The development of donor-acceptor (DA) based bulkheterojunction (BHJ) polymer solar cells (PSCs) has made significant progress in the past decade. In particular, power conversion efficiency (PCE) of 8–9% in a single junction device has been demonstrated.[1–5] Among various materials developed for BHJ devices, the multifused-ring conjugated polymers are particularly interesting due to their superior optical and electrical properties.[6–20] The highly fused aromatic/heteroaromatic units enhance effective conjugation of the polymer backbone to facilitate electron delocalization. Moreover, the covalently rigidified adjacent units could prevent rotational disorder to reduce reorganization energy, which results in enhanced charge carrier mobilities.[21–23] To date, numerous fused-ring, ladder type donor systems have been developed since the first report of thiophene-phenylene-thiophene (TPT)(also called indacenodithiophene (IDT)), based polymers by Ko et al.[6–8]

The PSC performance of one of the IDT based polymers, PIDT-BT, has been shown to reach 6.4% after careful solvent annealing.[24] This has inspired vigorous efforts to modify the IDT backbone, such as changing the bridging atoms or sidechains to improve polymer packing for better charge transport or light absorption.[9, 12, 25–27] Larger fused-rings by extending the IDT backbone,[10, 11, 16, 18, 19, 28–30] increasing the number of aromatic rings, or changing the sequence of tethered aromatic units on the backbone all have been tried and generated a wide variety of multiple fused-ring systems.[17, 20] For example, Cheng et al. have reported a novel seven-ring system by fusing a fluorene unit and two thiophene units together. Through proper side-chain modifications, the PCE of devices made from these