Chlorination is considered an appealing strategy aimed at improving the optoelectronic properties, reducing aggregation and thus enhancing device performance due to a low synthetic cost, a large C-Cl dipole moment and high pi-electron delocalization originating from an empty 3d orbital. To broaden the structural multiplicity of chlorinated conjugated polymers (CPs), herein, three monochlorinated benzothiadiazole-based medium bandgap (MBG) polymer donors, PClBDT-ClBT, PClBDT-TClBT, and PClBDT-DTClBT, were developed by varying the octylthiophene pi bridge number between chlorinated benzo[1,2-b:4,5-b ']dithiophene (ClBDT) and 5-chlorobenzo[c][1,2,5]thiadiazole (ClBT). Increasing the octylthiophene pi bridge number led to a blue-shifted maximum absorption peak, a reduced bandgap, a slightly elevated absorption coefficient, raised EHOMO, weakened molecular aggregation and a dominant face-on molecular orientation. As expected, importing an octylthiophene pi bridge impaired the VOC but gave rise to a suitable microstructural morphology, appropriate miscibility and an improved photophysical process. Therefore, the PClBDT-ClBT-based device exhibited a low PCE of only 1.42%, limited by low JSC and FF. Meanwhile, the PClBDT-TClBT-based device afforded a VOC of 0.87 V, a JSC of 22.45 mA cm-2, and a FF of 59.21% when paired with Y6, collectively contributing to the outstanding PCE as high as 11.52%. However, the PClBDT-DTClBT-based device gave a decreased VOC of 0.81 V and a JSC of 18.57 mA cm-2, but a higher FF of 62.99%, resulting in a decreased PCE of 9.48%. To the best of our knowledge, 11.52% efficiency is the highest PCE recorded among those of the chlorinated BT-based CPs in binary organic solar cells so far. Our findings facilitate a better understanding of the structure-property-performance relationship for precisely tuning the conjugated pi bridge which could effectively affect the energy level, microstructure, photophysical procedure, and thus boost the photovoltaic performance. Monooctylthiophene-pi-bridge PClBDT-TClBT achieved an outstanding PCE of 11.52%, and bisoctylthiophene-pi-bridge PClBDT-DTClBT exhibited a decreased PCE of 9.48% when paired with Y6. Unfortunately, PClBDT-ClBT exhibited an unsatisfactory PCE of 1.42%.
