Analytical electron microscopy reveals local molecular arrangements of PBDB-T:ITIC determining performance of current organic solar cells.

Dedicated to Professor Franz Effenberger on the occasion of his 90th birthday 1. Introduction Covalently linked donor-acceptor (D-A) dyads, triads, and multiads have been extensively investigated in the context of photoinduced energy and charge transfer (CT) processes, which are fundamental for natural photosynthesis, but as well important for the function of organic solar cells. [1,2] The nature and length of the molecular spacer bridging D and A, flexible nonconjugated or rod-like conjugated, polar or nonpolar, play an important role for the various fundamental processes such as excitation and energy transfer, CT, charge separation, and recombination. [3] CT is also dependent on the environment, [4] which severely comes into play for the photoinduced elementary processes of molecular D-A systems in the solid state, a scenario, encountered in organic electronic devices. In this respect, in thin films, aggregation of the molecules into supramolecular assemblies to form photoactive nanostructures with well-segregated A and D domains and their orientation relative to the substrate play a major role for efficient organic solar cells. [5] As a consequence, the tailoring of molecular properties in D-A systems is an important aspect in the overall structural design for tuning and controlling distance-dependent CT, whereby intermolecular interactions and self-assembly determine charge separation in thin films. The understanding and Single-material organic solar cells (SMOSCs) promise several advantages with respect to prospective applications in printed large-area solar foils. Only one photoactive material has to be processed and the impressive thermal and photochemical long-term stability of the devices is achieved. Herein, a novel structural design of oligomeric donor-acceptor (D-A) dyads 1-3 is established, in which an oligothiophene donor and fullerene acceptor are covalently linked by a flexible spacer of variable length. Favorable optoelectronic, charge transport, and self-organization properties of the D-A dyads are the basis for reaching power conversion efficiencies up to 4.26% in SMOSCs. The dependence of photovoltaic and charge transport parameters in these ambipolar semiconductors on the specific molecular structure is investigated before and after post-treatment by solvent vapor annealing. The inner nanomorphology of the photoactive films of the dyads is analyzed with transmission electron microscopy (TEM) and grazingincidence wide-angle X-ray scattering (GIWAXS). Combined theoretical calculations result in a lamellar supramolecular order of the dyads with a D-A phase separation smaller than 2 nm. The molecular design and the precise distance between donor and acceptor moieties ensure the fundamental physical processes operative in organic solar cells and provide stabilization of D-A interfaces.

Photoisomerization of chromophores usually shows significantly less efficiency in solid polymers than in solution as strong intermolecular interactions lock their conformation. Herein, we establish the impact of macromolecular architecture on the isomerization efficiency of main-chain-incorporated chromophores (i.e., α-bisimine) in both solution and the solid state. We demonstrate that branched architectures deliver the highest isomerization efficiency for the main-chain chromophore in the solid stateremarkably as high as 70% compared to solution. The macromolecular design principles established herein for efficient solid-state photoisomerization can serve as a blueprint for enhancing the solid-state isomerization efficiency for other polymer systems, such as those based on azobenzenes.

In the last few years, low-voltage TEM [1] was instrumental for changing paradigms of imaging using low energy electrons. For very thin and highly conductive samples low-voltage TEM is clearly advantageous, but one faces increasing ionization and thus increased beam damage for less conductive samples [2]. This conundrum can be overcome by further reducing electron energy and using SEM detection mechanisms instead of TEM or STEM.Based on the DELTA-technology [3] we demonstrate here the possibility of conventional SE and BSE imaging as well as electron spectroscopic imaging (SEM-ESI) at electron landing energies down to 50 eV, well below typical beam damaging energies. Such ultra-low landing energies require aberration correction and the development of new filter and detector technologies if high resolution SEM-ESI needs to be achieved. A novel SEM prototype combining all these technologies has now been developed in the framework of our DELTA-project. The correction of spherical and chromatic aberrations allows even for electrons at ultra-low energies high resolution imaging (better than 1nm at 50 eV). As less beam damage is observed we utilize the increased applicable electron dose for electron spectroscopy. Here we record either spectra of SE to obtain "work function / electron affinity contrast" [4,5], or spectra of BSE. The DELTA-SEM facilitates ESI in a similar way to energy loss imaging and spectroscopy in TEM. Fig. 1 shows the current DELTA-SEM prototype with its complex electron optics made possible by the Delta-shaped beam splitter and the electrostatic electron mirror. This SEM allows energy-filtered imaging (Fig. 2) using an electrostatic energy filter combined with a scintillator electron detector (DELTADetector ® design). Electrons of different energy range contribute selectively to the final image when the electric field in the DELTA-detector is varied. Fig. 3 shows an example of imaging and SE spectroscopy of semiconducting organic materials. Their SE intensity, compared to the gold substrate, and their ESI spectra correlate with the relative conductivity of the materials.Even more interesting than spectral imaging with SEs is the use of BSEs. While SE signals are higher than BSE signals at conventional landing energies, BSE gains on SE at very low energies. Fig. 4 shows the typical intensity profiles for BSE and SE of Al at 100 eV and 500 eV landing energies. Thus, at lower energies the BSE signal can be studied in detail: The zero-loss peak as the signal of immediate backscattering is trivial, whereas the spectrum at low energy loss can be interpreted as multiple elasticinelastic scattering. For Al the excitation of the surface plasmon can be identified (cf. Fig. 4).At present we work on the deconvolution of the energy spread function of the DELTA-SEM [5] and use landing energies down to 50 eV for spectral imaging of organic materials. Model simulations and experiments on the bulk heterojunction of organic solar cells [6] indicate the possibility to distinguish different materials and to utilize S...

Epoxy-based fiber-matrix composites based on a single-component curing system with pot times >>two weeks are described. Bisphenol-A-diglycidyl ether and hexahydrophthalic anhydride were used as epoxy matrix precursors; 1,3-dicyclohexyl-3,4,5,6-tetrahydropyrimidinium carboxylate (6Cy-CO 2 ) was used as latent pre-catalyst. Glass fiber-reinforced epoxy resins were obtained both via thermal curing under air and under vacuum-assisted resin infusion conditions. The high quality of the resulting composites and the absence of any air inclusion were confirmed by DSC and x-ray tomography. Rheological and kinetic data revealed that the 6Cy-CO 2 -based systems allow for an advanced processing and outrival commercial amine-based hardeners in terms of speed of curing.

It is demonstrated that the postfunctionalization of solid polymeric microspheres can generate fully and throughout functionalized materials, contrary to the expectation that core–shell structures are generated. The full functionalization is illustrated on the example of photochemically generated microspheres, which are subsequently transformed into polyradical systems. Given the all‐organic nature of the functionalized microspheres, characterization methods with high analytical sensitivity and spatial resolution are pioneered by directly visualizing the inner chemical distribution of the postfunctionalized microspheres based on characteristic electron energy loss signals in transmission electron microscopy (TEM). Specifically, ultrasonic ultramicrotomy is combined successfully with electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) during TEM. These findings open a key avenue for analyzing all‐organic low‐contrast soft‐matter material structures, while the specifically investigated system concomitantly holds promise as an all‐radical solid‐state functional material.

Reversible hydrogen uptake and the metal/dielectric transition make the Mg/MgH2 system a prime candidate for solid‐state hydrogen storage and dynamic plasmonics. However, high dehydrogenation temperatures and slow dehydrogenation hamper broad applicability. One promising strategy to improve dehydrogenation is the formation of metastable γ‐MgH2. A nanoparticle (NP) design, where γ‐MgH2 forms intrinsically during hydrogenation is presented and a formation mechanism based on transmission electron microscopy results is proposed. Volume expansion during hydrogenation causes compressive stress within the confined, anisotropic NPs, leading to plastic deformation of β‐MgH2 via (301)β twinning. It is proposed that these twins nucleate γ‐MgH2 nanolamellas, which are stabilized by residual compressive stress. Understanding this mechanism is a crucial step toward cycle‐stable, Mg‐based dynamic plasmonic and hydrogen‐storage materials with improved dehydrogenation. It is envisioned that a more general design of confined NPs utilizes the inherent volume expansion to reform γ‐MgH2 during each rehydrogenation.

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