Paper-based broadband flexible photodetectors with van der Waals materials.

Layered metal chalcogenide materials are exceptionally appealing in optoelectronic devices thanks to their extraordinary optical properties. Recently, their application as flexible and wearable photodetectors have received a lot of attention. Herein, broadband and high-performance paper-based PDs were established in a very facile and inexpensive method by rubbing molybdenum disulfide and titanium trisulfide crystals on papers. Transferred layers were characterized by SEM, EDX mapping, and Raman analyses, and their optoelectronic properties were evaluated in a wavelength range of 405-810 nm. Although the highest and lowest photoresponsivities were respectively measured for TiS (1.50 mA/W) and MoS (1.13 μA/W) PDs, the TiS-MoS heterostructure not only had a significant photoresponsivity but also showed the highest on/off ratio (1.82) and fast response time (0.96 s) compared with two other PDs. This advantage is due to the band offset formation at the heterojunction, which efficiently separates the photogenerated electron-hole pairs within the heterostructure. Numerical simulation of the introduced PDs also confirmed the superiority of TiS-MoS heterostructure over the other two PDs and exhibited a good agreement with the experimental results. Finally, MoS PD demonstrated very high flexibility under applied strain, but TiS based PDs suffered from its fragility and experience a remarkable drain current reduction at strain larger than ± 0.33%. However, at lower strains, all PDs displayed acceptable performances.