Defect Mitigation of Solution-Processed 2D WSe₂ Nanoflakes for Solar-to-Hydrogen Conversion

Abstract

Few-atomic-layer nanoflakes of liquid-phase exfoliated semiconducting transition metal dichalcogenides (TMDs) hold promise for large-area, high-performance, low-cost solar energy conversion, but their performance is limited by recombination at defect sites. Herein, we examine the role of defects on the performance of WSe thin film photocathodes for solar H production by applying two separate treatments, a pre-exfoliation annealing and a post-deposition surfactant attachment, designed to target intraflake and edge defects, respectively. Analysis by TEM, XRD, XPS, photoluminescence, and impedance spectroscopy are used to characterize the effects of the treatments and photoelectrochemical (PEC) measurements using an optimized Pt-Cu cocatalyst (found to offer improved robustness compared to Pt) are used to quantify the performance of photocathodes (ca. 11 nm thick) consisting of 100-1000 nm nanoflakes. Surfactant treatment results in an increased photocurrent attributed to edge site passivation. The pre-annealing treatment alone, while clearly altering the crystallinity of pre-exfoliated powders, does not significantly affect the photocurrent. However, applying both defect treatments affords a considerable improvement that represents a new benchmark for the performance of solution-processed WSe: solar photocurrents for H evolution up to 4.0 mA cm and internal quantum efficiency over 60% (740 nm illumination). These results also show that charge recombination at flake edges dominates performance in bare TMD nanoflakes, but when the edge defects are passivated, internal defects become important and can be reduced by pre-annealing.

Organic Semiconductor Based Devices for Solar Water Splitting

“…However, the fabrication of these high‐performance inorganic HER photocathodes is based on expensive and nonscalable processing techniques, such as atomic layer deposition, sputtering, and thermal evaporation. Indeed, while the innovative synthesis of these competing inorganic photocathodes via low cost methods has attracted considerable attention in recent years, the obtained photocurrent is comparable to that of the BHJ photocathode reported by Bourgeteau et al and Comas Rojas et al Thus, combining the aspects of photocurrent and low‐cost implementation, OS‐based photocathodes are on an equal position to their inorganic counterparts. In addition, the ability to tune the bandgap and the energy levels of the organic semiconductor gives advantages to control the light absorption and the photocurrent onset potential, as was recently demonstrated using subnaphthalocyanine and sexithiophene small molecule semiconductors .…”

Recent Advances and Emerging Trends in Photo‐Electrochemical Solar Energy Conversion

“…The use of these layers also adds complexity and cost to the device, which is then reflected in the final price of the generated hydrogen. Thus, the identification of materials that are intrinsically stable toward corrosion and generate (high) photovoltage at the semiconductor–electrolyte interface toward their respective water splitting reactions (true PEC systems, as in Figure b) is highly desirable …”

Recent progress of tungsten- and molybdenum-based semiconductor materials for solar-hydrogen production

“…Encouragingly, the solutionprocessed WSe 2 thin films prepared by a space-confined self-assembled (SCSA) thin-film deposition method using the solvent-exfoliated few-layer WSe 2 flakes as the precursor, exhibited a p-type photocurrent density of 1.0 mA cm −2 at 0 V vs. RHE under AM 1.5 G illumination [239]. Using Pt-Cu as the cocatalyst, a solution-processed 2D WSe 2 nanoflake photocathode exhibited a new benchmark photocurrent density of 4.0 mA cm −2 for solar-hydrogen production with an internal quantum efficiency of over 60% at the wavelength of 740 nm [240]. MoS 2 /WS 2 heterojunction photoanodes prepared by restacking the WS 2 and MoS 2 monolayers on FTO substrates exhibited a photocurrent density of 0.45 mA cm −2 and the oxygen evolution under simulated solar irradiation [241].…”