Nano Energy 36 (2017) 366–373
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Scalable, anisotropic transparent paper directly from wood for light management in solar cells
MARK
Chao Jia1, Tian Li1, Chaoji Chen1, Jiaqi Dai, Iain Michael Kierzewski, Jianwei Song, Yiju Li, ⁎ Chunpeng Yang, Chengwei Wang, Liangbing Hu Department of Materials Science and Engineering, University of Maryland College Park, College Park, MD, 20742, USA
A R T I C L E I N F O
A BS T RAC T
Keywords: Anisotropic Transparent paper Alignment High haze Solar cell Flexible electronics
The growing demand for flexible electronics and solar energy conversion devices has fueled a search for highquality paper-based materials with excellent mechanical flexibility and optical properties such as high transparency and haze. Despite the tremendous efforts have been dedicated to developing paper-based materials with high transparency or high haze, challenges still remain in achieving both due to the general exclusivity between them. Here, for the first time, we develop a novel anisotropic paper material possessing high mechanical flexibility and fantastic optical properties with both high transmittance (~90%) and high haze (~90%) simultaneously via a simple yet effective “top-down” approach by directly shear pressing the delignified wood material. The anisotropic transparent paper demonstrates a high efficiency as a light management coating layer for GaAs solar cell with a significant efficiency enhancement of 14% due to its excellent light management capability with both high transparency and high haze. The presented “top-down” approach is facile, scalable, cost-effective and “green”, representing a promising direction for developing flexible electronics, solar energy conversion devices and beyond.
1. Introduction Substrate materials with excellent mechanical flexibility, optical properties and biodegradability are urgently needed for the developing of flexible electronics and solar energy conversion devices [1–6]. Plastic as transparent flexible substrate has been ubiquitously used in electronic and optoelectronic devices due to their favorable mechanical properties, simple process technology, light weight and low cost [7–13]. However, plastic is neither biodegradable nor renewable, causing long-term sustainability concerns. Recently, cellulose-based transparent paper has drawn increased attention as an emerging flexible substrate due to its high biodegradability and renewability [2,3,14,15]. Cellulose-based transparent paper is generally fabricated through a “bottom-up” approach involving multiple steps - disintegrating the cellulose fibers using mechanical [16,17], chemical [18–21] or biological [22,23] methods, dispersing into solution and then reconstructing into transparent paper. Despite the advantages of the “bottom-up” approach including fine control of the cellulose structure and hybridizing with various additional components, the multi-step fabrication process would lower the fabrication efficiency, increase the cost, making it less competitive for large-scale applications [24–27].
⁎
1
Corresponding author. E-mail address:
[email protected] (L. Hu). These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.nanoen.2017.04.059 Received 17 March 2017; Received in revised form 30 April 2017; Accepted 30 April 2017 Available online 01 May 2017 2211-2855/ © 2017 Published by Elsevier Ltd.
In addition to mechanical flexibility and biodegradability, light management capability is also vital for flexible electronics and solar energy conversion devices, especially for thin film solar cells [28–31]. High broadband transmittance and haze are highly desirable for light management coating layer used in thin film solar cells, where both the absorption and utilization of sun light can be maximized [32–35]. However, achieving both high transmittance and high haze in a single material still remains a challenge due to the general exclusivity between transmittance and haze. Increasing the transmittance usually causes the sacrifice of haze. So far, only limited successes have been gained in achieving both high transmittance and haze in a single material [36]. Here, for the first time, we develop a simple yet effective “top-down” approach for fabricating an anisotropic flexible paper with both high transparency and high haze by directly shear pressing the pre-delignified wood material. The anisotropic wood-derived paper exhibits great potential as an efficient light management coating layer for GaAs thin film solar cells, demonstrated by a significant enhancement of 14% in solar energy conversion efficiency and 18% in short circuit density. The “top-down” approach presented here is facile, scalable, cost-effective, and “green”, representing an attractive direction for developing next-generation flexible electronics and solar energy conversion devices.
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Fig. 1. Graphical illustration of the design concept and “top-down” fabrication process for the anisotropic transparent paper directly from natural wood.
radial wood without delignification treatment. The original radial wood is yellowish (inset in Fig. 3a) due to the existence of lignin. Multiple channels with different sizes along the tree growth direction can be observed. After delignification treatment, the color of the wood sample become white and the structure become more porous due to the removal of lignin (Fig. S2). In addition to the shear pressing method, we have also performed vertical pressing as control experiments to investigate the influence of pressing method in the structure of the resultant paper. Vertical pressing of the delignified radial wood results in isotropic transparent paper, where the cellulose fibers are randomly distributed (Fig. 3d-f). By vertical pressing, the vertically aligned channels are completely crushed into a dense isotropic transparent paper. On the contrary, the alignment of cellulose fibers can be well preserved by shear pressing (Fig. 3g-i and Fig. S3), which can be observed both on the top surface and cross section of the anisotropic transparent paper. The anisotropic microstructure leads to an interesting anisotropic light management capability. A single mode green laser with collimated light spot was perpendicularly incident on the transparent paper with a spot size of around 200 µm. For the isotropic transparent paper, the scattering pattern is circular owning to the random fiber orientation (Fig. 4a). On the contrary, a large divergence angle for anisotropic transparent paper where cellulose fibers are aligned is observed across the alignment direction due to the light diffraction. Consequently, the transmittance scattering pattern is elliptical for the anisotropic transparent paper (Fig. 4d). The scattered light intensity distributions in the x and y directions are shown in Fig. 4b and e for isotropic transparent paper and anisotropic transparent paper, respectively. It can be observed that the scattering intensity of isotropic paper in x and y directions at different scattering angle is similar, which can be attributed to the random distribution of cellulose fibers (Fig. 4b). However, for the anisotropic paper, lower refractive index fluctuation in the y direction was obtained due to the aligned cellulose fibers in this direction (Fig. 4e). Small angle X-ray scattering (SAXS) was further employed to confirm the anisotropy of shear pressed transparent paper. From Fig. 4c we can see that the obtained SAXS pattern for vertically pressed transparent paper is uniform annulus, suggesting that cellulose fibers in the vertically pressed paper are randomly distributed with an isotropic feature. Unlike the vertically pressed paper, the SAXS pattern of the shear pressed transparent paper is an asymmetric annulus, confirming that the cellulose fibers in the shear pressed paper are orientated along with the shear press direction. In addition, the polarization effect of the anisotropic transparent paper was further evaluated by a polarizing microscope from Olympus
2. Results and discussion Wood is one of the most abundant resources on earth and widely used in green electronics, biological devices, energy storage and cellulose industry [37–39]. Here we used Basswood as the starting material to directly fabricate anisotropic transparent paper (see Experimental for synthesis details). Note that different types of wood possess similar anisotropic microstructures, so our "top-down" approach can also be used to prepare anisotropic transparent paper from other kinds of wood. Fig. 1 graphically illustrates the design concept and “top-down” fabrication process of the anisotropic transparent paper. Typically, the Basswood was first cut into slices vertically to its growth direction, then treated with NaClO solution to remove the lignin, resulted in the delignified wood with white color. The delignified wood was then shear pressed into paper by rolling a glass rod upon the delignified wood slice with a constant pressure. Compared with the traditional “bottom-up” paper fabrication process, the “top-down” process demonstrated here is quite straightforward, less time-consuming and more cost-effective. It is worth noting that the intrinsic alignment of cellulose fibers in natural wood can be well inherited by shear pressing, resulting in an anisotropic structure of the wood-derived paper. More attractively, the wood-derived paper is super flexible, highly transparent yet with high haze, holding great promise for solar cell application. The delignification efficiency is a major factor that determines the total efficiency of the whole “top-down” process (shear pressing is not the efficiency-determined step for this process is very quick, usually finished in a few minutes). Based on this consideration, we carried out control experiments to investigate the delignification efficiency by using vertically-cut (radial) wood and parallelly-cut (longitudinal) wood slices. Fig. 2a displays the schematic of the delignification process of the radial and longitudinal wood slices. Compared with longitudinal wood, radial wood has shorter channels, making it easier for the NaClO solution to enter the wood channels, and the decomposed products to transport into the outside mother solution. This can be confirmed by the color evolutions of the two samples during the delignification process recorded by digital camera (Fig. 2b and Fig. S1). The color of the wood slice gradually changed from yellow to white with increasing delignification time, indicating that the content of lignin diminished gradually. The radial wood slice became completely white in about three hours, which is 3 times shorter than that of the longitudinal wood (Fig. 2c), suggesting the high delignification efficiency of radial wood. The morphologies and microstructures of the original wood and derived paper materials are characterized by scanning electron microscopy (SEM). Fig. 3a-c shows the photo and SEM images of the original 367
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Fig. 2. Comparison of delignification from radial wood and longitudinal wood. (a) Schematic to show the lignin removal from radial wood and longitudinal wood, respectively. (b) Comparison of delignification process from radial wood and longitudinal wood using NaClO as the lignin removal chemical at ambient temperature. The wood color changes from yellow to white gradually with increasing delignification time. For fair comparison, both the radial wood and longitudinal wood have the same dimension of 44.5 mm×44.5 mm×0.8 mm. (c) Comparison of delignification time.
transmittance and high haze (Fig. S6). Fig. 5c plots the transmittance and haze of our anisotropic transparent paper compared with other haze papers from cellulose fibers [1,14,40,41] and some common plastics [42,43]. The haze of our anisotropic transparent paper is about 90%, much higher than that of other transparent substrates. The high transmittance along with the high haze renders our anisotropic transparent paper highly suitable as an efficient light management coating layer for thin film solar cells. GaAs solar cell has been intensively studied for the flexibility owning to the fact that its active absorption layer is typically less than 3 µm. Traditional dielectric antireflection layer is often brittle, largely restricting the flexible or wearable application of solar cells. In this work, we demonstrate for the first time the integration of anisotropic paper with both high transparency and high haze with potentially flexible solar cell device. Here the anisotropic transparent paper used for light management coating layer of solar cell has a thickness of about 100 µm. The current densityvoltage (J-V) curves of the GaAs solar cell without and with our anisotropic transparent paper coating layer are given in Fig. 5e. The electrical properties of the solar cell, including open circuit voltage (VOC), short circuit density (JSC), fill factor (FF, the maximum output power divided by the product of VOC and JSC) and the total conversion efficiency are obtained from the J-V curves, as shown in Table 1. The short circuit density increases by 18% for the solar cell with anisotropic transparent paper coating layer in contrast to the original GaAs solar cell. For the total conversion efficiency, a 14% enhancement is achieved by utilizing the anisotropic transparent paper as a light management coating layer. The forward scattering effect and index matching effect between air and GaAs can be used to explain these obtained results. On one hand, the transparent paper coating as an index matching layer reduces the light reflection, and more light can be forward guided through the coating layer to the surface of GaAs solar cell. On the other hand, the high haze of the transparent paper coating layer results in the diffusion of the incident light, thus the photon travelling path is extended and the light trapping possibility is improved, similar to the light management layers utilizing nano-sized patterning [32–35]. The excellent properties of the prepared anisotropic transparent paper will
(Fig. 4g). The polarization effect can be ascribed to the alignment of cellulose fibers in the anisotropic transparent paper. In comparison, the isotropic transparent paper did not exhibit this effect due to the random cellulose fiber orientation. Our developed anisotropic transparent paper directly made from shear pressing delignified radial wood exhibits a controllable manner of light management, which can function as a light diffuser with high optical transmittance where the distribution of light can be modulated. Large directional scattering was realized which can help cast an evenly distributed light along a desirable direction or following a desirable shape. In addition, the anisotropic transparent paper demonstrated a multi-scale alignment from molecular level crystalline cellulose chain alignment to microscale level fiber bundle alignment via a simple shear pressing process. The excellent optical properties from cellulose fiber alignment was demonstrated for the first time towards highly transparent cellulose paper with various optical functionalities, paving the way to incorporate more advanced photonic and optoelectronic applications. We also quantitatively investigated the optical properties of the anisotropic transparent paper, and determined the light transmittance and haze using a UV–vis spectrophotometer with an integrated sphere. From Fig. 5a and b we can see that the anisotropic transparent paper has very high light transmittance and haze. The light transmittance slightly increases with the wavelength, which can be up to ~90% when the wavelength is 800 nm. The inset in Fig. 5a and Fig. S4 show the anisotropic transparent paper placed directly on the substrate with colored words. When the anisotropic transparent paper is in contact with the substrate, the colored words on the substrate can be clearly seen, which indicates that the prepared transparent paper possesses a high light transmittance. Interestingly, the anisotropic transparent paper also exhibits a very high haze in the wavelength range of 400–800 nm. The haze can reach up to 93% at the wavelength of 400 nm and slightly decreases with the increasing wavelength. The inset in Fig. 5b and Fig. S5 display the anisotropic transparent paper placed 10 mm above the substrate with colored words. The colored words on the substrate become very fuzzy, indicating a high haze of the as made anisotropic transparent paper. Note that the isotropic transparent paper also possesses high 368
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Fig. 3. Microstructure characterizations of original radial wood, isotropic transparent paper and anisotropic transparent paper. (a) Top-view and (b, c) cross-section SEM images of original radial wood, inset in a shows the digital photo image of original radial wood. (d) Top-view and (e, f) cross-section SEM images of isotropic transparent paper, inset in d shows the digital photo image of isotropic transparent paper. (g) Top-view and (h, i) cross-section SEM images of anisotropic transparent paper, inset in g shows the digital photo image of anisotropic transparent paper.
open up new opportunities incorporating flexible paper substrate with anisotropic light management capabilities to optoelectronic devices.
4. Experimental section 4.1. Materials and chemicals Basswood was purchased from Walnut Hollow Company. Sodium hypochlorite solution (NaClO, 5%, Laboratory Grade) from Carolina Biological Supply Company was used to remove the lignin in Basswood. Membrane filters (0.65 µm DVPP) were provided by EMD Millipore Corporation. Ethanol (Pharmco-Aaper) and deionized (DI) water were employed to wash the delignified Basswood.
3. Conclusion In conclusion, we have developed a simple, scalable, and efficient way to fabricate anisotropic flexible paper with both high transparency and high haze by directly shear pressing the delignified wood. The delignification of the radial wood shows a high fabrication efficiency whereas the shear pressing method contributes to a good alignment of cellulose fibers in the resultant wood-derived paper. The structural anisotropy enables the transparent paper a unique anisotropic light scattering and polarization effect. In addition, the anisotropic flexible paper demonstrates both high transmittance (~90%) and high haze (~90%), representing one of the highest values among all transparent papers. A 14% enhancement in the total energy conversion efficiency and 18% improvement in the short circuit density for GaAs solar cell can be achieved by simply utilizing the anisotropic transparent paper as a light management coating layer due to the effective light scattering and improved light absorption enabled by the high transparency and high haze. Moreover, the “top-down” approach for preparing anisotropic transparent paper is facile, scalable, cost-effective and “green”, which will open new avenue to the development of flexible electronics, solar energy conversion devices and other applications involving flexible substrates.
4.2. Lignin removal from wood The wood slices were immersed in the NaClO solution with a concentration of 5 wt% and the reaction was carried out at ambient temperature until the wood slices became white completely. The mass ratio of Basswood and NaClO solution was 1:60. After that, the delignified white wood slices were rinsed in ethanol water solution (50 wt%) and washed for three times to remove the remaining chemicals. 4.3. Preparation of anisotropic transparent paper The delignified radial wood slice was placed on a PET film, and covered with a microporous membrane filter. The wood fibers were forced down to one direction by rolling a glass rod which was placed upon the wood slice with a constant pressure. All wood fibers in the wood slice were 369
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Fig. 4. Optical properties of isotropic transparent paper and anisotropic transparent paper. (a, d) The images of the scattered light spot for isotropic transparent paper and anisotropic transparent paper, respectively. (b, e) The scattered light intensity distributions in the x and y directions as shown in (a) and (d), respectively. (c, f) SAXS patterns of isotropic transparent paper and anisotropic transparent paper to confirm the isotropy of vertically pressed transparent paper and anisotropy of shear pressed transparent paper. (g) Polarizing microscope images of isotropic transparent paper (top) and anisotropic transparent paper (bottom). The polarization effect of the anisotropic transparent paper was demonstrated. The red arrows were used to show the cellulose fiber alignment direction.
4.4. Attachment of transparent paper on GaAs solar cell
attracted together due to the strong hydrogen bonding and formed an aligned wood slice. After that, the aligned wood slice was covered by some pieces of filter papers and pressed at room temperature for 5 h. After drying, the anisotropic paper with high transmittance and haze was obtained. For comparison, the isotropic paper was prepared by vertically pressing the delignified radial wood slice without the shear pressing step.
A drop of ethanol was dripped on the surface of prefabricated GaAs solar cell, and then the anisotropic transparent paper was firmly attached on the solar cell and dried.
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Fig. 5. Light transmittance and haze of the anisotropic transparent paper, and its application in GaAs solar cell. (a) The total transmittance of anisotropic transparent paper. Inset shows the transparent paper was placed directly on the substrate with colored words. (b) The haze of anisotropic transparent paper. Inset shows the transparent paper was placed 10 mm above the substrate with colored words. (c) Optical haze versus transmittance of our anisotropic transparent paper, other transparent papers from cellulose fibers [1,14,40,41] and some common plastics [42,43]. The haze of our transparent paper is about 90%, which is much higher than other substrates. (d) Schematic of the incident light distribution on solar cell. (e) Current density-voltage curves of GaAs solar cells without and with our anisotropic transparent paper coating layer.
integrated sphere (Lambda 35, Perkin Elmer, USA). The polarization effect of the transparent papers was characterized by a polarizing microscope. Current density-voltage curves of GaAs solar cells were measured by a computer controlled source meter (2400 Keithley) under illumination of Orel Solar Simulator (AM 1.5, 100 mW/cm 2) with a scan rate of 10 mV/s.
Table 1 Comparison of electrical properties of bare GaAs solar cell and the GaAs solar cell with our anisotropic transparent paper coating layer.
Bare GaAs solar cell GaAs cell with anisotropic transparent paper coating layer Enhancement (%)
Voc (V)
Jsc (mA/ cm2)
FF (%)
Efficiency (%)
0.904 0.907
17.08 20.17
75.06 76.05
12.21 13.94
0.33
18.09
1.32
14.17
Author contributions L. Hu, C. Jia, T. Li and C. Chen designed the experiments and analyzed the data. C. Jia, C. Wang and C. Chen contributed to the transparent papers preparation. T. Li, J. Song and I. Kierzewski carried out the optical property determination. J. Dai drew the schematics. Y. Li and C. Yang did the SEM characterization. All authors contributed to the manuscript writing.
4.5. Characterizations The morphologies of the products were determined using a field emission scanning electron microscopy (FESEM, HITACHI SU-70). The light scattering was determined by a 532 nm single mode laser DJ532-10 (Thorlabs Inc.) with stabilized output power as the incoming light source. The scattered light distribution in the 2D plane vertical to the light propagation direction was measured by a photodiode power sensor S130C from Thorlabs. Small angle X-ray scattering (SAXS) measurements were made using a Xenocs Xeuss SAXS/WAXS/GISAXS small angle X-ray scattering system. The light transmittance spectrum and haze was determined using a UV–Vis spectrophotometer with an
Acknowledgement Chao Jia would like to acknowledge the China Scholarship Council (CSC) for financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.nanoen.2017.04.059. 371
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Chao Jia received the B.S. degree in Packaging Engineering from Agricultural University of Hebei, Hebei, China in 2010. He obtained the M.S. degree in Mechanical Engineering from Jiangnan University, Jiangsu, China in 2013. He is currently a Ph.D. candidate in Materials Science and Engineering at Beijing Institute of Technology. From 2015 to 2017, he is an exchange Ph.D. student under the supervision of Prof. Liangbing Hu at University of Maryland College Park. His research interests include flexible electronics, nanomaterials and energy conversion.
Tian Li received the B.S. degree in Electrical Engineering from Huazhong University of Science and Technology, Hubei, China in 2010, and Ph.D. degree in Electrical and Computer Engineering from University of Maryland, College Park, USA, in 2015. She is currently a Postdoctoral Research Scholar with Dr. Liangbing Hu in University of Maryland, College Park, MD, USA. Her research interests include light and thermal energy harvesting and management. She received the ECE Distinguished Dissertation Fellowship and Outstanding Graduate Assistant Award in 2015 for the recognition of her Ph.D. work.
Chaoji Chen received his B.S. (2010) and Ph.D (2015) degree in Materials Science and Engineering from Huazhong University of Science and Technology (HUST), P.R.China. He is currently a postdoctoral researcher in HUST. His research focuses on nanomaterials for energy storage and water treatment.
Jiaqi Dai received his B.S degree in Materials Science and Engineering from Harbin Institute of Technology (2013), China. He is currently a Ph.D. candidate in materials engineering under the supervision of Prof. Liangbing Hu at University of Maryland, College Park, USA. His research mainly focus on nanotechnologies, advanced energy storage devices, and scientific visualizations.
Iain M. Kierzewski received the B.S. degree in Materials Science and Engineering from the University of Maryland (UMD), College Park, in 2012. He is currently pursuing a Ph.D. in Materials Science and Engineering at UMD. Since 2013, he has been a process engineer at the Adelphi Lab Center working on MEMS power components. His research interests include magnetic, electronic, and bio-inspired materials for energy harvesting applications.
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C. Jia et al. Jianwei Song is now a Ph.D. candidate in School of Light Industry and Engineering at South China University of Technology, and is currently an exchange Ph.D. Candidate in Department of Materials Science and Engineering at University of Maryland. His current research focuses on biomass for functional and structural materials.
Chengwei Wang received his B.S. (2011) from University of Science and Technology of China (USTC), P.R.China and Ph.D (2015) from Arizona State University in Materials Science and Engineering. He is currently an Assistant Research Scientist at Maryland University, College Park. His research focuses on solid state batteries and nanomaterials for ionic devices.
Yiju Li received his B.S. degree from Harbin Engineering University in 2013. He is currently an exchange Ph.D. student in University of Maryland, College Park. His research focuses on nanomaterials for electrochemical energy storage and conversion.
Liangbing Hu received his B.S. in applied physics from the University of Science and Technology of China (USTC) in 2002. He did his Ph.D. at UCLA, focusing on carbon nanotube based nanoelectronics. In 2006, he joined Unidym Inc as a co-founding scientist. He worked at Stanford University from 2009 to 2011, where he work on various energy devices based on nanomaterials and nanostructures. Currently, he is an associate professor at University of Maryland College Park. His research interests include nanomaterials and nanostructures, flexible and printed electronics, energy storage and conversion, and roll-to-roll nanomanufacturing.
Chunpeng Yang received his Ph.D. degree from University of Chinese Academy of Sciences in 2016 and B.S. degree from University of Science and Technology of China in 2011. He is currently a postdoctoral researcher in the group of Liangbing Hu in University of Maryland at College Park. His research focuses on materials for advanced energy-storage systems, such as lithium metal batteries and solid-state batteries.
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