of transistors, thus exhibiting higher photo sensitivity and lower noise levels than conventional photodiodes and photo conductors. [5][6][7][8][9][10] Although inorganic photo transistors (e.g., single crystalline silicon and InAlAs-InGaAs) have attracted considerable attention from both the industry and the global market, the high-temperature processing under vacuum, the poor compatibility with flexible substrates, and the limited sensing areas significantly restrict their further developments and practical applications. [11,12] Therefore, realizing low-cost, high-sensitivity, and highly flexible phototransistors in a large-scale area has quickly become an important research focus.Conjugated polymers (CPs) are wellknown and promising candidates for solution-processable and cost-effective semiconductor materials due to several attractive merits, including intrinsic flexibility, incorporation of functionality via molecular design, and ease of achieving large-scale and low-cost device fabrication through facile methods, such as spin coating, drop-casting, and inkjet printing. [13][14][15][16][17] In recent years, although the rational tuning of the electronic energy levels and band gaps is widely investigated, the charge carrier mobility of the reported CPs is still much lower than that of amorphous silicon (≈1 cm 2 V −1 s −1 ). Extensive studies have demonstrated that the wide absorption band of CPs determines the efficient photoinduced charge carrier generation under illumination, while the high charge carrier mobility ensures the swift charge transport and collection at the electrodes. [18][19][20] However, the strong π-π packing and high exciton binding energy (0.3-1 eV) desirable when aiming for high mobility of the CPs are generally detrimental for their use as the photoactive layer in the phototransistors. [21][22] Thus, how to balance the π-π packing and charge carrier mobility of the CPs is of crucial significance for fabricating high-performance phototransistors.Recent reports have demonstrated that diketopyrrolopyrrole (DPP)-based CPs are considered as highly prospective photoactive materials for solution-processed organic optoelectronic devices like organic solar cells (OSCs) and organic FETs (OFETs). [23][24][25][26][27] By selectively amalgamating the electrondeficient DPP units with proper electron-rich units, the D-A copolymers could show good planarity, controllable conjugated It is universally acknowledged that highly photosensitive transistors are strongly dependent on the high carrier mobility of polymer-based semiconductors. However, the polymer π-π stacking and aggregation, required to increase the charge mobility, conversely inhibit the dissociation of photogenerated charge carriers, in turn accelerating the geminate recombination of electron-hole pairs. To explore the effects of charge mobility and polymer stacking on the photoresponsivity of the phototransistors, here, two alternating copolymers are synthesized, namely P-PPAB-IDT and P-PPAB-BDT, by palladium-catalyzed Stille coupling of PP...