Multifunctional cellulose-paper for light harvesting and smart sensing applications

António T. Vicente , Andreia Araújo , Manuel J. Mendes *, Daniela Nunes , Maria J. Oliveira , Olalla Sanchez-Sobrado , Marta P. Ferreira , Hugo Águas , Elvira Fortunato and Rodrigo Martins *
CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa and CEMOP/UNINOVA, 2829-516 Caparica, Portugal. E-mail: mj.mendes@fct.unl.pt; rm@uninova.pt; amv17109@campus.fct.unl.pt; aca12741@campus.fct.unl.pt; daniela.gomes@fct.unl.pt; mj.oliveira@campus.fct.unl.pt; o.sanchez-sobrado@fct.unl.pt; mi.ferreira@campus.fct.unl.pt; hma@fct.unl.pt; emf@fct.unl.pt

First published on 9th March 2018

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From top-left to bottom-right the names are: Rodrigo Martins, António T. Vicente, Maria J. Oliveira, Olalla Sanchez-Sobrado, Daniela Nunes, Manuel J. Mendes, Marta P. Ferreira, Andreia Araújo, Hugo Águas and Elvira Fortunato

The Center of Materials Research, CENIMAT/i3N, associated with “Faculdade de Ciências e Tecnologia” of “Univ. NOVA de Lisboa” (FCT-NOVA), is devoted to materials science and engineering, including micro and nanotechnologies, and biotechnology. The center works in collaboration with CEMOP-UNINOVA, also integrated in i3N and FCT-NOVA, which is a R&D institution acting in direct application industry fields related to novel nanostructured materials for functional coatings, together with bottom up approaches targeting the next generation of nano-chips, RFIDs, photonic devices for ultra-high-speed communication, photovoltaics, nano-sensors for life science and environmental applications.

The authors belong to the Advanced Functional Materials for Micro and Nanotechnologies group led by R. Martins and E. Fortunato (MEON) of CENIMAT-CEMOP, which performs pioneering activities in novel materials and devices, in the fields of transparent and paper electronics, thin film solar cells, photonics, and optical bio-sensing. In the last 5 years the group published ∼250 peer-reviewed papers in international journals, capturing 16 M€ in projects, and hosted 5 ERCs (2 Advanced Grant, 1 Consolidator, 2 Starting Grants). MEON was recently awarded with prestigious prizes in recognition of outstanding contributions, such as: Lisbon Energy Live Innovation (Solar Tiles project); EU Patent Innovation (paper electronics); Exame Informática Innovation (solar cells on paper).

1 Introduction

Cellulose is the most abundant biopolymer on the Earth. Besides its traditional uses in books, newspapers, printing paper, packaging, or in traditional Korean/Japanese houses, it is nowadays envisaged for several thin film opto-electronic applications given its unique set of properties.¹ Cellulose is biocompatible, biodegradable, 100% recyclable, lightweight, flexible, foldable and low cost (0.3–0.6 cent per m²) when compared with the most common flexible substrates (e.g. polyethylene terephthalate – PET, polyimide – PI, polyethylene naphthalate – PEN) used in electronics which are above one order of magnitude more expensive.^2–4 Nevertheless, the use of cellulose in opto-electronic applications is challenging, namely the lower working temperature range, its surface roughness, porosity, or lower mechanical properties compared to certain polymers, which will require reengineering cellulose to be compatible with the intended application.

It is the high adaptability of thin film technologies that is fueling the growing interest in developing novel flexible platforms for fully autonomous intelligent devices. The demands for efficient regulation, reliable quality control, monitoring, and intelligent systems will require the incorporation of power-demanding flexible opto-electronic devices (e.g. sensors, logic circuits, antenna, lighting elements, and power systems) into clothing,⁵ personal objects,⁶ packages,⁷ diagnostic/monitoring platforms⁸ or even electronic-skin.⁹ Nevertheless, for thin film technology to be suitable for implementation on flexible substrates, such as paper, plastics, fabrics, and membranes, it must be adapted to allow conformal shaping and bending to some degree without losing function. In this way, besides incredibly broadening the applicability of thin film devices in various consumer electronic products, the technology is also made compatible with roll-to-roll (R2R) manufacture, the preferred industrial process for mass-production.

Solutions to power devices on textiles (electronic textiles or e-textiles),¹⁰ polymers,¹¹ or paper¹² have boomed in recent years, which shows how promising these segments can be for the market of thin film solar cells (TFSCs). Plastic substrates, such as PET, PI, and PEN, are the traditional options when considering flexible optoelectronics.^13–18 However, from an economic and raw material life-cycle perspective, these petroleum-based substrates are expensive and environmentally less attractive than other easily recyclable or biodegradable materials. Substrate materials which could be synthesized at low cost, from renewable feedstock, or energy-efficient carbon-based green materials (e.g. cellulose, starch, chitosan, collagen, soy protein, and casein), are particularly attractive to achieve sustainable technologies.¹⁹

Among the classes of carbon-based materials, paper, or cellulose-based materials (extracted from cotton, wood, hemp, algae, bacteria, among others²⁰), can be one of the best alternatives to ceramics, metal, glass, and polymer substrates given its biodegradability, cost effectiveness and abundance. Paper has been used ubiquitously since ancient times and, in the future, paper-supported photovoltaics could create other attractive new paradigms, including seamless integration into window shades, wall coverings, intelligent packaging and documents. Module installation may be as simple as cutting paper to size with scissors or tearing it by hand and then stapling it or gluing it. Additional cost savings can be anticipated given the low weight of paper and its ability to achieve a compact form factor by rolling or folding for facile transport from the factory to the point of use.²¹

Advantageously, the physical properties of cellulose-based materials can be easily engineered to a high degree, allowing the construction of ideal substrates with flat surface and good mechanical and chemical stability that are compatible with various fabrication processes and enable an inexpensive and scalable production,²² especially when accompanied by fast direct-write methodologies such as inkjet printing²³ for low cost disposable applications. In addition to the use of paper as a bendable mechanical support, it can also be engineered to exhibit beneficial optical properties for flexible optoelectronic devices, such as high transparency and haze (ratio between diffuse and total light intensity) to improve light transmission and coupling.

In light of the world of possibilities, this review explores recent progress concerning the applications that cellulose-based materials have in the field of opto-electronics, focusing on devices, which exploit light for either energy harvesting or sensing. Section 2 starts by overviewing fabrication methodologies and strategies to address the challenges of paper to obtain devices with comparable properties to those fabricated on conventional substrates. Section 3 evaluates thin film solar cell technology and latest breakthroughs in its adaptation to flexible platforms such as paper, together with promising innovative research pathways to boost the efficiency via light management/trapping solutions. The use of paper for optical bio-sensing applications is reviewed in Section 4, where focus is placed on Raman and photoluminescence-based detection. To complete the review, Section 5 comments on another important field outside opto-electronics, related to electronic circuitry, where the physical properties of paper are becoming of emerging interest. Lastly, the main conclusions and future prospects of these promising emergent technologies are presented in Section 6.

2 Paper engineering

The main application of paper in opto-electronics is in the form of a physical substrate to support the different functional materials. Another class of applications is the use of the paper porosity as a scaffold to immobilize photoactive nano-materials, where paper thereby becomes a more active part of the opto-electronic devices. This review covers recent advances in distinct paper-based technologies, focusing mainly on devices for solar energy harvesting and optical sensing. Nevertheless, in all the cases described here, it is crucial to properly modify the physical properties of paper (both bulk and surface) in order to optimize it for the targeted applications.

The most common type of paper engineering techniques consists in covering the natural porosity of paper surfaces with sealing layers, yielding a closed surface which does not allow penetration of the functional materials into the paper.²⁴ In general, most devices benefit from the surface smoothness of the substrate, as it enables the use of narrower and thinner features without risk of pinholes. In multilayer structures, for example thin film solar cells, excessive surface roughness can lead to non-uniform coverage of the different coatings and even penetration of one cell layer into another, making the devices inoperable due to short-circuiting caused by an excessive number of pinholes that connect the selective contact layers for electrons and holes. Barrier/sealing layers can not only provide a smooth surface but also act as encapsulants preventing oxygen or moisture from destroying the functionality of the patterned devices. A well-known example is the use of high-performance gas/moisture barriers in conventional paper cardboard products used in liquid packaging of beverages, where an alumina-coated aluminium barrier layer is conformally deposited onto the porous paper surface.²⁵ Besides roughness and encapsulation, the surface chemistry of paper can also play a role in the performance of functional materials in contact with it. While a chemically inert surface can be created through coating, to decouple the paper from the device, some surface chemical groups can potentially improve the performance of a functional material, for example, through doping.

When it comes to material engineering, cellulose-based materials are particularly versatile and highly adaptable via these or other approaches mentioned in the following sections.

2.1 Cellulose and its derivatives

Cellulose, mainly obtained from the skeletal component of plants, is an almost inexhaustible green material with an annual production of about 1.5 trillion tons.²⁶ The molecular structure of cellulose, (C₆H₁₀O₅)_n, is a polysaccharide consisting of a linear chain of glucose units linked together through β-1,4-glycosidic bonds²⁷ by a condensation reaction.²⁸ The cellulose chains are then organized into elementary fibrils (nanosized fibers), which aggregate into larger microfibrils and microfibrillar bands.^29,30 In microfibrils, the multiple hydroxyl groups on the glucose form hydrogen bonds with each other, holding the chains firmly together and contributing to their high tensile strength.^31,32 The solid-state structure of the microfibril is represented by the areas of both high (crystalline) and low (amorphous) order range. Variations in crystalline content and crystallite size dictate the differences in morphology, mechanical properties,³³ or thermal stability³⁴ of the resulting microfibril, which are then reflected in the final cellulosic product.³⁵

An in-depth review of cellulose materials, properties and fabrication methods can be found in the work of Moon et al.³⁵ Here, the main purpose is to provide a brief overview of the available cellulose materials to contextualize the topics under discussion.

Cellulose can be chemically modified to yield cellulose derivatives. These are widely used in various industrial sectors (e.g. rayon/viscose as a textile fiber used in the clothing sector, cellulose ethers in pharmaceuticals as an excipient, or cellulose gum in cosmetics and food) as thickeners, binding agents, adhesives, swelling agents, protective colloids, emulsion and suspension stabilizers, and film-forming agents.³⁶ Some of the most important cellulose derivatives are methyl cellulose (MC), hydroxypropylmethyl cellulose (HPMC), ethyl cellulose (EC), hydroxypropyl cellulose (HPC), and carboxymethyl cellulose (CMC). The cellulose derivatives get their names from the substituting groups that replace the free hydroxyl groups of cellulose.

Alternatively, cellulose can be purified/extracted from both cellulose I sources (such as wood fibers, cotton, and agricultural crops) and cellulose II sources (such as lyocell fibers)³⁷ to obtain fibers with characteristic dimensions and unique properties. By mechanical pressure, chemical (e.g. acid hydrolysis), or enzymatic pretreatments followed by high-pressure homogenization, the micrometer-sized cellulose fibers can be disintegrated to obtain microfibrillated cellulose (MFC), cellulose nanofibrillated (CNF), cellulose nanocrystalline (CNC), among other cellulose materials.³⁸ Another important nanocellulose material, named bacterial cellulose (BC), is synthesized from the fermentation of sugar, mainly by Gram-negative bacteria, such as the Gluconacetobacter xylinus (reclassified from Acetobacter xylinum).^39,40 Compared to regular cellulose materials, purified cellulose materials have a higher Young's modulus, dimensional stability, lower coefficient of thermal expansion (CTE), outstanding reinforcing potential, smoother surface, and transparency.⁴¹ Moreover, the reactive surface of –OH side groups facilitates grafting of chemical species to achieve surface functionalization.³⁵

The variety of envisioned applications include, for instance, barrier films, antimicrobial films, flexible displays, reinforcing fillers for polymers, biomedical implants, pharmaceuticals, fibers and textiles, energy storage, and templates for green electronic components.³⁵

The cost of nanocellulose, however, can be higher than that of traditional cellulose materials, due to the additional production steps that add to the energy and materials consumed.⁴² Prices based on the raw material cost of CNF range from 0.7 to 7 $ g⁻¹, considering a low weight nanopaper (20 g m⁻²), but the price is expected to decrease with industrialization.⁴³

2.2 Device fabrication on paper-based substrates

Despite all the envisioned applications of paper-based optoelectronics, implementation is not straightforward. For instance, devices like solar cells or OLEDs (organic light-emitting diodes), and printed electronics, require a smooth and non-porous substrate to prevent cracks, breaks and shunts in the films. Some applications and fabrication processes also require the substrate to withstand high temperatures (up to 250 °C) without undergoing degradation (e.g. sintering of Ag nanoparticles commonly used in nanocomposite inks⁴⁴). These and other challenges⁴⁵ are intrinsically linked to the properties of paper. Traditional paper, made of cellulose fibers with diameters of ∼20 μm, is usually extremely rough, with peak-to-valley roughness values of up to hundreds of micrometers.³⁸ Furthermore, most commercially available papers also add mineral fillers, seizers, and clays to fill the pores and optimize printability⁴⁶ (e.g. capillary action, ink drying and absorption), as well as pigments and fluorescent whitening agents to improve the whiteness of the paper and image quality.^21,46 All these additives can severely limit the quality of the devices fabricated on regular paper, especially if solution processes are involved.⁴⁷

Fortunately, there are several ways to overcome these challenges, such as smoothing the paper surface by cast-coating followed by supercalendering, as exemplified in Fig. 1. This process gives a smooth finishing to the paper surface, turning its microscopic porosity into nanoscopic roughness. It also decreases its wettability, which may be problematic for liquid deposition processes like printing, but can make it suitable for gas-phase coating by physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods that are typically used in thin film Si solar cell fabrication. This innovative approach allowed the realization of flexible a-Si:H solar cells on paper with sunlight-to-electricity conversion efficiencies (3.4%) similar to those (4.1%) attained on rigid (glass) substrates.⁴⁸

Nonetheless, there is nowadays a broad range of distinct strategies under development to tackle the issue of the high paper roughness and porosity and to allow coating its surface with different types of functional materials, as listed in the following sub-section.

2.3 Coating and printing techniques

A thorough overview of coating and printing techniques for solar cell applications was reported by Frederik Krebs.⁴⁹ Here, the goal is to briefly list the available coating and printing techniques compatible with cellulose-based substrates to contextualize the following sections dealing with devices fabricated on the same substrates.

Coatings can be applied on a variety of substrates using non-contact (e.g. inkjet) and contact (e.g. offset, flexographic, screen printing, and doctor blade) techniques (see Fig. 2).^50–52 These techniques open numerous possibilities to obtain, not only coated substrates, but also multilayer structures and devices. Multilayer structures, however, constrain the properties of the materials in use to not destroy or dissolve the previously cast layers. A small variation in properties (viscosity, surface tension, solid contents, evaporation rate etc.) of the solution, or of the substrate (surface energy, roughness, and porosity), can greatly change the coating/printing quality.⁵³ In an ideal process, the fabrication steps should be minimum, the materials environmentally friendly, and the final product recyclable.⁵⁴

The most common coating and printing methods are described below and depicted in Fig. 2, with special emphasis on those that are R2R compatible:

• Casting – casting is probably the simplest technique for film formation since it does not require any equipment. This technique simply involves the casting of a solution containing the desired material onto the surface of the substrate followed by solvent evaporation. However, it has limitations in the area coverage, lacking control over the film thickness and often picture framing effects are observed near the edges of the film or during drying.⁴⁹

• Dip coating – in dip coating, the substrate is dipped into the coating solution and a film is made either by removing the substrate from the solution or by draining the solution.⁵⁵ Film thickness can be controlled with several parameters, including the rate at which the substrate is immersed and removed from the liquid, the immersion time, the liquid and substrate intrinsic properties (concentration, viscosity, rate of interaction between the surface and the liquid etc.), and the number of times that the process is repeated.⁵⁶ There are advantages for the use of this technique such as good uniformity, very thin layers, large area coverage, and the simplicity of the method.^57–59 However, there is a substantial waste of materials and both sides of the substrate become coated. This technology has also been successfully employed in fabrication of solar cells. For example, Hu et al.⁵⁷ developed organic solar cells with a power conversion efficiency (PCE) of 3.93% and a fill factor of 63% using the dip coating technology.

• Spin coating – spin coating is a well-established technology commonly used, for instance, to coat silicon wafers with a photoresist, for the fabrication of sensors, casting protective coatings, optical coatings, and membranes.^60,61 Spin coating involves the application of a small volume of liquid on the surface followed by acceleration of the substrate with a chosen rotation speed producing a centrifugal force.⁶⁰ Due to the angular velocity of the substrate the excess liquid flows to the perimeter and is ejected, leaving behind a thin film on the substrate.⁴⁹ High reproducibility of perovskite solar cells was obtained by a complete spin-coating sequential solution deposition (spinning-SSD) process and it is a promising approach to achieve high-performance perovskite solar cells.⁶²

• Doctor blade – doctor blade is a continuous process that produces thin films on large area surfaces with a well-defined thickness and minimum waste of materials.^59,63 The doctor blade operates at a speed of up to several meters per minute and the films’ thickness can range from microns to several hundred microns.⁵⁹ Uses of this technique in the fabrication of organic solar cells can be found in the literature.^63,64

• Spray coating – in recent years, spray coating has been used as a viable technique for low-cost fabrication in many applications like solar cells.^65–67 In spray coating, the solution is forced through a nozzle by a high pressure, whereby a fine aerosol is formed which is accelerated towards the substrate with an inert carrier gas.⁶⁸ The quality of the coating depends on several process parameters such as the distance of the spray nozzle to the substrate, coating speed, and the number of sprayed layers.⁵¹

• Screen-printing – screen-printing is widely used due to its simplicity, speed, and compatibility with various substrates in which the ink is pushed through a fine mesh with a defined pattern producing functional structures with a large aspect ratio.⁵² This technique requires high-viscosity inks⁶⁹ with thixotropic (shear-thinning) behavior, as inks with lower viscosity can simply run through the mesh.⁵¹ The print resolution and print thickness depend on the density of the mesh and ink properties.⁵⁴ This technique is also scalable to the industrial level and R2R compatible.⁷⁰ For instance, screen printing has been used in the fabrication of conductive composites⁷¹ and transistors⁷² on paper substrates. In the photovoltaic industry, screen printing accounts for the majority of the metallization processes for silicon wafer solar cells.⁷³ Nevertheless, the organic photovoltaic (OPV) fabrication process often explores screen-printing to deposit active layers.^49,74

• Inkjet printing – inkjet printing is a digital noncontact printing technique, capable of reproducing complex patterns, which can also be used to deposit functional materials.⁴⁹ It is a low-cost technique, highly adaptable, and has low material consumption.^51,54 These materials, or inks, consist of a solute dissolved or otherwise dispersed in a solvent and can be classified into aqueous, non-aqueous, phase change, or UV-curable inks,⁴⁶ that are deposited in the form of droplets by a pressure pulse in the nozzle head.⁷⁵ Inkjet inks generally have low viscosities and low evaporation rate for fast droplet generation and to prevent clogging. Inkjet printing is being widely used to fabricate RFID antennas⁷⁶ and was also successfully implemented in the fabrication of solar cells.⁷⁷

• Gravure printing – gravure printing is commonly used to reproduce catalogs and magazines in high-volumes.⁵¹ This technique employs direct transfer of functional inks through physical contact of predefined engraved structures (metallic or a plastic roll) with the substrate, after which the excess ink is removed by a doctor blade.^78,79 The gravure rolls have a long lifetime but are expensive to produce, so this approach is mostly used in industrial mass printing.^52,80 Advantages of the technology include high printing speed (up to 15 m s⁻¹) and good printing resolutions due to the possibility of engraving different depths into the printer roller.⁵² The gravure printing technique can be applied to fabricate devices like organic solar cells,⁸¹ transistors,⁸² and OLEDs.⁸³

• Offset printing – offset (lithography) printing is one of the most common contact techniques. The roll is first chemically patterned and then covered with ink; however, the patterning creates surface sections that bind with the ink (by strong adhesive and cohesive forces) and form a thin film, and other sections that repel the ink.^52,80 The ink is then transferred to a substrate by high pressure. However, offset printing for printing electronics is limited by the required high viscosity of the ink, the transferring high pressure, and the typical presence of water.^54,84

• Flexographic printing – in flexographic printing, the print pattern is present as a protruding relief on a printing roll, made of rubber or a photopolymer.⁸⁵ The ink is first transferred from a reservoir onto the printing roll by an anilox cylinder with engraved microcavities embedded into the surface. The anilox cylinder supplies ink by contact with a fountain roller that is partly immersed in an ink bath.⁵¹ The pressures applied must be low to prevent excessive mechanical deformation of the protrusions which decrease the printing quality.⁵² A wide variety of inks (solvent-based, water-based, electron-beam curing inks, UV curing inks, etc.) can be printed by flexographic printing, whereas the typical viscosities are rather low, usually less than 500 mPa s.^51,52 The applicability of flexographic printing on printed electronic devices is reported, for instance, in the fabrication of OTFTs (organic thin-film transistors),⁸⁶ logic gates,⁸⁷ electroluminescent layers, and OPV.⁸⁸ In photovoltaics, flexographic printing is mainly used in front side metallization of silicon solar cells.⁸⁹ However, Hübler et al.⁸⁸ successfully fabricated a solar cell on paper with a PCE of 1.3% (see Section 3.2.2), where the transparent PEDOT:PSS [poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate)] anode was deposited by flexographic printing on top of the active layer of P3HT:PCBM [poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C₆₁ butyric acid methyl ester].

Table 1 summarizes the main distinctive features and evaluation parameters of the aforementioned wet-coating techniques.

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Coating and printing technologies are assisting and revolutionizing the field of flexible electronic devices by simplifying the process steps, reducing the waste of materials, lowering fabrication and maintenance costs, and speeding up production.⁸⁰

3 Paper-based photovoltaics and light management

In the last decade, references to the use of photovoltaics (PVs) to power printable electronics on paper started to emerge⁹¹ and it is nowadays a hot topic in the development of autonomous high-end applications,^92,93 introducing new directions for intelligent paper electronics. Taking into consideration the current technology stage of paper-based solar cells, where a single cell can generate a current of 5–20 mA cm⁻² and a voltage of 0.7–1.1 V, it is realistically conceivable that a simple integration of 2–3 rows of solar cells connected in parallel, and each row with 3–5 cells connected in series, can yield a power density output anywhere between 15 mW cm⁻² and 150 mW cm⁻², which is in line with the power requirements of many paper electronic systems under development. For example, in the work of Barr et al., the fabricated paper PV arrays produced >50 V.²¹ Tentzeris and Kawahara have roughly calculated the power specifications of future sensor devices in ICT (information and communications technologies) and μW Computing.²³ Most commonly used wireless sensor nodes (e.g. RFID-enabled sensor nodes) consume dozens of μW in sleep mode and hundreds of μW in active mode. Although the above study is directed towards scavenging of potential frequencies; such power requirements can be readily obtained by PVs to endow such devices with full autonomy. Moreover, next generations of these nodes consume significantly less power, for instance sensor nodes (sensor + readout circuit) are already able to absorb 1.2–1.8 μW in active mode⁹⁴ and full wireless nodes (sensor + readout + radio transmitter) are able to absorb 40 μW in active mode.⁹⁵ Kim et al. later explored such possibility in which is one of the first references to flexible solar powered wireless transmission devices fabricated on paper.⁹¹ Inkjet printing was used to fabricate the conductive circuit traces and the folded slot antenna. Autonomous operation was successfully achieved by powering the 800 MHz antenna-based beacon with an a-Si:H solar cell (drain current 4 mA and supply voltage 1.8 V).⁹¹

Among the numerous paper-based optoelectronic devices that could exploit PV power sources, OLED devices are one of the most studied.^22,96–99 The power requirements of OLEDs are already in line with those PVs can deliver. For instance, one of the most efficient OLEDs produced (external quantum efficiency of 11.7%) is reported in the work of Jung et al.¹⁰⁰ Here, they demonstrate the fabrication of OLEDs by inkjet printing with similar electrical performance to those deposited by vacuum processes. The OLED device with the lowest current density had a driving voltage at 1000 cd m⁻² of 6.3 V.

Paper-based batteries^101–103 and supercapacitors^26,104 coupled with PV is another promising strategy to extend autonomy and self-sufficiency of devices, when a light source is unavailable. For instance, Wee et al. demonstrated a novel printable module in which organic solar cells were integrated with an all-solid-state flexible supercapacitor.¹⁰⁵

The present section starts by introducing the current picture of the distinct solar cell technologies (Section 3.1), paying particular attention to thin film photovoltaics. This is the branch where paper can find most application in PVs, mainly as a flexible platform to mechanically support the solar cell layers. Therefore, the use of paper for PV substrates is the main focus here, where the chief technological challenges are identified: not only related to the adaptation of the paper materials and device fabrication conditions to allow stable operation on such substrates (Section 3.2), but also concerned with the improvement of their conversion efficiency via the implementation of advanced light trapping mechanisms (Section 3.3). Besides the use of paper as a substrate, there are other classes of applications where cellulose-based materials perform a more active role in the solar cells, as media to incorporate or immobilize nanostructures that assist in the sunlight-to-electricity conversion process (Section 3.4).

3.1 Current picture of solar cell technologies

Climate change poses one of the greatest threats to our life and is rapidly altering the dynamics of the Earth. To prevent irreversible damage to our planet, sustainability concerns must be taken into consideration in all our daily choices. Nowadays there is a great concern with the development of sustainable technologies to curb the negative impacts of humanity on the environment. This search for green technology promotes the manufacture of fully recyclable products, minimizes consumption of natural resources, and exploits renewable energy sources to power devices.

In the particular case of energy consumption, solar energy – the largest global renewable energy source¹⁰⁶ – is one of the most promising options,¹⁰⁷ given its sustainability and high adaptability. Depending on the intended application, solar energy is converted into other energy forms. The most efficient conversion is solar energy to heat, but a wider range of applications can be envisioned when solar energy is converted to electricity. This conversion can be done indirectly by mechanical work (e.g. with steam turbines, or a Stirling engine), or directly, using semiconducting materials that exhibit the photovoltaic effect, called photovoltaics. The direct conversion of solar energy into transportable and storable energy forms, by artificial photosynthesis/photocatalysts (e.g. to reduce CO₂ into renewable hydrocarbon solar fuels), or by photoelectrochemical cells (e.g. to produce hydrogen from water splitting),³² is also possible but still far from reaching industrial viability.

The global PV installed capacity in 2016 was of 301 GW¹¹⁰ and until 2040 it is expected to grow above 8% yearly.¹¹¹ Despite the numerous types of PV technologies (see Fig. 3), the market is dominated by first generation wafer-based crystalline silicon (c-Si) cells, which account for 94% of the total production in 2016.¹⁰⁸ Given the reliability, maturity, and continuous cost reduction of c-Si solar cells (in addition to the fact that Si is the second most abundant element in the Earth's crust), it is foreseeable that this standard PV technology will continue to lead the market in the near to mid-term future. The remainder of the PV market is held by second generation thin film solar cells (TFSCs), based on cadmium telluride (CdTe), copper indium gallium (di)selenide (CIGS) and silicon (either hydrogenated amorphous silicon, a-Si:H, or microcrystalline silicon, μc-Si:H).^112,113 TFSC technologies were developed to provide other important advantages compared to wafer-based solar cells (SCs):^114,115

• High production capacity and shorter energy pay-back time, given the reduced material consumption and energy input in the fabrication process (lower amount of purified semiconductor materials);

• Lower material and energy requirements lead to lower fabrication costs, thus reduced cost per watt of solar energy conversion, and lower levels of CO₂ equivalent emissions per kW h;

• The decommission and recycling stage is more favorable because the materials used as substrates are mostly composed of glass or plastics.

CdTe SCs take about ∼3% of the total market, while CIGS and silicon account for ∼2% and 1%, respectively.¹⁰⁸ The emerging TFSCs of third generation PVs have the potential to overcome the Shockley–Queisser limit for single bandgap and the cell efficiencies are already approaching those of commercialized second generation technologies. Particularly interesting is the case of perovskite solar cells (PSCs), the fastest-advancing solar technology, whose efficiencies soared from 3.8% in 2009¹¹⁶ to 22.1% in 2016.^117,118 In addition, a mechanically-stacked perovskite-on-silicon tandem solar cell has recently reached an efficiency of 26.4%, which rivals the current record efficiency of c-Si wafer-based cells of 26.7%.¹¹⁹

Wafer-based SCs require thick layers – 100 to 1000 times thicker than thin films – to efficiently absorb sunlight and are extremely fragile, which limits their applicability since they need to be mounted on rigid and heavy structures – features that also raise the balance of system (BOS) costs. Their limited applicability opened a market opportunity for PV solutions that can take advantage of thinner, flexible, and lightweight characteristics like those provided by second and third generation TFSCs. Small and flexible modules with power ranging from 3 to 50 W_P are used as portable battery chargers, in a variety of leisure products¹²⁰ and can be easily transported and installed in remote areas; there is also a rising interest in providing autonomy and self-sustainability to devices and sensors to achieve concepts such as the Internet of Things (IoT),¹²¹ wearable electronics,¹⁰ and smart environments in general.¹²² These electronic systems will contribute to our future lifestyles at the level of communications, logistics, and healthcare⁴⁸ and by being solar powered, the load to the energy grid will not increase.

Paper-based photovoltaics, as previously discussed, can be applied as an in situ power source for paper electronics. In the fabrication process of solar cell devices, cellulose can have three main purposes: (i) as matrix/binder/dispersion medium for polymer solutions; (ii) as a substrate for flexible (and, at times, transparent) SCs; (iii) to enhance surface and optical properties. The different uses of cellulose and solar cells produced are summarized in Tables 2, 3 and 4, respectively according to those categories. As can be seen, when used in polymer mixtures, most of them rely on ethyl cellulose; whereas as substrates for solar cells, numerous devices are fabricated on nanocellulose composites, as they are porous free, with nanometer scale roughness, and low impregnation volume, compared to traditional paper. These nanocellulose composites are also the preferable choice to enhance the optical properties of SCs, given their high transparency and haze.

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3.2 Paper as a photovoltaic substrate

The first step in the fabrication of solar cells is to choose the appropriate substrate according to the device requirements. By far, glass, for its rather low cost, transparency, and stability against the conditions of the fabrication process, is the most frequently-used substrate. However, its rigidity, weight, and thickness prevent the exploitation of advantageous potentialities (flexibility, lightweight, low material volume) of thin film solar cells. Flexible glass, in turn, is extremely fragile.

The subject of fabricating solar cells on flexible substrates is not novel. In the 1960s the first flexible solar cell arrays were fabricated for space power applications. These cells were made with thin silicon wafers (<180 μm) assembled on plastic substrates to provide mechanical support. In 1976, Wronski et al. successfully fabricated a Pt/a-Si:H Schottky barrier solar cell on stainless steel (SS).¹²³ At the beginning of the 80s decade, Staebler et al. successfully fabricated a single junction p-i-n SC on stainless steel (SS/p-i-n/ITO),¹²⁴ while Okinawa et al. produced SCs on polyimide substrates (PI/SS/p-i-n/ITO/Ag).¹²⁵ However, during the following decades, this subject was not explored in detail as the competition to maximize efficiency was hot, and new materials and multiple junctions were being developed. With the efficiency of silicon TFSCs reaching a bottleneck in the last decade, attention returned to reducing cost per Watt by using flexible substrates. Flexible substrates are generally cheaper than glass coated with TCO and their compatibility with R2R lowers the fabrication costs. The lightness of flexible substrates also leads to lower transportation costs.

For instance, recently, a-Si:H single junction TFSCs have achieved, routinely, efficiencies above 8% on both stainless steel¹²⁸ (∼9% for nc-Si:H¹²⁹ and up to 16.3% for triple junction – a-Si:H/a-SiGe:H/nc-Si:H,¹³⁰ see Fig. 4a) and plastic/polymeric substrates.^131–133 Despite the advantages of these flexible substrates, cellulose-based substrates can be much cheaper, sustainable, and easily recyclable (see Fig. 4b and c).¹³⁴ Furthermore, the mature coating technology of paper substrates can provide an opportunity for low cost R2R mass production.

3.3 Improving thin film solar cells with light management

Despite the considerable number of technological efforts described in the previous section to produce high performing TFSCs on flexible paper platforms, the best efficiencies attained so far are, in most cases, still below the record ones achieved on rigid glass substrates. The main reason is the low mechanical robustness of the TFSC structures when mounted on the rough paper surface and upon bending, resulting in film cracking, peeling and general increase of defect density. These aspects can be considerably improved by further reducing the cell thickness, since:^165,166

(1) The flexural rigidity of a film increases proportionally to the cube of its thickness;

(2) The peak strains associated with bending are proportional to the thickness;

(3) The ability to heterogeneously integrate PV films onto polymeric substrates (e.g. paper,⁴⁸ plastics¹⁶⁷) improves since the energy release rates for interface failure reduce linearly with thickness.

Therefore, highly-bendable TFSCs require ultra-thin thicknesses in order to enable their applicability in the flexible substrates of consumer-oriented products (e.g. wearable PVs, solar-powered intelligent packaging,²⁵ portable/disposable electronics, building-integrated PVs),¹⁶⁸ with efficiencies and stabilities comparable to state-of-the-art rigid devices. Besides, lowering the cells’ thickness brings additional advantages such as lower cost, lighter weight and faster fabrication, which are crucial at the industrial level allowing, for instance, large-scale roll-to-roll manufacturing. Moreover, thickness reduction can lead to higher open-circuit voltages (and consequently efficiencies) due to lower bulk recombination.^169,170

In this context, the development of optical strategies to boost the broadband light absorption in TFSCs, while allowing the reduction of their absorber thickness, is becoming of increasing importance.¹⁷¹ Many ideas and research efforts have been employed since the turn of the century to develop light-trapping (LT) solutions that allow the engineering of optically-thicker but physically-thinner devices, by amplifying their photocurrent generation and, consequently, efficiency.^170–173 Conventional LT strategies, as those applied in wafer-based devices that rely on textured rear/front surfaces, which provide anti-reflection and scattering,^48,173–175 can be detrimental to thin film PVs, since the increased roughness (hence surface area) leads to higher defect density in the PV material, which deteriorates the cells’ electrical transport via the increase of charge carrier trapping and recombination. Suitable alternatives for thin film PVs are, for instance, plasmonic back reflectors (PBR) or wave-optical dielectric front structures, not only due to their proven effectiveness but mainly because these LT structures are composed of arrays of nano/micro-particles that can be applied in any type of PV device by low-temperature (hence paper-friendly) patterning processes.

The plasmonic back-reflector (PBR) structure makes use of the intense light scattered from metal nanoparticles (NPs) sustaining surface plasmons, such as those resulting from monodisperse arrays of silver (Ag) or gold (Au) NPs.¹⁷⁶ The conventional technique employed to fabricate such NP structures is via ultra-thin film annealing, where a metallic precursor layer transforms into a drop-like NP array by a solid-state dewetting mechanism.^177–179 Nevertheless, the high temperatures (400–500 °C) required for the annealing treatment make this technique incompatible with the most common flexible substrates (as paper or PEN/PET) used in thin film PVs, as flexible materials can only withstand temperatures up to ∼150–200 °C without degradation. An alternative low-temperature (<120 °C) approach was demonstrated by Mendes et al.^180,181 who developed a wet coating method to precisely pattern arrays of spherical Au NP colloids, with more appropriate dimensions for pronounced far-field scattering, on the rear contact of any solar cell (see Fig. 9a–c). Nevertheless, metallic NPs can present significant parasitic absorption in the NIR range,¹⁷⁶ as discussed by Schuster et al.¹⁸² In this paper, the authors established a comparison between the LT efficiency in thin film solar cells produced by PBRs and that produced by dielectric diffractive nanostructures placed at the front, in an identical absorber configuration consisting of a 240 nm thick amorphous silicon layer. Both LT strategies show pronounced enhancement of the absorption in the red/NIR range, but parasitic absorption increases in the metal nanoparticles for the longer wavelengths, which reduces the overall performance of the plasmonic relative to the dielectric approach. This is clearly seen in the absorption measurements shown in Fig. 9d.

In view of that, dielectric-based structures applied in the cell's front are nowadays considered preferential LT approaches relative to PBRs.^25,171,183 Dielectric structures provide the highest LT effects in SCs when their dimensions are comparable to those of the illuminating wavelengths, thus they operate in the so-called regime of wave-optics (sometimes simply called photonics).^184,185 An important advantage of dielectric materials, relative to the previous metallic ones, is that they can be lossless (non-absorbing) in most parts of the solar spectrum. Therefore, the photonic elements can be incorporated on the top (front surface) of completed cells with flat layers. In this way, the structures do not increase the roughness or the surface area of the cell layers; and so, do not degrade the cells’ electric performance via increase of carrier recombination.

High refractive index media are often preferable for front-located LT structures,^186–188 since they provide the best light incoupling (i.e. minimum reflection) towards the absorber medium when their refractive index is comparable to that of such medium (e.g. Si with n ∼ 4).¹⁸⁵ Regarding the preferential geometry for the LT structures, it strongly depends on the refractive indices of both the photonic and absorbing materials of the cells as reported in the work of Mendes et al.¹⁸⁹ (see Fig. 10a). The physical mechanisms responsible for such enhancement, i.e. anti-reflection and scattering effects, are schematized in the diagram of Fig. 10b, providing a deeper understanding of the advantageous characteristics of the optimized geometries. The authors concluded that optimized structures, composed of TiO₂ half-prolates patterned on the cells' top surface, can yield two times higher photocurrent (up to 32.5 mA cm⁻² in 1.5 μm thick Si layer) than the same flat devices without an anti-reflection coating (ARC) or any LT scheme.

Following this theoretical work, Sanchez-Sobrado et al.¹⁸⁸ developed a low-cost soft-lithography method, known as colloidal-lithography (CL), to fabricate TiO₂-based micro-structures (Fig. 11). The method allows the formation of nano/micron-scale structures with a wide range of materials and is compatible with the PV industry scalability requirements, employing the 4 main steps illustrated in Fig. 11a: (i) deposition of periodic close-packed arrays of polystyrene spheres, which act as the mask pattern; (ii) shaping of the spheres and increasing their spacing via dry etching; (iii) infiltration of TiO₂ in the inter-particle spacing and (iv) removal of the polystyrene spheres to leave only the structured TiO₂ layer. The resultant array of wavelength-sized features acts as a nanostructured high-index anti-reflection coating, which not only suppresses the reflected light at short wavelengths but also increases the optical path length of the longer wavelengths, via light scattering, within the absorber. The measured optical absorptance of the a-Si:H sample with and without the TiO₂ nanostructure (NS) is plotted in Fig. 11b and a significant enhancement of the cell's photocurrent (27.3%) is anticipated with these TiO₂ structures, when compared the enhancement attained with a conventional indium zinc oxide (IZO) ARC.

Although the method developed by Sanchez-Sobrado et al. was tested with SC structures deposited on glass, it can be straightforwardly applied on paper-based substrates since it involves low temperature (<100 °C) steps and the wet-coating technique used to deposit the colloids can be easily adapted to prevent immersion of the substrate, for instance employing doctor blade surface patterning.

3.4 Paper as a binder for nanostructures

In addition to the use of paper-based compounds as substrates, encapsulants or light trapping media, another extensively-studied set of PV-related applications of cellulose materials is concerned with their use as scaffolds/binders to incorporate/immobilize different types of photo-active nanostructures that assist in the sunlight-to-electricity conversion mechanism. This section reviews some of the core advances in this class of applications, which historically were actually the first implementations of cellulose materials in the fields of opto-electronics. With the advancement of nanotechnology, the use of paper matrices for incorporation and/or immobilization of nanostructures has been progressing at a rapid pace.⁴⁹ Among the numerous benefits of paper matrices are the important advantages of reducing the dependency on petrochemical-based polymers, their biodegradability, cost effectiveness, and abundance.^45,199,200 The work of Matsubara et al. in 1995 marks the emergence of the research area of incorporation of nanostructures in paper matrices by a standard handsheet making method.²⁰¹ In this work, TiO₂-containing paper sheets were prepared by dispersing TiO₂ powder in paper pulp, which led to a highly efficient photocatalyst.

In the third-generation thin film PV field, metal oxide nanostructures (e.g. TiO₂, ZnO, Fe₂O₃/Fe₃O₄, CuO, ITO, SiO₂, MoO₂, and WO₃) can play a central role as photoactive elements. Nonetheless, these nanostructures have also shown great potential in other fields such as piezoelectric, magnetic, gas sensors, and bio-devices due to their unique optical, electronic, conductivity, catalytic and antimicrobial properties.⁴⁵ To properly disperse metal oxide nanostructures, or fabricate mesoporous films with intended properties and tailored to the chosen coating/printing method, the addition of a suitable surfactant or binder to the precursor paste, such as cellulose, can be crucial. Independently of the role played by the nanostructures, it is well known that the morphology, film thickness, porosity, and surface features (homogeneity, presence of cracks or aggregates, etc.) will significantly affect the performance, hence the fabrication method developed is essential to achieve solar cells with high efficiency.²⁰²

TiO₂ is by far the most studied metal oxide. Since Fujishima and Honda discovered in 1972 that TiO₂ can be used for water photolysis under UV light irradiation,^203,204 it has received great attention.³² The significant share of initial investigations focused on TiO₂ nanoparticles and they showed excellent performances in photocatalysis,²⁰⁵ hydrogen production, solar cells, adsorbents, and sensors due to their large surface, broadened band gap, and electron transport properties.²⁰⁶ TiO₂ plays three main roles in photovoltaic devices: (i) as antireflection coating or a scattering layer,²⁰⁷ (ii) the interlayer in organic photovoltaics (OPVs),²⁰⁸ (iii) as a selective contact layer of the device (mesoporous film) responsible for electron transport in dye sensitized solar cells (DSSCs), quantum dot solar cells (QDSCs), and perovskite solar cells.²⁰⁹

When using paper as a matrix for the incorporation of metal oxide nanostructures, adhesion occurs by weak interactions such as hydrogen bonding, and van der Waals forces, which poses retention problems. One way to solve this issue is through the use of suitable linkers, binders or retention aids for the incorporation/immobilization of the nanostructures in the paper matrices (see Fig. 13).^43,210 Alternatively, novel methodologies that avoid the use of binders, linkers or retention aids are being developed. For instance, the hydrothermal treatment (at 150 °C for 20 h) allows one to immobilize metal oxide nanostructures on the cellulose fibers of paper as reported by Chauhan et al.²¹¹ A non-hydrothermal and mass-producible synthesis of mesoporous TiO₂ spheres is also reported by Lee et al. where the concentration of ethyl cellulose controls the bulk calcination.²¹² Cellulose fibers have also been used as a template to prepare nanostructured TiO₂ hollow fibers to be applied in photocatalytic and dye-sensitized solar cells. These porous cellulose-templated TiO₂ nanostructures exhibited a significantly enlarged surface area and improved electron transport properties, whereas the PCE of the fabricated DSSC reached 7.2%.²¹³

Typical mesoporous semiconductor films comprise three main components, the metal oxide nanoparticles, a solvent and the surfactant (binder). The surfactant plays an important role in controlling the porosity, viscosity, rheology, and overall morphology properties of the pastes used mainly in OPVs and DSSCs,⁴³ which deeply influence the charge transport properties in the nanostructure matrix under illumination.²⁰² Cellulose is commonly used as a surfactant that enhances the interconnection of the nanoparticles, and does not leave undesired residues when the pastes undergo thermal processes.²¹⁴ Applications of these pastes range from electrodes to electrolytes. Ethyl cellulose (EC) is the binder usually chosen;^215–218 however, there are numerous studies on other cellulose materials applied to solar cell fabrication, such as hydroxyethyl cellulose (HEC),²¹⁹ hydroxypropyl cellulose (HPC),^220–222 cyanoethylated cellulose (CN-HPC),²²³ cellulose acetate (CA),^224,225 cellulose acetate butyrate (CAB),²²⁶ carboxymethyl cellulose (CMC),^227–230 trimethylsilyl-cellulose (TMSC),²⁰⁰ cellulose nanocrystalline (CNC),²³¹ microfibrillated cellulose (MFC),²²⁸ and bacterial cellulose.¹⁴¹

The importance of the properties of metal oxide pastes is reported in the work of Jiang et al., for instance.²³² They studied the influence of pore size, pore distribution and porosity of TiO₂ films prepared by changing the cellulosic thickener concentration in the pastes. The best results were achieved for a paste containing 15 wt% cellulosic thickener (60MP-50), which led to a DSSC with a PCE of 6.4%. The short-circuit photocurrent density (J_SC) was 13.0 mA cm⁻², the open-circuit photovoltage (V_OC) was 0.72 V, and the fill factor (FF) was 68.0%.

Likewise, Dhungel et al.²³³ obtained the best TiO₂ pastes when adding a cellulosic binder (ethyl cellulose, EC) along with the solvent, α-terpineol. The best DSSC had a PCE of 7.3%. Mori et al.²³⁴ also confirmed the importance cellulose has as a critical binder for TiO₂. Their TiO₂ dispersions, for DSSC electrodes, exhibited the best conversion efficiency when prepared using EC and α-terpineol (PCE = 5.07%, J_SC = 10.9 mA cm⁻², V_OC = 0.83 V, and FF = 56.0%). Recently, Maldonado-Valdivia et al.²⁰² also studied the importance EC has in the performance of TiO₂ photoelectrodes for DSSCs. The DSSCs with the highest PCE were systematically obtained with EC as a surfactant, instead of polyethylene glycol (PEG).²³³

Despite the general interest that TiO₂ mesoporous films have received, cellulose polymers are also considered a reliable thickener/binder for alternative mesoporous metal oxides.²³⁵ For example, alternatives such as Nb₂O₅,²²² and ZnO,^235–237 or mixed systems of mesoporous metal oxides like ZnO/SnO₂,²³⁸ have been explored over the last decade. The main reason behind the development of reliable substitutes to TiO₂ is the fact that TiO₂ has a high photocatalytic activity. Under UV light in natural sunlight, it decomposes organic materials in DSSCs during outdoor use and causes long-term reliability problems for the conversion efficiency.²³⁹ Zinc oxide, in particular, is a high candidate for photoanodic material in QDPVs and DSSCs given its advantageous intrinsic characteristics, such as a stable wurtzite crystal structure with a wide band gap (∼3.37 eV), high carrier mobility (∼115–155 cm² V⁻¹ s⁻¹), and large exciton binding energy (∼60 meV).²⁴⁰

Most of the studies published refer to a sole single cell component bearing a cellulose polymer; however, earlier this year, Bella et al.²⁴¹ moved from this usual approach, to interfacing different paper-based components within the same device (the photoanode and the electrolyte). Such a process gave rise to a DSSC with 3.55% efficiency and retained 96% of the efficiency value after 1000 h of accelerated aging test. Moreover, it is a step towards truly sustainable energy conversion devices.

Further applications of cellulose as a binder can be found for other types of solar cells, such as c-Si (mainly used in the preparation of high quality screen-printed metal paste electrodes),^242–245 CIGS/CIS,^246,247 CZTS/CZTSSe,²⁴⁸ or perovskites.²⁴⁹ It is interesting to note that one of the earliest references to the use of ethyl cellulose is in the work of Szlufcik et al.,²⁴² from 1988, where they employed it as a binder for screen-printed TiO₂ anti-reflection coating for c-Si solar cells (the improvement in J_SC and efficiency was more than 30%). Clemminck et al. also used ethyl cellulose to replace the commonly used binder at that time, propanediol, to obtain high quality screen printing CdS pastes for CdS-based solar cells.²⁵⁰

In the field of perovskite solar cells, Liu et al.²⁴⁹ fabricated a mesoporous TiO₂ film by annealing a mixture of EC:P25 TiO₂:α-terpineol and saw a 20% improvement in efficiency comparatively to the commonly used titanium isopropoxide. For QDPVs, Tian et al.²⁵¹ reported on homogeneously distributed CdS/CdSe quantum dots in a TiO₂ mesoporous film (see Fig. 13). The thickness and porosity of the film were optimized by adding 12 wt% EC and the PCE of the QDPV reached 4.62%. The major concern regarding perovskite solar cells is the stability of the organic–inorganic perovskite, since it is highly sensitive to moisture and light.²⁵² He et al. observed that the incorporation of EC into the perovskite film can significantly improve photostability and moisture stability.¹⁹⁴ The stability gain they found arises from the hydrogen bonds between the EC mesostructure and the crystal structure of CH₃NH₃PbI₃. The EC incorporated perovskite solar cell does not show degradation over 5 days under ambient indoor light and 60% RH.¹⁹⁴

Other mixtures where cellulose solutions have been used with PV applications are for instance in the preparation of polymer electrolyte membranes,²⁵³ nanocellulose aerogel membranes,²⁵⁴ hydrocalcites,²⁵⁵ electrospun nanofibers,²²⁴ and carbon counter electrodes.²⁵⁶

It is important to highlight the significant interest the employment of cellulose as a binder for carbon nanostructures has gained over the last years. The work of Cruz et al.²⁵⁷ focuses on the use of single-walled carbon nanohorns (SWNH) as counter electrodes of DSSCs (decorated with and without Pt nanoparticles). The counter electrode assembled with SWNH and 10 wt% of hydroxyethyl cellulose (HEC) had the highest electrocatalytic activity (the charge-transfer resistance, R_ct, reached141 Ω cm²). Applications for OPV can also be found. For example, Valentini et al.²⁵⁸ developed transparent and conductive CNC/graphene nanoplatelet (GNP) layers and Hu et al.⁹⁹ reported on the roll-to-roll production of PEDOT:PSS:graphene:ethyl cellulose (PEDOT:PSS:G:EC) electrodes (13 Ω sq⁻¹ and 78% optical transmittance) for flexible transparent electrodes.

Other developments over the last decade explored the fabrication of conductive papers and the functionalization of cellulose fibers also with photovoltaic applications. Surface functionalization allows the tailoring of particle surface chemistry to facilitate self-assembly, controlled dispersion within a wide range of matrix polymers, and control of both the particle–particle and particle–matrix bond strength.³⁵ For example, Small and Johnston developed photoluminescent cellulose fibers by adding ZnS crystals doped with Mn²⁺ and Cu²⁺ ions (emission at ∼600 nm and ∼530 nm, respectively).³¹ Sakakibara and Nakatsubo functionalized cellulose films with porphyrin for photocurrent generation, although the absorption band was very narrow (from 400 nm to 420 nm).²⁵⁹ Later on, the same research group addressed the limitation of the narrow absorption band by adding polypyridyl ruthenium(II) complexes (photocurrent generation range from 400 nm to 600 nm)²⁶⁰ as a new complementary material for porphyrin-bound, or phthalocyanine-bound (photocurrent generation range from 600 nm to 700 nm)²⁶¹ cellulose derivatives. Shi et al. reported the assembly of bacterial cellulose and polyaniline (PAni) to obtain electroconductive composite hydrogels (10⁻² S cm⁻¹),²⁶² which could be applied as flexible electrodes for solar cells. Embedding silver nanowires (Ag NWs) into transparent and conductive papers is another promising method to develop flexible transparent electrodes for solar cells, as shown by the work of Song et al.²⁶³ They successfully fabricated a paper using bamboo/hemp CNF and Ag NWs cross-linked by hydroxypropylmethyl cellulose (HPMC) with a sheet resistance of 1.90 Ω sq⁻¹ and transmittance above 80%, in the wavelength range from 500 nm to 800 nm.

Table 5 presents a selection of research works recently published regarding the use of cellulose in the production of pastes with PV application.

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4 Paper substrates for optical sensing

The area of sensors ascribes a high importance to paper substrates. From the healthcare perspective, point-of-care (PoC) tests, which are performed at or near the site of clinical care, provide unique opportunities to speed diagnosis and cost reduction, especially in developing countries. The strengths of paper-based microfluidics and sensors are their low-cost, disposability and minimal external equipment requirements.²⁶⁸ In the packaging area, the food and beverage industry is aligning its strategy with the consumers’ demands of more natural products with less additives, higher regulation, and quality control, to ensure food safety. Intelligent sensors can endow packages with the capability to acquire, store and transfer data, communicate and carry out logic functions, thereby contributing to increase consumer confidence in the products they eat and drink.⁷

The incorporation of electronics and microfluidics has the potential to generate new functions and devices in the field of lab-on-chip.^4,269 Nowadays, most of the devices are constructed by stacking electronic and microfluidic structures, which require multiple fabrication, assembly, mounting, and connection steps. Combining electronics and microfluidics on paper, the so-called lab-on-paper devices, has the advantage of exploring low-cost and scalable printing fabrication techniques with a substrate that is highly compliant (possibility of functionalizing cellulose, creating hydrophobic barriers or regions to contain biomolecules/reagents, etc.) and has intrinsic microfluidic transport mechanisms.^270,271 The fabrication of lab-on-paper devices explores various patterning techniques (e.g. photolithography,²⁷² laser treatment,²⁷³ inkjet printing,²⁷⁴ wax printing,^275,276 plasma treatment,²⁷⁷ and silanization²⁷⁸) to define channels and reaction zones onto paper. In the future, these devices are expected to perform more complex and a wider range of analysis, which could take advantage of PVs to power, for instance, a color sensor and display coupled to the lab-on-paper, to increase detection limits and display information/data analysis regarding the tests performed in the device. Hamedi et al. are already exploring co-fabrication processes to simultaneously engineer the electrical and fluidic components.²⁶⁹ In this work, they demonstrate a printed circuit board on paper, an electroanalytical device (coulometric measurement for ferrocyanide, and a glucose assay) and a paper battery. Regarding the power consumption of these devices, the electrochemical devices require a potential of 0.6 V; the electronic paper circuit (microcontroller based heater) requires a 5 V power source; whereas the paper battery has an energy output of 6 μW h.²⁶⁹ Further work on strategies to integrate PVs in these devices is feasible, given the power consumptions involved, in order to extend their self-sufficiency. Very recently, Pavinatto et al.²⁷⁹ have reported a printed and flexible impedance based biosensor for antioxidant detection, whereas the biological recognition layer (tyrosinase-containing ink, where CMC was selected as the viscosity enhancer) was deposited by large-area rotogravure. The finished biosensor was then encapsulated with a cellulose acetate dip-coating film to avoid dissolution.

The field of opto-fluidics can also take advantage of PV coupling. There is a growing focus on biological and chemical sensing, and significant research involving the implementation of opto-fluidic concepts using bulk optics and microchannels, that could see more complex design principles by exploiting power sources. Although this is an area that is yet to receive proper attention, there are exciting opportunities to combine opto-fluidic functionality with additional electrical, mechanical and magnetic elements.²⁸⁰ Erickson et al.²⁸¹ in their review discuss opportunities for opto-fluidics in the fields of photo-bioreactors and photo-catalytic reactors (for solar-energy-based fuel production), and liquid-based systems (for the collection and control of solar radiation); these are fields where PV can add value²⁸² by increasing energy/heat production rates, thus higher power densities and yield/revenues. For instance, Zimmerman et al. developed a concept for using a solar-collecting adsorbing substrate to provide the heat for a microfluidic-chip-based methanol reformation reaction to improve the efficiency of current micro-reactors.²⁸³

In view of the aforementioned exciting applications of paper materials for bio-detection, this section reviews two of the most researched technologies that allow such detection, using light-induced optical signals emitted by the analytes (probe molecules), based on either Surface Enhanced Raman Spectroscopy (SERS, Section 4.1) and Photoluminescence (Section 4.2) sensing. In the latter case, luminescent up-converting materials can also find application in the enhancement of solar cells’ efficiency, as they can provide improved spectral matching between the illuminating sunlight and the solar cells’ photo-current generation.

4.1 Plasmonic Raman sensing

4.2 Photoluminescence sensing

Photoluminescence sensing has emerged as an important and growing research field especially when it comes to biological and environmental areas. Moreover, photoluminescence techniques are very versatile and can be introduced to several analytes due to their high sensitivity and selectivity, as well as high spatial resolution.³⁷⁰ In this sense, semiconductors, with emphasis on semiconductor nanocrystals, play an expressive role, and their optical properties have been studied extensively over the years.^371–373 The optical and electronic properties of these small sized particles present quantum confinement effects, which is their major characteristic.³⁷³ This particularity leads to spatial enclosure of the electronic charge carriers within the nanocrystal,³⁷³ thus these materials exhibit unusually different behaviors compared to their bulk counterparts. Moreover, these materials allow tuning the light emission from ultraviolet to mid-infrared spectral ranges.³⁷³ Semiconductor nanocrystals are already commercially sold nowadays, for example in luminescent labels,³⁷⁴ electroluminescent devices,³⁷⁵ among others.

Doped semiconductor nanocrystals have also been largely investigated,^376,377 in which one of the most interesting doping categories in semiconductors are the magnetic ions, followed by luminescent activators. These latter ones have attracted the scientific community interest, mainly due to their ability to increase quantum luminescence efficiency of the semiconductor nanocrystals. Mn²⁺ or Eu²⁺ are examples of doping elements.^378,379 The photoluminescence up-conversion emission of ZnS:Mn²⁺ nanoparticles has been described by Chen et al.³⁸⁰ Moreover, Cu, Sn and In doping have also been reported.^381,382

The semiconductor nanocrystals have been added to different materials or substrates,^383,384 including paper. For instance, Small et al.³¹ reported the production of photoluminescent cellulose fibers having ZnS nanocrystals doped with Mn²⁺ and Cu²⁺ ions. This process did not influence the inherent properties of the fibers; however, it imparted photoluminescence properties to the coating. Another approach using fluorescently labeled cellulose nanocrystals for bioimaging has been reported by Dong et al.³⁸⁵

Despite these semiconductor nanocrystals, several other materials can be employed for photoluminescence sensing, which includes lanthanide-doped up-conversion materials. These materials are extremely interesting concerning reliability and stability; besides they can be obtained at the nanometer scale, and have their up-conversion emission precisely controlled, in terms of emission color, lifetime and intensity, which are the basic prerequisites for practical applications.³⁸⁶

5 Other paper applications in electronic circuitry

Together with the development of opto-electronic devices for photovoltaics and sensing, there is another class of related applications for electronic circuitry where paper is also emerging as a highly attractive material. Even though electronics is not a core subject in this article, it is nonetheless important to briefly comment the latest advances in this field concerning the use of paper-supported or paper-based materials, for completeness of the review.

To develop paper-based logic elements, significant research has been conducted towards the development of thin film transistors (TFTs, both organic and inorganic) that are efficient, reliable, and with low operation voltage requirements.^2,436 For instance, the development of low operation voltage (<3 V) organic field effect transistors (OFETs), based on naturally occurring materials (including cellulose), and fully compatible with printing fabrication methods are already a reality⁴³⁷ and can be realistically coupled with paper based solar cells. High-performance nonvolatile memory devices, with reliable data storage, low power consumption, and low manufacturing cost are also of key importance to realize intelligent optoelectronic devices.^438,439 In the field of security devices, Liao and co-workers developed, for the first time, a nonvolatile memory with a simple metal–insulator–metal device structure on paper using an all printing approach (the writing bias is +6 V and 100 μs width pulse, the erasing bias is a −3 V and 200 μs negative width pulse, and the reading voltage is 0.5 V).⁴⁴⁰ In another recent work, Nagashima et al.⁴⁴¹ demonstrated a Ag-decorated CNF substrate as an ultra-flexible (350 μm bent radius) nonvolatile resistive memory that can be electrically switched with 6 orders of ON/OFF resistance ratio. The readout voltages were 0.01 V for retention and 0.1 V for endurance, respectively.⁴⁴¹

Functionalization of cellulose also opens the door to realize devices build exclusively on cellulose multilayers/composites. By stacking different functionalized cellulose composite monolayers with tailored active functions (electroconductive, semiconductor, insulator), one can truly fabricate paper optoelectronic devices. One promising development in this subject is the work of Zang et al. where they report a paper-based ionic diode made of two oppositely charged MFC sublayers, and successfully rectified electric current.⁴⁴² Kawahara et al. reported low voltage operated electrochemical devices produced from electrically conducting polymers and polyelectrolytes cast together with CNF. The mechanical and self-adhesive properties of the films enable simple and flexible electronic systems by assembling the films into various kinds of components using a “cut and stick” method. This concept was demonstrated by detaching and reconfiguring one or several subcomponents by a “peel and stick” method to create yet another device configuration.⁴⁴³ A similar concept was also shown for iontronics, using innovative cellulose-based ion gel electrolytes (see Fig. 21), which is expected to impact several market sectors from health, packaging and printing to the battery industry.⁴⁴⁴ As reported by Cunha et al., this approach consists in implementing the electrolyte directly on flexible transistors on paper, in the form of a sticker, employing four steps: cut, transfer, stick and reuse.⁴⁴⁴

Furthermore, as paper-based electronics are easily recyclable, they do not have the disadvantages of producing electronic waste at the product's end-of-life stage, contrary to common electronic products. The recycling of cellulose-based materials is a mature process, and the recovered cellulose can reenter the device fabrication stage to minimize (ideally replace) the need for raw cellulose pulp. Alternatively, incineration can produce energy; and from char, metals can be recovered and reused. In 2013, Zhou et al. demonstrated that polymer solar cells fabricated on CNC can be recycled into individual components using a low-energy process at room temperature.³⁸ Recently, Jung et al.⁴⁴⁵ demonstrated an alternative method to break down nanocellulose substrates by fungal biodegradation (Postia placenta and Phanerochaete chrysosporium fungi). Fungus degradation was evaluated for pure CNF films and epoxy-coated CNF films and was found to be efficient for both films. The encapsulated electronic component could be recovered after a biodegradation period of 84 days.⁴⁴⁵ In a similar work, Seo et al. studied the fungal degradation (Postia placenta) of a TFT transistor fabricated on CNF.⁴⁴⁶

A broad range of near-future advances are currently anticipated in this booming field of paper electronics, such as improvement of device performance, bendability and reliability/stability; as well as the development of clever means to add recycling steps, instead of directly disposing the devices, to reduce material waste. These advances bring new exciting functionalities and added value to cellulose, while providing more sustainable and user-friendly technologies.

6 Conclusions

The advances reviewed here have shown that cellulose can be an appealing option for the next generation of optoelectronic and medical devices. This material can be multi-functional, contributing to the improvement of the performance and applicability of light harvesting technologies, as those aimed at power conversion and bio-detection. Such technologies can find widespread applicability in the market of consumer electronics. Besides wearable/portable electronics, the packaging and healthcare segments are two other main possibilities to integrate thin film devices. Here, paper-based materials have attracted a growing interest as platforms for solar-powered point-of-care tests and consumer diagnostics.

The combination of cellulose and solar cells is a hot topic nowadays that leaves ample room for photovoltaic innovation. Some of the biggest challenges to the widespread use of PV solar power in consumer electrical products are higher efficiencies, reproducibility, longer stability/lifetime and flexural capabilities for higher integration versatility. Thin film solar cells produced on flexible substrates, such as cellulose based materials, can open the possibility of achieving autonomous energy packaging systems;²⁵ thus, sharing the paper advantages, which include bendability, lightweight, recyclability, disposability, low price, among others.

Although the power conversion efficiency is still low compared to traditional TFSCs supported on glass, the performance of SCs on paper still has the potential to be much improved and more easily implemented in large-scale production. For that, two main R&D pathways are being pursued. One addresses the challenge of finding the optimum low-cost and scalable method for SC fabrication on the insulating, porous, and delicate nature of the fibrous paper material. The other concerns the application of light management structures, both for light trapping (via anti-reflection and scattering) and spectral matching (e.g. up-conversion), compatible with the cell architecture and promoting further enhancement of the generated photocurrent. Here, different optical approaches have been investigated to improve the light absorption capability of TFSCs, allowing the reduction of the thickness of the active layers without any detriment of the efficiency. This reduction of the commonly used thickness is important to achieve better structural quality, while reducing the manufacturing costs and enhancing the flexural properties of the devices. Moreover, light trapping structures can also be constructed in paper-based materials, allowing paper to serve not only as a mechanical support for the solar cells but also as an active optical medium.

Concerning optical sensing, the combination of paper substrates with the plasmonic properties of Au and Ag nanoparticles has been gaining a substantial interest for applications in SERS. The low cost of paper allied with its fibrous morphology gives SERS paper substrates soaking capabilities that are not possible to be attained by rigid SERS substrates, giving them a unique capability for many bio-applications. Paper substrates can also integrate microfluidics with SERS detection zones, making them a powerful tool for biosensor diagnostics that can be used in point of care tests. Moreover, the SERS fabrication techniques on paper are allowing these types of substrates to attain remarkably high enhancements that are already close to those that can be achieved with rigid substrates.

Another research line that was highlighted concerns the implementation of up-conversion materials in paper-based devices. Recently, UC materials have been used as fluorescent labels for molecular detection, medical analysis or even therapy. The flexibility and adaptability of these materials in terms of applications, together with their unique physical characteristics, make them prone to be integrated in many types of flexible functional devices. Another promising example is the incorporation of UC materials into thin film solar cells, which has demonstrated the possibility of overcoming one of their main optical drawbacks, i.e. the limitation of PV semiconductors to absorb photons with energy below their bandgap, by converting lower-energy photons (e.g. in the NIR range) to higher-energy photons (e.g. in the visible) that can generate a higher amount of photocurrent in the cells.

The complementary advances in the multi-disciplinary research topics reviewed here are likely to open a new era of environmentally-friendly, disposable and recyclable smart products, such as intelligent electronic labels for food control and medical diagnostic electronics (e.g. point-of-care tests), which have low energy consumption and can therefore operate autonomously powered by PV electricity. Such technological innovations, i.e. making paper intelligent and self-powered, shall open the way for the establishment of paper-based materials as key players in the present IoT revolution.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by FEDER funds, through the COMPETE 2020 Program, and national funds, through the Fundação para Ciência e Tecnologia (FCT), under the projects POCI-01-0145-FEDER-007688 (Reference UID/CTM/50025), ALTALUZ (Reference PTDC/CTM-ENE/5125/2014) and DISERTOX (Reference PTDC/CTM-NAN/2912/2014). The authors also acknowledge funding from the European Commission through the projects 1D-NEON (H2020-NMP-2015, grant 685758) and BET-EU (H2020-TWINN-2015, grant 692373). The work was also partially funded by the Nanomark collaborative project between INCM (Imprensa Nacional – Casa da Moeda) and CENIMAT/i3N. A. T. Vicente acknowledges the support from FCT and MIT-Portugal through the scholarship SFRH/ BD/33978/2009. A. Araújo, M. J. Mendes, D. Nunes and O. Sanchez-Sobrado also acknowledge funding from FCT through the grants SFRH/BD/85587/2012, SFRH/BPD/115566/2016, SFRH/BPD/84215/2012 and SFRH/BPD/114833/2016, respectively. The authors would also like to acknowledge Inês Cunha for assistance in the subject of electronic circuitry on paper.

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