Solution-Processed Self-Isolated C8-BTBT Coated Tungsten Diselenide Phototransistors for Enhanced UV Photosensitivity

Most transition metal dichalcogenide (TMD)-based phototransistors adopt additional light-absorbing layers not only to enhance optoelectronic properties but also to selectively absorb specific wavelengths (e.g., UV or IR detection). However, a major issue with traditional heterostructure phototransistors may be the increase in OFF current after depositing the light absorption layer. This increase is primarily due to surface charge transfer, which critically limits the signal-to-noise ratio (SNR). To address this challenge, we propose a self-isolation method that involves intentionally leaving photoresist (PR). We fabricated a self-isolated C8-BTBT-coated WSe₂ phototransistor to demonstrate the effectiveness of our self-isolation method. C8-BTBT (2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene), a p-type organic semiconductor, serves as an ultraviolet (UV) light-absorbing layer due to its relatively large bandgap (~3.84 eV). The type-I band alignment of the WSe₂/C8-BTBT heterostructure facilitates the transfer of photogenerated holes from C8-BTBT to the WSe₂ channel. As a result, a 1294% improvement in photosensitivity was observed under UV light illumination (λ_ex = 406 nm) after the C8-BTBT coating, while maintaining stable ON and OFF current levels. This simple technique allows for highly sensitive 2D photosensing applications.