Efficient organic near-infrared photodetectors based on lead phthalocyanine/C60 heterojunction

High efficiency organic small molecule near-infrared photodetectors (NIR-PDs) based on a lead phthalocyanine/C₆₀ planar heterojunction are demonstrated. The NIR-PDs show a broad-band response that extends to 1100 nm. The performance of the NIR-PDs is improved by using CuI as anode buffer layer. The optimized NIR-PD exhibits a response peak at about 900 nm with external quantum efficiencies (EQEs) of 19.7% at zero bias and 35.1% at −6 V, which are higher than other small molecule NIR-PDs reported. Comparable EQEs of 18.0% at zero bias and 33.2% at −6 V are found in the NIR-PD by further using 4,7-diphenyl-1,10-phenanthroline as cathode buffer layer. Meanwhile, the dark current is significantly reduced, which results in a high detectivity of 2.34 × 10¹¹ Jones at zero bias, which is among the highest detectivities reported for organic small-molecule NIR-PDs. Besides, the NIR-PDs show a reliable stability in ambient condition.     

Fast response organic photodetectors with high detectivity based on rubrene and C60

We presented a fast response organic photodetector based on high mobility material rubrene and C60 as donor and acceptor, respectively. It was found the diffusing time of the excitons limited the bandwidth in photodetectors composing of rubrene/C60 bilayer heterojunction. As introducing a mixing layer of rubrene and C60 between rubrene/C60 heterojunction, the response speed is greatly improved. The presented organic photodetectors finally showed a bandwidth of 15.6 MHz under a small reverse bias of −2 V. Moreover, the specific detectivity exceeded 10¹² Jones and the dark current density was as low as 1.5 nA cm⁻².     

3D Solar‐Blind Ga2O3 Photodetector Array Realized Via Origami Method

A 3D solar‐blind photodetector array is realized from amorphous Ga₂O₃ films grown on polyethylene terephthalate substrates via an origami route. The photodetector cells exhibit a dark current of 0.17 nA, and the peak responsivity is about 8.9 A W^?1 at 250 nm with a quantum efficiency of 4450%. The photodetector shows a distinct cut‐off wavelength at 268 nm with a solar‐blind ratio of more than two orders of magnitude (photocurrent ratio between 250 nm/300 nm). The photodetector cells reveal excellent electrical stability after thousands of bending cycles. All the photodetector cells of the 3D photodetector array have a highly consistent performance. In addition, the device can execute the functions of capturing a real‐time light trajectory and identifying multipoint light spatial distribution, which cannot be achieved in all the previously reported 2D solar‐blind photodetectors. The results suggest new pathways to fabricate 3D photodetectors from conventional semiconductor films, which may find potential applications in optical positioning, tracking, imaging and communications, etc.     

Low dark current and internal gain mechanism of GaN MSM photodetectors fabricated on bulk GaN substrate

Metal-semiconductor-metal ultraviolet photodetectors are fabricated on low-defect-density homoepitaxial GaN layer on bulk GaN substrate. The dislocation density of the homoepitaxial layer characterized by cathodoluminescence mapping technique is ∼5 × 10⁶ cm⁻². The photodetector with a high UV-to-visible rejection ratio of up to 1 × 10⁵ exhibits a low dark current of <2 pA at room temperature under 10 V bias. The photo-responsivity of the photodetector gradually increases as a function of applied bias, resulting in a photodetector quantum efficiency exceeding 100% at above medium bias. The photo-responsivity also shows a dependence on the incident optical power density and illumination conditions. The internal gain mechanism of the photodetector is attributed to photo-generated holes trapped at the semiconductor/metal interface as well as high-field-induced image-force lowering effect.     

Efficient Assembly of Bridged β‐Ga2O3 Nanowires for Solar‐Blind Photodetection

An increasing number of applications using ultraviolet radiation have renewed interest in ultraviolet photodetector research. Particularly, solar‐blind photodetectors sensitive to only deep UV (<280 nm), have attracted growing attention because of their wide applicability. Among recent advances in UV detection, nanowire (NW)‐based photodetectors seem promising, however, none of the reported devices possesses the required attributes for practical solar‐blind photodetection, namely, an efficient fabrication process, a high solar light rejection ratio, a low photocurrent noise, and a fast response. Herein, the assembly of β‐Ga₂O₃ NWs into high‐performance solar‐blind photodetectors by use of an efficient bridging method is reported. The device is made in a single‐step chemical vapor deposition process and has a high 250‐to‐280‐nm rejection ratio (～2 × 10³), low photocurrent fluctuation (<3%), and a fast decay time (<<20 ms). Further, variations in the synthesis parameters of the NWs induce drastic changes in the photoresponse properties, which suggest a possibility for tuning the performance of the photodetectors. The efficient fabrication method and high performance of the bridged β‐Ga₂O₃ NW photodetectors make them highly suitable for solar‐blind photodetection.     

Visible blind p-π-n-n ultraviolet photodetectors based on 4H-SiC homoepilayers

p⁺-π-n⁻-n⁺ ultraviolet photodetectors based on 4H-SiC homoepilayers have been presented. The growth of the 4H-SiC homoepilayers was carried out in a LPCVD system. The size of the active area of the photodetectors was 300×300 μm². The dark and illuminated I-V characteristics had been measured at reverse biases form 0 to 20 V at room temperature, and the illuminated current was at least two orders of magnitude than that of dark current below 13 V bias. The peak value zones of the photoresponse were located at 280-310 nm at different reverse biases, and the peak value located at 300 nm was 100 times greater than the cut-off response value in 380 nm at a bias of 10 V, which showed the device had good visible blind performance. A small red-shift about 5 nm on the peak responsivity occurred when reverse bias increased from 5 to 15 V.     

AlxGa1−xN/GaN multi-quantum-well ultraviolet detector based on p-i-n heterostructures

We report on characterization of a set of AlGaN/GaN multiple-quantum-well (MQW) photodetectors. The model structure used in the calculation is the p-i-n heterojunction with 20 AlGaN/GaN MQW structures in i-region. The MQW structures have 2 nm GaN quantum well width and 15 nm Al_xGa_1−xN barrier width. The cutoff wavelength of the MQW photodetectors can be tuned by adjusting the well width and barrier height. Including the polarization field effects, on increasing Al mole fraction, the transition energy decreases, the total noise increases, and the responsivity has a red shift, and so the detectivity decreases and has a red shift.     

Effect of dimensional parameters on the current of MSM photodetector

In this work, we present the influence of dimensional parameters on dark current and photocurrent of the metal-semiconductor-metal photodetector (MSM). MSM photodetectors of different sizes have been fabricated on GaAs (NID). The active area of MSM samples varies between 1×1 μm² and 10×10 μm² with equal electrodes spacing and finger widths (l=D) varying between 0.2 and 1 μm. The I(V) characterization in inverse and direct polarization in darkness shows good symmetry of curves, which shows the good performance of components and successful fulfillment of the Schottky contacts. The application of laser fiber of incident light power of 16 mW at wavelength of 850 nm for the illumination of the MSM photodetectors showed the evolution of the photocurrent ranging from 0.75 to 1.81 mA, respectively, for 1 to 0.2 μm electrodes spacing at 3 V and active area S=3×3 μm². We showed also that variation ranging from 0.45 to 2.5 mA, respectively, for S=1×1 μm² to S=10×10 μm² at 3 V and 0.3 μm electrodes spacing. The resistance of MSM photodetectors obtained evolved proportionally to the electrodes spacing (0.87 kΩ for D=0.2 μm and 2.27 kΩ for D=1 μm with S=3×3 μm²) and inversely proportional to the surface area (2.02 kΩ for S=1×1 μm², and 0.56 kΩ for S=10×10 μm² with 0.3 μm inter electrodes spacing).     

Optical properties of photodetectors based on single GaN nanowires with a transparent graphene contact

We report the fabrication and optical and electrical characterization of photodetectors for the UV spectral range based on single p–n junction nanowires with a transparent contact of a new type. The contact is based on CVD-grown (chemical-vapor deposition) graphene. The active region of the nitride nanowires contains a set of 30 radial In_0.18Ga_0.82N/GaN quantum wells. The structure is grown by metal-organic vaporphase epitaxy. The photodetectors are fabricated using electron-beam lithography. The current–voltage characteristics exhibit a rectifying behavior. The spectral sensitivity of the photodetector is recorded starting from 3 eV and extending far in the UV range. The maximal photoresponse is observed at a wavelength of 367 nm (sensitivity 1.9 mA/W). The response switching time of the photodetector is less than 0.1 s.     

Direct Polarimetric Image Sensor and Wide Spectral Response Based on Quasi-1D Sb2S3 Nanowire

Polarized photodetectors with wide spectral detection and ultra-fast photoresponses based on anisotropic semiconductors have potential applications in military and civilian fields and have been widely studied in recent years. The dual advantages of low-symmetry crystal structure and special electronic band-structure make Sb₂S₃ the perfect choice for polarized photodetection. In this work, the optical, vibrational, and optoelectronic anisotropy of the high-quality orthorhombic Sb₂S₃ nanowires are systematically investigated by experimental and theoretical studies. The metal-semiconductor-metal photodetectors based on a single Sb₂S₃ nanowire exhibit good polarization sensitivity in a broadband range from ultraviolet to near-infrared (360 to 1550 nm) and the obtained maximum dichroic ratio is 2.54 at 638 nm. The polarization-sensitive photocurrent mapping results show that the photocurrent is mainly derived from the Schottky junction at the interface between Au and Sb₂S₃. The effective separation of the photo-generated carriers near the Schottky junction gives a photodetector response time of 470 µs. The direct polarimetric imaging demonstrates that the gray value of the image obtained by the imaging system is sensitive to the object's polarized direction. This natural sensitivity of the Sb₂S₃-based photodetector to polarized objects makes it possible to image polarized objects directly as an image sensor.     

A Paper-Based Wearable Photodetector for Simultaneous UV Intensity and Dosage Measurement

Photodetectors, which convert the light signal into other forms of signal, have been under the spotlight of research for many years because they are widely applied in monitoring, communication, and imaging. Most of the currently available photodetectors can output electrical signals to indicate the transient light intensity, while some display color change to reveal the absorbed light dosage. However, there is no device that can tell the transient light intensity and accumulated light dosage at the same time. Here, a paper-based wearable photodetector that can simultaneously measure transient light intensity and accumulated light dosage is reported. The phosphomolybdic acid/citric acid system, whose color change can be observed by the naked eye, is designed as the photochromic material to combine with photodetective materials (using 2D Sr₂Nb₃O₁₀ and ZnO nanoparticle as examples) on paper. Such paper-based photodetector fully utilizes natural hygroscopicity and softness of paper, showing decent flexibility. Its optoelectronic signal remains stable even after 1000 cycles of bending. To the best of one's knowledge, this is the first photodetector that can tell light intensity and dosage simultaneously. This work introduces a new type of wearable photodetector by structure design and material selection, shedding light on more novel works for convenient and practical photodetection.     

High‐Performance Near‐IR Photodetector Using Low‐Bandgap MA0.5FA0.5Pb0.5Sn0.5I3 Perovskite

Photodetectors with ultrafast response are explored using inorganic/organic hybrid perovskites. High responsivity and fast optoelectronic response are achieved due to the exceptional semiconducting properties of perovskite materials. However, most of the perovskite‐based photodetectors exploited to date are centered on Pb‐based perovskites, which only afford spectral response across the visible spectrum. This study demonstrates a high‐performance near‐IR (NIR) photodetector using a stable low‐bandgap Sn‐containing perovskite, (CH₃NH₃)_0.5(NH₂CHNH₂)_0.5Pb_0.5Sn_0.5I₃ (MA_0.5FA_0.5Pb_0.5Sn_0.5I₃), which is processed with an antioxidant additive, ascorbic acid (AA). The addition of AA effectively strengthens the stability of Sn‐containing perovskite against oxygen, thereby significantly inhibiting the leakage current. Consequently, the derived photodetector shows high responsivity with a detectivity of over 10¹² Jones ranging from 800 to 970 nm. Such low‐cost, solution processable NIR photodetectors with high performance show promising potential for future optoelectronic applications.     

X-ray imager using solution processed organic transistor arrays and bulk heterojunction photodiodes on thin,flexible plastic substrate

We describe the fabrication and characterization of large-area active-matrix X-ray/photodetector array of high quality using organic photodiodes and organic transistors. All layers with the exception of the electrodes are solution processed. Because it is processed on a very thin plastic substrate of 25 μm thickness, the photodetector is only 100 μm thick. When combined with an 300-μm-thick X-ray scintillator, this gives a thin, low-weight and shatterproof X-ray detector of ca. 400 μm thickness. We demonstrate X-ray imaging under conditions that are used in medical applications.     

Micro Ceramic Cell Analyzer (MCCA) - Preliminary results

This paper describes the preliminary results of a detection module for a Micro Ceramic Cell Analyzer (MCCA) development and fabrication process. The detector with integrated light source (electroluminescence diode) and photodetector (light-to-voltage converter) is made using Low Temperature Co-fired Ceramics (LTCC) technology. The presented device is assigned for use in fluorescence activated cell detection. Its performance is examined experimentally for two test solutions, which consist of Escherichia coli and Saccharomyces cervisiae cells. Investigated biological material is marked using the DAPI (4′, 6-diamidino-2′-phenylindole, dihydrochloride) fluorescent stain. After exposure to a 370 nm excitation light, the cells bonded with DAPI fluorescent blue (460 nm). Using the photodetector, fluorescence intensity was found to be proportional to the number of the cells in the test solutions.     

Advantage of further scaling in gate dielectrics below 0.5 nm of equivalent oxide thickness with La2O3 gate dielectrics

N-type metal-oxide-semiconductor field-effect transistor (MOSFET) with an equivalent oxide thickness (EOT) of 0.37 nm has been demonstrated with La₂O₃ as a gate dielectric for the first time. Despite the existence of parasitic capacitances at gate electrode and inversion layer in the channel, a sufficient drain current increment in both linear and saturation regions have been observed, while scaling the gate oxide from 0.48 to 0.37 nm in EOT. Therefore, continuous scaling of EOT below 0.5 nm is still effective for further improvement in device performance.     

Visible blind p-i-n ultraviolet photodetector fabricated on 4H-SiC

The 4H-SiC visible blind p-i-n ultraviolet (UV) photodetector has been designed, fabricated and characterized. The dark I-V characteristics of the detector were carried out at room temperature. It was found that the photocurrent of detector was at least two orders of magnitude higher than the dark current. The photon response spectrum of the detector was measured and calibrated. The ratio of responsivity at 275 nm to that at 375 nm was nearly 100, which implied that the photodetector has a great improved visible blind performance.     

High-performance omnidirectional self-powered photodetector constructed by CsSnBr3/ITO heterostructure film

Omnidirectional photodetectors attract enormous attention due to their prominent roles in optical tracking systems and omnidirectional cameras. However, it is still a challenge for the construction of high-performance omnidirectional photodetectors where the incident light can be effectively absorbed in multiple directions and the photo-generated carriers can be effectively collected. Here, a high-performance omnidirectional self-powered photodetector based on the CsSnBr₃/indium tin oxide (ITO) heterostructure film was designed and demonstrated. The as-fabricated photodetector exhibited excellent self-powered photodetection performance, showing responsivity and detectivity up to 35.1 ​mA/W and 1.82 ​× ​10¹⁰ Jones, respectively, along with the smart rise/decay response time of 4 ​ms/9 ​ms. Benefitting from the excellent photoelectric properties of the CsSnBr₃ film as well as the ability of the CsSnBr₃/ITO heterostructure to efficiently separate and collect photo-generated carriers, the as-fabricated photodetector also exhibited excellent omnidirectional self-powered photodetection performance. All the results have certified that this work finds an efficient way to realize high-performance omnidirectional self-powered photodetectors.     

High‐Performance Flexible Broadband Photodetector Based on Organolead Halide Perovskite

Organolead halide perovskites have attracted extensive attentions as light harvesting materials for solar cells recently, because of its high charge‐carrier mobilities, high photoconversion efficiencies, low energy cost, ease of deposition, and so on. Herein, with CH₃NH₃PbI₃ film deposited on flexible ITO coated substrate, the first organolead halide perovskite based broadband photodetector is demonstrated. The organolead halide perovskite photodetector is sensitive to a broadband wavelength from the ultraviolet light to entire visible light, showing a photo‐responsivity of 3.49 A W^?1, 0.0367 A W^?1, an external quantum efficiency of 1.19×10³%, 5.84% at 365 nm and 780 nm with a voltage bias of 3 V, respectively. Additionally, the as‐fabricated photodetector exhibit excellent flexibility and robustness with no obvious variation of photocurrent after bending for several times. The organolead halide perovskite photodetector with high sensitivity, high speed and broad spectrum photoresponse is promising for further practical applications. And this platform creates new opportunities for the development of low‐cost, solution‐processed and high‐efficiency photodetectors.     

OLED compatible water-based nanolaminate encapsulation systems using ozone based starting layer

Highly efficient nanolaminate diffusion barriers made of TiO₂/Al₂O₃ multilayers using low temperature atomic layer deposition optimized for organic light emitting diodes are presented. Water vapour transmission rates (WVTR) show values of the order of 10⁻³ g/m²/d at 38 °C, 90% RH on planarized PEN webs (pPEN) when ozone is used as the oxidizing precursor for Al₂O₃ deposition. OLED encapsulated with such diffusion barriers display few dark spots observed over 2000 h after deposition and for aging under ambient conditions. Diffusion barriers deposited using water as the oxidizing precursor for Al₂O₃ result in at least 10 times lower WVTR on pPEN webs (10⁻⁴ g/m²/d). However, these water based diffusion barriers are incompatible with OLEDs such that the latter show extensive black spot formation (areas of no visible luminescence) immediately after deposition. Finally through the growth of these initial black spots, more than 40% loss in initial luminescence occurs after merely 900 h of operation. In this report, we introduce a new methodology for OLED encapsulation using a two step process where 10 nm thick ozone Al₂O₃ based nanolaminate diffusion barrier is followed by a 90 nm thick water Al₂O₃ based diffusion barrier (keeping TiO₂ precursors always the same). With this novel diffusion barrier stack, no visible black spot growth is observed over 2000 continuous operation hours under ambient conditions. Simultaneously, high OLED luminescence representing 90% of the initial luminescence value, which is measured at t = 0 is maintained after 2000 h of operation. Low WVTR values in the 10⁻⁴ g/m²/d range on pPEN webs are consistently measured in these essentially water based barrier layers with only 10 nm thick starting ozone Al₂O₃ based nanolaminate diffusion barriers. The results reported here have implications on developing methodologies for ultra high performance, OLED compatible diffusion barriers by ALD.     

Optimization of Solubility,Film Morphology and Photodetector Performance by Molecular Side‐Chain Engineering of Low‐Bandgap Thienothiadiazole‐Based Polymers

A series of donor–acceptor (D‐A) type low‐bandgap polymers containing the terthiophene and thieno[3,4‐b]thiadiazole units in the main chain but different numbers of identical side chains are designed and synthesized in order to study the effect of side chain on the polymer properties and optimize the performance of polymer photodetectors. Variation in the side chain content can influence the polymer solubility, molecular packing, and film morphology, which in turn affects the photodetector performance, particularly with regard to the photoresponsivity and dark current. X‐ray diffraction patterns indicate that molecular ordering increases with more side chains. Atomic force microscopy shows that appropriate morphology of the active layer in the polymer photodetector is necessary for high photocurrent and low dark current. Using BCP as a hole blocking layer (10 nm), the photodetector based on P4 exhibits the optimized performance with specific detectivity of 1.4 × 10¹² Jones at 800 nm, which is among the best reported values for polymer photodetectors and even comparable to that of a silicon photodetector.