by Allison Kerley
Bernardi et al. (2013) investigated the absorbance of graphene and three different monolayer transition metal dichalcogenides (TMDs)—MoS2, MoSe2, and WS2—alone and in various combinations as the active layer in ultrathin photovoltaic (PV) devices. In calculating the upper limits of the electrical current density (measured in mA/cm2), each material can contribute to the total absorption of a device. The authors found that subnanometer thick graphene and TMD monolayers can absorb the equivalent short-circuit currents of 2–4.25 mA/cm2, while 1 nm thick Si, GaAs, and P3HT (commonly used materials in current PV devices) were found to generate currents between 0.1–0.3 mA/cm2. Further testing suggested that the high absorption of the monolayer MoS2 is due in part to the dipole transitions between localized d orbitals (which were estimated to contribute 2–5% of the total visual absorption) and escitonic coupling of dipole transitions (which were estimated to contribute 5–10% of the total visual absorption). The authors also compared the absorption of TMD monolayers and graphene against their bulk counterparts, bulk TMDs and graphite respectively. Both grapheme and MoS2 monolayers were found to have absorption values higher than their bulk counterparts by a factor of 2–3. To further examine the applicability of subnanometer thick TMDs and graphene as the active layer in PV devices, the authors examined the performances of a hypothetical device with a bilayer composed of two stacked monolayers of MoS2/graphene as the active layer. Their calculations found the maximum short-circuit current to be 4.3 mA/cm2 and the maximum open circuit voltage to be approximately 0.3 eV. They also examined the feasibility and performance of a hypothetical 1 nm thick PV device with an active layer composed of a monolayer of MoS2 stacked on a monolayer of WS2, and found the maximum short-circuit current to be approximately 3.5 mA/cm2 and the maximum open circuit voltage to be approximately 1 V. The authors found that a single TMD monolayer with a subnanometer thickness can absorb as much sunlight as 50 nm of the commonly used Si.
The authors note that while it is not, strictly speaking, possible to “define a macroscopic absorption coefficient in the layer-normal direction for a single layer of MoS2. By definition this quantity should be averaged over several unit cells of the material”, so they calculated the equivalent absorption coefficient α for a monolayer of MoS2 using the equation where A is the absorbance and is the layer thickness in Å. The monolayer MoS2 was found to have an equivalent absorbance of 1–1.5×106 cm –1 and graphene was found to have an absorbance of 0.7×106 cm –1, while bulk MoS2 was found to have an absorbance of approximately 0.1–0.6×106 cm –1 and graphite was found to have an absorbance of approximately 0.2–0.4×106 cm –1.
The authors found that in order for a bilayer of MoS2/graphene to work, as graphene is a (semi)metal and MoS2 a semiconductor, a Schottky barrier must be formed. Using Density Functional Theory (DFT), the authors computed the workfunction value to be eV for a monolayer of MoS2. In addition, the authors note that the maximum short-circuit current for the MoS2/graphene layer (4.3 mA/cm2) differs slightly from the sum of monolayer currents found for graphene and MoS2 individually (5.9 mA/cm2) and attribute the difference to the use of different theories used to compute the individual and combined currents. The Bethe-Salpeter equation (BSE) was used to calculate the currents for the individual monolayers and independent-particle theory was used to calculate the current for the stacked MoS2/graphene layer.
The authors found that a bilayer MoS2/WS2 interface can realize PV operation by the use of a type-II heterojunction. In addition they predict that due to the interaction between the two TMD monolayers, there would be significant changes in the bandstructure and absorption spectrum of the MoS2/WS2 interface as compared to their individual monolayers.
The authors emphasize that although power density is not a conventional unit for judging the effectiveness of PV devices, they it is crucial to understanding the limits in solar cells with the smallest possible thickness, as well as to estimating the energy achievable from a unit volume or weight of an active layer material.
Bernardi, M., Palummo, M., Grossman, J., 2013. Extroardinary sunlight absorption and one nanometer thick photovoltaics using two-dimensional monolayer materials. Nano Letters 13, 3664–3670. Full paper at: http://bit.ly/1lsCzgO