Tuning Morphology and Defect Density in Self-Assembled Thin-Films of Solvent-Exfoliated WSe2 for Photoelectrochemical Hydrogen Production

Abstract

The layered semiconducting transition metal dichalcogenides can be exfoliated into atomically-thin 2D sheets offering promising opto-electronic characteristics for application in solar energy conversion. However, the challenges to fabricate high-quality thin films of these 2D sheets using scalable and cost-effective methods limit their practical application. Here we present novel solution-based approaches for large-area semiconducting films of liquid exfoliated WSe2 with controllable flake alignment and defect density, which is leveraged to advance the understanding of the morphological and structural factors on the optoelectronic performance in devices. Specifically, we develop a thin film self-assembly method employing spatial confinement of WSe2 nanoflakes which affords overlapping-free morphology and superior charge transfer character over films with aggregations.[1, 2] The critical roles of the flake edge density, flake lateral size and thickness on the photogenerated charge transport and transfer are established by both experimental (by solution-based flake size sorting) and theoretical (by anisotropic charge transport simulation) routes.[3] In addition, we recently develop approaches to reduce charge recombination at internal crystal defects and surface dangling bonds by applying pre-annealing and surfactant treatments, respectively, which affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe2. Solar photocurrents for H2 evolution up to 4.0 mA cm–2 (at 0V vs RHE, AM 1.5G illumination), and internal quantum efficiency over 60% are reported for 10 nm thick WSe2 photoelectrodes. [1] Yu X.; Prevot, M. S.; Guijarro, N.; Sivula, K. Nat. Commun. 2015, 6, 7596. [2] Yu X.; Prévot, M. S.; Sivula, K. Chem. Mater. 2014, 26, 5892. [3] Yu, X.; Sivula, K. Chem. Mater. 2017, 29, 6863.