# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 3 Photodetectors  
Photodetectors convert light into electrical signals and are at the heart of many applications, such as optical communications, imaging and sensing. Compared to graphene photodetectors [30], TMD-based devices offer lower dark currents and higher photoresponsivities (the photoresponsivity \(R=I_{ph}/P_{opt}\) measures the electrical output current \(I_{ph}\) per optical input power \(P_{opt}\)), though they work only in the near-infrared and visible spectral regimes. Similarly to graphene, several physical mechanisms can give rise to a photoresponse in TMDs. Photoelectric [31; 32] and photo-thermoelectric (Seebeck) [33] effects have been identified in devices operated under short-circuit conditions. The photoelectric effect relies on the separation of photoexcited carriers at the built-in electric field in lateral [34; 35] or vertical [36; 37] p-n junctions, or at the Schottky barriers at lateral TMD/metal interfaces [38], similarly to traditional photodiodes. The electric field can also be produced by application of an external voltage without driving a large dark current due to the semiconducting TMD behavior. Optical illumination of TMD/metal interfaces can produce a photovoltage due to the different Seebeck coefficients of the metal contact and the TMD channel [33; 39]. The photovoltage is defined as \(V_{PTE}=\left(S_{1}-S_{2}\right)\Delta T_{e}\), where \(S_{1,2}\) is the thermoelectric power in two differently doped regions of the TMD and \(\Delta T_{e}\) is the electron temperature difference between the regions. The Seebeck coefficient is related to the conductivity through the Mott formula \(S=\left(\pi^{2}k_{B}^{2}T/3e\right)\left(d\text{ln}(\sigma\left(E\right))/dE\right)\), where \(k_{B}\) is the Boltzmann constant, \(T\) is the temperature, \(e\) is the electron charge and \(\sigma\left(E\right)\) is the conductivity as a function of energy. The derivative has to be evaluated at the Fermi energy \(E_{F}\). Large numbers for the Seebeck coefficient in MoS\({}_{2}\), tunable over several orders of magnitude by an external electric field, were reported by [PERSON] _et al._[33]. Both photoelectric and photo-thermoelectric effects lead to moderate responsivities of the order of mA/W. Figure 2a (inset) shows a TMD field-effect transistor structure that has been employed by several groups for MoS\({}_{2}\)-, MoSe\({}_{2}\)-, WS\({}_{2}\)-, and WSe\({}_{2}\)-based photodetection [31; 32; 40; 41; 42; 43; 44; 45]. The same device structure has also been used in conjunction with other 2D semiconductors, such as GaSe, GaTe, In\({}_{2}\)Se\({}_{3}\), black phosphorus and several others [46; 47; 48; 49; 50; 51; 52]. Wavelength-dependent studies have shown that the photocurrent follows the TMD absorption spectrum. It was further demonstrated that the spectral response depends on the number of TMD layers [43]. For example, MoS\({}_{2}\) mono- and bilayer devices are effective for green visible light detection, whereas three-layer MoS\({}_{2}\) is better suited for detection of red visible light. Studies have also been performed using chemical vapor deposition grown TMDs [42]. By applying an external voltage, high photoresponsivities of more than 10\({}^{3}\) A/W and high photogain [40; 41; 42] have been achieved in these devices via internal amplification of the current. The photocurrent is then given as the difference between the current under optical illumination and the dark current (see Figure 2a, main panel). The term photogain refers to the fact that the number of circuit electrons generated per photoexcited carrier can be larger than one. The photogain in TMDs results from capturing of photoexcited carriers into trap states that are present either in the underlying (oxide) substrate or in the TMD itself [41]. The former is exacerbated by the high surface-to-volume ratio in 2D materials, and oxide surface treatments have been used to modify this effect [40]. The traps within the bandgap of TMDs arise from band tail states, induced by disorder or structural defects (see Figure 2b).