Session:

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) MX₂such as MoS₂, WSe₂, and NbSe_2,etc. show strong light-matter interaction which results in very high absorptance and photogeneration. Due to these excellent optoelectronic properties, TMDCs are being used for various applications e.g., transistors, memory devices, photovoltaics etc. In particular, TMDCs are promising as candidates for flexible and ultra-light photovoltaics. A single MoS₂ monolayer has a thickness of 0.65 nm and can absorb up to 10% of light at wavelengths in the visible and IR spectrum. Apart from high absorption, monolayer MoS₂ has a direct bandgap of 1.85 eV.
Due to the quantum nature of 2D MoS₂, the monolayer-like optoelectronic properties cannot be achieved from films synthesized with multiple layers. To maintain monolayer-like properties for multi-layer films, our strategy is to stack individual MoS₂ monolayers. These monolayers are synthesized on a sapphire substrate using chemical vapor deposition (CVD) with S and MoO₃ precursors carried by Ar gas at 750^oC. After achieving high-quality uniform monolayer MoS₂ films, optoelectronic properties were measured. The absorption spectra of a monolayer MoS₂ shows three peaks A, B, and C at 652 nm, 616 nm, and 431 nm respectively with a C peak absorption of 17.5 %. The absorption spectra were then compared with the computed absorption of the monolayer MoS₂ using measured n and k and the transfer matrix method (TMM). The relative error between model and experiment is <5%. Raman scattering for the monolayer shows E¹_2G and A_1G peaks at 383.5cm^-1 and 405 cm^-1 respectively. The normalized PL intensity of as-grown monolayer MoS₂ shows the peak at 670 nm.
A surface-energy-assisted transfer technique was then used for the stacking of individual monolayers from CVD-grown films. The obtained C-peak of absorption spectra for bi-layer, 3-layers, and 4-layers of stacked MoS₂ is 31%, 41%, and 49% respectively which is in accordance with the computed results using TMM. Raman scattering for bi-layer, 3-layers, and 4-layers of stacked MoS₂ shows peaks at 383.5cm^-1 and 405 cm^-1 like the monolayer scattering, as opposed to the well-known Raman peak shift in multi-layer MoS₂. Meanwhile the PL of the stacked films show a peak at 670 nm but with reduced intensity.
Schottky PV contacts were then fabricated using Ti and Pt on a SiO₂/Si substrate as an asymmetric work function interlocking-finger type device onto which the MoS₂ monolayer film is transferred. A band offset of -0.9 eV arises between the Fermi levels of Ti and MoS₂ at their interface while an offset of 0.41 eV arises at the Pt-MoS₂ interface. Efficiency of 0.000018% for a monolayer has been achieved in preliminary devices, with short-circuit current density (J_sc) of 0.0006 mA/cm²under 1 sun AM1.5D illumination. With much improved electronic transport, we project one monolayer to achieve J_sc as high as 1.49 mA/cm² in AM0 conditions, based on measured photon absorption. By taking advantage of our demonstrated ability to stack layers while maintaining monolayer-like optical properties, using TMM we project that with 120 layers, the J_sc can be improved to 11 mA/cm² for a 2D MoS₂-based PV device.
Currently, we are focused on conserving the PL of the stacked monolayers to maintain high electronic quality in these 2D material stacks. For that purpose, we are optimizing the transfer process by applying different dry transfer techniques e.g., stamping methods, thermal release transfer (TRT), as well as improved wet transfer techniques. In addition to that, we are also working to measure the carrier concentration and mobility for monolayer and stacked monolayer structures using the Van der Pauw method. The results of this stacked monolayer 2D film analysis will be implemented in our COMSOL Multiphysics model of multiple monolayer Schottky 2D PV. The monolayer films will then be stacked on the fabricated Schottky PV devices to enhance their efficiency.

Co-Author(s): Timothy Ismael (Tulane), Noah Hill (The College of Wooster), Claire Luthy (Tulane), Matthew Escarra (Tulane)

Muhammad Aamir Abbas

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