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Phosphorus-doped zinc oxide p–n homojunction thin film for flexible piezoelectric nanogenerators
Author’s Accepted Manuscript `Phosphorus-doped Zinc Oxide p-n Homojunction Thin Film for Flexible Piezoelectric Nanogenerators Yang Hyeog Kwon, Doo-Hee Kim, Han-Ki Kim, Junghyo Nah www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(15)00388-2 http://dx.doi.org/10.1016/j.nanoen.2015.10.009 NANOEN984
To appear in: Nano Energy Received date: 3 August 2015 Revised date: 13 October 2015 Accepted date: 13 October 2015 Cite this article as: Yang Hyeog Kwon, Doo-Hee Kim, Han-Ki Kim and Junghyo Nah, `Phosphorus-doped Zinc Oxide p-n Homojunction Thin Film for Flexible Piezoelectric Nanogenerators, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.10.009 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.
`Phosphorus-doped Zinc Oxide p-n Homojunction Thin Film for Flexible Piezoelectric Nanogenerators
Yang Hyeog Kwona†, Doo-Hee Kimb†, Han-Ki Kimb,*, Junghyo Naha,*
a
Department of Electrical Engineering, Chungnam National University, Daejeon, 34134, South Korea b
Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do, 26035, South Korea *Corresponding authors: [email protected], [email protected]
KEYWORDS: Piezoelectric nanogenerator; ZnO pn homojunction; flexible electronics
Abstract The performance of zinc oxide (ZnO)-based piezoelectric nanogenerators (PENGs) has been largely limited by piezoelectric potential screening effect due to excess electrons in ZnO. To address this problem, we report here a method that can greatly enhance the performance of ZnO PENGs by reducing excess electrons. We formed ZnO p-n homojunction thin film, composed of unintentionally doped n-ZnO and phosphorus-doped pZnO, on the ITO/Ag/ITO(IAI) coated flexible substrate with a low sheet resistance of 3.03 Ohm/square and a high optical transmittance of 88.16%, where were prepared by roll-to-roll sputtering. The fabricated PENG with a p-n homojunction demonstrates the output power up to ~140 μW, which is approximately two orders of magnitude higher than that of the PENG only with ZnO. Besides, the output performance related with the p-ZnO and n-ZnO thickness ratio and the roles of p-n junction formation were also systematically investigated. The results here introduce a new method to further extend performance limit of ZnO-based PENGs.
1
Introduction Harvesting energy from independent energy resources has been of interest due to its potential application in electronic devices [ 1]. Piezoelectric nanogenerator (PENG) in particular can generate electricity from various physical movements existing in human motions and environment [2,3]. Zinc oxide (ZnO) is one of the well-known piezoelectric materials [4,5], which has wurtzite crystal structure with a direct wide energy band gap (3.37eV). Due to its non-toxicity and relatively low cost, ZnO have been widely studied as a piezoelectric energy harvesting material [6,7,8]. However, ZnO PENG’s output power is relatively low in comparison to other PENGs using different piezoelectric materials such as BaTiO3[9,10], PZT [11], due to its low piezoelectric coefficient [12,13]. Large electron concentration exhibited in ZnO due to the n-type unintentional doping by oxygen vacancies or zinc interstitials [14,15]. These electrons can screen piezoelectric potential generated by mechanical deformation or applied stress, resulting in low piezoelectric output power generation. To address this problem, several methods have been introduced to date: p-type doping [16,17,18], p-n junction formation [19,20], and surface treatment [21, 22] have been applied to reduce excess electron concentration in ZnO in order to boost piezoelectric potential. In particular, reliable excess electron concentration reduction has been successfully demonstrated by forming a p-n junction with p-type materials and ZnO, enhancing the piezoelectric output performance. Various p-type materials including P3HT [23, 24], ZnS [25], NiO [26], PEDOT:PSS [27], and p-Si [28] have been adapted to form a p-n heterojunction. However, relatively few attempts have been made to form ZnO p-n homojunction due to absence of high quality p-ZnO film [29]. The homojunction formation is more suited for PENGs due to its chemical stability, mechanical durability, and improved piezoelectric power generation. 2
In this work, we fabricated the PENGs with unintentionally doped n-ZnO and phosphorus-doped p-ZnO:P [30,31] homojunction and demonstrated related performance enhancement of the PENGs. Besides, the maximum performance has been achieved by optimizing thickness ratio between ZnO and p-ZnO:P layers. The ZnO p-n homojunction PENG demonstrated the output voltage and current up to ~24 V and ~6 μA, respectively, at the applied force of 0.5 MPa. This is approximately two orders of magnitude higher output power in comparison to the output power of the PENGs only with a ZnO layer.
Experimental Method Transparent ITO/Ag/ITO multilayer coating on PET substrate. Flexible ITO/Ag/ITO (IAI) multilayer electrodes were sputtered on a flexible PET substrate with a thickness of 125 μm by using lab-scale roll-to-roll (RTR) sputtering at room temperature. By using an unwinding and rewinding system, the flexible PET substrate continuously passed over the rectangular ITO and Ag targets. In addition, tension of the flexible PET substrate was controlled using a load cell in the rolling system. Prior to the RTR sputtering, the surface of the PET substrate was pre-treated at a constant pulsed DC power of 550 W by irradiation of Ar+ ions to improve the surface morphology of the PET substrate and the adhesion with the bottom ITO film. After ion-beam pretreatment of the PET substrate, a bottom ITO layer was sputtered at a constant Ar/O2 flow ratio of 30/2 sccm, a DC power of 550 W, and a working pressure of 0.5 mTorr using a rectangular ITO target with geometry of 200 100 mm2 placed below the rotating cooling drum at a distance of 100 mm. After sputtering of bottom ITO layer, the 12 nm thick Ag layer was also RTR sputtered on the bottom ITO layer at a constant Ar flow ratio of 30 sccm, a DC power of 380W, and a working 3
pressure of 1 mTorr using a rectangular Ag metal target. Finally, a top ITO layer was sputtered onto the Ag interlayer under deposition conditions identical to those used to sputter the bottom ITO layer. ZnO layer deposition The ZnO and p-type ZnO:P films were deposited on flexible PET substrate using RF magnetron sputtering system with multi-cathode guns at room temperature. Unintentionally doped ZnO layer was first sputtered on the IAI multilayer electrode at a constant RF power of 100 W applied to a 3 inch ZnO target, a Ar/O2 flow rate of 20/2 sccm, and working pressure of 9 mTorr. During the RF magnetron sputtering of the ZnO layers, the PET substrate was constantly rotated at a speed of 20 rpm to ensure uniform thickness of the ZnO layers. The pZnO:P layer was then sputtered on the unintentionally doped n-ZnO layer using 1 wt% P2O5 doped ZnO ceramic target without breaking a vacuum as a function of thickness. The pZnO:P layer was grown under identical RF power, Ar/O2 flow rate and working pressure to the n-ZnO layer. Figure S1 shows continuous n-ZnO and p-ZnO:P sputtering process using tilted multi-cathode guns. Piezoelectric nanogenerator fabrication process Polydimethylsiloxane (PDMS) is spin-coated on a pre-cleaned ITO-coated PET substrate at 3500 rpm for 30 s. The PDMS spin-coated film is then soft-cured in oven at 85 ºC for 5 min. Subsequently, soft-cured PDMS covered substrate is carefully attached on the ZnO deposited IAI substrate to avoid forming air bubbles in PDMS layer between the two substrates. Lastly, the attached device is placed in vacuum chamber to remove air bubbles in PDMS layer, followed by curing in oven at 85 ºC for 3 hr.
4
Device characterization The output performances of our PENGs were measured using SR 570 (Stanford Research Systems) low noise current amplifier and Lecroy Waverunner LT 354. The electrical characteristics of diodes were measured using Agilent 4145B semiconductor analyzer. The compressive force was periodically applied on the PENGs using the pushing machine. The crystal structure of the ZnO layers was examined by synchrotron X-ray scattering (XRS) at the GI-WAXS beam line of a Pohang Light Source (PLS) and by high-resolution transmission electron microscopy (HRTEM: JEM-2100F). The wavelength of the incident Xrays was set to 1.243 Å by a double bounce Si (111) monochromator.
Results and discussion The ZnO p-n homojunction PENG fabrication process is described in Fig. 1(a). The PET film is first cleaned with acetone, IPA, and deionized (DI) water, followed by bottom electrode layer deposition using RF magnetron sputtering. The bottom electrode layer on PET substrate is composed of ITO/Ag/ITO (IAI) layers. The IAI-coated PET substrate can provide good electrical conductivity and reliability under periodically applied stress in comparison to ITOcoated PET substrate. The n-ZnO and p-ZnO:P layers were then subsequently deposited on the IAI substrate. Next, polydimethylsiloxane (PDMS) is spin-coated on the ITO-coated PET substrate, which is subsequently attached on the active ZnO layers. Here, the role of PDMS layer is two-fold. First, it works as an adhesive layer, bonding the top electrode layer and the ZnO layer on the bottom electrode layer. More importantly, it works as an insulation layer in the PENG, preventing carrier transport between the electrodes through active ZnO layers. Thus, the generated piezoelectric potential can be used to drive electrons toward an external 5
circuit, accumulating the electrons at the interface region between the top electrode and PDMS until the piezoelectric potential balances with these electrons.
Finally, the sample is
cured in the oven to firmly bond the top electrode layer and the ZnO layer. The detailed device fabrication process is described in the experimental method section. Fig 1(b) is a scanning electron micrograph (SEM) of device’s cross section shows the ZnO p-n junction layers deposited by RF magnetron sputtering. The fabricated device shows the good flexibility and transparency as shown in Fig. 1(c). In detail, the transmittance of the active layer was over 67% in visible wavelengths range after the ZnO layers are deposited on the IAI substrate [Fig. S2]. This indicates that the device can potentially be integrated with photonic devices or photoelectric energy harvesting devices. After deposition of active ZnO layers, the X-ray diffraction (XRD) patterns were investigated to confirm the crystal structures of the n-ZnO and p-ZnO layers [Figure 2(a)]. The 2θ peaks can be observed at (002) and (102) in all the ZnO films. We note that the relatively higher (002) peak was obtained in the ZnO films deposited on the IAI layer, indicating that IAI substrate is a good platform to fabricate PENGs. Using the n-ZnO and pZnO layers, the p-n diode was fabricated and electrically characterized to confirm the formation of the p-n junction. Obvious rectifying behavior can be observed in the p-n homojunction diode [Fig. 2(b)]. Specifically, approximately two orders of magnitude higher forward current than reverse current was obtained in the ZnO p-n homojunction diode. The diode only with a ZnO thin film, on the other hand, shows similar forward and reverse currents of ~ 2 μA under ±2V biases. The results here clearly indicate the formation of ZnO p-n homojunction. Besides, the ideality factor was determined by fitting the data in the forward bias region, using the diode equation, reverse bias saturation current,
is the electron charge, 6
[
(
)
], where
is a
is applied voltage across of the
diode,
is ideality factor,
is Boltzmann’s constant,
is absolute temperature. The
ideality factor of the ZnO p-n homojunction diode is ~ 4, which is a typical value observed in ZnO p-n diode prepared by sputter deposition and indicate that the carrier transport is mainly dominated by carrier recombination in the depletion region. We note that p-type conduction of p-ZnO layer was also confirmed by fabricating back-gated p-ZnO thin film transistor [Fig. S3]. The working principles of the PENGs with ZnO and ZnO p-n homojunction are described in Fig. 3(a) and (b), respectively. When the compressive force is applied on the top of the ZnO PENG, this separates the centers of positive and negative charges, generating electric dipole moment. After cancellation of interior charges of opposite signs, the charges remain at both surfaces, responsible for the polarization. Positive piezoelectric potential (V+) is set up at the ZnO/PDMS interface, and a negative piezoelectric potential (V-) is induced at the IAI/ZnO interface, deforming the energy band from the initial condition (dashed line) as denoted by solid line in Fig. 3(a). This piezoelectric potential drives the electrons toward the top ITO electrode through an external circuit from the bottom IAI electrode, accumulating them at the top electrode until they are balanced by the piezoelectric potential. As the applied force is removed, the induced piezoelectric potential instantly disappears and the accumulated electrons flow back to the bottom electrode. However, in ZnO PENG excess electrons in the ZnO layer significantly screen the positive dipoles, reducing the generated piezoelectric potential. In the p-ZnO and n-ZnO homojunction PENG, overall working mechanism is similar to that of ZnO PENG. However, different from the ZnO PENG, p-ZnO and n-ZnO layer form a depletion region between the two layers after recombination of electrons and holes in the layers. This can effectively reduce excess electron concentration in ZnO and enhance piezoelectric potential. Therefore, when the compressive force is applied on the top 7
of the ZnO p-n homojunction PENG, it can further deform energy band near the interfaces by comparison to ZnO PENG [Fig. 3(b)]. Furthermore, overall structure of ZnO p-n homojunction PENG can be modeled as three capacitors connected in series when compressed: the one is at IAI/n-ZnO interface and the others are at n-ZnO/p-ZnO junction and p-ZnO/PDMS interface. Thus, the total capacitance of n-ZnO/p-ZnO PENG is expected to be less than that of ZnO only PENG with the same thickness, where the depletion region is formed only near the interfaces. Consequently, output voltage increase is expected in the ZnO p-n homojunction PENG due to the enhanced piezoelectric potential coupled with the reduced total capacitance. For experimental verification, two different PENGs were fabricated, where the one device has only a ZnO layer and the other has a ZnO p-n junction. Fig. 3(c) and (d) show the output voltages and currents of these two PENGs, illustrated in Fig. 3 (a), (b). Here, the output voltage and current were measured with a 1 MΩ load resistor between the two electrodes. The two PENGs have the same total layer thickness of 400 nm, where the ZnO p-n homojunction PENG is composed of a 200 nm thick p-ZnO layer and a 200 nm thick n-ZnO layer. As demonstrated in Fig. 3(c) and (d), much higher output voltage and current were obtained in the ZnO p-n homojunction PENG, which is at least 6-fold higher voltage and two-fold higher current than the values obtained in the PENG only with ZnO, resulting in an order of magnitude higher output power. These results are well matched with the predictions in Fig. 3(a) and (b). Besides, to demonstrate how the reduced electron concentration affects on piezoelectric output in the PENG, we also fabricate the p-ZnO PENG with the same layer thickness [Fig. S4(a), (b)]. Although the output voltage and current of the p-ZnO PENG is lower than those of the ZnO p-n homojunction PENG, it is still higher than those of ZnO PENG due to reduction in excess electron concentration by recombination with holes. Inversely positioned p-n junction in the device structure can also significantly affect the output performance. The ZnO n-p homojunction PENG, composed of 8
the p-ZnO on the bottom and the n-ZnO on the top, exhibits much lower output voltage and current by comparison to the PENG with a p-n junction [Fig. S5]. In the n-p junction PENG, the positive piezoelectric potential induced at the n-ZnO/PDMS interface easily vanishes since electron concentration increases as the energy band bends downward with the compressive force. The negative piezoelectric potential at the p-ZnO/IAI interface is also diminished due to the increasing hole concentration with the applied compressive force. Thus, generated output voltage and current are similar to or even lower than those of ZnO PENG even after recombination of excess electrons with holes in p-ZnO. To further investigate the output performance related with n-ZnO and p-ZnO thickness ratios, the n-ZnO thickness was kept at 200 nm and only p-ZnO thickness was varied from 200 nm to 500 nm [Fig. 4(a), (b)]. The results show that the maximum output voltage and current can be obtained when a 300 nm-thick p-ZnO is deposited on the n-ZnO layer. Presumably, donated hole concentration from a 200 nm-thick p-ZnO layer was not enough to sufficiently remove excess electrons in n-ZnO by recombination. Reduced output voltage in the PENG with a 500 nm-thick p-ZnO can be possibly attributed to increased layer stiffness. This indicates that there exists optimal thickness ratio between p-ZnO and n-ZnO to maximize the output power. Fig. 4(c) summarizes the output power as a function of p-ZnO layer thickness. We found that the output power reaches ~140 μW at p-ZnO : n-ZnO = 300 nm : 200 nm. Similarly, the output power was also investigated as a function of n-ZnO thickness while the p-ZnO thickness was kept at a 100 nm [Fig. S6]. The output power of the PENG gradually decreases with the increase of the n-ZnO thickness, which is then saturated at a 400 nm. Considering the optimal thickness ratio between p-ZnO and n-ZnO above, the maximum output power is expected to occur at the n-ZnO thickness less than a 100 nm. Next, the stability of the PENG was investigated by cyclic measurement, using the PENG with a 9
200 nm-thick n-ZnO and a 200 nm-thick p-ZnO [Fig. 4(d)]. Consistent output voltage and current were measured over 5000 cycles without signal degradation. We note here that the PDMS capping layer and IAI substrate adopted in the PENG contributed to enhance stability and durability of the device. Fig. 4(e)-left shows a schematic circuit diagram consisting of single PENG, rectifying diode bridge, and a blue light emitting diode (LED). When the PENG is compressed periodically, it can instantly light up a blue LED as shown in Fig. 4(e)right.
Conclusions In this report, we have examined ZnO p-n homojunction to enhance the performance of PENGs. We formed a ZnO p-n homojunction thin film, composed of unintentionally doped n-ZnO and phosphorus-doped p-ZnO to alleviate the excess electron screening effect. The ZnO p-n junction PENG demonstrated the output power up to ~140 μW, which is approximately two orders of magnitude higher compared to ZnO PENG, resulted from the reduction in screening effect and total capacitance. In addition, the output performance related with n-ZnO and p-ZnO thickness ratio and the roles of p-n junction formation were also systematically investigated. The work here introduces a new method that can be served as a guidance to further extend performance limit of ZnO-based piezoelectric nanogenerators.
Acknowledgments This research was supported by Basic Science Research Program through the National Research
Foundation
of
Korea
(NRF-2015R1A1A1A05027235)
2015R1A2A2A01002415).
10
and
(NRF
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Vitae
Yang Hyeog Kwon received his B.S degree in Department of Electrical Engineering from Chungnam National University, Korea in 2015. He is currently Master course student supervised by Professor Junghyo Nah. His research topic is piezoelectric and triboelectric nanogenerators and self-powered energy storage devices.
Doo-Hee Kim received his B.S degree in Department of Advanced Materials Science Engineering for Information and Electronics from Kyung Hee University, Korea in 2015. He is currently Master course student supervised by Professor Han-Ki Kim. His current research topic is growth of p-type and n-type oxide thin films for flexible and transparent nanogenerators.
Han-Ki Kim received his Ph.D. in Materials Science and Engineering from Gwangju Institute of 13
Science and Technology in 2003. From 2003 to 2005, he worked in Samsung SDI as a principle researcher and developed TCO materials for AMOLEDs. Kim was an assistant professor at Kumoh National Institute of Technology from 2005 to 2008. Since 2009, he is a professor in Department of Advanced Materials Science and Engineering for Information and Electronics and Kyung Hee Fellow at Kyung Hee University, Yongin, Korea. His research interests include transparent electrode materials, flexible and transparent electronics and printable electronics. He has published over 205 articles and holds 100 patents.
Junghyo Nah received Ph.D. in Electrical and Computer Engineering from the University of Texas at Austin in 2010. He worked as postdoctoral scholar at University of California at Berkeley form 2011 to 2012. He joined the faculty of the Department of Electrical Engineering, Chungnam National University, Daejeon, Korea, in 2012. His research interests include piezoelectric and triboelectric energy harvesting devices, flexible electronics, and high performance transistors using low dimensional semiconductor nanomaterials.
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Figure caption
Figure 1. (a) Schematic representations of the piezoelectric nanogenerator fabrication process: IAI roll-to-roll sputter deposition on a PET substrate, ZnO p-n junction formation by RF magnetron sputtering, and bonding the active ZnO layer with an ITO-deposited topside electrode. (b) The scanning electron micrograph of cross-sectional view of the p-n homojunction layer. Scale bar: 200 nm (c) The fabricated ZnO p-n homojunction PENG, demonstrating transparency and flexibility.
15
Figure 2. (a) XRD patterns of the sputter-deposited layers by RF magnetron sputtering. Enhanced peak along the n-ZnO (002) was observed. (b) Current-voltage (I-V) characteristics of the n-ZnO and ZnO p-n homojunction diode at bias voltage ±2V. The p-n junction diode shows greatly enhanced rectifying behavior.
16
Figure 3. Energy band diagram and schematic representations of the output power generation mechanism of (a) n-ZnO PENG and (b) ZnO p-n junction PENG. When the force is applied, the positive and negative piezoelectric potentials are formed near the surfaces, deforming the energy band as a consequence. The negative piezoelectric potential pushes electrons toward the topside ITO electrode, accumulating them on the electrode until they are balanced by the piezoelectric potential. For the PENG with a ZnO p-n homojunction, overall generation mechanism is the same as n-ZnO PENG. However, reduction in excess electron screening effect and total capacitance further enhances the output performance of the PENG. (c), (d) Output voltage and current of the PENG, consisting of n-ZnO (left) or ZnO p-n homojunction (right), having the same total layer thickness of 400 nm. Significantly improved output performance can be observed in the PENG with ZnO p-n homojunction. The constant compressive force (0.5MPa) normal to the surface was applied during the measurement.
17
Figure 4. (a) Output voltage and (b) current of the PENGs with different p-ZnO layer thicknesses. The n-ZnO film thickness was fixed at 200 nm. (c) Output power of the ZnO p-n homojunction PENGs with different n-ZnO and p-ZnO thickness ratios. (d) The PENG, consisting of 200 nm-thick n-ZnO and a 200 nm-thick p-ZnO, exhibits stable output voltage and current under compressive force (0.5MPa) over 5000 cycles. (e) (Left) schematics of the circuit diagram consisting of a single PENG, rectifying diode bridge, and a blue light emitting diode (Right) LED is instantly turned on when compressive force is applied on the PENG.
18
Graphical abstract
highlights High performance piezoelectric nanogenerator was demonstrated by adopting ZnO p-n homojunction. Forming a p-n junction with phosphorus-doped ZnO on the ZnO layer successfully reduced excess electrons in ZnO. The p-n homojunction formation can effectively reduce the total capacitance under applied force, resulting in enhanced output power generation. The ITO/Ag/ITO substrate provides good conductivity, transparency, flexibility, and reliability under periodically applied stress.
19
PII: DOI: Reference:
S2211-2855(15)00388-2 http://dx.doi.org/10.1016/j.nanoen.2015.10.009 NANOEN984
To appear in: Nano Energy Received date: 3 August 2015 Revised date: 13 October 2015 Accepted date: 13 October 2015 Cite this article as: Yang Hyeog Kwon, Doo-Hee Kim, Han-Ki Kim and Junghyo Nah, `Phosphorus-doped Zinc Oxide p-n Homojunction Thin Film for Flexible Piezoelectric Nanogenerators, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.10.009 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.
`Phosphorus-doped Zinc Oxide p-n Homojunction Thin Film for Flexible Piezoelectric Nanogenerators
Yang Hyeog Kwona†, Doo-Hee Kimb†, Han-Ki Kimb,*, Junghyo Naha,*
a
Department of Electrical Engineering, Chungnam National University, Daejeon, 34134, South Korea b
Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin-si, Gyeonggi-do, 26035, South Korea *Corresponding authors: [email protected], [email protected]
KEYWORDS: Piezoelectric nanogenerator; ZnO pn homojunction; flexible electronics
Abstract The performance of zinc oxide (ZnO)-based piezoelectric nanogenerators (PENGs) has been largely limited by piezoelectric potential screening effect due to excess electrons in ZnO. To address this problem, we report here a method that can greatly enhance the performance of ZnO PENGs by reducing excess electrons. We formed ZnO p-n homojunction thin film, composed of unintentionally doped n-ZnO and phosphorus-doped pZnO, on the ITO/Ag/ITO(IAI) coated flexible substrate with a low sheet resistance of 3.03 Ohm/square and a high optical transmittance of 88.16%, where were prepared by roll-to-roll sputtering. The fabricated PENG with a p-n homojunction demonstrates the output power up to ~140 μW, which is approximately two orders of magnitude higher than that of the PENG only with ZnO. Besides, the output performance related with the p-ZnO and n-ZnO thickness ratio and the roles of p-n junction formation were also systematically investigated. The results here introduce a new method to further extend performance limit of ZnO-based PENGs.
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Introduction Harvesting energy from independent energy resources has been of interest due to its potential application in electronic devices [ 1]. Piezoelectric nanogenerator (PENG) in particular can generate electricity from various physical movements existing in human motions and environment [2,3]. Zinc oxide (ZnO) is one of the well-known piezoelectric materials [4,5], which has wurtzite crystal structure with a direct wide energy band gap (3.37eV). Due to its non-toxicity and relatively low cost, ZnO have been widely studied as a piezoelectric energy harvesting material [6,7,8]. However, ZnO PENG’s output power is relatively low in comparison to other PENGs using different piezoelectric materials such as BaTiO3[9,10], PZT [11], due to its low piezoelectric coefficient [12,13]. Large electron concentration exhibited in ZnO due to the n-type unintentional doping by oxygen vacancies or zinc interstitials [14,15]. These electrons can screen piezoelectric potential generated by mechanical deformation or applied stress, resulting in low piezoelectric output power generation. To address this problem, several methods have been introduced to date: p-type doping [16,17,18], p-n junction formation [19,20], and surface treatment [21, 22] have been applied to reduce excess electron concentration in ZnO in order to boost piezoelectric potential. In particular, reliable excess electron concentration reduction has been successfully demonstrated by forming a p-n junction with p-type materials and ZnO, enhancing the piezoelectric output performance. Various p-type materials including P3HT [23, 24], ZnS [25], NiO [26], PEDOT:PSS [27], and p-Si [28] have been adapted to form a p-n heterojunction. However, relatively few attempts have been made to form ZnO p-n homojunction due to absence of high quality p-ZnO film [29]. The homojunction formation is more suited for PENGs due to its chemical stability, mechanical durability, and improved piezoelectric power generation. 2
In this work, we fabricated the PENGs with unintentionally doped n-ZnO and phosphorus-doped p-ZnO:P [30,31] homojunction and demonstrated related performance enhancement of the PENGs. Besides, the maximum performance has been achieved by optimizing thickness ratio between ZnO and p-ZnO:P layers. The ZnO p-n homojunction PENG demonstrated the output voltage and current up to ~24 V and ~6 μA, respectively, at the applied force of 0.5 MPa. This is approximately two orders of magnitude higher output power in comparison to the output power of the PENGs only with a ZnO layer.
Experimental Method Transparent ITO/Ag/ITO multilayer coating on PET substrate. Flexible ITO/Ag/ITO (IAI) multilayer electrodes were sputtered on a flexible PET substrate with a thickness of 125 μm by using lab-scale roll-to-roll (RTR) sputtering at room temperature. By using an unwinding and rewinding system, the flexible PET substrate continuously passed over the rectangular ITO and Ag targets. In addition, tension of the flexible PET substrate was controlled using a load cell in the rolling system. Prior to the RTR sputtering, the surface of the PET substrate was pre-treated at a constant pulsed DC power of 550 W by irradiation of Ar+ ions to improve the surface morphology of the PET substrate and the adhesion with the bottom ITO film. After ion-beam pretreatment of the PET substrate, a bottom ITO layer was sputtered at a constant Ar/O2 flow ratio of 30/2 sccm, a DC power of 550 W, and a working pressure of 0.5 mTorr using a rectangular ITO target with geometry of 200 100 mm2 placed below the rotating cooling drum at a distance of 100 mm. After sputtering of bottom ITO layer, the 12 nm thick Ag layer was also RTR sputtered on the bottom ITO layer at a constant Ar flow ratio of 30 sccm, a DC power of 380W, and a working 3
pressure of 1 mTorr using a rectangular Ag metal target. Finally, a top ITO layer was sputtered onto the Ag interlayer under deposition conditions identical to those used to sputter the bottom ITO layer. ZnO layer deposition The ZnO and p-type ZnO:P films were deposited on flexible PET substrate using RF magnetron sputtering system with multi-cathode guns at room temperature. Unintentionally doped ZnO layer was first sputtered on the IAI multilayer electrode at a constant RF power of 100 W applied to a 3 inch ZnO target, a Ar/O2 flow rate of 20/2 sccm, and working pressure of 9 mTorr. During the RF magnetron sputtering of the ZnO layers, the PET substrate was constantly rotated at a speed of 20 rpm to ensure uniform thickness of the ZnO layers. The pZnO:P layer was then sputtered on the unintentionally doped n-ZnO layer using 1 wt% P2O5 doped ZnO ceramic target without breaking a vacuum as a function of thickness. The pZnO:P layer was grown under identical RF power, Ar/O2 flow rate and working pressure to the n-ZnO layer. Figure S1 shows continuous n-ZnO and p-ZnO:P sputtering process using tilted multi-cathode guns. Piezoelectric nanogenerator fabrication process Polydimethylsiloxane (PDMS) is spin-coated on a pre-cleaned ITO-coated PET substrate at 3500 rpm for 30 s. The PDMS spin-coated film is then soft-cured in oven at 85 ºC for 5 min. Subsequently, soft-cured PDMS covered substrate is carefully attached on the ZnO deposited IAI substrate to avoid forming air bubbles in PDMS layer between the two substrates. Lastly, the attached device is placed in vacuum chamber to remove air bubbles in PDMS layer, followed by curing in oven at 85 ºC for 3 hr.
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Device characterization The output performances of our PENGs were measured using SR 570 (Stanford Research Systems) low noise current amplifier and Lecroy Waverunner LT 354. The electrical characteristics of diodes were measured using Agilent 4145B semiconductor analyzer. The compressive force was periodically applied on the PENGs using the pushing machine. The crystal structure of the ZnO layers was examined by synchrotron X-ray scattering (XRS) at the GI-WAXS beam line of a Pohang Light Source (PLS) and by high-resolution transmission electron microscopy (HRTEM: JEM-2100F). The wavelength of the incident Xrays was set to 1.243 Å by a double bounce Si (111) monochromator.
Results and discussion The ZnO p-n homojunction PENG fabrication process is described in Fig. 1(a). The PET film is first cleaned with acetone, IPA, and deionized (DI) water, followed by bottom electrode layer deposition using RF magnetron sputtering. The bottom electrode layer on PET substrate is composed of ITO/Ag/ITO (IAI) layers. The IAI-coated PET substrate can provide good electrical conductivity and reliability under periodically applied stress in comparison to ITOcoated PET substrate. The n-ZnO and p-ZnO:P layers were then subsequently deposited on the IAI substrate. Next, polydimethylsiloxane (PDMS) is spin-coated on the ITO-coated PET substrate, which is subsequently attached on the active ZnO layers. Here, the role of PDMS layer is two-fold. First, it works as an adhesive layer, bonding the top electrode layer and the ZnO layer on the bottom electrode layer. More importantly, it works as an insulation layer in the PENG, preventing carrier transport between the electrodes through active ZnO layers. Thus, the generated piezoelectric potential can be used to drive electrons toward an external 5
circuit, accumulating the electrons at the interface region between the top electrode and PDMS until the piezoelectric potential balances with these electrons.
Finally, the sample is
cured in the oven to firmly bond the top electrode layer and the ZnO layer. The detailed device fabrication process is described in the experimental method section. Fig 1(b) is a scanning electron micrograph (SEM) of device’s cross section shows the ZnO p-n junction layers deposited by RF magnetron sputtering. The fabricated device shows the good flexibility and transparency as shown in Fig. 1(c). In detail, the transmittance of the active layer was over 67% in visible wavelengths range after the ZnO layers are deposited on the IAI substrate [Fig. S2]. This indicates that the device can potentially be integrated with photonic devices or photoelectric energy harvesting devices. After deposition of active ZnO layers, the X-ray diffraction (XRD) patterns were investigated to confirm the crystal structures of the n-ZnO and p-ZnO layers [Figure 2(a)]. The 2θ peaks can be observed at (002) and (102) in all the ZnO films. We note that the relatively higher (002) peak was obtained in the ZnO films deposited on the IAI layer, indicating that IAI substrate is a good platform to fabricate PENGs. Using the n-ZnO and pZnO layers, the p-n diode was fabricated and electrically characterized to confirm the formation of the p-n junction. Obvious rectifying behavior can be observed in the p-n homojunction diode [Fig. 2(b)]. Specifically, approximately two orders of magnitude higher forward current than reverse current was obtained in the ZnO p-n homojunction diode. The diode only with a ZnO thin film, on the other hand, shows similar forward and reverse currents of ~ 2 μA under ±2V biases. The results here clearly indicate the formation of ZnO p-n homojunction. Besides, the ideality factor was determined by fitting the data in the forward bias region, using the diode equation, reverse bias saturation current,
is the electron charge, 6
[
(
)
], where
is a
is applied voltage across of the
diode,
is ideality factor,
is Boltzmann’s constant,
is absolute temperature. The
ideality factor of the ZnO p-n homojunction diode is ~ 4, which is a typical value observed in ZnO p-n diode prepared by sputter deposition and indicate that the carrier transport is mainly dominated by carrier recombination in the depletion region. We note that p-type conduction of p-ZnO layer was also confirmed by fabricating back-gated p-ZnO thin film transistor [Fig. S3]. The working principles of the PENGs with ZnO and ZnO p-n homojunction are described in Fig. 3(a) and (b), respectively. When the compressive force is applied on the top of the ZnO PENG, this separates the centers of positive and negative charges, generating electric dipole moment. After cancellation of interior charges of opposite signs, the charges remain at both surfaces, responsible for the polarization. Positive piezoelectric potential (V+) is set up at the ZnO/PDMS interface, and a negative piezoelectric potential (V-) is induced at the IAI/ZnO interface, deforming the energy band from the initial condition (dashed line) as denoted by solid line in Fig. 3(a). This piezoelectric potential drives the electrons toward the top ITO electrode through an external circuit from the bottom IAI electrode, accumulating them at the top electrode until they are balanced by the piezoelectric potential. As the applied force is removed, the induced piezoelectric potential instantly disappears and the accumulated electrons flow back to the bottom electrode. However, in ZnO PENG excess electrons in the ZnO layer significantly screen the positive dipoles, reducing the generated piezoelectric potential. In the p-ZnO and n-ZnO homojunction PENG, overall working mechanism is similar to that of ZnO PENG. However, different from the ZnO PENG, p-ZnO and n-ZnO layer form a depletion region between the two layers after recombination of electrons and holes in the layers. This can effectively reduce excess electron concentration in ZnO and enhance piezoelectric potential. Therefore, when the compressive force is applied on the top 7
of the ZnO p-n homojunction PENG, it can further deform energy band near the interfaces by comparison to ZnO PENG [Fig. 3(b)]. Furthermore, overall structure of ZnO p-n homojunction PENG can be modeled as three capacitors connected in series when compressed: the one is at IAI/n-ZnO interface and the others are at n-ZnO/p-ZnO junction and p-ZnO/PDMS interface. Thus, the total capacitance of n-ZnO/p-ZnO PENG is expected to be less than that of ZnO only PENG with the same thickness, where the depletion region is formed only near the interfaces. Consequently, output voltage increase is expected in the ZnO p-n homojunction PENG due to the enhanced piezoelectric potential coupled with the reduced total capacitance. For experimental verification, two different PENGs were fabricated, where the one device has only a ZnO layer and the other has a ZnO p-n junction. Fig. 3(c) and (d) show the output voltages and currents of these two PENGs, illustrated in Fig. 3 (a), (b). Here, the output voltage and current were measured with a 1 MΩ load resistor between the two electrodes. The two PENGs have the same total layer thickness of 400 nm, where the ZnO p-n homojunction PENG is composed of a 200 nm thick p-ZnO layer and a 200 nm thick n-ZnO layer. As demonstrated in Fig. 3(c) and (d), much higher output voltage and current were obtained in the ZnO p-n homojunction PENG, which is at least 6-fold higher voltage and two-fold higher current than the values obtained in the PENG only with ZnO, resulting in an order of magnitude higher output power. These results are well matched with the predictions in Fig. 3(a) and (b). Besides, to demonstrate how the reduced electron concentration affects on piezoelectric output in the PENG, we also fabricate the p-ZnO PENG with the same layer thickness [Fig. S4(a), (b)]. Although the output voltage and current of the p-ZnO PENG is lower than those of the ZnO p-n homojunction PENG, it is still higher than those of ZnO PENG due to reduction in excess electron concentration by recombination with holes. Inversely positioned p-n junction in the device structure can also significantly affect the output performance. The ZnO n-p homojunction PENG, composed of 8
the p-ZnO on the bottom and the n-ZnO on the top, exhibits much lower output voltage and current by comparison to the PENG with a p-n junction [Fig. S5]. In the n-p junction PENG, the positive piezoelectric potential induced at the n-ZnO/PDMS interface easily vanishes since electron concentration increases as the energy band bends downward with the compressive force. The negative piezoelectric potential at the p-ZnO/IAI interface is also diminished due to the increasing hole concentration with the applied compressive force. Thus, generated output voltage and current are similar to or even lower than those of ZnO PENG even after recombination of excess electrons with holes in p-ZnO. To further investigate the output performance related with n-ZnO and p-ZnO thickness ratios, the n-ZnO thickness was kept at 200 nm and only p-ZnO thickness was varied from 200 nm to 500 nm [Fig. 4(a), (b)]. The results show that the maximum output voltage and current can be obtained when a 300 nm-thick p-ZnO is deposited on the n-ZnO layer. Presumably, donated hole concentration from a 200 nm-thick p-ZnO layer was not enough to sufficiently remove excess electrons in n-ZnO by recombination. Reduced output voltage in the PENG with a 500 nm-thick p-ZnO can be possibly attributed to increased layer stiffness. This indicates that there exists optimal thickness ratio between p-ZnO and n-ZnO to maximize the output power. Fig. 4(c) summarizes the output power as a function of p-ZnO layer thickness. We found that the output power reaches ~140 μW at p-ZnO : n-ZnO = 300 nm : 200 nm. Similarly, the output power was also investigated as a function of n-ZnO thickness while the p-ZnO thickness was kept at a 100 nm [Fig. S6]. The output power of the PENG gradually decreases with the increase of the n-ZnO thickness, which is then saturated at a 400 nm. Considering the optimal thickness ratio between p-ZnO and n-ZnO above, the maximum output power is expected to occur at the n-ZnO thickness less than a 100 nm. Next, the stability of the PENG was investigated by cyclic measurement, using the PENG with a 9
200 nm-thick n-ZnO and a 200 nm-thick p-ZnO [Fig. 4(d)]. Consistent output voltage and current were measured over 5000 cycles without signal degradation. We note here that the PDMS capping layer and IAI substrate adopted in the PENG contributed to enhance stability and durability of the device. Fig. 4(e)-left shows a schematic circuit diagram consisting of single PENG, rectifying diode bridge, and a blue light emitting diode (LED). When the PENG is compressed periodically, it can instantly light up a blue LED as shown in Fig. 4(e)right.
Conclusions In this report, we have examined ZnO p-n homojunction to enhance the performance of PENGs. We formed a ZnO p-n homojunction thin film, composed of unintentionally doped n-ZnO and phosphorus-doped p-ZnO to alleviate the excess electron screening effect. The ZnO p-n junction PENG demonstrated the output power up to ~140 μW, which is approximately two orders of magnitude higher compared to ZnO PENG, resulted from the reduction in screening effect and total capacitance. In addition, the output performance related with n-ZnO and p-ZnO thickness ratio and the roles of p-n junction formation were also systematically investigated. The work here introduces a new method that can be served as a guidance to further extend performance limit of ZnO-based piezoelectric nanogenerators.
Acknowledgments This research was supported by Basic Science Research Program through the National Research
Foundation
of
Korea
(NRF-2015R1A1A1A05027235)
2015R1A2A2A01002415).
10
and
(NRF
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Vitae
Yang Hyeog Kwon received his B.S degree in Department of Electrical Engineering from Chungnam National University, Korea in 2015. He is currently Master course student supervised by Professor Junghyo Nah. His research topic is piezoelectric and triboelectric nanogenerators and self-powered energy storage devices.
Doo-Hee Kim received his B.S degree in Department of Advanced Materials Science Engineering for Information and Electronics from Kyung Hee University, Korea in 2015. He is currently Master course student supervised by Professor Han-Ki Kim. His current research topic is growth of p-type and n-type oxide thin films for flexible and transparent nanogenerators.
Han-Ki Kim received his Ph.D. in Materials Science and Engineering from Gwangju Institute of 13
Science and Technology in 2003. From 2003 to 2005, he worked in Samsung SDI as a principle researcher and developed TCO materials for AMOLEDs. Kim was an assistant professor at Kumoh National Institute of Technology from 2005 to 2008. Since 2009, he is a professor in Department of Advanced Materials Science and Engineering for Information and Electronics and Kyung Hee Fellow at Kyung Hee University, Yongin, Korea. His research interests include transparent electrode materials, flexible and transparent electronics and printable electronics. He has published over 205 articles and holds 100 patents.
Junghyo Nah received Ph.D. in Electrical and Computer Engineering from the University of Texas at Austin in 2010. He worked as postdoctoral scholar at University of California at Berkeley form 2011 to 2012. He joined the faculty of the Department of Electrical Engineering, Chungnam National University, Daejeon, Korea, in 2012. His research interests include piezoelectric and triboelectric energy harvesting devices, flexible electronics, and high performance transistors using low dimensional semiconductor nanomaterials.
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Figure caption
Figure 1. (a) Schematic representations of the piezoelectric nanogenerator fabrication process: IAI roll-to-roll sputter deposition on a PET substrate, ZnO p-n junction formation by RF magnetron sputtering, and bonding the active ZnO layer with an ITO-deposited topside electrode. (b) The scanning electron micrograph of cross-sectional view of the p-n homojunction layer. Scale bar: 200 nm (c) The fabricated ZnO p-n homojunction PENG, demonstrating transparency and flexibility.
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Figure 2. (a) XRD patterns of the sputter-deposited layers by RF magnetron sputtering. Enhanced peak along the n-ZnO (002) was observed. (b) Current-voltage (I-V) characteristics of the n-ZnO and ZnO p-n homojunction diode at bias voltage ±2V. The p-n junction diode shows greatly enhanced rectifying behavior.
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Figure 3. Energy band diagram and schematic representations of the output power generation mechanism of (a) n-ZnO PENG and (b) ZnO p-n junction PENG. When the force is applied, the positive and negative piezoelectric potentials are formed near the surfaces, deforming the energy band as a consequence. The negative piezoelectric potential pushes electrons toward the topside ITO electrode, accumulating them on the electrode until they are balanced by the piezoelectric potential. For the PENG with a ZnO p-n homojunction, overall generation mechanism is the same as n-ZnO PENG. However, reduction in excess electron screening effect and total capacitance further enhances the output performance of the PENG. (c), (d) Output voltage and current of the PENG, consisting of n-ZnO (left) or ZnO p-n homojunction (right), having the same total layer thickness of 400 nm. Significantly improved output performance can be observed in the PENG with ZnO p-n homojunction. The constant compressive force (0.5MPa) normal to the surface was applied during the measurement.
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Figure 4. (a) Output voltage and (b) current of the PENGs with different p-ZnO layer thicknesses. The n-ZnO film thickness was fixed at 200 nm. (c) Output power of the ZnO p-n homojunction PENGs with different n-ZnO and p-ZnO thickness ratios. (d) The PENG, consisting of 200 nm-thick n-ZnO and a 200 nm-thick p-ZnO, exhibits stable output voltage and current under compressive force (0.5MPa) over 5000 cycles. (e) (Left) schematics of the circuit diagram consisting of a single PENG, rectifying diode bridge, and a blue light emitting diode (Right) LED is instantly turned on when compressive force is applied on the PENG.
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Graphical abstract
highlights High performance piezoelectric nanogenerator was demonstrated by adopting ZnO p-n homojunction. Forming a p-n junction with phosphorus-doped ZnO on the ZnO layer successfully reduced excess electrons in ZnO. The p-n homojunction formation can effectively reduce the total capacitance under applied force, resulting in enhanced output power generation. The ITO/Ag/ITO substrate provides good conductivity, transparency, flexibility, and reliability under periodically applied stress.
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