of benchmark organic optoelectronic materials [11]. On transfer to a lacey carbon support film for electron microscopy, the draping of the OSC crystals on the support film introduces a small amount of sample bending. This bending defines a set of diffraction conditions that produce ribbons of bright intensity running across images of the film referred to as bend contours. These bend contours exhibit an abrupt shift or break on crossing dislocations unless they satisfy the invisibility criterion. Our 4D-STEM approach specifically supports simultaneous analysis of many lattice planes approximately parallel to the crystal direction perpendicular to the film, i.e. [001] for p-terphenyl and [101] for anthracene. Plotting and fitting the magnitude of breaks in the bend contours as a function of the corresponding diffraction vectors (g) for each plane determines the Burgers vector. For instance, this analysis establishes mixed-type b = [010] dislocations in p-terphenyl and anthracene. These generalizable methods make an analysis of the Burgers vectors of dislocations in beam-sensitive OSC films a routine process. The capability to measure the character and type of dislocations will provide experimental input for models of distortions at these structural defects and enables assessing methods for inhibiting dislocation formation during crystal growth and reducing or removing their deleterious contributions to device performance.