Cocrystal Engineering of Organic Semiconductors for Photovoltaic Applications: Modeling Excited-State Properties of a Charge Transfer Cocrystal of a Dicarbazole Donor and a Fluoranil Acceptor

With the recent advancements
in lightweight, flexible, and environmentally
benign organic supramolecular aggregates for various optoelectronic
applications, cocrystals of aromatic π-donors and π-acceptors
have emerged as promising n-type semiconductors and
near-infrared absorbers for enhanced photovoltaic properties. Herein,
we demonstrate the electron-dominant charge transport and wide absorption
spanning from ultraviolet (UV) to NIR-I region (375–800 nm)
of a cocrystal with π-donor 4,4′-bis(carbazol-9-yl)biphenyl
(CBP) and π-acceptor 1,4-tetrafluoro-p-benzoquinone
(fluoranil) as the components. The crystal packing in CBP:(fluoranil)2 is characterized by mixed stacks of alternative CBP and fluoranil
molecules tethered by strong face-to-face π···π
stacking interactions. The electron-dominant charge transport in the
CBP:(fluoranil)2 cocrystal is governed by the “superexchange”
hopping mechanism along the D–A mixed π-stack and is
dominated by factors like the energy and symmetry of the frontier
molecular orbitals of the CBP and fluoranil moieties. The narrow bandgap
(≈1.2 eV) and the high value of the superexchange electron
transfer integral (≈100 meV) confirm the potential application
of this cocrystal as the active layer material in n-type organic field
effect transistors (OFETs). In addition, the strong absorption spanning
from the UV to near-infrared region, narrow and direct bandgap, and
low exciton binding energy indicate that the CBP:(fluoranil)2 cocrystal can also be exploited for photovoltaic applications. The
electron–hole distribution offset, exciton size, and one-electron
transition density matrix analyses confirm facile charge transfer
exciton generation and dissociation leading to free charge carriers.
The calculated value of spectroscopy-limited maximum efficiency (SLME)
from periodic density functional theory (DFT) calculations for this
cocrystal shows that it can reach a photoconversion efficiency (PCE)
of 31%, implying its potential applicability as a practical photovoltaic
material.