# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 8 Conclusions  
We finally compare the performance of TMD optoelectronic devices with today's technologies and discuss potential future applications and remaining challenges. Many applications in optoelectronics (photovoltaic solar cells, display panels, _etc._) require large-area dimensions for which traditional semiconductors--especially III-Vs--are too costly. Alternative technologies, such as e.g., organic semiconductors, are thus currently employed, but these exhibit rather low material quality and suffer from degradation issues. TMDs, on the other hand, are crystals with high material quality and stability. In addition, they are bendable, thus providing opportunities for novel applications, such as wearable devices. The prospects for commercialization will depend on device reliability and means for low-cost and large-volume production. The growth of TMD monolayers has made large progress in recent years [117, 118, 119, 120, 121, 122, 123, 124, 125, 126], but routes to control the chemical doping are still to be devised. Direct growth on plastic substrates does not seem feasible. Therefore, techniques need to be developed to transfer TMDs from the growth substrate to a target substrate on an industrial scale [124]. Besides the aforementioned technological advantages, TMDs also offer new physics. The large exciton binding energy could be exploited for devices that allow the  
Figure 9: **(a)** Source drain bias dependence (\(V_{x}\)) of the Hall voltage (\(V_{H}\)) for a monolayer MoS\({}_{2}\) device for different light polarizations demonstrating the valley Hall effect. Reproduced with permission from [114], Copyright The American Association for the Advancement of Science, 2014. **(b)** Lateral p-n diode with opposite current flow direction (top) and corresponding dominant excitonic emission (bottom), representing the contribution to electroluminescence from \(K\) and \(K^{\prime}\) valleys. Reproduced with permission from [65], Copyright The American Association for the Advancement of Science, 2014.  
controlled generation and manipulation of excitons at room temperature. The short radiative lifetime of excitons provides the opportunity for high-speed modulation of TMD-based light sources for data communications. The ability to generate, control and read-out valley polarizations in monolayer TMDs, may lead to devices for quantum computation with valley-qubits. Moreover, the endless number of possibilities of stacking different 2D materials into van der Waals heterostructures could allow for the realization of materials with tailored properties and functionalities. Already today, TMD photodetectors exhibit photoresponsivities that are comparable to that of state-of-the-art 3D semiconductor devices. In addition, the integration into multi-pixel arrays seems feasible. The lack of a killer application, however, is an issue. Because of their rather large bandgap, TMDs work mainly in the visible spectral regime, which is well covered by the highly mature Si technology. In the infrared, hybrid devices that combine TMDs with other materials (e.g., quantum dots) could offer a low-cost alternative to micro-bolometers for imaging. Research on TMD light-emitters has made large progress recently. Efficiencies are comparable to that of state-of-the-art organic light-emitting diodes. Among all TMDs studied so far, WS\({}_{2}\) has the largest bandgap and emits red visible light. For full-color displays, green and blue visible light emission would additionally be required. Given the large exciton binding energies in TMDs, it might be possible to shift the emission to somewhat shorter wavelengths (e.g., by engineering of the dielectric environment), but achieving blue light emission does not seem feasible. Other 2D semiconductors, that possess larger bandgaps, may eventually allow overcoming this limitation. The optical semi-transparency of such displays might find application in augmented reality and related applications. First experiments on TMD-based photovoltaics are encouraging.