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Hybrid Energy Cell with Hierarchical Nano/Micro Architectured Polymer Film to Individually/Simultaneously Harvest Mechanical, Solar, and Wind Energies Bhaskar Dudem, Yeong Hwan Ko, Jung Woo Leem, Joo Ho Lim, and Jae Su Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09785 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Applied Materials & Interfaces

Hybrid Energy Cell with Hierarchical Nano/Micro Architectured Polymer Film to Individually/Simultaneously Harvest Mechanical, Solar, and Wind Energies Bhaskar Dudem, Yeong Hwan Ko, Jung Woo Leem, Joo Ho Lim, and Jae Su Yu* Department of Electronics and Radio Engineering, Kyung Hee University, 1732 Deogyeongdaero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea

ABSTRACT We report the hierarchical nano/micro architectured (HNMA)-polydimethylsiloxane (PDMS) film-based hybrid energy cells with multi-functionality to simultaneously harvest mechanical, solar, and wind energies. These films consist of nano/micro dual-scale architectures (i.e., nanonipples on inverted micro pyramidal arrays) on the PDMS surface. The HNMA-PDMS is replicable by a facile and cost-effective soft imprint lithography (SIL) using a nanoporous anodic alumina oxide (AAO) film formed on the micro-pyramidal (MP) structured silicon substrate. The HNMA-PDMS film plays multifunctional roles as a triboelectric layer in nanogenerators and an antireflection (AR) layer for dye-sensitized solar cells (DSSCs) as well as a self-cleaning surface.

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This film is employed to triboelectric nanogenerator (TENG) devices, fabricated by laminating it on the indium tin oxide-coated polyethylene terephthalate (ITO/PET) as a bottom electrode. The large effective contact area emerged from the densely packed hierarchical nano/micro architectures of PDMS film leads to the enhancement of TENG device performance. Moreover, the HNMA-PDMS/ITO/PET with high transmittance of > 90% also results in the highlytransparent TENG devices. By placing the HNMA-PDMS/ITO/PET with the zinc oxide nanowires-coated ITO/PET on the top glass substrate of DSSCs, the device is able to add the functionality of TENG devices, thus creating a hybrid energy cell. The hybrid energy cell can successfully convert the mechanical, solar, and wind energies into electricity, simultaneously or independently. To specify the device performance, the effects of external pushing frequency and load resistance on the output of TENG devices are also analyzed, including the photovoltaic performance of the hybrid energy cells.

KEYWORDS: hybrid energy cell, triboelectric nanogenerator, hierarchical nano/micro architectured PDMS, dye-sensitized solar cell, nanoporous anodic alumina oxide

1. INTRODUCTION To efficiently convert the solar radiation energy into electrical power, various types of photovoltaic devices have been developed by boosting their conversion efficiencies, scaling up the size or lowering the cost of the device, and improving the reliability.1-4 In particular, the dyesensitized solar cell (DSSC) is promising for commercialization due to its less weight, better transparency, and cost-effective manufacturing.5,6 However, the power conversion efficiencies (PCEs) of DSSCs are still relatively low compared to the other inorganic material-based solar


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cells (e.g., PCEs > 10-30%) due to certain limitations including the light absorption of the dyes and the interface charge separation between dyes and titanium dioxide (TiO2) electrodes.7,8 One of the effective strategies for enhancing the device performance is to increase the light absorption of the active layer in DSSCs by employing an efficient antireflective (AR) and light scattering layer on the cover of transparent substrates such as glasses and plastics.9,10 Recently, many studies have been reported on the nanostructures (e.g., nanogratings,10,11 nanowires,12,13 and nanoholes,14 etc.) at the external surface of solar cells as a AR layer. These nanostructured AR layers can considerably suppress the surface Fresnel reflections in wide ranges of incident light wavelengths and angles due to the effective gradient-refractive-index (GRIN) profile between air and the top surface of solar cells.15,16 Additionally, the microstructured AR layers also successfully help to enhance the light absorption in solar cells by the extension of effective light path length.17,18 Moreover, the hierarchical architectures consisting of nanosturctures on microstructures have been predominantly studied to simultaneously increase total transmittance and diffuse light scattering due to the combination of their AR abilities.19-21 However, to directly form nano- and micro-structures on the surface of transparent substrates, the complex and costeffective patterning techniques such as photolithography,22 electron-beam lithography,23,24 nanoimprint lithography,25,26 and laser interference lithography,27,28 as well as a subsequent dry etching process are required. Thus, it is necessary to develop efficient AR layers in an effective way including simple and cost-effective methods. Energy harvesting from nanogenerators is also one of the foremost interests to efficiently convert vibrational, rotational, and mechanical energies into electricity based on the piezoelectric and triboelectric effects.29-31 Especially, triboelectric nanogenerators (TENGs) have been a promising candidate for realization because of its simple design and cost-effective fabrication


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with high output performance.22,32-34 The TENG device operates on the basis of flow of electron as driven by the triboelectric effect induced electrostatic charges on the surfaces of two different triboelectric materials.35,36 The efficiency of TENG devices can be enhanced by introducing triboelectric materials with high surface roughness or large contact area. Since it can be utilized to increase the friction between the triboelectric materials, large numbers of electrostatic charges are induced.37,38 Hence, the polymer layer with a high-roughened hierarchical nano/micro architecture (HNMA) can be beneficial for high-performance TENGs. Besides, the introduction of the HNMA polymer can also lead to the improved transparency of flexible TENG devices because of its efficient AR ability, as mentioned above.21,22 Recently, engineers have been strongly trying to get high-performance hybrid energy systems by integrating different two or three energy conversion devices into a single system (i.e., hybrid cell).39-44 This is because the hybrid cell can efficiently gather more amount of usable energy than conventional devices. At present, this research is an early phase of development. Therefore, there have been several reports on the hybrid energy cells with a combination of NGs (e.g., triboelectric or piezoelectric NGs) and solar cells (silicon solar cells or DSSCs) consisting of various configurations (fiber or tandem) for harvesting solar and mechanical energies, simultaneously or independently, in previous works.39-42 But, there are no reports on hybrid energy cells designed by utilizing a multifunctional (i.e., triboelectric, AR, self-cleaning abilities) polymer film with HNMA. In this study, we fabricated the HNMA-polydimethylsiloxane (PDMS) film and utilized it to design the simple and cost-effective hybrid energy cells. Introduction of the HNMA-PDMS brought about multiple advantages such as a triboelectric layer in TENGs and a protective AR layer in DSSCs in high-performance hybrid energy devices. Additionally, the manufacturing process of the novel hierarchical architectures consisting of


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nanonipples on inverted micro pyramidal arrays involves the facile and cost-effective soft imprint lithography (SIL) technique, unlike the expansive and complicated patterning techniques followed by a dry etching, as discussed above. So, a new hybrid energy cell was designed by integrating two different energy conversion devices (i.e., TENG and DSSC) with the multifunctioned HNMA-PDMS. To investigate the device feasibility of the HNMA-PDMS film, it was laminated on the indium tin oxide-coated polyethylene terephthalate (ITO/PET) for TENGs, and their output device performance was explored, including their solar cell performance. The optical and surface wetting behaviors of the HNMA-PDMS were also analyzed.

2. EXPERIMENTAL AND OPTICAL SIMULATION DETAILS 2.1. Fabrication of HNMA PDMS: Figure 1a illustrates the schematic diagram of the fabrication procedure for the HNMA-PDMS using an anodic aluminium oxide on the micro-pyramidal structured silicon (AAO/MP-Si) mold via the SIL technique. Initially, (100) Si substrates cleaned by acetone, methanol, and de-ionized (DI) water with a gently blowing dry nitrogen (N2) gas were prepared, and then they were subsequently rinsed using a mixed solution consisting of buffer oxide etchant (BOE), 5 wt% hydrofluoric (HF) acid solution, and DI water, followed by a N2 gas. To make the MP-Si, a typical wet-chemical etching process was performed with the alkaline etchant solution mixed by potassium hydroxide (20 mL), isopropyl alcohol (10 mL), and DI water (200 mL) at a temperature of ∼80 °C.45,46 Then, an aluminum (Al) thin film with a thickness of 300 nm was deposited by using an electron-beam evaporation system at room temperature. In addition, the AAO template on the MP-Si substrate was formed by the anodization of the Al thin film under an applied voltage of 80 V in a 5 wt% phosphoric acid (H3PO4) solution at 10 °C for 15 min. Afterwards, to widen the nanopore size of AAO, the anodized samples were dipped into the 5 wt% H3PO4 solution for 5 min at room temperature,


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thus obtaining the HNMA patterns of AAO/MP-Si as a mold.47 Subsequently, for the realization of the HNMA-PDMS film, the HNMA patterns on the surface of molds were transferred onto the PDMS film by a SIL method. A hard PDMS (h-PDMS; mixture of VDT-731, SIP 6831, SIT7900, and HMS-301, Gelest Inc.) was spin-coated on the surface of AAO/MP-Si molds, and then the samples were cured in an oven at 75 °C for 25 min. Here, the h-PDMS has been typically used to successfully replicate the highly-dense nano- and micro-structures on the mold to prevent any pattern distortion and deformation becasue of its high modulus (~ 9 N/mm2) and low surface energy.48 Next, a soft PDMS (s-PDMS; Sylgard 184) solution was poured on the hPDMS/AAO/MP-Si mold and cured again for 2 h at 75 °C. The s-PDMS not only plays a role as a protection layer of stiff h-PDMS, but also allows manual applications in a nondestructive manner. Finally, the PDMS (h-PDMS/s-PDMS) film was carefully peeled off from the mold, thus creating the 100 µm-thick HNMA-PDMS. Certainly, the HNMA-PDMS film was very well laminated on the surface of ITO/PET substrate and also further utilized as a bottom electrode of the TENGs. 2.2. Fabrication of ZnO NWs: To obtain the vertically-aligned zinc oxide nanowires (ZnO NWs) on the surface of ITO/PET substrate, the cost-effective and simple hydrothermal synthesis method was used. Prior to the synthesis of ZnO NWs, the ITO/PET substrates were cleaned with methanol, and DI water, and subsequently dried with N2 gas blowing. After that, a 30 nm-thick ZnO thin film as a seed layer was deposited on the ITO/PET by using an RF magnetron sputtering system in argon (Ar) environment at 6 mTorr of process pressure and 100 W of RF power. Then, for the growth of ZnO NWs, the ZnO seed/ITO/PET was dipped into the ZnO precursor solution consisted of zinc nitrate hexahydrate and hexamethylenetetramine with equal molar concentrations (25 mM) dissolved in 200 mL of DI water, and then the samples were


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placed into the oven at 90 °C for 6 h, following that the samples were washed by DI water several times. The ZnO NWs/ITO/PET were used as a top electrode of the TENGs. 2.3. Preparation of TENGs and DSSCs: Figure 1b illustrates the schematic diagram of the archshaped TENG integrated with the HNMS-PDMS and ZnO NWs. Firstly, to design the archshaped TENG, two ITO/PET substrates with different sizes were cleaned with isopropyl alcohol and dried with N2 gas blowing. After that, the HNMA-PDMS film with a size of 2 × 4 cm2 was laminated on one ITO/PET substrate with a size of 3 × 5 cm2, which is utilized as a bottom electrode of the TENG. On the other hand, another ITO/PET substrate with a size of 3 × 6 cm2, deposited by the ZnO NWs with a size of 2 × 5 cm2, was utilized as a top electrode. Finally, the arch-shaped TENG device with a total size of 3 × 5 cm2 was prepared by combining the top and bottom electrodes using a cellophane tape, as shown in Figure 1b. In order to simultaneously harvest both the solar and mechanical energies, hybrid energy cells were realized by laminating the TENG device onto the prototype DSSCs as a protective cover layer, as shown in Figure 1c. Herein, a typical titanium dioxide (TiO2)-based DSSC was utilized. For the fabrication of the DSSC with an active area of 0.5 × 0.5 cm2, initially, the TiO2 particles (PST-18NR, CCIC) were coated with ∼15 µm of thickness onto the surface of fluorine doped tin oxide (FTO) deposited glasses (i.e., FTO/glass) by a doctor-blade process using polyimide tapes as a spacer. Then, the samples were annealed at 500 °C for 2 h in a furnace. Next, the TiO2-coated FTO/glass was dipped into a 3 × 10−4 M solution of ruthenium (II) dye (Solaronix, N719) in ethanol and kept at room temperature for 24 h in dark. To prepare a counter electrode, the platinum paste (counter PT-1, Dyesol) was coated onto another FTO/glass using the same doctor-blade method, followed by annealing treatment at 500 °C for 2 h. Subsequently, an electrolyte (Dyesol, electrolyte HPE) was injected and sealed by using a hot press.


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2.4. Characterized Instrumentation: The surface morphological properties of the fabricated HNMA-PDMS film and ZnO NWs grown on the ITO/PET substrate were observed by using a field-emission scanning electron microscope (FE-SEM; LEO SUPRA 55, Carl Zeiss). The crystalline properties of ZnO NWs were studied by using an X-ray diffractometer (XRD; Mac Science, M18XHF-SRA). The optical properties of the samples were observed by using a UVVis-NIR spectrophotometer (Cary 5000, Varian). To investigate the surface wetting behavior of the HNMA-PDMS film, a contact angle measurement system (Phoenix-300, SEO Co., Ltd.) was employed. For all the devices, the output voltage and current were estimated by using a programmable multimeter (Keithley 2000) and a picoammeter (Keithley 6487), respectively. The external pushing force applied on the TENG device to generate the output voltage or current was monitored by using both the load indicator (BONGSHIN, Inc.) and load cell. To evaluate the photovoltaic performance (i.e., current density-voltage (J-V) curves) of DSSCs and hybrid energy cells, a solar simulator (SUN 3000, ABET) with a 1000 W Xe short arc lamp was used. Incident photon to current conversion efficiency (IPCE) spectra of the devices were also explored. 3. RESULTS AND DISCUSSION Figure 2a shows the top-view and cross-sectional SEM images of the (i) AAO/MP-Si mold and (ii) HNMA-PDMS. The high-magnification SEM images of the corresponding samples were also shown in the insets of Figure 2a. From the SEM image of the AAO/MP-Si mold, as shown in Figure 2a-(i), it can be observed that the AAO film consisting of nanopores was well uniformly incorporated into the randomly distributed MP arrays with various microscale dimensions on the Si surface over a large area. For the AAO film, the average diameter of nanopores was ∼ 100 nm and the film thickness was ∼ 300 nm, respectively. On the other hand, for the MP arrays, the


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bottom size was roughly estimated to be range of 1.9-2.5 µm. The overall height of the AAO/MP on the Si surface was in the range of 0.6-2 µm. Using the simple SIL technique, the HNMA patterns of AAO/MP-Si mold were negatively well transferred onto the surface of PDMS film without any serious pattern deformations and distortions, thus producing the HNMA-PDMS film, as shown in Figure 2a-(ii). From the SEM images, the top size of the inverted MP arrays and overall depth of HNMA on the surface of PDMS film were observed in the ranges of 1.9-2.5 µm and 0.6-2 µm, respectively. The average diameter and height of the nanostructures (i.e., nanonipples) were approximately 100 nm and 140 nm, respectively. For the HNMA-PDMS, the dimensions of the nano- and micro-structures were nearly similar to those of the AAO/MP-Si mold. But, the height of the nanonipples on the HNMA-PDMS is slightly different from the depth of nanopores on the AAO/MP-Si mold. This may be a reason why the h-PDMS solution cannot be easily squeezed into the very small and deep nanopores (i.e., diameter of ∼ 100 nm and thickness of ∼ 300 nm) of the AAO film, and resultantly the patterns of nanopores on the AAO film were poorly transferred into the surface of the PDMS film though the h-PDMS was generally used to replicate the nanoscale patterns. Figure 2b shows the top-view SEM image of the ZnO NWs on ITO/PET substrate. The inset of Figure 2b also confirms that the ZnO NWs were grown vertically on the ITO/PET substrate with an average diameter and height of ∼ 50 nm and ∼ 410 nm, respectively. From the XRD measurements of ZnO NWs on ITO/PET, as shown in Figure 2c, at 2θ = 34.5°, a strong XRD peak of ZnO (002) was observed, indicating the wellcrystallized ZnO NWs grown perpendicularly on the seed layer along the c-axis of the hexagonal wurtzite structure.49 Furthermore, the two broad XRD peaks at 46.83 and 53.74° of PET were also observed, respectively.


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Figure 3a and 3b show the measured total transmittance spectra of the flat PDMS, HNMAPDMS, and ZnO NWs on ITO/PET substrate, in comparison with the bare ITO/PET. The insets of Figure 3a and 3b also show the measured total reflectance spectra of the corresponding samples. As can be seen in Figure 3a, the flat PDMS laminated on the ITO/PET slightly enhanced the average total transmittance (Tavg) value to ~ 83.8%, compared to the bare ITO/PET (i.e., Tavg ~ 80.3%) in the wavelength range of 350-1000 nm. The flat PDMS can reduce the surface reflection of the ITO/PET because of the step GRIN profile in constituent materials (i.e., air/PDMS/ITO/PET). Similarly, a lower average reflectance (Ravg) value of ~ 12.5% was obtained compared to the bare ITO/PET (i.e., Ravg ~ 17.6%). However, the HNMA-PDMS with the combination of nano- and micro-architectures exhibited the higher Tavg value of ~ 92.1% (lower Ravg ~ 9.5%). This is attributed to two main effects followed by the increased optical path lengths and the effective GRIN effect due to the HNMA.50-53 For the HNMA-PDMS, the MP arrays were mostly used to diffract the light into several beams and prevent the light from escaping back to air, and thus they enhanced the transmittance of the HNMA-PDMS.17 On the contrary, the nanonipples with a subwavelength scale have the continuously and linearly GRIN profile between air and the bulk MP-PDMS. The optical transmittance of MP-PDMS was also measured and compared with the HNMA-PDMS, as shown in Figure S1 of the Supporting Information. The MP-PDMS exhibited the Tavg value of ~ 87.7 %, which is lower than that (Tavg ~ 92.1%) of the HNMA-PDMS. Therefore, the HNMA-PDMS with high Tavg (>90%) was utilized to enhance the transparency of the TENG device as a bottom electrode as well as a protective AR layer on the DSSC, simultaneously. Furthermore, the ZnO NWs deposited on the ITO/PET also exhibited the relatively higher Tavg (lower Ravg) value of ~ 84.2% (~ 7.7%) than those of the bare ITO/PET, and thus it also can improve the transparency of TENG device. The


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ZnO NWs/ITO/PET was employed as a top electrode, leading to the increased transmittance (i.e., Tavg ~ 72.2%) of the TENG device with ZnO NWs compared to TENG device without ZnO NWs (i.e., Tavg ~ 67.5%), as shown in Figure S1. Compared to our previous work,54 the present TENG device exhibits high transparency due to the introduction of HNMA-PDMS and ZnO NWs as bottom and top electrodes, respectively. Moreover, the TENG device performance is also enhanced, which has been discussed in the succeeding text. Figure 3c shows the photographs of the flat PDMS and HNMA-PDMS on ITO/PET placed under white fluorescent light. The water droplets on the surface of the corresponding samples are also shown in Figure 3c. The white florescent light was strongly reflected on the surface of the flat PDMS laminated on ITO/PET while it was relatively weekly reflected on the surface of the HNMA-PDMS, verifying the efficient AR behavior (i.e., high transparency) of the HNMA-PDMS. Also, the surface wettability of the flat PDMS and HNMA-PDMS was explored. The flat PDMS showed a hydrophobic surface with a water contact angle (θCA) of ~ 98°. On the other hand, the surface of HNMA-PDMS exhibited a superhydrophobic behavior (i.e., θCA ~ 151°). This is ascribed to the significantly increased surface roughness caused by the combination of nano- and microarchitectures on the surface of PDMS film, as can be explained by the Cassie-Baxter theory.55 The HNMA-PDMS film with a superhydrophobic surface can be also useful to maintain the device performance due to a self-cleaning function in real outdoor environment.56,57 The operating principle of the TENG device incorporated with both the HNMA-PDMS as the bottom electrode and vertically-aligned ZnO NWs as the top electrode was schematically illustrated in Figure 4a. When an external pushing force was applied to the top electrode of the arch-shaped TENG, two electrodes of ZnO NWs and HNMA-PDMS were contacted with each other. As a result, the opposite electrostatic charges are induced on each electrode due to the


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triboelectric tendency.58,59 On the other hand, by releasing the applied force, the two electrodes are moved apart from each other, and then electrostatic potential difference is induced across the electrodes. Such a potential difference across the electrodes starts to drive the electrons from the bottom to top electrode while the current in opposite direction. After the electrodes revert back into their original positions, the triboelectric charge distribution reaches electrostatic equilibrium. Afterwards, when the external pushing force is applied again onto the TENG device, the reversely induced potential difference across the electrodes can start to drive the electrons from the top to bottom electrode and the flow of these electrons produces the positive current. The output voltage and current curves generated by the TENG devices with and without the ZnO NWs are shown in Figure 4b and 4c, respectively. During these measurements, the external pushing force was maintained constantly at 0.2-0.5 kgf, under 0.5 Hz of external pushing frequency. The averaged output voltage and current values of the TENG device without ZnO NWs were observed as ∼ 14.6 V and ∼ 0.9 µA, respectively. On the contrary, the integration of ZnO NWs with the TENG device led to the further improved output voltage and current values of ∼ 18.2 V and ∼ 1.4 µA, respectively. This is mainly ascribed to the high surface roughness introduced by the ZnO NWs on the surface of ITO/PET as the top electrode. The ZnO NWs with high surface roughness enable to easily collect and transport the electrostatic charges generated from the electrodes, which results in the enhanced electrostatic potential and output device performance of the TENG.49 Figure 5a shows the photographic images for the pushing test for the TENG device. By using a load cell, a pushing force is applied externally onto the top electrode to generate the electrostatic charges on the electrode surfaces according to the triboelectric tendency, as explained in Figure 4a. Figure 5b shows the J-V characteristics of the bare DSSC in comparison


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with the DSSCs with HNMA-PDMS and TENG devices with and without ZnO NWs as a protective cover-layer. The schematic diagram of the DSSC laminated with the TENG device is also shown in the inset of Figure 5b. The device characteristics (open circuit voltage, VOC; short circuit current density, JSC; fill factor, FF; PCE) of the corresponding DSSCs are summarized in Table 1. As shown in Figure 5b, for the bare DSSC, the VOC and JSC values were observed to be about 0.7 V and 16.98 mA/cm2, respectively, indicating the PCE value of ~ 7.36%.

By

laminating the HNMA-PDMS on the DSSC, the PCE value was increased to ~ 7.64%. This is mainly attributed to the enhancement of the JSC value from ~ 16.98 to ~ 17.85 mA/cm2 caused by the increased transmission over a wide wavelength range (i.e., 400-800 nm) by laminating the HNMA-PDMS with efficient AR ability on the DSSC. On the other hand, for the DSSC integrated with two different TENG devices (i.e., with and without ZnO NWs as the top electrode), the relatively lower PCE values of ~ 6.06 and ~ 5.67%, respectively, were achieved. This is ascribed to the reduced transmittance caused by the TENG device structure with the air gap between the two electrodes (i.e., ZnO NWs/ITO/PET and HNMA-PDMS/ITO/PET) compared to the HNMA-PDMS/DSSC, as shown in Figure S1. From this, the TENG can interrupt the light incidence into the active layer of DSSCs, resulting in the decrease in JSC. In addition, the significant reduced JSC values also lead to the slightly lower VOC values due to the relation between the VOC and JSC.60 Nevertheless, although the PCE values of devices combined with TENG (with ZnO NWs) and DSSC were lower than those of the bare DSSC and the DSSC with the HNMA-PDMS, the resultant hybrid energy cell can have high device performance, which will be discussed in Figure 5d and 5e. The IPCE spectra of the corresponding DSSCs were also followed with the same behavior of J-V curves (Figure 5b), as shown in Figure S2a. The output voltage of the bare DSSC and the DSSC with TENG device (with ZnO NWs) as a


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protective cover-layer was observed constantly to be about 0.7 V, as shown in Figure S2b. These results clearly conclude that, even after the lamination of the TENG device on DSSC, the output voltage is almost similar. This indicates that the lamination of TENG device does not affect the electrical and material properties of DSSCs.61 A bridge rectification circuit was employed to convert the AC signals produced by TENG into the DC signals. The obtained rectified output voltage pulse of the device was about ∼17.5 V, as shown in Figure 5c. Furthermore, both the DSSC and rectified TENG devices were connected in series to design the integrated hybrid energy cell, as illustrated in Figure S3, which can continuously provide the output voltage/current when either solar or mechanical energy is available. The output performance of these hybrid energy cells can be mainly affected by the internal resistance of the device, especially largely contributed by TENG rather than DSSC.62,63 Figure 5d shows the output voltage of the hybrid energy cell by simultaneously illuminating the solar light and applying the mechanical force of 0.3-0.6 kgf on it by using the transparent glass slide. Here, the output signals of TENG were rectified. It can be clearly observed that the DSSC and the TENG can operate simultaneously/individually to harvest the solar and mechanical energies, respectively. Moreover, the output voltage of the hybrid energy cell was dramatically increased to be ∼ 20 V, which is much larger than those of the individual bare DSSC (∼ 0.7 V) and TENG (∼ 18 V) devices. This output voltage of hybrid energy cell is also relatively high compared to the results reported in previous literatures.39-42 Thus, our experimental results provide a promising potential in terms of the feasibility and the improvement in the energy conversion from individual device modules. The corresponding output current of the hybrid energy cell was about 3.1 mA in Figure S4. To investigate the practical application of the device in outdoor environment, we fabricated the hybrid energy cell consisting of a TENG and four DSSCs and measured its output performance


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under both the illumination of light and external mechanical forces using different sources, as shown in Figure S5. Firstly, the hybrid energy cell was illuminated with the solar light, exhibiting the output voltage of ∼ 1.8 V (as shown in Figure 5e). The observed voltage is responsible for only DSSCs, which can support that the hybrid energy cell can harvest the individual energies (i.e., solar energy), even in the absence of the mechanical energy. With the solar light, the mechanical energy was also applied by gently touching with a finger or pen (i.e., pushing force < 0.3 kgf), and its output voltage response was observed. When the finger or pen (i.e., opaque objects) was introduced to apply the pressure on hybrid energy cell, it might block the light path reaching to the DSSCs, which leads to the voltage drop. But, when the finger or pen (i.e., opaque objects) was removed or pressure released, the voltage may suddenly fluctuate and be a maximum value of ∼ 8-12 V, as shown in Figure 5e. The corresponding measurements were clearly demonstrated in the Video 1 (Supporting Information). This is mainly attributed to the simultaneous response from both the DSSCs and TENG devices. Besides, the hybrid energy cell was also exposed to wind (i.e., wind flow speed < 4 m/s), which is an existing energy source in working (or outdoor) environment of solar systems. This wind energy can be easily converted to mechanical energy by fluctuating the flexible top electrode (ZnO NWs/ITO/PET) of TENG device. As a result, the output voltage of hybrid energy cell was abruptly enhanced from 1.8 to ∼ 10 V (as shown in Figure 5e). Therefore, from these results, it is clear that the hybrid energy cell can simultaneously/individually harvest the solar, mechanical, and wind energies to effectively get the maximum output. Figure 6a shows the measured output voltage and current curves of the TENG device at external pushing frequencies from 0.5 to 3 Hz under the applied external pushing force range of 0.2-0.5 kgf. From these output voltage and current curves, as the frequency increased from 0.5 to


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3 Hz, the average output voltage and current values were increased from 18 to 29 V and 1.76 to 2.82 µA, respectively. This is mainly ascribed to the large accumulation of residual charges on both the electrodes because of fastest pushing cycles. Thus, the improved residual charges across the electrodes are desirable to enhance the triboelectric potential and output performance of the TENG device. The reliability of the TENG device was also studied by measuring the output voltage and current of the TENG device for more than 30 min (∼ 1000 cycles) under the applied external force range of 0.2-0.5 kgf at 0.5 Hz, as shown in Figure 6b. Despite the operation time of 30 min (∼ 1000 cycles), the output voltage and current of the TENG device combined with HNMA-PDMS and ZnO NWs were maintained without any significant degradation on the entire device performance. These results suggest that both the ZnO NWs (top electrode) and HNMAPDMS (bottom electrode) are expected to continuously generate the triboelectric charges without any predominant material deformation. Figure S6 shows the SEM images of the ZnO NWs after 1000 repetitive external pushing cycles. From these SEM images, it can be observed that most of the ZnO NWs remained stable during the 1000 repetitions of external pushing cycles. Furthermore, in order to analyze the effect of external load resistance on the output performance of the TENG device, the measured average (or maximum) current peak values and power densities were examined at different external loads from 50 KΩ to 200 MΩ, as shown in Figure 6c. As the external load resistance was increased from 50 KΩ to 200 MΩ, the average output current value was gradually decreased from 2.18 to 0.4 µA under the external pushing force range of 0.2-0.5 kgf at 0.5 Hz. Also, the average power density (Waverage) was calculated according to the following equation:64


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N

Waverage = R ∑ i =1

∫

t2 i

t1i

J i 2 (t )dt

N (t2i − t1i )

,

where R and J(t) are the external load resistance and the output current density as a function of time, respectively. N is the number of peaks and (t2i-t1i) is the interval time of the ith peak. As shown in Figure 6b, the calculated Waverage value was increased by increasing the load resistance from 50 KΩ to 60 MΩ, and then it reached the maximum value. With further increasing the load resistance at values of above 60 MΩ, the Waverage value was gradually decreased. Thus, at the load resistance of 60 MΩ, the maximum Waverage value of 0.65 mW/m2 was obtained.

4. CONCLUSION In summary, we have designed the hybrid energy cell (i.e., TENG+DSSC) incorporated with the HNMA-PDMS film. The HNMA on the PDMS film was easily replicated from the mold consisting of hierarchical AAO film with nanopores on the MP arrayed Si substrate by a facile and cost-effective SIL method. The HNMA-PDMS plays multifunctional roles such as a triboelectric layer for NGs and an AR layer for DSSCs, including a self-cleaning ability. The HNMA-PDMS with a large effective contact area was employed as the triboelectric layer to enhance the output performance of the TENG device. Under the applied external pushing force of 0.2-0.5 kgf, the TENG with HNMA-PDMS and ZnO NWs on ITO/PET as the bottom and top electrodes, respectively, exhibited ∼ 18.2 V of output voltage and ∼ 1.4 µA of output current at 0.5 Hz of pushing frequency. Furthermore, the HNMA-PDMS with high transparency (i.e., T > 90%) over a wide wavelength range of 400-800 nm was used as an AR layer on the DSSC to increase its efficiency. Besides, it can be also utilized as a self-cleaning surface due to its superhydrophobicity (i.e., θCA ~151°). Therefore, the introduction of HNMA-PDMS with


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effective multifunctionality into the hybrid energy cell (TENG+DSSC) can lead to the high device output performance. From the obtained results, the hybrid energy cell is expected to simultaneously/individually convert the mechanical, solar, and wind energies into electricity efficiently. Supporting Information. Optical transmittance of the MP-PDMS and entire TENG devices, IPCE spectra of bare DSSC and DSSC with TENG devices, schematic diagram of hybrid energy cell, output current of the DSSC and hybrid cell, photographs related to hybrid energy cells, and SEM images of ZnO NWs after the TENG device was tested for 1000 repetitive pushing cycles. The movie file (Video 1) was also attached to clarify the simultaneous/individual enery harvesting behaviour of hybrid energy cell. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author *Email: [email protected] (Prof. J. S. Yu)

ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013R1A2A2A01068407).

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Table Table 1. Device characteristics of the bare DSSC, the DSSCs with HNMA-PDMS and TENG devices with and without ZnO NWs as a cover layer. a

DSSC (cover-layer)

VOC [V]

JSC [mA cm−2]

FF [%]

PCE [%]

Bare

0.70072±0.001b 16.98030±0.23 61.8609±1.03 7.3605±0.03

HNMA-PDMS

0.70073±0.001

17.85473±0.19 61.0843±1.06 7.6425±0.05

TENG without ZnO NWs

0.68396±0.002

12.75549±0.22 65.0180±1.01 5.6723±0.02

TENG with ZnO NWs

0.69898±0.002

13.31417±0.18 65.0851±1.05 6.0571±0.06

a

For each case, more than five DSSC devices were fabricated in the same fabrication facilities and all the DSSCs were characterized by solar simulator under 1-sun AM1.5G illumination. b Mean value ± standard deviation.


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Figures

Figure 1. Schematic diagrams of (a) the fabrication procedure for the HNMA-PDMS using an AAO/MP-Si mold via the SIL technique, (b) the arch-shaped TENG integrated with the HNMAPDMS and ZnO NWs, and (c) the hybrid energy cell consisting of TENG and DSSC.


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Figure 2. (a) Top-view and cross-sectional SEM images of the (i) AAO/MP-Si mold and (ii) HNMA-PDMS. (b) SEM images of the ZnO NWs on ITO/PET. (c) XRD patterns of ZnO NWs on ITO/PET. The insets of (a) and (b) also show the high-magnification SEM images of the corresponding samples.


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Figure 3. Measured total transmittance spectra of the (a) flat PDMS and HNMA-PDMS on ITO/PET, and (b) ZnO NWs on ITO/PET, in comparison with the bare ITO/PET. The insets of (a) and (b) also show the measured total reflectance spectra of the corresponding samples. (c) Photographic images of the flat PDMS and HNMA-PDMS on ITO/PET substrates and the water droplet on the surface of the corresponding samples.


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Figure 4. (a) Schematic diagrams for operating mechanism of the triboelectric nanogenerator combined with the HNMA-PDMS as the bottom electrode and the vertically-aligned ZnO NWs as the top electrode. Measured output voltage and current of the HNMA-PDMS based TENGs (b) without and (c) with ZnO NWs as the top electrode.


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Figure 5. (a) Photographic images for the pushing test for the TENG device. (b) J-V characteristics of the bare DSSC, the DSSCs with HNMA-PDMS and TENG devices (with and without ZnO NWs) as a cover layer. The schematic diagram of the DSSC laminated with the TENG is also shown in the inset of (b). (c) Output voltage of the TENG device, after rectification. Output voltage of the hybrid energy cell by combining the (d) one or (e) four DSSCs and a TENG device in series to simultaneously/individually harvest the solar, mechanical, and wind energies.


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a

3

30

2

10

Current (µA)

Voltage (V)

20

0 -10 0.5 Hz

-20 -30

1 Hz

2 Hz

20

40

60

1 0 -1 -2

80

1 Hz

0.5 Hz

> 3 Hz

External pushing force: 0.2-0.5 kgf

0

100

b

2 Hz

External pushing force: 0.2-0.5 kgf

0

10

20

30

Time (sec)

40

50

60

> 3 Hz

70

80

90 100

Time (sec)

25

3

20 2

15

Current (µA)

10 5 0 -5

External pushing force: 0.2-0.5 kgf External pushing frequency: 0.5 Hz

1

10

-2

100

External pushing force: 0.2-0.5 kgf External pushing frequency: 0.5 Hz

1

1000

10

Average of peak Current (µA)

Time (sec)

c

2.4

0.7

2.0

0.6

1.6

0.5 0.4

1.2

0.3 0.8

0.2

0.4

0.1 External pushing force: 0.2-0.5 kgf External pushing frequency: 0.5 Hz

0.0 0

100

1000

Time (sec)

40

80

120

160

0.0

2

-20

0

-1

-10 -15

1

Power Density (mW/m )

Voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

Load Resistance (MΩ)

Figure 6. (a) Measured output voltage and current curves under different pushing frequencies from 0.5 to 3 Hz at 0.2-0.5 kgf of external pushing force. (b) Reliability of the device was tested by measuring the output voltage and current of the TENG for ∼ 1000 repetitions of external pushing. (c) Average output peak current and power density values of the TENG device, depending on the load resistance under the applied external pushing force of 0.2-0.5 kgf at the cyclic pushing frequency of 1 Hz.


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Table of Contents (TOC)


34

Micro ... - ACS Publications

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