# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 6 Photovoltaics  
Photovoltaic cells are used to convert solar energy into electricity. Solar cells that rely on inorganic or organic semiconductors have been developed over the last decades. Recently, TMDs are reconsidered as alternative for photovoltaics because of their band gap in the visible part of the electromagnetic spectrum and strong optical absorption [91]. Moreover, TMDs are interesting from a technological point of view, as they are chemically stable, environmentally sustainable and can potentially be produced at low cost. Calculations of graphene/MoS\({}_{2}\)[92] and WS\({}_{2}\)/MoS\({}_{2}\)[93] monolayer heterojunction solar cells predict a maximum power conversion efficiency of \(\sim\)1%, limited by the, in absolute numbers, low optical absorption of the atomically thin materials. Absorption enhancement is therefore necessary and can be achieved by stacking of 2D materials, using few-layer sheets or exploiting plasmonic effects [94]. The photovoltaic properties of various bulk and thin film TMD crystals have been studied since the 1980s [95]. Following the demonstration of monolayer graphene and TMD crystals, photovoltaic  
Figure 7: **(a)** Photoluminescence (PL) spectrum of localized emitters showing narrow emission lines. The inset shows a high-resolution spectrum of the brightest line in the main panel. (**b**) PL intensity map of the narrow emission lines centered around 1.719 eV. The dashed line indicates the monolayer. (c) Second order correlation measurement of the PL of a localized emitter showing photon antibunching. (**a–c**) reproduced with permission from [86], Copyright Nature Publishing Group, 2015.  
properties of ultrathin layers have been investigated. An electrostatically induced p-n junction diode device (compare the device in Figure 5a) shows photovoltaic response when operated in diode configuration [34; 35]. Power conversion efficiency \(\eta=P_{in}/P_{el}\) (\(P_{in}\) incident light power; \(P_{el}\), electrical output power) of the solar cells was found to be on the order of 0.5% limited by the low absorption in the monolayer and recombination of charge carriers. In a similar device structure, [PERSON] _et al._[96] demonstrated \(\eta>\) 14% and fill factors of ~70% at AM-1.5 spectral irradiation, using multi-layer MoSe\({}_{2}\). Similar results were obtained by other groups [97] and for a MoS\({}_{2}\)/black phosphorus solar cell [98]. Schottky junctions can also create lateral electric fields. [PERSON] _et al._[38] studied MoS\({}_{2}\) field-effect devices with two different contact metals. Due to different work functions in gold and palladium, photogenerated electron-hole pairs are separated by the built-in field at the contact region and a give rise to a photovoltaic effect. Lateral electrode arrangement prevents easy scalability of TMD photovoltaic cells, and it is therefore desirable to fabricate vertical junctions. Stacking single or few-layer sheets of TMD crystals on top of each other results in p-n junction heterostacks, held together by van der Waals forces. A photovoltaic response in such devices was reported by [PERSON] _et al._[36], [PERSON] _et al._[37], [PERSON] _et al._[99] and [PERSON] _et al._[100]. The device forms a p-n junction at the interface between two TMD monolayers (_i.e._, MoS\({}_{2}\) and WSe\({}_{2}\)), as depicted in Figure 8a. Due to different electron affinities of both materials, a type-II heterojunction is formed, meaning that the lowest energy electron and highest energy hole states are located in the MoS\({}_{2}\) and WSe\({}_{2}\) layers, respectively. Electrical measurements, while increasing the illumination power from 180 W/m\({}^{2}\) to 6400 W/m\({}^{2}\), showed the shift of the \(I\)-\(V\) curve, and a maximum electrical power of \(P_{el}\sim\) 15 pW was extracted from the solar cell (depicted in the main panel and inset of Figure 8b). As the doping concentration in the monolayer sheets is determined by extrinsic factors, a gate voltage is needed to tune the stack to a regime in which a vertical p-n junction is formed.