High‐Performance Phototransistor Based on a 2D Polybenzimidazole Polymer

Abstract Photodetectors are fundamental components of modern optoelectronics, enabling the conversion of light into electrical signals. The development of high‐performance phototransistors necessitates materials with both high charge carrier mobility and robust photoresponse. However, achieving both in a single material poses challenges due to inherent trade‐offs. Herein, this study introduces a polybenzimidazole‐(1,3‐diazole)‐based 2D polymer (2DPBI), synthesized as few‐layer, crystalline films covering ≈28 cm 2 on the water surface at room temperature, with large crystalline domain sizes ranging from 110 to 140 µm 2 . The 2DPBI incorporates a π‐conjugated photoresponsive porphyrin motif through a 1,3‐diazole linkage, exhibiting enhanced π‐electron delocalization, a narrow direct band gap of ≈1.18 eV, a small reduced electron–hole effective mass ( m * = 0.171 m 0 ), and a very high resonant absorption coefficient of up to 10 6 cm −1 . Terahertz spectroscopy reveals excellent short‐range charge carrier mobility of ≈240 cm 2 V −1 s −1 . Temperature‐dependent photoconductivity measurements and theoretical calculations confirm a band‐like charge transport mechanism. Leveraging these features, 2DPBI‐based phototransistors demonstrate an on/off ratio exceeding 10 8 , photosensitivity of 1.08 × 10 7 , response time of 1.1 ms, and detectivity of 2.0 × 10 13 Jones, surpassing previously reported standalone few‐layer 2D materials and are on par with silicon photodetectors. The unique characteristics of 2DPBI make it a promising foundation for future optoelectronic devices.