High transparency and triboelectric charge generation properties of nano-patterned PDMS

Abstract

Triboelectric charge generation and high transparency properties of the nano-patterned PDMS, which was stamped by silicon subwavelength grating structures as a mold, on indium tin oxide (ITO) coated polyethylene terephthalate (PET) were investigated for triboelectric nanogenerators. At visible wavelengths, the nano-patterned PDMS on ITO coated PET (i.e., ITO/PET) exhibited high transmittances of >85%, together with a theoretical analysis using the RCWA simulation. When the ITO/PET was compressed onto the nano-patterned PDMS on ITO/PET, the output voltage and current density were reliably generated with a short interval time of 0.17 s, which yielded −5.2/5.4 V and −0.57/0.74 μA cm⁻², respectively, under a low external pushing force. The effect of pushing frequency on the output voltage and current density was also investigated.

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1 Introduction

Energy harvesting technology has attracted great interest in renewable and green energy applications including self-powered electronic devices, touch sensors and artificial skin.^1–3 There have been many efforts to achieve high output power in electric power generators, which convert ambient waste energy into electricity, using piezoelectrics, trioelectrics, electromagnetics and pyroelectrics.^4–7 Among them, flexible polymer materials-based triboelectric nanogenerators which have many advantages of high output power, scalability, simple design and cost-effective fabrication have been considered as a promising supplementary power source. In recent years, the extensively invented high output triboelectric nanogenerators were demonstrated using the patterned surface or nanowires of polymer materials such as polydimethysiloxane (PDMS) or Kapton.^8,9 The working principle of such nanogenerators involves the triboelectric charge generation, separation, induction and neutralization from the deformation of triboelectric materials, thus creating flowing charge electrons and resulting in a triboelectric power. In this principle, the surface morphology of triboelectric material plays a key role in the reliable device performance because the interface roughness of the surface was closely related to the friction area and separation efficiency between the triboelectric polymer materials and electrodes.¹⁰

On the other hand, pyramid shaped micro-patterns or nanowire arrays of PDMS have been demonstrated to effectively drive such triboelectric materials based on the principle of nanogenerators. Here, the PDMS was transferred from the micro-patterned silicon (Si) or nano-porous anodic aluminum oxide as a mold, which is a simple and reproducible process.^11–13 Furthermore, the patterned PDMS may enable manufacture of highly transparent triboelectric nanogenerators with transparent conductive oxide coated film such as indium tin oxide (ITO). Indeed, the nano-patterned PDMS on the ITO coated polyethylene terephthalate (PET) can be a good antireflective coating film according to effective medium theory.¹⁴ Unfortunately, there is very little or no study on the transparency and charge generation properties of nano-patterned PDMS based triboelectric nanogenerators. In this paper, we fabricated the inverted moth-eye nano-patterned PDMS on ITO/PET using Si subwavelength grating structured mold and investigated their transmittance and triboelectric characteristics. The rigorous coupled-wave analysis (RCWA) simulation was also performed to theoretically analyze the transparency property of the nano-patterned PDMS.

2 Experimental details

To obtain the inverted moth-eye shape of nano-patterned PDMS, we used the closely-packed hexagonal grating structure of Si substrate as a mold, fabricated by laser interference lithography and dry etching technique as reported in our previous work.¹⁵ For pattern transfer, the hard PDMS (h-PDMS), formed by mixing and degassing four components (VDT-731, SIP 6831.1, SIT7900, and HMS-301, Gelest Inc.), was spin-coated on the Si mold and then cured in oven. Subsequently, the soft PDMS (s-PDMS: base resin and curing agent, weight ratio = 10 : 1) was covered on the h-PDMS/Si mold and cured again. After that, the inverted moth-eye nano-patterned PDMS consisting of h-PDMS/s-PDMS was obtained by peeling off from the Si mold. Finally, the nano-patterned PDMS was spontaneously laminated on the ITO/PET substrate. Here, the ITO film with 200 nm was deposited via the RF magnetron sputtering.

2.1 Characterization

For structural and morphological properties, scanning electron microscope (SEM; LEO SUPRA 55, Carl Zeiss) and atomic force microscope (AFM, XE150, PSIA) images were measured. The transmittance was characterized by using a UV-vis-NIR spectrophotometer (Cary 5000, Varian) with an integrating sphere. For theoretical analysis of the nano-patterned PDMS on ITO/PET substrate, optical modeling and simulation were performed by the rigorous coupled-wave analysis (RCWA) method using a commercial software (DiffractMOD 3.1, Rsoft Design Group). To measure the output short-circuit current and open-circuit voltage of the fabricated triboelectric nanogenerators, multimeter and picoammeter (Keithley 2000 and 6487) were employed and the external pushing force was monitored by using an indicator with load cell (BONGSHIN, Inc.).

3 Results and discussion

Prior to characterization of transparency properties of the nano-patterned PDMS-based triboelectric nanogenerator structure, we calculated the transmittance of the PDMS samples with different pattern sizes on ITO/PET via the RCWA simulation where the dispersion property of each material was considered for more exact calculations.¹⁶ The Fig. 1 shows the three-dimensional (3D) modelling of inverted moth-eye nano-patterned PDMS on ITO/PET and the contour plot of the calculated transmittance spectra as a function of pattern size. The thicknesses of PDMS, ITO, and PET were set to 1.5 mm, 200 nm and 137 μm, respectively. The height of patterns was fixed to 175 nm, based on the experimental results (Fig. 2). As shown in the contour plot, for the patterned PDMS on ITO/PET, the region of low transmittances of <81% was increased at visible wavelengths of 420–650 nm with increasing the pattern size of PDMS from 300 to 600 nm. In fact, the transparency of such triboelectric structure depended strongly on Fresnel reflection losses between the patterned PDMS and air.¹⁷ When the wavelength of incident light is shorter than the pattern size of medium, the structure provides an effective graded refractive index profile, thus leading to a high transparency with efficiently reduced surface reflection.¹⁴ For this reason, the contour plot indicates that the PDMS with nano-pattern sizes (<310 nm) could result in high transmittances above 85% over a wide wavelength region of 450–800 nm.

Fig. 1 3D modelling of inverted moth-eye nano-patterned PDMS on ITO/PET and contour plot of the calculated transmittance spectra as a function of pattern size of PDMS using the RCWA simulation. Based on experimental results, the thicknesses of PDMS, ITO, and PET were set to 1.5 mm, 200 nm and 137 μm, respectively.

Fig. 2 (a) 5 μm × 5 μm scan AFM images and (b) measured transmittance spectra of the (i) bare and (ii) nano-patterned PDMS on ITO/PET in range of wavelengths from 300 to 1100 nm. The insets of (b) also show the schematic diagram of the corresponding structure and the tilted view of FE-SEM image for the fabricated nano-patterned PDMS.

The Fig. 2 shows the (a) 5 μm × 5 μm scan AFM images and (b) measured transmittance spectra of the (i) bare and (ii) nano-patterned PDMS on ITO/PET. The insets of (b) also show the schematic diagram of the corresponding structure and the tilted view of FE-SEM image of the fabricated nano-patterned PDMS. Comparing with both the AFM images, it is clear that the surface of PDMS was nano-patterned regularly with a 308 nm of pattern size. By an analysis of image statistics, the surface areas of bare and nano-patterned PDMS were obtained as 25.07 and 38.4 μm², respectively. This increased surface area can be expected to offer an improved triboelectric property due to the large friction area. As shown in Fig. 2b, it is commonly observed that the transmittance curves were somewhat fluctuated from 790 to 865 nm because the detector was changed from photomultiplier (300–800 nm) to InGaAs (800–1100 nm) detector. The bare PDMS on ITO/PET exhibited the increased transmittance as compared with the ITO/PET without PDMS. This can be explained by the fact that the refractive index of PDMS (1.4) is lower than that of ITO film (2.3–1.7) at wavelengths of 300–1100 nm and so the PDMS relaxes the large refractive index contrast between air and the ITO film. For the nano-patterned PDMS on ITO/PET, the transmittance was more increased as wavelength was increased from 350 to 1100 nm, which well agreed with the calculation results in Fig. 1. According to effective medium theory, the nano-patterned PDMS could provide the graded effective refractive index profile where the effective refractive index (n_eff) was estimated by n_eff = [f_PDMSn_PDMS^2/3 + f_airn_air^2/3]^3/2, where f_PDMS/n_PDMS and f_air/n_air are the volume fraction and refractive index for each PDMS and air.¹⁸ When the light passes through from air to the nano-patterned PDMS (i.e., smaller size than incident wavelength), the f_PDMS is gradually increased but the f_air is decreased, thus leading to a slow increase of n_eff. Therefore, it provided a high transparency property by reducing the Fresnel reflection between air to the nano-patterned PDMS.

The Fig. 3a shows the (i) schematic illustration of the nano-patterned PDMS-based triboelectric nanogenerator. To characterize the triboelectric characteristics, two rectangular ITO/PET substrates were utilized for each flexible electrode. Herein, the glass slide was placed between two electrodes, which sustained to be separated under pushing and releasing. While the 0.3 kgf of external force was applied to the top electrode of the triboelectric nanogenerator, the pushing force and voltage/current were investigated by monitoring the indicator and multimeter/picoammeter. The (ii) of Fig. 3a shows the photographic image of bare PDMS on ITO/PET and nano-patterned PDMS on ITO/PET under illumination of white fluorescent light. In comparing to bare PDMS on ITO/PET, it is clearly observed that the nano-patterned PDMS on ITO/PET exhibited more transparent property with somewhat coloured light, which caused by the diffraction at the inverted moth-eye nano-patterns. To examine the triboelectric characteristics for nano-patterned and bare PDMS, the measured output voltages of triboelectric nanogenerators under 0.5 Hz of external pushing frequency are shown in Fig. 3b. Herein, we obtained reasonable output voltage/current curves by many repetitions of the pushing test. Above 5000 times under low external pushing force of 0.3 kgf, the surface morphology of the nano-patterned PDMS was not degraded and the output voltage/current remained almost unchanged. As a result, it is evident that the nano-patterned PDMS considerably improved triboelectric properties. When the top electrode was touched and rubbed with the nano-patterned and bare PDMS, the positive voltage peaks were observed as approximately 5.4 and 1.2 V, respectively. Then, the negative voltage peaks of −5.2 and −1.4 V were shown as soon as the top electrode was separated from each PDMS. In addition, it can be noted that the nano-patterned PDMS produced a short interval time from touching to separating moment. The interval times of nano-the patterned and bare PDMS were estimated to 0.17 and 1.22 s, respectively. In fact, the PDMS possessed adhesive surface property to various solids including the glass and ITO/PET due to the van der Waals forces.¹⁹ Therefore, the bare PDMS was laminated to the top-electrode of ITO/PET and slowly separated by spacer. Whereas, the nano-patterned PDMS rapidly detached from the top-electrode after pressing, which can be caused by the fact that their sharp morphology of the patterns minimizes the contact area with top electrode after leasing from the deformation, which helps to easily detach the top electrode. Owing to this short interval time, the nano-patterned PDMS enables the triboelectric nanogenerator to operate under high external pushing frequency.

Fig. 3 (a) (i) Schematic illustration of the nano-patterned PDMS based triboelectric nanogenerator and (ii) photographic image of bare PDMS on ITO/PET and nano-patterned PDMS on ITO/PET under illumination of white fluorescent light, and (b) the measured output voltages of triboelectric nanogenerators under 0.5 Hz of external pushing frequency and 0.3 kgf of external pushing force.

The Fig. 4a shows the schematic diagram and photographic images for the mechanism of triboelectric nanogenerators with nano-patterned PDMS: (i) pressing, (ii) releasing and electrical equilibrium and (ii) separation. When the top electrode is compressed by mechanical pushing force, the positive and negative electrostatic charges are generated and distributed at each surface of the nano-patterned PDMS and ITO film according to triboelectric tendency.²⁰ And these oppositely distributed charges form an inner electric potential and induce a flow of free charges by an electrical equilibrium as depicted in the (ii) of Fig. 4a. Then, the free charges reversely flow across the two electrodes due to the neutralization process when the top electrode was separated from the nano-patterned PDMS. The Fig. 4b shows the measured current densities of triboelectric nanogenerators under 0.5 Hz of external pushing frequency and 0.3 kgf of external pushing force. The peak directions of both current densities at pushing and separating moments were well consistent with the charge flow of the schematic diagram in Fig. 4a. The negative/positive peaks observed alternated with each other. The nano-patterned and bare PDMS revealed the negative/positive current density peaks corresponded to −0.57/0.74 μA cm⁻² and −0.17/0.11 μA cm⁻², respectively.

Fig. 4 (a) Schematic diagram for the mechanism of triboelectric nanogenerators with nano-patterned PDMS: (i) pressing, (ii) releasing and electrical equilibrium and (iii) separation, and (b) measured output current densities of triboelectric nanogenerators under 0.5 Hz of external pushing frequency and 0.3 kgf of external pushing force. The photographic images for the pushing test with a triboelectric nanogenerator are shown in (a).

The Fig. 5 shows the measured (a) voltage and (b) current density of the triboelectric nanogenerator with nano-patterned PDMS under 0.3 kgf of external pushing force at different external pushing frequencies from 1 to 5 Hz. It is clearly observed that the reliable output voltage and current density could be obtained from the nano-patterned PDMS-based triboelectric nanogenerator under different external pushing frequencies, which is caused by a short interval time as explained in Fig. 3b. As the external pushing frequency was increased from 1 to 3 Hz, the both output voltage and current density were gradually increased, which would be caused by fact that some charges remained and were accumulated by an incomplete electrical neutralization for fast pushing cycles, which increased the triboelectric potential.^5,21 However, at 5 Hz, the output voltage and current density were slightly increased due to the fast pushing cycles. This is because the external pushing force was applied subsequently before the separated top electrode and deformed nano-patterned PDMS were recovered.

Fig. 5 Measured (a) voltage and (b) current density of the triboelectric nanogenerator with nano-patterned PDMS under 0.3 kgf of external pushing force at different external pushing frequencies from 1 to 5 Hz.

To investigate the output performance of the nano-patterned PDMS-based triboelectric nanogenerators with different external loads, the output current density and average power (W_average) were characterized. Fig. 6 shows measured current densities of the nano-patterned PDMS-based triboelectric nanogenerators with different external loads of 5, 10, 24 and 50 kΩ under external pushing force of 0.3 kgf and external pushing frequency of 1 Hz. As the resistance of external load was increased, the current density was gradually decreased. For considering the output power, we assumed that the electric energy was equivalent to the Joule heating energy. Therefore, the W_average was calculated using the equation of , where N, R, and I(t) are the number of peaks, the resistance of external load and the output current as a function of time. And the time between t_1i and t_2i is the interval time of ith peak of measured current. As shown in the inset of Fig. 6, the W_average was relatively high at 10 kΩ of external load. For unit area of 1 cm², the 0.77 μW was obtained as a maximum W_average value.

Fig. 6 Measured current densities of the nano-patterned PDMS-based triboelectric nanogenerators with different external loads of 5, 10, 24 and 50 kΩ under external pushing force of 0.3 kgf and external pushing frequency of 1 Hz. The inset also shows the calculated W_average value as a function of resistance of external loads.

4 Conclusions

We investigated high transparency property and reliable triboelectric characteristics of the inverted moth-eye nano-patterned PDMS-based nanogenerators, fabricated by a simple pattern transfer process with Si mold. According to effective medium theory, the nano-patterned PDMS on ITO/PET could provide a high transparency and it was also theoretically analyzed using the RCWA simulation. For analyzing the triboelectric characteristics of the nano-patterned PDMS, two ITO/PET electrodes were used as each electrode. Under 0.3 kgf of low external force, the nano-patterned PDMS exhibited the improved voltage and current peaks due to the large friction area. Additionally, the reliable output voltage and current density were observed at different external pushing frequencies from 1 to 5 Hz because of short interval time by the nano-patterned PDMS.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (no. 2013-068407).

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