DFT insights into optoelectronic properties of a 1 : 1 perylene : TCNQ cocrystal as a near-infrared absorber for photovoltaic applications

Abstract

With the advent of lightweight photovoltaic materials with room-temperature solution processing ability, organic materials have emerged as a particularly promising family of candidates. Organic cocrystals comprising π-electron-rich donors and π-electron-deficient acceptors show immense potential within the field of thin film photovoltaic devices, possessing strong and broad optical absorption covering the visible and near-infrared region of the solar spectrum, intrinsic semiconductor properties, and solution-processing ability under ambient conditions. However, they have been scarcely studied as materials for photovoltaic devices. Herein, we investigated the excited-state features and electron/hole transport properties of a 1 : 1 cocrystal of π-donor perylene and π-acceptor 7,7′,8,8′-tetracyanoquinodimethane (TCNQ). The donor and acceptor molecules form infinite π-stacks via strong face-to-face π⋯π stacking interactions to facilitate charge transfer. The absorption spectrum of this cocrystal shows quite broad absorption from the UV to the near infrared regions (320–1150 nm) owing to charge transfer. The time-dependent DFT study indicates efficient charge transfer and exciton generation and dissociation. The bandgap (0.92 eV) and the exciton binding energy (0.12 eV) values of this cocrystal are ideal for photovoltaic applications, while its theoretically calculated spectroscopic limited maximal efficiency (SLME) is 24% at 1000 nm thickness, indicating its future applicability. The electron (45 meV) and hole (48 meV) transfer integral values along the π⋯π stacking direction indicate ambipolar charge transport, while the low internal reorganization energy values of perylene (147 meV) and TCNQ (255 meV) are favourable for fast charge carrier transport, making the cocrystal a suitable candidate for photovoltaic applications. The 1 : 1 perylene : TCNQ cocrystal represents an intriguing example of an indirect bandgap organic material with a high theoretical maximum photovoltaic efficiency at low film thickness owing to the very similar values of its indirect bandgap and direct allowed bandgap. This theoretical study unveils the photophysical and charge transport properties of an organic cocrystal with strong potential for photovoltaic applications.