High-Performance Phototransistor Based on a 2D Polybenzimidazole Polymer

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 cm² on the water surface at room temperature, with large crystalline domain sizes ranging from 110 to 140 µm². 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 m₀), and a very high resonant absorption coefficient of up to 10⁶ cm^-1. Terahertz spectroscopy reveals excellent short-range charge carrier mobility of ≈240 cm² 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 10⁸, photosensitivity of 1.08 × 10⁷, response time of 1.1 ms, and detectivity of 2.0 × 10¹³ 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.