Graphene, Transition Metal Dichalcogenides, and Perovskite Photodetectors Graphene, Transition Metal Dichalcogenides, and Perovskite Photodetectors

Recent years have witnessed a tremendous progress in 2D materials photodetector. Their unique properties including wide photoresponse wavelength, passivated surfaces, strong interaction with incident light, and high mobility enable dramatical superiority in photo- detector application. The photophysics, device structure, working mechanism, and performance of three kinds of 2D materials photodetectors including graphene, transition metal dichalcogenides (TMDs), and halide perovskite are discussed in detail to give a profound understanding for the readers. In addition, we highlight the challenges and opportunities faced in studying 2D materials photodetector, and various strategies are summarized to help on the development of photodetection research and find ways to approach major problems.


Introduction
Two-dimensional (2D) materials are a class of materials derived from layered van der Waals solids. The in-plane atoms are held together by tight covalent or ionic bonds along 2D directions to form atomic layers, while the atomic layers are bonded together by weak van der Waals interactions along the out-of-plane direction [1][2][3]. Nicolosi et al. summarized different types of layered materials, which can be grouped into diverse families. The simplest are the atomically thin, hexagonal sheets of graphene and hexagonal boron nitride (h-BN). Other layered materials also include transition metal dichalcogenides (TMDs), black phosphorus (BP), metal halides (such as PbI 2 and MgBr 2 ), layered metal oxides (such as MnO 2 and MoO 3 ), layered double hydroxides (LDHs) [such as Mg 6 Al 2 (OH) 16 ], layered III-VIs (such as InSe and GaS), layered V-VIs (such as Bi 2 Te 3 and Sb 2 Se 3 ), and halide perovskites [4]. Graphene is composed of a single layer of carbon atoms bonded together in a hexagonal honeycomb lattice, which was firstly obtained by exfoliating graphite using an adhesive tape in 2004 [5]. Then graphene becomes the most famous 2D layered material on the basis of its appealing electronic, optical, mechanical, and thermal properties, and thus Geim and Novosolov were awarded the Nobel Prize in Physics in 2010.
2D materials have many distinctive properties including mechanical stiffness, strength and elasticity and high electrical and thermal conductivity. In this chapter, the optical and electrical properties of 2D materials are cared for photodetector application. Compared with traditional bulk materials, 2D materials have several superiorities as photodetectors. Firstly, 2D materials are sensitive over a wide range of the electromagnetic spectrum including ultraviolet (UV), visible, infrared (IR), and terahertz (THz) (see Figure 1a) [6]. The bandgap and photoresponse range of different 2D material photodetectors are summarized in Figure 1b. Graphene photodetector can sense over an ultrawide spectrum from UV to IR and even THz due to its zero bandgap and linear dispersion near the Dirac point [7,8]. TMD photodetectors have a photoresponse over a spectrum from visible to near infrared [9]. The bandgap of BP can be increased from 0.3 eV of bulk to around 2 eV of monolayer, corresponding to near infrared and visible [10]. Halide perovskite photodetectors have a photoresponse over a spectrum from UV to near infrared [11]. Secondly, the surfaces of 2D materials are free of dangling bonds, facilitating the construction of vertical heterojunctions without the restriction of lattice matching and avoiding surface leakage current due to their naturally passivated surfaces, and noise current can be decreased to fairly small order [12]. Thirdly, 2D semiconductors with ultrathin thickness can still strongly interact with incident light resulting from the singularities effect near the conduction and valence band edges [13]. Therefore, 2D semiconductor films are ideal materials for photodetector application.

Graphene photodetector
Graphene is an appealing material for optoelectronic applications. The concentrations of electrons and holes can be as high as 10 13 cm −2 , and their mobilities μ can exceed 15,000 cm 2 V −1 s −1 even under ambient conditions [14]. Due to its zero bandgap and linear dispersion near the Dirac point, graphene exhibits an extremely broadband absorption from UV to THz. Despite of the atomic layer thickness, graphene absorbs 2.3% of incident photons without significant wavelength dependence [6].

The physical mechanisms
The physical mechanisms of graphene photodetector including the photoconductive effect, the photovoltaic effect, the photo-thermoelectric effect, the bolometric effect, the photogating effect, and the plasma-wave-assisted mechanism (also called Dyakonov-Shur mechanism) have been reported (Figure 2) [15]. Based on the sensing mechanism, photodetectors can be divided into two categories: photon detectors and thermal detectors. Photodetectors that operate via the photovoltaic effect are usually called photodiodes. A photodiode photodetector can function at two modes under illumination: the photovoltaic mode at zero bias and the photoconductive mode under reverse bias [3,16]. The charge-separation mechanism is shown in Figure 3. Photovoltaic effect is based on the separation of photogenerated electron-hole pairs by a built-in electric field at junctions, and electrons and holes are collected at opposite electrodes, which generates a considerable photocurrent (short-circuit current). A photodiode working in photovoltaic mode has the lowest dark current, and an improved detectivity can be obtained. In photoconductive mode, the external electric field with a moderate biasing voltage has the same direction as the built-in one, which increases the separation efficiency of the electron-hole pairs, as well as the response speed due to reduced carrier transit time and lowered diode capacitance. The photo-thermoelectric effect is also called Seebeck effect. Pronounced thermoelectric photocurrent can be generated in graphene by hot carriers, and the temperature differences between carriers are able to produce a significant photoresponse [17]. The bolometric effect is associated with the resistance change produced by heating associated with the incident photons. Instead of generating photocurrent under zero bias, it only modifies the conductance of graphene under high external bias [18][19]. A d.c. voltage is generated Figure 2. Schematic of the four kinds of graphene photodetection mechanisms [15]. in a field-effect transistor in response to a THz radiation oscillation field through plasma-wave rectification, which was first proposed by Dyakonov and Shur [20]. The graphene photodetector based on plasma-wave-assisted mechanism can be used to integrate with silicon photonics.

The structure and performance
Here, all graphene photodetectors are based on the photoconductor and photovoltaic effects. The external bias voltage is not desired for graphene photodetectors because a large dark current can be generated as graphene has high conductivity. The built-in electric fields can be introduced by constructing a homogeneous p-n junction, a metal-graphene Schottky junction, or a tunneling barrier junction. Surface chemical modification methods can be adopted for the controllable n-or p-type doping in graphene. Liu et al. fabricated a graphene p-n junction photodiode photodetector through a selected-area chemical doping process, showing its IR detection ability [21]. A metal-graphene-metal (MGM) photodetector can be achieved by taking advantage of the work-function difference between graphene and a contacting metal. In 2009, Xia et al. reported the first graphene photodetector with a 40 GHz bandwidth and a 0.5 mA W −1 photoresponsivity (Figure 4(a, b)) [22]. Then they fabricated a MGM photodetector with asymmetric electrodes (Ti and Pd) to extend the operation region, and a photoresponsivity of 6.1 mA W −1 , a bandwidth of 16 GHz, and a data rate of 10 Gbit s −1 were obtained (Figure 4 (c, d)) [23]. In this device, photocurrent is generated by local illumination of one of the metal/graphene interfaces of a back-gated graphene FET. The field arises from charge transfer from the respective contact metal to graphene, and it can be adjusted by choosing proper metal. A graphene double-layer tunneling barrier junction can be used to achieve ultra-broadband and highsensitivity photodetection. The device is composed of two graphene layers sandwiching a

TMD photodetector
2D transition metal dichalcogenides (TMDs) include MX 2 (M = Mo, W; X = S, Se, Te) which is formed by covalently bonded X-M-X 2D hexagonal trilayers, and neighboring layers bond with each other via weakly van der Waals forces [25]. Bulk TMDs possess an indirect band gap of 0.9-1.4 eV. Only monolayer TMDs have direct band gap, and the indirect band gap of TMDs is thickness dependent resulting from strong interlayer coupling and quantum confinement effect, contributing to efficient light absorption and emission [26]. The absorption spectrum of TMDs is well matched to the solar spectrum, and the absorption coefficient is typically on the order of 10 4 -10 6 cm −1 so that more than 95% of the sunlight is absorbed for TMD films with sub-micrometer thickness. The mobility of TMDs increases with the number of layers, typically ranging from 0.2 to 34,000 cm 2 V −1 s −1 [27]. The charge-carrier density depends on the doping levels and recombination centers, and the typical value is 10 12 cm −2 [28]. Moreover, monolayer TMDs have long-lived excitons because of high binding energies, such as 320 meV for excitons in WS 2 and 20 meV for trions in MoS 2 , which are several times higher than those in bulk [29,30].

MoS 2 photodetector
Monolayer MoS 2 has a direct bandgap of 1.8 eV, large carrier mobility above 200 cm 2 V −1 s −1 , strong photoluminescence, good chemical stability and mechanical flexibility, and so on [3]. MoS 2 -based photodetectors are always in the form of a FET configuration. In 2012, Yin et al. developed a mechanically exfoliated monolayer MoS 2 phototransistor, which has a photoresponsivity of 7.5 mA/W for 750 nm light and a response time of 50 ms [31]. As shown in Figure 6a, the cut-off wavelength of photodetector is at ≈670 nm, which agrees well with the bandgap of the monolayer MoS 2 . After that, several factors have been investigated to explore how they affect the photoresponse performance. First, the charge trapping in MoS 2 or at the MoS 2 /substrate interface plays a key role in the photoresponse process. Lopez-Sanchez et al. obtained a high responsivity up to 880 A/W from a monolayer MoS 2 phototransistor (Figure 6b), but the response time can be several seconds, which is influenced by the surroundings of MoS 2 [32]. This is a specific example to demonstrate the effect of trap states on responsivity and response time. Second, the responsivity and response speed are highly dependent on the layer number of MoS 2 . Khan et al. reported an increased responsivity and response speed with increasing numbers of MoS 2 layers. They also demonstrated that responsivity is significantly enhanced in N 2 gas environment compared with that in atmospheric environment, indicating that the charge-carrier density can be affected by the oxygen (Figure 6c, d) [33]. Third, the preparation methods including mechanically exfoliated, chemical vapor deposition (CVD), liquid exfoliation, and solution synthesis can have a significant effect on photoresponse (Figure 6e, f). Similar performance has been reported for mechanically exfoliated and CVD methods, while inferior performance is obtained for solution-processed MoS 2 . Besides, electrode contact and surface modification are also reported to affect the performance of MoS 2 photodetector, and several strategies are employed to enhance the photoresponse performance [32,34,35]. The photodetector with low Schottky barrier exhibits a very high photo gain, but response speed is slow. On the contrary, a very short response time can be obtained for the photodetector with high Schottky-contact barrier, but the gain decreases by several orders of magnitude [36].

WS 2 photodetector
Monolayer WS 2 has a direct bandgap of 1.98 eV instead of an indirect bandgap of 1.4 eV in the bulk, but its carrier mobility of 0.2 cm 2 V −1 s −1 is lower than bulk and it also has strong photoluminescence due to high exciton binding energy [26]. WS 2 is also reported to have much smaller effective electron mass, providing better transistor performance than Si as well as MoS 2 [37]. Besides, WS 2 as a potential electronic material is inert, nontoxic, and environmentally friendly. The photoresponse performance of WS 2 photodetector is also affected by the same factors as MoS 2 including the charge trapping induced by substrate and atmosphere environment, the layer number, the preparation methods, electrode contact, and surface modification. Perea-López et al. have studied the photoresponse of CVD-grown few-layer WS 2 photodetector (Figure 7a). The responsivity is highly dependent on the excitation wavelength, and the highest photoresponse wavelength is 647 nm. The device shows a maximum photocurrent of 0.6 μA, a responsivity of 92 μA W −1 , and a fast response speed of ≈5 ms. The low responsivity is attributed to the high resistances ranging between 10 9 and 10 12 Ω resulting from the low mobility [38]. Lee et al. reported a mechanically exfoliated WS 2 photodetector with a photoresponsivity of 0.27 A/W and an on/off ratio of 10 2 -10 3 [37]. Huo et al. reported that the responsivity of mechanically exfoliated WS 2 photodetector can increase from 5.7 A W −1 in vacuum to a maximum of 884 A W −1 in an NH 3 atmosphere (Figure 7b). The high sensitivity on the surrounding environment can be ascribed to the electron transfer to absorbed gas molecules, demonstrating the role of charge trapping [39]. As shown in Figure 7c, Yao et al. prepared

Perovskite photodetector
Recent years have witnessed the rapid development of the organic-inorganic hybrid halide perovskites. Their exceptional properties including an adjustable spectral absorption range, high carrier mobility, long diffusion lengths, and the affordability of fabrication render them one of the most exceptional optoelectronic materials for photoelectrical device applications especially in photodetectors [41]. Organic-inorganic hybrid halide perovskites have the same chemical formula ABX 3 . The A cation can be organic species including CH 3 CH 2 NH 3 + (EA), HC(NH 2 ) 2 + (FA), and CH 3 NH 3 (MA) or inorganic species such as Cs, Rb, K, and Na, and their ionic radiuses decrease gradually. The A cation is thought no direct contribution toward electronic properties, but its size can alter the degree of distortion that in turn affects electronic properties [42]. The B metal cation can be group IV metal such as Pb 2+ , Sn 2+ , and Ge 2+ in a divalent oxidation state, and the Pb 2+ has superior stability than Sn 2+ and Ge 2+ . The halide anion can be the most effectively varied component. In general, with increasing ionic radius, a narrower bandgap can be obtained, resulting in that absorption spectra shift to longer wavelengths. Perovskites have a large absorption coefficient, so strong optical absorbance across the entire visible spectrum can be achieved for a thin thickness less than 500 nm, facilitating the collection of charge carriers [43]. Moreover, MAPbI 3 has been reported with the carrier diffusion lengths up to 100 nm for both electrons and holes, and exceeding 1 μm has been reported in the MAPbI 3 − x Cl x , resulting from long carrier lifetime and large carrier mobility [44,45]. So far, perovskite photodetectors have been widely reported from different aspects including composition engineering such as the organic-inorganic hybrid MAPbX 3 (X = Cl, Br, I) and allinorganic CsPbX 3 (X = Cl, Br, I) perovskites, morphology such as compact polycrystalline films and nanocrystal films (quantum dots, nanocubes, nanowires, nanoplatelets, and nanosheets), device structure such as photoconductor and photodiode, and so on. Compared with organicinorganic perovskite, all-inorganic perovskite exhibits better air stability.

2D perovskite photodetector
2D halide perovskites with different morphologies including microdisks, nanoplatelets, and nanosheets (NSs) have been synthesized via both CVD and colloidal chemistry methods. Many features including high photoluminescence (PL) quantum yield, quantum confinement effect, increased exciton binding energy, and long electron diffusion length have been demonstrated.  [46]. By comparing photoresponse performance in Figure 8b and c, MAPbBrI 2 has a larger photocurrent than MAPbBr 3 under the same irradiation condition, which may result from its stronger light absorption. Next, Liu et al. synthesized triangle 2D MAPbI 3 NSs with thinner thickness of several nanometers and larger lateral sizes of several micrometers based on two-step CVD method. A photoconductor photodetector was fabricated in the form of a FET configuration (Figure 8d and e). High responsivities of 22 12 A W −1 at 1 V bias voltage were obtained under irradiations of 405 nm laser with 0.07 mW cm −2 . The response or recovery time was shorter than 20 or 40 ms upon 405 nm irradiation (Figure 8f and g) [47]. The excellent photoresponse properties make this 2D perovskite promise for photodetection applications.
2D all-inorganic perovskites are also reported to fabricate high-performance photodetectors. In particular, CsPbBr 3 NSs has been widely studied on synthesis, photophysics, electrical property, and device application due to its high air stability. 2D CsPbBr 3 perovskite is formed by incorporating Cs + cations into a 2D sheet of PbBr 4 2− octahedra of which six Br atoms surround each Pb atom and four in-plane Br atoms are being shared by two octahedrons [11]. Bulk CsPbBr 3 has been reported to possess a thermodynamically orthorhombic structure at room temperature, and it can transform to tetragonal and cubic phases at 88 and 130°C, respectively [48]. However, the cubic phase could be preserved at room temperature owing to a large number of surfaces. Both cubic and orthorhombic phases are reported for CsPbBr 3 NSs by different groups [49,50]. In terms of well-controlled synthesis, Akkerman et al.  (Figure 9a-c). At the same time, high carrier mobility of 77.9 cm 2 V −1 s −1 was obtained by the Hall effect and transient photoconductivity measurements [54]. There are some reports on 2D CsPbBr 3 NSs photodetector application. In 2016, Song et al. The response and recovery time were measured to be 0.019 or 0.025 ms, respectively. A high stability and outstanding flexibility (>10,000 cycles) were also obtained [55]. This work demonstrates that all-inorganic perovskite CsPbBr 3 NSs have a great potential in photodetector application. Next, Lv et al. reported a similar result on CsPbBr 3 NSs photodetectors [56]. Then, our group reported that the responsivity could be improved by the annealing, which was demonstrated to increase coupling energy but decrease exciton binding energy resulting from ligand rearrangement. Tunneling instead of resonant energy transfer as a dominant way of exciton dissociation contributed to a high charge transfer rate. This exciton dissociation mechanism was proposed based on microscopic KPFM results (Figure 9d). As shown in Figure 9e and f, the response and recovery time were short, and the largest responsivity of 0.53 A W −1 could be obtained under the 525 nm irradiation with the intensity of 0.009 mW cm −2 [54].

Perovskite/graphene hybrid photodetector
Despite that the perovskites with different compositions, morphologies, and structures have been reported as photodetectors with high performance, their responsivity can be further improved by constructing hybrid structure. A promising strategy is based on hybrid phototransistors, where light is efficiently absorbed by perovskite layer while photogenerated carriers can transfer into conducting layer such as graphene, and high gain can be obtained from one carrier that recirculate many times within the lifetimes of the opposite charges [3]. For this hybrid photodetector, graphene is firstly transferred on the FET configuration substrate, and then perovskites including polycrystalline film or nanocrystal can be deposited on the graphene. In 2015, Lee et al. fabricated the MAPbI 3 /graphene hybrid photodetector, and a dramatic PL quenching was observed for hybrid film resulting from effective charge-carrier transfer through π-π interaction between perovskite and sp 2 hybridized graphene (Figure 10a, b). The device exhibited a broad spectral photoresponsivity across the visible range. The photoresponsivity and effective quantum efficiency (EQE) were 180 A/W and 5 × 10 4 % at a relatively high illumination power of 1 μW, respectively (Figure 10c). The device also exhibited good on-off switching, and response and recovery times were 87 and 540 ms (Figure 10d) [57]. The success of this novel perovskite/ graphene hybrid photodetector is a powerful evidence to demonstrate efficient hybrid structure. Instead of compact polycrystalline perovskite film, Wang et al. reported a high-performance perovskite/graphene hybrid photodetector consisting of graphene covered with a layer of dispersive MAPbBr 2 I islands. A photoconductive gain of 10 9 , a responsivity of 6.0 × 10 5 A W −1 , and a broadband detection across UV-visible are obtained. The photoresponse mechanism was discussed deeply. Under illumination, electron-hole pairs were generated in perovskite and then separated at the heterojunction interface. The holes were transferred to graphene sheets, and electrons were trapped in perovskite with a quite long lifetime, inducing a photogating effect [58]. Then, Qian et al. fabricated a nitrogen-doped graphene quantum dots (NGQDs)-MAPbI 3 / reduced graphene oxide (rGO) transistor, exhibiting a high responsivity of 1.92 × 10 4 A/W and a short response and recovery time of 10 ms. The outstanding performance was attributed to that NGQDs offer an effective path for electron transfer from perovskite to the rGO [59]. Despite high responsivity, perovskite/graphene hybrid photodetectors suffer from a large dark current about mA order because of ultrahigh conductivity of bottom graphene layer, resulting in high noise level.
Besides the lateral photoconductor structure, a vertical graphene/perovskite/graphene (GPG) photodetector has also been fabricated, and higher responsivity is achieved due to its shorter carrier extraction length. Duan's group fabricated a series of single 2D material heterojunction. Wang et al. reported a graphene/CsPbBr 3 microplate/graphene sandwiched vertical photodetector by growing CsPbBr 3 microplates on an exfoliated graphene flake, followed by the dry transfer of a second layer of graphene (Figure 11a). As shown in Figure 11b, the photocurrent rapidly increased with the light power, and the photocurrent versus bias curves were nonlinear, indicating the presence of a contact barrier. The responsivity decreases with the increasing light power density, and the maximum responsivity exceeds 10 5 A/W (Figure 11c) [60]. Besides, Cheng et al. fabricated a graphene/MAPbI 3 /graphene photodetector, delivering a high responsivity of 950 A W −1 and a high photoconductive gain of 2200 at 1 V bias under a 1.52 nW irradiation of 532 nm laser (Figure 11d and e). It also exhibited good and reproducible photo-switching behavior with response and recovery time of 22 and 37 ms. The photocurrent mapping showed uniformly contribution from the overlapping area, indicating that the majority of photoinduced carriers travel in vertical direction (Figure 11f) [61].

Perovskite/TMDs hybrid photodetector
Compared with perovskite/graphene hybrid photodetector, perovskite/TMD hybrid photodetector has a significant lower dark current. TMD layer serves as both light absorbers and carrier transport layer, while perovskite as sensitized layer can enhance light absorbance and extend the detection range. The heterojunction formed between perovskite and TMD layer can promote the carrier transfer, contributing to a high responsivity and fast response speed. In 2016, Ma et al. fabricated a hybrid perovskite MAPbI 3 and TMDs material WS 2 planar photoconductor detector for the first time. Both MAPbI 3 and WS 2 layers contacted with Au electrodes, which could transport photocarriers as parallel channels (Figure 12a). From the band structures of hybrid bilayer in Figure 12b, photogenerated charges were separated at the interface due to the Fermi level difference, which reduced the charge recombination and enhanced the carrier separation. The significant PL quenching for both MAPbI 3 and WS 2 was observed, indicating efficient charge transfer at the interface and exciton dissociation (Figure 12d). Furthermore, they proposed the photoresponse mechanism. In dark, the charge transfer at the interface lowers (raises) the Fermi level of WS 2 (perovskite), leading to the formation of depletion regions, suppressing the dark current. Under the light illumination, excitons are generated in the perovskite film, and electrons are transferred to WS 2 film, which raises (lowers) the Fermi level of WS 2 (perovskite) and reduces the Schottky barriers at contacts, resulting in the high photocurrent (Figure 12c). The device exhibited a high on/off ratios of ≈10 5 and a high responsivity of ≈17 A W −1 . Owing to the high mobility of the WS 2 monolayer and the efficient interfacial charge separation, the response speed of the hybrid photodetector was enhanced by four orders of magnitude compared to the reference perovskite single layer (Figure 12e and f) [62].   At the same time, Kang et al. developed an ultrahigh-performance MoS 2 /MAPbI 3 hybrid photodetector by using mechanically exfoliated 25-nm-thick MoS 2 nanoflakes [63]. Similar with the WS 2 /MAPbI 3 hybrid bilayer, the photogenerated electrons in perovskite are thought to reach the MoS 2 layer through diffusion (Figure 13a). The significant PL quenching for hybrid bilayer was also observed, indicating the charge transfer at the interface (Figure 13c). Under the bias voltage and laser illumination, electron-hole pairs generated in perovskite were separated based on the ambipolar charge transport property of the perovskite and diffuse into the MoS 2 region, followed by the carrier collection step caused by the electric field in the Ti-MoS 2 junctions (Figure 13b). The photocurrent of hybrid photodetector has increased by one order than MoS 2 photodetector when V G was −30 V (Figure 13d). The photoresponsivity of the hybrid photodetector and their ratios as a function of wavelength is shown in Figure 13e.
In the case of the 520 nm laser, the photoresponsivity was highly enhanced by factors of 7.7 (from 636 to 4.9 × 10 3 A W −1 ). The response and recovery time were also decreased for hybrid photodetector, but they are quite long compared with WS 2 /MAPbI 3 hybrid photodetector (Figure 13f). Then, Lu et al. reported a WSe 2 /MAPbI 3 hybrid photodetector with a photoresponsivity of 110 A W −1 , a high EQE of 2.5 × 10 4 %, and a high D* of 2.2 × 10 11 Jones [64]. In all, the outstanding photoresponse performance of perovskite/TMDs hybrid photodetector demonstrates this strategy as an efficient route to improve the performance of 2D material photodetector.

Summary and outlook
The distinctive optical and electrical properties enable 2D materials as an ideal and powerful candidate in photodetector application. The superiorities of 2D materials include a wide range of photoresponse wavelength (graphene is from UV to THz, TMDs is from visible to near infrared, halide perovskite is from UV to near infrared), facilitating the construction of vertical heterojunctions due to their naturally passivated surfaces, and the strong interaction with incident light resulting from the singularities effect near the conduction and valence band edges. This chapter has introduced the photophysics, device structure, working mechanism, and performance of three kinds of 2D materials photodetectors including graphene, TMDs, and halide perovskite. For graphene photodetector, four kinds of physical mechanisms are discussed, and the graphene p-n junction and metal-graphene Schottky junction are based on photovoltaic and photoconductor effects, respectively. These two effects also work for most of the 2D materials photodetectors. Generally, graphene photodetector has a wide sensitive wavelength and fast response speed, but it also suffers from a low responsivity and a high dark current. For TMD photodetector, there are the same performance influence factors, such as the charge trapping induced by substrate and atmosphere environment, the layer number, the preparation methods, electrode contact, and surface modification. However, the photoresponse performance is also determined by the photophysical parameters including mobility and absorption coefficient. For example, MoS 2 photodetector has a higher responsivity than WS 2 because of its high mobility. For 2D halide photodetector, the high absorption and mobility promote the high photoresponse performance of MAPbBr 3 nanoplatelets and CsPbBr 3 nanosheets. Furthermore, both the perovskite/graphene and perovskite/TMD hybrid structures are employed to enhance the photoresponse based on reduced the charge recombination and enhanced the carrier separation due to band difference. In summary, the 2D materials show their unique superiority in photodetector application.
In spite of the great success for 2D materials photodetectors, it is admitted that lots of challenges still remain. The dark current of graphene photodetector is too large, resulting in a high noise. The sensitive to environment such as oxygen and water is an unfavorable factor to TMDs and perovskite photodetectors, which is the origin of the device instability. To solve these problems, the novel 2D materials, approaches, and hybrid structure should be developed to meet the requirement for practical applications. First, the responsivity and response speed can be enhanced by exploring novel 2D materials with controllable preparation method. Second, novel approaches for chemical doping and surface treatment should be investigated, which has been proved as a feasible strategy for improving or manipulating device performance. Third, new hybrid photodetectors should be focused to demonstrate the increased responsivity resulting from efficient carriers transfer. Both materials engineering and device structure design are important to achieve high and reliable photoresponse performance for pursuing practical application of 2D materials photodetectors in the future.