Interfacial Molecular Packing Determines Exciton Dynamics in Molecular Heterostructures: The Case of Pentacene–Perfluoropentacene

Abstract

The great majority of electronic and optoelectronic devices depend on interfaces between p-type and n-type semiconductors. Finding matching donor-acceptor systems in molecular semiconductors remains a challenging endeavor because structurally compatible molecules may not necessarily be suitable with respect to their optical and electronic properties, and the large exciton binding energy in these materials may favor bound electron-hole pairs rather than free carriers or charge transfer at an interface. Regardless, interfacial charge-transfer exciton states are commonly considered as an intermediate step to achieve exciton dissociation. The formation efficiency and decay dynamics of such states will strongly depend on the molecular makeup of the interface, especially the relative alignment of donor and acceptor molecules. Structurally well-defined pentacene-perfluoropentacene heterostructures of different molecular orientations are virtually ideal model systems to study the interrelation between molecular packing motifs at the interface and their electronic properties. Comparing the emission dynamics of the heterosystems and the corresponding unitary films enables accurate assignment of every observable emission signal in the heterosystems. These heterosystems feature two characteristic interface-specific luminescence channels at around 1.4 and 1.5 eV that are not observed in the unitary samples. Their emission strength strongly depends on the molecular alignment of the respective donor and acceptor molecules, emphasizing the importance of structural control for device construction.

Electrostatic Interactions Shape Molecular Organization and Electronic Structure of Organic Semiconductor Blends

“…24 Conversely, vacuum co-deposition of PEN and PFP (on SiO x ) yields films intermixed on the molecular scale. There, the growth of a crystalline mixed PEN:PFP phase with 1:1 stoichiometry on SiO x has been discussed in numerous studies, as deduced from specular (XRD) and grazing incidence X-ray diffraction (GIXRD), 18,28 electron diffraction, 29 optical absorption 18,28,30,31 and vibrational spectroscopy. 32 This mixed phase has been found to be remarkably stable 33 and has been shown to prevail also in the excess of one of the two components.…”

Charge Transfer Excitation and Asymmetric Energy Transfer at the Interface of Pentacene–Perfluoropentacene Heterostacks

“…In addition, we also investigate an intermixed blend of both molecules in an equimolar ratio, in the following referred to as bulk heterojunction consistent with Ref. 22. Here, the molecules adopt an upright molecular orientation (total thickness 40 nm) with respect to the substrate with a -stack packing motif within the layers (25).…”

“…The emission and exciton dynamics of these samples have been reported in our previous work. (22) The high sensitivity of PLE-spectroscopy allows us to examine both, the absorption energy of the CT-state itself as well as the coupling of all observed absorption features to the CTexciton system and, thereby, to obtain complementary information on its formation process. The PLE spectra in this work are obtained by measuring the photoluminescence intensity of the CT exciton depending on the photon energy of the exciting laser.…”

“…whereas the latter displays a predominantly face-on alignment at the interface. (22) The structural details at the internal interface are as of yet unknown. Indications for a herringbone-type alignment in the lying heterostack are reported,(24) while the recent grazing incidence Xray diffraction study of a 1:1 blend of PEN: PFP finds a -stack arrangement between PEN and PFP (25).…”

Advances in Functional Nanomaterials Science

“…[191] Moreover, structural and optical properties of molecular acceptor/donor heterosystems can be conveniently studied at the examples of PEN/PFP and PEN/C 60 . [192][193][194] Not less important remain colloidal QDs as model nanoclusters in which quantization effects severely determine optical properties. Recently, it was investigated, how the emission of CdS QDs can be tuned with silver particles.…”