Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook

Abstract

Two-dimensional (2D) semiconductors provide a unique opportunity for optoelectronics due to their layered atomic structure, electronic and optical properties. To date, a majority of the application-oriented research in this field has been focused on fieldeffect electronics as well as photodetectors and light emitting diodes. Here we present a perspective on the use of 2D semiconductors for photovoltaic applications. We discuss photonic device designs that enable light trapping in nanometer-thickness absorber layers, and we also outline schemes for efficient carrier transport and collection. We further provide theoretical estimates of efficiency indicating that 2D semiconductors can indeed be competitive with and complementary to conventional photovoltaics, based on favorable energy bandgap, absorption, external radiative efficiency, along with recent experimental demonstrations. Photonic and electronic design of 2D semiconductor photovoltaics represents a new direction for realizing ultrathin, efficient solar cells with applications ranging from conventional power generation to portable and ultralight solar power. *Corresponding author: haa@caltech.eduKeywords: Transition metal dichalcogenides, heterostructures, light-trapping, ShockleyQuessier, nanophotonics, 2D materials Since the isolation of graphene as the first free-standing two-dimensional (2D) material (from graphite), the class of layered 2D materials with weak van der Waals inter-planar bonding has expanded significantly. Two-dimensional materials now span a great diversity of atomic structure and physical properties. Prominent among these are the semiconductor chalcogenides of transition and basic metals (Mo, W, Ga, In, Sn, Re etc.) 1-3 , as well as layered allotropes of other p-block elements of the periodic table such as P, As, Te etc. 4 The availability of atomic layer thickness samples of stable, passivated, and dangling bond free semiconductor materials ushers in a new phase in solid state device design and optoelectronics.1, 5-8 A notable feature of the metal chalcogenide 2D semiconductors is the transition from an indirect bandgap in bulk to direct bandgap (Eg) in monolayer form, resulting in a high photoluminescence quantum yield (PL QY) [9][10] in turn corresponding to high radiative efficiency. This combined with the bandgap ranging from visible to near infrared part of the spectrum (1.1 to 2.0 eV) 1 makes the chalcogenides

Device physics of van der Waals heterojunction solar cells

“…Würfel et al, initially developed for organic solar cells [46]. In brief, carrier accumulation due to poor transport properties leads to a quasi-Fermi level splitting Gd% = 7,& − 7,9 in the electron and hole transport layers that differs from the externally applied voltage:…”

Near-Direct Bandgap WSe2/ReS2 Type-II pn Heterojunction for Enhanced Ultrafast Photodetection and High-Performance Photovoltaics

“…Layered vdW materials have large absorption coefficients in the visible range for mono and few layers (nearly 10 5 cm −1 for 600 nm wavelength). 25 While the absorption is ∼ 10% for monolayers, 26 it is significantly higher at ∼ 40% for thicker flakes (∼ 25 nm). 25 This makes FL ReS2 a more appropriate choice for the top, light facing material in the WSe2/ReS2 pn heterostructure stack.…”

“…25 While the absorption is ∼ 10% for monolayers, 26 it is significantly higher at ∼ 40% for thicker flakes (∼ 25 nm). 25 This makes FL ReS2 a more appropriate choice for the top, light facing material in the WSe2/ReS2 pn heterostructure stack. Further, ultrathin FL TMDs of group-6 (here, WSe2) show excellent modulation of the Fermi level with applied gate voltage, 27 in contrast to FL ReS2.…”

Tuning the Carrier Confinement in GeS/Phosphorene van der Waals Heterostructures

“…To gain more physical insight, the projected band structures and corresponding band decomposed charge densities of the VBM and CBM are also calculated, as shown in Figure S6 Small 2018, 14,1703536 ( Supporting Information) and the insets in Figure 4a. Initially, the type-I-like alignment is shown (pink region).…”

“…[1][2][3][4][5][6] A variety of heterostructures based on graphene, hexagonal boron nitride, transition-metal dichalcogenide (TMD), and so forth, have been extensively studied and shown promising applications in photocatalysis, optoelectronics, field-effect transistors, and many other fields. [7][8][9][10][11][12][13][14][15] Phosphorene, a new and promising 2D semiconducting material, has been successfully fabricated through mechanical or liquid exfoliation from bulk black phosphorus. [16,17] Further producing highquality nanosheets toward practical largescale applications, especially in a scalable fashion, are still on the way.…”