Impact of End-Group Chlorination on Charge Generation and Recombination Patterns in Organic Solar Cells with Wide-Band-Gap Acceptors

Cl-substituted nonfullerene acceptors (NFAs) exhibit planar conformations that can enhance molecular packing and electronic coupling. In this study, a chlorinated wide-band-gap NFA, YO-TCl, with tetrahalogen-substituted end groups, is introduced and compared with ZY-4Cl to systematically examine how the Cl-substituted end groups impact the blend morphology, charge generation, and extraction in their correlated bulk heterojunction (BHJ) devices. Compared to ZY-4Cl, with two chlorine atoms in each end group, YO-TCl possesses a downshifted highest occupied molecular orbital (HOMO) and red-shifted absorption. Organic solar cells (OSCs) are fabricated using PBDB-T as donors and the two wide-band-gap NFAs as acceptors. From ZY-4Cl to YO-TCl, power conversion efficiencies (PCEs) from 7.6 to 13.2% are obtained, which can be attributed to the better film morphology of the YO-TCl-based bulk heterojunction (BHJ) film. Taking into account the efficiency disparities between ZY-4Cl- and YO-TCl-based BHJ devices, we specifically focus on investigating the molecular packing properties of the organic films and their effects on charge transport, recombination, and extraction in the resulting BHJ devices. Our analyses reveal that ZY-4Cl- and YO-TCl-based devices exhibit significantly different charge generation, recombination, and extraction characteristics, which stem from distinct aggregation and phase distribution within the BHJ blend films.