Charge‐Generating Mode Control in High‐Performance Transparent Flexible Piezoelectric Nanogenerators

In this work, we demonstrate the mode transition of charge generation between direct‐current (DC) and alternating‐current (AC) from transparent flexible (TF) piezoelectric nanogenerators (NGs), which is dependent solely on the morphology of zinc oxide (ZnO) nanorods without any use of an AC/DC converter. Tilted ZnO nanorods grown on a relatively low‐density seed layer generate DC‐type piezoelectric charges under a pushing load, whereas vertically aligned ZnO nanorods on a relatively high‐density seed layer create AC‐type charge generation. The mechanism for the geometry‐induced mode transition is proposed and characterized. We also examine the output performance of TF‐NGs which employ an indium zinc tin oxide (IZTO) film as a TF electrode. It is demonstrated that an IZTO film has improved electrical, optical, and mechanical properties, in comparison with an indium tin oxide (ITO) film. Enhanced output charge generation is observed from IZTO‐based TF‐NGs when TF‐NGs composed of only ITO electrodes are compared. This is attributed to the higher Schottky barrier and the lower series resistance of the IZTO‐based TF‐NGs. Thus, by using IZTO, we can expect TF‐NGs with superior mechanical durability and power generating performance.     

Unidirectional High‐Power Generation via Stress‐Induced Dipole Alignment from ZnSnO3 Nanocubes/Polymer Hybrid Piezoelectric Nanogenerator

The extremely stable high‐power generation from hybrid piezoelectric nanogenerator (HP‐NG) based on a composite of single‐crystalline piezoelectric perovskite zinc stannate (ZnSnO₃) nanocubes and polydimethylsiloxane without any electrical poling treatment is reported. The HP‐NG generates large power output under only vertical compression, while there is negligible power generation with other configurations of applied strain, such as bending and folding. This unique high unidirectionality of power generation behavior of the HP‐NG provides desirable features for large‐area piezoelectric power generation based on vertical mechanical compression such as moving vehicles, railway transport, and human walking. The HP‐NGs of ZnSnO₃ nanocubes exhibit high mechanical durability, excellent robustness, and high power‐generation performance. A large recordable output voltage of about 20 V and an output current density value of about 1 μA cm^?2 are successfully achived, using a single cell of HP‐NG obtained under rolling of a vehicle tire.     

High‐Performance Piezoelectric,Pyroelectric, and Triboelectric Nanogenerators Based on P(VDF‐TrFE) with Controlled Crystallinity and Dipole Alignment

Poly(vinylidenefluoride‐co‐trifluoroethylene) (P(VDF‐TrFE)), as a ferroelectric polymer, offers great promise for energy harvesting for flexible and wearable applications. Here, this paper shows that the choice of solvent used to dissolve the polymer significantly influences its properties in terms of energy harvesting. Indeed, the P(VDF‐TrFE) prepared using a high dipole moment solvent has higher piezoelectric and pyroelectric coefficients and triboelectric property. Such improvements are the result of higher crystallinity and better dipole alignment of the polymer prepared using a higher dipole moment solvent. Finite element method simulations confirm that the higher dipole moment results in higher piezoelectric, pyroelectric, and triboelectric potential distributions. Furthermore, P(VDF‐TrFE)‐based piezoelectric, pyroelectric, and triboelectric nanogenerators (NGs) experimentally validate that the higher dipole moment solvent significantly enhances the power output performance of the NGs; the improvement is about 24% and 82% in output voltage and current, respectively, for piezoelectric NG; about 40% and 35% in output voltage and current, respectively, for pyroelectric NG; and about 65% and 75% in output voltage and current for triboelectric NG. In brief, the approach of using a high dipole moment solvent is very promising for high output P(VDF‐TrFE)‐based wearable NGs.     

Layer‐by‐Layer Controlled Perovskite Nanocomposite Thin Films for Piezoelectric Nanogenerators

Perovskite nanoparticle‐based nanocomposite thin films strictly tailored using unconventional layer‐by‐layer (LbL) assembly in organic media for piezoelectric nanogenerators (NGs) are demonstrated. By employing sub‐20‐nm BaTiO₃ nanoparticles stabilized by oleic acid ligands (i.e., OA‐BTO_NPs) and carboxylic acid (COOH)‐functionalized polymers, such as poly(acrylic acid) (PAA), the resulting OA‐BTO_NP/PAA nanocomposite multilayers are prepared by exploiting the high affinity between the COOH groups of PAA and the BTO_NPs. The ferroelectric and piezoelectric performance of the (PAA/OA‐BTO_NP)_n thin films can be precisely controlled by altering the bilayer number, inserted polymer type, and OA‐BTO_NP size. It is found that the LbL assembly in nonpolar solvent media can effectively increase the quantity of adsorbed OA‐BTO_NPs, resulting in the dramatic enhancement of electric power output from the piezoelectric NGs. Furthermore, very low leakage currents are detected from the (PAA/OA‐BTO_NP)_n thin films for obtaining highly reliable power‐generating performance of piezoelectric NGs.     

Ultrathin Nanogenerators as Self‐Powered/Active Skin Sensors for Tracking Eye Ball Motion

Ultrathin piezoelectric nanogenerator (NG) with a total thickness of ≈16 μm is fabricated as an active or self‐powered sensor for monitoring local deformation on a human skin. The NG was based on an anodic aluminum oxide (AAO) as an insulating layer grown on a thin Al foil by anodization, on which a thin film made of aligned ZnO nanowire compacted arrays is grown by solution chemistry. The performance of the NG is characterized with the assistance of the finite element method (FEM) simulation. The extremely thin NG is attached on the surface of an eyelid, and its output voltage/current characterizes the motion of the eye ball underneath. Since there is no external power needed for the operation of the NG, this self‐powered or active sensor can be effective in monitoring sleeping behavior, brain activities, and spirit status of a person as well as any biological associated skin deformation.     

Resistive Switching Memory Integrated with Nanogenerator for Self‐Powered Bioimplantable Devices

Resistive random access memory (ReRAM) devices powered by piezoelectric nanogenerators (NGs) have been investigated for their application to future implantable biomedical devices. Biocompatible (Na_0.5K_0.5)NbO₃ (NKN) films that are grown at 300 °C on TiN/SiO₂/Si and flexible TiN/Polyimide (TiN‐PI) substrates are used for ReRAM and NGs, respectively. These NKN films have an amorphous phase containing NKN nanocrystals with a size of 5.0 nm. NKN ReRAM devices exhibit typical bipolar switching behavior that can be explained by the formation and rupture of oxygen‐vacancy filaments. They have good ReRAM properties such as a large ratio of R_HRS to R_LRS as well as high reliability. The NKN film grown on flexible TiN‐PI substrate exhibits a high piezoelectric strain constant of 50 pm V^?1. The NKN NG has a large open‐circuit output voltage of 2.0 V and a short‐circuit output current of 40 nA, which are sufficient to drive NKN ReRAM devices. Stable switching properties with a large ON/OFF ratio of 10² are obtained from NKN ReRAM driven by NKN NG.     

Highly Porous Polymer Aerogel Film‐Based Triboelectric Nanogenerators

A novel class of high performance polymer porous aerogel film‐based triboelectric nanogenerators (A‐NGs) is demonstrated. The A‐NGs, made of a pair of highly porous polymer films, exhibit much higher triboelectric outputs than the corresponding dense polymer film‐based triboelectric nanogenerators (D‐NGs) under the same mechanical stress. The triboelectric outputs of the A‐NGs increase significantly with increasing porosity, which can be attributed to the increase in contact area and the electrostatic induction in the porous structure, thereby leading to additional charges on the porous surface. Remarkably, the A‐NG fabricated using porous chitosan aerogel film paired with the most porous polyimide (with a porosity of 92%) aerogel film demonstrates a very high voltage of 60.6 V and current of 7.7 µA, corresponding to a power density of 2.33 W m^?2, which is sufficient to power 22 blue light‐emitting‐diodes (LEDs). This is the first report on triboelectric nanogenerators (TENGs) employing porous polymer aerogel films as both positive and negative materials to enhance triboelectric outputs. Furthermore, enhancing the tribopositive polarity of the cellulose aerogel film via silanization using aminosilane can dramatically improve the triboelectric performance. Therefore, this study provides new insights into investigating porous materials with tunable triboelectric polarities for high performance TENGs.     

Performance Optimization of Vertical Nanowire‐based Piezoelectric Nanogenerators

The integrated nanogenerator (NG) based on vertical nanowire (NW) arrays is one of the dominant designs developed to harvest mechanical energy using piezoelectric nanostructures. Finite element method (FEM) simulations of such a NG are developed using ZnO NWs in compression mode to evaluate its performances in term of piezoelectric potential generated, capacitance, induced mechanical energy, output electrical energy, and efficiency. This evaluation is essential to correctly understand NG operation. Three main issues are highlighted. The mechanical and electrical structures of the NG as an integrated system are optimized, and strategies for concentrating the mechanical strain field in the NWs and increasing the force sensitivity are developed. In addition, the influence of NWs length and diameter on NG performances is investigated. The optimization results in a piezoelectric nano composite material where global performances are improved by mean of long and thin NWs.     

White Light‐Emitting Diode From Sb‐Doped p‐ZnO Nanowire Arrays/n‐GaN Film

A whole interfacial transition of electrons from conduction bands of n‐type material to the acceptor levels of p‐type material makes the energy band engineering successful. It tunes intrinsic ZnO UV emission to UV‐free and warm white light‐emitting diode (W‐LED) emission with color coordinates around (0.418, 0.429) at the bias of 8–15.5 V. The W‐LED is fabricated based on antimony (Sb) doped p‐ZnO nanowire arrays/Si doped n‐GaN film heterojunction structure through one‐step chemical vapor deposition with quenching process. Element analysis shows that the doping concentration of Sb is ≈1.0%. The I–V test exhibits the formation of p‐type ZnO nanowires, and the temperature‐dependent photoluminescence measurement down to 4.65 K confirms the formation of deep levels and shallow acceptor levels after Sb‐doping. The intrinsic UV emission of ZnO at room temperature is cut off in electroluminescence emission at a bias of 4–15.5 V. The UV‐free and warm W‐LED have great potential application in green lights program, especially in eye‐protected lamp and display since television, computer, smart phone, and mobile digital equipment are widely and heavily used in modern human life, as more than 3000 h per year.     

Biocompatible Poly(lactic acid)‐Based Hybrid Piezoelectric and Electret Nanogenerator for Electronic Skin Applications

Human machine interface (HMI) devices, which can convert human motions to electrical signals to control/charge electronic devices, have attracted tremendous attention from the engineering and science fields. Herein, the high output voltage from a nonpiezoelectric meso‐poly(lactic acid) (meso‐PLA) electret‐based triboelectric nanogenerator (NG) is combined with the relatively high current from a double‐layered poly(l ‐lactic acid) (PLLA)‐based piezoelectric nanogenerator (PENG) for an E‐skin (electronic skin) (HMI) device application. The hybrid NG with a cantilever structure can generate an output voltage of 70 V and a current of 25 µA at the resonance frequency of 19.7 Hz and a tip load of 4.71 g. Moreover, the output power of the hybrid NG reaches 0.31 mW, which is 11% higher than that from the PLLA‐based PENG. Furthermore, it is demonstrated that the PLA‐based hybrid NG can be used to turn a light‐emitting diode light on and off through an energy management circuit during a bending test. Finally, it is demonstrated that the PLA‐based woven E‐skin device can generate the output signals of 35 V (V_oc) and 1 µA (I_sc) during an elbow bending test. The advantages of biocompatible, ease of fabrication, and relatively high output power in the hybrid NG device show great promise for future E‐skin applications.     

Triboelectric and Piezoelectric Effects in a Combined Tribo‐Piezoelectric Nanogenerator Based on an Interfacial ZnO Nanostructure

In this contribution, combined triboelectric and piezoelectric generators (TPEG) with a sandwich structure of aluminum‐polydimethylsiloxane/polyvinylidene fluoride composite‐carbon (Al‐PPCF‐Carbon) are fabricated for the purpose of mechanical energy harvesting. Improved by the surface modification of PPCF with zinc oxide (ZnO) nanorods through a hydrothermal method, the TPEG generates an open‐circuit voltage (V_oc) of ≈40 V, a short‐circuit current (I_sc) of 0.28 μA with maximum power density of ≈70 mWm^?2_, and maximum conversion efficiency of 34.56%. Subsequently, in order to understand the transduction mechanism of the triboelectric and piezoelectric effects, analyses focusing on the potential composition ratio in the final output and the impact of ZnO interfacial nanostructure are carried out. The observed potential ratio between triboelectric and piezoelectric effects is 12.75:1 and the highest potential improvement by ZnO nanorods of 21.8 V is achieved by the TPEG fabricated with spacer. Finally, the relationships between the voltage, power density, conversion efficiency, and the external load resistances are also discussed. Overall, the fabricated TPEG is proved to be a simple and effective nanogenerator in mechanical energy conversion with enhanced output potential and conversion efficiency.     

Exploiting In Situ Redox and Diffusion of Molybdenum to Enable Thin‐Film Circuitry for Low‐Cost Wireless Energy Harvesting

Direct additive fabrication of thin‐film electronics using a high‐mobility, wide‐bandgap amorphous oxide semiconductor (AOS) can pave the way for integration of efficient power circuits with digital electronics. For power rectifiers, vertical thin‐film diodes (V‐TFDs) offer superior efficiency and higher frequency operation compared to lateral thin‐film transistors (TFTs). However, the AOS V‐TFDs reported so far require additional fabrication steps and generally suffer from low voltage handling capability. Here, these challenges are overcome by exploiting in situ reactions of molybdenum (Mo) during the solution‐process deposition of amorphous zinc tin oxide film. The oxidation of Mo forms the rectifying contact of the V‐TFD, while the simultaneous diffusion of Mo increases the diode's voltage range of operation. The resulting V‐TFDs are demonstrated in a full‐wave rectifier for wireless energy harvesting from a commercial radio‐frequency identification reader. Finally, by using the same Mo film for V‐TFD rectifying contacts and TFT gate electrodes, this process allows simultaneous fabrication of both devices without any additional steps. The integration of TFTs alongside V‐TFDs opens a new fabrication route for future low‐cost and large‐area thin‐film circuitry with embedded power management.     

Mesoporous Piezoelectric Polymer Composite Films with Tunable Mechanical Modulus for Harvesting Energy from Liquid Pressure Fluctuation

Harvesting mechanical energy from biological systems possesses great potential for in vivo powering implantable electronic devices. In this paper, a development of flexible piezoelectric nanogenerator (NG) is reported based on mesoporous poly(vinylidene fluoride) (PVDF) films. Monolithic mesoporous PVDF is fabricated by a template‐free sol–gel‐based approach at room temperature. By filling the pores of PVDF network with poly(dimethylsiloxane) (PDMS) elastomer, the composite's modulus is effectively tuned over a wide range down to the same level of biological systems. A close match of the modulus between NG and the surrounding biological component is critical to achieve practical integration. Upon deformation, the composite NG exhibits appreciable piezoelectric output that is comparable to or higher than other PVDF‐based NGs. An artificial artery system is fabricated using PDMS with the composite NG integrated inside. Effective energy harvesting from liquid pressure fluctuation (simulating blood pressure fluctuation) is successfully demonstrated. The simple and effective approach for fabricating mesoporous PVDF with tunable mechanical properties provides a promising route toward the development of self‐powered implantable devices.     

Embossed Hollow Hemisphere‐Based Piezoelectric Nanogenerator and Highly Responsive Pressure Sensor

Harvesting energy using piezoelectric materials such as ZnO, at nanoscale due to geometrical effects, are highly desirable for powering portable electronics, biomedical, and healthcare applications. Although one‐dimensional nanostructures such as nanowires have been the most widely studied for these applications, there exist a limited number of piezomaterials that can be easily manufactured into nanowires, thus, developing effective and reliable means of preparing nanostructures from a wide variety of piezomaterials is essential for the advancement of self‐powered devices. In this study, ZnO embossed hollow hemispheres thin film for highly responsive pressure sensors and nanogenerators are reported. The asymmetric hemispheres, formed by an oblique angle deposition, cause an unsymmetrical piezoelectric field direction by external force, resulting in the control of the current direction and level at about 7 mA cm^‐2 at normal force of 30 N. The nanogenerators repeatedly generate the voltage output of ≈0.2 V, irrespective of the degree of symmetry. It is also demonstrated that when one piece of hemisphere layer is stacked over another to form a layer‐by‐layer matched architecture, the output voltage in nanogenerators increases up to 2 times.     

Solution‐Processed Extremely Efficient Multicolor Perovskite Light‐Emitting Diodes Utilizing Doped Electron Transport Layer

A specially designed n‐type semiconductor consisting of Ca‐doped ZnO (CZO) nanoparticles is used as the electron transport layer (ETL) in high‐performance multicolor perovskite light‐emitting diodes (PeLEDs) fabricated using an all‐solution process. The band structure of the ZnO is tailored via Ca doping to create a cascade of conduction energy levels from the cathode to the perovskite. This energy band alignment significantly enhances conductivity and carrier mobility in the CZO ETL and enables controlled electron injection, giving rise to sub‐bandgap turn‐on voltages of 1.65 V for red emission, 1.8 V for yellow, and 2.2 V for green. The devices exhibit significantly improved luminance yields and external quantum efficiencies of, respectively, 19 cd A^?1 and 5.8% for red emission, 16 cd A^?1 and 4.2% for yellow, and 21 cd A^?1 and 6.2% for green. The power efficiencies of these multicolor devices demonstrated in this study, 30 lm W^?1 for green light‐emitting PeLED, 28 lm W^?1 for yellow, and 36 lm W^?1 for red are the highest to date reported. In addition, the perovskite layers are fabricated using a two‐step hot‐casting technique that affords highly continuous (>95% coverage) and pinhole‐free thin films. By virtue of the efficiency of the ETL and the uniformity of the perovskite film, high brightnesses of 10 100, 4200, and 16,060 cd m^?2 are demonstrated for red, yellow, and green PeLEDs, respectively. The strategy of using a tunable ETL in combination with a solution process pushes perovskite‐based materials a step closer to practical application in multicolor light‐emitting devices.     

Patterning of Flexible Transparent Thin‐Film Transistors with Solution‐Processed ZnO Using the Binary Solvent Mixture

Flexible transparent thin‐film transistors (TTFTs) have emerged as next‐generation transistors because of their applicability in transparent electronic devices. In particular, the major driving force behind solution‐processed zinc oxide film research is its prospective use in printing for electronics. Since the patterning that prevents current leakage and crosstalk noise is essential to fabricate TTFTs, the need for sophisticated patterning methods is critical. In patterning solution‐processed ZnO thin films, several points require careful consideration. In general, as these thin films have a porous structure, conventional patterning based on photolithography causes loss of film performance. In addition, as controlling the drying process is very subtle and cumbersome, it is difficult to fabricate ZnO semiconductor films with robust fidelity through selective printing or patterning. Therefore, we have developed a simple selective patterning method using a substrate pre‐patterned through bond breakage of poly(methyl methacrylate) (PMMA), as well as a new developing method using a toluene–methanol mixture as a binary solvent mixture.     

Solution‐Deposited Zinc Oxide and Zinc Oxide/Pentacene Bilayer Transistors: High Mobility n‐Channel,Ambipolar, and Nonvolatile Devices

A solution processed n‐channel zinc oxide (ZnO) field effect transistor (FET) was fabricated by simple dip coating and subsequent heat treatment of a zinc acetate film. The field effect mobility of electrons depends on ZnO grain size, controlled by changing the number of coatings and zinc acetate solution concentration. The highest electron mobility achieved by this method is 7.2 cm² V^?1 s^?1 with On/Off ratio of 70. This electron mobility is higher than for the most recently reported solution processed ZnO transistor. We also fabricated bilayer transistors where the first layer is ZnO, and the second layer is pentacene, a p‐channel organic which is deposited by thermal evaporation. By changing the ZnO grain size (or thickness) this type of bilayer transistor shows p‐channel, ambipolar and n‐channel behavior. For the ambipolar transistor, well balanced electron and hole mobilities are 7.6 × 10^?3 and 6.3 × 10^?3 cm² V^?1 s^?1 respectively. When the ZnO layer is very thin, the transistor shows p‐channel behavior with very high reversible hysteresis. The nonvolatile tuning function of this transistor was investigated.     

衬底温度对ZnO薄膜氧缺陷的影响

采用射频磁控溅射在石英玻璃和单晶硅Si(100)衬底上制备了ZnO薄膜，研究了衬底温度对ZnO薄膜中氧缺陷的影响。实验发现，ZnO薄膜c轴取向性随温度的升高而增强；当衬底温度达到550。C时，XRD谱上仅出现一个强的(002)衍射峰和一个弱的(004)衍射峰，显示ZnO具有优异c轴取向性。同时，随着温度的升高，ZnO薄膜的紫外透射截止边带向高波长方向漂移，其电导率也随衬底温度的升高逐渐增大，表明薄膜中的氧缺陷逐渐增多。这种氧缺陷是由于ZnO的氧平衡分压高于Zn所致，可通过提高溅射气体中氧含量来改善。     

ZnO薄膜的掺杂和转型的研究进展

本征的 Zn O为高阻材料 ,电阻率高达 1 0 12 Ω· cm,如何对 Zn O进行掺杂 ,精确控制其电学性能 ,制备高质量的 n型和 p型材料是实现 Zn O应用的关键。目前 ,研究表明通过掺杂 ,n型 Zn O的制备已经能够精确控制 ,但由于 Zn O中存在的缺陷 ,如氧空位和锌间隙原子 ,Zn O天然呈 n型 ,p型的制备还有一定难度 ,本文报道了这方面研究的最新进展