Flexible Vertical p-n Diode Photodetectors with Thin N-type MoSe₂ Films Solution-Processed on Water Surfaces

Two-dimensional (2D) nanosheets of transition metal dichalcogenides (TMDs) are of significant interest for potential photoelectronic applications. However, the fabrication of solution-processed arrays of mechanically flexible thin TMD films-based vertical type p-n junction photodetectors over a large area is a great challenge. Our method is based on controlled solvent evaporation of MoSe₂ suspension spread on water surface. Single or few-layered MoSe₂ nanosheets modified with the dispersant amine-terminated poly(styrene) (PS-NH₂) were homogeneously deposited and stacked on water upon solvent evaporation, giving rise to uniform MoSe₂/PS-NH₂ composite films that can be readily transferred onto other substrates. A p-n junction vertical diode of Al/p-type Si/p-type poly(9,9-di-n-octylfluorenyl-2,7-diyl)/n-type MoSe₂ composite/Au stacked from bottom to top exhibited characteristic rectifying current behavior upon voltage sweep with a rectification ratio of 10³. Subsequent illumination of near-infrared light on the device resulted in a substantially enhanced dark current of approximately 10³ times greater than that of the nonexposed device. The photodetection performance, that is, switching time, responsivity, and detectivity, were 100.0 ms, 2.5 AW^-1, and 2.34 × 10¹⁴, respectively. Furthermore, the performance of mechanically flexible photodetectors devices was comparable with that of the devices fabricated on the hard Si substrate even after 1000 bending cycles at a bending diameter of 7.2 mm.