Kevin Sivula1

1, Ecole Polytechnique Federale Lausanne, Lausanne, , Switzerland

Given their established robustness and favorable optoelectronic properties, the semiconducting transition metal dichalcogenides (TMDs, e.g. MoS2 and WSe2) are attractive for optoelectronic applications including solar energy conversion (photovoltaic and photoelectrochemical solar fuel production).[1] Recent advances in the liquid-phase exfoliation (LPE) of semiconducting TMDs into mono- or few-layered 2D nanoflake dispersions suggests that inexpensive roll-to-roll processing can be used to prepare TMD-based devices inexpensively over large area.[2] However, the high concentration of defects in these materials act as recombination sites for photogenerated carriers and limit the performance. In this presentation the challenges with charge transport, separation, recombination, and interfacial transfer in LPE TMD nanoflake thin film devices will be discussed with respect to the 2D flake size and defect passivation/charge extraction treatments.[3] Our results give insight into the roles of both edge and internal defects and suggest routes for improvement. Overall it is shown that LPE semiconducting TMDs with suitable defect mitigation can achieve internal quantum efficiency for photon harvesting similar to bulk single crystal samples. Specifically, we show that WSe2 nanoflake thin films achieve absorbed-photon-to-current efficiency over 50% and photocurrent densities for solar water reduction at 4 mA cm–2 under standard testing conditions.[4]

[1] X. Yu, K. Sivula, ACS Energy Lett. 2016, 1, 315.
[2] X. Yu, M. S. Prevot, N. Guijarro, K. Sivula, Nat. Commun. 2015, 6, 7596.
[3] X. Yu, K. Sivula, Chem. Mater. 2017, 29, 6863.
[4] X. Yu, N. Guijarro, M. Johnson, K. Sivula, Nano Lett. 2018, 18, 215.