- Open Access
Application of ferroelectric materials for improving output power of energy harvesters
© The Author(s) 2018
- Received: 10 September 2018
- Accepted: 14 October 2018
- Published: 2 November 2018
In terms of advances in technology, especially electronic devices for human use, there are needs for miniaturization, low power, and flexibility. However, there are problems that can be caused by these changes in terms of battery life and size. In order to compensate for these problems, research on energy harvesting using environmental energy (mechanical energy, thermal energy, solar energy etc.) has attracted attention. Ferroelectric materials which have switchable dipole moment are promising for energy harvesting fields because of its special properties such as strong dipole moment, piezoelectricity, pyroelectricity. The strong dipole moment in ferroelectric materials can increase internal potential and output power of energy harvesters. In this review, we will provide an overview of the recent research on various energy harvesting fields using ferroelectrics. A brief introduction to energy harvesting and the properties of the ferroelectric material are described, and applications to energy harvesters to improve output power are described as well.
- Energy harvesting
- Triboelectric effect
- Photovoltaic effect
Two of the most important trends in recent electronic technology have been the size reduction and functional improvement of mobile electronic devices. Mobile electronic devices are small, portable, and contain a variety of information that is instantly accessible at all times, including the ability to share and communicate information wirelessly. These devices are becoming even smaller and lighter so that they can be wearable or attachable to objects that can be used daily, such as a watch, glasses, or clothes. All devices that are based on microelectronic technology require a lot of external power supply due to their increased functions, and batteries are the most important power source for mobile electronic devices. However, batteries take up increasingly significant parts of the overall device volume and weight as the electronic devices are miniaturized. Moreover, battery technology is limited in energy capacity per volume for supplying sufficient energy to a mobile electronic device.
Therefore, many studies have been focused on reducing power consumption and designing energy efficient devices to reduce the sizes but extend the lifetimes of the batteries. Nevertheless, the batteries must be replaced or recharged after being discharged, and this is an obstacle to realizing always-on wearable electronic devices. In order to make up for this problem, we need to develop an energy harvesting system that can harvest and reuse energy sources from the ambient environment. Energy harvesters convert various environmental energy sources such as mechanical stress, vibration, light, and heat, etc. to electrical energy. Each energy source can be converted to electrical energy by each coupled-physical phenomenon such as piezoelectric, triboelectric photovoltaic, and thermoelectric (or pyroelectric) effects. The amount of output energy obtained from piezoelectric effect is ~ 5.92 μW/cm2 , triboelectric effect is ~ 0.7 mW/cm2 , photovoltaic effect is ~ 22.1 mW/cm2 , and pyroelectric effect is 1.4 μW/cm2 . The working principle of each energy harvester is different, but basically, electric current is generated by internal polarization or potential. Therefore, increasing the polarization density is important for improving output power of energy harvester. Conventional materials have limitation in increasing internal polarization because of low polarization density. Moreover hardness of the conventional materials hinders application to wearable devices. However introducing novel materials with strong and permanent polarization, ferroelectric materials, can overcome these limitations. In this review article, ferroelectric materials in energy harvesters are addressed. Ferroelectric materials have permanent dipole moments once electric field is applied, so polarization density can be increased through the insertion of ferroelectric materials. First, we will briefly describe the types of ferroelectric materials as well as the basic theory of energy harvesting technologies. Then, recent applications of ferroelectric materials in energy harvesting devices are discussed.
The ferroelectricity can be tested by measuring polarization as a function of electric field. Ferroelectric materials have spontaneous polarization, and this varies with external electric field, so in a polarization versus electric field curve, a hysteresis loop is shown (Fig. 1b). However, the ferroelectricity is shown only after the phase transition below a certain temperature, called Curie temperature (TC). Above the Curie temperature, ferroelectricity disappears and paraelectricity is shown.
2.1 Perovskite ferroelectric materials
These materials have perovskite structures, like BaTiO3, whose general chemical formula is ABO3, where A and B atoms are cations. Normally the A cation has radius of 1.2–1.6 Å and B cation has one of 0.6–0.7 Å. A atoms are positioned at the cube corner and oxygen atoms are positioned at the face center and form an octahedron surrounding the B atom which is positioned at the body center, as graphically illustrated in Fig. 1c . Under electric field, the position of B cation shifts, then the geometrically unbalanced electrical charge forms a dipole moment.
2.2 Ilmenite ferroelectric materials
The ilmenite ferroelectric materials have the same chemical formula as perovskite materials, ABO3, e.g. LiNbO3 and LiTaO3. However, the A cation is too small to fill the position of the perovskite crystal coordinate [6, 7]. Oxygen atoms comprise hexagonal close-packed layer, and A and B atoms are positioned at the octahedron site between layers (Fig. 1d) .
2.3 Polymeric ferroelectric materials
The first discovered and the most representative polymeric ferroelectric material is polyvinylidene fluoride (PVDF) [8–10]. Polymers have long carbon backbone, so their structure is complex and has a lot of configurations depending on whether the neighboring carbon bonds are trans or gauche. Among the configurations of PVDF described in Fig. 1e , the β-phase has all trans configuration. The fluorine atoms have the strongest electronegativity, resulting C–F polar bond so that PVDF molecule has dipole moment in perpendicular direction to its carbon chain. However, the dipole moments of the pristine polymer chains are not arranged in single direction, so the net polarization is zero. Therefore, a strong electric field is required to arrange the dipole moments of chains, which is called electrical poling. In addition, copolymer with trifluoroethylene (10–46%) helps the formation of β-phase.
Energy harvesting utilizes various energy sources, including mechanical, thermal, and solar energies. Each energy source can be converted to electrical energy through each coupled-physical phenomenon, but basic principle is same: the variation of the internal dipole moment or potential. Therefore, the introduction of ferroelectric materials to energy harvesters can increase dipole moments and potential in the devices due to the strong polarization in the ferroelectric materials so that conversion efficiency can be enhanced.
3.1 Piezoelectric energy harvesting
3.1.1 Piezoelectric effect
Due to the coupling effect of mechanical strain and electric charge separation in piezoelectric effect, the piezoelectric effect has been exploited to convert mechanical energy. Energy harvesting using the piezoelectric effect was first introduced by Wang’s group using piezoelectric semiconducting ZnO nanowires in 2006 . Since then, a lot of research on piezoelectric energy harvesters, called piezoelectric nanogenerators (PENG), has been reported. Many researchers have attempted to enhance the output performance of PENG by designing new devices. Among the various factors to increase output performance, the development of a material with a high piezoelectric coefficient is the most important.
Ferroelectric materials have piezoelectricity as well, and their piezoelectric coefficient is relatively high (d33 of PMN-PT ferroelectric ceramic is 630 pC/N ). Initially, dipole moments in ferroelectric material are randomly aligned so it has neither polarization nor piezoelectricity. However, once strong electric field is applied, dipole moments are aligned in a single direction and piezoelectricity is formed. Therefore, PENGs made of ferroelectric materials have been reported.
3.1.2 Thin film perovskite PENG
After fabrication, BTO film was poled with an electric field of 100 kV/cm for 15 h at 140 °C in order to align ferroelectric polarization and enhance the piezoelectric output. The piezoelectric coefficient (d33) was characterized using piezoresponse force microscopy (PFM) and compared the value prior to and after poling. By the poling process with external electric field, ferroelectric polarization becomes stronger. As shown in Fig. 2b, before poling the measured d33 is 40 pm/V but after poling it increased up to 105 pm/V which fits with the previous report (d33 = 30–100 pm/V [24, 25]). Besides, hysteresis loop of piezo response in the inset clearly shows ferroelectric behavior of BTO after poling.
The PENG with BTO thin film on flexible substrate is driven by compressive force and bending. Then, tensile stress is applied to BTO film. The deformation of the BTO film by tensile stress generates piezoelectric polarization and induces charge induction in electrode resulting in electrical current (Fig. 2c). Figure 2d shows the output current (~ 10 nA) and voltage (0.3 V) of flexible BTO PENG with 1350 MIM structure arrays by periodic bending and unbending. The BTO based PENG shows the possibility of ferroelectric ceramic material for flexible and high output energy harvesters through thin film deposition and the transferring technique.
3.1.3 Ferroelectric-polymer composite PENG
In order to further enhance output power, other ferroelectric materials with higher piezoelectric coefficients such as PZT and PMN-PT were used for energy harvesters [1, 18, 21, 22]. The high-power PENGs with thin film ferroelectric materials were integrated on flexible plastic substrate and utilized for bio-implantable devices [19, 21]. Although the output power of PENGs successfully increased up to an open-circuit voltage of 200 V and short-circuit current density of 150 μA/cm2 , ferroelectric materials are deposited as thin film, resulting in the limitation of output power and fabricating large area devices. Moreover, rigid ferroelectric thin film cannot be used under strong force.
Well-mixed ZnSnO3:PDMS composite was fabricated into PENG and the output power was measured by applying vertical compressive force using vehicle, as described in Fig. 3b. The open-circuit voltage of 20 V and short-circuit current density of 1 μA/cm2 were measured. Detailed working mechanism is described in Fig. 3c. At low strain, most of the strain occurs in PDMS matrix so the actual strain in nanocubes are small. As a result, randomly aligned minor piezoelectric potential is induced. However, at a high strain, enough compressive force is applied to nanocubes, so piezoelectric polarization is generated and aligned in a single direction due to stress-induced poling effect [27–30]. Therefore, this result shows high performance of ferroelectric-polymer composite PENG and its promising application in large area and under high pressure.
3.1.4 Polymeric ferroelectric based PENG
The ferroelectric powder-embedded polymer composite shows high output power and mechanical stability but is not acceptable for low magnitude and frequency input force. Poly(vinylidene fluoride) (PVDF) is one of the representative ferroelectric polymers. Its copolymer poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) has a high piezoelectric coefficient of d31 = 25 pC/N, d33 = 40 pC/N  and its flexibility is promising for application in fully flexible, foldable, twistable, and stretchable PENG . Previously reported PENGs comprised of plastic substrate and metal electrode have limitation in flexibility and stretchability. However, the semi-metallic two-dimensional carbon material, graphene, is a promising electrode for flexible electrode due to its high mechanical durability and elasticity (1 TPa) .
Furthermore, the output voltage was investigated with wind flow (Fig. 4c). With wind speed of 0.5–3 m/s, peak to peak output voltage of P(VDF-TrFE) PENG increased from 0.3 V to 3.9 V while PENG on PEN substrate increased from 0.1 V to 0.5 V. Detailed analyses of single peaks are compared in Fig. 4c(ii). When the continuous wind flows, P(VDF-TrFE) PENG on PDMS substrate flutters because of its flexibility and sensitivity to low magnitude strain so continuous output signals are observed. However, PENG on PEN substrate shows signal only when wind is turned on and off instantly as depicted in Fig. 4c(iii) and (iv). In conclusion, ferroelectric polymer shows very promising result as a high output piezoelectric energy harvester.
3.2 Triboelectric energy harvesting
3.2.1 Triboelectric effect
Triboelectric effect is the charge exchange between two materials through contact or rubbing each other. Although the detailed mechanism of triboelectric effect remains elusive, there are four possibilities: electron transfer, ion transfer, material transfer, and mechano-chemistry . Triboelectric charging occurs by complex of these four phenomena. Numerical prediction of triboelectric charging is not possible yet because there are too many factors to determine triboelectric effect, but triboelectric charge polarity is predictable using triboelectric series [34, 35].
Static charges by triboelectric effect have been considered as disturbance to human health and especially industry because the charges have an effect on electric devices. Therefore, there have been efforts to prevent the triboelectric effect. However, prof. Zhong Lin Wang’s group invented a new type of energy harvester called a triboelectric nanogenerator (TENG) which exploits the triboelectric effect in 2012 . TENG extend energy harvesting field more widely due to its simple structure, light weight, and high output power.
As can be seen in the equation, triboelectric charge density is the most important factor when designing TENG. The triboelectric charge density is determined only by surface property of material. Previously, many researchers have tried to increase the number of fluorine atoms by using Teflon film , plasma treatment  or self-assembled monolayer (SAM)  to enhance output power. The ferroelectric materials have spontaneous polarization so they can enhance the output power of TENG. There have been TENGs supported by ferroelectric polarization.
3.2.2 Controllable charge transfer by ferroelectric polarization
In TENG, there is charge transfer between two materials. Generally, the amount and polarity of charge is determined by material properties, especially work function. However, work function is hardly modulated, so controlling triboelectric effect of existing material is very limited. Introduction of ferroelectric material can control and increase triboelectric charging behavior due to its switchable and controllable polarization.
The triboelectric behavior affected by ferroelectric polarization was also investigated in an energy harvesting device (TENG). Following the fabrication of TENGs with P(VDF-TrFE) film, each devices was positively or negatively polarized as shown in Fig. 5c, and periodic force was applied to TENGs in order to obtain triboelectric output voltage. The P(VDF-TrFE) films with different applied bias voltage have different direction of polarization so output voltage directions different as well. Moreover, both positively and negatively polarized P(VDF-TrFE) film shows high output voltage than bare P(VDF-TrFE) film because of well-oriented polarization. Conventionally triboelectric property is fixed and determined by tribo-series, but this result shows it can be modified in ferroelectric materials.
3.2.3 Ultrahigh triboelectric charge density in TENG by ferroelectric layer
In order to increase the output power of TENG, researchers have tried post treatments such as ionized-air injection , self-assembled monolayer . It was found that surface charge density of 240 μC/m2 can be obtained by ionized-air injection but long-term stability is not secured. Ferroelectric materials have permanent polarization, so it is expected that the output performance can be enhanced through the use of ferroelectric materials with long-term stability.
The application of TENG with BT layer in electronic devices exhibited in Fig. 6c. The supercapacitor is charged by closing K1 and opening K2 in an equivalent circuit, described in Fig. 6c(i). The charged voltage in supercapacitor is 21.49 mV for a charging time of 10 s in atmosphere, but the voltage increases up to 80.36 mV in high vacuum conditions, as shown in Fig. 6c(ii). The high output TENG in high vacuum shows rising voltage of supercapacitor even while the watch and humidity-temperature meter are working (Fig. 6c(iii)). On the other hand, when the TENG works in atmosphere, the supercapacitor hardly charged (Fig. 6c(iv)) or the charged voltage decreased (Fig. 6c(vi)). The high output TENG also shows the ability to light 32 light-emitting diodes (LEDs) when working in high vacuum (Fig. 6c(vii)), but only two LEDs were lit in atmosphere (Fig. 6c(viii) and (ix)). The introduction of ferroelectric layer in TENG shows the enhanced charge density beyond the limit of conventional materials, particularly in high vacuum where breakdown voltage increases.
3.3 Pyroelectric energy harvesting
3.3.1 Pyroelectric effect
For energy harvesting from thermal energy, the thermoelectric effect has been used [46–48]. However, it requires a spatial gradient in temperature, so it is not applicable when the temperature of material varies . Therefore, when there is time-dependent variation of temperature, pyroelectric energy generator (PEG) can be used for energy harvesting [4, 50–55].
3.3.2 Pyroelectric energy harvesting from hot/cold water
Heat energy is one of the most prevalent types of wasted energy, and there have been many researches to harvest the heat energy. Conventionally, thermoelectric technology has been exploited to convert heat energy to electric energy, but temperature gradient should be maintained for thermoelectric effect and conversion efficiency is still low . Therefore, pyroelectric energy generator (PEG) was invented , but output power of inorganic pyroelectric material-based PEGs is still low (voltage of 22 V, and current of 170 nA) due to the low pyroelectric coefficient of PZT (− 80 nC/cm2K) . However, polymeric ferroelectric material, PVDF is promising material for PEG because of its high pyroelectric coefficient (200 μC/cm2K) , and its flexibility enables it to be applied for flexible and stretchable PEG .
3.3.3 Highly stretchable piezoelectric-pyroelectric coupled energy harvester
Recently, the electronic devices are required to be flexible and stretchable as well for application of wearable electronics [57–61]. As mentioned before, ferroelectric polymer P(VDF-TrFE) has a lot of advantageous features for such an application. Especially stretchability is one of the most unique properties of P(VDF-TrFE) among the ferroelectric materials. Besides, dual properties of pyroelectricity and piezoelectricity in P(VDF-TrFE) can realize high output energy harvester by hybridization. The stretchable hybrid energy harvester is successfully realized through the introduction of micro-line pattern structure and combining piezoelectric and pyroelectric effect . However, the piezoelectric and pyroelectric effect output was produced by each independent energy source.
3.4 Photovoltaic energy harvesting
3.4.1 Ferroelectric effect in photovoltaic cell
Solar energy is one of the most abundant energy sources in the earth, and photovoltaic cells using solar energy are currently the most prevalent energy harvesting technology. In order to increase the output power of photovoltaic cells, controlling electronic properties like energy band structure or junction is essential [65, 66]. Ferroelectric materials have switchable spontaneous polarization once electric field is applied. When the ferroelectric layer is introduced in photovoltaic cell, the polarization induces internal electric field resulting in aid separation of excited carriers. Therefore, there have been reports about ferroelectric-inserted photovoltaic devices. Moreover, it is found that recently developed photovoltaic material, organic halide perovskite has ferroelectric polarization.
3.4.2 Ferroelectric coupling on energy-band structure
3.4.3 Ferroelectric behavior in halide perovskite solar cell
Recently, one of the most promising materials for photovoltaic cells is halide perovskite [84, 85]. The power conversion efficiency (PCE) of perovskite photovoltaic cell that fabricated with two-step sequential deposition and vapor evaporation method achieved 15% in 2013 [86, 87]. After that, much higher PCEs of perovskite photovoltaic cell have been reported [3, 88]. The halide perovskite material, CH3NH3PBI3 (MAPbI3) has unique properties such as ambipolar self-doping property , high permittivity , I–V hysteresis , and slow dynamics . It is expected that MAPbI3 has ferroelectricity , but this is still ambiguous as of now.
The unique properties of the ferroelectric effect, particularly its spontaneous, switchable, and permanent polarization have attracted many researchers to develop many application devices, and energy harvesting technologies have exploited the unique properties of ferroelectric material. Energy harvesters convert various energy sources to electrical energy. Ferroelectric polarization can have an important role to increase output power of energy harvesters by enhancing internal potential. Strong ferroelectric polarization produces high piezoelectric potential and surface potential. For mechanical stability and robustness in PENG and TENG, the oxide ferroelectric materials were deposited in thin film or imbedded in polymer matrix. Using oxide ferroelectric powder-polymer composite and ferroelectric polymer P(VDF-TrFE), very highly stable PENG and stretchable PENG were developed. In the case of TENG, controlling the surface potential is crucial. The ferroelectric polarization modified and attracted more charges, resulting in higher output power. With high output TENG, it was demonstrated that electronic devices such as smart watch and humidity-temperature meter can be driven and charged simultaneously. In addition, ferroelectric polymer P(VDF-TrFE) which has pyroelectricity was micro-patterned in order to develop stretchable PEG, and hybrid pyroelectric effect and piezoelectric effect for high output. Finally, it was found that the energy level at junction in photovoltaic cell can be adjusted to increase VOC and JSC. Recently, ferroelectric polarization was discovered in MAPbI3 which is considered as a promising photovoltaic material and studied.
Nowadays, the importance of energy harvesting technologies has become larger due to the prevalence of the mobile electronics and the fact that their power consumption is very high. However, present energy storage technology cannot cover the power consumption needs, so the output power of energy harvester must be improved. There are several factors to determine output power of energy harvesters, but the development of a proper material is a key factor. The introduction of ferroelectric material will give way for improvement in designing material system in energy harvester and realize alternative powering system in near future.
TYK, SKK, and SWK wrote manuscript. All authors designed figure sets and analyzed literatures. SWK supervised the overall conception. All authors read and approved the final manuscript.
Tae Yun Kim: Dr. Tae Yun Kim is postdoctoral researcher in the School of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). He received his PhD degree under the supervision of Prof. Sang-Woo Kim in SKKU. His research interests include characterization and simulation of nano-materials for electronic devices such as sensors and energy harvesters.
Sung Kyun Kim: Dr. Sungkyun Kim works as a postdoctoral researcher with Prof. Sohini Kar-Narayan at University of Cambridge, United Kingdom. He received his PhD degree under the supervision of Prof. Sang-Woo Kim in SKKU in 2017. His research interests include atomic force microscopy studies of piezoelectric, triboelectric and ferroelectric materials and characterization of 2-D materials and polymer based energy harvester.
Sang-Woo Kim: Dr. Sang-Woo Kim is a Professor in the Department of Advanced Materials Science and Engineering at Sungkyunkwan University (SKKU). His recent research interest is focused on piezoelectric/triboelectric nanogenerators, photovoltaics, 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. He is currently serving as an Associate Editor of Nano Energy (Elsevier) and an Executive Board Member of Advanced Electronic Materials (Wiley).
The authors acknowledge financial supports from the Industrial Strategic Technology Development Program (10052668, Development of wearable self-powered energy source and low-power wireless communication system for a pacemaker), the Technology Innovation Program (10065730, Flexible power module and system development for wearable devices), and “Human Resources Program in Energy Technology (20154030200870)” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea).
The authors declare that they have no competing interests.
Availability of data and materials
The review is based on the published data and sources of data upon which conclusions have been drawn can be found in the reference list.
Funding was provided by Industrial Strategic Technology Development Program (Grant No. 10052668), Technology Innovation Program (Grant No. 10065730), Korea Institute of Energy Technology Evaluation and Planning (Grant No. 20154030200870)
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