DFT insights into opto-electronic properties of near infra-red absorber 1:1 perylene:TCNQ cocrystal towards photovoltaic application

Abstract

With the advent of photovoltaic materials with room temperature solution processing ability and light-weightiness, organic materials have emerged as the promising candidates. Organic cocrystals comprising π-electron rich donor and π-electron deficient acceptor hold immense potential for thin film photovoltaic devices for strong and broad optical absorption covering the visible and near infra-red region of solar spectrum, intrinsic semiconductor property, and the solution processing ability at ambient conditions yet they are under-represented as photovoltaic materials. Herein, we have investigated the excited state features and electron/hole transport properties of 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 staking interaction 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. Time dependent DFT study indicates efficient charge transfer exciton generation and dissociation. The bandgap (0.92 eV) and the exciton binding energy (0.12 eV) values of this cocrystal are ideal for the photovoltaic applications, while the theoretically calculated spectroscopic limited maximal efficiency (SLME) of the cocrystal is 24 % at 1000 nm thickness giving an indication of practical applicability. The electron (45 meV) and hole (48 meV) transfer integral values along the π··π stacking direction indicate ambipolar charge transport, while low values of the internal reorganization energy of perylene (147 meV) and TCNQ (255 meV) are favourable for the fast transport of charge carriers making the cocrystal a suitable candidate for photovoltaic applications. The 1:1 perylene:TCNQ cocrystal poses an intriguing example of organic indirect bandgap material having high value of the theoretical maximum photovoltaic efficiency at low film thickness, owing to very similar values of indirect bandgap and direct allowed bandgap. This theoretical study unveils the photo-physical and charge transport properties of an organic cocrystal with strong potential for photovoltaic applications.