Flexible UV Research Articles

UV-Responsive Screen-Printed Porous ZnO Nanostructures on Office Paper for Sustainable and Foldable Electronics

The fabrication of low-cost, flexible, and recyclable electronic devices has been the focus of many research groups, particularly for integration in wearable technology and the Internet of Things (IoT). In this work, porous zinc oxide (ZnO) nanostructures are incorporated as a UV sensing material into the composition of a sustainable water-based screen-printable ink composed of carboxymethyl cellulose (CMC). The formulated ink is used to fabricate flexible and foldable UV sensors on ubiquitous office paper. The screen-printed CMC/ZnO UV sensors operate under low voltage (≤2 V) and reveal a stable response over several on/off cycles of UV light exposure. The devices reach a response current of 1.34 ± 0.15 mA and a rise and fall time of 8.2 ± 1.0 and 22.0 ± 2.3 s, respectively. The responsivity of the sensor is 432 ± 48 mA W−1, which is the highest value reported in the literature for ZnO-based UV sensors on paper substrates. The UV-responsive devices display impressive mechanical endurance under folding, showing a decrease in responsivity of only 21% after being folded 1000 times. Their low-voltage operation and extreme folding stability indicate a bright future for low-cost and sustainable flexible electronics, showing potential for low-power wearable applications and smart packaging.

Research on piezo-phototronic effect in ZnO/AZO heterojunction flexible ultraviolet photodetectors

A novel Al-doped (3 wt%) ZnO (AZO) UV photodetector (PD) and ZnO/AZO heterojunction double-layer UV PD were prepared on a polyethylene terephthalate (PET) substrate by utilizing the radio frequency (RF) sputtering technology and conventional photolithography. Because the ZnO/AZO heterojunction possesses the ability to improve the separation of the photogenerated electron-hole pairs, the responsivity of the ZnO/AZO PD is higher than that of the pure AZO PD under the same bias voltage. Moreover, the responsivity of the obtained ZnO/AZO PD showed a 1.13 fold increase from 5.91 to 6.70 A/W when the tensile variable ɛ was 0.16. These findings also provide a new alternative for the preparation of flexible UV PDs with piezo-phototronic effect, which is low-cost and offers environmental sustainability.

Flexible ultraviolet detector with robust ZnO nanoparticle nanoassemblies on catechol‐functionalized polysiloxane nanofilms

AbstractAs a result of the vast Young's moduli difference between an inorganic semiconducting channel and flexible substrates, flexible optoelectronic devices readily lose their functionality through material delamination and local fracturing, which lead to short‐circuiting of devices. For this study, we synthesized a catechol‐containing polysiloxane (CFPS) adhesive and applied it to ZnO nanoparticle (NP) assembly on plastic substrates for flexible UV detector applications. The 30 nm thick CFPS adhesive can anchor 70 nm thick ZnO NPs strongly through a coordination bond, thereby forming an ultra‐stable ZnO NP channel layer. A peeling test of ZnO NP layer was conducted using transparent tape (Scotch®; 3 M Inc.). The ZnO NPs were firmly immobilized, reflecting the outstanding mechanical stability of CFPS adhesives. A UV detector also exhibited stable photo‐response performance even after a thousand iterations of bending with 3 mm curvature radii. The result indicates the polycyclosiloxane‐based flexible device as promising for wearable detector applications.

Piezo-phototronic effect triggered flexible UV photodetectors based on ZnO nanosheets/GaN nanorods arrays

Here in, we report on the fabrication of a flexible ultraviolet photodetector composed of two dimensional (2D) ZnO nanosheets (NSs) and one dimensional (1D) GaN nanorods (NRs). ZnO NSs were grown on the surface of GaN NRs to form 2D/1D heterojunction via simple hydrothermal method. A striking photoresponse was observed across 2D-ZnO/1D-GaN heterojunction under UV illumination. The photoresponsitivity and detectivity of the photodetector enhanced by two order magnitude than that of 1D GaN NRs. Furthermore, under a tensile strain the photoresponsivity of this photodetector was also tuned through piezo-phototronic effect arising from ZnO NSs. Due to lowering of barrier height under the application of strain, more photogenerated electron-hole pairs can transport through the heterojunction without recombination, resulting in enhanced photorespositivity.

Synthesis and photochromic behavior of comb-like acrylate polymer nanoparticle containing spiropyran

Spiropyran (SP) materials have attracted considerable interest owing to sensitive and efficient photochromic properties, but it is a challenge to obtain photochromic materials with excellent sensitive UV-responsivity and photodegradation resistance durability. In this study, a series of comb-like acrylate polymer nanoparticle latexes containing SP are fabricated via miniemulsion polymerization. The influence of the length of the alkyl side chain of comb-like acrylate polymer on some photochromic properties of SP is investigated. Compared with the short-side chain poly(butyl acrylate) (PBA) nanoparticles latexes, the comb-like acrylate polymer nanoparticle latexes with longer side chain possess better photoresponsivity and more excellent photodegradation resistance because the strong interaction and chain flexibility of comb-like alkyl side chain provide effective spaces for the SP→MC or MC→SP photochemical isomerization. The investigations of photochromic nanoparticle latexes guide an approach to reversible optical data storage, multipurpose photo-responsive sensing system and show promising application in the field of ultraviolet intensity indicator and flexible wearable UV protective clothing.

Visible-blind ultraviolet narrowband photomultiplication-type organic photodetector with an ultrahigh external quantum efficiency of over 1 000 000.

A visible-blind ultraviolet (UV) photodetector can detect UV signals and is not interfered with by visible light or infrared light in the environment. In order to realize high-performance visible-blind UV organic photodetectors (OPDs), we design photomultiplication-type (PM-type) OPDs by using a novel strategy. Firstly, wide bandgap organic semiconductor materials, which do not absorb visible light, are selected as donors to absorb UV light. Secondly, a very small amount of C60 is used as an acceptor to trap photogenerated electrons. These accumulating electrons near the Al electrode form a potential, which leads to band bending and narrowing of the interface barrier, thereby assisting hole-tunneling injection to form a multiplication. The fabricated visible-blind UV PM-type OPDs with donor/acceptor doping ratio of 50 : 1 exhibit a narrowband response with full-width at half-maximum (FWHM) of approximately 36 nm, an ultrahigh external quantum efficiency of 1.08 × 106% and a remarkable specific detectivity of 1.28 × 1014 jones at 335 nm wavelength under -14 V bias. The UV-to-visible rejection ratio exceeds 103 by adjusting the donor/acceptor mixing ratios. The devices made with other wide bandgap organic materials also showed similar performance, indicating that this device structure provides an effective method for the preparation of high-performance visible-blind UV PM-type OPDs. In addition, we prepared a flexible visible-blind UV PM-type OPD based on a PET substrate and integrated it with a flexible OLED to fabricate a wearable UV monitor, which can visually detect the intensity of UV light.

Optimizing the Electronic Structure of In2O3 through Mg Doping for NiO/In2O3 p-n Heterojunction Diodes.

In2O3 is a wide bandgap oxide semiconductor, which has the potential to be used as an active material for transparent flexible electronics and UV photodetectors. However, the high concentration of unintentional background electrons existing in In2O3 makes it hard to be modulated by the electric field or form p-n heterojunctions with a sufficient band-bending width at the interface. In this work, we report the reduction of the background electrons in In2O3 by Mg doping (Mg-In2O3) and thereby improve the device performance of p-n diodes based on the NiO/Mg-In2O3 heterojunction. In particular, Mg doping compensates the free electrons in In2O3 and reduces the electron concentration from 1.7 × 1019 cm-3 without doping to 1.8 × 1017 cm-3 with 5% Mg doping. Transparent p-n heterojunction diodes were fabricated based on p-type NiO and n-type Mg-In2O3. The device performance was considerably enhanced by Mg doping with a high rectification ratio of 3 × 104 and a remarkable high breakdown voltage of >20 V. High-resolution X-ray photoelectron spectroscopy was used to investigate the interfacial electronic structure between NiO and Mg-In2O3, revealing a type II band alignment with a valence band offset of 1.35 eV and a conduction band offset of 2.15 eV. A large built-in potential of 0.98 eV was found for the undoped In2O3 but decreased to 0.51 eV for 5% Mg doping of In2O3. The NiO/Mg-In2O3 diodes with an improved rectification ratio and wider depletion region provide the possibility of achieving photodetectors with rapid photoresponse.

Optical performance appraisal of mechanically flexible and visibly clear PVP-PVA/calcium doped zirconium oxide nanocomposites for UV shielding applications

Herein, we report the successful development and optical performance evaluation of mechanically flexible, optically clear and UV-shielding poly vinyl pyrrolidone (PVP) - poly vinyl alcohol (PVA) (50:50) nanocomposite (NC) thick films, containing 0.0, 0.5, 1.0, 2.0 and 4.0 Wt% calcium doped zirconium oxide (CaZrO3) nanofillers. The solution combustion synthesized nanofillers and aqueous solution casted NCs were characterized for their gross structural, micro-structural, surface morphological, mechanical and optical properties, with special emphasis on UV-shielding performance. Consequently, electronic spectral studies were employed to investigate the optical changes in PVP-PVA films, that emerge due to doping with CaZrO3 nanofillers. While, Scanning Electron Microscopy (SEM) was used to probe the surface morphological changes. Similarly, Fourier Transform Infrared (FTIR) spectroscopy was used to characterize the molecular interactions of polymer nanocomposites (PNCs), while Powder X-ray Diffraction (P-XRD) technique was employed to validate the changes in their micro-structural properties. Finally, tensile measurements and photo-catalytic degradation studies with indigo carmine (C16H8N2Na2O8S2) established the ability of PVP-PVA/CaZrO3 NCs for possible applications as mechanically flexible UV protective materials. The, UV-transilluminator measurements validated the principle associated with the observed UV shielding behaviours to be nanofiller/polymer synergism induced UV to visible photonic down-conversions.

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

AbstractA 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.

Flexible Self-Powered Real-Time Ultraviolet Photodetector by Coupling Triboelectric and Photoelectric Effects.

The portable UV photodetector is used to timely remind humans of overexposure to UV radiation. However, the traditional UV photodetector cannot meet the practical demands, and the power supply problem hinders its further development. In this work, we demonstrated a flexible, transparent, and self-powered UV photodetector by coupling of triboelectric and photoelectric effects. The device integrates a flexible ZnO nanoparticle (NP) UV photodetector, a transparent- and flexible-film-based TENG (TFF-TENG), commercial chip resistors, and LEDs on the PET thin film. The TFF-TENG could harvest mechanical energy from finger tapping and sliding motion and power the ZnO NP UV photodetector to realize self-powered detection. The voltage of the constant resistors connected with the UV photodetector in series changes from 0.5 to 19 V under the UV light with power intensities increasing from 0.46 to 21.8 mW/cm2, and the voltage variation is reflected by the number of LEDs directly. The excellent flexibility and transparency of the device could extend its application scenarios; for example, such a portable device could be applied to real-time monitoring of the UV radiation to remind humans of intense UV light.

Performance modulation of a MgZnO/ZnO heterojunction flexible UV photodetector by the piezophototronic effect

The modulation of the performance of a dual-band MgZnO/ZnO heterojunction flexible UV photodetector by the piezophototronic effect.

Ultrasensitive Fiber-Based ZnO Nanowire Network Ultraviolet Photodetector Enabled by the Synergism between Interface and Surface Gating Effects.

A flexible UV photodetector with a high on/off ratio is extremely important for environmental sensing, optical communication, and flexible optoelectronic devices. In this work, a flexible fiber-based UV photodetector with an ultrahigh on/off ratio is developed by utilizing the synergism between interface and surface gating effects on a ZnO nanowire network structure. The synergism between two gating effects is realized by the interplay between surface band bending and the Fermi level through the nanowire network structure, which is proved through the control experiments between the ZnO micro/nanowire photodetector and micro/nanowire junction photodetector, and the corresponding Kelvin probe force microscopy (KPFM) measurements. The on/off ratio of the fiber-based ZnO nanowire network UV photodetector reaches 1.98 × 108 when illuminated by 1.0 mW cm-2 UV light, which is 20 times larger than the largest reported result under the same UV illumination. This new UV sensor also has a high resolution to UV light intensity change in the nW cm-2 range. Furthermore, when the fiber-based photodetector is curved, it still shows excellent performance as above. This work gives a new effective route for the development of a high-performance UV photodetector or other optoelectronic detection devices.

ZnO@graphene QDs with tuned surface functionalities formed on eco-friendly keratin nanofiber textile for transparent and flexible ultraviolet photodetectors

Abstract We demonstrate the ZnO@graphene core@shell quantum dot (ZGQDs) fabricated by selectively engineering of oxygenated functional groups on the graphene shells with eco-friendly keratin textile for transparent and flexible UV photodetector devices. The ZGQDs with the octylamine (ZGQDs-OA) and the graphene sheet were employed as an active material and an electron transport channel. The functional groups on the surrounding graphene shell in ZGQDs were passivated by the OA, resulting in a good absorbance in UV light and excellent carrier transport properties. The passivation of the functional groups on the graphene shell by OA drastically enhanced the photocurrent. Optical transmittance of the ZGQDs-OA/graphene sheets/PMMA/keratin nanofiber textile composites structures was approximately 77% at the visible spectrum range. Current–voltage (I-V) and current–time (I-t) measurements on the UV photodetector under illumination state in bending condition exhibited the excellent ON/OFF switching states and stability. The photosensitivity and responsivity of the photodetector with ZGQDs-OA were found to be around 13.10 and 68.64 μAmW−1, respectively, which is much higher than that without the passivation. The improved performance of the photodetector with ZGQDs-OA could be attributed to the efficient electron transport to the electrode by the passivation of the graphene on ZnO QDs.

Improved Detectivity of Flexible a-InGaZnO UV Photodetector via Surface Fluorine Plasma Treatment

Amorphous InGaZnO (a-IGZO) UV photodetectors with a metal-semiconductor-metal structure were fabricated on polyethylene terephthalate (PET) substrates, and the effects of surface fluorine plasma treatment on a-IGZO were investigated. With this treatment, the oxygen vacancies (Ovac’s) at the a-IGZO surface were effectively passivated, thus reducing the shallow-level donor states and electron traps. Accordingly, the direct and/or trap-assisted tunnelings through the metal/a-IGZO Schottky barrier were suppressed, leading to a significant reduction in the dark current. On the other hand, the photo current was nearly unchanged if the treatment time was less than 20min. It is due to the simultaneous reductions in hole trap and recombination center, both of which are related to the Ovac’s at deep levels. As a result, a high detectivity of ${4.36} \times {10}^{{13}}$ Jones was realized with an increase of nearly 10 times. Moreover, a highly uniform trap distribution was achieved, and it could be beneficial to the photoconductive reliability of the device.

High-Responsivity MSM Solar-Blind UV Photodetector Based on Annealed ITO/Ag/ITO Structure Using RF Sputtering

In this paper, a novel ITO/Ag/ITO (IAI) multilayer UV photodetector was successfully prepared via RF magnetron sputtering technique to achieve the dual-benefit of improved photoresponse and outstanding visible blindness properties. The influence of the annealing process on the performance of the elaborated IAI UV photodetector was carried out. Moreover, the structural and photoelectrical properties of the sputtered tri-layered structures with and without annealing effects were also analyzed. It was found that the elaborated UV -photodetector provides near perfect UV absorbance of 96%. This achievement is attributed to the improved light trapping capability and tunable plasmonic resonance effects induced by the efficient optical coupling between free-space UV photons and Ag plasmons. It was also revealed that the heat treatment paves a new path toward preparing efficient visible-blind UV photodetectors, where the annealed sample demonstrated a superior rejection ratio of 225 as compared to that of the unannealed counterpart. Besides, electrical characterization underlined the ability of the annealed IAI UV photodetector at 500°C for offering exciting opportunities, where it exhibits a high responsivity of 3.8mA/W at self-powered condition. Therefore, by well optimizing both IAI multilayer geometry and annealing conditions, we were able to elaborate an ultrathin photodetector suitable for high-performance and flexible UV sensing applications.

Development of Al Doped ZnO Nanowalls Based Flexible, Ultralow Voltage UV Photodetector

We report an Al-doped ZnO nanowalls network based ultralow voltage, flexible UV photodetector (PD). The PD is synthesized over an Al foil by using a simple and low-cost hydrothermal process. The device has shown photocurrent to a dark-current ratio of 349, high responsivity (265 mA·W–1 at just 0.1 V applied bias), and specific detectivity of 4.5 × 1010 cm·Hz1/2·W–1 at 350 nm. The incident light power has been kept as low as 396 μW·cm–2 throughout the measurements. The external quantum efficiency was 1827.8% at 5 V applied bias. The proposed ultralow voltage UV PD demands very low power and, therefore, could potentially lengthen the battery discharge time. Additionally, a prominent UV sensitive piezoelectric response, having dark voltage to photo voltage ratio of 24.5, has been demonstrated that has made the device a potential candidate for fast response time, self-powered PD.

ZnO Thin Film Flexible UV Photodetectors: Regulation on the ZnO/Au Interface by Piezo‐Phototronic Effect and Performance Outcomes

AbstractThe ZnO flexible ultraviolet photodetectors (UV PDs) with poly(ethylene terephthalate) (PET) substrate are manufactured successfully at room temperature, and the effect of strain on PDs performance are studied. When subjected to 0.2 tensile stress, the responsivity of PDs is increased by 20% and sensitivity is increased by 770%. This is due to the influence of piezo‐phototronic effect on the transport behavior of photogenerated carriers at the interface of ZnO/Au Schottky junction. The polarization charge generated by the piezoelectric element breaks the original thermodynamic equilibrium. As a consequence, the charge at the interface is being redistributed, and the Schottky barrier height (SBH) and depletion layer width change, thus regulating the interface characteristics. This effect has value for the optimization and regulation of the UV PDs performance. Furthermore, current study proves that the piezo‐phototronic effect not only exists in ZnO nanomaterials, but also has a significant piezo‐phototronic effect on high‐quality thin film ZnO materials. In addition, this study also provides an extension field for flexible UV PDs based on piezo‐phototronic effect of semiconductor materials.

Accelerated ultraviolet aging of structural and luminescent properties of the ureasil-polyether hybrid materials U-PEO:Eu3+ and U-PPO:Eu3+

In this paper, we evaluate the role played by the polyether type and chain length on the complexation of Eu3+ by the urea group of siloxane-polyether organic-inorganic hybrid materials based on polyethylene oxide (U-PEO, mw = 1900 g mol−1) and polypropylene oxide (U-PPO, mw = 230, 2000, and 4000 g mol−1). Investigation was made of the effect of the photooxidation of the PEO and PPO chains, induced by artificially accelerated ultraviolet aging (λ ≥ 300 nm), with the decomplexation of Eu3+ from the urea-polyether array. Fourier transform infrared (FTIR) and photoluminescence (PL) spectra evidenced loss of the ligand-to-metal charge transfer (LMCT) when the Eu3+ cations were unable to disrupt the strong hydrogen-bonded urea/urea aggregates formed using shorter U-PPO chains (230 g mol−1) or semicrystalline U-PEO (1900 g mol−1). FTIR analysis showed that formate was the main photoproduct formed continuously during the artificial ultraviolet aging of U-PEO (1900 g mol−1) and U-PPO (2000 g mol−1), with concomitant decreases of Eu3+ complexed to urea bridges, spatially correlated siloxane nodes, and the Judd-Ofelt parameter Ω2 (R). The kinetics of all these parameters were described by exponential laws with comparable time constants, evidencing the sensitivity of the Eu3+ photoluminescence to the elimination/formation of the hydrogen-bonded urea-polyether array during the photooxidation of PPO chains. In the case of U-PEO:Eu3+, different time constant values were observed, indicating the existence of a faster process governed by the facilitated transport of the oxidative species through the amorphous moieties of the semicrystalline U-PEO. This sensitivity of structural and PL properties arising from the photoinduced degradation of ureasil-polyether films can be exploited in the fabrication of new transparent, flexible, and biocompatible UV dosimeters and sensors.

Piezo-phototronic effect for enhanced sensitivity and response range of ZnO thin film flexible UV photodetectors

In this work, we fabricated a ZnO thin film flexible UV photodetector with a room-temperature process and investigated it with an emphasis on the influence of different external stresses. The photodetector exhibited enhancing photocurrent, sensitivity, responsivity, and response range with a tensile strain. This may originate from the improving collection ability of photogenerated carriers with the change of Schottky barrier height, which is induced by the piezo-phototronic effect. The as-obtained thin film flexible photodetector belongs to the family of photoresponse-controlled optoelectronic devices and has applications in a myriad of useful ways.

Synthesis of free-standing Ga2O3 films for flexible devices by water etching of Sr3Al2O6 sacrificial layers**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572033, 51572241, 61774019, 61704153, and 11404029), the Fund of State Key Laboratory of IPOC (BUPT), and the Open Fund of IPOC (BUPT), Beijing Municipal Commission of Science and Technology, China (Grant No. SX2018-04).

Flexible electronic devices have attracted much attention due to their practical and commercial value. Integration of thin films with soft substrate is an effective way to fabricate flexible electronic devices. Ga2O3 thin films deposited directly on soft substrates would be amorphous mostly. However, the thickness of the thin film obtained by mechanical exfoliation method is difficult to control and the edge of the film is fragile and easy to be damaged. In this work, we fabricated free-standing Ga2O3 thin films using the water-soluble perovskite Sr3Al2O6 as a sacrificial buffer layer. The obtained Ga2O3 thin films were polycrystalline. The thickness and dimension of the films were controllable. A flexible Ga2O3 solar-blind UV photodetector was fabricated by transferring the free-standing Ga2O3 film on a flexible polyethylene terephthalate substrate. The results displayed that the photoelectric performances of the flexible Ga2O3 photodetector were not sensitive to bending of the device. The free-standing Ga2O3 thin films synthesized through the method described here can be transferred to any substrates or integrated with other thin films to fabricate electronic devices.