Toward Large-Area Solar Energy Conversion with Semiconducting 2D

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Towards Large-area Solar Energy Conversion with Semiconducting 2D Transition Metal Dichalcogenides Xiaoyun Yu, and Kevin Sivula ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00114 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016

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Towards Large-area Solar Energy Conversion with Semiconducting 2D Transition Metal Dichalcogenides Xiaoyun Yu and Kevin Sivula Laboratory for Molecular Engineering of Optoelectronic Nanomaterials(LIMNO), Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland. Corresponding Author *[email protected]

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ABSTRACT. The recently-discovered unique optoelectronic properties and chemical robustness of mono- or few-layer two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) such as MoS2, WS2, MoSe2, and WSe2 have opened new possibilities for solar energy conversion devices. However solar energy conversion on a globally-relevant scale with TMDs requires the ability to fabricate high-performance large-area thin films of these materials using scalable, low-cost techniques, which remains an ongoing challenge. In this perspective the classic properties as well as the new opportunities afforded by the 2D-nature of TMDs for photovoltaic and photoelectrochemical application are presented. State-of-the-art methods for preparing TMD thin films and devices over large areas are compared and scrutinized. The need for increased fundamental understanding of defect states, grain boundaries, interfacial effects, photo-generated

charge

carrier

dynamics,

and

improved

control

over

thin

film

morphology/quality are identified as challenges remaining to be addressed, and routes to enabling global-scale solar energy conversion with these materials are suggested.

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Traditional photovoltaic solar energy conversion devices based on established inorganic semiconductors (i.e. crystalline silicon) may not be able to meet the renewable energy production targets needed to mitigate climate change in the next 30-40 years.1 Accordingly, the development of inexpensive semiconducting materials that can be processed by facile, scalable and economical techniques into high-performance devices capable of robust operation for the decade time scale is an urgent goal. While this ambition represents a significant engineering challenge, its fulfilment is the key to enabling economically-viable, global-scale solar energy conversion. Of the many emerging semiconductor material systems that could meet this goal,2,3 a class of materials with unique properties and application potential is based on ultra-thin 2D semiconductor materials—the transition metal dichalcogenides (TMDs). Semiconducting transition metal dichalcognides exemplified most commonly by MoS2, WS2, MoSe2, and WSe2, in the 2H-crystal arrangement, have attracted interest for solar energy conversion for decades in their bulk form owing to their suitable band gap (1-2 eV) and high absorption coefficients (105-106 cm–1) suitable for harvesting a large fraction of solar photons.4 Moreover, Mo, W, and S are major industrial elements with suitable natural abundance for largescale production. Indeed, even for selenium-based semiconducting TMDs, e.g. WSe2, the potential annual electricity production from the known reserves of the raw materials greatly exceeds the global energy use,5 suggesting that TMDs could easily meet the energy demand on the terawatt scale. This perspective article will briefly examine the properties of semiconducting TMDs that have motivated the initial studies of these materials for solar energy conversion, as well as present the new opportunities afforded by the 2D-nature of TMDs. The potential for application of 2D-TMDs will be scrutinized based on the state-of-the-art methods for preparing


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TMD thin films over large areas, and the challenges remaining to be addressed to enable globalscale solar energy conversion with 2D TMDs will be finally discussed. From an atomic arrangement perspective, bulk semiconducting TMD materials can be described as the stacking of closely packed hexagonal sheets of chalcogenide anions in the cdirection with each second interlayer space occupied by metal atoms through trigonal prismatic coordination (see Figure 1a). This structure results in the chalcogenide surface of the sandwiched formation being non-charged and inert, helping to make the bulk crystal highly stable in environmental conditions including under photoelectrochemical operation in aqueous and organic solutions. In addition, the crystal structure is stable under illumination as band-gap excitations are based on nonbonding orbital transitions which do not perturb the crystal structure.6 These properties, together with the aforementioned band-gap energy and absorption coefficient made bulk single crystal TMDs a common subject of investigation as photovoltaic and photoelectrochemical devices from the 1970s to the 1990s. Indeed, for photovoltaic devices a power conversion efficiency of 7.5% under standard conditions was reported with a Au/pWSe2/ZnO device structure.7 While the same TMD was shown to convert 14% of the power of red light irradiation (150 mW cm–2) in a rechargeable photoelectrochemical configuration with the I-/I3- redox couple.8 These single crystal-based devices and others published during this period had active areas in the millimeter-squared size range. However, since single crystals semiconducting TMDs were deemed not technically or economically feasible for large area applications, interest shifted to more feasible polycrystalline samples.9,10 Unfortunately, attempts to make devices with polycrystalline TMD films resulted in only minor success, as the recombination of photo-generated carriers limited device performance.4,11,12 Much effort was accordingly placed in understanding the role of defect states on the poor device performance.


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Metallic defects including edges resulting from crystal termination on any plane besides the inert chalcogenide (001) crystal planes, grain boundaries and stacking faults were found to cause, device shunting, Fermi level pinning, and, of course, photo-generated carrier recombination.13-15 To enable efficient solar energy conversion with practical (non-single crystal) TMD films, charge extraction techniques need to be leveraged to favor photo-generated charge harvesting over recombination. Possible strategies include methods to increase diffusion length through defect passivation,16 or band structure engineering.17 Alternatively, decreasing the charge transport distance between the points of free carrier generation and collection will reduce the transport time and can also reduce recombination in TMD materials. This latter possibility can be accomplished by nano-structuring the TMD. Given the crystal structure of TMDs and the corresponding weak van-der-Waals (vdW) interactions between the chalcogenide layers perpendicular to the crystal c-axis, thinning the material in this direction is readily accomplished. Moreover since breaking the vdW interactions does not induce defects or surface states due to the unique layered crystal structure of TMDs, thinning the material by layer exfoliation is an obvious choice for nanostructuring. Indeed, the exfoliation of bulk TMD crystals into single or few atomic layer flakes or sheets with thickness ranging from sub-nanometer to a few nanometers but with lateral dimensions ranging from tens of nanometers to millimeters (See Figure 1b) is readily accomplished. A common and facile method to exfoliate TMDs is by simple mechanical means (e.g. the scotch tape method18). Indeed, the investigation of the optoelectronic properties of mechanically exfoliated 2D TMD layers has revealed unique optical and electrical properties which have been summarized in a number of recent review articles.19-22 Of particular interest for solar energy conversion, it has been shown that single or few layer TMDs exhibit superior light harvesting characteristics compared to their bulk counterparts, due to strong light-


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matter interactions (from dipole-allowed localized d-state transitions)23 while also alleviating the disadvantageous anisotropic electronic properties intrinsic to their bulk counterparts (i.e. charge carrier transport across the layers three orders of magnitude slower than the transport along the layer plane24). For example, a single layer of TMD was estimated to exhibit two orders magnitude higher sunlight absorption than the most commonly used semiconductors for solar energy conversion (Si and GaAs). Thus, a single TMD monolayer with sub-nanometer thickness could potentially generate 4.5 mA cm–2 photocurrent under standard illumination conditions, which is equivalent to the photocurrent produced by 50 nm thick Si or 15 nm GaAs.25 Moreover, given the inertness of the chalcogenide surface, atomic layer heterojunctions can be constructed without lattice strain that can lead to defects and recombination.26 Indeed, photoactive p-n WSe2/MoS2 heterojunctions have been demonstrated with TMD monolayers that were mechanically exfoliated from bulk crystals and stacked on top of each other.27 Most recently a MoTe2/MoS2 p-n heterojunction (see device structure schematic in Figure 2a and optical image of actual device in Figure 2b) demonstrated photovoltaic performance with an open circuit photovoltage up to 300 mV with 800 nm illumination (Figure 2c) and an external quantum efficiency (EQE) of up to 85% under short circuit conditions with blue light, which was higher than a Si diode at the same illumination wavelength (Figure 2d).28 Overall, these initial device demonstrations on 2D TMDs are encouraging and provide a path for the direct use of these TMD-based photovoltaics in nano-scale electronics (e.g. highfrequency rectification and photovoltaic-switching elements). Despite this, the active areas demonstrated in these mechanically-exfoliated flake based-devices are typically in the 100 µm2 range. Indeed, to enable solar energy conversion on a globally relevant scale with semiconducting TMD materials, methods are needed to prepare the attractive ultra-thin 2D layers


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over large areas in technically facile and economically feasible approaches. Next, we discuss relevant routes to achieve large area semiconducting 2D TMD thin films. Similar to other thin film PV technologies (CdTe, amorphous silicon, CIGS, etc…) physical or chemical vapor deposition techniques (e.g. sputtering or CVD) can be employed to fabricate thin film TMD layers. The most obvious benefits to preparing TMDs using these techniques are the ability to fabricate large-area films with precise thickness and composition control. Common CVD methods use MoO3 and S precursors29,30 at 600-700°C to deposit on insulating (e.g. sapphire) substrates or to simply expose Mo coated SiO2 substrates to sulfur vapor at similar temperatures to produce MoS2 with suitable semiconducting properties for use in schottkyjunction PV cells.31 H2S has also been used as a sulfur source to prepare large-area MoS2 with good semiconducting properties.32 Overall the drawbacks of these methods are the high temperature used and the need to transfer the prepared TMD layers to a conductive substrate for application in electronic devices. Sputtering has also been employed to prepare large-area films of TMDs, and while sputtered films can potentially be prepared directly on conductive substrates, poor photoactivity has limited this application (e.g. for WS2 photovolatics33). Despite the drawbacks of the sputtering and CVD based approaches, we note that advances with the deposition of large-area mono or few-layer films using these techniques continues to be reported in the literature.34-37 While not all of these methods are being used to produce photoactive devices, likely the future will bring advances in the performance of these films for solar energy conversion. In contrast to the vapor-based depositions methods of preparing large area TMD thin films, liquid-based wet-chemical approaches can be used. Solution-processed precursors (e.g. ammonium thiomolybdate, (NH4)2MoS4) can be simply coated onto insulating substrates via dip


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coating and converted into MoS2 thin films (under thermal annealing in the presence of S).38 Optimization of this technique recently afforded photoactive films of MoS2 at lower temperatures (450°C) over wafer-scale areas (Figure 3, top), suitable for deposition on flexible polyimide substrates. However, for photodetectors made with this technique (see Figure 3, bottom) only about 2 µA cm–2 of photocurrent was extractable with 12.5 mW cm–2 illumination.39 Further examination of the crystal structure, grain boundaries and defects of these films are needed to gain insight into the origin of the recombination. Given the attractive potential for solution-based processing techniques to offer extremely lowcost deposition suitable for rapid roll-to-roll large area film formation, many other approaches have been investigated. Nano-structured 2D TMDs can be directly prepared on conductive substrates using a hydrothermal method at 220°C,40 or 2D nanoflakes dispersed in solution can be prepared by hot injection methods,41 electrochemical exfoliation,42 or by simply exfoliating bulk powders in liquid with chemical agents43 or with ultrasonication/shearing forces.44,45 The “chemical” exfoliation methods have the benefit of a higher yield of large (typically over 1 µm in lateral dimension) monolayer flakes. Surface defects created during chemical exfoliation also allow for the attachment of surfactants which can stabilize the dispersion of the 2D flakes in solution. As a drawback, this method also transforms the atomic arrangement of the TMD into the 1T crystal form, destroying the semiconducting properties of the material.46 The 2H form can be mostly restored through post annealing treatment, however, and modest photoactivities of thin film devices prepared from this route have been measured.47,48 In contrast to chemical exfoliation, the shearing or sonication-based methods, termed “solvent assisted” exfoliation preserve the 2H crystal structure and the semiconducting properties of the 2D flakes. The flake size typically ranges from a few tens to hundreds of nanometers in lateral dimension with this


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approach and the yield of monolayer flakes is much lower compared to chemical exfoliation. Importantly, the conditions used for solvent assisted exfoliation leave the vdW surfaces better intact compared to the chemical approach. This creates a challenge with the attachment of surfactants that could prevent the rapid aggregation and precipitation of the 2D flakes. Recently this challenge has been addressed through the attachment of metal acetate salts49 which were shown to improve the dispersibility of the 2H-TMD flakes in a variety of common laboratory solvents. In addition our group has investigated the effects of different chain lengths on alkyltrichlorosilane surfactants on the dispersibility and electronic properties of solvent exfoliated 2H MoS2.50 Despite the ability to increase the stability of the dispersion while still retaining the semiconducting properties of the flakes, restacking/aggregation of the flakes was still observed during the processing of the dispersions into thin films for optoelectronic investigations. Indeed, while attempts have been made to integrate solvent-exfoliated TMDs into devices for solar energy conversion,51-54 the resulting thin films are typically formed of large aggregates of TMDs, which is a morphology far away from the small-area monolayer or few-layer flake films used to demonstrate the promising solar energy conversion devices on the small-area mechanically exfoliated 2D flakes discussed above. Our group has recently developed a self-assembly technique to overcome the challenge of controlled film formation with dispersions of semiconducting 2D TMD materials. The space confined self-assembly (SCSA) technique55 (Figure 4a) uses a liquid-liquid interface to confine the flakes and prevent their restacking during film formation. The mild conditions also allow thin film formation on flexible plastic substrates (Figure 4b) and could reasonably be scaled to arbitrary size using roll-to-roll type processing. While the film formed using this technique does not form a continuous atomic layer of TMDs consisting rather of 2D flakes in an edge-to-edge


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configuration (Figure 4c), charge transfer in the lateral direction could be measured, suggesting that the edges of the 2D flakes are in electrical contact. Moreover the SCSA method affords TMD nanoflake films with superior contact to the conductive substrate even compared to a traditional Langmuir-Blodgett “aggregate compacted” (AC) type of deposition (see Figures 4d and e for cross sectional SEM comparison of the two methods). Indeed, this morphology difference was found to be important for extracting photogenerated charges from semiconducting WSe2 films for solar hydrogen production (Figure 4f). While large capacitive photocurrent transients with stable current densities only in the tens of µA cm–2 range were observed for films prepared with the AC method, the SCSA method gave films with stable photocurrents on the order of 1 mA cm–2. Overall the simplicity and scalability of the semiconducting TMD film formation with the solvent-exfoliation plus space-confined self-assembly approach gives promise to the prospect of ultra-low cost solar energy conversion with TMDs. However, it should be noted that the photocurrent density produced by WSe2 films prepared by the SCSA technique was significantly lower than single crystal photoelectrodes, which gave about 20 mA cm–2 under similar measurement conditions.56 While this is partially due to the smaller light absorption of the nano-flake films, the internal quantum efficiencies of the SCSA film (ca. 10%) remains much lower than the single crystal case (ca. 60%) for this application suggesting that further development is needed to bring solution-processed large-area semiconducting TMD films up to state-of-the-art conversion efficiency. To enable high performance, large area and low cost semiconducting TMD-based devices, much fundamental understanding still needs to be gained and engineering advances also need to be accomplished. Firstly, while perhaps not economically feasible for global-scale application, the continuing development of CVD-based approaches for large-area films will afford insights into


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the material factors that govern performance, given their ability to produce high-quality defect free mono- or few-layer single crystals over the centimeter length scale. Specifically, wafer scale CVD films will give insights into how grain boundaries and defects affect optical and electrical properties in large-area devices.57 The precise thickness control of CVD-based approaches may also facilitate the study of the effects of exciton binding energy and trion formation over large area 2D films. In general increasing the fundamental understanding of solvent exfoliated 2D TMD materials will also be necessary for TMDs processed in this way to obtain commercially-relevant performance. However, current understanding lags behind that of the mechanically exfoliated or CVD-based materials. Fundamental insights into the nature of the defects present and their density is a primary concern. Indeed while solvent exfoliated TMDs are sometimes claimed to be defect free, the drastically lower performance in photoactive devices and charge carrier mobility44 point to the importance of charge carrier recombination at trap sites. Most obviously, exfoliation induces a significant amount of edges, and elucidations into the electronic nature of the edge sites are still needed. Some recent understanding of the roles of steps and defects on terraces in TMDs used for photoelectrochemical devices has indeed encouragingly suggested that their presence, even in large concentration, should not necessarily affect device performance to a large degree.58 Basic understanding of the photo-induced charge carrier dynamics by transient absorption/luminescence spectroscopy,59 and terahertz spectroscopy60 will also be important to understand the role of defects, exiton/trion formation, and the roles of heterojunctions and substrate interfaces in solvent-exfoliated TMDs. Calculation-based approaches will also be critical in the understanding of these defects and the electronic structure in general in TMD materials.25,61 Moreover, calculation-based research can help to identify new


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promising TMD materials62,63 and heterojunction structures64 for future development. For instance, the local density and green function approximations (LDA and G0W0) were recently used in density functional theory to catalog dozens of promising 2D TMDs based on their bandgaps and energy levels (see Figure 5). Future synthetic efforts to prepare these materials and their fundamental optoelectronic investigation will be invaluable to advancing the field of TMDs for solar energy conversion. From an engineering standpoint, a host of possible routes toward the improvement of solution processed TMD thin films remain to be more fully explored. Simply varying the flake size via post-exfoliation techniques like force-controlled centrifugation,65 or density gradient ultracentrifugation,66,67 can give insight into the role of the concentration of edge defects and potentially improve charge carrier harvesting. Specific tools for defect engineering like selective etching,16 passivation by chemical methods (e.g. sulfurization, surfactant binding),68,69 and systematic studies of tuning the electronic structure by surface modification49,70,71 will also likely lead to higher internal quantum efficiency in solar energy conversion devices. Moreover engineering charge selective blocking layers, optical spacers, and optimizing the device electrodes similar to demonstrations with organic photovoltaics,72 will likely provide additional opportunities to increase device efficiency. Overall if the currently-observed relatively low performance of large-area TMD-based devices can be overcome through further fundamental understanding and material/interface engineering, ultrathin devices can attain high internal quantum efficiency and reasonable overall performance suitable for applications where stability and good transparency are important. For example building integrated photovoltaics73 provide an evident application. In this case, based on the attractive optoelectronic properties of a monolayer TMD, ubiquitous low-cost smart windows


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with almost complete transparency and a power conversion efficiency of ca. 5-10% using graphene as a charge collecting electrode74 is not an unreasonable goal. Moreover, for higher conversion efficiency, but non-transparent applications the external efficiency can likely be improved by light-trapping techniques such as plasmon-enhanced absorption, strain engineering, and/or vertical stacking.75 The intrinsic stability of TMD materials, based on their atomic structure, makes them also promising for application in harsh environments like the direct solarto-chemical energy conversion in emerging photoelectrochemical and photocatalytic-based applications.3 Indeed, single crystal WSe2 based photocathodes for solar H2 production have already been established as uniquely stable without protection layers with performance consistent with a solar-to-chemical energy conversion of ca. 15% in a tandem cell configuration with a photoanode for water oxidation.56 The previously-mentioned calculation-based predictions may also be useful for this application. Indeed the materials shown in Figure 5 are shown with respect to the water reduction and oxidation potentials, and suggests that 1T-NiSe2 and ZrSe2 could be new materials for investigation. With all of these attractive potential applications there is certainly an encouraging future for low-cost large area TMD-based solar energy conversion devices.


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Figure 1. (a) The ball-and-stick crystal structure of generic TMD material in the 2H crystal structure illustrates how the transition metal atoms (light blue) are bonded with the chalcogenide atoms (dark yellow) creating layers where only vdW forces between the layers. (b). Transmission electron micrograph image of solvent-exfoliated MoS2 few layer nanoflake. The number of layers of each part of the flake are indicated (reprinted from Chem. Mater. 2014, 26, 5892-5899)


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Figure 2. Schematic 3D view of (a) a Mo-based p–n diode comprised of n-MoS2 and p-MoTe2 nanoflakes on glass. b) Optical microscopy (OM) images of a p–n diode on a glass substrate. (Black and red dashed lines indicate MoTe2 and MoS2 nanoflake, respectively). Direct imprinting was used to realize the MoS2 /MoTe2 heterojunction p–n diode. c) Photoinduced I–V curve of heterojunction p–n diode on glass under dark, and IR (800 nm) illuminations on a linear scale. d) External quantum efficiency (EQE) characteristics of the p–n junction diode under RGB LEDs and IR (800 nm) illumination, obtained at zero volt as a function of illumination wavelength. For comparison, the EQE of a typical Si photodiode is added (purple line). (Reproduced with permission from A. Pezeshki, S. H. H. Shokouh, T. Nazari, K. Oh, S. Im. Adv. Mater. 2016, 28, 3216-3222. copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)


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Figure 3. (top) A photograph of homogeneous MoS2-based visible-light photodetector arrays on a 4 inch SiO2/Si wafer. (bottom) Time-dependent photocurrents of a photodetector device (channel length and width of 20 and 250 µm, respectively) at V = 20 V for different illumination light powers. (Reproduced with permission from Y. R. Lim, W. Song, J. K. Han, Y. B. Lee, S. J. Kim, S. Myung, S. S. Lee, K.-S. An, C.-J. Choi, J. Lim. Adv. Mater. 2016, DOI: 10.1002/adma.201600606 copyright 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)


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Figure 4. The space-confined self-assembly approach for 2D TMD thin film deposition and results with WSe2. a) shows a schematic of the deposition method (HEX = hexane, EG = ethylene glycol). Step 1: Injection of WSe2 dispersion, Step 2: Flake confinement and selfassembly, Step 3: Hexane removal, Step 4: Ethylene glycol removal and film deposition, Step 5: Drying at 150°C. b) displays a photograph of a single-flake-layer WSe2 thin film deposited on flexible Sn:In2O3 (ITO) coated PET plastic and c) shows a representative TEM image of a single-flake-layer WSe2 layer SCSA thin film deposited on a carbon-coated TEM grid. Panels (d) and (e) show cross-sectional SEM images for the SCSA and the (aggregated-compacted) AC film, respectively. The scale bars are 400 nm. f) Photoelectrochemical characterization of the WSe2 thin films prepared by the SCSA and AC techniques under linear scanning voltammetry (LSV) with intermittent illumination (1 sun, 100 mW cm–2) after Pt deposition. A horizontal axis corresponding to zero current is indicated for each curve. (adapted from X. Yu, M. S. Prevot, N. Guijarro, K. Sivula. Nat. Commun. 2015, 6, 7596)


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Figure 5. Position of the valence band maximum and conduction band minimum relative to the vacuum level (set to zero) for LDA and G0W0 approximations in DFT calculated monolayer TMDs in the 1T and 2H crystal arrangement. In both cases, spin−orbit splitting of the bands has been taken into account. The hydrogen and oxygen evolution potentials at pH 7 are shown by green dashed lines. (adapted from F. A. Rasmussen, K. S. Thygesen. J. Phys. Chem. C 2015, 119, 13169-13183). AUTHOR INFORMATION *[email protected] http://limno.epfl.ch/ Notes The authors declare no competing financial interest. AUTHOR BIOGRAPHIES Xiaoyun Yu received her master degree from Sun Yat-sen University in China, supervised by Prof. Daibin Kuang. She is now a PhD candidate under supervision of Prof. Kevin Sivula in


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EPFL, Switzerland. Her research currently focuses on 2D thin film processing and chalcogenide material applications in solar energy conversion. Kevin Sivula obtained a BChE from the University of Minnesota followed by a PhD in Chemical Engineering from the University of California, Berkeley under the supervision of Prof. Jean Fréchet. He is currently a Tenure-Track Assistant Professor in the Institute of Chemical Sciences and Engineering at EPFL.

ACKNOWLEDGMENT The authors acknowledge the School of Basic Sciences at EPFL for financial support.

REFERENCES (1) Jean, J.; Brown, P. R.; Jaffe, R. L.; Buonassisi, T.; Bulovic, V. Pathways for Solar Photovoltaics. Energy Environ. Sci. 2015, 8, 1200-1219. (2) Polman, A.; Knight, M.; Garnett, E. C.; Ehrler, B.; Sinke, W. C. Photovoltaic Materials: Present Efficiencies and Future Challenges. Science 2016, 352, aad4424. (3) Sivula, K.; van de Krol, R. Semiconducting Materials for Photoelectrochemical Energy Conversion. Nat. Rev. Mater. 2016, 1, 15010. (4) Aruchamy, A. Photoelectrochemistry and Photovoltaics of Layered Semiconductors, Springer; 1992. ISBN-10: 0792315561


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Toward Large-Area Solar Energy Conversion with Semiconducting 2D

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