Novel p–p Heterojunctions Self‐Powered Broadband Photodetectors with Ultrafast Speed and High Responsivity

Novel inorganic/organic self‐powered UV–vis photodetectors based on single Se microtube and conducting polymers—polyaniline (PANI), polypyrrole (PPy), and poly(3,4‐ethylenedioxythiophene) (PEDOT)—are fabricated. The conducting polymers are directly coated on the surface of a single Se microtube via a facile and low‐cost in situ polymerization method. The integrated Se/PANI photodetector with 45‐nm‐thick PANI layer shows excellent self‐powered behavior under UV–vis light illumination. In particular, it exhibits high on/off ratio of 1.1 × 10³, responsivity (120 mA W^?1), large detectivity (3.78 × 10¹¹ Jones), and ultrafast response speed (rise time of 4.5 µs and fall time of 2.84 ms) at zero bias at 610 nm (0.434 mW cm^?2)‐light illumination. Moreover, the individual Se/PPy and Se/PEDOT self‐powered photodetectors also exhibit fast and stable responses, including responsivity of 70 and 5.5 mA W^?1, rise time of 0.35 and 1.00 ms, fall time of 16.97 and 9.78 ms, respectively. Given the simple device architecture and low cost fabrication process, this work provides a promising way to fabricate inorganic/organic, high‐performance, self‐powered photodetectors.     

Energy Harvesting for Nanostructured Self‐Powered Photodetectors

Harvesting the available forms of energies in the environment to create self‐powered nanosystems is now becoming a technological reality. Self‐powered nanodevices and nanosystems are expected to play a crucial role in the future development of nanotechnology because of their specific role in fundamental studies and nanotechnological applications, mainly due to their size‐dependent properties and independent, sustainable, maintainance‐free operation. As a new field in self‐powered nanotechnology‐related research, self‐powered photodetectors have been developed which exhibit a much faster photoresponse and higher photosensitivity than the conventional photoconductor‐based photodetectors. Herein, the energy‐havesting techniques are discussed and their prospects for application in self‐powered photodetectors are summarized. Moreover, potential future directions of this research area are highlighted.     

High‐Performance Ferroelectric Polymer Side‐Gated CdS Nanowire Ultraviolet Photodetectors

An efficient ferroelectric‐enhanced side‐gated single CdS nanowire (NW) ultraviolet (UV) photodetector at room temperature is demonstrated. With the ultrahigh electrostatic field from polarization of ferroelectric polymer, the depletion of the intrinsic carriers in the CdS NW channel is achieved, which significantly reduces the dark current and increases the sensitivity of the UV photodetector even after the gate voltage is removed. Meanwhile, the low frequency noise current power of the device reaches as low as 4.6 × 10^?28 A² at a source‐drain voltage V_ds = 1 V. The single CdS NW UV photodetector exhibits high photoconductive gain of 8.6 × 10⁵, responsivity of 2.6 × 10⁵ A W^?1, and specific detectivity (D*) of 2.3 × 10¹⁶ Jones at a low power density of 0.01 mW cm^?2 for λ = 375 nm. In addition, the spatially resolved scanning photocurrent mapping across the device shows strong photocurrent signals near the metal contacts. This is promising for the design of a controllable, high‐performance, and low power consumption ultraviolet photodetector.     

Response Enhancement of Self‐Powered Visible‐Blind UV Photodetectors by Nanostructured Heterointerface Engineering

The development of efficient photodetectors (PDs) for ultraviolet (UV) light is of great importance for many applications. In this paper, a novel approach is proposed for boosting the performances of self‐powered PDs. Visible‐blind UV‐A PDs are built by combining a mesoporous TiO₂ layer with a Spiro‐OMeTAD layer. The nanostructured heterointerface is engineered by inserting a self‐assembled layer of organic modifiers. By choosing 4‐nitrobenzoic acid (NBA), the responsivity is boosted by 70% compared to the pristine devices. It achieves 64 mA W^?1 at 0 V bias, 380 nm, and 1 mW cm^?2. The PD displays a very high sensitivity (>10⁴), a fast response time (<3 ms), a high stability, and repeatability. 4‐chlorobenzoic acid, 4‐methoxy benzoic acid, 4‐nitro benzoic acid, and β‐alanine surface modifiers are studied by a combined experimental and theoretical approach. Their dipole moment is calculated. Their presence induces a step in the vacuum energy and the formed dipole field dramatically affects the charge transfer and then the photocurrent/photoresponse of the device. The higher responsivity of the NBA‐modified PD is thus explained by the better and faster electron charge transfer toward the electrical contact on TiO₂.     

Solution‐Processed Self‐Powered Transparent Ultraviolet Photodetectors with Ultrafast Response Speed for High‐Performance Communication System

Transparent ultraviolet (UV) photodetectors are an essential component of next‐generation “see‐through” electronics. However, the current photodetectors often suffer from relatively slow response speeds and high driving voltages. Here, all‐solution‐processed UV photodetectors are reported that are facilely prepared from environmentally friendly and abundant materials. The UV photodetectors are composed of a titanium dioxide thin film as the photosensitive layer sandwiched between two different transparent electrodes to form asymmetric Schottky junctions. The photodetector with high optical transparency can operate at zero bias because of spontaneous separation of photogenerated electron–hole pairs by the built‐in electric field. The resulting self‐powered photodetector displays high sensitivity to broadband UV light (200–400 nm). In particular, an ultrafast response speed up to 44 ns is obtained, representing a significant improvement over those of the conventional transparent photodetectors. Moreover, the photodetector has been successfully applied, for the first time, in a UV communication system as the self‐powered signal receiver. This work uniquely combines the features of high optical transparency and self‐power ability into UV photodetectors and would enable a broad range of optoelectronic applications.     

Self‐Limited Epitaxial Growth of Ultrathin Nonlayered CdS Flakes for High‐Performance Photodetectors

2D nonlayered materials that possess appealing properties are entering the researchers' vision. However, direct access to the 2D level of these materials is still a great challenge due to the instrinsic isotropic chemical bond. This work presents the initially self‐limited epitaxial growth of ultrathin nonlayered CdS flakes (as thin as 6 nm) on mica substrate with a large domain size (>40 µm) by employing In₂S₃ as the passivation agent. Besides, the thickness and sizes of the products could be tunable by the addition level of In₂S₃ amount. The growth mechanism is evidenced via experiments and theoretical calculations, which is attributed to the surface distortion effect of In–S motif and the preference of local environments for In on the CdS (0001) surface. The photodetector designed on CdS flake demonstrates a high photoswitching ratio (up to 10³), a high detectivity (D* ≈ 2.71 × 10⁹ Jones), and fast photoresponse speed (τ_R = 14 ms, τ_D = 8 ms). The as‐proposed self‐limited epitaxial growth method opens a new avenue to synthetize 2D nonlayered materials and will promote their further applications in novel optoelectronic devices.     

Highly Sensitive Low‐Bandgap Perovskite Photodetectors with Response from Ultraviolet to the Near‐Infrared Region

It is a great challenge to obtain broadband response perovskite photodetectors (PPDs) due to the relatively large bandgaps of the most used methylammonium lead halide perovskites. The response range of the reported PPDs is limited in the ultraviolet–visible range. Here, highly sensitive PPDs are successfully fabricated with low bandgap (≈1.25 eV) (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite as active layers, exhibiting a broadband response from 300 to 1000 nm. The performance of the PPDs can be optimized by adjusting the thicknesses of the perovskite and C₆₀ layers. The optimized PPDs with 1000 nm thick perovskite layer and 70 nm thick C₆₀ layer exhibit an almost flat external quantum efficiency (EQE) spectrum from 350 to 900 nm with EQE larger than 65% under ?0.2 V bias. Meanwhile, the optimized PPDs also exhibit suppressed dark current of 3.9 nA, high responsivity (R ) of over 0.4 A W^?1, high specific detectivity (D* ) of over 10¹² Jones in the near‐infrared region under ?0.2 V. Such highly sensitive broadband response PPDs, which can work well as self‐powered conditions, offer great potential applications in multicolor light detection.     

MoS2/Si Heterojunction with Vertically Standing Layered Structure for Ultrafast,High‐Detectivity,Self‐Driven Visible–Near Infrared Photodetectors

As an interesting layered material, molybdenum disulfide (MoS₂) has been extensively studied in recent years due to its exciting properties. However, the applications of MoS₂ in optoelectronic devices are impeded by the lack of high‐quality p–n junction, low light absorption for mono‐/multilayers, and the difficulty for large‐scale monolayer growth. Here, it is demonstrated that MoS₂ films with vertically standing layered structure can be deposited on silicon substrate with a scalable sputtering method, forming the heterojunction‐type photodetectors. Molecular layers of the MoS₂ films are perpendicular to the substrate, offering high‐speed paths for the separation and transportation of photo‐generated carriers. Owing to the strong light absorption of the relatively thick MoS₂ film and the unique vertically standing layered structure, MoS₂/Si heterojunction photodetectors with unprecedented performance are actualized. The self‐driven MoS₂/Si heterojunction photodetector is sensitive to a broadband wavelength from visible light to near‐infrared light, showing an extremely high detectivity up to ≈10¹³ Jones (Jones = cm Hz^1/2 W^?1), and an ultrafast response speed of ≈3 μs. The performance is significantly better than the photodetectors based on mono‐/multilayer MoS₂ nanosheets. Additionally, the MoS₂/Si photodetectors exhibit excellent stability in air for a month. This work unveils the great potential of MoS₂/Si heterojunction for optoelectronic applications.     

Boosting Photocurrent via Heating BiFeO3 Materials for Enhanced Self‐Powered UV Photodetectors

BiFeO₃ (BFO) is a potentially important Pb‐free ferroelectric with a narrow bandgap and is expected to become a novel photodetector. The photocurrent in BFO₃ strongly depends on the temperature but only a few studies have investigated in detail the relationships between photocurrent and temperature. Here, the temperature‐dependent photocurrent and the corresponding photosensing properties of a Ag/BFO/indiumtin oxide (ITO) photodetector based on an optimized planar‐structured electrode configuration are investigated. The photocurrent and responsivity of the BFO₃‐based photodetector can first be increased and then be decreased with increasing temperature. The largest photocurrent and responsivity can reach 51.5 µA and 6.56 × 10^?4 A W^?1 at 66.1 °C, which is enhanced 126.3% as compared with that at room temperature. This may be caused by the temperature‐modulated bandgap and barrier height in Ag/BFO/ITO device. This study clarifies the relationship between photosensing performance and the operating temperature of BFO‐based photodetector and will push forward the application of ferroelectric materials in photoelectric field.     

Low‐Bandgap Methylammonium‐Rubidium Cation Sn‐Rich Perovskites for Efficient Ultraviolet–Visible–Near Infrared Photodetectors

Solution‐processed and low‐temperature Sn‐rich perovskites show their low bandgap of about 1.2 eV, enabling potential applications in next‐generation cost‐effective ultraviolet (UV)–visible (vis)–near infrared (NIR) photodetection. Particularly, the crystallization (crystallinity and orientation) and film (smooth and dense film) properties of Sn‐rich perovskites are critical for efficient photodetectors, but are limitedly studied. Here, controllable crystallization for growing high‐quality films with the improvements of increased crystallinity and strengthened preferred orientation through a introducing rubidium cation into the methylammonium Sn‐Pb perovskite system (65% Sn) is achieved. Fundamentally, the theoretical results show that rubidium incorporation causes lower surface energy of (110) plane, facilitating growth in the dominating plane and suppressing growth of other competing planes. Consequently, the methylammonium‐rubidium Sn‐Pb perovskite photodetectors simultaneously achieve larger photocurrent and lower noise current. Finally, highly efficient UV–vis–NIR (300–1100 nm) photodetectors with record‐high linear dynamic range of 110 and 3 dB cut‐off frequency reaching 1 MHz are demonstrated. This work contributes to enriching the cation selection in Sn‐Pb perovskite systems and offering a promising candidate for low‐cost UV–vis–NIR photodetection.     

Environmentally Robust Black Phosphorus Nanosheets in Solution: Application for Self‐Powered Photodetector

Large‐size 2D black phosphorus (BP) nanosheets have been successfully synthesized by a facile liquid exfoliation method. The as‐prepared BP nanosheets are used to fabricate electrodes for a self‐powered photodetector and exhibit preferable photoresponse activity as well as environmental robustness. Photoelectrochemical (PEC) tests demonstrate that the current density of BP nanosheets can reach up to 265 nA cm^?2 under light irradiation, while the dark current densities fluctuate near 1 nA cm^?2 in 0.1 M KOH. UV–vis and Raman spectra are carried out and confirm the inherent optical and physical properties of BP nanosheets. In addition, the cycle stability measurement exhibits no detectable distinction after processing 50 and 100 cycles, while an excellent on/off behavior is still preserved even after one month. Furthermore, the PEC performance of BP nanosheets‐based photodetector is evaluated in various KOH concentrations, which demonstrates that the as‐prepared BP nanosheets may have a great potential application in self‐powered photodetector. It is anticipated that the present work can provide fundamental acknowledgement of the performance of a PEC‐type BP nanosheets‐based photodetector, offering extendable availabilities for 2D BP‐based heterostructures to construct high‐performance PEC devices.     

Flexible and Self‐Powered Lateral Photodetector Based on Inorganic Perovskite CsPbI3–CsPbBr3 Heterojunction Nanowire Array

Self‐powered perovskite photodetectors mainly adopt the vertical heterojunction structure composed of active layer, electron–hole transfer layers, and electrodes, which results in the loss of incident light and interfacial accumulation of defects. To address these issues, a self‐powered lateral photodetector based on CsPbI₃–CsPbBr₃ heterojunction nanowire arrays is designed on both a rigid glass and a flexible polyethylene naphthalate substrate using an in situ conversion and mask‐assisted electrode fabrication method. Through adding the polyvinyl pyrrolidone and optimizing the concentration of precursors under the pressure‐assisted moulding process, both the crystallinity and stability of perovskite nanowire array are improved. The nanowire array–based lateral device shows a high responsivity of 125 mA W^?1 and a fast rise and decay time of 0.7 and 0.8 ms under a self‐powered operation condition. This work provides a new strategy to fabricate perovskite heterojunction nanoarrays towards self‐powered photodetection.     

Temperature Performance of Doping‐Free Top‐Gate CNT Field‐Effect Transistors: Potential for Low‐ and High‐Temperature Electronics

High‐performance top‐gate carbon nanotube (CNT) field‐effect transistors (FETs) have been fabricated via a doping‐free fabrication process in which the polarity of the CNT FET is controlled by the injection of carriers from the electrodes, instead of using dopants. The performance of the doping‐free CNT FETs is systemically investigated over a wide temperature range, from very low temperatures of down to 4.3 K up to 573 K, and analyzed using several temperature‐dependent key device parameters including the ON/OFF state current and ratio, carrier mobility, and subthreshold swing. It is demonstrated that for ballistic and quasi‐ballistic CNT FETs, the operation of the CNT FETs is largely independent of the presence of dopant, thus avoiding detrimental effects due to dopant freeze‐out at low temperature and dopant diffusion at high temperature, and making it possible to use doping‐free CNT FETs in both low‐ and high‐temperature electronics. A new method is also proposed for extracting the band‐gap and diameter of a semiconducting CNT from the temperature dependent OFF‐state current and shown to yield results that are consistent with AFM measurements.     

Ultrahigh‐Performance Flexible and Self‐Powered Photodetectors with Ferroelectric P(VDF‐TrFE)/Perovskite Bulk Heterojunction

Flexible and self‐powered perovskite photodetectors attract widespread research interests due to their potential applications in portable and wearable optoelectronic devices. However, the reported devices mainly adopt an independent layered structure with complex fabrication processes and high carrier recombination. Herein, an integrated ferroelectric poly(vinylidene‐fluoride‐trifluoroethylene) (P(VDF‐TrFE)) and perovskite bulk heterojunction film photodetector on the polyethylene naphthalate substrate is demonstrated. Under the optimum treatment conditions (the polarization voltage and time, and the concentration of P(VDF‐TrFE)), the photodetector exhibits a largely enhanced performance compared to the pristine perovskite device. The resulting device exhibits ultrahigh performance with a large detectivity (1.4 × 10¹³ Jones) and fast response time (92/193 µs) at the wavelength of 650 nm. The improved performance is attributed to the fact that the polarized P(VDF‐TrFE)/perovskite hybrid film provides a stronger built‐in electric field to facilitate the separation and transportation of photogenerated carriers. These findings provide a new route to design self‐powered photodetectors from the aspect of device structure and carrier transport.     

Semitransparent,Flexible, and Self‐Powered Photodetectors Based on Ferroelectricity‐Assisted Perovskite Nanowire Arrays

Transparent and flexible photodetectors hold great promise in next‐generation portable and wearable optoelectronic devices. However, most of the previously reported devices need an external energy power source to drive its operation or require complex fabrication processes. Herein, designed is a semitransparent, flexible, and self‐powered photodetector based on the integrated ferroelectric poly(vinylidene‐fluoride‐trifluoroethylene) (P(VDF‐TrFE)) and perovskite nanowire arrays on the flexible polyethylene naphthalate substrate via a facile imprinting method. Through optimizing the treatment conditions, including polarization voltage, polarization time, and the concentration of P(VDF‐TrFE), the resulting device exhibits remarkable detectivity (7.3 × 10¹² Jones), fast response time (88/154 µs) at zero bias, as well as outstanding mechanical stability. The excellent performance is attributed to the efficient charge separation and transport originating from the highly oriented 1D transport pathway and the polarization‐induced internal electric field within P(VDF‐TrFE)/perovskite hybrid nanowire arrays.     

Self‐Powered Flexible TiO2 Fibrous Photodetectors: Heterojunction with P3HT and Boosted Responsivity and Selectivity by Au Nanoparticles

A novel inorganic–organic heterojunction (TiO₂/P3HT (poly(3‐hexylthiophene)) is easily prepared by a combination of anodization and vacuumed dip‐coating methods, and the constructed flexible fibrous photodetector (FPD) exhibits high‐performance self‐powered UV–visible broadband photoresponse with fast speed, high responsivity, and good stability, as well as highly stable performance at bending states, showing great potential for wearable electronic devices. Moreover, Au nanoparticles are deposited to further boost the responsivity and selectivity by regulating the sputtering intervals. The optimal Au/TiO₂/P3HT FPD yields an ≈700% responsivity enhancement at 0 V under 350 nm illumination. The sharp cut‐off edge and high UV–visible rejection ratio (≈17 times higher) indicate a self‐powered flexible UV photodetector. This work provides an effective and versatile route to modulate the photoelectric performance of flexible electronic devices.     

Comprehensive Pyro‐Phototronic Effect Enhanced Ultraviolet Detector with ZnO/Ag Schottky Junction

As a coupling effect of pyroelectric and photoelectric effect, pyro‐phototronic effect has demonstrated an excellent tuning role for fast response p–n junction photodetectors (PDs). Here, a comprehensive pyro‐phototronic effect is utilized to design and fabricate a self‐powered and flexible ultraviolet PD based on the ZnO/Ag Schottky junction. By using the primary pyroelectric effect, the maximal transient photoresponsivity of the self‐powered PDs can reach up to 1.25 mA W^?1 for 325 nm illumination, which is improved by 1465% relative to that obtained from the steady‐state signal. The relative persistent secondary pyroelectric effect weakens the height of Schottky barrier, leading to a reduction of the steady‐state photocurrent with an increase in the power density. When the power density is large enough, the steady‐state photocurrent turns into a reverse direction. The corresponding tuning mechanisms of the comprehensive pyro‐phototronic effect on transient and steady‐state photocurrent are revealed based on the bandgap diagrams. The results may help us to further clarify the mechanism of the pyro‐phototronic effect on the photocurrent and also provide a potential way to optimize the performance of self‐powered PDs.     

Surface‐Modified Low‐Temperature Solid Oxide Fuel Cell

This paper reports both experimental and theoretical results of the role of surface modification on the oxygen reduction reaction in low‐temperature solid oxide fuel cells (LT‐SOFC). Epitaxial ultrathin films of yttria‐doped ceria (YDC) cathode interlayers (<10–130 nm) are grown by pulsed laser deposition (PLD) on single‐crystalline YSZ(100). Fuel cell current–voltage measurements and electrochemical impedance spectroscopy are performed in the temperature range of 350 °C ≈ 450 °C. Quantum mechanical simulations of oxygen incorporation energetics support the experimental results and indicate a low activation energy of only 0.07 eV for YDC, while the incorporation reaction on YSZ is activated by a significantly higher energy barrier of 0.38 eV. Due to enhanced oxygen incorporation at the modified Pt/YDC interface, the cathodic interface resistance is reduced by two‐fold, while fuel cell performance shows more than a two‐fold enhancement with the addition of an ultrathin YDC interlayer at the cathode side of an SOFC element. The results of this study open up opportunities for improving cell performance, particularly of LT‐SOFCs by adopting surface modification of YSZ surface with catalytically superior, ultrathin cathodic interlayers.