The first solution processable PDI-based polymer acceptor P(PDI2DD-DTT) (P1) composed of alternating dithienothiophene(DTT) and PDI units (Fig. 4) was synthesized by Zhan and coworkers in 2007 [52]. Benefiting the highly ordered p-stackedstructures and extended absorption coverage induced from theintramolecular low-energy charge-transfer transition suggestedby the D–A character of the polymer (from 300 nm to 700 nmand extending into the near-IR), the resulting fullerene-free OSCsbased on P(PDI2DD-DTT) showed a PCE over 1% with a Jsc of4.2 mA cm2, a Voc of 0.63 V and a fill factor (FF) of 39%. Byemploying binary additives, including a nonvolatile-conjugatedmolecule PDI-2DTT and DIO to facilitate mixing of donor andacceptor and to improve phase separation, Zhan et al. fabricatedPSC devices consisting of low-bandgap polymer donor PBDTTTC-T and P(PDI2DD-DTT) and achieved a higher PCE of 3.45%[64]. Later on, Zhou’s group fabricated PSC devices by pairing P(PDI2DD-DTT) with a p-type polymer BDDT (also known asPBDTBDD-T or PBDB-T) and obtained a PCE of 3.49% [65].