From Y6 to BTPT-4F: a theoretical insight into the influence of the individual change of fused-ring skeleton length or side alkyl chains on molecular arrangements and electron mobility

Abstract

Organic solar cells (OSCs) based on non-fullerene acceptor (NFA) Y6 have drawn tremendous attention due to the great progress in their power conversion efficiencies (PCEs). Recently, experimental researchers found that the performance of BTPT-4F (like Y6 but having different side alkyl chains and a shorter fused-ring skeleton simultaneously) based OSCs is much worse than that of Y6 based OSCs. However, it is still confusing how the individual change of the fused-ring skeleton length or side alkyl chains of Y6 affects the photovoltaic properties, especially molecular arrangements and electron mobility. Therefore, we have modelled possible molecular arrangements of Y6, BTPT-4F, and two new intermediate NFAs, named BTPTT-4F-C8 and BTPTT-4F-C12, to systematically study the effects. The computed results could explain the experiment well, and show that terminal/central side alkyl chains and the length of the fused-ring skeleton all have noticeable influences on molecular planarity and arrangements. The outputs also illustrate that extending the fused-ring skeleton can increase electron mobility more than engineering side alkyl chains, and high electron mobility mainly depends on the formation of close face-on stacking of end groups and overlapping of partially flat backbones. Last but not least, both BTPTT-4F-C12 and BTPTT-4F-C8 are superior to Y6 in dipole moment, absorption spectra, charge separation, and electron mobility.