Roll-to-Roll Deposition of Semiconducting 2D Nanoflake Films of Transition Metal Dichalcogenides for Optoelectronic Applications

Exfoliated transition metal dichalcogenide (TMD) nanomaterials possess remarkable and tunable semiconducting properties which make them competitive for use in ultrathin, flexible devices such as photodetectors, sensors, and photoelectrodes for solar energy conversion. Until now, the large-scale production of such devices has presented a challenge that limits their commercialization and broad application. We demonstrate here, for the first time, a procedure for the roll-to-roll (R2R) deposition of two-dimensional (2D) TMD nanoflake thin films via a continuous liquid–liquid interfacial self-assembly method. Remarkably, no solvent removal is required during film deposition, nor is there any nanoflake accumulation in the solvent bath, making continuous operation feasible. In our prototype deposition system described herein, TMD nanoflakes (9 nm average flake thickness, 50–500 nm long) are self-assembled into large area films up to 100 mm in width that are reproducibly printed at 10 mm s–1 with nanoflake loadings of 35 mg m–2. Optically uniform coverage on transparent conductive oxide-coated flexible plastic substrates is shown, and key printing parameters to afford the required single-flake-layer deposition are identified. Photoelectrochemical testing verifies optoelectronic activity of the MoS2 nanoflake films achieving photocurrent densities for iodide oxidation of 40 μA cm–2 at +0.1 V (vs an Ag/Ag+ reference) under 1 sun illumination. In addition, we show that these R2R films are robust and that our method works for multiple (successive) flake layer depositions by preparing large area MoS2/WSe2 heterojunction nanoflake films. This demonstration represents an important milestone in advancing large-scale production of 2D TMD films toward low-cost, high-performance optoelectronic devices.