Enhancing the photovoltaic performance of heteroheptacene-based nonfullerene acceptors through the synergistic effect of side-chain engineering and fluorination

As two essential components of acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors, both end-groups and side-chains can affect the performance of the resulting nonfullerene polymer solar cells (PSCs). Here, two new nonfullerene acceptors, namely M1 and M4, are developed to investigate the synergistic effect of side-chains and end-groups on the photovoltaic properties of nonfullerene acceptors. Compared with M1 with nonfluorinated end-groups and shorter side-chains, M4 exhibits a red-shifted absorption, lowered energy levels, more ordered molecular packing and increased crystallinity due to its longer side-chains as well as fluorinated end-groups. Moreover, when blended with the polymer donor of PM6, the M4-based blend shows an enhanced face-on molecular orientation and more favorable phase separation in comparison with the M1-based counterpart thereby leading to increased charge carrier mobilities and decreased charge recombination for the PSCs based on M4. As a consequence, a significantly enhanced power conversion efficiency (PCE) of 14.75% with an increased short-circuit current density of 23.44 mA cm(-2) and a fill factor of 71.52% is achieved for the M4-based devices outperforming that of the M1-based devices (PCE = 8.06%). This study provides an important strategy to improve the photovoltaic properties of nonfullerene acceptors.