Exciton engineering using atomic-scale defects in stacked heterostructures
Van der Waals based solar cells have recently been assembled by stacking two-dimensional semiconductors molybdenium disulfide MoS2) and tungsten diselenide (WSe2) on top of each other [Figure]. A p-n contact forms between the two adjacent semiconducting layers . Many-body states such as trions and excitons are key for understanding the complex physics upon illumination. Interface properties prove quite sensitive to the local environment [Figure], and can strongly be modified by finite twist angles , strain or defects .
© Florian Libisch
Left: Van-der-Waals based solar cell, with excitons forming at the interface between the top
and bottom layer. Right: Photocurrent of a solar cell, as a function of gate and bias voltage 
Upon illumination of semiconducting heterobilayers, a photocurrent is created by optical excitation of electron-hole pairs at the p-n junction, followed by separation of electrons and holes into different layers. Details of this separation process, and the subsequent charge extraction, are still unclear, but key to improve device efficiency. We aim to understand and exploit the role of defects and strain modulations. This requires a complete microscopic picture of exciton formation and separation at the p-n junction of the heterostructure, to engineer a more efficient solar cell. The relative alignment angle of the different layers φ yields another tunable degree of freedom, that can strongly influence the electronic structure and consequently the properties of excitons.
We will use our tight-binding approach for the individual TMD layers, parametrized using density functional theory. Using a Poisson solver, we account for the gate-dependent potential throughout the heterostructure. We have recently used tight-binding eigenstates with a Bethe-Salpeter like equation to describe defect-bound excitons in WSe2 . We will extend this approach to multi-layered heterostructures, to investigate intra-layer and inter-layer excitons, and their sensitivity to defects. We investigate moiré effects for different twist angles using effective potentials parametrized from density functional theory, similar to our approach for graphene on hexagonal boron nitride or MoS2 . Embedding approaches allow replacing substrates and contacts with effective potentials.
The groups of Grasser and Filipovic will provide electrostatic device simulations and simulations of charge transport in MoS2 films, respectively, for a quantitative treatment of gate contacts and the dielectric environment. The Mueller group will create and characterize optical devices. Together with the Madsen group, we will transfer ab-initio defect descriptions to tight binding. We will compare our relaxed geometries with measurements from the Kotakoski group.
Florian Libisch is a theoretical condensed matter physicist. His research focuses on the simulation of realistic nanodevices including strain, edges, defects, and substrate interactions. Quantum transport is described by a tailored Green’s function approach, enabling the simulation of large-scale structures (up to 1μm2), and direct quantitative comparison to STM measurements, phonon renormalization in moiré superstructures or photoluminescence of single photon emitters. Embedding approaches allow combining different levels of theory within one simulation, enabling the high-level modeling of local excitations.
- M. M. Furchi, A. Pospischil, F. Libisch, J. Burgdörfer und T. Müller. Photovoltaic Effect in an Electrically Tunable van der Waals Heterojunction. Nano Letters 14, 4785-4791 (2014). DOI: 10.1021/nl501962c.
- J. Quan, L. Linhart, M.-L. Lin, D. Lee, J. Zhu, C.-Y. Wang, W.-T. Hsu, J. Choi, J. Embley, C. Young, T. Taniguchi, K. Watanabe, C.-K. Shih, K. Lai, A. H. MacDonald, P.-H. Tan, F. Libisch, und X. Li. Phonon renormalization in reconstructed MoS2 moiré superlattices. Nature Materials 20, 1100-1105 (2021), DOI: 10.1038/s41563-021-00960-1.
- L. Linhart, M. Paur, V. Smejkal, J. Burgdörfer, T. Müller, and F. Libisch. Localized Intervalley Defect Excitons as Single-Photon Emitters in WSe2. Physical Review Letters 123, 146401 (2019). DOI: 10.1103/physrevlett.123.146401.