We revealed the relationship between the London dispersion components of three-dimensional non-fullerene acceptors and photocurrent generation efficiency in bulk-heterojunction-type organic photovoltaics.
In
organic semiconductors, the hole and electron transport occurs
through the intermolecular overlaps of highest occupied molecular
orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), respectively.
A measure of such intermolecular electronic coupling is the transfer
integral, which can experimentally be observed as energy level splittings
or the width of the respective energy bands. Quantum chemistry textbooks
describe how an energy level splits into two levels in molecular dimers,
into three levels in trimers and evolves into an energy band in infinite
systems, a process that has never been observed for the LUMO or beyond
dimers for the HOMO. In this work, our new technique, low-energy inverse
photoelectron spectroscopy, was applied to observe the subtle change
of the spectral line shape of a LUMO-derived feature while we used
ultraviolet photoelectron spectroscopy to investigate the occupied
states. We show at first that tin-phthalocyanine molecules grow layer-by-layer
in quasi-one-dimensional stacks on graphite, and then discuss a characteristic
and systematic broadening of the spectral line shapes of both HOMO
and LUMO. The results are interpreted as energy-level splittings due
to the intermolecular electronic couplings. On the basis of the Hückel
approximation, we determined the transfer integrals for HOMO–1,
HOMO, and LUMO to be ≤15 meV, (100 ± 10) meV, and (128
± 10) meV, respectively.
The energy band structure of the conduction band (energy–momentum relation of electrons) is crucial to understanding the electron transport of crystalline materials. In this paper, we describe an angle-resolved low-energy inverse photoelectron spectroscopy (AR-LEIPS) apparatus that examines the conduction band structures of materials sensitive to the electron beam, such as organic semiconductors and organic–inorganic hybrid perovskites. The principle of this apparatus is based on AR inverse photoelectron spectroscopy. To minimize radiation damage and improve energy resolution, we employed our previous approach used in LEIPS [H. Yoshida, Chem. Phys. Lett. 539–540, 180 (2012)]. We obtained an overall energy resolution of 0.23 eV with a momentum resolution of 0.9 nm−1 at the electron kinetic energy of 2 eV or higher.
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