Exploring the Impact of Heavy Atom Substitutions (Se, Te) on the CB16-Based Non- Fullerene Acceptor for High-Performance Organic Solar Cells: DFT and TD-DFT Study.

Heavy atom substitution is a promising strategy for enhancing organic solar cells (OSCs) performance. This study explores the optoelectronic impact of such substitution on the recently synthesized non-fullerene acceptor CB16 (C). Specifically, two derivatives, C and C, were designed, replacing sulfur (S) with two distinct heavy heteroatoms selenium (Se) and tellurium (Te), respectively. Their photophysical and optoelectronic characteristics have been investigated through density functional theory (DFT) and time-dependent DFT simulations using the HSEH1PBE/6-311G(d,p) method. The computational investigations covered a wide range of properties, including electronic structure and bonding characteristics, electronic density and stability, reorganization energy, absorption properties and non-linear optical response. Interestingly, in both C and C, the energy difference between the highest occupied and the lowest unoccupied molecular orbitals (HOMO-LUMO) was significantly reduced, decreasing from1.71 eV in C to 1.64 eV in C and 1.54 eV in C. Similarly, the simulation results showed several enhanced properties in both derivatives compared to C, with C demonstrating markedly greater improvements such as enhanced dipole moment (2.35D), red-shifted absorption indicating enhanced light-harvesting potential, and extended excited state lifetime (4.94 ns) owing it to its low hole reorganization energies (λh ~ 0.173 eV). Analysis of molecular orbitals, transition density matrix, and bonding characteristics suggests that Se and Te substitutions facilitate improved intramolecular charge transfer and enhanced non-linear optical response and provide a higher photovoltaic potential. These findings highlight the effectiveness of heavy atom substitution in tuning optoelectronic properties and the potential of targeted heavy tellurium atom substitution for the development of next-generation high-performance organic solar cells.