Recently, research on graphene based photodetectors has drawn substantial attention due to ultrafast and broadband photoresponse of graphene. However, they usually have low responsivity and low photoconductive gain induced by the gapless nature of graphene, which greatly limit their applications. The synergetic integration of graphene with other two-dimensional (2D) materials to form van der Waals heterostructure is a very promising approach to overcome these shortcomings. Here we report the growth of graphene-Bi2Te3 heterostructure where Bi2Te3 is a small bandgap material from topological insulator family with a similar hexagonal symmetry to graphene. Because of the effective photocarrier generation and transfer at the interface between graphene and Bi2Te3, the device photocurrent can be effectively enhanced without sacrificing the detecting spectral width. Our results show that the graphene-Bi2Te3 photodetector has much higher photoresponsivity (35 AW(-1) at a wavelength of 532 nm) and higher sensitivity (photoconductive gain up to 83), as compared to the pure monolayer graphene-based devices. More interestingly, the detection wavelength range of our device is further expanded to near-infrared (980 nm) and telecommunication band (1550 nm), which is not observed on the devices based on heterostructures of graphene and transition metal dichalcogenides.

material. Graphene has been demonstrated to be an effective channel material for phototransistor because of its broadband light absorption, fast response time, and ultrahigh carrier mobility. [1][2][3] However, the relatively low absorption cross-section, fast recombination rate and the absence of gain mechanism that can generate multiple charge carriers from one incident photon have limited the responsivity of pure graphene-based phototransistor [ 4,5 ] to ≈10 −2 A W −1 which is much lower than that of commercial Si photodiode. [ 6 ] So far, the rapid development of graphene-based photodetection has focused on enhancement of the light absorption in graphene by variant approaches such as plasmonic coupling [ 7 ] and microcavity confi nement. [8][9][10] Nevertheless, a key to ultrasensitive graphene-based photodetection is the implementation of photoconductive gain which could afford the ability to generate multiple electrical carriers per single incident photon.Until now, the photoconductive gain for improved sensitivity has not been observed in pure grapehene-based photodetector. Alternatively, the hybridization of graphene with a gain material or the formation of a heterostructure has been proved to be an effective approach to enhance the photodetection performance. For example, the mixtures of graphene with TiO 2 [ 11 ] or quantum dots [ 12 ] have shown greatly improved photoconductive gain but the synthesis of gain material needs complicated processes. The formation of vertical heterostructure of graphene and layered transition metal dichalcogenides (TMDs) such as MoS 2 , [ 13,14 ] WS 2 , [ 15 ] and WSe 2 [ 16,17 ] can achieve very high quantum effi ciency upon light illumination due to effective photoexcited carrier separation at the interface. However, the fabrication of these devices is expensive and lack of scalability as it demands delicately controlled sample transfer technique which has low-yield and multiple lithography procedures.Recently, mixed organic-inorganic halide perovskites have emerged as a new class of light harvesting material for highly effi cient solar cells with confi rmed effi ciency of 19.2%. [ 18 ] This family of perovskite materials take the form of ABX 3 (A = CH 3 NH 3 + ; B = Pb 2+ ; X = Cl − /I − /Br − ) and show large absorption cross-section, long photocarrier diffusion length, and high charge carrier mobility. [ 19 ] These unique photoelectrical properties enable many photonic and optoelectronic applications such as random lasing, [ 20 ] light emitting diode, [ 21 ] and Graphene is an attractive optoelectronic material for light detection because of its broadband light absorption and fast response time. However, the relatively low absorption cross-section, fast recombination rate, and the absence of gain mechanism have limited the responsivity of pure graphene-based phototransistor to ≈10 −2 A W −1 . In this work, a photoconductive gain of ≈10 9 electrons per photon and a responsivity of ≈6.0 × 10 5 A W −1 are demonstrated in a hybrid photodetector that consists of monolayer g...

Molybdenum disulphide (MoS2), which is a typical semiconductor from the family of layered transition metal dichalcogenides (TMDs), is an attractive material for optoelectronic and photodetection applications because of its tunable bandgap and high quantum luminescence efficiency. Although a high photoresponsivity of 880–2000 AW−1 and photogain up to 5000 have been demonstrated in MoS2-based photodetectors, the light absorption and gain mechanisms are two fundamental issues preventing these materials from further improvement. In addition, it is still debated whether monolayer or multilayer MoS2 could deliver better performance. Here, we demonstrate a photoresponsivity of approximately 104 AW−1 and a photogain of approximately 107 electrons per photon in an n-n heterostructure photodetector that consists of a multilayer MoS2 thin film covered with a thin layer of graphene quantum dots (GQDs). The enhanced light-matter interaction results from effective charge transfer and the re-absorption of photons, leading to enhanced light absorption and the creation of electron-hole pairs. It is feasible to scale up the device and obtain a fast response, thus making it one step closer to practical applications.

A gradient heterosturcture is one of the basic methods to control the charge flow in perovskite solar cells (PSCs). However, a classical route for gradient heterosturctures is based on the diffusion technique, in which the guest ions gradually diffuse into the films from a concentrated source of dopants. The gradient heterosturcture is only accessible to the top side, and may be time consuming and costly. Here, the “intolerant” n‐type heteroatoms (Sb3+, In3+) with mismatched cation sizes and charge states can spontaneously enrich two sides of perovskite thin films. The dopants at specific sides can be extracted by a typical hole‐transport layer. Theoretical calculations and experimental observations both indicate that the optimized charge management can be attributed to the tailored band structure and interfacial electronic hybridization, which promote charge separation and collection. The strategy enables the fabrication of PSCs with a spontaneous graded heterojunction showing high efficiency. A champion device based on Sb3+ doped film shows a stabilized power‐conversion efficiency of 21.04% with a high fill factor of 0.84 and small hysteresis.

tuning strategy [14,15] with improved moisture tolerance. On the other hand, we have first demonstrated a surface functionalization method of MAPbI 3 (MA = CH 3 NH 3 + ) film with tetra-alkyl ammonium molecules which could tremendously enhance the humid stability of the perovskite device even under very harsh condition (90% relative humidity). [16] Furthermore, a series of hydrophobic molecules, such as alkylphos phonic acid ω-ammonium chloride, [17] dodecyltrimethoxysilane (C 12 -silane), [18] polystyrene, [19,20] polymethyl methacrylate, [21] and phenylethylammonium iodide, [22] were developed to enhance the moisture tolerance of perovskites. In addition, encapsulation was another effective way to protect perovskites form degradation induced by ambient moisture. [23][24][25][26] However, it was found that the organic layers with insulated alkyl groups usually limit the carrier extraction and thus result in slight loss of PCEs in our experiments.To protect the perovskite devices with high efficiency, surface-functionalized molecules must combine excellent electrical conductivity and hydrophobic properties. Among various molecules, thiophene derivatives enable the electron-rich conjugated π system, which consist of four 2p orbital electrons from carbon atoms and two lone electron pairs from sulfur atom. [27] These thiophene-based derivatives or polythiophene derivatives are usually used as sufficient hole extraction materials, together with the advantage of their highest occupied molecular orbital (HOMO). [28] Moreover, unlike siloxanes and amines, thiophenes can be directly coordinated to the lead atom by the lone pair of electrons offered by the sulfur atom, which may be directly interacted with the valance band of perovskite. [29] Therefore, if we modify perovskite surface with such molecules, device with high efficiency and stability is expected.In this work, we reported a new strategy to fabricate moisturetolerant and high-performance PSCs by employing 3-alkylthiophene derivatives as the multifunctional surface layer. This class of molecules contains unique delocalized conjugated π systems and hydrophobic alkyl groups that can enhance the charge transfer at perovskite/2,2′-7,7′-tetrakis[N,N-di(4-methoxyphenyl) amino]-9,9′-spirobifluorene (Spiro-OMeTAD) interface and protect the inner perovskite. Planar heterojunction devices utilizing Although the efficiency of perovskite solar cells (PSCs) is close to crystalline silicon solar cells, the instability of perovskite, especially in humid condition, still hinders its commercialization. As an effective method to improve their stability, surface functionalization, by using hydrophobic molecules, has been extensively investigated, but usually accompanied with the loss of device efficiencies owing to their intrinsic electrical insulation. In this work, for the first time, it is demonstrated that 3-alkylthiophene-based hydrophobic molecules can be used as both water-resistant and interface-modified layers, which could simultaneously enhance both stability and perform...