2 research outputs found

    Electrostatic Effects at Organic Semiconductor Interfaces: A Mechanism for “Cold” Exciton Breakup

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    Exciton dissociation at organic semiconductor interfaces is an important process for the design of future organic photovoltaic (OPV) devices, but at present, it is poorly understood. On the one hand, exciton breakup is very efficient in many OPVs. On the other, electron–hole pairs generated by an exciton should be bound by Coulombic attraction, and therefore difficult to separate in materials of such low dielectric. In this paper, we highlight several electrostatic effects that appear commonly at organic/organic interfaces. Using QM/MM simulations, we demonstrate that the electric fields generated in this fashion are large enough to overcome typical electron–hole binding energies and thus explain the high efficiencies of existing OPV devices without appealing to the existence of nonthermal (“hot”) carrier distributions. Our results suggest that the classical picture of flat bands at organic/organic interfaces is only qualitatively correct. A more accurate picture takes into account the subtle effects of electrostatics on interfacial band alignment

    Charge Transfer or J‑Coupling? Assignment of an Unexpected Red-Shifted Absorption Band in a Naphthalenediimide-Based Metal–Organic Framework

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    We investigate and assign a previously reported unexpected transition in the metal–organic framework Zn<sub>2</sub>(NDC)<sub>2</sub>(DPNI) (<b>1</b>; NDC = 2,6-naphthalenedicarboxylate, DPNI = dipyridyl-naphthalenediimide) that displays linear arrangements of naphthalenediimide ligands. Given the longitudinal transition dipole moment of the DPNI ligands, J-coupling seemed possible. Photophysical measurements revealed a broad, new transition in <b>1</b> between 400 and 500 nm. Comparison of the MOF absorption spectra with that of a charge transfer (CT) complex formed by manual grinding of DPNI and H<sub>2</sub>NDC led to the assignment of the new band in <b>1</b> as arising from an interligand CT. Constrained density functional theory utilizing a custom long-range-corrected hybrid functional was employed to determine which ligands were involved in the CT transition. On the basis of relative oscillator strengths, the interligand CT was assigned as principally arising from π-stacked DPNI/NDC dimers rather than the alternative orthogonal pairs within the MOF