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.