Article pubs.acs.org/cm
Photogenerated Charge Harvesting and Recombination in Photocathodes of Solvent-Exfoliated WSe2 Xiaoyun Yu and Kevin Sivula* Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, 1015 Lausanne, Switzerland S Supporting Information *
ABSTRACT: Understanding and optimizing the effects of edge states and nanoflake dimensions on the photon harvesting efficiency in ultrathin transition-metal dichalcogenide (TMD) semiconductor photoelectrodes is critical to assessing their practical viability for solar energy conversion. We present herein a novel filtration-based separation approach to systematically vary the TMD nanoflake dimensions and edge density of solutionprocessed large-area multiflake WSe2 photocathodes. Photoelectrochemical measurements in both aqueous electrolyte (for water reduction) and a sacrificial redox system, together with a continuum-based charge transport model, reveal the role of the edge sites and the effects of the flake size on the light harvesting, charge transport, and recombination. A selective passivation technique using atomic layer deposition is developed to address detrimental recombination at flake edges. Edge-passivated WSe2 films prepared with the smallest flakes (∼150 nm width, 9 nm thickness) demonstrate an internal quantum yield of 60% (similar to bulk single-crystal results). An optimized (1 sun) photocurrent density of 2.64 mA cm−2 is achieved with 18-nm-thick flakes (700 nm width) despite transmitting ∼80% of the accessible photons. Overall, these results represent a new benchmark in the performance of solution-processed TMDs and suggest routes for their development into large-area low-cost solar energy conversion devices.
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20 nm to 1000 nm in width and up to 40 nm in thickness)18,19 implies a large density of flake edge sites, which can severely limit device performance, compared to bulk monocrystalline electrodes. For example, solution-processed WSe2 employed toward photoelectrochemical solar hydrogen production gave internal quantum photon conversion efficiency of ca. 10%,20 which was significantly lower than bulk WSe2 single crystals (ca. 60%) measured under similar conditions.21 Similarly poor and variable performance has been observed in the optoelectronic properties of solvent-exfoliated MoS2 and a variety of other TMDs,10,22,23 indicating that overcoming photogenerated charge carrier recombination in solution-processed TMD flakes remains a major challenge. Indeed, while the vdW (basal plane) surfaces of TMD crystals are known to be suitable for photogenerated charge transfer,24 recombination has been reported to dominate at edge defects, which have a metallic nature.25,26 Moreover, the anisotropic charge carrier transport in TMDs preferentially directs photogenerated carriers along the layers to step-edge defects or flake edges (as the charge carrier mobility is up to 103 times greater1 through the layers
INTRODUCTION Given their favorable optoelectronic properties and chemical robustness, the semiconducting transition-metal dichalcogenides (TMDs)1 have attracted considerable attention as active materials in solar energy conversion devices over many years.2 Moreover, the TMD crystal structure, which consists of a layer of metal cations sandwiched between two layers of chalcogen atoms forming stacks of weakly bound van der Waals (vdW) layers, facilitates the exfoliation of bulk single crystals into monolayered or few-layered two-dimensional (2D) sheets or flakes.3−5 This feature has recently opened new opportunities for semiconducting TMDs in functional devices by offering control over the nature and energy of the band gap,6−8 and the ability to form vdW heterojunctions with few defects and a high tolerance for lattice mismatch.9−11 While the exfoliated TMDs have demonstrated promising solar energy conversion device performance over small active areas,12−14 their widespread application is currently limited by the difficulty to prepare highperformance large-area devices using scalable techniques.15 In this regard, the solvent-assisted exfoliation methods3,5,16 are particularly promising, as they can produce dispersions of TMD flakes that retain their semiconducting 2H crystal structure (avoiding structural transition to the metallic 1T phase) and can be processed into thin-film devices.17 However, the dimensions of solvent-exfoliated flakes (typically ranging from © 2017 American Chemical Society
Received: May 16, 2017 Revised: July 24, 2017 Published: July 26, 2017 6863
DOI: 10.1021/acs.chemmater.7b02018 Chem. Mater. 2017, 29, 6863−6875
Article
Chemistry of Materials
Figure 1. Exfoliated WSe2 flake size separation: (a) schematic of the multistage track-etched membrane separation column developed for this work; (b−e) TEM images of WSe2 SCSA films prepared from Fractions A (panel (b)), B (panel (c)), C (panel (d)), and D (panel (e)); (f−i) corresponding flake lateral size distribution statistics, in which the vertical dashed lines indicate the pore size of the filtration membrane(s) that nominally bound the flake size.
ultracentrifugation18,32,33 and liquid cascade centrifugation34), these approaches are limited in scalability and their ability to afford samples containing exclusively large flakes. Alternatively for this work, we developed a sieve-based filtration method to separate TMD flakes using track-etched membranes with welldefined pore size.35 Since existing polycarbonate track-etched membranes are not stable in pure N-methyl-2-pyrrolidone (NMP, the exfoliation solvent), a ternary solvent mixture (NMP:iPrOH:H2O = 7:2:1) was optimized to balance TMD flake dispersibility and membrane durability. In this mixture, we found the pores of the polycarbonate track-etched membranes to remain unchanged after 30 min soaking (see Figure S1 in the Supporting Information). A multistage pressure-driven liquid filtration column (see schematic in Figure 1a) was designed and implemented to afford TMD flake separation into several different size ranges upon the introduction of a crude flake dispersion in a semicontinuous fashion. Membranes with decreasing pores diameter (Dp) values of 1, 0.6, and 0.2 μm were used and constant agitation of the column was applied to prevent filter cake formation. The separation resulted in fractions of dispersed flakes as the stage retentates with optical appearance consistent with varying flake dimensions (see Figure S2 in the Supporting Information). While we found that flakes passing though the membrane with the smallest size (Dp = 0.2 μm, coded as Fraction E) resulted in a successful size selection (see Figure S3 in the Supporting Information), the low fluid permeability of this membrane restricted the permeate flow to 1.6 eV) for Fraction A to only 14% for Fraction D. Despite the relatively low absorption implying a small film thickness, we note that, because of the initial low yield of monolayer sheets typically observed in solvent-assisted exfoliation34 and our purification process, which removes ultrathin flakes that also have small lateral dimension ( 1.6 eV) into solar fuel. The small thickness required for this performance and the relatively high transmittance also suggests that these WSe2 photocathodes could be used as top cells72 in tandem configurations for overall solar water splitting or in building integrated device applications.
(Fractions A−D, largest to smallest). The application of these fractions as single-flake layer photocathodes (prepared via a solution-based self-assembly method) under different PEC testing conditions indicated significant recombination at the flake edge sites, and it suggested the critical role of the flake size on the extraction of photogenerated charges. In addition, a minority charge carrier transport simulation was employed to elucidate precisely how the anisotropic optoelectronic properties of WSe2, which produce an edge-terminating “dead” zone in the flakes, lead to a tradeoff between light absorption and charge extraction that is strongly dependent on flake size. Interestingly, the optimum flake dimensions were found to match well with the accessible size range of the solventexfoliated flakes, suggesting an intrinsic link between the mechanical and optoelectronic properties in WSe2. While the largest flakes of Fraction A were predicted to deliver the highest photocurrent density, a disaccord between the experimental PEC performance and the simulation results suggested that catalytically active “hot” edge sites are highly active under sacrificial redox conditions but significantly less when driving solar water reduction. Passivation of the edge defects via the ALD of Al2O3 improved the PEC performance of all flake fractions while further revealing the limitations of nonedge (internal) defects in the largest flakes of Fraction A. The optimum flake size was determined experimentally to be Fraction B (ca. 700 nm width × 18 nm thick flakes), where passivated photoelectrodes produced (1 sun) photocurrent densities up to 2.64 mA cm−2 (in the presence of an electron scavenger), despite transmitting ca. 80% of the accessible photons. The maximum internal quantum yield was measured with passivated flakes from the thinnest Fraction D (150 nm × 9 nm) to be over 60%, which is comparable to the quantum efficiency reported in state-of-the-art single-crystal p-type WSe2 photocathodes under similar conditions. While further work in the development of passivation and cocatalyst coating techniques are still needed, our results represent a new benchmark in the performance of solution-processed WSe2 and give promise to realizing inexpensive and high-performance TMDs for large-area photoelectrochemical or photovoltaic energy conversion applications.
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EXPERIMENTAL METHODS
WSe2 Liquid Exfoliation. Commercially available WSe2 powder (AlfaAesar,
