Efficient polymer solar cells enabled by alkoxy-phenyl side-chain-modified main-chain-twisted small molecular acceptors

Four new main-chain-twisted acceptor-donor-acceptor-type (A-D-A) non-fullerene acceptors (i-mO-4F, i-mO-4Cl, i-pO-4F, and i-pO-4Cl) were designed and synthesized, featuring the introduction of meta-hexyloxy-phenyl and para-hexyloxy-phenyl side-chains in the fused-ring core as well as 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile and 2-(5,6-dichlorine-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile as the end-capping groups. All four small molecules exhibited broad absorption spectra in the range of 300 nm to 820 nm with a corresponding small optical bandgap of 1.55 eV (i-mO-4F), 1.53 eV (i-mO-4Cl), 1.52 eV (i-pO-4F), and 1.48 eV (i-pO-4Cl), which ensured total complementary absorption with a wide bandgap polymer donor material (PBDB-T). Although i-mO-4Cl possessed a finely bathochromic-shifted absorption spectrum, its poor morphology with PBDB-T in the blend films led to a low power conversion efficiency (PCE) of 7.41%. In contrast, the optimal PBDB-T:i-mO-4F-, PBDB-T:i-pO-4F-, and PBDB-T:i-pO-4Cl-based devices achieved promising PCEs of 14.02%, 13.47%, and 10.98%, respectively. The obtained significantly different results for these two types of NFAs, on the one hand, demonstrate the synergistic effect of the hexyloxy-phenyl side-chain modification and non-planar main-chain strategy to construct highly efficient non-fullerene acceptors for polymer solar cells, and on the other hand, reveal the influence of the fluorinated and chlorinated end-groups on the morphologies and performances of the resultant main-chain-twisted NFAs.