An All-Organic Self-Powered Photodetector with Ultraflexible Dual-Polarity Output for Biosignal Detection

High-performance self-powered broadband photodetector based on AgxOy@n-Si heterojunction

Thin silver oxide AgxOyfilm (p-type) was deposited via DC magnetron sputtering onto n-type silicon substrate and integrated into a pn heterojunction architecture. Structural (XRD, XPS and EDX), optical ultraviolet-visible-near infrared and morphological analysis (SEM) of the AgxOyfilm were investigated in detail. Electrical measurements revealed that the AgxOy/n-Si pn heterojunction as a self-driven photodetector device exhibits a high photoresponse both in visible light and in UV, IR and yellow lights. It was also observed that under visible light the photocurrent increased with increasing light intensity, higher at higher intensities. Furthermore, the photodetector exhibits high sensitivity to the incident light of 365 nm with responsivity as 1061 mA W-1for -1.5 V. The highest specific detectivity value for the conditions illuminated by LED with wavelength of 590 nm is 9.77 × 1012cm·Hz1/2·W-1(Jones) for zero bias. Experimental results show that the AgxOy/n-Si heterojunction has great potential for practical applications as self-driven and high-performance photodetectors.

Enhanced Exciton Delocalization in Organic Near-Infrared Photodetectors via Solid Additive-Mediated J-Aggregation

Near-infrared organic photodetectors (NIR-OPDs) have emerged as increasingly significant in optoelectronics, offering unparalleled advantages for applications in health monitoring and night vision. The development of self-powered devices with low dark currents and enhanced NIR sensitivity involves complex engineering that requires careful material selection, defect state density control, and environmental consideration. In this study, an innovative approach is introduced that utilizes solid additive (DIB) to induce improvements in the J-aggregation morphology and exciton delocalization in acceptor molecules. The goal is to broaden the response spectrum of the device and augment its detection capabilities. The key findings revealed that solid additive exhibit an electrostatic affinity for acceptors, which facilitates their orderly face-to-face stacking and controls the π-π stacking distance. These enhanced intermolecular interactions lead to the delocalization of electron-hole pairs, reduced exciton recombination, and increased charge separation efficiency. Consequently, the modified devices exhibited exceptional specific detectivity, exceeding 1014 Jones across the wavelength range of 695-860 nm, thereby establishing a new standard for NIR response in organic photodetection. Overall, this study successfully addressed the compatibility challenges associated with self-powered NIR-OPDs, thereby expanding their potential applications in various settings.

Self-Powered Photodetectors with High Stability Based on Se Paper/P3HT:Graphene Heterojunction

Photodetectors based on selenium (Se) have attracted significant attention because of their outstanding optoelectronic characteristics, including their rapid reactivity and high photoconductivity. However, the poor responsivity of pure Se limits their further development. In this study, a novel Se-P/P3HT:G photodetector was designed and fabricated by combining an organic semiconductor made of poly-3-hexylthiophene mixed with graphene (P3HT:G) with self-supporting Se paper (Se-P) via spin-coating process. The device possesses a dark current of around 4.23 × 10-12 A and self-powered characteristics at 300-900 nm. At zero bias voltage and 548 nm illumination, the Se-P/P3HT:G photodetector demonstrates a maximum photocurrent of 1.35 × 10-9 A (745% higher than that of Se-P at 0.1 V), a quick response time (16.2/27.6 ms), an on/off ratio of 292, and a maximum detectivity and responsivity of 6.47 × 1011 Jones and 34 mA W-1, respectively. Moreover, Se-P/P3HT:G exhibits superior environmental stability. After one month, the photocurrent value of the Se-P/P3HT:G device held steady at 91.4% of its initial value, and even following pre-treatment at 140 °C, the on/off ratio still remained 17 (at a retention rate of about 5.9%). The excellent thermal stability, environmental reliability, and optoelectronic performance of this heterojunction structure offer a useful pathway for the future advancement of high-performance optoelectronic devices.

Recent Progress in Photodetectors: From Materials to Structures and Applications

Photodetectors are critical components in a wide range of applications, from imaging and sensing to communications and environmental monitoring. Recent advancements in material science have led to the development of emerging photodetecting materials, such as perovskites, polymers, novel two-dimensional materials, and quantum dots, which offer unique optoelectronic properties and high tunability. This review presents a comprehensive overview of the synthesis methodologies for these cutting-edge materials, highlighting their potential to enhance photodetection performance. Additionally, we explore the design and fabrication of photodetectors with novel structures and physics, emphasizing devices that achieve high figure-of-merit parameters, such as enhanced sensitivity, fast response times, and broad spectral detection. Finally, we discuss the demonstration of new applications enabled by these advanced photodetectors, including flexible and wearable devices, next-generation imaging systems, and environmental sensing technologies. Through this review, we aim to provide insights into the current trends and future directions in the field of photodetection, guiding further research and development in this rapidly evolving area.

Versatile Papertronics: Photo-Induced Synapse and Security Applications on Papers

Paper is a readily available material in nature. Its recyclability, eco-friendliness, portability, flexibility, and affordability make it a favored substrate for researchers seeking cost-effective solutions. Electronic devices based on solution process are fabricated on paper and banknotes using PVK and SnO2 nanoparticles. The devices manufactured on paper substrates exhibit photosynaptic behavior under ultraviolet pulse illumination, stemming from numerous interactions on the surface of the SnO2 nanoparticles. A light-modulated artificial synapse device is realized on a paper at a low voltage bias of -0.01 V, with an average recognition rate of 91.7% based on the Yale Face Database. As a security device on a banknote, 400 devices in a 20 × 20 array configuration exhibited random electrical characteristics owing to the local morphology of the SnO2 nanoparticles and differences in the depletion layer width at the SnO2/PVK interface. The security Physically Unclonable Functions (PUF) key based on the current distribution extracted at -1 V show unpredictable reproducibility with 50% uniformity, 48.7% inter-Hamming distance, and 50.1% bit-aliasing rates. Moreover, the device maintained its properties for more than 210 days under a curvature radius of 8.75 mm and bias and UV irradiation stress conditions.

Ultrahigh Responsivity and Robust Semiconducting Fiber Enabled by Molecular Soldering-Governed Defect Engineering for Smart Textile Optoelectronics

Semiconducting fibers (SCFs) are of significant interest to design next-generation wearable and comfortable optoelectronics that seamlessly integrate with textiles. However, the practical applications of current SCFs are always limited by poor optoelectronic performance and low mechanical robustness caused by uncontrollable multiscale structural defects. Herein, a versatile in situ molecular soldering-governed defect engineering strategy is proposed to construct ultrahigh responsivity and robust wet-spun MoS2 SCFs, by using a π-conjugated dithiolated molecule to simultaneously patch microscale sulfur vacancies within MoS2 nanosheets, diminish mesoscale interlayer voids/wrinkles, promote macroscale orientation, build long-range photoelectron percolation bridges, and provide n-doping effect. The derived MoS2 SCFs exhibit over two orders of magnitude higher responsivity (144.3 A W-1) than previously reported fiber photodetectors, 37.3-fold faster photoresponse speed (52 ms) than pristine counterpart, and remarkable bending robustness (retain 94.2% of the initial photocurrent after 50 000 bending-flattening cycles). Such superior robustness and photodetection capacity of MoS2 SCFs further enable large-scale weaving of reliable smart textile optoelectronic systems, such as direction-identifiable wireless light alarming system, modularized mechano-optical communication system, and indoor light-controlled IoT system. This work offers a universal strategy for the scalable production of mechanically robust and high-performance SCFs, opening up exciting possibilities for large-scale integration of wearable optoelectronics.

High Sensitive and Stable UV-Vis Photodetector Based on MoS2/MoO3 vdW Heterojunction

The development of new high-performance photodetectors (PDs) is currently focused on achieving small size, low power consumption, low cost, and large bandwidth. Two-dimensional (2D) materials and heterostructures offer promising approaches for the future development of optoelectronic devices. However, there has been limited research on 2D wide-bandgap semiconductor heterostructures. In this study, we successfully constructed a MoS2/MoO3 vdW heterojunction PD. This PD exhibited excellent response and significant photovoltaic behavior in the ultraviolet (UV) to visible (Vis) range. Under 365 nm UV light and 1 V bias voltage, the PD demonstrated a high responsivity of 645 mA/W, a high specific detectivity of 8.98 × 1010 Jones, and fast response speeds of 55.9/59.6 ms. At 0 V bias voltage, the responsivity reached as high as 157 mA/W. Furthermore, the PD exhibited remarkable stability in its performance. These outstanding characteristics can be attributed to the strong internal electric field created by the type II heterojunction structure and the chemical stability of the materials. This work opens a route for the application of 2D wide-bandgap semiconductor materials in optoelectronic devices.

Deterministic Fabrication and Quantum-Well Modulation of Phase-Pure 2D Perovskite Heterostructures for Encrypted Light Communication

Deterministic integration of phase-pure Ruddlesden-Popper (RP) perovskites has great significance for realizing functional optoelectronic devices. However, precise fabrications of artificial perovskite heterostructures with pristine interfaces and rational design over electronic structure configurations remain a challenge. Here, the controllable synthesis of large-area ultrathin single-crystalline RP perovskite nanosheets and the deterministic fabrication of arbitrary 2D vertical perovskite heterostructures are reported. The 2D heterostructures exhibit intriguing dual-peak emission phenomenon and dual-band photoresponse characteristic. Importantly, the interlayer energy transfer behaviors from wide-bandgap component to narrow-bandgap component modulated by comprising quantum wells are thoroughly revealed. Functional nanoscale photodetectors are further constructed based on the 2D heterostructures. Moreover, by combining the modulated dual-band photoresponse characteristic with double-beam irradiation modes, and introducing an encryption algorithm mechanism, a light communication system with high security and reliability is achieved. This work can greatly promote the development of heterogeneous integration technologies of 2D perovskites, and could provide a competitive candidate for advanced integrated optoelectronics.

Pyroelectric Photoconductive Diode for Highly Sensitive and Fast DUV Detection

Detecting high-energy photons from the deep ultraviolet (DUV) to X-rays is vital in security, medicine, industry, and science. Wide bandgap (WBG) semiconductors exhibit great potential for detecting high-energy photons. However, the implementation of highly sensitive and high-speed detectors based on WBG semiconductors has been a huge challenge due to the inevitable deep level traps and the lack of appropriate device structure engineering. Here, a sensitive and fast pyroelectric photoconductive diode (PPD), which couples the interface pyroelectric effect with the photoconductive effect based on tailored polycrystal Ga-rich GaOx (PGR-GaOx) Schottky photodiode, is first proposed. The PPD device exhibits ultrahigh detection performance for DUV and X-ray light. The responsivity for DUV light and sensitivity for X-ray are up to 104 A W-1 and 105 µC Gyair -1 cm-2, respectively. Especially, the interface pyroelectric effect induced by polar symmetry in the depletion region of the PGR-GaOx can significantly improve the response speed of the device by 105 times. Furthermore, the potential of the device is demonstrated for imaging enhancement systems with low power consumption and high sensitivity. This work fully excavates the potential of the pyroelectric effect for detectors and provides a novel design strategy to achieve sensitive and high-speed detectors.

Advancing Detectivity and Stability of Near-Infrared Organic Photodetectors via a Facile and Efficient Cathode Interlayer

Near-infrared (NIR) organic photodetectors (OPDs) are pivotal in numerous technological applications due to their excellent responsivity within the NIR region. Polyethylenimine ethoxylated (PEIE) has conventionally been employed as an electron transport layer (hole-blocking layer) to suppress dark current (JD) and enhance charge transport. However, the limitations of PEIE in chemical stability, processing conditions, environmental impact, and absorption range have spurred the development of alternative materials. In this study, we introduced a novel solution: a hybrid of sol-gel zinc oxide (ZnO) and N,N'-bis(N,N-dimethylpropan-1-amine oxide)perylene-3,4,9,10-tetracarboxylic diimide (PDINO) as the electron transport layer for NIR-OPDs. Our fabricated OPD exhibited significantly improved responsivity, reduced internal traps, and enhanced charge transfer efficiency. The detectivity, spanning from 400 to 1100 nm, surpassed ∼5 × 1012 Jones, reaching ∼1.1 × 1012 Jones at 1000 nm, accompanied by an increased responsivity of 0.47 A/W. Also, the unpackaged OPD remarkedly demonstrated stable JD and external quantum efficiency (EQE) over 1000 h under dark storage conditions. This innovative approach not only addresses the drawbacks of conventional PEIE-based OPDs but also offers promising avenues for the development of high-performance OPDs in the future.

All-solution-processed ultraflexible wearable sensor enabled with universal trilayer structure for organic optoelectronic devices

All-solution-processed organic optoelectronic devices can enable the large-scale manufacture of ultrathin wearable electronics with integrated diverse functions. However, the complex multilayer-stacking device structure of organic optoelectronics poses challenges for scalable production. Here, we establish all-solution processes to fabricate a wearable, self-powered photoplethysmogram (PPG) sensor. We achieve comparable performance and improved stability compared to complex reference devices with evaporated electrodes by using a trilayer device structure applicable to organic photovoltaics, photodetectors, and light-emitting diodes. The PPG sensor array based on all-solution-processed organic light-emitting diodes and photodetectors can be fabricated on a large-area ultrathin substrate to achieve long storage stability. We integrate it with a large-area, all-solution-processed organic solar module to realize a self-powered health monitoring system. We fabricate high-throughput wearable electronic devices with complex functions on large-area ultrathin substrates based on organic optoelectronics. Our findings can advance the high-throughput manufacture of ultrathin electronic devices integrating complex functions.

A bionic self-driven retinomorphic eye with ionogel photosynaptic retina

Bioinspired bionic eyes should be self-driving, repairable and conformal to arbitrary geometries. Such eye would enable wide-field detection and efficient visual signal processing without requiring external energy, along with retinal transplantation by replacing dysfunctional photoreceptors with healthy ones for vision restoration. A variety of artificial eyes have been constructed with hemispherical silicon, perovskite and heterostructure photoreceptors, but creating zero-powered retinomorphic system with transplantable conformal features remains elusive. By combining neuromorphic principle with retinal and ionoelastomer engineering, we demonstrate a self-driven hemispherical retinomorphic eye with elastomeric retina made of ionogel heterojunction as photoreceptors. The receptor driven by photothermoelectric effect shows photoperception with broadband light detection (365 to 970 nm), wide field-of-view (180°) and photosynaptic (paired-pulse facilitation index, 153%) behaviors for biosimilar visual learning. The retinal photoreceptors are transplantable and conformal to any complex surface, enabling visual restoration for dynamic optical imaging and motion tracking.

Air-stable self-powered photodetector based on TaSe2/WS2/TaSe2 asymmetric heterojunction with surface self-passivation

Two-dimensional (2D) transition metal dichalcogenides are highly suitable for constructing junction photodetectors because of their suspended bond-free surface and adjustable bandgap. Additional stable layers are often used to ensure the stability of photodetectors. Unfortunately, they often increase the complexity of preparation and cause performance degradation of devices. Considering the self-passivation behavior of TaSe2, we designed and fabricated a novel self-powered TaSe2/WS2/TaSe2 asymmetric heterojunction photodetector. The heterojunction photodetector shows excellent photoelectric performance and photovoltaic characteristics, achieving a high responsivity of 292 mA/W, an excellent specific detectivity of 2.43 × 1011 Jones, a considerable external quantum efficiency of 57 %, a large optical switching ratio of 2.6 × 105, a fast rise/decay time of 43/54 μs, a high open-circuit voltage of 0.23 V, and a short-circuit current of 2.28 nA under 633 nm laser irradiation at zero bias. Moreover, the device also shows a favorable optical response to 488 and 532 nm lasers. Notably, it exhibits excellent environmental long-term stability with the performance only decreasing ∼ 5.6 % after exposed to air for 3 months. This study provides a strategy for the development of air-stable self-powered photodetectors based on 2D materials.

Recent advances in 2D transition metal dichalcogenide-based photodetectors: a review

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as a highly promising platform for the development of photodetectors (PDs) owing to their remarkable electronic and optoelectronic properties. Highly effective PDs can be obtained by making use of the exceptional properties of 2D materials, such as their high transparency, large charge carrier mobility, and tunable electronic structure. The photodetection mechanism in 2D TMD-based PDs is thoroughly discussed in this article, with special attention paid to the key characteristics that set them apart from PDs based on other integrated materials. This review examines how single TMDs, TMD-TMD heterostructures, TMD-graphene (Gr) hybrids, TMD-MXene composites, TMD-perovskite heterostructures, and TMD-quantum dot (QD) configurations show advanced photodetection. Additionally, a thorough analysis of the recent developments in 2D TMD-based PDs, highlighting their exceptional performance capabilities, including ultrafast photo response, ultrabroad detectivity, and ultrahigh photoresponsivity, attained through cutting-edge methods is provided. The article conclusion highlights the potential for ground-breaking discoveries in this fast developing field of research by outlining the challenges faced in the field of PDs today and providing an outlook on the prospects of 2D TMD-based PDs in the future.

Tunable Periodic Nanopillar Array for MAPbI3 Perovskite Photodetectors with Improved Light Absorption

In the field of optoelectronic applications, the vigorous development of organic-inorganic hybrid perovskite materials, such as methylammonium lead triiodide (MAPbI3), has spurred continuous research on methods to enhance the photodetection performance. Periodic nanoarrays can effectively improve the light absorption of perovskite thin films. However, there are still challenges in fabricating tunable periodic patterned and large-area perovskite nanoarrays. In this study, we present a cost-effective and facile approach utilizing nanosphere lithography and dry etching techniques to create a large-area Si nanopillar array, which is employed for patterning MAPbI3 thin films. The scanning electron microscopy (SEM) and X-ray diffraction (XRD) results reveal that the introduction of nanopillar structures did not have a significant adverse effect on the crystallinity of the MAPbI3 thin film. Light absorption tests and optical simulations indicate that the nanopillar array enhances the light intensity within the perovskite films, leading to photodetectors with a responsivity of 11.2 A/W and a detectivity of 7.3 × 1010 Jones at 450 nm in wavelength. Compared with photodetectors without nanostructures, these photodetectors exhibit better visible light absorption. Finally, we demonstrate the application of these photodetector arrays in a prototype image sensor.

Manipulating Surface Band Bending of III-Nitride Nanowires with Ambipolar Charge-Transfer Characteristics: A Pathway Toward Advanced Photoswitching Logic Gates and Encrypted Optical Communication

The operational principle of semiconductor devices critically relies on the band structures that ultimately govern their charge-transfer characteristics. Indeed, the precise orchestration of band structure within semiconductor devices, notably at the semiconductor surface and corresponding interface, continues to pose a perennial conundrum. Herein, for the first time, this work reports a novel postepitaxy method: thickness-tunable carbon layer decoration to continuously manipulate the surface band bending of III-nitride semiconductors. Specifically, the surface band bending of p-type aluminum-gallium-nitride (p-AlGaN) nanowires grown on n-Si can be precisely controlled by depositing different carbon layers as guided by theoretical calculations, which eventually regulate the ambipolar charge-transfer behavior between the p-AlGaN/electrolyte and p-AlGaN/n-Si interface in an electrolyte environment. Enabled by the accurate modulation of the thickness of carbon layers, a spectrally distinctive bipolar photoresponse with a controllable polarity-switching-point over a wide spectrum range can be achieved, further demonstrating reprogrammable photoswitching logic gates "XOR", "NAND", "OR", and "NOT" in a single device. Finally, this work constructs a secured image transmission system where the optical signals are encrypted through the "XOR" logic operations. The proposed continuous surface band tuning strategy provides an effective avenue for the development of multifunctional integrated-photonics systems implemented with nanophotonics.

Self-powered all-quantum dot based broadband photodetectors for color imaging and heart rate monitoring

All-quantum dot based self-powered broadband (300–1000 nm) photodetectors (SnO2 QDs/PbS-I QDs/PbS-EDT QDs) were successfully fabricated, showing great potential in long-distance color recognition imaging and human heartbeat monitoring.

Emerging Schemes for Advancing 2D Material Photoconductive-Type Photodetectors

By virtue of the widely tunable band structure, dangling-bond-free surface, gate electrostatic controllability, excellent flexibility, and high light transmittance, 2D layered materials have shown indisputable application prospects in the field of optoelectronic sensing. However, 2D materials commonly suffer from weak light absorption, limited carrier lifetime, and pronounced interfacial effects, which have led to the necessity for further improvement in the performance of 2D material photodetectors to make them fully competent for the numerous requirements of practical applications. In recent years, researchers have explored multifarious improvement methods for 2D material photodetectors from a variety of perspectives. To promote the further development and innovation of 2D material photodetectors, this review epitomizes the latest research progress in improving the performance of 2D material photodetectors, including improvement in crystalline quality, band engineering, interface passivation, light harvesting enhancement, channel depletion, channel shrinkage, and selective carrier trapping, with the focus on their underlying working mechanisms. In the end, the ongoing challenges in this burgeoning field are underscored, and potential strategies addressing them have been proposed. On the whole, this review sheds light on improving the performance of 2D material photodetectors in the upcoming future.

Ultrathin Self-Powered Heavy-Metal-Free Cu-In-Se Quantum Dot Photodetectors for Wearable Health Monitoring

Mechanically deformable photodetectors (PDs) are key device components for wearable health monitoring systems based on photoplethysmography (PPG). Achieving high detectivity, fast response time, and an ultrathin form factor in the PD is highly needed for next-generation wearable PPG systems. Self-powered operation without a bulky power-supply unit is also beneficial for point-of-care application. Here, we propose ultrathin self-powered PDs using heavy-metal-free Cu-In-Se quantum dots (QDs), which enable high-performance wearable PPG systems. Although the light-absorbing QD layer is extremely thin (∼40 nm), the developed PD exhibits excellent performance (specific detectivity: 2.10 × 1012 Jones, linear dynamic range: 102 dB, and spectral range: 250-1050 nm at zero bias), which is comparable to that of conventional rigid QD-PDs employing thick Pb-chalcogenide QD layers. This is attributed to material and device strategies─materials that include Cu-In-Se QDs, a MoS2-nanosheet-blended poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer, a ZnO nanoparticle electron transport layer, Ag and ITO electrodes, and an ultrathin form factor (∼120 nm except the electrodes) that enable excellent mechanical deformability. These allow the successful application of QD-PDs to a wearable system for real-time PPG monitoring, expanding their potential in the field of mobile bioelectronics.

Polarity Conversion of the Ag2S/AgInS2 Heterojunction by Radical-Induced Positive Feedback Polydopamine Adhesion for Signal-Switchable Photoelectrochemical Biosensing

Efficient tuning of the polarity of photoactive nanomaterials is of great importance in improving the performance of photoelectrochemical (PEC) sensing platforms. Herein, polarity of the Ag2S/AgInS2 heterojunction is converted by radical-induced positive feedback polydopamine (PDA) adhesion, which is further employed to develop a signal-switchable PEC biosensor. In the nanocomposites, Ag2S and AgInS2 achieve electron-hole separation, exhibiting a strong anodic PEC response. Under the irradiation of light, the Ag2S/AgInS2 heterojunction is able to produce superoxide radical and hydroxyl radical intermediate species, leading to the polymerization of dopamine (DA) and the subsequent adhesion of PDA onto the Ag2S/AgInS2 heterojunction (Ag2S/AgInS2@PDA). By constructing a new electron-transfer pathway with PDA, the polarity of the Ag2S/AgInS2 heterojunction is converted, and the PEC response changes from anodic to cathodic photocurrents. In addition, since the photoreduction activity of PDA is stronger than that of the Ag2S/AgInS2 heterojunction, more superoxide radical can be produced by Ag2S/AgInS2@PDA once PDA is generated, thereby promoting the generation of PDA. Consequently, a positive feedback mechanism is established to enhance the polarity conversion of the Ag2S/AgInS2 heterojunction and amplify the responding to DA. As a result, the bioanalytical method is capable of sensitively quantifying DA in 10 orders of magnitude with an ultralow limit of detection. Moreover, the applicability of this biosensor in real samples is identified by measuring DA in fetal bovine serum and compared with a commercial ELISA method. Overall, this work offers an alternative perspective for adjusting photogenerated carriers of nanomaterials and designing high-performance PEC biosensors.

Supramolecular interface decoration on a polymer conductor for an intrinsically stretchable near-infrared photodiode

Stretchable photodiodes with near-infrared (NIR) response face the challenge of material deficiency. A supramolecular cathode with excellent optical, tensile and electrical properties was proposed. Together with a stretchable organic heterojunction, we developed an intrinsically stretchable NIR photodiode with high detectivity over 1011 Jones and that remained functional under 100% strain.

Self-Powered Wavelength-Dependent Dual-Polarity Response Photodetector Based on CdS@PEDOT:PSS@Au Sandwich-Structured Core-Shell Nanorod Arrays

Self-powered operation and multifunctionality have significantly oriented the development of photodetectors (PDs), which could be realized through nanoarchitecture construction and energy band structure design. Herein, a self-powered wavelength-dependent dual-polarity response PD based on (CdS@PEDOT:PSS@Au) sandwich-structured core-shell nanorod arrays (NRAs) is proposed. The synthesis approach of this three-layer heterostructure consists of a hydrothermal reaction, spin coating, and thermal evaporation. The n-CdS/p-PEDOT:PSS junction and the PEDOT:PSS/Au Schottky junction at the interfaces provide two photocurrent driving forces in opposite directions, and their contribution to the net photocurrent is controlled by the incident light wavelength due to the different light absorption ranges of the CdS core and the PEDOT:PSS shell. As a result, the polarity of the photocurrent switches from negative to positive as the wavelength increases. In addition, the response speed of negative photocurrents (∼10 ms) is faster than that of positive photocurrents (∼100 ms), which is consistent with the underlying mechanism of the dual-polarity response. Furthermore, color discrimination and imaging capabilities are demonstrated by deploying the PDs as sensing pixels and recognizing green and red patterns. The sandwich-structured core-shell NRA heterojunction system introduces a novel idea for dual-polarity response PDs.

Highly Efficient Van Der Waals Heterojunction on Graphdiyne toward the High-Performance Photodetector

Graphdiyne (GDY), a new 2D material, has recently proven excellent performance in photodetector applications due to its direct bandgap and high mobility. Different from the zero-gap of graphene, these preeminent properties made GDY emerge as a rising star for solving the bottleneck of graphene-based inefficient heterojunction. Herein, a highly effective graphdiyne/molybdenum (GDY/MoS2 ) type-II heterojunction in a charge separation is reported toward a high-performance photodetector. Characterized by robust electron repulsion of alkyne-rich skeleton, the GDY based junction facilitates the effective electron-hole pairs separation and transfer. This results in significant suppression of Auger recombination up to six times at the GDY/MoS2 interface compared with the pristine materials owing to an ultrafast hot hole transfer from MoS2 to GDY. GDY/MoS2 device demonstrates notable photovoltaic behavior with a short-circuit current of -1.3 × 10-5 A and a large open-circuit voltage of 0.23 V under visible irradiation. As a positive-charge-attracting magnet, under illumination, alkyne-rich framework induces positive photogating effect on the neighboring MoS2 , further enhancing photocurrent. Consequently, the device exhibits broadband detection (453-1064 nm) with a maximum responsivity of 78.5 A W-1 and a high speed of 50 µs. Results open up a new promising strategy using GDY toward effective junction for future optoelectronic applications.

Arrays of Fresnel Nanosystems for Enhanced Photovoltaic Performance

Omnidirectional broadband absorption of the solar radiation is pivotal to solar energy harvesting and particularly to low-cost non-tracking photovoltaic (PV) technologies. The current work numerically examines the utilization of surface arrays composed of Fresnel nanosystems (Fresnel arrays), which are reminiscent of the known Fresnel lenses, for the realization of ultra-thin silicon PV cells. Specifically, the optical and electrical performances of PV cells integrated with Fresnel arrays are compared with those of a PV cell incorporated with an optimized surface array of nanopillars (NP array). It is shown that the broadband absorption of specifically tailored Fresnel arrays can provide an enhancement of ∼20% over that of an optimized NP array. The performed analysis suggests that broadband absorption in ultra-thin films decorated with Fresnel arrays is driven by two light trapping mechanisms. The first is light trapping governed by light concentration, induced by the arrays, into the underlying substrates, which increases the optical coupling between the impinging illumination and the substrates. The second mechanism is light trapping motivated by refraction, as the Fresnel arrays induce lateral irradiance in the underlying substrates, which increases the optical interaction length and hence the overall probability for optical absorption. Finally, PV cells incorporated with surface Fresnel arrays are numerically calculated, with short-circuit current densities (J_sc) which are ∼50% higher than that of a PV cell incorporated with an optimized NP array. Also, the effect of increased surface area, due to the presence of Fresnel arrays, and its effect on surface recombination and open-circuit voltage (V_oc) are discussed.

Low Dark Current Operation in InAs/GaAs(111)A Infrared Photodetectors: Role of Misfit Dislocations at the Interface

We demonstrate an extended short-wave infrared (e-SWIR) photodetector composed of an InAs/GaAs(111)A heterostructure with interface misfit dislocations. The layer structure of the photodetector consists simply of an n-InAs optical absorption layer directly grown with a thin undoped-GaAs spacer layer on n-GaAs by molecular beam epitaxy. The lattice mismatch was abruptly relaxed by forming a misfit dislocation network at the initial stage of the InAs growth. We found high-density threading dislocations (1.5 × 10⁹ cm^-2) in the InAs layer. The current-voltage characteristics of the photodetector at 77 K had a very low dark current density (<1 × 10^-9 A cm^-2) at a positive applied voltage (electrons flow from n-GaAs to n-InAs) of up to ∼+1 V. Simulation of the band structure revealed that the direct connection of GaAs and InAs and the formation of interfacial states by the misfit dislocations play significant positive roles in suppressing dark current. Under illumination with e-SWIR light at 77 K, a clear photocurrent signal was observed with a 2.6 μm cutoff wavelength, which is consistent with the bandgap of InAs. We also demonstrated e-SWIR detection at room temperature with a 3.2 μm cutoff wavelength. The maximum detectivity at 294 K exceeds 2 × 10⁸ cm Hz^0.5 W^-1 for the detection of e-SWIR light at 2 μm.

Soft Electronics for Health Monitoring Assisted by Machine Learning

Due to the development of the novel materials, the past two decades have witnessed the rapid advances of soft electronics. The soft electronics have huge potential in the physical sign monitoring and health care. One of the important advantages of soft electronics is forming good interface with skin, which can increase the user scale and improve the signal quality. Therefore, it is easy to build the specific dataset, which is important to improve the performance of machine learning algorithm. At the same time, with the assistance of machine learning algorithm, the soft electronics have become more and more intelligent to realize real-time analysis and diagnosis. The soft electronics and machining learning algorithms complement each other very well. It is indubitable that the soft electronics will bring us to a healthier and more intelligent world in the near future. Therefore, in this review, we will give a careful introduction about the new soft material, physiological signal detected by soft devices, and the soft devices assisted by machine learning algorithm. Some soft materials will be discussed such as two-dimensional material, carbon nanotube, nanowire, nanomesh, and hydrogel. Then, soft sensors will be discussed according to the physiological signal types (pulse, respiration, human motion, intraocular pressure, phonation, etc.). After that, the soft electronics assisted by various algorithms will be reviewed, including some classical algorithms and powerful neural network algorithms. Especially, the soft device assisted by neural network will be introduced carefully. Finally, the outlook, challenge, and conclusion of soft system powered by machine learning algorithm will be discussed.

Light-Induced Bipolar Photoresponse with Amplified Photocurrents in an Electrolyte-Assisted Bipolar p-n Junction

The p-n junction with bipolar characteristics sets the fundamental unit to build electronics while its unique rectification behavior constrains the degree of carrier tunability for expanded functionalities. Herein, a bipolar-junction photoelectrode employed with a gallium nitride (GaN) p-n homojunction nanowire array that operates in electrolyte is reported, demonstrating bipolar photoresponse controlled by different wavelengths of light. Significantly, with rational decoration of a ruthenium oxides (RuO_x ) layer on nanowires guided by theoretical modeling, the resulting RuO_x /p-n GaN photoelectrode exhibits unambiguously boosted bipolar photoresponse by an enhancement of 775% and 3000% for positive and negative photocurrents, respectively, compared to the pristine nanowires. The loading of the RuO_x layer on nanowire surface optimizes surface band bending, which facilitates charge transfer across the GaN/electrolyte interface, meanwhile promoting the efficiency of redox reaction for both hydrogen evolution reaction and oxygen evolution reaction which corresponds to the negative and positive photocurrents, respectively. Finally, a dual-channel optical communication system incorporated with such photoelectrode is constructed with using only one photoelectrode to decode dual-band signals with encrypted property. The proposed bipolar device architecture presents a viable route to manipulate the carrier dynamics for the development of a plethora of multifunctional optoelectronic devices for future sensing, communication, and imaging systems.

Degradable optical resonators as in situ microprobes for microscopy-based observation of enzymatic hydrolysis

Optical resonators work as precise physical and chemical sensors. Here, we assemble a whispering gallery mode resonator from a natural polymer, fibroin protein, and successfully observe its catalytic degradation reaction as a spectral shift. This methodology will contribute to the precise in situ observation of biological reactions by optical microscopy.

Organic and quantum dot hybrid photodetectors: towards full-band and fast detection

Photodetectors hold great application potential in many fields such as image sensing, night vision, infrared communication and health monitoring. To date, commercial photodetectors mainly rely on inorganic semiconductors, e.g., monocrystalline silicon, germanium, and indium selenide/gallium with complex and costly fabrication, which are hardly compatible with wearable electronics. In contrast, organic conjugated materials provide great superiority in flexibility and stretchability. In this Highlight, the unique properties of organic and quantum dot photodetectors were firstly discussed to reveal the great complementarity of the two technologies. Subsequently, the recent advance of organic/quantum dot hybrid photodetectors was outlined to highlight their great potential in developing broadband and high-performance photodetectors. Moreover, the multiple functions (e.g., dual-band detection and upconversion detection) of hybrid photodetectors were highlighted for their promising application in image sensing and infrared detection. Lastly, we present a forword-looking discussion on the challenges and our insights for the further advancement of hybrid photodetectors. This work may spark enormous research attention in organic/quantum dot electronics and advance the commercial applications.

In Situ Encapsulated Moiré Perovskite for Stable Photodetectors with Ultrahigh Polarization Sensitivity

Nanostructures provide a simple, effective, and low-cost route to enhance the light-trapping capability of optoelectronic devices. In recent years, nano-optical structures have been widely used in perovskite optoelectronic devices to greatly enhance the device performance. However, the inherent instability of perovskite materials hinders the practical application of these nanostructured optoelectronic devices. Here, in situ encapsulated moiré lattice perovskite photodetectors (PDs) by two nanograting-structured soft templates with relative rotation angles is fabricated. The confinement growth of the two nanograting templates leads to crystal growth with moiré lattice structure, which improves the light-harvesting ability of the perovskite crystal, thereby improving the device performance. The PD exhibits responsivity to 1026.5 A W^-1 . The Moiré lattice-perovskite-based PD maintained 95% of the initial performance after 223 days. After being continuously sprayed with water moist for 180 min, the performance is maintained at 95.7% of its initial level. The nanograting structure endows the device with high polarization sensitivity of I_max /I_min as high as 9.1.

Interfacial Assembly of Ti3 C2 Tx /ZnIn2 S4 Heterojunction for High-Performance Photodetectors

Two-dimensional (2D) materials have emerged as prospective candidates for electronics and optoelectronics applications as they can be easily fabricated through liquid exfoliation and used to fabricate various structures by further subsequent processing methods in addition to their extraordinary and unique optoelectronic properties. Herein, the Ti3 C2 Tx /ZIS heterostructure with nanometer-thick Ti3 C2 Tx -MXene and ZnIn2 S4 (ZIS) films is fabricated by successive interfacial assembly of liquid exfoliated 2D MXene and ZnIn2 S4 nanoflakes. Benefiting from the superior light-harvesting capability and low dark current of ZnIn2 S4 , the limited absorbance, large scattering coefficient, and high dark current disadvantages of MXene are ameliorated. Meanwhile, the separation and transport of photogenerated carriers in ZnIn2 S4 are improved due to the excellent electrical conductivity of Ti3 C2 Tx nanoflakes. As a result, the as-prepared Ti3 C2 Tx /ZIS heterostructure photodetector has excellent optoelectronic characteristics in terms of a high responsivity of 1.04 mA W-1 , a large specific detectivity up to 1 × 1011  Jones, a huge on/off ratio at around 105 , and an ultralow dark current at ≈10-12  A. This work demonstrates a convenient method to construct heterostructured photodetectors by liquid exfoliated 2D nanoflakes, the as-fabricated Ti3 C2 Tx /ZIS heterostructured photodetectors show promising application potential for low-cost, reliable, and high-performance photodetectors.

Anisotropic photoresponse behavior of a LaAlO3 single-crystal-based vacuum-ultraviolet photodetector

Nowadays, vacuum-ultraviolet (VUV) photodetectors (PDs) have attracted extensive attention owing to their potential applications in space exploration, radiation monitoring, and the semiconductor industry. Benefiting from its intrinsic ultra-wide band-gap, chemical robustness, and low-cost features, LaAlO₃ shows great promise in developing next-generation compact, cheap, and easy-to-fabricate VUV PDs. In this work, we report the unique anisotropic photoresponse behavior of LaAlO₃ single crystals for VUV photodetection applications. First of all, with the guidance of density functional theory (DFT) calculations along with the comprehensive material characterization, the anisotropic carrier transport behavior of LaAlO₃ single crystals was confirmed. Thereafter, after exploring the metal-semiconductor-metal (MSM) device configuration along different substrate orientations, including (100), (110), and (111)-LaAlO₃ single crystals, we found that the (110)-LaAlO₃ VUV PD exhibits the best device performance under VUV illumination, with a responsivity of 2.23 mA W^-1, a high detectivity of 3.72 × 10¹¹ Jones, and a photo-to-dark-current ratio of 5.48 × 10³. This work not only provides a feasible avenue to explore the anisotropic optoelectronic behavior of ultra-wide band-gap semiconductors but also expands the application of the low-cost oxide perovskite family in the field of VUV photodetection.

MultiShapeC, an algorithm to assess concentration in multi-shape nanoparticle samples: nanosilver, a case study

Shape, size, and dispersity play a crucial role in the calculation of colloidal nanoparticle concentrations, which results in remarkable differences in the determination of parameters like Stern–Volmer constants. In this work, we propose an algorithm named MultiShapeC to include the variability in shapes and polydispersity in the concentration calculation. This algorithm was validated using the quenching of carbazole fluorescence emission by silver nanoparticles.

An algorithm to include multi-shape and polydispersity in the nanoparticle concentration calculation is presented.