Strong Photon Absorption in 2-D Material-Based Spiral Photovoltaic Cells
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Strong Photon Absorption in 2-D Material-Based Spiral Photovoltaic Cells Mohammad Hossein Tahersima1 Volker J. Sorger1 1 The George Washington University, Department of Electrical & Computer Engineering, 800 22nd Street NW, Washington, United States of America, 20052 ABSTRACT Atomically thin transition-metal dichalcogenides (TMD) hold promise for making ultrathin-film photovoltaic devices with a combination of excellent photo-absorption and mechanical flexibility. However, reported absorption for photovoltaic cells based on TMD materials is still just a few percent of the incident light due to their sub-wavelength thickness leading to low cell efficiencies. Here we discuss that taking advantage of the mechanical flexibility of two dimensional (2D) materials by rolling their Van der Waal heterostructures such as molybdenum disulfide (MoS2)/graphene (Gr)/hexagonal boron nitride (hBN) to a spiral solar cell, leads to strong light matter interaction allowing for solar absorptions up to 90%. The optical absorption of a 1 µm-long hetero-material spiral cell consisting of the aforementioned hetero stacks is about 50% stronger compared to a planar MoS2 cell of the same thickness; although the volumetric absorbing material ratio is only 6%. We anticipate these results to provide guidance for photonic structures that take advantage of the unique properties of 2D materials in solar energy conversion applications. INTRODUCTION Isolation of 2D materials [1,2] and recent research results in Van der Waals heterostructures [5-10] has provided new opportunities to form complete 2D material systems, made of metallic (graphene), insulating (hBN) and semiconducting (MoS2 or WS2) 2D materials, that enable functional devices with atomic thickness in a wide range of optical applications. The family of Transition Metal Dichalcogenides (TMD) contains four 2D crystals of molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2) that are stable at ambient conditions [3], and semiconducting with optical bandgaps in the range of 1~2 eV. Taking a closer look at monolayer TMDs reveals some exceptional properties relating to interactions with light; quantum confinement in perpendicular direction to the plane of 2D material causes the band gap of MoS2, for instance, to increase from an indirect band gap of 1.2 eV for bulk material to a direct band gap of 1.8 eV for monolayers, which is accompanied by a 104 fold photoluminescence (PL) enhancement [4,15]. Such thickness-dependent bandgap tunability of MoS2 has enabled tunable photo-detectors for absorption in different wavelengths, where single- and double-layer MoS2 absorbs green light, while triple-layer absorbs in the red visible spectrum [11]. Despite being atomically thin, hence visually transparent, TMDs are promising sunlight absorbers originated from their rich Van Hove singularity peaks in their density of states [6]. Because of their atomic thickness, optical and electrical properties of TMDs greatly alter by perturbations such as n
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