Optimizing non-fullerene acceptor molecules constituting fluorene core for enhanced performance in organic solar cells: a theoretical methodology

Context

Looking for novel outstanding performance materials suitable for organic solar cells, we constructed a range of non-fullerene acceptors (NFAs) evolved from the recently synthesized acceptor molecule identified as DICTIF, structured around fluorene core where 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene) propanedinitrile presented the terminals end-groups. Employing density functional theory (DFT) and time dependent-DFT (TD-DFT) simulations, we have simulated the impact of altering the end groups of DICTIF molecule by five assorted acceptors molecules, for the purpose of exploring their opto-electronic properties and their performance in organic solar cell (OSC) applications. We proved that the designed non-fullerene acceptors provide enhanced efficiency compared to the synthesized molecule, such as planar geometries and narrower energy gap ranging from 1.51 to 1.95 eV. A red shift in absorption was observed for all tailored molecules (λ_max = 583.5–711.4 nm) as compared to the reference molecule (λ_max = 578 nm).Various decisive factors such as frontier molecular orbitals (FMOs), exciton binding energy (EB), absorption maximum (λ_max), open circuit voltage (V_OC), reorganization energies (RE), transition density matrix (TDM), reduced density gradient (RDG), and electron-hole overlap have also been computed for analyzing the performance of NFAs. Low reorganizational energy values facilitate charge mobility which improves the conductivity of all the designed molecules. This study showed that our novel tailored molecules might be suitable candidates for the fabrication of highly efficient photovoltaic materials.

Methods

After testing various hybrid functionals, optimized geometries were assigned using DFT HSEH1PBE/6-31G(d) level of theory. Electronic excitations and absorption spectra were investigated using the TD-DFT MPW1PW91/6-31G(d) level of theory. We ascertained that HSEH1PBE/6-31G(d) level of theory yield the closest calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the DICTIF to the corresponding experimental ones and that TD-MPW1PW91//6-31G(d) was the most suitable level of theory for exploring electronic excitations and finding the maximum of absorption (λ_max).