High-performance near-infrared polymeric phototransistors realized by combining cross-linked polymeric semiconductors and bulk heterojunction bilayer structures

Flexible polymeric phototransistors highly responsive to near-infrared (NIR) light are crucial for the room temperature biological imaging due to some unique properties of organic semiconductors, such as light weight, low cost, and good biocompatibility. However, the low mobility and/or the high dark off-current (DOC) of low-bandgap polymeric phototransistors limit their NIR light detectability. This work proposes the development of a solution-processable bilayer heterostructure (BHS) for polymeric phototransistors, which uses cross-linked polymeric semiconductors (CLPS) as the bottom conducting channel and a bulk heterojunction (BHJ) as the photoactive layer. The BHS architecture ensures fast carrier transport and efficient exciton separation. Besides, the electron-trapping effect generated between the CLPS and BHJ results in a significantly lower DOC compared with other controlled devices. Thus, the BHS phototransistor based on low-bandgap polymer can detect NIR light with wavelengths of 808 nm, 1064 nm, and even 1550 nm, going from NIR-I (750–1000 nm) to NIR-II (1000–1700 nm). Meanwhile, its responsivity (R) is two or three orders of magnitude higher than the controlled device. Furthermore, an ultra-flexible NIR phototransistor array (~5 μm thick) using this strategy could detect image barcodes with different gray levels, demonstrating its potential application in flexible NIR imaging.