Photoconductivity of Bulk‐Film‐Based Graphene Sheets

Time‐resolved photoconductivity measurements are carried out on graphene films prepared by using soluble graphene oxide. High photocurrent generation efficiency is observed for these graphene‐based films, and the relationships between their photoconductivity and different preparation methods, incident light intensity, external electric field, and photon energies are investigated. Higher photoconductivity is observed with higher photon energy at same incident light intensity. By fitting the experimental data to the Onsager model, the primary quantum yields for charge separation to generate bound electron–hole pairs and the initial ion‐pair thermalization separation distance are calculated.     

Few‐Layer GeAs Field‐Effect Transistors and Infrared Photodetectors

The family of 2D semiconductors (2DSCs) has grown rapidly since the first isolation of graphene. The emergence of each 2DSC material brings considerable excitement for its unique electrical, optical, and mechanical properties, which are often highly distinct from their 3D counterparts. To date, studies of 2DSC are majorly focused on group IV (e.g., graphene, silicene), group V (e.g., phosphorene), or group VIB compounds (transition metal dichalcogenides, TMD), and have inspired considerable effort in searching for novel 2DSCs. Here, the first electrical characterization of group IV–V compounds is presented by investigating few‐layer GeAs field‐effect transistors. With back‐gate device geometry, p‐type behaviors are observed at room temperature. Importantly, the hole carrier mobility is found to approach 100 cm² V^?1 s^?1 with ON–OFF ratio over 10⁵, comparable well with state‐of‐the‐art TMD devices. With the unique crystal structure the few‐layer GeAs show highly anisotropic optical and electronic properties (anisotropic mobility ratio of 4.8). Furthermore, GeAs based transistor shows prominent and rapid photoresponse to 1.6 µm radiation with a photoresponsivity of 6 A W^?1 and a rise and fall time of ≈3 ms. This study of group IV–V 2DSC materials greatly expands the 2D family, and can enable new opportunities in functional electronics and optoelectronics based on 2DSCs.     

Perovskite/Organic Bulk‐Heterojunction Integrated Ultrasensitive Broadband Photodetectors with High Near‐Infrared External Quantum Efficiency over 70%

Ultraviolet‐visible‐near infrared (UV‐Vis‐NIR) broadband detection is important for image sensing, communication, and environmental monitoring, yet remains as a challenge in achieving high external quantum efficiency (EQE) in the broad spectrum range. Herein, sensitive broadband integrated photodetectors (PDs) with high EQE levels are reported. The organic bulk‐heterojunction (OBHJ) layer, based on a NIR sensitive organic acceptor, is employed to extend the response spectrum of the perovskite PDs. A key strategy of introducing dual electron transport materials respectively for Vis and NIR regions into the active layer of integrated PDs is applied. Further combined with the proper energy level alignment and reasonable distribution of PC₆₁BM in the active layer, the extraction and transport of photo induced charges in between perovskite and OBHJ is promoted efficiently. The integrated PD with the optimized structure exhibits an EQE mostly beyond 70% in the Vis–NIR region, which is the highest value among the ever reported solution‐processable broadband PDs. The highest responsivity is 0.444 and 0.518 A W^?1 in the Vis and NIR region, respectively. The specific detectivity is beyond 10¹⁰ Jones in the range from 340 to 940 nm, enabling the device to detect weak signals in the UV to NIR broad region.     

Carbon Nanotube‐Quantum Dot Nanohybrids: Coupling with Single‐Particle Control in Aqueous Solution

A strategy is reported for the controlled assembly of organic‐inorganic heterostructures consisting of individual single‐walled carbon nanotubes (SWCNTs) selectively coupled to single semiconductor quantum dots (QDs). The assembly in aqueous solution was controlled towards the formation of monofunctionalized SWCNT‐QD structures. Photoluminescence studies in solution, and on surfaces at the single nanohybrid level, showed evidence of electronic coupling between the two nanostructures. The ability to covalently couple heterostructures with single particle control is crucial for the design of novel QD‐based optoelectronic and light‐energy conversion devices.     

High‐Responsivity Graphene/InAs Nanowire Heterojunction Near‐Infrared Photodetectors with Distinct Photocurrent On/Off Ratios

Graphene is a promising candidate material for high‐speed and ultra‐broadband photodetectors. However, graphene‐based photodetectors suffer from low photoreponsivity and I_light/I_dark ratios due to their negligible‐gap nature and small optical absorption. Here, a new type of graphene/InAs nanowire (NW) vertically stacked heterojunction infrared photodetector is reported, with a large photoresponsivity of 0.5 AW^?1 and I_light/I_dark ratio of 5 × 10², while the photoresponsivity and I_light/I_dark ratio of graphene infrared photodetectors are 0.1 mAW^?1 and 1, respectively. The Fermi level (E_F ) of graphene can be widely tuned by the gate voltage owing to its 2D nature. As a result, the back‐gated bias can modulate the Schottky barrier (SB) height at the interface between graphene and InAs NWs. Simulations further demonstrate the rectification behavior of graphene/InAs NW heterojunctions and the tunable SB controls charge transport across the vertically stacked heterostructure. The results address key challenges for graphene‐based infrared detectors, and are promising for the development of graphene electronic and optoelectronic applications.     

Phase‐Engineered Type‐II Multimetal–Selenide Heterostructures toward Low‐Power Consumption,Flexible, Transparent,and Wide‐Spectrum Photoresponse Photodetectors

Phase‐engineered type‐II metal–selenide heterostructures are demonstrated by directly selenizing indium‐tin oxide to form multimetal selenides in a single step. The utilization of a plasma system to assist the selenization facilitates a low‐temperature process, which results in large‐area films with high uniformity. Compared to single‐metal–selenide‐based photodetectors, the multimetal–selenide photodetectors exhibit obviously improved performance, which can be attributed to the Schottky contact at the interface for tuning the carrier transport, as well as the type‐II heterostructure that is beneficial for the separation of the electron–hole pairs. The multimetal–selenide photodetectors exhibit a response to light over a broad spectrum from UV to visible light with a high responsivity of 0.8 A W^?1 and an on/off current ratio of up to 10². Interestingly, all‐transparent photodetectors are successfully produced in this work. Moreover, the possibility of fabricating devices on flexible substrates is also demonstrated with sustainable performance, high strain tolerance, and high durability during bending tests.     

High‐Performance Flexible Photodetectors based on High‐Quality Perovskite Thin Films by a Vapor–Solution Method

Organometal halide perovskites are new light‐harvesting materials for lightweight and flexible optoelectronic devices due to their excellent optoelectronic properties and low‐temperature process capability. However, the preparation of high‐quality perovskite films on flexible substrates has still been a great challenge to date. Here, a novel vapor–solution method is developed to achieve uniform and pinhole‐free organometal halide perovskite films on flexible indium tin oxide/poly(ethylene terephthalate) substrates. Based on the as‐prepared high‐quality perovskite thin films, high‐performance flexible photodetectors (PDs) are constructed, which display a nR value of 81 A W^?1 at a low working voltage of 1 V, three orders higher than that of previously reported flexible perovskite thin‐film PDs. In addition, these flexible PDs exhibit excellent flexural stability and durability under various bending situations with their optoelectronic performance well retained. This breakthrough on the growth of high‐quality perovskite thin films opens up a new avenue to develop high‐performance flexible optoelectronic devices.     

Novel UV–Visible Photodetector in Photovoltaic Mode with Fast Response and Ultrahigh Photosensitivity Employing Se/TiO2 Nanotubes Heterojunction

A feasible strategy for hybrid photodetector by integrating an array of self‐ordered TiO₂ nanotubes (NTs) and selenium is demonstrated to break the compromise between the responsivity and response speed. Novel heterojunction between the TiO₂ NTs and Se in combination with the surface trap states at TiO₂ help regulate the electron transport and facilitate the separation of photogenerated electron–hole pairs under photovoltaic mode (at zero bias), leading to a high responsivity of ≈100 mA W^?1 at 620 nm light illumination and the ultrashort rise/decay time (1.4/7.8 ms). The implanting of intrinsic p‐type Se into TiO₂ NTs broadens the detection range to UV–visible (280–700 nm) with a large detectivity of over 10¹² Jones and a high linear dynamic range of over 80 dB. In addition, a maximum photocurrent of ≈10⁷ A is achieved at 450 nm light illumination and an ultrahigh photosensitivity (on/off ratio up to 10⁴) under zero bias upon UV and visible light illumination is readily achieved. The concept of employing novel heterojunction geometry holds great potential to pave a new way to realize high performance and energy‐efficient optoelectronic devices for practical applications.     

In Situ Fabrication of Vertical Multilayered MoS2/Si Homotype Heterojunction for High‐Speed Visible–Near‐Infrared Photodetectors

c2D transition metal dichalcogenides (TMDCs)‐based heterostructures have been demonstrated to achieve superior light absorption and photovoltaic effects theoretically and experimentally, making them extremely attractive for realizing optoelectronic devices. In this work, a vertical multilayered n‐MoS₂/n‐silicon homotype heterojunction is fabricated, which takes advantage of multilayered MoS₂ grown in situ directly on plane silicon. Electrical characterization reveals that the resultant device exhibits high sensitivity to visible–near‐infrared light with responsivity up to 11.9 A W^–1. Notably, the photodetector shows high‐speed response time of ≈30.5 µs/71.6 µs and capability to work under higher pulsed light irradiation approaching 100 kHz. The high response speed could be attributed to a good quality of the multilayer MoS₂, as well as in situ device fabrication process. These findings suggest that the multilayered MoS₂/Si homotype heterojunction have great potential application in the field of visible–near‐infrared detection and might be used as elements for construction of high‐speed integrated optoelectronic sensor circuitry.     

Atomically Thin Oxyhalide Solar‐Blind Photodetectors

2D wide‐bandgap semiconductors demonstrate great potential in fabricating solar‐blind ultraviolet (SBUV) photodetectors. However, the low responsivity of 2D solar‐blind photodetectors still limits their practical applications. Here, high‐responsivity solar‐blind photodetectors are achieved based on 2D bismuth oxychloride (BiOCl) flakes. The 2D BiOCl photodetectors exhibit a responsivity up to 35.7 A W^?1 and a specific detectivity of 2.2 × 10¹⁰ Jones under 250 nm illumination with 17.8 µW cm^?2 power density. In particular, the enhanced photodetective performances are demonstrated in BiOCl photodetectors with increasing ambient temperature. Surprisingly, their responsivity can reach 2060 A W^?1 at 450 K under solar‐blind light illumination, maybe owing to the formation of defective BiOCl grains evidenced by in situ transmission electron microscopy. The high responsivity throughout the solar‐blind range indicates that 2D BiOCl is a promising candidate for SBUV detection.     

Strategies toward High‐Performance Solution‐Processed Lateral Photodetectors

Due to their low cost and ease of integration, solution‐processed lateral photodetectors (PDs) are becoming an important device type among the PD family. In recent years, enormous effort has been devoted to improving their performances, and great achievements have been made. A summary of the core progress, especially from the perspective of design principles and device physics, is necessary to further the development of the field, but is currently lacking. Here, to address this need, first, the working mechanism of PDs and the device figures‐of‐merit are introduced. Second, by classifying the active materials into four categories, including inorganic, organic, hybrid, and perovskite, the developed strategies toward high performance are discussed respectively. To close, the common physical rules behind all these strategies are generalized, and suggestions for future development are given accordingly.