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Hybrid Energy Harvesters: Toward Sustainable Energy Harvesting

Hanjun Ryu, Hong-Joon Yoon, and Sang-Woo Kim*

DOI: 10.1002/adma.201802898

systems and makes them inconvenient for users.[9–11] In addition, the periodic exchange of the primary battery causes an enormous waste of resources and complex maintenance problems.[12–14] Although the ultralow power consumption system and the high capacity battery can extend the WSN system’s operation time, it cannot ensure continuous operation of the system for decades.[15] Thus, an energy harvesting system that converts wasted ambient environment energy into valuable electric energy is one of the important technolo-gies for a future sustainable society.[16–18]

There are various energy harvesting systems, such as piezoelectric,[19–22] tribo-electric,[23–25] thermoelectric,[26–31] pyro-electric,[32–34] photovoltaic,[35–37] and water evaporation-based energy harvesting sys-tems[38,39] and those using green energy sources such as solar, wind, wave, heat, and vibrations. These renewable energy har-vesting systems have been receiving great attention in the field of research into renew-able and sustainable energy harvesters (EHs) to realize self-powering smart WSN systems and self-charging electronics.[40–46]

The output performance of energy harvesting systems has rapidly improved, and WSN systems and energy har-vesting systems have been combined. These two advancements have resulted in extend operation times of small electronic devices. Specialized EH, which uses one kind of green energy, is efficiently converting energy, but this system is fatally flawed because it is influenced by weather conditions. For example, bio-mechanical energy-based harvesters can generate energy both indoor and outdoors, but specific targeted biomechanical move-ment is required.[47] Thus, unexpected mechanical energy or other thermal and solar energy is wasted. Similarly, solar EHs can effectively harvest energy under illumination from the sun, but constantly changing weather conditions place constraints on solar cell performance.[48] Furthermore, thermal energy gen-erated by mechanical energy or solar energy is wasted without additional thermal EHs.[49] Therefore, to prevent the useless wasting of energy by a single energy harvesting system, utilizing plural green energy harvesting systems is a countermeasure for sustainable energy harvesting, so that otherwise wasted energy is fully utilized with high energy conversion efficiency, which can power WSN systems at anytime, anywhere.

Nature and artificial energies, such as solar, wind, wave, heat, machine vibration, automobile noise continuously exist, so

Recently, sustainable green energy harvesting systems have been receiving great attention for their potential use in self-powered smart wireless sensor network (WSN) systems. In particular, though the developed WSN systems are able to advance public good, very high and long-term budgets will be required in order to use them to supply electrical energy through temporary batteries or connecting power cables. This report summarizes recent significant progress in the development of hybrid nanogenerators for a sustainable energy harvesting system that use natural and artificial energies such as solar, wind, wave, heat, machine vibration, and automobile noise. It starts with a brief introduction of energy harvesting systems, and then summarizes the different hybrid energy harvesting systems: integration of mechanical and photovoltaic energy har-vesters, integration of mechanical and thermal energy harvesters, integration of thermal and photovoltaic energy harvesters, and others. In terms of the reported hybrid nanogenerators, a systematic summary of their structures, working mechanisms, and output performances is provided. Specifically, electromagnetic induction, triboelectric, piezoelectric, photovoltaic, thermoelectric, and pyroelec-tric effects are reviewed on the basis of the individual and hybrid power perfor-mances of hybrid nanogenerators and their practical applications with various device designs. Finally, the perspectives on and challenges in developing high performance and sustainable hybrid nanogenerator systems are presented.

Energy Harvesters

H. Ryu, H.-J. Yoon, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 16419, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201802898.

1. Introduction

Recently, technical innovation has led to the development of smart wireless sensor network (WSN) systems, which are being used in enhancing the public welfare for the future of society.[1–5] Although these innovative WSN systems advance the public welfare, their construction and maintenance require an astronomical budget. Especially, powering the WSN systems is a challenge, due to the difficulty of connecting power cables with multitudinous sensors, so a temporary powering system based on WSNs is one of the candidates.[6–8] However, because of the limited capacity of batteries, an energy source without a self-powering function limits the sustainable operation of WSN

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mechanical, solar, and thermal energy harvesting systems can coexistence and can generate energy continuously (see Figure 1). For example, in the daytime, sunlight is the main energy source of hybrid energy harvesting devices, while other mechanical and thermal energy sources assist to enhance output performance of the devices. However, at night or on a rainy day, mechanical energy harvesting systems or thermal energy harvesting systems demonstrate better performance than photovoltaic harvesting systems, thus hybrid EHs can sustainably harvest energy. Such complementary hybrid generators create sustainable energy har-vesting system for the permanent WSN systems.

Herein, we introduce recent research progress in the hybridi-zation of energy harvesting systems and present a simple but effective method for sustainable energy harvesting. First, energy harvesting systems are categorized and introduced by input energy sources, such as mechanical energy, solar energy, and thermal energy. Piezoelectric, triboelectric, and electromagnetic generators belong to the mechanical energy harvesting group; thermoelectric and pyroelectric generators belong to the thermal energy harvesting group; solar cells belong to the solar energy har-vesting group. Second, various integration research and enhance-ments of the generator’s performances are described. Finally, this research progress will introduce recent research trends and give perspective into overcoming the present obstacles.

2. Energy Harvesting Systems

2.1. Mechanical Energy Harvesting System

2.1.1. Electromagnetic Induction-Based Generator

Mechanical energy that comes from elements of natural and artificial environments, such as wind, waves, machine vibration, and human motion, are the main driving force in electromag-netic generators (EMGs); coil, magnetic fields, and mechanical displacement of devices are key factors in generating electric energy using conductors and magnets (see Figure 2a). Faraday’s law of electromagnetic induction deals with inducing electric flow through conductors by interaction with a magnetic flux (ΦB) and an electromotive force (εemg).[50] This electromagnetic gen-erator output voltage can be derived by the following equation.

ε =− Φd

demg

BNt

(1)

where N is the number of turns in a coil. Therefore, high con-ductive coil design and optimized device structure are required for a high-performance EMG. Since the EMG was discovered, it has been utilized for everything from small portable genera-tors to large plant-scale generators, and EMGs have extensively contributed to the development of modern society.

2.1.2. Piezoelectric Effect-Based Generator

Another mechanical energy-based nanogenerator is the piezo-electric nanogenerator (PENG). Stress/strain and a piezoelectric constant are key factors in generating electric energy using

piezoelectric materials (see Figure 2b). The piezoelectric effect is the conversion of mechanical energy into electric energy by breaking central symmetry of crystal structure. In breaking the central symmetry of the material by external force, piezo-electric materials generate internal potential, which results in the potential difference across the material, and electric charges flow through external circuits. This piezoelectric polarization charge density can be expressed as

ρ =p pd X (2)

where ρp is the polarization charge density, dp is the piezo-electric coefficient, and X is the applied stress. The electric field

Hanjun Ryu is a Ph.D. student under the supervision of Prof. Sang-Woo Kim at School of Advanced Materials Science & Engineering, Sungkyunkwan University (SKKU). His current research is fabrication and character-ization of pyroelectric and triboelectric nano generators for energy harvesting, port-able self-powered devices.

Hong-Joon Yoon is pursuing Ph.D. course under the super-vision of Prof. Sang-Woo Kim at School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU). His research interests are fabrica-tions and characterizations of piezoelectric and triboelectric nanagenerators for energy harvesting and their applica-tions in self-powered devices.

Sang-Woo Kim is a pro-fessor in the Department of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). His recent research interest is focused on piezoelectric/triboelectric nanogenerators, photo-voltaics, and 2D materials including graphene, MoS2 etc. Now he is a director of

SAMSUNG-SKKU Graphene/2D Research Center and is leading National Research Laboratory for Next Generation Hybrid Energy Harvester. He was the Conference Chair of the 4th NGPT (Nanogenerator Piezotronics) in 2018.

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and potential can be derived by the charge density as expressed by the following equation.

ρε

∇ = pE (3)

where ∇E is the divergence of the electric field, and ε is the permittivity. Therefore, optimized design of the piezoelectric material and device structure is essential for high PENG per-formance. Prof. Wang, of the Georgia Institute of Technology, invented PENG in 2006 by vertically pressing a ZnO nanowire

using an atomic force microscope (AFM) tip,[19] and recently, output power performance of specially designed PENGs, con-sidering above mentioned factors have increased output power from 0.5 pW to 0.7 mW since 2006.[19,51]

2.1.3. Triboelectric Effect-Based Generator

The other mechanical energy-based nanogenerator is the tribo-electric nanogenerator (TENG), which is based on triboelectri-fication and electrostatic induction. Surface charge density is a

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Figure 1. A schematic description of a sustainable hybrid energy harvesting system using natural and artificial energies.

Figure 2. Schematic illustrations of various energy harvesting systems based on a) the electromagnetic induction, b) piezoelectric effect, c) triboelectric effect, d) photovoltaic effect, e) thermoelectric effect, and f) pyroelectric effect. (inspired from ref. [17]).



key factor in generating electric energy using triboelectric mate-rials (see Figure 2c). Surface triboelectric charge polarities that form on materials are determined by the triboelectric series, and these charges induce electric flow through external circuits with specific potential differences.[52] Thus, when two different materials come into contact, the interface of the materials transfers a charge, which results in the generation of charges with opposite polarities on the surfaces. After the formation of surface triboelectric charges, the relative movement of the two triboelectric materials induces the flow of electric charges between electrodes to maintain the electrostatic equilibrium. The triboelectric potential VT can be derived by

ρε

= −TT

0

Vd

(4)

where ρT is the triboelectric charge density, ε0 is the vacuum permittivity, and d is the gap distance between two triboelectric materials. The triboelectric current IT can be expressed by

= ∂∂

+ ∂∂t

T TT

TTI C

VV

C

t (5)

where CT is the capacitance of the triboelectric system and VT is the generated triboelectric voltage across the two electrodes. The degree of the TENG’s mechanical deformation defines both voltage and current output performance, so design of the structure and triboelectric material selection are significant for high TENG performance. Prof. Wang invented TENG in 2012 by simple materials friction,[53] and recently, output power performance of the innovative TENG system, considering the above mentioned factors, has increased generated power from 2 µW to 50 W since 2012.[53,54]

2.2. Solar Energy Harvesting System

A solar cell, or photovoltaic cell, is one of the promising green energy harvesters converting infinite solar energy into prac-tical electricity by photovoltaic effect[55] (see Figure 2d). When sunlight irradiates a solar cell, absorbed light energy in the semiconductor generates electron–hole pairs, which separate and transport charges to the electrodes. Work function of the p-, n-type semiconductor and electrodes is a critical factor in deciding charge transport ability and generation of voltage. Thus, developing materials and modifying the work function are required for high energy conversion efficiency. The conver-sion efficiency (ηsolar) of the solar cell is one of the standards in evaluating the output performance of solar cells. The conver-sion efficiency can be derived by

η ( ) = = × × ×%FF

100solarmax

in

OC SC

in

P

P

V J

P (6)

where Pmax is the maximum output performance, Pin is input solar energy, VOC is the open-circuit voltage, JSC is the short-circuit current, and FF is the fill factor. The FF is the max-imum power from the solar cell, which is achieved from the I–V curve. Power performance of all the solar cells, which are classified as silicon-based solar cells, dye-sensitized solar cells (DSSCs), and organic solar cells (OSCs), are evaluated using the same standard. Since solar cells were invented, their energy

conversion efficiency has developed from a few percent to over 40%, due to developing materials.[56]

2.3. Thermal Energy Harvesting System

2.3.1. Thermoelectric Effect-Based Generator

Thermal energy that comes from various natural and artificial environments abundant and can drive thermoelectric nanogen-erators (see Figure 2e). The thermoelectric nanogenerator is based on the Seebeck effect: the induction of electrons and holes diffusion by temperature gradient through p- and n-type semi-conductors. Thus, the Seebeck coefficient determines the power performance of the output voltage of the thermoelectric nano-generator, and output voltage is derived by followed equation.

α= ∆ThermoV T (7)

where VThermo is the thermoelectric output voltage, ∆T is the temperature gradient, and α is the Seebeck coefficient. For comparing thermoelectric nanogenerators, the ZT value, which is the thermoelectric material’s figure of merit, is a well-known standard, which is expressed as

σακ κ

=+

2

e l

ZTT

(8)

where σ is the electrical conductivity, T is the mean operating temperature, κe is the electronic contributed thermal con-ductivity, and κl is lattice contributed thermal conductivity. Thus, high electrical conductivity by doping, and using low thermal conductivity materials, will enhance thermoelectric power performance. Since the thermoelectric nanogenerator was invented, the ZT values have developed from 0.2 to over 2.5, due to developing materials.[57] For instance, tellurium-free chalcogenides and Cu2Se had ZT values of 1.6 at 923 K and 1.5 at 1000 K, respectively. Cu2S and Cu2Te had ZT values of up to 2.1 at 1000 K and SnSe had a ZT value of 2.62 at 923 K.

2.3.2. Pyroelectric Effect-Based Generator

The other thermal energy-based nanogenerator is the pyroelec-tric effect based generator, which is based on spontaneous polari-zation change by continuous temperature change (see Figure 2f). Because every pyroelectric material has piezoelectric property, temperature change induces a pyroelectric effect, as well as a piezoelectric effect by thermal expansion of the pyroelectric material. Thus, the pyroelectric coefficient is measured under a constant stress condition, preventing the thermal expansion induced piezoelectric effect, the so-called secondary pyroelectric effect. The pyroelectric coefficient (Ppyro) is explained as

ρ= d

dpyrop

T (9)

where ρ is the spontaneous polarization and T is the temperature. The electric current output by the pyroelectric effect is driven by

µ= =d

d

d

dpyroI

Q

tp A

T

t (10)

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where Q is the induced charge, µ is the absorption coefficient of radiation, and A is the surface area. Thus, the development of high pyroelectric coefficient materials and the optimization of easily heated and cooled device structures are significant for the high performance of pyroelectric nanogenerators.

3. Hybrid Energy Harvesting Systems

Hybrid EHs for sustainable energy harvesting have received atten-tion as new candidates for renew-able EHs because of the high performance and coupling effect between energy harvesting sys-tems. Integration of mechanical and photovoltaic EHs,[58–64] inte-gration of mechanical and thermal EHs,[65–69] integration of thermal and photovoltaic EHs,[70–74] and integration of various EHs are representative demonstrations of hybrid EHs.[75–89]

3.1. Integration of Mechanical and Photovoltaic Energy Harvesters

Since Xu et al. reported on mechanical and photovoltaic hybrid generators in 2009,[90] hybrid energy harvesters have been intensively developed, based on piezoelectric, tribo-electric, and photovoltaic effects. Yang et al. reported on a trans-parent flexible polymer based TENG and a pyramid patterned silicon (Si) solar cell combined hybrid generator for sustainable and simultaneous energy har-vesting (see Figure 3).[91] Using the transparent polymer, polydi-methysiloxane (PDMS) nanowire array as a triboelectric material and patterned Si, a double elec-trode-mode TENG was able to harvest mechanical energy (see Figure 3a). A 200 nm diameter and 500 nm depth anodic aluminum oxide (AAO) template was used for fabricating the PDMS nanowire. To maintain the transparency of the TENG, indium tin oxide (ITO) transparent

film was used as an electrode. The Si micropyramid-patterned solar cell consisted of an Al electrode, a p-type doped Si layer, an n-type doped Si layer, SiN film, a silver (Ag) grid, and the

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Figure 3. a) A schematic image of the hybrid energy nanogenerator. b) SEM image of the PDMS nanowire arrays. c) SEM image of the Si pyramids. d) Current density (J)–voltage (V) curves of the Si solar cell with/without PET–PDMS polymer under the 100 mW cm−2 light illumination intensity condition. e) Output voltage performance of the hybrid nanogenerator with integrated 12 solar cells. f) Energy charging curve by the hybrid nanogenerator and the constant current discharge curve of a Li-ion battery. g) The voltage charging curve of a Li-ion battery, and real photo image of a red laser diode operated by Li-ion battery. h) The voltage charging curve of commercial cell phone’s battery. All panels reproduced with permission.[91] Copyright 2013, American Chemical Society.



ITO electrode.[92] The light absorption was improved, and the triboelectrification was enhanced by the patterned Si. In addi-tion, the patterned Si served as the protection layer of the Si solar cell. Both the TENG and the Si solar cell had ITO coelec-trodes on the Si pattern. Figure 3b shows an image by scan-ning electron microscopy (SEM) of the patterned PDMS that revealed the diameter of the nanopattern for enhancing power performance, and Figure 3c shows a surface SEM image of the pyramid-patterned Si. To compare the power performance of the solar cell with/without the TENG device, the Si solar cell was illuminated under one sun condition. Compare to the other Si nanowire heterojunction solar cell, the Si solar cell without the TENG device generated 0.6 V, 35 mA cm−2 and had a 16% energy conversion efficiency, which was much larger than demonstrated in the previous report (see Figure 3d).[93] How-ever, when the TENG device was assembled on the Si solar cell, the energy conversion efficiency decreased from 16% to 14% due to disturbing sunlight illumination by the TENG. Thus, a single Si solar cell device generated about 0.6 V under sunlight illumination and the TENG generated ≈2.5 V under periodic mechanical input energy. When both sunlight illumination and mechanical motion were applied simultaneously, output voltage of the hybrid device was ≈2.7 V.

To demonstrate future applications of the hybrid energy device, 12 devices were integrated to enhance the output perfor-mance. In addition, a charging energy storage unit, a lithium (Li)-ion battery used to store harvested energy, was demon-strated, which is essential for larger energy consumption devices, and it prolongs the lifetime of WSNs. Figure 3e shows the integrated hybrid cells’ output performance over 13 V, and 10.6 mA. Using the hybrid cells, a Li-ion battery was success-fully recharged from 1.54 to 3.60 V in 1.3 h (see Figure 3f). Under a constant 10 mA discharge current, the battery was discharged to 1.54 V after 580 s which meant that the stored electric capacity of the battery was around 1.61 mAh. Figure 3g demonstrates the operation of the red laser diode, which is widely used as a commercial pointer for presentations and is powered by recharged the Li-ion battery. Hybrid energy gen-erators also recharge commercialized cell phones from 2.63 to 3.50 V in 9 h (see Figure 3h). Although there were some observed peaks during recharge of the battery, these peaks were not seen in another Li-ion battery. Therefore, hybridization of solar cells and mechanical energy harvesters can operate inde-pendently, while simultaneous operation of the devices boosted the output voltage performance.

Another mechanical and solar hybrid energy harvester was reported by Yoon et al.[94] It utilized serially integrated hybrid devices that consisted of direct current (DC) type 2D ZnO nanosheets as a PNG (DC-PNG) and an OSC that used poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC60BM). Thus, each part of the piezoelectric and photovoltaic hybrid generator could operate individu-ally, or they could operate simultaneously. Figure 4a shows a schematic image of the flexible hybrid generator, which used vertical mechanical energy and sunlight energy. Figure 4b,c shows a cross-sectional field-emission SEM (FE-SEM) image of the DC-PNG and OSC, respectively. The flexible DC-PNG consisted of a top electrode with 0.5 µm high ZnO nanosheests as piezoelectric materials, and 20 nm high Zn:Al layered

double hydroxide (LDH) layer on the bottom electrode. The ZnO was grown by the hydrothermal method on the bottom aluminum(Al) electrode (see Figure 4b). The inverted structure OSC consisted of an 80 nm high Ag anode, a 20 nm high MoO3 (electrode blocking layer)/200 nm high P3HT:PC60BM (active layer)/50 nm height ZnO film (electron transport layer)/200 nm height ITO cathode (see Figure 4c). A circuit diagram of the hybrid generator is shown in Figure 4d, in which the OSC and DC-PNG had series connection.

The power performance of the hybrid generator was evalu-ated by measuring the open-circuit voltage (Voc) and Jsc. The OSC generated 0.6 Voc and 10.2 mA cm−2 under light condi-tions, and the DC-PNG generated 0.4 V and 22 µA cm−2 under periodic vertical compressive pressure with/without a light. The Zn:Al LDH layer that formed between the ZnO and Al electrode achieved a DC-type output piezoelectric signal. When the ZnO was compressed by external force, the contained posi-tive charges in LDH layer were compensated by the negative charges from the ZnO nanosheets. Compensation of the ZnO and LDH charges resulted in an accumulation of huge nega-tive charges at the interface between the ZnO and LDH, which created a high potential difference and generated DC property. Because of the unique property of the DC-PNG, the hybrid generator did not require an additional electric management component and it increased energy management efficiency. Figure 4e shows output performance of the hybrid device under various conditions, such as the existence of a mechanical force and dark/light illumination. Without both light and mechanical compression, the hybrid generator kept its initial status. When light irradiated the hybrid device, the generated photovoltaic voltage rose to 0.4 V, and when a mechanical force was applied in conjunction with light, the output voltage of the hybrid device increased to 0.7 V. After both light and mechanical energy were disappeared, the hybrid generator’s output voltage rapidly decreased to 0 V. When only mechanical pressure was applied to the device, it generated voltage even when the OSC was not operating. Output current of the hybrid device showed a trend similar to the voltage output performance.

Figure 4f,g shows the piezoelectric potential–photovoltaic coupling effect, which enhanced the OSC performance. When pressure was applied to the hybrid device, the output voltage of the device increased, and it remained constant until the pres-sure released. Figure 4f shows magnified voltage pulses of the hybrid generator that was affected by the piezoelectric potential. When the PNG and the OSC were connected in series, the gold (Au) electrode was connected to the cathode of the OSC, and the Al electrode was connected to the OSC’s anode. The Schottky barrier (φ) was formed at the interface between the Au electrode and the ZnO nanosheets. As shown in Figure 4g, large nega-tive charges at the interface of ZnO/LDH built up, and corre-sponding positive charges were accumulated at the interface of ZnO/Au.[95] A modified band structure of the ZnO nanosheets led to the piezoresistance effect, which caused significant elec-tron flow enhancement from the OSC through the PNG. Con-sequently, when pressing the device, the generated voltage of the device increased. Furthermore, the remaining time of the piezoelectric potential in the ZnO nanosheets was extended without being fully screened by the free carriers, while pre-serving the pressure.[96,97] Therefore, the conduction band of the

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ZnO neighboring the Al electrode was elevated by the remnant polarization, and current flowed through the PNG from the Au electrode to the Al electrode. Thus, a serially integrated OSC and PNG could be used as simultaneous energy harvesters without additional electric components to manage the current flow.

The other mechanical and solar hybrid energy harvester was reported by Chen et al. which was a fiber-type solar cell and a TENG-based simultaneous energy harvesting fabric.[98] They focused on the sustainable and foldable power generators by fabricating all-solid hybrid power textiles using economically

Adv. Mater. 2019, 31, 1802898

Figure 4. a) A schematic image of the hybrid nanogenerator. b) Cross-sectional SEM image of ZnO nanosheets. c) Cross-sectional SEM image of the inverted OSC. d) A schematic circuit of the hybrid nanogenerator. e) Voltage and current output performance of the hybrid nanogenerator with con-trolled light and mechanical force conditions. f) Voltage output performance of the piezoelectricphotovoltaic effects coupled hybrid nanogenerator. g) Energy band diagram of the ZnO when pressure is applied. All panels reproduced with permission.[94] Copyright 2015, Elsevier Ltd.



competitive materials and large scalable fabrication processes. To fabricate the energy harvesting fabrics, polymer fiber based solar cells were used as the basic part in fabricating TENGs, so fibers harvested both mechanical energy and light energy simul-taneously. All solid power textiles were a single-layer-interlaced structure mixing a fiber-type solar cell and a TENG for con-verting mechanical energy and ambient solar energy into elec-tricity, as shown in Figure 5a,b. Figure 5c shows an SEM image of the photoanode and copper (Cu)-coated polymer fiber, which was a counter electrode. It was observed that it assembled on polymer fibers by a low temperature wet process. All electrodes were compatible with high throughput textile fabrication. The wire shaped photoanode strings with Cu-coated polytetrafluor-oethylene (PTFE) strips and Cu electrodes were woven by a

weaving machine to make a hybrid textile type power generator via a shuttle-flying process.[99] Both the photovoltaic textile and the fabric TENG utilized a Cu electrode to simplify the structure design, and a lightweight polymer was used as the substrate of the photoanode to improve mechanical strength and flexibility. These components of the hybrid generator worked simultane-ously to greatly increase energy harvesting. The woven hybrid power textile enhanced with additional commercial fibers had a 0.32 mm thickness. It was flexible, aesthetically pleasing, and had wearable properties (see Figure 5d).

Since Chen et al. optimized the textile structure with regard to the weaving patterns of the photovoltaic and TENG fabrics, and the intercomponent electrical connection. The 4 cm by 5 cm size textile generator that consisted of the 80% TENG and

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Figure 5. a) Schematic images of the fabric TENG. b) Schematic images of the photovoltaic textile. c) SEM images of the fabric TENG. d) A real photo image of the photovoltaic textile. e) Output performance of the wearable hybrid power textile under various conditions. f) Output power performances of the individual and hybrid nanogenerators depend on the load resistances. g) Real photo images of applications using the hybrid power textile. All panels reproduced with permission.[98] Copyright 2016, Springer Nature.



20% photovoltaic cell had comprehensively optimized and eval-uated the power performances. The photovoltaic textile, which consisted of a series 15 wires connected to the solar cell, and the plain weaving patterns of the TENG textile were electrically connected with a current regulated unit. Figure 5e shows the power performances of the energy harvesting textile under dif-ferent circumstances. During the sunlight exposure of the tex-tile, the photovoltaic section was capable of harvesting energy by converting solar irradiance, and the TENG section also oper-ated under mechanical excitation. When the textile was pressed with sunlight, for instance, and when people walked or exer-cised outside during the daytime, it sustainably and simultane-ously generated power from both human biomechanical energy and sunlight. As shown in Figure 5f, external load resistors were utilized to further evaluate the output power performance of the device. Instantaneous peak power of the hybrid textile and each generator were maximized at a matched resistance. The hybrid power textile stably generated an average power output of 0.5 mW under a wide range of load resistances (from 10 kΩ to 10 MΩ). Therefore, hybridization and optimization of the solar cell and the TENG fabrics evidently better output per-formance than the individual components. The optimum load resistances of the device were expanded as well, which was a significant improvement for a power generator of small elec-tronics that had varying operation resistances.

Figure 5g shows various shapes of hybrid devices as sus-tainable and wearable power sources for commercial elec-tronic devices. Colorful, lightweight, and thin woven textiles with various patterns were fabricated into clothes. As hybrid generators with outdoor activity under natural daylight, the woven fabric harvested power that was able to charge a 2 mF capacitor to around 2 V within 1 min. These hybrid power fab-rics were also able to directly charge a cell phone and continu-ously run an electronic watch. It is worth noting that energy harvesting textiles were able to utilize not only wearable appli-cations but others such as smart flag type wind, temperature, and humidity sensors. They demonstrated a power harvesting flag that was able to harvest sunlight and wind to recharge personal electronics and create an electrochemical reaction for self-powered water splitting. The hybrid power textiles also demonstrated the ability to generate power from dim sun-light and fine wind cast off by a moving car in the city on a cloudy day, which indicated its decent and sustainable ability to operate even in various environments. In addition, as a sus-tainable energy generator, not only the power performance of the device under various conditions but its robustness was sig-nificant. Chen et al. examined the mechanical, chemical, and humidity stability of the hybrid device for practical applica-tions. Humidity is one of the critical factors in the use of the TENG. When the humidity rose from 10% to 90%, the power performance of the hybrid device decreased to ≈73.5% of its original power performance. However, when the humidity decreased and the textile was subsequently dried, the output performance of the TENG was recovered.[100,101] Thus, not only increasing the power performance of the hybrid energy gener-ators but encapsulating layer or encapsulated structure of the device to prevent environmental contamination and negative environmental impact were ideal for sustainable energy har-vesting systems.[102]

3.2. Integration of Mechanical and Thermal Energy Harvesters

Lee et al. reported a highly stretchable, piezoelectric, and pyroe-lectric hybrid NG (HSNG) based on ferroelectric and stretchable polymers, as well as carbon-based electrodes for simultaneous and robust energy harvesting (see Figure 6).[103] Figure 6a shows a schematic image of the HSNG, which was composed of three layers: the micropatterned poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)] polymer, as a piezoelectric and pyroelectric energy generating layer; micropatterned PDMS–carbon nanotube (CNTs) composite polymer, as a substrate and bottom electrode; and graphene as a top electrode. To fab-ricate the PDMS–CNT composite, the liquid PDMS elastomer, cross-linker, and multiwalled CNTs were mixed and degassed. The composite PDMS was spin coated on the line-patterned Si master mold, which was fabricated by the photolithography and etching processes. After polymerization of the PDMS–CNT in an oven at 80 °C, the micropatterned PDMS–CNT composite was detached from the Si master mold. A spin coater was used to directly coat the PDMS–CNT with the high crystalline P(VDF-TrFE) Think film. Then this was annealed at 140 °C for 2 h. To prevent thermal damage, the oven was naturally cooled to room temperature (RT). The formation of β phase P(VDF-TrFE) was confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy measurements.[104,105] The top electrode, multilayer graphene, was transferred onto the top of the P(VDF-TrFE) ferroelectric layer. Especially, an efficient pyroelectric energy harvesting system requires fast temperature transition, thus the graphene electrode took advan-tage of superhigh thermal conductivity and ultralow thickness. Therefore, when heat was applied to the graphene electrode side, higher pyroelectric output performance was obtained than when it was applied the PDMS–CNT side, due to thermal conductivity. A photo image of the fabricated HSNG shows the high stretchability of the device (see Figure 6b). Figure 6c shows the HSNG’s high compatibility with various parts of the human body, demonstrating that the HSNG is suitable for wearable and robotic applications. As shown in Figure 6d, the piezoelectric output voltage of the HSNG was generated by stretching and releasing, and its pyroelectric output voltage was generated by external temperature heating and cooling.

To evaluate the individual HSNG’s power performance, the piezoelectric and the pyroelectric output performance were sepa-rately measured under different strains and at different thermal gradients. Because both the piezoelectric and pyroelectric effects are based on the internal polarization alternation, accurate con-trol of the mechanical and thermal energy could enhance the output performance. Lee et al. analyzed the polarization behav-iors of HSNG with regard to stretching, compressing, releasing, heating, and cooling. Figure 6e,f shows the enhancement of the output voltage from the HSNG by controlling the piezoelectric and pyroelectric effects. As shown in Figure 6e, the piezoelectric output voltage was −0.7 V under stretching and releasing actions, and the pyroelectric output voltage was −0.4 V under thermal heating and cooling. When the HSNG was released and heated at the same time, output voltage was enhanced up to −1.1 V, which was the sum of the piezoelectric and pyroelectric output voltage. On the other hand (see Figure 6f), the piezoelectric output was over −1 V under compressing, and the pyroelectric output

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voltage was –0.4 V under thermal heating. When the HSNG was compressed and heated at the same time, output voltage was enhanced up to –1.5 V. It is worth noting that the unique design and utilization of both the piezoelectric and pyroelectric properties of P(VDF-TrFE) could harvest not only mechanical energy but thermal energy with the effect of coupling. These piezoelectric and pyroelectric effects increased the internal polar-ization change of the P(VDF-TrFE). As Lee et al. demonstrated, strain- and temperature-induced polarization changes could be amplified, which would result in a larger current flow through the external circuit. In this study, they utilized a micropatterned structure to enhance the stretchability, durability, and robust-ness of the HSNG. When the HSNG was stretched up to 30%, it

generated stable output performances. Furthermore, the nonpat-terned hybrid generator had significant damage, such as cracks and wrinkles, after being stretched by 30%, but the HSNG had a small number of wrinkles and cracks under same mechanical conditions. Therefore, the microline-patterned HSNG was seen to be a mechanically durable, stretchable, sustainable, effective energy harvesting system that harvested thermal and mechan-ical energy.

The other mechanical and thermal hybrid energy harvester was reported by Wang et al. It harvested rotation mechanical energy and scavenging wasteful rotation induced thermal energy.[106] Wang et al. hybridized EMG, TENG, and thermo-electric generating systems for simultaneous energy harvesting.

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Figure 6. a) Schematic images and SEM images of the hybrid nanogenerator. b) A real photo image of the hybrid nanogenerator. c) Real photo images of the attached hybrid nanogenerators on the various part of the human body. d) Output voltage of the piezoelectric and pyroelectric nanogenerator. e) Output voltage of the stretch mode PENG, the heat–cool condition pyroelectric nanogenerator, and the hybrid nanogenerator. f) Output voltage of the compressed mode PENG, the heat–cool condition pyroelectric nanogenerator, and the hybrid nanogenerator. All panels reproduced with permis-sion.[103] Copyright 2013, John Wiley and Sons.



Usually, mechanical heat is mostly wasted energy, since it is a thermal energy created by friction heat during rotation, so scav-enging not only mechanical energy but thermal energy could increase the energy conversion efficiency of the hybrid gen-erator. For the mechanical energy harvesting, triboelectric and electromagnetic effects were utilized, and thermoelectric effect was utilized to harvest thermal energy. Rotation energy drove both the TENG and EMG, and induced friction heat created a thermal gradient through the thermoelectric generator (ThEG). Because it used three different energy harvesting systems, the hybrid generator required power management circuits for practical applications. As shown in the schematic image of the hybrid generator (see Figure 7a), the device consisted of rota-tors and stators. The EMG consisted of layer 1, to which eight magnets were attached to an acrylic disk and arranged with alternative magnetic polarity, and layer 5 that had a series of eight connected coils whose positions corresponded to the posi-tions of the magnets on the acrylic disk. The TENG consisted of later 2, with the radially arrayed Cu triboelectric materials, and layer 4 that had the radially arrayed Cu electrodes. This was attached under layer 3, which was a polyamide freestanding layer. The ThEGs constituted layer 6, which was fixed under layer 5 to scavenge wasted thermal energy, and layer 7, which was a heat transfer plate. The diameter of the hybrid nanogen-erator was 14.5 cm. When the rotator rotated and the stator was fixed, every generating system operated; layer 1 rotated and induced fluctuations of the magnetic flux through coils in layer 5, which result in current flowing through the EMG electrodes.

While layer 2 rotated, the Cu gratings on the rotator and the polyamide freestanding film formed triboelectric charges with opposite polarity. Because of Cu gratings, the unbalanced sur-face triboelectric charges on layer 2 induced a current flow through the TENG external electrodes. Because of friction heat, the temperature of the stator surface increased, and this thermal energy transferred to layer 6, which created a thermal gradient to drive the ThEG. Because of the heat source and heat sink (layer 7), electrons and holes were able to move from the high temperature electrode side to the low temperature elec-trode side, inducing a power output from the ThEG.

Figure 7b shows the EMG current and voltage output perfor-mance in which the peak current of the EMG was 4.5 mA and the peak voltage was 26.5 V, under 1000 revolutions per minute (RPM). The TENG output performance is shown in Figure 7c, in which the peak current was 3.6 mA and the peak current was 58.7 V. Figure 7d shows the ThEG output performance after the rotator rotated. Because the ThEG required a friction heat induced temperature gradient, output performance of the ThEG was able to increase with continuous operation of the hybrid nanogenerator, where the current and voltage outputs were increased up to 57.7 mA and 1.2 V, respectively, after 220 s. Because of the different impedance property of the energy har-vesting systems, optimum resistance for maximizing power performance of the systems was investigated by measuring the current output performance under different load resistances (see Figure 7e–g). Figure 7e shows output performance of the EMG, in which the largest output performance, 26.5 mW, was

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Figure 7. a) A schematic diagram of the hybrid nanogenerator. b) Output current and voltage of the EMG. c) Output current and voltage of the TENG. d) Output current and voltage of the ThEG. e) The TENG output current and power at the load resistances. f) The EMG output current and power at the load resistances. g) The ThEG output current and power under a load resistance of 30 Ω. j) The temperature change curve of the top and bottom sides of the ThEG. h) The enlarged output current and energy of the EMG. i) The enlarged output current and energy of the TENG. All panels repro-duced with permission.[106] Copyright 2016, Elsevier Ltd.



obtained under the 6 kΩ load resistance condition. As presented in Figure 7f, the maximized power performance of the TENG was 238.9 mW under a 40 kΩ load resistance. For the maximized ThEG power performance, load resistance was established at 30 Ω, and the largest power output performance was 14.5 mW after 280 s (see Figure 7g). Figure 7j shows the temperatures of both the inner and outer side of the ThEG when the hybrid generator was working. Thus, during the mechanical rota-tion motion, the fabricated ThEG was effectively scavenging the wasted thermal energy. A comparison of the EMG and the TENG power performances was calculated by output energy under the load resistance condition (see Figure 7h,i). The EMG produced 0.2 mJ in 15 ms, while the TENG generated 1.8 mJ in 15 ms. Wang et al. evaluate the hybrid generators with increasing rotation speed, up to 3000 RPM. Output of the hybrid nanogenerator increased due to the optimum impedance drop of the TENG and the rapid increase in temperature. For the practical usage of the hybrid generator, a proper power manage-ment circuit was required. For instance, to charge an Li-ion bat-tery, output voltages of the three energy harvesting system were regulating as a same voltage, so that the output current was elevated. Thus, a hybridized nanogenerator consisting of the EMG, the TENG, and the ThEG was able to scavenge mechan-ical and thermal energy with high efficiency. This hybrid nano-generator structure was able to operate the ThEG using not only mechanical energy but natural thermal energy for the sustain-able energy harvesting. Scavenging thermal energy inside the device might increase the TENG performance because thermal energy disturbs the formation of triboelectric charges.

3.3. Integration of Thermal and Photovoltaic Energy Harvesters

Park et al. reported a hybrid generator using ferroelectric film and poly(3,4-ethylenedioxythiophene)s (PEDOTs) based on pyroelectric, thermoelectric, and photovoltaic effects[107] (see Figure 8). For hybridizing, the photovoltaic cell with other har-vesters, thermal energy harvesters are one of the strong candi-dates because sunlight irradiation is accompanied by thermal energy. For this thermal energy harvesting system, Park et al. utilized the photothermal effect of PEDOTs for the pyroelectric energy harvester and the thermoelectric effect. Figure 8a is a schematic image of the hybrid energy generator, which was a hybrid of photovoltaic, photothermal, pyroelectric, and thermo-electric generators. Specially designed photothermal materials that absorb and convert the wide solar spectrum were required to harvest superfluous heat from the sun. Conducting polymer thin films (CPs) were suitable candidates for the hybrid gen-erators because of their light absorption range with high pho-tothermal conversion.[108,109] Especially, PEDOTs, which act as multienergy harvesters, easily optimized their transparency and photothermal conversion property by controlling the doping states. Thus, the sun irradiation-driven photothermal heat was converted into electric energy through both pyroelectric and thermoelectric generators operating at the same time. Fur-thermore, a photovoltaic power cell was able to be combined with the photothermally driven thermoelectric and pyroelectric generators for boosting output performance of the solar energy-based power generating system. The high open-circuit voltage

in the solar cell was needed to enhance the energy conversion efficiency, as well as to drive electronics requiring high voltages. In addition, a compact hybrid photovoltaic–phtothermal/pyro-electric–thermoelectric energy cell behaved in a way similar to a tandem solar cell system, which utilized the full solar spectrum for energy harvesting.

A conductive PEDOT film acted as the photothermal conver-sion layer and electrode of the pyroelectric generator. Additional treatment was performed on the PEDOT and P(VDF-TrFE), the combination of which was abbreviated as PP-PEDOT, in order to enhance the photothermal effect. The PP-PEDOT with dense film morphology had a high electrical conductivity, which resulted in higher photothermal conversion efficiency,[110] and the thickness of the top and bottom PP-PEDOT electrodes was optimized because the thickness of the electrodes affected heat generation. To transfer a heat efficiently, a pyroelectric generator was attached on the thermoelectric generator that had been coated with a thermal paste. Thus, almost all the remaining heat energy in the pyroelectric generator was transfer efficiently to the thermoelectric generator through the thermal paste. The pyroe-lectric generator converted near infrared (NIR) irradiation energy into electric energy immediately, but output performance was decreasing due to a decrease in the temperature differential (dT) over time (dt). The heat inside the hybrid generator dispersed to create thermal balance; therefore, the thermoelectric generator with the heat sink was able to convert dispersed heat energy into electric energy using the formed thermal gradient. Therefore, when the hybrid device was irradiated with sunlight, the DSSC harvested ultraviolet (UV) light and visible range light, while NIR light was transmitted to the pyroelectric and thermoelectric generators to convert thermal energy into electric energy. Over 50% of 700−1000 nm range of light was transmitted through the DSSC (filled area), and transmitted light in the 400–1600 nm range was absorbed by the section of thermoelectric and pyro-electric generators as shown in Figure 8b. Whether or not the DSSC absorbed the part of sunlight, the pyroelectric generator was not significantly affected, but overall energy conversion efficiency increased from 9.72% (with the DSSC only) to 11.7% (with hybridization) as a result of increased open-circuit voltage.

To manage the power of the hybrid generator, a simple cir-cuit, composed of capacitors and a bridge rectifier was pre-pared. The pyroelectric output performance appeared stable, regardless of whether the light was on or off after rectification (see Figure 8c). As shown in Figure 8d, the pyroelectric gen-erator successfully charged a capacitor steadily. The DSSC and the thermoelectric hybrid generator were combined and gener-ated energy using a light-driven electron flow and a thermal-driven electron flow. The photovoltaic output performance under sunlight steadily decreased due to the fast recombina-tion phenomenon, but the hybrid generator output voltage steadily increased until it had a higher voltage value than the photo voltaic cell alone, due to complementation by the ther-moelectric generator (see Figure 8e). The charging behavior of the capacitor also confirmed the enhanced hybrid output performance (see Figure 8f). The hybrid generator showed an enhanced and stable performance with the two generators complementing each other. This demonstrates that the hybrid generator effectively used the full solar irradiation spectrum with high energy conversion efficiency. The series of connected

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photovoltaic–thermoelectric generators showed 20% enhanced power performance, and pyroelectric output power was additional harvested energy. As demonstrated in this paper, irradiation by the sun with additional heat energy will enhance output, and thermal energy alone will also drive the hybrid generator.

3.4. Integration of Various Energy Harvesters

Zhang et al. reported a mechanical, thermal, and solar inte-grated hybrid nanogenerator[111] (see Figure 9). To utilize a multifunctional energy conversion material, PbZrxTi1-xO3

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Figure 8. a) A schematic image of the hybrid nanogenerator. b) The sunlight intensity and UV–vis–NIR transmittance spectra of the DSSC. c) Output voltage of the pyroelectric nanogenerator after rectification. d) A capacitor charging behavior using the pyroelectric nanogenerator. e) Output voltage of photovoltaic cell and series connected photovoltaic–thermoelectric nanogenerators device. f) Output charge and energy of a capacitor charged by photovoltaic cell and series connected photovoltaic–thermoelectric nanogenerators device. All panels reproduced with permission.[107] Copyright 2015, American Chemical Society.



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(PZT), they developed one structure-based, multifunctional hybrid nanogenerator. Though PZT was a well-known material for use in piezoelectric and pyroelectric nanogenerators,[112,113] it was noble structure that multienergy source harvesting structure using PZT for sustainable energy harvesting. By integrating pyroelectric, piezoelectric, and triboelectric nano-generators, and a photovoltaic cell into one device using iden-tical electrodes (see Figure 9a), the hybrid nanogenerators were able to simultaneously/individually harvest various envi-ronmental energies. The nanogenerator mainly consisted of three parts. PZT acted as the piezoelectric-, pyroelectric-, and photovoltaic-active layer. Polyamide not only generated tribo-electrification with fluorinated ethylene propylene (FEP) but applied strain to the PZT. Ag was uses as the bottom electrode of the PZT, and the top electrode consisted of ITO with an Ag nanowire (AgNWs)/PDMS composite layer, and a thermoelec-tric generator was attached beneath the hybrid nanogenerator.

Zhang et al. evaluated the individual energy harvesting sys-tems’ electrical properties to understand hybrid nanogenera-tors. The pyroelectric nanogenerator generated short circuit peak current, 480 nA, and peak voltage was over 100 V. At the optimal load resistance condition (0.2 GΩ), the pyroelectric nanogenerator achieved a maximum output power of 13 µW. The photovoltaic cell was measured using a full-spectrum solar simulator; output peak current was 890 nA, and output peak voltage was 60 V. However, due to the solar irradiation-induced heat effect, the pyroelectric output was coupled with the photovoltaic output performance. After excluding this pyroelectric effect, the output performance of photovoltaic cell was saturated at 170 nA and 48 V. At the optimal load resist-ance condition (0.1 GΩ), the photovoltaic cell achieved a max-imum output power of 7 µW. The piezoelectric–triboelectric coupled nanogenerator generated a short circuit peak current of 3.8 µA. At the optimal load resistance condition (8 kΩ), the piezoelectric–triboelectric coupled nanogenerator achieved a maximum output power of 44 nW at an air flow speed of about 15 m s−1. The performance of the hybrid nanogenerator with different combinations of individual energy harvesting systems was measured under the same environmental condi-tions. The pyroelectric and photovoltaic combination nanogen-erator (PyENG + PVC), the pyroelectric and piezo-triboelectric coupling nanogenerator (PyENG + TPiENG), the photovol-taic and piezotriboelectric coupling nanogenerator (PVC + TPiENG), and the pyroelectric, photovoltaic, and piezo-tribo-electric coupling nanogenerator (PyENG + PVC + TPiENG) output performances are shown in Figure 9b–j. Output peak current and voltage of the PyENG + PVC hybrid nanogen-erator, which generated higher current output than either the pyroelectric nanogenerator or photovoltaic cell alone, were 1 µA and 91 V, respectively (see Figure 9b,c). It is worth noting that thermal energy and solar energy were able to operate the hybrid nanogenerator simultaneously/individually. As for the PyENG + TPiENG nanogenerator, the PVC + TPiENG nanogenerator, and the PyENG + PVC + TPiENG nanogenerator, these three

hybrid nanogenerators showed alternative current output sim-ilar to that of the piezotriboelectric coupled nanogenerator with enhanced peak current output performance. As illustrated in Figure 9d–i, Zhang et al. recognized that the photovoltaic cell led to a 50 V voltage platform; the pyroelectric nanogenerator promoted the peak output voltage; and the piezo-triboelectric coupled nanogenerator promoted the peak output current. Thus, the hybrid nanogenerator generated peak current of 5 µA, platform voltage of 50 V, and peak voltage of 80 V. In addition, Figure 9j shows the capacitor-charging behavior of the three individual nanogenerators and the hybrid nanogen-erator. The hybrid nanogenerator showed a higher charging rate and output current than the other individual nanogenera-tors. Therefore, integration of the energy harvesting systems to utilize various energy sources for sustainable and enhanced power generation is a simple and effective method. Based on the hybrid energy harvesting systems, this one material-based nanogenerator is able to demonstrate small, simple, low-cost, and effective performance.

4. Perspective and Summary

The recent research results of mechanical, thermal, and solar energy hybrid generators are described in this progress report. Further development of hybrid nanogenerators requires the development of innovative techniques for creating positive feedback between energy harvesting systems for high power conversion efficiency. For example, the coupling effect between energy conversions is one of the promising candidates. Yang et al. utilized piezo-phototronic coupling effect to enhance the solar energy conversion efficiency[114] (see Figure 10a). When the negative piezoelectric potential in ZnO was generated, the conduction band level of ZnO lifted up, which resulted in decreased output open-circuit voltage by a piezo-phototronic effect. On the contrary, when the positive piezoelectric poten-tial in ZnO was generated, the conduction band level of ZnO lifted down, which resulted in an increased open-circuit voltage output by a piezo-phototronic effect. Similarly, the piezo-phototronic, ferroelectric potential in the P3HT also tuned the conduction and valance band, which resulted in modified open-circuit voltage.[115,116] Lee et al. utilized thermal expansion of the substrate to couple the piezoelectric and pyroelectric proper-ties[117] (see Figure 10b). Due to the differential thermal expan-sion coefficient between patterned substrate and patterned ferroelectric materials, the thermal expansion-induced strain on the ferroelectric material generated the piezoelectric and pyroelectric at the same time. Thus, it affected a spontaneous polarization change in the ferroelectric material, so a thermally induced strain coupling effect enhanced the pyroelectric output performance to approximately four to five times higher. Phan et al. reported the piezoelectric potential screening effect by UV light and its protecting method[118] (see Figure 10c). It is important to ensure stable output performance while the

Figure 9. a) A schematic image of the hybrid nanogenerator. b) Output current of the PyENG, PVC, and PyENG + PVC. c) Output voltage of the PyENG + PVC. d) Output current and e) voltage of the PyENG + TPiENG hybrid nanogenerator. f) Output current and g) voltage of the PVC + TPiENG hybrid nanogenerator. h) Output current and i) voltage of the PyENG + PVC + TPiENG hybrid nanogenerator. j) Accumulated charges curve by the nano-generators. All panels reproduced with permission.[111] Copyright 2016, John Wiley and Sons.



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piezoelectric nanogenerator is operated outdoors. Without pas-sivation on the ZnO, O2 molecules were possibly adsorbed by the ZnO, and it transformed into O2

− ions. When the ZnO was exposed to UV radiation, holes were generated that neutralized the O2

− ions, and the remaining electrons negatively affected the piezoelectric potential of the ZnO.

Passivation on the ZnO effec-tively reduced adsorption of the O2 molecules, which resulted in decreased dissociation of electron–hole pairs by sun irradiation. This ensured the stable piezoelectric output performance, which was 25 times higher than that of the ZnO device without passivation. For the efficient power management of the hybrid nanogenerators, addi-tional power management inte-grated circuit (PMIC) researches are essential because of different electric properties of harvesters such as impedance, frequency, polarity, and capacity. For example, thermoelectric generators and solar cells generate DC output, but piezoelectric, triboelectric, electro-magnetic, and pyroelectric gen-erators generate alternative current (AC) output. Because of this funda-mental difference, each harvester requires a different type of PMIC. Though recent hybrid generator papers have used individual PMICs for plural energy harvesters, such complex and inefficient systems decrease the power conversion efficiency of the whole system.

Figure 10. (a) Schematic band diagrams of P3HT/ZnO with negative and positive piezoelectric charges. The blue and red lines indicate the modified energy band by the piezoelectric potential. Repro-duced with permission.[108] Copyright 2011, American Chemical Society (b) Temperature changes of the device and correspond voltage output performances of the pyroelectric nanogenerator and the hybrid nanogenerator. Reproduced with permission.[111] Copyright 2015, John Wiley and Sons (c) Schematic images of O2 adsorption on the ZnO surface and screening effect with/without passivation under UV light illumination. Reproduced with permission.[112] Copyright 2012, Royal Society of Chemistry.



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Thus, without specialized PMICs, the hybrid energy harvesting system has inherent limitations on maximum energy conver-sion efficiency.

With the future development of PMICs for the hybrid energy harvesters, sustainable hybrid energy harvesting systems become one of the important renewable energy technologies in the future. However, there remain the following challenges. Energy storage of hybrid energy harvesters should ensure long cycle stability. If unstable energy storage cannot maintain perfor-mance, the life of WSN systems will be limited even the energy harvesters can supply the WSN systems with permanent energy. Specialized PMICs must be demanded for high efficient system. Because of ultralow power consumption systems, microscale energy management will significantly enhance the system effi-ciency. Finally, whether or not a single device can transform an energy harvesting system depends on the external environ-mental condition. For example, if a device mainly harvests solar energy during daytime, when the sun goes down, the device transforms from a solar cell into another generator to harvest mechanical or thermal energy. Transformable single energy har-vesters will offer high volume efficiency, 24 h sustainable energy harvesting, user convenient, ease of utilize at various places and simple power management. Thus, a transformable single energy harvester will become a sustainable energy harvester in the future, without additional components. A systematic analysis of these challenges can be resolved by theoretical and experimental approaches, and multidisciplinary research from material science, physics, and electronics will advance the sus-tainable hybrid energy harvesting system in near future.

AcknowledgementsThis work was supported by the Technology Innovation Programs (10052668, “Development of wearable self-powered energy source and low-power wireless communication system for a pacemaker” and 10065730, “Flexible power module and system development for wearable devices”) funded by the Ministry of Trade, Industry and Energy (MOTIE, Republic of Korea) and by the GRRC program of Gyeonggi province (GRRC Sungkyunkwan 2017-B05, Development of acoustic sensor based on piezoelectric nanomaterials).

Conflict of InterestThe authors declare no conflict of interest.

Keywordscoupling effects, hybrid energy harvesters, mechanical energy, solar energy, thermal energy

Received: May 6, 2018Revised: September 3, 2018

Published online: February 26, 2019

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