Dynamic Effects on the Charge Transport in an Organic Near-Infrared Absorber Material

In a theoretical study, combining molecular dynamics simulations, quantum-chemical calculations, and charge migration simulations based on Marcus theory, we investigate the electronic structure, its fluctuations, and the charge transport of a promising organic near-infrared absorber material: 7,7-difluoro-7<i>H</i>-5,9-diphenyldiisoindolo­[2,1-<i>c</i>:1′,2′-<i>f</i>]­[1,3,5,2]­triazaborinine-6-ium-7-uide (Ph₂-benz-BODIPY), which is already successfully used as the donor material in organic solar cells. For the crystalline, defect-free phase, we find a one-dimensional hole transport characteristic with a mobility of 0.53 cm²/(V s) and a two-dimensional electron transport characteristic with a smaller mobility of 0.15 cm²/(V s). The attachment of the phenyl rings to the molecular core tends to improve the electron conduction by reducing the internal reorganization energy and by increasing the intermolecular coupling. In contrast, such functionalization tends to impair the hole transport as the highest occupied molecular orbital couples dominantly to the dynamics of the phenyl rings and the annulated benzene rings

Tuning Near-Infrared Absorbing Donor Materials: A Study of Electronic, Optical, and Charge-Transport Properties of aza-BODIPYs

The class of 4,4′-difluoro-4-bora-3a,4a,8-triaza-s-indacenes (aza-BODIPYs) are promising near-infrared absorber materials which are successfully used in organic solar cells to extend their absorption to the near-infrared regime. We computationally studied electronic properties, internal reorganization energies, and the optical properties of more than 100 promising candidates and derived design principles, including novel functionalization routes, to improve their performance as donor materials. We synthesized and characterized several of the promising molecules, confirming the predicted trends. The best charge transport properties and absorption characteristics are obtained for naphthalene-annulated molecular cores due to optimally delocalized frontier molecular orbitals. Further optimization can be achieved by α-functionalization with fluorinated groups, β-functionalization with accepting substituents, and modification of the borondifluoride group. For such molecules, we predict a bathochromic shift in the absorption, which should not significantly reduce the open-circuit voltage. Torsional restriction of α-substituents by carbon bridges can further improve both charge transport and absorption. The theoretically and experimentally observed independence of most of the functionalization strategies makes BODIPYs an ideal material class for tailor-made absorber materials that can cover a broad range of absorption, charge transport, and energetic regimes, calling for further exploration in organic solar cell applications, fluorescence microscopy, and photodynamic therapy

<i>In Situ</i> Observations of Free-Standing Graphene-like Mono- and Bilayer ZnO Membranes

ZnO in its many forms, such as bulk, thin films, nanorods, nanobelts, and quantum dots, attracts significant attention because of its exciting optical, electronic, and magnetic properties. For very thin ZnO films, predictions were made that the bulk wurtzite ZnO structure would transit to a layered graphene-like structure. Graphene-like ZnO layers were later confirmed when supported over a metal substrate. However, the existence of free-standing graphene-like ZnO has, to the best of our knowledge, not been demonstrated. In this work, we show experimental evidence for the <i>in situ</i> formation of free-standing graphene-like ZnO mono- and bilayer ZnO membranes suspended in graphene pores. Local electron energy loss spectroscopy confirms the membranes comprise only Zn and O. Image simulations and supporting analysis confirm that the membranes are graphene-like ZnO. Graphene-like ZnO layers are predicted to have a wide band gap and different and exciting properties as compared to other ZnO structures

Graphene Coatings for the Mitigation of Electron Stimulated Desorption and Fullerene Cap Formation

Graphene already has numerous applications in transmission electron microscopy. Here, we extend its application in electron microscopy by demonstrating its potential to stop electron induced desorption in nonconducting samples, where in essence charge build-up leads to surface atom desorption. Graphene films provide a conduction pathway to prevent charge build-up and do not interfere with the imaging process allowing the direct imaging of specimens sensitive to electron induced desorption. We also show that small graphene flakes on the surface of MgO transform to fullerenes or hemispherical fullerenes. The hemispherical fullerenes anchor to the MgO surface and are of particular interest as they suggest it should be possible to nucleate single walled carbon nanotubes on the surface of oxide supports without the need of a catalyst particle

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

Absorption Tails of Donor:C₆₀ Blends Provide Insight into Thermally Activated Charge-Transfer Processes and Polaron Relaxation

In disordered organic semiconductors, the transfer of a rather localized charge carrier from one site to another triggers a deformation of the molecular structure quantified by the intramolecular relaxation energy. A similar structural relaxation occurs upon population of intermolecular charge-transfer (CT) states formed at organic electron donor (D)–acceptor (A) interfaces. Weak CT absorption bands for D–A complexes occur at photon energies below the optical gaps of both the donors and the C₆₀ acceptor as a result of optical transitions from the neutral ground state to the ionic CT state. In this work, we show that temperature-activated intramolecular vibrations of the ground state play a major role in determining the line shape of such CT absorption bands. This allows us to extract values for the relaxation energy related to the geometry change from neutral to ionic CT complexes. Experimental values for the relaxation energies of 20 D:C₆₀ CT complexes correlate with values calculated within density functional theory. These results provide an experimental method for determining the polaron relaxation energy in solid-state organic D–A blends and show the importance of a reduced relaxation energy, which we introduce to characterize thermally activated CT processes