Energy storage and solar cells devices printed on paper: an exciting opportunity

Paper is a fascinating material: it is abundant, harmless, cheap and useful. Humanity has relied on paper for storing information for centuries. The new challenge posed for paper consists of its future use as a substrate for organic electronic devices. This is one of the pivotal developments for paper uses, that will enable cheap and more environmentally friendly electronics. The added value of accessing portable, bendable and lightweight devices represents also a novelty and opportunity.

The paper “Printed Solar Cells and Energy Storage Devices on Paper Substrates by Francesca Brunetti, Alessandra Operamolla, Sergio Castro‐Hermosa, Giulia Lucarelli, Valerio Manca, Gianluca M. Farinola and Thomas M. Brown¹ was published in Advanced Functional Materials at the beginning of the year and represents a comprehensive review on the state-of-art research on printed photovoltaic devices or batteries and supercapacitors on paper and paper-like substrates. The paper is a contribution from researchers from the University of Rome Tor Vergata and from the University of Bari Aldo Moro, who are involved for years in the topic of light-harvesting devices, photovoltaic conversion, and energy storage devices. Issues as device stability and active layers deposition strategies are critically assessed, reporting the most significative examples from the literature.

In particular, the authors try to trace a future perspective on the true possibilities of recycling devices deposited on paper, pointing at the current lack of strategy and technology due to a very fragmentary and not-focused research effort.

The review was commented in an article by Michael Berger that appeared in the magazine NanoWerk.

Schermata 2019-07-09 alle 23.48.29

References. ¹ F. Brunetti, A. Operamolla, S. Castro-Hermosa, G. Lucarelli, V. Manca, G. M. Farinola, T. M. Brown, Advanced Functional Materials, 2019, 29 (21),1806798.

Copyrights of this article belong to Alessandra Operamolla. All rights reserved.

About “Haze” in cellulose nanopaper

The opaqueness of standard paper originates from the presence of empty spaces among fibers: the presence of air and other fillers yields a material with a discontinuous refractive index. This causes scattering of light in the backward direction and is the reason why we cannot look through a sheet of paper. This limit represents, actually, one of the paper’s most important properties: it is relying on the opaqueness of paper that we are using it for centuries as a substrate for the transmission of knowledge. We cannot be disturbed by the object behind a page of a book or a document, therefore we can read it carefully and undisturbed.

On the contrary, transparency in cellulose nanopaper is one of its most intriguing properties. It arises from the reduced interstice dimension between fibers: the more homogeneous diffraction index deriving from nanopaper composition will minimize backscattering in favor of transmission of light in the forward direction.

In addition to transparency, cellulose nanopaper often presents another fundamental effect, defined as “haze”. Haze is defined as the ratio between the light transmitted through nanopaper (deviated from the incident light beam of an angle above 2.5°) and the total transmitted light intensity. A high haze allows light coupling and responses not depending on the angle of the incident light.1 Haze is tunable on the dependence of the dimension of the nanocelluloses chosen to produce the paper. This property is considered particularly beneficial when the active layer of a solar cell is deposited on cellulose nanopaper since haze can increase the path length of light in the light-harvesting layers.2 The following figure shows in panel a) a representation of what the haze effect consists of. The panel b) shows a nanopaper 1cmx1cm sample placed on a periodic table: transparency, in this case, is the relevant property. The panel c) shows the same nanopaper sample lifted from the surface: as far as the distance with the object behind the nanopaper increases, the haze manifests its effects.

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Figure 1. a) Schematic description of the haze effect on nanopaper. b) Transparency in nanopaper is particularly observed when an object is placed close to the nanopaper. c) Haze in nanopaper is observed on far objects.

References. 1 Y. Yao, J. Tao, J. Zou et al., “Light management in plastic-paper hybrid substrate towards high-performance optoelectronics,” Energy & Environmental Science, 2016, 9, 2278–2285. 2 Z. Fang, H. Zhu, W. Bao, et al., “Highly transparent paper with tunable haze for green electronics,” Energy & Environmental Science, 2014, 7, 3313–3319.

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The solarleaf project publishes outcomes on cellulose nanopaper modification on the journal Soft Matter

It was released today as a Just Accepted Manuscript the article “Tailoring water stability of cellulose nanopaper by surface functionalization”. The publication is a study on cellulose nanopaper topochemical functionalization, attained via dipping the nanopaper in an organic solution of acyl chlorides.

The surface chemistry of cellulose nanocrystals represents a hot topic in the nanocelluloses research.

You can access the article at this link.

THIS ARTICLE IS PART OF AN ACTION OF DISSEMINATION OF KNOWLEDGE AND RESULTS OF THE PROJECT SOLARLEAF, CO-FOUNDED BY COHESION AND DEVELOPMENT FUND 2007-2013 – APQ RESEARCH PUGLIA REGION “REGIONAL PROGRAMME SUPPORTING SMART SPECIALIZATION AND SOCIAL AND ENVIRONMENTAL SUSTAINABILITY – FUTUREINRESEARCH”, PROT. F6YRAO1.

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Cellulose nanopaper as potential substrate for printed electronics

An optically transparent paper is appealing for a series of applications and can be attained by processing cellulose nanocrystals by standard paper-making procedures. The nano-dimensioned cellulose crystals produce a thin film with suppression of light scattering, thanks to the reduced dimension of interstices among nanofibers.

The nanofibers can be obtained by a top-down approach from various sources, including wood pulp. After removal of lignin and hemicellulose, the wood pulp can be processed by mechanic treatment or acid digestion. The process has the effect of removing the amorphous cellulose region, yielding nanocrystals with regular dimensions. The cartoon reported in Figure 1 represents how the process of acid hydrolysis can remove the amorphous cellulose and yield crystalline cellulose nanorods. The dimension, molecular weight and other properties of cellulose nanofibers are directly dependent on the nature of the pristine cellulose source (i.e. wood pulp, cotton, flax, hemp etc.)

idrolisi

Fig. 1: Acid hydrolysis of cellulose.

Cellulose nanocrystals with 150 nm length were used by M. Nogi et al.1 to prepare transparent nanopaper. The nanofibers were isolated from wood flour previously subjected to removal of lignin and hemicellulose. A preparation procedure adapted from the paper industry, including mechanical compression under vacuum at a pressure of 160 MPa, was used to produce high-quality nanopaper with limited water and voids content. A surface polishing process was also carried out to improve the transparency and aesthetic of the nanopaper. A powerful comparison of the nanopaper with a piece of filter paper is shown in Figure 2. Noteworthy, the scanning electron microscopy micrographs of the two samples show likewise morphologies with the main difference in the scale-bar: while appreciation of a normal paper sheet morphology is possible with a 50X magnification, revealing the presence of interwoven fibers with average length of 30 μm, the nanopaper demands for a higher magnification, down to the nanometer scale, to appreciate the presence of tinier nanorods that reveal reciprocal interconnection by their long dimension. The standard paper usually presents voids that must be leveled with fillers or adhesives. Nanopaper presents much more film-like surface morphology, with a very smooth and defect-free surface.

adv. mater fig. 1

Fig. 2: Nanopaper (left) and paper (right) comparison. Optically transparent paper (left) is composed by 150 nm cellulose nanofibers. The upper left inset, with a scale bar of 100 nm, shows a SEM micrograph of the surface of transparent nanopaper. Conventional cellulose paper (right) is composed by 30 μm pulp fibers. The upper right inset, with a scale bar of 200 μm, shows the SEM micrograph of the surface of a sample of filter paper. Reproduced from Advanced Materials, Ref. 1, with permission from John Wiley and Sons.

With such a nice smooth surface morphology, deposition of organic material on top of nanopaper is possible. The limited presence of voids gives a valuable opportunity to deposit high quality organic thin films and opens the way to the fabrication of organic devices deposited on transparent nanopaper. Figure 3 shows thin films of organic conductive polymers deposited on nanopaper, as performed by spin coating in our laboratories at the Chemistry Department of the University of Bari.

spin cast

Fig. 3: PEDOT:PSS (left) and a polythiophene-based organic polymer (right) spin cast on nanopaper (supported on a glass slide)

References: 1 M. Nogi, S. Iwamoto, A. N. Nakagaito, H. Yano, “Optically Transparent Nanofiber Paper” Adv. Mater. 2009, 21, 1595–1598, DOI: 10.1002/adma.200803174

THIS ARTICLE IS PART OF AN ACTION OF DISSEMINATION OF KNOWLEDGE AND RESULTS OF THE PROJECT SOLARLEAF, CO-FOUNDED BY COHESION AND DEVELOPMENT FUND 2007-2013 – APQ RESEARCH PUGLIA REGION “REGIONAL PROGRAMME SUPPORTING SMART SPECIALIZATION AND SOCIAL AND ENVIRONMENTAL SUSTAINABILITY – FUTUREINRESEARCH”, PROT. F6YRAO1.

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Ashby plots help comparing properties and potentialities of natural materials

Evaluation of materials properties for a selected application necessarily requires a method to compare different material sources. This is particularly true in the case of natural materials, those biocompatible and renewable constituents that can be extracted from biologic tissues.

Natural materials often come in the form of composites. Just think, for instance, about a human hair, which is composed of nanostructures of the protein keratin. Another even more pertinent example is given by the bone tissue, a nanocomposite formed by the inorganic hydroxyapatite and the organic protein collagen. It is clear that the combination of the two components, self-assembled with a precise macrostructure, endows bone tissues with the necessary properties for their function. As well, when materials are composites deriving from nano-objects, the dimension is a parameter that we need to consider in evaluating their potential performances.

A powerful tool for helping in materials selection is offered by the Ashby plot1. This is a scatter plot displaying at the same time two properties of materials or classes of materials. It is convenient because it gives useful information not only on which material displays the highest (or the lowest) property reported on the x- or y-axes, but also which one presents the highest ratio between the two properties. It is useful, in addition, to compare properties values with relation to material dimensions or density and offers also the possibility to condense a large body of information into a compact, but accessible form.

A three-dimensional Ashby chart in which the third dimension is represented by density can be easily transposed into a bi-dimensional scatter plot by normalizing by density the other two properties. This is the case for the chart reproduced in the Figure 1a, with permission from Springer Nature. The two properties normalized by density are the specific strength (y-axis, fracture toughness normalized by density) and the specific stiffness (x-axis, Young’s modulus divided by density). The ratio strength/stiffness represents the strain at which the material ceases to be linearly elastic and signifies the “mechanical efficiency”, meaning the use of the least mass of material to do the most structural work. The chart shows different regions for natural (pink, deep blue, green and orange) and synthetic (only in blue) materials. They are enclosed in envelopes because data for a given family of materials (polymers, for example) usually cluster together.

Ashby plot

Fig. 1: Ashby plots for natural and synthetic materials. a) specific properties normalized by density allow easy comparison with synthetic materials like polymers and ceramics; b) this chart shows how nanocomposites can display dramatically improved properties with respect to the starting building blocks. Reproduced from Nature Materials, Ref. 2, with permission from Springer Nature.

Looking at the chart, it is clear that synthetic materials can reach high values of strength and toughness. For instance, ceramics, Kevlar, and carbon fibers display values that are much higher than those of the best natural materials. This is commonly true, but with exceptions: for instance, silk can reach the extraordinary toughness of 1,000 MJ m–3 with a modulus of 10 GPa3, but also nanocelluloses can reach exceptional values, approaching those recorded for Kevlar4. These properties increase their overall appeal if one considers that all-natural structural materials use a limited chemical palette of inexpensive building blocks, the ones described above for human hair and bone tissue, whose individual properties are often meagre (for instance keratin or collagen): however, these building-blocks are typically arranged in a hierarchical architecture which is formed at ambient temperatures and displays enhanced properties. This is well represented in chart 1b: bone and nacre are natural composite materials with toughness values that far exceed those of their constituents (hydroxyapatite and collagen for bone, and aragonite and nacre protein for nacre). Their much higher fracture toughness is related to the microarchitecture of the final composite material. Generally speaking, composite materials dominate both charts: this justifies the wide interest towards these materials in the aerospace field and justifies our interest in natural nanostructured materials, including nanocelluloses.

References: 1 Michael F. Ashby, “Materials selection in mechanical design, Fourth Ed.”, 2011, Butterworth-Heinemann, Elsevier Ltd. 2 Ulrike G. K. Wegst, Hao Bai, Eduardo Saiz, Antoni P. Tomsia, “Bioinspired structural materials” Nature Materials, Volume 14, issue 1, 2014, 23-36, DOI: 10.1038/nmat4089. 3 J. M. Gosline, P. A. Guerette, C. S. Ortlepp, K. N. Savage, “The mechanical design of spider silks: From fibroin sequence to mechanical function” J. Exp. Biol., Volume 202, 1999, 3295-3303. 4 Robert J. Moon, Ashlie Martini, John Nairn, John Simonsen, Jeff Youngblood, “Cellulose nanomaterials review: structure, properties and nanocomposites” Chem. Soc. Rev., 2011, 40, 3941–3994.

THIS ARTICLE IS PART OF AN ACTION OF DISSEMINATION OF KNOWLEDGE AND RESULTS OF THE PROJECT SOLARLEAF, CO-FOUNDED BY COHESION AND DEVELOPMENT FUND 2007-2013 – APQ RESEARCH PUGLIA REGION “REGIONAL PROGRAMME SUPPORTING SMART SPECIALIZATION AND SOCIAL AND ENVIRONMENTAL SUSTAINABILITY – FUTUREINRESEARCH”, PROT. F6YRAO1.

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About cellulose

Formal discovery of cellulose, the structural material of plants, dates 1838 in the work of Anselme Payen,1 a French chemist also known for the discovery of the enzyme diastase. Cellulose technological importance is mainly connected to paper industry, even if cellulose derivatives, like rayon and cellophane, are part of our life for decades. Additional challenges are connected to industrial and academic research on the conversion of cellulose into biofuels or into other high-value products.

From the chemical point of view, cellulose is an organic polymer. Yes, organic, as every component of living cells and living organisms that are made of chemical compounds organized in a smart and a self-reproducing complex architecture. So, cellulose is a natural and renewable material, a natural polymer and a chemical compound.

Cellulose is a linear polymer of β-D-glucopyranoside units connected to each other via 1,4-O-glycoside bridges. (Fig. 1) Cellulose linearity is given by the equatorial disposition of the anomeric bonds. In addition, cellulose is unbranched. The further rigidity is conferred to the polymer chain by the existence of intramolecular H-bonds from the 3-OH groups to ring oxygen atoms. This intramolecular interaction is made possible by rotation of each repeating unit of approximately 180° with respect to the neighbors. An additional intramolecular H-bond interaction may be established between 2-OH groups and O(6). The linearity confers high propensity to establish further intermolecular H-bonds: this aspect is important, because during the biosynthesis, and depending on the conditions of biosynthesis, cellulose chains spontaneously aggregate into fibers. It is the cooperative formation of multiple interchain hydrogen bonds which is responsible for fibers formation and of cellulose scarce solubility.

schema cellulosa
Fig.1 Chemical structure of cellulose

Cellulose aggregates (fibers) produced by plants and bacteria are part of a well-organized hierarchical structure, possessing featureless as well as crystalline domains, kept together by multiple H-bonds. This particular self-assembled hierarchical structure, alternating amorphous and rigid crystalline domains, endows natural organisms with the necessary functionality, flexibility and mechanical strength/weight performances to carry out their biological function. In the final assembly, cellulose fibers are embedded in a matrix of featureless polymers. In most plants, the matrix contains amorphous cellulose as well as hemicellulose and lignin. In the following cartoon, different magnifications on the structure of a leave show the presence of a plant cell (tens of micrometers), a polymeric matrix, normally containing cellulose fibers and amorphous cellulose, hemicellulose and lignin, and the structure of a cellulose fibers bundle.

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1 Anselme Payen, “Mémoire sur la composition du tissu propre des plantes et du ligneux”, Comptes Rendus, 1838, 7, 1052-1056.

This article is part of an action of dissemination of knowledge and results of the project SolarLeaf, CO-FOUNDED BY COHESION AND DEVELOPMENT FUND 2007-2013 – APQ RESEARCH PUGLIA REGION “REGIONAL PROGRAMME SUPPORTING SMART SPECIALIZATION AND SOCIAL AND ENVIRONMENTAL SUSTAINABILITY – FUTUREINRESEARCH”, PROT. F6YRAO1.
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Why Nanopaper?

Considering the outmost attention put by the European Community on biodegradable and renewable plastics, the cellulose nanopaper (CNP), consisting of transparent and flexible paper sheets that are completely biodegradable, can find straightworward technological application in packaging, coating and many other applications.

Nanopaper is a self-standing transparent film, that can be produced by deposition of nanocellulose crystals or fibers. With regard to chemical composition, differently from ordinary paper, nanopaper is formed only by crystalline cellulose, and has negligible content of amorphous cellulose, lignin or hemicellulose. Being a highly pure and crystalline material, composed as well by nanodimensioned objects, it presents new properties, including transparency, but also high specific surface area (SSA).

Nanopaper presents high flatness, especially if compared to normal paper, that makes it suitable as a substrate for organic electronics thin film devices. Its potentialities in printed electronics, whose market is estimated to be $73 billions in 2025, are one of the aspects that make CNP a very interesting product for future industrial developments.

Copyrights of this article belong to Alessandra Operamolla. All rights reserved.