Rational design of non-fullerene acceptors via side-chain and terminal group engineering: a computational study

We investigated the optoelectronic and photovoltaic properties of three types of acceptor-donor-acceptor-based non-fullerene acceptor (NFA) molecules for organic solar cell (OSC) applications. Density functional theory and its time-dependent variant were employed to compute the quadrupole moment perpendicular to the π-system (Q<inf>20</inf>), open circuit voltage (V<inf>OC</inf>), and other relevant solar cell parameters. The role of functionalization in the acceptor unit on the overall device performance was explored by incorporating halogen and methoxy-based electron-withdrawing groups. The electronegativity differences between the halogen atoms and the methoxy group demonstrated contrasting effects on the energy levels, molecular orbitals, and absorption maximum. We observed a trade-off between short-circuit current (J<inf>SC</inf>) and V<inf>OC</inf>, which was further substantiated by an inverse correlation between Q<inf>20</inf> and V<inf>OC</inf>. We found an optimum value of Q<inf>20</inf> in the range of 80 to 130 ea<inf>0</inf>² to achieve an optimized solar cell performance. Among the designed systems, Se-derived NFAs with a small band gap, red-shifted absorption maximum, high-oscillator strength, small exciton binding energy, and optimum Q<inf>20</inf> turned out to be potential candidates for future applications. These criteria can be generalized to design and screen next-generation non-fullerene acceptors to achieve improved OSC performance.