Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

Opitz, Andreas; Banerjee, Rupak; Grob, Stefan; Gruber, Mark; Hinderhofer, Alexander; Hörmann, Ulrich; Kraus, Julia; Linderl, Theresa; Lorch, Christopher; Steindamm, Andreas; Topczak, A. K.; Wilke, Andreas; Koch, Norbert; Pflaum, Jens; Schreiber, Frank; Brütting, Wolfgang

doi:10.1007/978-3-319-28338-8_4

Cited by 2 publications

(4 citation statements)

References 87 publications

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“…The downward bending of the spectral profiles of the pristine donor compounds around 1.5 eV is an artifact due to the grating and detector changeover in the spectrometer. Note that the magnitude of k xy for DIP 51,52 and for 6T 21,53 compares very well with the literature. Ellipsometry.…”

Section: ■ Experimental Sectionsupporting

confidence: 85%

Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

et al. 2018

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In this work, two novel donor/acceptor (D/A) complexes, namely, diindenoperylene (DIP)/1,3,4,5,7,8-hexafluoro-tetracyanonaphthoquinodimethane (F6TCNNQ) and alpha-sexithiophene (6T)/F6TCNNQ, are studied. The D/A complexes segregate in form of π–π stacked D/A cocrystals and can be observed by X-ray scattering. The different conformational degrees of freedom of the donor molecules, respectively, seem to affect the thin-film crystalline texture and composition of the D/A mixtures significantly. In equimolar mixtures, for DIP/F6TCNNQ, the crystallites are mostly uniaxially oriented and homogeneous, whereas for 6T/F6TCNNQ, a mostly 3D (isotropic) orientation of the crystallites and coexistence of domains of pristine compounds and D/A complex, respectively, are observed. Using optical absorption spectroscopy, we observe for each of the two mixed systems a set of new, strong transitions located in the near-IR range below the gap of the pristine compounds: such transitions are related to charge-transfer (CT) interactions between donor and acceptor. The optical anisotropy of domains of the D/A complexes with associated new electronic states is studied by ellipsometry. We infer that the CT-related transition dipole moment is perpendicular to the respective π-conjugated planes in the D/A complex.

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Section: ■ Experimental Sectionsupporting

confidence: 85%

Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

et al. 2018

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“…polymer HJ from similar experiments [52,[108][109][110] using CV measurements or detection of charge transfer emission/absorption to determine an estimate for the photovoltaic gap ΔE PV [107,111,112]. This bandgap-voltage offset is significantly larger for OPVCs than for most inorganic pn-junction solar cells [113] as well as for the emerging perovskite cells [114] and one of the reasons for the lower performance of OPVCs [53]. A clear limitation for maximisation of the photovoltaic gap ΔE PV is given due to the necessary HOMO-HOMO and LUMO-LUMO offset.…”

Section: Concluding Remarks and Outlookmentioning

confidence: 99%

“…The clear discrimination between charge transfer from the substrate into the films of the PHJ can be achieved by careful analysis and considering electrostatic modelling [68,77,79]. The cases presented here were contact doping [69], long range charge transfer through an unpinned layer [53] or charge transfer at the interface of two pinned layers However, electrostatic modelling can be applied successfully only if the real morphology is known. As mentioned before, structural compatibility and related reorientation [54,55], structural disorder induced by the deposition process [87,128] and the surface/interface energies [55] have to be taken into account.…”

Section: Concluding Remarks and Outlookmentioning

confidence: 99%

“…Understanding these behaviours at the organic/electrode and organic/organic interface are performed on model systems and can be transferred also to other devices. PHJs are used here to reduce the before mentioned challenges and the focus lies on materials used in organic donor/acceptor systems for solar cell applications [49,52,53]. The energy level alignment is drawn very often with a constant electrostatic potential through the whole device (see figure 2).…”

Section: Introductionmentioning

confidence: 99%

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Energy level alignment at planar organic heterojunctions: influence of contact doping and molecular orientation

Opitz

2017

J. Phys.: Condens. Matter

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Planar organic heterojunctions are widely used in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. The energy level alignment in the devices plays an important role in obtaining the aspired gap arrangement. Additionally, the π-orbital overlap between the involved molecules defines e.g. the charge-separation efficiency in solar cells due to charge-transfer effects. To account for both aspects, direct/inverse photoemission spectroscopy and near edge x-ray absorption fine structure spectroscopy were used to determine the energy level landscape and the molecular orientation at prototypical planar organic heterojunctions. The combined experimental approach results in a comprehensive model for the electronic and morphological characteristics of the interface between the two investigated molecular semiconductors. Following an introduction on heterojunctions used in devices and on energy levels of organic materials, the energy level alignment of planar organic heterojunctions will be discussed. The observed energy landscape is always determined by the individual arrangement between the energy levels of the molecules and the work function of the electrode. This might result in contact doping due to Fermi level pinning at the electrode for donor/acceptor heterojunctions, which also improves the solar cell efficiency. This pinning behaviour can be observed across an unpinned interlayer and results in charge accumulation at the donor/acceptor interface, depending on the transport levels of the respective organic semiconductors. Moreover, molecular orientation will affect the energy levels because of the anisotropy in ionisation energy and electron affinity and is influenced by the structural compatibility of the involved molecules at the heterojunction. High structural compatibility leads to π-orbital stacking between different molecules at a heterojunction, which is of additional interest for photovoltaic active interfaces and for ground-state charge-transfer.

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Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

Cited by 2 publications

References 87 publications

Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

Thin-Film Texture and Optical Properties of Donor/Acceptor Complexes. Diindenoperylene/F6TCNNQ vs Alpha-Sexithiophene/F6TCNNQ

Energy level alignment at planar organic heterojunctions: influence of contact doping and molecular orientation

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