Polymer Nanocomposite Coatings for Organic Solar Cells

For a sustainable society, the supply of cheap and green electricity is indispensable (Smalley 2005). For this purpose, one of the most explored technologies that eventually could overcome the energy problem is solar cells or photovoltaic cells (PVs). These devices convert the electromagnetic energy radiation from the sun into electrical energy, which, in principle, could meet the energy needs (Krebs 2008).

In general, conventional PVs are silicon-based p-type (electron-deficient) and n-type (electron-rich) semiconductor junctions, with maximum power conversion efficiency (PCE) around 25%, and the commercial ones around 16% (O’Regan and Gratzel 1991; Philipps and Bett 2016). However, drawbacks of these PVs are their high cost and fragility (Markavart 2005).

Based on the above, alternatives have been explored with organic compounds, i.e., organic photovoltaics (OPVs) (Thompson and Frechet 2008). In OPVs, the most important component is the active layer, which basically is a conjugated polymer nanocomposite coating. The conjugated polymers are the matrix, while nanomaterials like fullerene derivatives PC6()BM, PC71BM and indene-C60 bisadduct (ICBA) (Wadsworth et al. 2019) are the reinforcements (Figure 3.7).

In order to obtain better PCE values in an OPV, there are some paths that can be addressed. One is the chemical modification and engineering of the polymers and nanomaterials, and the other is to find a way to produce better characteristics of the coating. These strategies can be used as a simple and effective modification strategy to optimize the active-layer nanocomposite morphology through the enhancement of crystallinity of the resulting molecules, improved planarity, better molecular orientation, etc. Dramatically improving the PCE enhances the morphological miscibility in the active-layer nanocomposite coating.

Here we present some of the strategies that can be taken in order to achieve a better and improved active layer performance.

D–A Strategy and Halogenation

D-A Strategy: The ultimate advantages of conjugated polymers are their tunable properties such as band gap, molecular energy levels, carrier mobility, morphology, among others. A general strategy to synthesize a conjugated polymer is by combining an

OPV general architecture

FIGURE 3.7 OPV general architecture.

electron-rich monomer (donor) and an electron-deficient monomer (acceptor), in order to polymerize a D-A alternating conjugated polymer. When the donor monomer is bonded to the acceptor monomer, it results in a D-A conjugated polymer that has a linear combination of molecular orbitals, i.e., the HOMO (highest occupied molecular orbital) of the polymer is similar to the HOMO of the donor monomer, while the LUMO (lowest unoccupied molecular orbital) of the polymer is similar to the LUMO of the acceptor monomer, resulting in a lower band gap and therefore capable of use in a wider solar spectrum, as seen in Figure 3.8.

Halogenation: This is another approach to improve and control the properties of the conjugated polymers; for example, fluorinated materials show enhanced extinction coefficient and higher charge carrier mobility compared to their nonfluorinated analogues. Also, fluorination as a simple and effective molecular modification strategy is used to optimize the active-layer nanocomposite morphology through the enhanced crystallinity of the resulting molecules, improved planarity, and better molecular orientation.

Small Molecules

In comparison with the traditional fullerene derivatives, small molecules (SM) possess higher reproducibility in synthesis and PV performance and more direct and reliable analysis in the structure-property relationships. Nevertheless, when compared to polymers, SM frequently exhibits lower extinction coefficient, lower hole mobility, and poorer coating-forming properties. To overcome these shortcomings of SM donors, researchers have made many successful attempts, especially the molecular fluorination (Fan et al. 2019). Some examples of conjugated polymers and SMs can be seen in Figure 3.9.

Band gap engineering by D-A strategy

FIGURE 3.8 Band gap engineering by D-A strategy.

Chemical structures of representative

FIGURE 3.9 Chemical structures of representative (a) SM-donors, (b) polymer acceptors, and (c) SM-acceptors used in OPVs. Reprinted with permission from Fan et al., “Fluorinated photovoltaic materials for high-performance organic solar cells”. Chemistry - An Asian Journal. Sep 16:14(18):3085-3095, 2019.

Polymer Nanocomposite Coatings with 2D Materials

To obtain a polymer-based nanocomposite that can be applied as a coating and also provide the function of an active layer in a PV, one needs to consider what kind of acceptor materials can be used. In this regard, since the discovery of the one- atom thin graphene in 2004 (Novoselov et al. 2004), the so-called 2D materials have attracted much attention, because of their huge range of properties. Two-dimensional materials present strong in-plane covalent bonds, whereas they are stacked vertically by weak van der Waals (vdW) forces. The electronic, optical, mechanical, and thermal properties of these materials are strongly dependent on the number of layers and therefore the thickness, exhibiting a drastic change when they are in a monolayer or in few layers in comparison to its bulk counterpart (Sun et al. 2019). In order to achieve solution processing, 2D materials are often required to be chemically modified or functionalized. As a good example of this, one can consider graphene derivatives like GO and rGO, which have been already used in the OPVs as photoactive layer (Liu et al. 2008; Mendez-Romero et al. 2019). Figure 3.10 shows a nonco- valent functionalization approach of GO with octadecylamine (ODA) from which the obtained GO-ODA can be easily transferred to organic solvents like oDCB or toluene (Mendez-Romero et al. 2019). With this method, apart from obtaining GO soluble in organic solvents, the introduction of the ODA groups prevents the restacking of the material due to steric hindrance.

Nevertheless, the introduction of oxygen functional groups induces a rise in GO’S band gap up to ~3eV (Mendez-Romero et al. 2019) and a high electrical resistance (~10u Q.) (Mendez-Romero et al. 2019), which is detrimental for the carrier transport and final performance of the OPV devices. To overcome this issue, a reduction where some of the oxygen groups are removed can be conducted, thereby obtaining a material called rGO. Figure 3.11 shows an easy and straightforward method to conduct the chemical and thermal reduction of the GO-ODA to obtain rGO-ODA that maintains its solubility in organic solvents. In this way, the rGO-ODA can be mixed with some polymer, ensuring a good dispersion that later can be translated into obtaining a polymer nanocomposite coating without agglomeration or segregation, enabling it to be used as a photoactive layer.

Other 2D materials that are suitable to be used in OPV devices are the family of TMDCs because of their intrinsic semiconducting nature (Sun et al. 2019). Among TMDCs, one of the most studied materials, MoS2 exhibits a band gap change from ~1.29eV as a multilayer to ~1.9eV when it is a monolayer (Singh et al. 2017). This band gap change also shifts from indirect to direct as the number of layers decreases, which is something desirable for PV applications (Splendiani et al. 2010).

As mentioned before, to make feasible the fabrication of the solar cells, liquid- phase processing is desired. Obtaining 2D TMDCs by liquid-phase exfoliation (LPE) has proved to be an interesting approach that would allow these materials to be combined with organic materials and form 2D organic nanocomposites, which can be coated and used in different parts of the OPVs like the photoactive layer.

Nanocomposite Coatings for Conductive Inks

In the last decades, printed electronics has become a low-cost, easy, quick, and reliable way to manufacture functional devices; e.g., sensors, photodetectors, solar cells, organic light-emitting diodes, and supercapacitors (Li, Lemme, and Ostling 2014).

Therefore, the use of conductive inks is increasing in demand and need for better capabilities. Current printing techniques make their elaboration in a faster, cheaper, and easier way, becoming an option for the replacement of photolithography manufacturing systems that requires multiple manufacturing steps as well as the use of chemical reagents and expensive equipment (Kamyshny et al. 2005).

Electrostatic functionalization of GO with ODA, transferred to oDCB or toluene

FIGURE 3.10 Electrostatic functionalization of GO with ODA, transferred to oDCB or toluene. Reprinted with permission from Mendez-Romero et al., “Functionalized reduced graphene oxide with tunable band gap and good solubility in organic solvents”. Carbon 146:491-502, 2019.

Chemical and thermal reduction of functionalized graphene oxide to generate functionalized reduced graphene oxide

FIGURE 3.11 Chemical and thermal reduction of functionalized graphene oxide to generate functionalized reduced graphene oxide (rGO-ODA). Reprinted with permission from Mendez-Romero et al., “Functionalized reduced graphene oxide with tunable band gap and good solubility in organic solvents”. Carbon 146:491-502, 2019.

The conductive inks as conductive coatings are manufactured with a polymer matrix and the incorporation of metallic particles such as Ag, Cu, Au, and Al, whose conductivities are on the order of 107 S/m (Kamyshny and Magdassi 2014). But, these kinds of inks require further steps and curing at high temperatures, making them not optimal for many applications.

Therefore, the development of new nanocomposite materials with the addition of nanostructures belonging to the 2D materials family such as graphene, carbon nanotubes (CNT), transition metal dichalcogenides (TMDs), black phosphorous (BP), and hexagonal boron nitride (h-BN) have become relevant for their novel electrical, thermal, mechanical, and optical properties, which are attributed to the quantum confinement of its nature due to their dimensions (McManus et al. 2017).

Additionally, 2D materials can be dispersed in a wide variety of organic solvents without chemically reacting with the environment, avoiding changes in their intrinsic structure (restacking of the layers). This allows formulation of an ink organic solvents as stable and uniform coatings, and additives can be used without modifying the properties of these materials, generating inks with rheological properties adjustable to different types of printing techniques or substrates.

Actually, there are many printing techniques such as inkjet printing, screen printing, rotary screen, flexographic printing, and gravure printing; each of these techniques gives us different characteristics in the resolution and performance of printed patterns (Hu et al. 2018), as we can see in Figure 3.12.

Comparison between the resolution and the production speed of the different printing techniques

FIGURE 3.12 Comparison between the resolution and the production speed of the different printing techniques. Reprinted with permission from G. Hu et al., “Functional inks and printing of two-dimensional materials,” Chein. Soc. Rev. 47(2015):3265—3300, 2018.

In order to use any of these printing techniques, we must have certain considerations, such as the size and nature of the substrate, ink concentration, as well as the volume of ink needed for each of the different printing techniques.

An ink system based on polymer nanocomposites possess three main components:

  • • Functional materials that give the main characteristics - in this particular case, a polymer nanocomposite of metallic NPs or 2D materials.
  • • A solvent, sometimes called a vehicle, where the material is dissolved or dispersed depending on the nature of the first.
  • • An additive that modifies the rheological properties of viscosity and surface tension.

Functional material: The amount of material present in an ink is defined by its solubility, dispersibility, and stability (e.g., graphene, rGO, carbon nanotubes). For inks that do not contain dissolved materials, the main problem is agglomeration and subsequent coagulation and sedimentation of the materials. The stability of a system is related to the balance between attractive forces and van der Waals forces, which must be counteracted by repulsive forces. There are two main mechanisms to overcome this issue: (1) stabilization by adsorbed charges on the surface that forms a layer, avoiding interaction between particles and (2) stabilization by steric effects, where interactions are prevented by forming a physical barrier through adsorption of a molecule on the surface of the particle (Arao and Kubouchi 2015).

Solvents: The solvent is the vehicle that dissolves or suspends the functional material and other components of the ink. Some inks are water based, they are widely used on substrates like paper and textiles, due to ink-substrate compatibility. However, they have disadvantages in their formulation, due to their need of stabilizers, additives, and buffers to adjust the rheological parameters (Hutchings and Martin 2012).

Solvent-based inkjet inks are the most widespread in industrial graphic applications because of their fast-drying time, print quality, image durability, and compatibility with a wide range of substrates (e.g., metal, glasses, ceramics, plastics). In many formulations, mixtures of various solvents are used, which enable tailoring ink properties such as viscosity, evaporation rate, and surface tension; these properties facilitate obtaining homogenous coatings free of defects (Kamyshny and Magdassi 2012).

Additives: It modifies the physicochemical properties of the medium to make it compatible with some of the processing methods; in some inks, it maintains the stability of the dispersed material. Some additives are as follows:

  • • Humidifiers mainly are used in water-based inks to prevent drying and clogging of the ink in the nozzle at the air-ink interface. The main chemical characteristic that a humectant must satisfy is the presence of hydrophilic groups in its structure, such as hydroxy, carboxy, or amine, in order to adsorb water and delay the evaporation of the solvent during the expulsion of the ink.
  • • Surfactants, sometimes called wetting agents, serve two major purposes within ink formulation. First, they are used to stabilize the dispersion of particles in the medium avoiding material agglomeration. Second, it is the control of surface tension, a very important feature for using an ink in a printing system.
  • • Adhesion promoters are used to generate compatibility between the substrate and the ink. They may simply dry and solidify by solvent evaporation or can require some curing in order to produce cross-linking. The most common polymers used are acrylics, alkyds, cellulose, and resins such as vinyl chloride/vinyl acetate, acrylic resins, rubber resins, and polyketone resins (Hutchings and Martin 2012; Zhan et al. 2017).

Rheological performance: The precise formulation of the ink components defines the physical properties of the inks. Rheology is the science of flow and deformation. Flow properties of coatings are critical for the proper application and appearance of films. Depending on how stress is applied to a fluid, there are several types of flow. Of major importance in coatings is flow under shear stress (Jones, Nichols, and Pappas 2017). The two physical properties that dominate the behavior of an ink in an injection system are viscosity and surface tension (Kamyshny and Magdassi 2012).

Viscosity: It is defined as the resistance that a fluid opposes to the movement when a tangential force (shear or shear force) is applied to the surface. The fluid forms a gradient of velocities (Figure 3.13), because the external zones of the liquid perceive a greater deformation compared to the internal zones of the fluid (Coussot 2012).

Viscosity is a parameter for classifying inks and their use in any printing process. Inks can be separated into two groups of low viscosity and high viscosity. Each group needs a different printing process, as shown in Table 3.1 (Zolek-Tryznowska 2016).

Surface tension: It is a manifestation of the intermolecular forces within the liquids, which make them stay together and is defined as the amount of energy needed to increase the surface of a liquid. In the matter of conductive inks, this rheological property is related to the ability of the ink to coat the substrate once the ink-substrate interaction occurs. The ink must be in a range between 25 and 50 m/Nm, this range is adjusted to the characteristics of inks used in injection systems (Hutchings and Martin 2012; Cummins and Desmulliez 2012; Lyklema 1999).

Scheme of a fluid velocity profile

FIGURE 3.13 Scheme of a fluid velocity profile.

TABLE 3.1

Typical Ink Composition for Common Printing Technologies

Technique

Pigment (wt%)

Binder (wt%)

Solvent (wt%)

Additive (wt%)

Viscosity (mPa s)

Inkjet

5-10

5-20

65-95

1-5

4-30

Screen

12-20

45-65

20-30

1-5

lk-lOk

Gravure

12-17

20-35

60-65

1-2

100-lk

Flexo

12-17

40-45

25-45

1-5

1 k—2 k

Source: Data from Hu. G., J. Kang, L.W. Ng, et al. 2018. Functional inks and printing of two-dimensional materials. Chemical Society Reviews 47 (9): 3265-3300.

Conclusions

Over the last decades, the role of functional coatings has considerably changed. Polymer coating technology has evolved by the increase in scientific and technological understanding of important principles. The development and design of polymer nanocomposites- enabled coatings has gained research importance as they offer the promise of incremental and disruptive improvements to products and processes. Polymer nanocomposite coatings represent properties with incredible practical applications for mechanical, optical, and electronic products. Polymer coatings based on nanocomposites offer significant product performance and cost-saving advantages with functional features.

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