Abstract
Perylenediimide (PDI) derivatives have been widely studied as electron acceptor alternatives to fullerenes in organic photovoltaics (OPVs) because of their tunable absorption in the visible range, inexpensive synthesis, and photochemical stability. A common motif for improving device efficiency involves joining multiple PDIs together through electron-rich linkers to form a twisted acceptor-donor-acceptor molecule. Molecular features such as ring fusion are further employed to modify the structure locally and in films. These synthetic efforts have greatly enhanced OPV device efficiencies, however it remains unclear how the increasingly elaborate structural modifications affect the photophysical processes integral to efficient photon-to-charge conversion. Here we carry out a systematic study of a series of PDI dimers with thienoacene linkers in which the twist angle, linker length, and degree of ring fusion are varied to investigate the effects of these structural features on the molecular excited states and exciton recombination dynamics. Spectroscopic characterization of the dimers suggest that ring fusion causes greater coupling between the donor and acceptor components and greatly enhances the lifetime of a thienoacene to PDI charge transfer state. The lifetime of this CT state also correlates well with the linker-PDI dihedral angle, with smaller dihedral angle resulting in longer lifetime. DFT and two-photon absorption TDDFT calculations were developed in-house to model the ground state and excited transitions, providing theoretical insight into the reasons for the observed photophysical properties and identifying the charge transfer state in the excited state absorption spectra. These results highlight how the longevity of the excited state species, important for the efficient conversion of excitons to free carriers in OPV devices, can be chemically tuned by controlling ring fusion and by using steric effects to control the relative orientations of the molecular fragments. The results provide a successful rationalization of the behavior of solar cells involving these acceptor molecules.
Original language | English (US) |
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Pages (from-to) | 7133-7143 |
Number of pages | 11 |
Journal | Chemical Science |
Volume | 11 |
Issue number | 27 |
DOIs | |
State | Published - Jul 21 2020 |
Funding
We acknowledge Dr Matthew Kelley for performing the preliminary DFT calculations. We acknowledge support from the Ultrafast Initiative of the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This research was also supported in part through the computation resources and staff contributions provided for the Quest high performance computing facility at Northwestern University which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology.
ASJC Scopus subject areas
- General Chemistry