Self‐Powered Si/CdS Flexible Photodetector with Broadband Response from 325 to 1550 nm Based on Pyro‐phototronic Effect: An Approach for Photosensing below Bandgap Energy

Cadmium sulfide (CdS) has received widespread attention as the building block of optoelectronic devices due to its extraordinary optoelectronic properties, low work function, and excellent thermal and chemical stability. Here, a self‐powered flexible photodetector (PD) based on p‐Si/n‐CdS nanowires heterostructure is fabricated. By introducing the pyro‐phototronic effect derived from wurtzite structured CdS, the self‐powered PD shows a broadband response range, even beyond the bandgap limitation, from UV (325 nm) to near infrared (1550 nm) under zero bias with fast response speed. The light‐induced pyroelectric potential is utilized to modulate the optoelectronic processes and thus improve the photoresponse performance. Lasers with different wavelengths have different effects on the self‐powered PDs and corresponding working mechanisms are carefully investigated. Upon 325 nm laser illumination, the rise time and fall time of the self‐powered PD are 245 and 277 µs, respectively, which are faster than those of most previously reported CdS‐based nanostructure PDs. Meanwhile, the photoresponsivity R and specific detectivity D* regarding to the relative peak‐to‐peak current are both enhanced by 67.8 times, compared with those only based on the photovoltaic effect‐induced photocurrent. The self‐powered flexible PD with fast speed, stable, and broadband response is expected to have extensive applications in various environments.     

Piezo‐Phototronic Matrix via a Nanowire Array

Piezoelectric semiconductors, such as ZnO and GaN, demonstrate multiproperty coupling effects toward various aspects of mechanical, electrical, and optical excitation. In particular, the three‐way coupling among semiconducting, photoexcitation, and piezoelectric characteristics in wurtzite‐structured semiconductors is established as a new field, which was first coined as piezo‐phototronics by Wang in 2010. The piezo‐phototronic effect can controllably modulate the charge‐carrier generation, separation, transport, and/or recombination in optical‐electronic processes by modifying the band structure at the metal–semiconductor or semiconductor–semiconductor heterojunction/interface. Here, the progress made in using the piezo‐phototronic effect for enhancing photodetectors, pressure sensors, light‐emitting diodes, and solar cells is reviewed. In comparison with previous works on a single piezoelectric semiconducting nanowire, piezo‐phototronic nanodevices built using nanowire arrays provide a promising platform for fabricating integrated optoelectronics with the realization of high‐spatial‐resolution imaging and fast responsivity.     

Regulation of Charge Carrier Dynamics in ZnO Microarchitecture‐Based UV/Visible Photodetector via Photonic‐Strain Induced Effects

A feasible, morphological influence on photoresponse behavior of ZnO microarchitectures such as microwire (MW), coral‐like microstrip (CMS), fibril‐like clustered microwire (F‐MW) grown by one‐step carrier gas/metal catalyst “free” vapor transport technique is reported. Among them, ZnO F‐MW exhibits higher photocurrent (I_Ph) response, i.e., I_{Ph/ZnO F‐MW} > I_{Ph/ZnO CMS} > I_{Ph/ZnO MW}. The unique structural alignment of ZnO F‐MW has enhanced the I_Ph from 14.2 to 186, 221, 290 µA upon various light intensities such as 0 to 6, 11, 17 mW cm^?2 at λ_{405 nm}. Herein, the nature of the as‐fabricated ZnO photodetector (PD) is also demonstrated modulated by tuning the inner crystals piezoelectric potential through the piezo‐phototronic effect. The I_Ph response of PD decreases monotonically by introducing compressive strain along the length of the device, which is due to the synergistic effect between the induced piezoelectric polarization and photogenerated charge carriers across the metal–semiconductor interface. The current behavior observed at the two interfaces acting as the source (S) and drain (D) is carefully investigated by analyzing the Schottky barrier heights (Φ_SB). This work can pave the way for the development of geometrically modified strain induced performances of PD to promote next generation self‐powered optoelectronic integrated devices and switches.     

Piezo‐Phototronic Effect Controlled Dual‐Channel Visible light Communication (PVLC) Using InGaN/GaN Multiquantum Well Nanopillars

Visible light communication (VLC) simultaneously provides illumination and communication via light emitting diodes (LEDs). Keeping a low bit error rate is essential to communication quality, and holding a stable brightness level is pivotal for illumination function. For the first time, a piezo‐phototronic effect controlled visible light communication (PVLC) system based on InGaN/GaN multiquantum wells nanopillars is demonstrated, in which the information is coded by mechanical straining. This approach of force coding is also instrumental to avoid LED blinks, which has less impact on illumination and is much safer to eyes than electrical on/off VLC. The two‐channel transmission mode of the system here shows great superiority in error self‐validation and error self‐elimination in comparison to VLC. This two‐channel PVLC system provides a suitable way to carry out noncontact, reliable communication under complex circumstances.     

Piezo‐Phototronic Effect on Selective Electron or Hole Transport through Depletion Region of Vis–NIR Broadband Photodiode

Silicon underpins nearly all microelectronics today and will continue to do so for some decades to come. However, for silicon photonics, the indirect band gap of silicon and lack of adjustability severely limit its use in applications such as broadband photodiodes. Here, a high‐performance p‐Si/n‐ZnO broadband photodiode working in a wide wavelength range from visible to near‐infrared light with high sensitivity, fast response, and good stability is reported. The absorption of near‐infrared wavelength light is significantly enhanced due to the nanostructured/textured top surface. The general performance of the broadband photodiodes can be further improved by the piezo‐phototronic effect. The enhancement of responsivity can reach a maximum of 78% to 442 nm illumination, the linearity and saturation limit to 1060 nm light are also significantly increased by applying external strains. The photodiode is illuminated with different wavelength lights to selectively choose the photogenerated charge carriers (either electrons or holes) passing through the depletion region, to investigate the piezo‐phototronic effect on electron or hole transport separately for the first time. This is essential for studying the basic principles in order to develop a full understanding about piezotronics and it also enables the development of the better performance of optoelectronics.     

Progress in Piezo‐Phototronic‐Effect‐Enhanced Light‐Emitting Diodes and Pressure Imaging

Wurtzite materials exhibit both semiconductor and piezoelectric properties under strains due to the non‐central symmetric crystal structures. The three‐way coupling of semiconductor properties, piezoelectric polarization and optical excitation in ZnO, GaN, CdS and other piezoelectric semiconductors leads to the emerging field of piezo‐phototronics. This effect can efficiently manipulate the emission intensity of light‐emitting diodes (LEDs) by utilizing the piezo‐polarization charges created at the junction upon straining to modulate the energy band diagrams and the optoelectronic processes, such as generation, separation, recombination and/or transport of charge carriers. Starting from fundamental physics principles, recent progress in piezo‐phototronic‐effect‐enhanced LEDs is reviewed; following their development from single‐nanowire pressure‐sensitive devices to high‐resolution array matrices for pressure‐distribution mapping applications. The piezo‐phototronic effect provides a promising method to enhance the light emission of LEDs based on piezoelectric semiconductors through applying static strains, and may find perspective applications in various optoelectronic devices and integrated systems.     

Recent Progress on Piezotronic and Piezo‐Phototronic Effects in III‐Group Nitride Devices and Applications

Wurtzite‐structured III‐group nitrides, like GaN, InN, AlN, and their alloys, present both piezoelectric and semiconducting properties under straining owing to the polarization of ions in a crystal with non‐central symmetry. The piezoelectric polarization charges are created at the interface when a strain is applied. As a result, a piezoelectric potential (piezopotential) is produced, which is used as a “gate” to tune/control the charge transport behavior across a metal/semiconductor interface or a p‐n junction. This is called as piezotronic effect. A series of piezotronic devices and applications have been developed, such as piezotronic nanogenerators (NGs), piezotronic transistors, piezotronic logic devices, piezotronic electromechanical memories, piezotronic enhanced biochemical, and gas sensors and so on. With the flourished development of piezotronic effect, the piezo‐phototronic effect, as the three‐way coupling of piezoelectric polarization, semiconductor properties, and optical excitation, utilizes the piezopotential to modulate the energy band profile and control the carrier generation, transportation, separation, and/or recombination for improving performances of optoelectronic devices. This paper intends to provide an overview of the rapid progress in the emerging fields of piezotronics and piezo‐phototronics, covering from the fundamental principles to devices and applications. This study will provide important insight into the potential applications of GaN based electronic/optoelectronic devices in sensing, active flexible/stretchable electronics/optoelectronics, energy harvesting, human‐machine interfacing, biomedical diagnosis/therapy, and prosthetics.

     

Translucent Photodetector with Blended Nanowires–Metal Oxide Transparent Selective Electrode Utilizing Photovoltaic and Pyro‐Phototronic Coupling Effect

ZnO is a potential candidate for photodetection utilizing the pyroelectric effect. Here, a self‐biased and translucent photodetector with the configuration of Cu₄O₃/ZnO/FTO/Glass is designed and fabricated. In addition, the pyroelectric effect is effectively harvested using indium tin oxide (ITO), silver nanowires (AgNWs), and a blend of AgNWs‐coated ITO as the transparent selective contact electrode. The improved rise times are observed from 1400 µs (bare condition; without the selective electrode) to 69, 60, 7 µs, and fall times from 720 µs (bare condition) to 80, 70, 10 µs for corresponding ITO, AgNWs, and AgNWs‐coated ITO contact electrodes, respectively. Similarly, the responsivity and detectivity are enhanced by about 4.39 × 10⁷ and 5.27 × 10⁵%, respectively. An energy band diagram is proposed to explain the underlying working mechanism based on the workfunction of the ITO (4.7 eV) and AgNWs (4.57 eV) as measured by Kelvin probe force microscopy, which confirms the formation of type‐II band alignment resulting in the efficient transport of photogenerated charge carriers. The functional use of the transparent selective contact electrode can effectively harness the pyro‐phototronic effect for next‐generation transparent and flexible optoelectronic applications.     

A Self‐Powered and Flexible Organometallic Halide Perovskite Photodetector with Very High Detectivity

Flexible and self‐powered photodetectors (PDs) are highly desirable for applications in image sensing, smart building, and optical communications. In this paper, a self‐powered and flexible PD based on the methylammonium lead iodide (CH₃NH₃PBI₃) perovskite is demonstrated. Such a self‐powered PD can operate even with irregular motion such as human finger tapping, which enables it to work without a bulky external power source. In addition, with high‐quality CH₃NH₃PBI₃ perovskite thin film fabricated with solvent engineering, the PD exhibits an impressive detectivity of 1.22 × 10¹³ Jones. In the self‐powered voltage detection mode, it achieves a large responsivity of up to 79.4 V mW^?1 cm^?2 and a voltage response of up to ≈90%. Moreover, as the PD is made of flexible and transparent polymer films, it can operate under bending and functions at 360 ° of illumination. As a result, the self‐powered, flexible, 360 ° omnidirectional perovskite PD, featuring high detectivity and responsivity along with real‐world sensing capability, suggests a new direction for next‐generation optical communications, sensing, and imaging applications.     

Photovoltaic–Pyroelectric Coupled Effect Induced Electricity for Self‐Powered Photodetector System

Ferroelectric materials have demonstrated novel photovoltaic effect to scavenge solar energy. However, most of the ferroelectric materials with wide bandgaps (2.7–4 eV) suffer from low power conversion efficiency of less than 0.5% due to absorbing only 8–20% of solar spectrum. Instead of harvesting solar energy, these ferroelectric materials can be well suited for photodetector applications, especially for sensing near‐UV irradiations. Here, a ferroelectric BaTiO₃ film‐based photodetector is demonstrated that can be operated without using any external power source and a fast sensing of 405 nm light illumination is enabled. As compared with photovoltaic effect, both the responsivity and the specific detectivity of the photodetector can be dramatically enhanced by larger than 260% due to the light‐induced photovoltaic–pyroelectric coupled effect. A self‐powered photodetector array system can be utilized to achieve spatially resolved light intensity detection by recording the output voltage signals as a mapping figure.     

UV Photodetectors Based on BiOCl Nanosheet Arrays: The Effects of Morphologies and Electrode Configurations

A facile chemical bath method is adopted to grow bismuth oxychloride (BiOCl) nanosheet arrays on a piece of Cu foil (denoted as BiOCl‐Cu) and isolated BiOCl nanosheets are collected by ultrasonication. A self‐supporting BiOCl film is obtained by the removal of Cu foil. Photodetectors (PDs) based on these BiOCl materials are assembled and the effects of morphologies and electrode configurations on the photoelectric performance of these PDs are examined. The BiOCl nanosheet PD achieves high responsivities in the spectral range from 250 to 350 nm, while it presents quite a small photocurrent and slow response speed. The BiOCl film PD yields low photocurrents and near‐unity on–off ratios, demonstrating poor photoelectric performance. The photocurrent of the BiOCl‐Cu PD with both electrodes on the BiOCl film is much higher than those of these above‐mentioned PDs, and the response times are fast. Meanwhile, the BiOCl‐Cu PD with separate electrodes on the BiOCl film and Cu foil achieves even higher photocurrents and presents a self‐powering characteristic, depicting the improved photodetecting performances induced by the specific morphology and distinct electrode configuration. These results would promote the applications of BiOCl nanostructures in the photoelectric devices.     

A Mechanoresponsive Phase‐Changing Electrolyte Enables Fabrication of High‐Output Solid‐State Photobioelectrochemical Devices from Pigment‐Protein Multilayers

Exploitation of natural photovoltaic reaction center pigment proteins in biohybrid architectures for solar energy harvesting is attractive due to their global abundance, environmental compatibility, and near‐unity quantum efficiencies. However, it is challenging to achieve high photocurrents in a device setup due to limitations imposed by low light absorbance by protein monolayers and/or slow long‐range diffusion of liquid‐phase charge carriers. In an attempt to enhance the photocurrent density achievable by pigment proteins, here, an alternative solid‐state device architecture enabled by a mechanoresponsive gel electrolyte that can be applied under nondenaturing conditions is demonstrated. The phase‐changing electrolyte gel provides a pervading biocompatible interface for charge conduction through highly absorbing protein multilayers that are fabricated in a simple fashion. Assembled devices exhibit enhanced current stability and a maximal photoresponse of ≈860 µA cm⁻², a fivefold improvement over the best previous comparable devices under standard illumination conditions. Photocurrent generation is enhanced by directional energy transfer through extended layers of light‐harvesting complexes, mimicking the modular antenna/transducer architecture of natural photosystems, and by metastable radical pair formation when photovoltaic reaction centers are embedded throughout light‐harvesting regions of the device.     

Large‐Scale Horizontally Aligned ZnO Microrod Arrays with Controlled Orientation,Periodic Distribution as Building Blocks for Chip‐in Piezo‐Phototronic LEDs

A novel method of fabricating large‐scale horizontally aligned ZnO microrod arrays with controlled orientation and periodic distribution via combing technology is introduced. Horizontally aligned ZnO microrod arrays with uniform orientation and periodic distribution can be realized based on the conventional bottom‐up method prepared vertically aligned ZnO microrod matrix via the combing method. When the combing parameters are changed, the orientation of horizontally aligned ZnO microrod arrays can be adjusted (θ = 90° or 45°) in a plane and a misalignment angle of the microrods (0.3° to 2.3°) with low‐growth density can be obtained. To explore the potential applications based on the vertically and horizontally aligned ZnO microrods on p‐GaN layer, piezo‐phototronic devices such as heterojunction LEDs are built. Electroluminescence (EL) emission patterns can be adjusted for the vertically and horizontally aligned ZnO microrods/p‐GaN heterojunction LEDs by applying forward bias. Moreover, the emission color from UV‐blue to yellow‐green can be tuned by investigating the piezoelectric properties of the materials. The EL emission mechanisms of the LEDs are discussed in terms of band diagrams of the heterojunctions and carrier recombination processes.     

Carbon Nanolights in Piezopolymers are Self‐Organizing Toward Color Tunable Luminous Hybrids for Kinetic Energy Harvesting

Herein, an all‐solid‐state sequential self‐organization and self‐assembly process is reported for the in situ construction of a color tunable luminous inorganic/polymer hybrid with high direct piezoresponse. The primary inorganic self‐organization in solid polymer and the subsequent polymer self‐assembly are achieved at high pressure with the first utilization of piezo‐copolymer (PVDF‐TrFE) as the host matrix of guest carbon quantum dots (CQDs). This process induces the spontaneous formation of a highly ordered, microscale, polygonal, and hierarchically structured CQDs/PVDF‐TrFE hybrid with multicolor photoluminescence, consisting of very thermodynamic stable polar crystalline nanowire arrays. The electrical polarization‐free CQDs/PVDF‐TrFE hybrids can efficiently harvest the environmental available kinetic mechanical energy with a new large‐scale group‐cooperation mechanism. The open‐circuit voltage and short‐circuit current outputs reach up to 29.6 V cm^?2 and 550 nA cm^?2, respectively. The CQDs/PVDF‐TrFE–based hybrid nanogenerator demonstrates drastically improved durable and reliable features during the real‐time demonstration of powering commercial light emitting diodes. No attenuation/fluctuation of the electrical signals is observed for ≈10 000 continuous working cycles. This study may offer a new design concept for progressively but spontaneously constructing novel multiple self‐adaptive complex inorganic/polymer hybrids that promise applications in the next generation of self‐powered autonomous optoelectronic devices.     

Light‐Induced Reversible Control of Ferroelectric Polarization in BiFeO3

Manipulation of ferroic order parameters, namely (anti‐)ferromagnetic, ferroelectric, and ferroelastic, by light at room temperature is a fascinating topic in modern solid‐state physics due to potential cross‐fertilization in research fields that are largely decoupled. Here, full optical control, that is, reversible switching, of the ferroelectric/ferroelastic domains in BiFeO₃ thin films at room temperature by the mediation of the tip‐enhanced photovoltaic effect is demonstrated. The enhanced short‐circuit photocurrent density at the tip contact area generates a local electric field well exceeding the coercive field, enabling ferroelectric polarization switching. Interestingly, by tailoring the photocurrent direction, via either tuning the illumination geometry or simply rotating the light polarization, full control of the ferroelectric polarization is achieved. The finding offers a new insight into the interactions between light and ferroic orders, enabling fully optical control of all the ferroic orders at room temperature and providing guidance to design novel optoferroic devices for data storage and sensing.     

Graphene‐Based Integrated Photovoltaic Energy Harvesting/Storage Device

Energy scavenging has become a fundamental part of ubiquitous sensor networks. Of all the scavenging technologies, solar has the highest power density available. However, the energy source is erratic. Integrating energy conversion and storage devices is a viable route to obtain self‐powered electronic systems which have long‐term maintenance‐free operation. In this work, we demonstrate an integrated‐power‐sheet, consisting of a string of series connected organic photovoltaic cells (OPCs) and graphene supercapacitors on a single substrate, using graphene as a common platform. This results in lighter and more flexible power packs. Graphene is used in different forms and qualities for different functions. Chemical vapor deposition grown high quality graphene is used as a transparent conductor, while solution exfoliated graphene pastes are used as supercapacitor electrodes. Solution‐based coating techniques are used to deposit the separate components onto a single substrate, making the process compatible with roll‐to‐roll manufacture. Eight series connected OPCs based on poly(3‐hexylthiophene)(P3HT):phenyl‐C61‐butyric acid methyl ester (PC₆₀BM) bulk‐heterojunction cells with aluminum electrodes, resulting in a ≈5 V open‐circuit voltage, provide the energy harvesting capability. Supercapacitors based on graphene ink with ≈2.5 mF cm^?2 capacitance provide the energy storage capability. The integrated‐power‐sheet with photovoltaic (PV) energy harvesting and storage functions had a mass of 0.35 g plus the substrate.     

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.     

Nanoscale Insights into Photovoltaic Hysteresis in Triple‐Cation Mixed‐Halide Perovskite: Resolving the Role of Polarization and Ionic Migration

Triple‐cation mixed‐halide perovskites of composition Cs_x(FA_yMA_1?y)_1?xPb(I_zBr_1?z)₃ (CsFAMA) have been reported to possess excellent photovoltaic efficiency with minimal hysteresis; in this work, nanoscale insight is shed into the roles of illumination‐induced polarization and ionic migration in photovoltaic hysteresis. By examining the concurrent evolution of ionic distribution and spontaneous polarization of CsFAMA under light illumination using dynamic‐strain‐based scanning probe microscopy, strong linear piezoelectricity arising from photoenhanced polarization is observed, while ionic migration is found to be not significantly increased by lightening. Nanoscale photocurrents are mapped under a series of biases using conductive atomic force microscopy, revealing negligible difference between forward and backward scans, and local IV curves reconstructed from principal component analysis show minimal hysteresis of just 1%. These observations at the nanoscale are confirmed in a macroscopic perovskite solar cell made of CsFAMA, exhibiting a high efficiency of 20.11% and with hysteresis index as small as 3%. Ionic migration, polarization, and photocurrent hysteresis are thus directly correlated at the nanoscale, and photoenhanced polarization in triple‐cation mixed‐halide perovskites is established, which does not contribute to the photovoltaic hysteresis.     

2D In2S3 Nanoflake Coupled with Graphene toward High‐Sensitivity and Fast‐Response Bulk‐Silicon Schottky Photodetector

Silicon‐based electronic devices, especially graphene/Si photodetectors (Gr/Si PDs), have triggered tremendous attention due to their simple structure and flexible integration of the Schottky junction. However, due to the relatively poor light–matter interaction and mobility of silicon, these Gr/Si PDs typically suffer an inevitable compromise between photoresponsivity and response speed. Herein, a novel strategy for coupling 2D In₂S₃ with Gr/Si PDs is demonstrated. The introduction of the double‐heterojunction design not only strengthens the light absorption of graphene/Si but also combines the advantages of the photogating effect and photovoltaic effect, which suppresses the dark current, accelerates the separation of photogenerated carriers, and brings photoconductive gain. As a result, In₂S₃/graphene/Si devices present an ultrahigh photoresponsivity of 4.53 × 10⁴ A W^?1 and fast response speed less than 40 µs, simultaneously. These parameters are an order of magnitude higher than pristine Gr/Si PDs and among the best values compared with reported 2D materials/Si heterojunction PDs. Furthermore, the In₂S₃/graphene/Si PD expresses outstanding long‐term stability, with negligible performance degradation even after 1 month in air or 1000 cycles of operation. These findings highlight a simple and novel strategy for constructing high‐sensitivity and ultrafast Gr/Si PDs for further optoelectronic applications.