Epitaxially grown ultrathin organic semiconductors on graphene show great promise as highly efficient phototransistors. The devices exhibit a strong photoresponse down to the limit of a monolayer organic crystal, with a photoresponsivity higher than 10(4) A W(-1) and a photoconductive gain over 10(8) . The excellent performance is attributed to the high quality of the organic crystal and interface, a unique feature of van der Waals epitaxy.

Interfacing light-sensitive semiconductors with graphene can afford high-gain phototransistors by the multiplication effect of carriers in the semiconductor layer. So far, most devices consist of one semiconductor light-absorbing layer, where the lack of internal built-in field can strongly reduce the quantum efficiency and bandwidth. Here, we demonstrate a much improved graphene phototransistor performances using an epitaxial organic heterostructure composed of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) and pentacene as the light-absorbing layer. Compared with single light-absorbing material, the responsivity and response time can be simultaneously improved by 1 and 2 orders of magnitude over a broad band of 400-700 nm, under otherwise the same experimental conditions. As a result, the external quantum efficiency increases by over 800 times. Furthermore, the response time of the heterostructured phototransistor is highly gate-tunable down to sub-30 μs, which is among the fastest in the sensitized graphene phototransistors interfacing with electrically passive light-absorbing semiconductors. We show that the improvement is dominated by the efficient electron-hole pair dissociation due to interfacial built-in field rather than bulk absorption. The structure demonstrated here can be extended to many other organic and inorganic semiconductors, which opens new possibilities for high-performance graphene-based optoelectronics.

has been a challenge. [5,6] To enhance the absorption and photoresponse of graphene devices, researchers provide a series of strategies to interface graphene with light-absorbing semiconductors. [7][8][9][10][11][12][13][14][15][16] Early experimental studies on hybrid devices mainly focus on using one semiconductor layer, including colloidal quantum dots, [7,8] perov skites, [9] organic polymers, [10] single crystals, [16] 2D materials, [17] silicon, and other traditional materials. [11] More recently, improvement of device performance has been made by introducing PN junction bilayer absorbing layer. Incorporating graphene with a perovskite/ organic heterojunction or organic PN junction [14,15] is reported to improve both the photo responsivity and bandwidth. However, the limited narrow spectral range of light-absorbing layer causes ultrahigh photoconductive gain but at the same time sacrifices the detection spectral range. [18] In addition, a number of chemical approaches have been reported to synthesize the conjugated polymers/small molecules (typically with a bandgap of less than 1.6 eV) with appropriate energy gap and desirable photoelectric properties, but the device performance is still restricted. [19] So far the spectral range of graphene-based high gain photodetection is limited to typically 400-700 nm. [9,10,14,15,20,21] Herein, we explore a broadband (405-1550 nm) graphene/ organic semiconductor heterojunction phototransistors with bi-directional photoresponse (both positive and negative photocurrents) for the first time. Instead of broadening the absorption range of the semiconductor layer, our devices exploit ultrasensitive photoresponse at visible region, and the inverse photoresponse at near-infrared region without the need for cryogenics or adjusting gate voltage. Taking organic small molecule C 60 /pentacene heterojunction as the light-absorption layer, we achieve a highest responsivity of 9127 A W −1 , response time down to 275 µs, and external quantum efficiency up to 11.5% in visible regime and over 1800 A W −1 (0.063%) in near-infrared regime. Compared with previous work, our phototransistors not only have large built-in electric field at the C 60 /pentacene interface for high quantum efficiency, but also maintain an ultrasensitive response to the near-infrared region. The wavelength-dependent bi-directional response enables us to analyze the device mechanism. Our devices have potential applications in hyperspectral imaging.A graphene-semiconductor heterojunction is very attractive for realizing highly sensitive phototransistors due to the strong absorption of the semiconductor layer and the fast charge transport in the graphene. However, the photoresponse is usually limited to a narrow spectral range determined by the bandgap of the semiconductor. Here, an organic heterojunction (C 60 /pentacene) is incorporated on graphene to realize a broadband (405-1550 nm) phototransistor with a high gain of 5.2 × 10 5 and a response time down to 275 µs. The visible and near-infrared parts of the photor...

Solar-blind ultraviolet (UV) photodetectors (PDs) have attracted tremendous attention in the environmental, industrial, military, and biological fields. As a representative III-nitride material, AlGaN alloys have broad development prospects in the field of solar-blind detection due to their superior properties, such as tunable wide bandgaps for intrinsic UV detection. In recent decades, a variety of AlGaN-based PDs have been developed to achieve high-precision solar-blind UV detection. As integrated optoelectronic technology advances, AlGaN-based focal plane arrays (FPAs) are manufactured and exhibit outstanding solar-blind imaging capability. Considering the rapid development of AlGaN detection techniques, this paper comprehensively reviews the progress on AlGaN-based solar-blind UV PDs and FPAs. First, the basic physical properties of AlGaN are presented. The epitaxy and p-type doping problems of AlGaN alloys are then discussed. Diverse PDs, including photoconductors and Schottky, metal–semiconductor–metal (MSM), p-i-n, and avalanche photodiodes (APDs), are demonstrated, and the physical mechanisms are analyzed to improve device performance. Additionally, this paper summarizes imaging technologies used with AlGaN FPAs in recent years. Benefiting from the development of AlGaN materials and optoelectronic devices, solar-blind UV detection technology is greeted with significant revolutions.

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility and flexibility, making it appealing for ultrafast, broadband and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures have been exploited to improve the device performance. Particularly, van der Waals (vdW) heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With the hybrid structures, Gr hybrid photodetectors can operate from ultraviolet (UV) to terahertz (THz), with significantly improved R (up to 10 9 A W -1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues has been successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures towards highperformance photodetectors and integrated optoelectronic applications is reviewed.