Photoresponsive Transistors Based on a Dual Acceptor-Containing Low-Bandgap Polymer

In this Article, low-bandgap pTTDPP-BT polymers based on electron-accepting pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP) and benzothiadiazole (BT) and electron-donating thienothiophene (TT) moieties were synthesized. Phototransistors have been fabricated using ambipolar-behaving pTTDPP-BT polymers as active channel materials. The electrical and photoresponsive properties of the pTTDPP-BT phototransistors were strongly dependent on the film annealing temperature. As-spun pTTDPP-BT phototransistors exhibited a low hole mobility of 0.007 cm²/(V·s) and a low electron mobility of 0.005 cm²/(V·s), which resulted in low photocurrent detection due to the limited transport of the charge carriers. Thermal treatment of the polymer thin films led to a significant enhancement in the carrier mobilities (hole and electron mobilities of 0.066 and 0.115 cm²/(V·s), respectively, for 200 °C annealing) and thus significantly improved photoresponsive properties. The 200 °C-annealed phototransistors showed a wide-range wavelength (405-850 nm) of photoresponse, and a high photocurrent/dark-current ratio of 150 with a fast photoswitching speed of less than 100 ms. This work demonstrates that a dual acceptor-containing low band gap polymer can be an important class of material in broadband photoresponsive transistors, and the crystallinity of the semiconducting polymer layer has a significant effect on the photoresponse characteristics.