Electron blocking layer-based interfacial design for highly-enhanced triboelectric nanogenerators

Author’s Accepted Manuscript Electron Blocking Layer-based Interfacial Design for Highly-enhanced Triboelectric Nanogenerators Hyun-Woo Park, Nghia Dinh Huynh, Wook Kim, Choongyeop Lee, Youngsuk Nam, Sangmin Lee, Kwun-Bum Chung, Dukhyun Choi www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(18)30338-0 https://doi.org/10.1016/j.nanoen.2018.05.024 NANOEN2732

To appear in: Nano Energy Received date: 10 April 2018 Revised date: 10 May 2018 Accepted date: 10 May 2018 Cite this article as: Hyun-Woo Park, Nghia Dinh Huynh, Wook Kim, Choongyeop Lee, Youngsuk Nam, Sangmin Lee, Kwun-Bum Chung and Dukhyun Choi, Electron Blocking Layer-based Interfacial Design for Highlyenhanced Triboelectric Nanogenerators, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.05.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electron Blocking Layer-based Interfacial Design for Highly-enhanced Triboelectric Nanogenerators Hyun-Woo Parka1, Nghia Dinh Huynha,1, Wook Kima, Choongyeop Leea, Youngsuk Nama, Sangmin Leec, Kwun-Bum Chungb,* and Dukhyun Choia,* a

Department of Mechanical Engineering, Kyung Hee University, 1732 Deogyeong-daero,

Giheung-gu, Yongin-Si, Gyeonggi-do 446-701, South Korea. b

Department of Physics and Semiconductor Science, Dongguk University, Seoul, 100-715,

Korea c

School of Mechanical Engineering, Chung-Ang University, Seoul, 06974, Korea

[email protected] (D. Choi) [email protected] (K.-B. Chung) *Corresponding author.

ABSTRACT The key to enhance the output power from triboelectric nanogenerators (TENGs) is to control the surface charge density of tribo-materials. In this study, we introduce an electron blocking layer (EBL) between a negative tribo-material and an electrode to dramatically enhance the output power of TENGs. For the first time, we suggest that the tribo-potential can be significantly reduced by the presence of interfacial electrons; electrostatically induced positive charges at the interface beneath a negative tribo-material can be screened out by the electrons, thereby 1

These authors are Equally contributed.

decreasing the surface charge density. By employing an EBL between a negative tribo-material and an electrode, we can maintain a high surface charge density at the surface of the negative tribo-material. Furthermore, an EBL with high permittivity can enhance the polarization of the tribo-material, resulting in an improved surface charge density. As a proof of concept, polydimethylsiloxane (PDMS) and aluminum (Al) are used as a negative tribo-material and an electrode, respectively. A TiOx EBL is then deposited in between these materials by radio frequency (RF) sputtering. Due to the coupling effects of the electron blocking and enhanced polarization, the output peak power from the TENG with a TiOx EBL reaches approximately 2.5 mW at 3 Hz and 5 N, which is 25 times larger than that of a TENG without an EBL. To understand the improved behavior of the TENG with a TiOx EBL, we investigate the correlations between the output behavior of the TENG and the physical properties of the surface/interface of TiOx and PDMS (e.g., the surface potential, dielectric properties, and electronic structures). We expect that our results can provide a novel design way to significantly improve the output performance of TENGs.

Graphical abstract

Keywords: Electron Blocking Layer; Triboelectric Nanogenerators; Surface Charge Density; Polarization; Electronic Structure

1. Introduction Energy harvesting technologies utilizing solar, waves, wind, or mechanical energies have attracted considerable attention for self-powered small electronics with low power consumption, such as sensors, wearable devices, electronic skins, and body-implantable devices [1-3]. Among them, triboelectric nanogenerators (TENGs), which are operated by the coupling effects of triboelectrification and electrostatic induction, are emerging due to their low-cost, light-weight, high degree of freedom for material selection, high power, and high applicability [4-6]. Since invented in 2012 by Wang’s group [4], TENGs are operated by four basic working modes of TENGs: vertical contact-separation, in-plane contact-sliding, single-electrode, and freestanding triboelectric-layer [7]. Among the various working modes, the vertical contact-separation mode was first introduced and has become the primary mode because this mode can provide a long lifetime, low-cost manufacturing and stable/high output performance. Basically, TENGs consist of two dissimilar dielectric films and electrodes at the top and bottom of a device. Physical contact between the two dielectric films induces triboelectric charges that can generate a potential drop when it separates by mechanical force, which drives electron flow between the electrodes via the external circuit [8]. For a wide range of applications using sustainable power sources, the need for higher output performance still remains as one of the most important and critical issues. Since the first report, many researchers have demonstrated various methods for obtaining high-performance TENGs, such as modification of the relative permittivity of triboelectric materials through doping with

high permittivity nanoparticles (e.g., TiO2, BaTiO3, and SrTiO3 [9,10], changing the surface area via nano-patterning [11], pore formation [12], and treating the surface of triboelectric materials using corona discharge [13] or plasma treatment [14]. These methods can be used to obtain higher output performance, but there is still a limit to practical application due to problems related to the complexity, high cost, and high production time, low reproducible and temporary nature of the treatments. Recently, the surface and/or interface between the adjacent layer and energy harvesting materials have also been considered as key parameters; these can be modified and designed by simple, low cost, practical, highly reproducible and permanent treatments methods. Cui et al. showed the improvement of the output performance of TENGs by adjusting the depth distribution of the triboelectric charges in the frictional layer [15]. C. Wu et al. also demonstrated enhanced charge density in the frictional layer by adopting reduced graphene oxide (rGO) acting as electrontrapping sites [16]. However, those studies have only focused on the frictional layer to improve the surface charge density or the residence time of charge carriers in the frictional layer. Moreover, previous TENG studies did not elucidate the detailed physical mechanism analysis according to the interface states; there is a lack of parametric investigation regarding the output performance.

A few years ago, our group reported [17] that the piezoelectric power generation was enhanced by inserting the polymer interlayer due to the enhanced piezoelectric potential via free carrier passivation on the piezoelectric surface. Based on this report to block the loss of induced electric fields, we focused on the interface between the frictional layer and the bottom electrode in TENGs for enhancing the surface charge density of the frictional layer. Since electrostatically

induced positive charges at the interface beneath a frictional layer (i.e. a negative tribo-material) can be screened out by interfacial electrons from the bottom electrode, the surface charge density created by triboelectrification can be decreased, thereby reducing the output power. In this work, we report that the output performance of TENG can be significantly enhanced by employing an electron blocking layer (EBL). By employing an EBL between a negative tribo-material and an electrode, the initial high surface charge density at the surface of a negative tribo-material can be maintained. Furthermore, the polarization of the tribo-material can be improved by using an EBL with high permittivity, resulting in an enhanced surface charge density. As a demonstration, a TiOx layer was sputtered on an aluminum (Al) electrode and then polydimethylsiloxane (PDMS) was attached to the TiOx/Al substrate as a negative tribo-material. To clearly understand the significantly enhanced output power of TENGs with a TiOx EBL, we investigated the surface/interface characteristics of TiOx and PDMS, including the dielectric properties, surface potential, and electronic structure. We further examine the effects of the oxygen partial pressure (O2 p.p.) during sputtering and the thickness of the TiOx. It was shown that the TiOx EBL grown at an O2 p.p. of 0% with a thickness of 100 nm had an output power that was 25 times higher than the TENG without an EBL.

2. Experimental 2.1. Deposition process of an EBL: An aluminum plate with a 3x3 cm2 area was used as the bottom electrode for the creation of the vertical contact-separation mode of the TENG. The TiOx EBL (20 nm to 140 nm thick) was deposited by an RF sputtering system without substrate heating. An oxygen-deficient, three-inch

TiOx (dark grey color) was used, and the RF power and oxygen partial pressure were set to 150 W and changed from 0 to 2%, respectively, which were controlled by using Ar gas and O2 gas flow rate ratios of 20:0, 20:0.2, and 20:0.4 sccm. In order to further verify our concept, we used AlOx and HfOx, as an EBL by using the same process. 2.2. Fabrication process of the flat PDMS layer: A flat, 200-µm thick PDMS layer was fabricated by simple imprint lithography (SIL) technique using a doctor-blade and a Teflon template as a mold. The Teflon template was cleaned with acetone, ethanol, and deionized (DI) water, consecutively, and then dried by gently blowing it with dry nitrogen (N2) gas. The surface of the Teflon mold was then treated with heptadecafluoro-1, 1, 2, 2-tetrahydrodecyltrichlorosilane (HDFS) to make its surface hydrophobic. In this way, the Teflon template mold (with a hydrophobic surface) can be utilized to prevent the PDMS layer sticking to the mold. PDMS was prepared as a mixture of base resin and curing agent (Sylgard 184A: Sylgard 184B, Dow Corning Co.) with a weight ratio of 10:1. The PDMS mixture was then laminated to the Teflon mold by a doctor-blade technique and cured at 80 C for 4 hours. The 200-µm-thick, flat PDMS layer was then carefully peeled off the Teflon mold. 2.3. Fabrication and output measurement of the TENGs: To design the vertical contact-separation mode TENG device, the top electrode was prepared with a 3x3 cm2, 80-µm-thick commercial aluminum foil, which was cleaned by ethanol and dried in stream N2 gas. The aluminum foil was then attached to the polylactic acid (PLA) substrate with double-sided foam tape. Subsequently, to prepare the TENG bottom electrode, a flat PDMS film (~200-µm-thick, 3x3 cm2) was carefully laminated onto the aluminum plate (for the

reference control sample), and onto the TiOx-deposited aluminum (0.5-mm-thick) without any air gaps in between the layers. The electrode was then cured at 100 oC for 8 h, and the dried bottom electrode was then attached to the PLA substrate with double-sided foam tape. The surface morphology of the TiOx-deposited aluminum plate and the flat PDMS film was examined using field emission scanning electron microscopy (FE-SEM; LEO SUPRA 55, Carl Zeiss). To measure the TENG output, a force was applied using a pushing tester (JIPT-100, Junil Tech). The TENG output voltage was measured using a Tektronix MDO3052 mixed-domain oscilloscope with an input impedance of 40 MΩ. Current measurements were captured using a Stanford Research Systems SR570 low-noise current preamplifier connected to the Tektronix MDO3052 mixed domain oscilloscope. The TENG output was measured at a working frequency of 3 Hz and a force of 5 N, the gap in between the two electrodes was 10 mm. 2.4. Electronic structure and surface potential measurements: Changes in the electronic structures, such as the conduction band features and oxygen vacancies related to the band edge states below the conduction band, were investigated using X-ray absorption spectroscopy (XAS). XAS experiments were performed at the 2A beamline in the Pohang Accelerator Laboratory (PAL), Korea. In addition, the triboelectric charges of PDMS, according to the TiOx EBL, were quantitatively studied using Kelvin probe force microscopy (KPFM; NX10, Park Systems Corp.) and contact potential difference (CPD) measurements.

3. Result and Discussion The output power of TENGs is dominated by the surface charge density formed on the surface of tribo-materials. Here, for the first time, we suggest that TENGs might produce a

reduced surface charge density due to interfacial electrons between a negative tribo-material and an electrode. An example can be discussed by using a simple model of a contact-type TENG consisting of Al-PDMS/Al, where PDMS and a bottom Al layer were used as a negative tribomaterial and a bottom electrode, respectively (see Figure. 1). As shown in Figure. 1a-(i), the negative surface charges (σ) created by triboelectrification induce positive charges beneath the negative tribo-material. Due to the abundance of free electrons at the interface, the positive charges can be screened out, thereby causing a decrease in the surface charge density on the surface of the negative tribo-material. However, such screening troubles can be prevented by employing an electron blocking layer (EBL) between the negative tribo-material and the bottom electrode, as shown in the right schematic of Figure. 1a-(i). Since an EBL has much fewer carriers than Al, the screening problem can be overcome. Thus, the surface charge density in a TENG with an EBL can be maintained, producing an enhanced output power. As mentioned in the introduction, the surface charge density can be improved by using high-permittivity particles in the negative tribo-materials, to form a composite, due to the enhanced polarization. Normally, EBLs are oxide materials (e.g., aluminum oxide (Al 2O3), titanium oxide (TiO2), and hafnium oxide (HfO2)), and their permittivity is very high. Therefore, an EBL with a high permittivity can further enhance the output power by improving the polarization of tribo-materials, as shown in Figure. 1a-(ii). Finally, we expect that the coupling effects of electron blocking and enhanced polarization can be accomplished by employing an EBL with high permittivity to significantly enhance the output performance of TENGs. In order to prove our concept, we employed a TiOx EBL between PDMS and an Al electrode; the relative permittivity (εr) of this EBL was carefully examined. First, we analyzed the changes of the surface morphology caused by the TiOx deposition. As seen in Figure. S1

(Supporting Information), the FE-SEM images showed no significant changes in the surface morphology after the deposition of a 100-nm-thick TiOx film on the Al substrate. Additionally, to measure the εr values of the TiOx EBL and PDMS, capacitance-voltage (C-V) analysis was performed on the metal-insulator-metal (M-I-M) structure, as shown in Figure. S2 (Supporting Information). The capacitance value of the TiOx film with a thickness of 100 nm was measured by using a top electrode with an area of 0.049 mm2. The εr values of the TiOx film and PDMS were calculated by from the C-V curves according to the following equation: (1)

Here, dfilm is film thickness, Cmax is maximum capacitance, A is an area of the top electrode, and ε0 is permittivity of free space. As shown in the equation, since the capacitance values are inversely proportional to the thickness of the film, PDMS with a thickness of 200 μm has a very small capacitance because of the thick thickness. To overcome this problem, the top electrode on the PDMS was increased in size to 28.27 mm2. The εr values of the TiOx film and PDMS were ~21 and ~4.7, respectively, which are reasonable compared to a previous report [18,19]. Finally, it was confirmed that we can successfully prepare a TiOx EBL with a high εr. To demonstrate the concept of the coupling effects for electron blocking and enhanced polarization, we compared the surface charge density of TENGs with and without a TiOx EBL. As shown in Figure. 1b, a normal TENG without an EBL showed a surface charge density of about 15 µC/m2 at a frequency of 3 Hz and a pushing force of 5 N. Surprisingly, we found that the surface charge density of a TENG with an EBL reached 30 µC/m2, which is 2.5 times higher than the value demonstrated by just a TiOx layer. To theoretically understand the output power enhancement of the TENG with a TiOx EBL, a COMSOL Multiphysics simulation was

conducted. Figure. 1c shows the corresponding results for the triboelectric potential distributions of TENGs with and without a TiOx EBL. The details of the COMSOL Multiphysics simulation parameters and equations are provided in the Supporting Information. As a result, the COMSOL simulations showed that the TENG with the TiOx EBL has a triboelectric potential distribution that is 2.5 times higher than the TENG without a TiOx EBL. Finally, we compared the experimental results of the output voltage and current. A TENG without an EBL produced a peak output voltage of ~50 V and a peak output current of 2 µA. As expected, a TENG with a TiOx EBL showed dramatically enhanced output power, where the output voltage and current were about 272 V and 9.1 µA, respectively. In order to clearly demonstrate the effect of the EBL on other TENG systems, we performed additional tests on TENGs with and without AlOx HfOx, and TiOx EBL. As shown in Figure. S3 (Supporting Information), the output performance of all TENGs with an EBL were significantly enhanced compared to TENGs without an EBL. We also found that the output performances of TENGs are strongly related to the dielectric constant of each EBL material. This result verifies that our concept of employing an EBL is not limited to TiOx but can be used with other materials that have a low electron density and a high relative permittivity. To demonstrate the present approach of enhancing the output performance of TENGs by applying a TiOx EBL, the amount of triboelectric charge on the PDMS surface with and without a TiOx EBL is investigated by contact potential difference (CPD) analysis, before and after contact with the top Al electrode. The CPD analysis method served to measure the different contact potentials induced on the material surface before and after a contact and separation cycle. Before friction with the top Al electrode, the surface potential values of the PDMS without and with the TiOx EBL were 815 mV (Figure 2a) and 395 mV (Figure 2c), respectively. The most

plausible origin of this remarkable reduction in the surface potential of the PDMS with the TiOx EBL can be attributed to the enhanced polarization of PDMS caused by the TiOx EBL, canceling out the electrons present on the PDMS surface. In other words, a low surface potential value before friction with the top Al electrode can indicate that PDMS polarization is enhanced. After friction with the top Al electrode, the surface potential value of the PDMS without the TiOx EBL was 954 mV (Figure. 2b), which was 130 mV greater than it was before friction. However, in the case of PDMS with the TiOx EBL, the surface potential value of 1035 mV (Figure. 2d) was significantly increased by 640 mV (compared to what it was before friction with the top Al electrode). The significant change in the surface potential with the TiOx EBL, as determined by the CPD method analysis, is reasonable considering the previous output performance and charge density of the TENG with the TiOx EBL. Based on the above results, the TiOx EBL can maintain a high surface charge density at the surface of PDMS due to the coupling effect of electron blocking and enhanced polarization. More discussion is provided below by considering the correlation between the output performance of the TENG and the electronic structures (e.g., the conduction band features and oxygen vacancy-related band edge state below the conduction band of the TiOx EBL) according to the oxygen partial pressure and EBL thickness. Figure. 3a and 3b show the output voltage and current characteristics of the TENG with a TiOx EBL as a function of the oxygen partial pressure during the deposition process of the EBL (measured under a relative humidity of ~38%). All TENGs with TiOx EBLs increased the output voltage and current compared to the TENG without the TiOx EBL, regardless of the oxygen partial pressure. In addition, no significant changes in the surface properties, due to changes in the oxygen partial pressure, were observed in the SEM images, as shown in Figure. S4

(Supporting Information). Interestingly, as the oxygen partial pressure increased during the deposition process of the TiOx EBL, the output performances of the TENG decreased, as shown in Figure. 3a and Figure. 3b. In order to elucidate the cause of this trend, the correlation between the electronic structure of the TiOx EBL and the energy conversion mechanism of the TENG with the TiOx EBL were investigated. Figure. 3c shows the hybridized molecular orbital structures in the conduction band of the TiOx EBL as a function of the oxygen partial pressure measured by XAS. The normalized intensities of the oxygen K-edge spectra of TiOx EBLs directly reflect the molecular orbital hybridization between the Ti orbital (3d and 4sp states) and O orbital (2p states) based on the local atomic bonding symmetry [20]. The oxygen K-edge spectra could be deconvoluted into six distinct Gaussian absorption peaks, especially for the second derivative of the oxygen K-edge spectra, which is used to determine the energy level of the molecular orbital states, as shown in Figure. 3c (blue line). The qualitative energetic orders of the corresponding hybridized molecular orbital between Ti 3d, 4sp and O 2p are characterized as 2t2g (Ti 3d; O 2pπ) < 3eg (Ti 3d; O 2pσ) < 1a1g (Ti 4s; O 2pσ) < 3t1u (Ti 4p; O 2p) [21]. As the oxygen partial pressure increased from 0% to 2%, the relative area of the conduction band decreased. This extended conduction band area can enhance charge transport due to the increase in the unoccupied absorption states caused by adding the hybridized states. Figure. 3d shows another meaningful result regarding the analysis of the band edge state below the conduction band. As the oxygen partial pressure decreased from 2% to 0%, the oxygen vacancy-related band edge states were dramatically reduced. This trend can be clearly seen based on the results of the relative area of the band edge state in Figure. 3e. These changes in the amount of oxygen vacancy-related band edge states can affect the electron blocking properties of the TiOx EBL;

this is the case because the degree of negative charge trapping, including electrons and oxygen ions, can change (Figure. 3f). As a result, the increase of the band edge states is correlated with the improvement of the electron blocking properties of the TiOx EBL. Figure. 4a and 4b show the output voltage and current characteristics of the TENG with a TiOx EBL as a function of the EBL thickness (measured under a relative humidity of ~65%). In particular, the TiOx EBL according to the thickness was deposited at 0% oxygen partial pressure. In addition, the TiOx EBL deposited on the Al substrate showed different colors for different TiOx EBL thicknesses, as observed in the optical images shown in Figure. S5 (Supporting Information). This general phenomenon is due to changes of the interference effect caused by the different film thicknesses. As the EBL thickness increased from 20 nm to 100 nm, the output voltage and current significantly increased from 128 V and 3.5 μA to 272 V and 9.1 μA, respectively. However, at TiOx EBL thicknesses over 100 nm, the output voltage and current are drastically decreased to 162 V and 3.88 μA, respectively. To investigate how the physical structure of the TiOx film changes at different thicknesses, XRD analysis was performed, as shown in Figure. S6 (Supporting Information). The XRD spectra was measured by a θ–2θ X-ray diffractometer and normalized by Al (200) from an Al substrate. The XRD results indicate that the thickness of the TiOx film has no effect on the physical structure; an amorphous structure was preserved. When the thickness of the EBL increases, the electron blocking properties can be improved due to the increase in the relative amount of oxygen vacancies. However, considering the total capacitance of capacitors connected in series, the thickness of the TiOx EBL is inversely proportional to the total capacitance:

Here, Ctot is the total capacitance, εr,TiOx and εr,PDMS are the relative permittivity of each material, and dTiOx and dPDMS are the thicknesses of each layer. Therefore, the change in the thickness of the TiOx EBL has a trade-off relationship between the electron blocking properties and the total capacitance. Furthermore, to understand the change in the TENG output performance according to the EBL thickness in terms of the electronic structure, hybridized molecular orbital structure analysis was also performed, as shown in Figure. 4c. The relative area of eg/t2g+eg and 4sp/3d+4sp molecular orbitals were calculated based on the relative area under each of the corresponding Gaussian curves (as shown in fig 3(c)). The ratio of the eg state and 4sp state of the conduction band changes as the thickness of the EBL is altered. The increase in the number of eg states indicates enhanced molecular orbital symmetry of z2 and x2−y2 in the 3d orbital, which induces the uniform distribution of d-orbital states. This change in d-orbital ordering can enhance the charge transport due to the symmetric distribution of unoccupied states in the five-fold directional 3d orbital [22,23]. The other increase in the relative ratio of the 4sp orbital can also cause an increase in charge transport due to the enhancement of the spherically symmetrical s-orbital compared to the directional d-orbital [24]. Considering the relative ratio of eg and the 4sp state based on the XAS results, when the thickness of the TiOx EBL is 100 nm, optimized charge transportation is expected (Figure. 4d). This superior charge transportation with a 100-nm-thick TiOx EBL can lead to easily move of electron within the TiOx EBL. Even if with the same amount of oxygen vacancies in the TiOx EBL, it is possible to reduce the trapping time (τT) of electrons by the oxygen vacancies due to the easy transportation of the electrons and increase the trap velocity (νT), which can improve the electron blocking properties of the TiOx EBL.

4. Conclusions In summary, for the first time, we demonstrated the critical effects of an EBL on TENG performance. Due to the coupling effect of the electron blocking and enhanced polarization, a TiOx EBL with a high permittivity enhanced the TENG output power by a factor of 25 (272 V and 9.1 µA at 3 Hz and 5 N). To understand the behaviors of the TENG performance when using an EBL, we controlled the oxygen partial pressure and the thickness of the TiOx EBL during growth. Finally, the TiOx layer grown at an O2 p.p. of 0% with a thickness of 100 nm provided the best performance. Based on electronic structure studies, it was determined that the oxygen vacancies and the charge transport properties of the TiOx EBL were critical parameters for enhancing the electron blocking characteristics, producing significantly enhanced output powers in the TENGs. Thus, employing an EBL in TENGs may provide a promising way to create higher output powers from TENGs through a highly reproducible and cost-effective method.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MSIP) (No.2016R1A4A1012950), (No.2014M3A7B4052202), (No. 2017R1D1A1B03032375), and (No. 2017R1A6A3A11029892).

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Figure 1. Coupling effects of the EBL and high permittivity to dramatically enhance the output power of TENGs. (a) Schematic illustration of the working principle of TENGs and the role of the multifunctional TiOx EBL, (b) charge density, (c) COMSOL Multiphysics simulation results, and (d) output voltage and current of the TENGs with and without the TiOx EBL.

Figure 2. The different surface potentials of PDMS: (a) without and (b) with a TiOx EBL. Measurements were made before and after friction with the top Al electrode.

Figure 3. Effects of the output power of TENGs caused by the oxygen partial pressure. (a) Output voltage and (b) current of the TENGs with a TiOx EBL as a function of the oxygen partial pressure during the deposition process of the EBL (measured under a relative humidity of ~38%). (c) Deconvoluted oxygen K-edge XAS spectra, (d) enlargement of the XAS spectra for band edge states below the conduction band, and (e) relative area of oxygen vacancy-related band edge state of the TiOx EBL as a function of the oxygen partial pressure. (f) Schematic illustration of the electron trap mechanism according to the oxygen vacancies.

Figure 4. Effects on the output power of TENGs caused by varying the TiOx thickness. (a) Output voltage and (b) current of the TENGs with a TiOx EBL as a function of the TiOx EBL thickness (measured under a relative humidity of ~65%). (c) Normalized oxygen K-edge XAS spectra and (d) the relative ratio of eg/(eg + t2g) states and the relative molecular orbital ratio of 4sp/(3d+4sp) as a function of the TiOx EBL thickness.

-Highlights 

We first report the critical effects of an electron blocking layer (EBL) between a negative tribo-material and a bottom electrode to significantly enhance TENG performance.



A high surface charge density at the surface of a negative tribo-material by electron blocking property of an EBL



An EBL with high permittivity can enhance the polarization of the tribo-material



An improved surface charge density and contributing to the enhanced output power.