End-capped group modification on cyclopentadithiophene based non-fullerene small molecule acceptors for efficient organic solar cells; a DFT approach

Four acceptor-donor-acceptor (A-D-A) type cyclopentadithiophene core-based non-fullerene small acceptor molecules were designed with the objective to improve the proficiency of photovoltaic cells. A comprehensive density functional theory (DFT) analysis was done by employing B3LYP functional with 6-31G(d,p) basis set to study optoelectronic properties of R as well as M1-M4 molecules, while the time-dependent self-consistent field (TDSCF) was utilized to analyze their excited state calculations. Several essential characteristics must be refined in order to enhance the efficiency of small molecular acceptors, i.e., the density of states (DOS), HOMO-LUMO band gap, transition density matrix (TDM), dipole moment, reorganization energy, light-harvesting efficiency, and open-circuit voltage, etc. In comparison to the R molecule, all the derived molecules show better maximum absorption (in chloroform solvent) with a range of 886-951 nm and a smaller band gap with a range of 1.65-1.55 eV M2 retains the least exciton binding energy of 0.24 eV, and amongst all the investigated molecules M3 molecule has the least interaction coefficient values so, it possesses better charge transport probability. The reorganization energy values in eV for both electron (0.00579) and hole (0.00737) are the least for M3 molecule, so this molecule exhibits better charge mobility for electron and hole. V-OC of R and M1-M4 molecule was calculated by theoretically computing the values of their complexes with PTB7-Th donor molecule.

Automated summary

Four acceptor-donor-acceptor (A-D-A) type cyclopentadithiophene core-based non-fullerene small acceptor molecules were designed to improve the efficiency of photovoltaic cells. Density functional theory (DFT) and time-dependent self-consistent field (TDSCF) calculations were performed to analyze the optoelectronic properties and excited state calculations of the molecules. The designed molecules showed improved absorption, smaller band gap, and better charge transport probability compared to the reference molecule. The reorganization energy values were also lower in one of the molecules, indicating better charge mobility. The open-circuit voltage (V-OC) was calculated for the molecules in complex with a donor molecule.