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