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Low open-circuit voltages significantly limit the power conversion efficiency of organic photovoltaic devices. Typical strategies to enhance the open-circuit voltage involve tuning the HOMO and LUMO positions of the donor (D) and acceptor (A), respectively, to increase the interfacial energy gap or to tailor the donor or acceptor structure at the D/A interface. Here, we present an alternative approach to improve the open-circuit voltage through the use of a zinc chlorodipyrrin, ZCl [bis(dodecachloro-5-mesityldipyrrinato)zinc], as an acceptor, which undergoes symmetry-breaking charge transfer (CT) at the donor/acceptor interface. DBP/ZCl cells exhibit open-circuit voltages of 1.33 V compared to 0.88 V for analogous tetraphenyldibenzoperyflanthrene (DBP)/C60-based devices. Charge transfer state energies measured by Fourier-transform photocurrent spectroscopy and electroluminescence show that C60 forms a CT state of 1.45 ± 0.05 eV in a DBP/C60-based organic photovoltaic device, while ZCl as acceptor gives a CT state energy of 1.70 ± 0.05 eV in the corresponding device structure. In the ZCl device this results in an energetic loss between E(CT) and qV(OC) of 0.37 eV, substantially less than the 0.6 eV typically observed for organic systems and equal to the recombination losses seen in high-efficiency Si and GaAs devices. The substantial increase in open-circuit voltage and reduction in recombination losses for devices utilizing ZCl demonstrate the great promise of symmetry-breaking charge transfer in organic photovoltaic devices.

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Li-ion cathodes based on conversion reactions such as iron fluoride (FeF 2 ) can achieve in principle high specific capacity. However, significant capacity fading is observed upon cycling. This has been attributed in part to the formation and continuous growth of a solid electrolyte interphase (SEI) layer at the cathode/electrolyte interface. In this work, scanning transmission electron microscopy, electron energy loss spectroscopy, selected area electron diffraction, and X-ray photoelectron spectroscopy were used in combination to study both the structural changes of the FeF 2 /C active material and the growth and evolution of the SEI layer upon cycling. Two main sources of capacity loss have been found. An increasing amount of Fe 2+ appeared trapped inside the SEI layer with increasing cycle number, thus resulting in the loss of active material. In addition, reconversion is strongly impeded with increasing cycle number, leaving untransformed LiF and Fe 0 upon delithiation. This correlates with the irreversible growth of a SEI layer that limits electronic and ionic transport.

The electronic structure of the Zn(II)-5-(3,5-dicarboxyphenyl)-10,15,20-triphenylporphyrin dye (ZnTPP-Ipa), chemisorbed onto ZnO(112 j 0) and TiO 2 (110) single-crystal surfaces, has been investigated by means of density functional theory (DFT) and by electron spectroscopy methods in an ultra-high-vacuum environment. The core levels (Ti 2p and Zn 2p) as well as the valence band have been probed using X-ray and ultraviolet photoemission spectroscopies, whereas the conduction band has been evaluated from inverse photoemission spectroscopy. The calculated density of states for the gas phase molecule compares well to the experimentally determined electronic structure, allowing both a simple understanding of the adsorbate electronic properties and a direct determination of the ZnTPP-Ipa frontier orbitals with respect to the substrates' band edges.

The relation between energy alignment, adsorption geometry, and electron transfer between a chromophore and an oxide surface has been explored for a series of Zn(II) tetraphenylporphyrin derivatives adsorbed on TiO2(110) and ZnO(112̅0) surfaces. The electronic occupied and unoccupied structure has been obtained using UV-photoemission and inverse photoemission spectroscopies. From these results, a full picture of the energetics at the chromophore–oxide interface was established. The alignment of the molecular levels relevant for optical transition was found independent of the functionalization of the meso-phenyl groups. However, to explain the observation of different optical properties and electron transfer efficiencies of these different dyes, the adsorption geometry of two of these dyes was determined using scanning tunnel microscopy and near edge absorption fine structure spectroscopy. Functionalization of the meso-phenyls with COOH groups in the meta-position results in the ZnP macrocycle adsorbed parallel to the surface. Functionalization of the meso-phenyl groups with COOH groups in the para position results in a bounding geometry where the ZnP macrocycle makes an angle of ∼50° from the surface normal. This geometry, which allows face-to-face stacking of the porphyrin rings, opens a new electronic channel for exciton delocalization that competes with direct electron injection into the substrate conduction band.