Stacked Janus Solar-Cell devices were constructed using QuantumATK NanoLab GUI by stacking layers of the Janus transition metal dichalcogenide (TMD) MoSSe, with a built-in structural cross-plane (cp) asymmetry. The asymmetry generates a cp dipole built into the 2D layer and this dipole can be stacked by putting these Janus layers on top of each other. This creates a p-n junction which is used to separate generated electrons (e-) and holes (h+) in the bottom and top layers, which are then picked up by graphene electrodes to generate a photocurrent between the layers. Importantly, graphene electrodes do not screen the cp dipole in contrast to metal electrodes.
QuantumATK first-principles DFT and DFT-NEGF approaches in the new Photocurrent Module framework were used to calculate band structure, transmission, transport channels, photocurrent density, and EQE as a function of photon energy for the stacked Janus and 20 nm Si p-n junction solar-cell devices. As shown in the figure above, stacked Janus solar-cell devices generate a larger photocurrent (dominated by a cp transport channel) than the 20-40 thicker silicon device. Interestingly, the device generates current at photon energies below the band gap of the monolayer Janus MoSSe due to the stacking of dipoles. The authors suggest, that one could combine MoSSe Janus layers with silicon thin films in order to improve silicon’s weak absorption of light in the low energy regime. Furthermore, the authors propose to investigate other Janus materials, such as CrSSe and ZrSSe, for potential photovoltaic applications.
All the methods used in the paper are available in the QuantumATK O-2018.06 release and they are described in:
 M. Palsgaard, T. Markussen, T. Gunst, M. Brandbyge, and K. Stokbro, "Efficient First-Principles Calculation of Phonon-Assisted Photocurrent in Large-Scale Solar-Cell Devices", Phys. Rev. App. 10, 014026 (2018).
 M. Palsgaard, T. Gunst, T. Markussen, K. S. Thygesen, and M. Brandbyge, "Stacked Janus Device Concepts: Abrupt pn-Junctions and Cross-Plane Channels", Nano Lett. 18, 7275 (2018).