The liquid-phase exfoliation of semiconducting transition metal dichalcogenide (TMD) powders into 2D nanosheets represents a promising route toward the scalable production of ultrathin high-performance optoelectronic devices. However, the harsh conditions required negatively affect the semiconducting properties, leading to poor device performance. Herein we demonstrate a gentle exfoliation method employing standard bulk MoS 2 powder (pressed into pellets) together with the electrochemical intercalation of a quaternary alkyl ammonium. The resulting nanosheets are produced in high yield (32%) and consist primarily of mono-, bi-, triatomic layers with large lateral dimensions (>1 μm), while retaining the semiconducting polymorph. Exceptional optoelectronic performance of nanosheet thin-films is observed, such as enhanced photoluminescence, charge carrier mobility (up to 0.2 cm 2 V −1 s −1 in a multisheet device), and photon-to-current efficiency while maintaining high transparency (>80%). Specifically, as a photoanode for iodide oxidation, an internal quantum efficiency up to 90% (at +0.3 V vs Pt) is achieved (compared to only 12% for MoS 2 nanosheets produced via ultrasonication). Further using a combination of fluorescence microscopy and high-resolution scanning transmission electron microscopy (STEM), we show that our gently exfoliated nanosheets possess a defect density (2.33 × 10 13 cm −2 ) comparable to monolayer MoS 2 prepared by vacuum-based techniques and at least three times less than ultrasonicated MoS 2 nanoflakes. Finally, we expand this method toward other TMDs (WS 2 , WSe 2 ) to demonstrate its versatility toward high-performance and fully scalable van der Waals heterojunction devices.

Cu 2 O is a model p-type semiconductor for photocathodes in photoelectrochemical (PEC) water splitting cells. However, major challenges remain in controlling its deposition into thin and homogeneous semitransparent films. Herein, we report a new route to construct thin homogeneous Cu 2 O layers on transparent fluorine doped tin oxide (FTO) substrates via the in situ electroconversion of CuSCN to Cu 2 O. We highlight the morphology control of the resulting converted Cu 2 O thin films while demonstrating that they maintain promising performance for solar-driven hydrogen production with a maximum incident photon to current efficiency (IPCE) reaching 60% (at 0 V vs RHE and 450 nm) for a 180 nm thick film and integrated solar photocurrents up to 4 mA cm −2 . In addition, altering the deposition conditions (e.g., applied potential, electrolyte compositions, and pH) gives important insight into the mechanism and operation of the electroconversion process.

Gas diffusion electrodes are essential components of common fuel and electrolysis cells but are typically made from graphitic carbon or metallic materials, which do not allow light transmittance and thus limit the development of gas‐phase based photoelectrochemical devices. Herein, the simple and scalable preparation of F‐doped SnO2 (FTO) coated SiO2 interconnected fiber felt substrates is reported. Using 2–5 µm diameter fibers at a loading of 4 mg cm−2, the resulting substrates have porosity of 90%, roughness factor of 15.8, and Young's Modulus of 0.2 GPa. A 100 nm conformal coating of FTO via atmospheric chemical vapor deposition gives sheet resistivity of 20 ± 3 Ω sq−1 and loss of incident light of 41% at illumination wavelength of 550 nm. The coating of various semiconductors on the substrates is established including Fe2O3 (chemical bath deposition), CuSCN and Cu2O (electrodeposition), and conjugated polymers (dip coating), and liquid‐phase photoelectrochemical performance commensurate with flat FTO substrates is confirmed. Finally, gas phase H2 production is demonstrated with a polymer semiconductor photocathode membrane assembly at 1‐Sun photocurrent density on the order of 1 mA cm−2 and Faradaic efficiency of 40%.

While employing charge transport layers (CTLs) in optoelectronic devices including hybrid perovskite solar cells is essential for high performance, using solution-processed organic semiconductor-based CTLs poses challenges for device fabrication. Cross-linking the organic semiconductor is a viable approach, but cross-linked CTLs require additional development for practical application. Here a facile in situ strategy to prepare a cross-linked electron transport layer (ETL) is demonstrated by employing a semiconducting cross-linker composed of perylenediimide-diazide, together with the fullerene (C60) derivative coded as PCBM. Varying the PCBM ratio the cross-linking conditions affords a tunable ETL that exhibits solvent tolerance, suitable electron mobility (>10–4 cm2 V–1 s–1), and hydrophobicity at cross-linking temperatures as low as 120 °C. In both the n–i–p structure and the p–i–n perovskite solar cells employing our ETL, stable power generation and hysteresis-free performance are achieved. Optimized p–i–n devices with our cross-linked ETL gave power conversion efficiency of over 12% (active area of 1.07 cm2). These results suggest that robust n-type semiconducting films obtained with our cross-linking method are promising as ETLs in practical optoelectronic applications.

The remarkable semiconducting properties of 2D transition metal dicalcogenides (TMDs) are of great interest to researchers hoping to harness such traits for optoelectronic and photoelectrochemical (PEC) applications. 2D MoS2, WSe2, and related materials have been incorporated in ultrathin devices including transistors1, LEDs2, solar cells3, and photoelectrodes4,5. While typical approaches like chemical vapor deposition produce pristine monolayer or few-layer 2D TMD sheets and flakes, these methods are expensive and energy intensive making them unsuitable for large scale implementation. Thus, developing scalable solution processable methods to prepare high-quality 2D TMD materials is crucial. Typical solution-based production approaches have historically yielded TMDs with poor semiconducting behavior. Recently a novel electrointercalation technique was reported by Lin et al. wherein large organic cations are inserted in between MoS2 layers in a bulk single crystal thereby exfoliating it to thin nanoflakes while maintaining benchmark semiconducting performance. Despite this advance, there remains a lack of information concerning the optoelectronic properties of these flakes and their ability to be used in solar energy conversion devices. Herein we use the electrointercalation of large cation tetraheptylammonium bromide (THAB) to exfoliate a variety of bulk TMDs, investigate their optoelectronic properties, and process them into photoelectrodes for solar-to-chemical energy conversion. When compared to alternative solution processing methods such as ultrasonication and shear-mixing exfoliation, these THAB-assisted exfoliated flakes show increased photocurrents, photovoltages, and internal quantum efficiencies. We show that this is, in part, a result of thinner 2D flakes with larger lateral dimensions. Raman, photoluminescence, and X-ray spectroscopy suggest that this technique results in a lower defect density when compared to traditional methods. These results suggest that THAB-intercalated 2D TMDs are promising for scalable solar energy applications. References: 1. Lin, Z.; Liu, Y.; Halim, U.; Ding, M.; Liu, Y.; Wang, Y.; Jia, C.; Chen, P.; Duan, X.; Wang, C.; Song, F.; Li, M.; Wan, C.; Huang, Y.; Duan, X. Nature 2018, 562 (7726), 254–258. 2. Palacios-Berraquero, C.; Barbone, M.; Kara, D. M.; Chen, X.; Goykhman, I.; Yoon, D.; Ott, A. K.; Beitner, J.; Watanabe, K.; Taniguchi, T.; Ferrari, A. C.; Atatüre, M. Nat. Commun. 2016, 7 (1), 12978. 3. Akama, T.; Okita, W.; Nagai, R.; Li, C.; Kaneko, T.; Kato, T. Sci. Rep. 2017, 7 (1), 11967. 4. Wells, R. A.; Johnson, H.; Lhermitte, C. R.; Kinge, S.; Sivula, K. ACS Appl. Nano Mater. 2019, 2 (12), 7705–7712. 5. Yu, X.; Prévot, M. S.; Guijarro, N.; Sivula, K. Nat. Commun. 2015, 6, 7596.