**1.2. Paper electronics and its thermal management**

**1. Introduction**

34 Nanostructured Solar Cells

to the paper.

**1.1. Printing technology**

The e-market for flexible and printed large-area electronics is rapidly growing and it is expected to become a \$69 billion market in the next 10 years [1]. The growth is manly supported by the organic light-emitting diodes (OLEDs) and conductive ink industries. Nevertheless, as technology emerges from R&D, new market opportunities with huge growth potential will appear. Two potential markets are the food and health. These industries are facing a paradigm shift as society demands more regulation, quality control, and smart systems to improve life quality while being environmentally friendly and allowing continuous user interface. Printed electronics can be the key to address such demands by imparting products with solutions to acquire, store, and transfer data, communicate and carry out logic functions

Printing technologies enable electronics to be readily integrated as a part of other printed media by processing them in the same press. This renders possible low cost products such as radio frequency identification (RFID), intelligent packaging, food quality control devices, or disposable diagnostic kits. Functionality and performance of printed electronics are not intended to compete with silicon-based electronics, nevertheless, mass-printing methods offers economic advantages for large-scale production of appropriate products. Printing technology is highly customizable, it is compatible with the preferable fabrication method in industry—the roll-to-roll (R2R)—does not require large vacuum chambers and has lower capital investment costs when compared with other production methods. It is estimated that a printed electronics facility will cost 100 times less than a conventional silicon electronics plant. There are various printing techniques, such as inkjet, screen, flexography, gravure, or offset printing, and their features expand the range of applications. The selection of the printing method is dictated either by the requirements concerning printed films or the level of printing system complexity. In the field of electronics, printing techniques are used to apply coatings, to deposit precise patterning, or even to develop microstructures [3]. Inkjet printing patterns material by expelling from the nozzle one picoliter droplet of ink at a time, as the printhead moves over the substrate. It is a method suitable for low-viscosity inks (1–20 cP). Screen printing is a highly versatile technique given its simplicity and reproducibility. To print, a squeegee transfers the ink through a patterned screen onto a substrate. Gravure (or rotogravure), flexography, and offset printing use a rotary printing press. Gravure printing is the most popular process for flexible packaging manufacturing and it consists in applying ink to an engraved cylinder, which is then transferred (directly, or indirectly through a transfer roll) to the substrate. The flexographic technique prints on flexible substrates by ink transfer (with low viscosity, 50–500 cP) from a laser-etched flexible relief plate. Offset printing is the preferable method for newspaper printing. It works on the principle of oil and water repulsion. A plate is damped first in water (nonimage area) and then ink (image area); the ink adheres to the print area, then it is transferred to a rubber blanket and from it

to take decisions, where recyclability and low cost are key vectors [2].

Electronic devices fabricated on plastics are currently a booming field of research [4–10]. Polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) foils are the most used substrates for printed electronics, because of their high smoothness and good electrical and barrier properties [11, 12]. Physical properties, such as roughness, absorptive capacity, temperature resistance, and flexibility, are also critical. PET foil meets most of these requirements but PEN is also an interesting option given its greater dimensional and thermal stability [13]. PI (polyimide, named Kapton) is another widely used polymeric substrate for flexible thin film solar cells, due to its remarkable chemical and thermal robustness (withstands up to 300– 400°C) and bendability [14]. Nonetheless, for low-cost devices, the price of these synthetized materials can be high and its use is ecologically questionable, since they are not biodegradable and are produced from oil-based raw materials.

On the other hand, paper is potentially useful for some specific applications and markets, such as those already mentioned of intelligent packaging and diagnostic kits [15], where low cost (€0.001 dm−2 compared with €25 dm−2 for silicon or €1–10 dm−2 for polymer substrates) and sustainability are highly desirable. Cellulose is the most abundant natural biopolymer on earth, which is recyclable, biodegradable, as well as nontoxic. It is a carbohydrate polymer made up of repeating β-D-glucopyranose units and consists of three hydroxyl groups per anhydroglucose unit (AGU), giving the cellulose molecule a high degree of functionality. The knowledge of the molecular structure of cellulose is of prime importance as it explains the characteristic properties of cellulose, such as hydrophilicity, chirality, biodegradability, and high functionality [16], which are key factors as far as device feasibility is concerned.

Cellulose fibers have remarkable properties, in particular their thermal and mechanical dimensional stability when compared to plastics, for instance. This is of particular interest if alignment is required when printing different functional materials on top of paper substrates. Paper is commonly used as a dielectric for capacitors [17] and supercapacitors [18], as permeable membranes in liquid electrolyte batteries [19, 20], or just as the physical support of energy storage devices [21], as an organic thin film transistor (OTFTs) [22], printed sensors, and RFID tags [23], printed batteries [24], inorganic powder electroluminescence devices [25], foldable printed circuit boards [26], oxide TFTs [27], and flexible low-voltage electric double-layer TFTs [28].

Many end-user devices will then require power sources, either to display information, integration with other devices, or simply to improve their processing capabilities and complexity. Though recent advances in paper batteries aim to suppress those power requirements, full autonomy can only be achieved by coupling a power generator, such as a solar cell. Hence, one can envision the interconnection of several paper functionalities, to accomplish self-sufficient electronic intelligent paper, or enable disposable sensors of complex laboratory functions (lab-on-chip), as seen in **Figure 1**.

Nevertheless, the substrate requirements for printed electronics are much more demanding than for image printing, hence cellulose substrates have numerous challenges. Homogenous, pinhole-free layers are essential to ensure the desired functionality of the deposited layers, and conventional paper substrates present limitations such as roughness on a length scale of micrometers, porosity, and hydrophilic characteristics. Research has focused on modifying papers for the specific requirements of printed electronics and the paper surface can be modified in a thermal-mechanical manner (i.e., calendered or coated). Normally, the coatings consist of pigments and binders in an aqueous dispersion. However, rendering paper a suitable substrate for active materials can be expensive and may require materials and techniques that are neither cheap nor environmentally friendly [29, 30], diluting the advantage of using a paper substrate over plastic foils for mass production. Moreover, there are reports of chemical interaction/reaction with the selected coating, which degrades the electrical performance of the organic functional layer, as observed when printing PEDOT:PSS on paper [31]. This means that it is difficult to develop a coating, which is chemically compatible with all the inks required to print electronic devices based on organic materials. Another important aspect to consider is the process temperature of the functional materials being deposited on paper which must stay below 200°C in order to avoid cellulose degradation [32].

**Figure 1.** Concept of the multiple integration of devices built with or on paper to perform complex functions. A solar cell coupled with a battery assures full autonomy to a device that requires energy to perform logic operations at the electronic control unit (CMOS), given the signals received from the sensor unit, and then distributes the processed information to be saved in a papermemory or transmitted by an RFID antenna to another device. Other devices/functions can be added according to the final application, such as a display to visually convey messages to the user, or a lab-on-paper diagnostic device.

Though not always using printing deposition techniques, recently, some attempts envisaged to use paper in electronic applications to explore its potential as substrate for low-cost flexible devices [33, 34]. These can complement the functional inks, but their electric performance and stability hardly meet the requirements without specific deposition conditions and encapsulation [29, 35].

pinhole-free layers are essential to ensure the desired functionality of the deposited layers, and conventional paper substrates present limitations such as roughness on a length scale of micrometers, porosity, and hydrophilic characteristics. Research has focused on modifying papers for the specific requirements of printed electronics and the paper surface can be modified in a thermal-mechanical manner (i.e., calendered or coated). Normally, the coatings consist of pigments and binders in an aqueous dispersion. However, rendering paper a suitable substrate for active materials can be expensive and may require materials and techniques that are neither cheap nor environmentally friendly [29, 30], diluting the advantage of using a paper substrate over plastic foils for mass production. Moreover, there are reports of chemical interaction/reaction with the selected coating, which degrades the electrical performance of the organic functional layer, as observed when printing PEDOT:PSS on paper [31]. This means that it is difficult to develop a coating, which is chemically compatible with all the inks required to print electronic devices based on organic materials. Another important aspect to consider is the process temperature of the functional materials being deposited on paper which must stay below 200°C in order to avoid cellulose degradation [32].

**Figure 1.** Concept of the multiple integration of devices built with or on paper to perform complex functions. A solar cell coupled with a battery assures full autonomy to a device that requires energy to perform logic operations at the electronic control unit (CMOS), given the signals received from the sensor unit, and then distributes the processed information to be saved in a papermemory or transmitted by an RFID antenna to another device. Other devices/functions can be added according to the final application, such as a display to visually convey messages to the user, or a lab-on-paper diagnostic

device.

36 Nanostructured Solar Cells

Electronics on paper has been investigated since the 1960s, when TFTs were deposited on paper substrates on a roll inside a vacuum chamber [36, 37]. The envisioned applications were flexible circuits for credit cards, electric sensors, toys, and hobby kits; but without any industrial or commercial success since the technology appeared during the boom of the siliconbased CMOS devices that fueled a revolution in the so-called information, communication and telecommunication (ICT) fields. Since then, research on paper-based field effect transistors (FETs) [38–42] has shown that paper could be used as an active component (dielectric). This has established a new approach to the challenge of *paper electronics*, where performance and stability issues are addressed using inorganic semiconductors, not requiring any special paper surface preparation nor device passivation. Likewise, devices with oxide functional materials can also benefit from the fact that cellulose is chemically more compatible with oxides than with many organic semiconductors [43], they bind properly at low temperatures and the natural porosity of paper can be explored in practical applications. Cellulose also provides a high surface area matrix for memories and sensors, for instance, as well as the capacity of electronic charge accumulation at the fiber surface for the integration of electronic components, as demonstrated by our recent research activity [41].

So far, some of the materials and techniques proposed to build electronic devices on paper are assumed to be compatible with low temperature processing and printing deposition. However, there are no reports yet of fully printed devices on paper that are 100% compatible with roll-to-roll processing. Thus, the solution for this challenge is not only the development of materials that can be printed at low temperatures but also to assure that their energy consumption is not high, so that the power sources to be used (solar cells, primary green batteries, and RF energy harvesting) can efficiently meet the energy needs of the device. The integration of multiple fabrication techniques can also maximize the device performance.

When developing electrical circuits, thermal and power dissipation issues, when a large number of electronic circuits/devices are integrated on an area of square millimeters, are another concern, particularly when using organic inks given their sensibility to temperature. Here, the supply voltage used, the overall current passing through the circuit, and the operating frequencies of such circuits are key factors. Resistors along the circuit are the main source of the heat generated and it corresponds to a bottleneck that forces the use of local heat sinks for enhanced dissipation. To overcome this limitation, paper electronics may not target complex and highly demanding applications but rather niche of low power and low frequency applications, where expected power generation remains very low. For instance, a reasonable target for printed paper electronics is to have no more than 10–50 transistors per cm<sup>2</sup> , also because the integration density on paper is driven by the resolution of the printing/depositing processes (tens of microns), which are far lower than the traditional photolithography techniques used in the silicon microelectronic field (tens of nanometers).

Finally, contrary to plastic substrates, paper exhibits high dimensional stability over temperature, which is advantageous for electronic components by not introducing complex thermal parasitic effects in the electronic devices' behavior. For example, an optically transparent paper made of cellulose nanofibers has a coefficient of thermal expansion (CTE) of <8.5 ppm [44], which is much lower than plastic (PEN: 18–20 ppm; PET: 22–25 ppm). Besides, paper can also endure annealing cycles at 200°C for short time periods without any noticeable change in physical characteristics.
