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.

Abstract

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 <i>n</i>-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-<i>p</i>-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 <i>face-to-face</i> 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.