# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 6 Photovoltaics
Charge transfer between individual TMD sheets was found to be ultrafast (<100 fs) and highly efficient (almost 100%) [36; 37; 101; 102; 103]. Figure 8: Photovoltaic cells. (**a**) Schematic device layout of a vertical heterojunction device with lateral contacts. A p-n junction is formed at the interface between an n-doped MoS\({}_{2}\) sheet and a p-doped WSe\({}_{2}\) sheet. Reproduced with permission from [36], Copyright Nature Publishing Group, 2014. (**b**) Electrical (main panel) and optical (inset) characteristics of the solar cell. The incident laser power is increased from 180 W/m\({}^{2}\) (red curve) to 6400 W/m\({}^{2}\) (blue curve). Reproduced with permission from [36]. (**c**) Schematic device layout of a vertical heterojunction with graphene electrodes on bottom and top, which allow for vertical carrier extraction. Reproduced with permission from [37], Copyright Nature Publishing Group, 2014. (**d**) Schematic of the bandstructure. The horizontal arrows indicate exciton dissociation and charge carrier diffusion. Reproduced with permission from [37]. Under bias, a diode-like rectification behavior was observed, although the origin of rectification is different from the mechanism in a traditional 3D semiconductor diode. A forward bias voltage drives electrons (holes) in MoS\({}_{2}\) (WSe\({}_{2}\)) to the area of the heterojunction, where they either recombine or overcome the conduction (valence) band offsets to be injected into the other layer and diffuse towards the contacts. In an ideal diode, diffusion dominates over recombination, but it was found that in atomically thin p-n junctions recombination plays a substantial role [36]. The lack of a depletion region in vertical heterojunctions implies that majority carriers in the individual layers are in close proximity to each other, which results in strong interlayer recombination [36; 100]. The photovoltaic response of such vertical devices under white light illumination can be explained as follows: Absorption of photons creates electron-hole pairs (excitons) in both layers, which then relax to minimum energy, driven by the type-II band offset. Subsequently, the relaxed charge carriers diffuse towards the contacts, resulting in a photocurrent. During this diffusion process, interlayer recombination occurs that reduces the efficiency of the solar cell. Measurements show an efficiency and a fill factor of \(\sim\)0.2% and \(\sim\)50%, respectively, for such heterostructures [36]. Scalability can be achieved by vertical contacts using graphene as electrode material, as schematically depicted in Figure 8c. A schematic band diagram is shown in Figure 8d. Electrodes on the top and bottom of the p-n junction allow for vertical carrier extraction and also increase the overall efficiency of the cell. [PERSON] _et al._[37] report multilayer heterojunctions, which, compared to their monolayer counterparts, show about an order of magnitude higher EQE. Due to the thickness-dependent competition between generation, collection and recombination of charge carriers, further optimization of the layer thickness probably leads to even higher efficiencies. Vertical p-n junction devices using other material combinations, such as GaTe/MoS\({}_{2}\)[104] and organohalide perovskite/WSe\({}_{2}\)[105], the formation of a p-n junction by surface charge induced doping [106], as well as organic/inorganic material junctions [107; 108; 109] have been demonstrated and gave similar results.