# A bird’s eye view on the two-dimensional landscape

### Francesco Bonaccorso, Vittorio Pellegrini

1 A brief journey in the last fifteen years of flatlandia

After 16 years from its isolation and first experimental studies, graphene is emerging as a multi- functional material mature to enter different market areas addressing diverse applications. The unique mechanical, electrical, thermal and optical properties of graphene are suggesting key applications in batteries, smart coatings, composites and optoelectronic devices. While the so-called killer application has not been identified yet, the availability of a large-scale synthesis process is pivotal for any successful commercial exploitation of the material, compatible with the industrial requirements of mass production and repeatability. To date, the synthesis relies on two main routes: the bottom-up and the top-down approaches. The chemical vapour deposition (CVD) is the most representative and industrially relevant bottom-up technique leading to high-quality large-area graphene. Today the approach allows a continuous (roll-to-roll) production that enables first the production of graphene on a copper (Cu) substrate and then the transfer of the two-dimensional crystal from Cu to the targeted substrate chosen according to the final application. Despite the high cost of the CVD method, it is becoming suitable for high-value-added applications, e.g., photonics, electronics and flexible electronics. Integration of graphene in current semiconductor technology still represents a challenge.

On the contrary, in the top-down approach, graphite crystals are exfoliated or peeled off to achieve ultra-thin flakes. The most commonly used top-down method compatible with large-scale production is the liquid phase exfoliation (LPE). In the LPE method, graphite is exfoliated in liquid solvents (organic or water-based) by exploiting cavitation or shear forces to extract single- (SLG), few-layers (FLG) graphene flakes and graphene nanoplatelets (GNPs) (number of layers >10). The outcome of this method is a liquid suspension with a distribution of SLG, FLG and GNPs (depending on the type of the LPE approach used) with a concentration ranging typically from 1 to 10 g/L. The LPE process can be scaled up and the exfoliated flakes can be deposited or printed on different substrates using well-known techniques, e.g., ink-jet printing, flexography, spray-coating. Also the LPE method can be applied for the exfoliation of other layered crystals such as hexagonal boron nitride (h-BN) black phosphorous, and transition metal mono- and di-chalcogenides. Recently, our group has proposed and patented the use of the wet-jet mill (WJM) process to exfoliate different layered crystals, for the large-scale production of high-quality 2D crystals, see fig. 1. The WJM exploits high pressure (180–250 MPa) to force the passage of the solvent/layered-crystal mixture through perforated disks, with adjustable hole diameters (0.3–0.1 mm, named nozzle), strongly enhancing the effectiveness of the shear forces. The resulting exfoliated graphene flakes have average thickness of less than 3 nm (less than 10 layers) and main lateral size distribution of ~300–500 nm. The main advantage of the WJM compared to all the aforementioned techniques is the process time of the sample, therefore enabling higher production capability at lower cost. Based on this patented process we founded BeDimensional Spa, which is now in the process of industrializing the production of 2D crystals.

In this article, we concentrate on the production of 2D crystals by LPE. We first comment on the key aspects of material quality and then we overview applications in composites, coatings and energy.

2 Not always (only) graphene

When a new material is discovered or proposed for new applications, it takes a while to develop appropriate methods for high-quality and low-cost production. A representative example is offered by aluminum, which was firstly produced in powder form (30 grams) in 1829 by the German chemist Friedrich Woehler. However, its industrial production able to provide reproducible quality occurred more than 50 years later. At that stage, at the end of the XIX century, aluminum began to be used in several key applications (trains, cars and airplanes), thus creating a real incentive for the development of new industries. Graphene might follow the same path although its development is expected to be much faster. Infact after 16 years from the isolation of the first crystal of graphene, a few low-cost methods for the large-scale (tons) or large-area (meter square) production of high-quality graphene for the manufacturing market (composites, coatings, energy) or optoelectronic/photonics field, are already available. A European roadmap published in 2015 (and currently under revision) prioritizes the most promising areas of technological developments and highlights the emerging methods for large-scale production of graphene and other two-dimensional materials. Among them, the area of energy applications appears as one of the most promising.

When addressing the quality of graphene that is produced nowadays, mainly two different aspects should be considered. The first one is the presence of impurity atoms or chemical species that, due to the production method, remain adsorbed on the surface of the flake or trapped between the flakes, for the case of multi-layered crystals. Those impurities have different consequences such as the reduction of thermal and/or electrical conductivities, the modification of the optical absorption spectral properties, a deterioration of the mechanical properties and a possible negative environmental impact. For example, the CVD on catalytic Cu substrates is the growth method currently exploited to obtain large-area monolayer graphene for optoelectronic applications. However, this method yields metallic contamination levels that are typically well above the specifications requested for wafer integration of such graphene layers and therefore alternative insulating CVD substrates (such as sapphire) are being explored. Another example is given by the production of graphene oxide (GO), an insulating material that, however, can be made conductive by appropriate post-treatments allowing for removal of the oxygen functional groups, obtaining reduced graphene oxide (RGO). In case of graphene flakes obtained by LPE in organic solvents, the challenge is the removal of the solvent molecules by efficient drying methods.

The second aspect to consider is the actual thickness of the produced graphene flakes, i.e., the average number of layers of the exfoliated flake. While this aspect is not relevant for the CVD growth that has the capability to grow a polycrystalline graphene layer on large areas but with atomic thickness, it instead becomes very critical for industrial methods targeting production of flakes at the tons scale. According to the newly established ISO certification, FLG at room temperature is defined as a material composed of less than 10 atomic layers, a thickness region characterized by specific fingerprints in the Raman spectra. Many studies have highlighted the importance of such atomic-thickness regime when the material is used in manufacturing applications such as, for example, filler in thermoplastic composites or in smart coatings.

The importance of the degree of exfoliation, i.e., thickness or number of layers in the classification of graphene flakes produced in large amounts (i.e., by LPE, for example) has been recently highlighted by Kauling et al. The work demonstrates that the so-called graphene material produced by 60 different worldwide manufacturers is actually composed mainly of multilayer graphene (MLG) or GNPs, which are not optimal for most applications.

The considerations reported above highlight the crucial importance of the quality assessment of the two-dimensional crystals produced for commercial applications. One of the forthcoming challenges in order to close the gap between lab-scale prototypes and commercial products will be to combine material quality certification, reproducibility in the industrial production line and cost.

This will be pivotal for the commercial exploitations of the potential applications outlined in the following sections.

3 Being smart with 2D coatings and composites

3.1 Composites
Graphene has been deeply investigated for its use in coatings and composites due to the very large potential markets. In fact, composites with enhanced properties compared to the bare polymers, or coatings can be used in several markets that benefits from material developments.

The research and development goal is focused towards the design and realization of structural materials that offer functionalities in term of mechanical strength, thermal and electrical conductivities, protection to corrosion and barrier properties to gas/liquid/salt.

In this context, graphene-based materials have been used to enhance the properties of the bulk material, e.g., a polymer, ceramic, concrete, or to improve the properties of surfaces. In composites, graphene-based materials are mostly used as powders or flakes in the form of: FLG and MLG, GNPs, GO or RGO. For coatings instead, films of mainly FLG, RGO and GO are mostly used for anticorrosion coatings or electromagnetic interference (EMI) shielding and anti-triboelectric coatings. These films have also been exploited as gas/liquid separation membranes (e.g., water desalination membrane).

At the industrial level, the introduction of a new material is not always a simple task to be accomplished. However, in the case when there is no need to change the production technology, this introduction can be straightforward. Many suppliers are investigating the use of pre-dispersions in different forms, i.e., solid, liquids or polymers, used as intermediates (e.g., masterbatch for the thermoplastic industry and pre-pregs or resin dispersions for thermosets and graphene-based dispersion in solids for the rubber industry) for test and development of graphene-enabled final products.

It is important to highlight the fact that the addition of graphene to a composite determines a cost increase, depending on the loading and quality of the graphene-based material itself. It is therefore important to determine an objective evaluation of cost/benefit towards both the state of the art and the competing technologies. To this extent, although the cost is predictable, the benefits need to be clear. In fact, it is not sufficient to compare the composite with the bare/original material, but instead a deep comparison with competing technologies is needed. This has to be done at the industrial level, i.e., often tests are carried out only in laboratories, being key to show the potential in demonstrators.

The cost as well as the comparison to competing technologies becomes even more severe with consumer polymers, e.g., polyetilene, polypropylene, acrylonitrile butadiene styrene (ABS), etc., in which the polymer costs ~1–6 €/kg. The addition of a graphene-based material at a loading ~1–3% can significantly enhance the cost of the final material. Thus, the comparison with other materials showing comparable properties is mandatory. Moreover, the fine tuning of the flake morphologies, i.e., surface area ( A ) and thickness ( t ), see fig. 2, to reduce the filler content and optimize the performances of the final product is a critical aspect to address.

In this context, one example comes from our group. Figure 2b summarizes the impact of different flake morphologies i.e., A and t on the mechanical properties enhancement of polymer composites loaded with graphitic materials having different aspect ratio. In particular, our group has shown that there is a linear relation between the mechanical reinforcement at ultra-low (0.1%) filler loading and the $A/t^2$ ratio for three representative 2D fillers, i.e., graphene, h-BN, and tungsten disulphide (WS2) produced by LPE. Significant enhancements above what can be obtained by other conventional fillers (glass fibers, for example) at high loading (typically above 20% in loading) are achievable in such ultra-low doping regime only when the average thickness of the flakes is around 2–3 nm (less than 10 layers) and $A/t^2$ is > 4.0 × 104. In the GNPs regime (thickness above 3nm) the improvements become marginal, as shown in fig. 2b.

Graphene-based materials have also been proposed as replacement of conductive carbon black in composites to improve electrical conductivity. In this context, the key advantage relies on saving active material. In fact, lower loadings, compared to carbon black, are needed to achieve better performance, with consequent reduction of material used. However, the calculation is simple, if the loading is reduced by a factor of 10, graphene should cost less than 10 times carbon black (~1–2 €/kg for carbon black used as filler in the rubber industry, and ~16–60€/kg for conductive carbon black). Also in this case, a double effort is needed, i.e.: i) optimization of the fine-tuning of the flake morphologies to reduce the filler content and optimize the performances of the final product and ii) reduction of the production costs.

The introduction of graphene in these market sectors has still many challenges. Firstly, the cost/benefit ratio is not yet clear. This aspect is crucial but it becomes critically important for cost-sensitive markets such as consumer and automotive. In addition, the graphene properties that are eventually transferred to bulk material are far from being the ideal ones. Here, the production of the right quality material and its possible functionalization are currently the main bottlenecks. Lack of international standards determines uncertainty of quality assurance and quality control of the current graphene supply, which hinders market uptake, because very different materials in terms of price and quality are sold as “graphene”. From the other side of the value chain, i.e., the end-users, it is often unclear which material (i.e., quality, loading, functionalization) is best suited for the target application. Moreover, end-users only do testing, but integration and innovation are performed at component or prepreg/masterbatch supplier level. In addition, there are several challenges directly related to the processing. In many cases, there is a lack of technical expertise for applications and finding the right formulation/processing is rather time consuming. For example, for thermal and electrical conductive composites there are key parameters to be considered/controlled. In fact, to transport high electrical current, isotropic bulk transport is needed, requiring a 3D architecture to increase the overall transported power and current. On the contrary, anisotropy can be an added value, but it can induce an extra barrier, as it is more complex to be designed than isotropy. In addition, there is the issue with scalability. In fact, the formulation/processing must be consolidated at industrial scale, considering the reproducibility of large-scale homogenization, dispersion and mixing in the respective solvent or matrix throughout the processes. There are, non-secondary, other aspects to be considered for the establishment of graphene in these markets. Firstly, we have to consider the infancy of the supply/value chain, which fights against established and conservative supply chains. Furthermore, life cycle health and safety as well as end of life properties need to be considered. Last but not least, the addition of graphene determines a significant change in coloration of the final products, even at low loadings. Since a black colour is often not desirable, a different option is needed. In this case we have already the solution with the possibility to use h-BN, as well as other 2D crystals, for many applications.

3.2 Coatings
A coating covers only the surface of the sample/host, thus the functionality is only active at the surface. There are multiple applications for coatings such as conductive inks for printed electronics applications and transparent, flexible and conductive films, coatings for electronics, sensing and photonics applications, coatings for membranes and filtering, and coatings for photocatalysis, photovoltaics (PV), fuel cells, batteries and supercapacitors. An exhaustive and comprehensive analysis is behind the scope of this paper and the readers are invited to refer to specialized reviews in each of these sectors. Here, we briefly discuss the large-scale (~m2) graphene-based coatings/varnishes/paints for industrial applications such as multifunctional smart coatings and surfaces in the construction industry, automotive, aerospace and marine applications.

In terms of functionalities, anti-corrosion is one of the most important applications addressed by coatings, with the marine industry that represents the largest market for anti-corrosion coatings. The barrier coatings market for packaging, the anti-microbial coatings, as well as the coated fabrics are also of interest for graphene-based materials. In particular, fabrics are coated to increase flame retardancy, change their wettability, or make them resistant against wear.

Graphene-based materials are ideal candidates for coatings due to their properties such as thinness, large aspect ratio, flexibility, stretchability, chemical inertness and anisotropy. These properties confer multi-functionality to the coating (e.g., thermal and electrical conductivity, flexibility and barrier against corrosion/gases/liquids or flame retardancy). In addition, graphene-based coatings have shown lubrication and tribological enhancement in hard disk coatings, electrical contacts or extreme pressure conditions.

Although from a technological point of view graphene grown by CVD can be applied on different substrates as coating, the industrialization of graphene-based coatings is strongly linked to the possibility of applying long-lasting coatings on a large area, and therefore, most of the opportunities will come from paints or inks. These can be deposited exploiting different techniques such as printing, spraying, dipping, impregnation or layer-by-layer deposition, see fig. 3. For coatings and inks, graphene-based materials can be used as additives similar to composites to enhance properties of common coatings. The added value of graphene-based coatings relies on the fact that it can satisfy both technological issues, e.g., heat dissipation for the increasing integration of electronics, and also market need, e.g., reduced use of de-icing agents in de-icing coatings for aircrafts.

Even though the technological advances of graphene-based coatings are relevant, several issues exist and need to be addressed. The challenges identified and discussed for graphene-based composite materials are also valid for coatings and have to be considered for the market penetration of such technology. It is important to highlight that for the markets addressed by graphene-based coatings, there are already established technologies/materials/additives that work well. Many industries in the field are rather conservative and as such new entries are considered only when two requirements are fulfilled, i.e., they are mature enough and the state of the art has reached its limit. Further, the uniqueness of graphene-based materials mainly lies in the combination of properties, but for specific uses in which the multi-functionality is not necessarily needed, other coatings might work equally well or might be more cost effective. The latter is a critical point because coatings are mostly mass-produced and cost sensitive. Thus the cost/performance ratio plays a key role. Contrarily to bulk materials, the amount of material needed for a functional coating is low, thus the cost of graphene-based materials could not be an unaffordable factor. On the contrary, the production cost for both formulation and application of the coating could instead be much more relevant. However, once the formulation and process for inks/paints/coatings is established for a certain application, the cost will be reduced and could have an affordable value for many market sectors.

As in the case of any new coating that is approaching the market, also the graphene-based ones must fulfill the requirement for reproducibility and quality of the coatings, reliability, durability, integrity and low wear. These properties have to be demonstrated not only at the proof of concept stage but, and more importantly, at the industrial level. This is of utmost importance for films on solid substrates and for textile coatings, which have to withstand hundreds of washing cycles.

The possibility to functionalize graphene-based materials is, on the one hand, a great opportunity to fine-tune and optimize the ink/paint/paste formulation as well as the graphene-based coating and a real necessity (i.e., stability for transport and shelf life). On the other hand, the functionalization poses a barrier in terms of the manifold of potential solutions for a given application. In addition, the functionalization processes need to be scalable industrially.

4 A new flat way to store energy

Energy storage is one of the main challenges that our society is facing today. Rechargeable or secondary batteries, as opposite to primary ones that cannot be recharged, are at the core of a range of applications, from portable electronics to automotive systems. Alessandro Volta was the first in 1796 to demonstrate a practical battery, the so-called Voltaic pile, capable to exploit the energy delivered by spontaneous chemical redox reactions to produce electric power in a controlled way. Batteries have evolved significantly since Volta’s time. Remarkably similar to the architecture introduced by Volta, the modern battery is an electrochemical cell composed of three crucial elements: a negative electrode (anode) able to accommodate ions during charging and to release electrons to the external circuit during discharge; a positive electrode (cathode) that is reduced during discharge; and an electrolyte solution containing dissociated salts, which enable ions transfer between the two electrodes. Each of these components is vital to develop to perfection in order to reach the high performances required by consumers. Nowadays, Li-ion batteries (LIBs) are the best options to guarantee the progressive expansion of different pivotal technologies such as those of hybrid/full electric vehicles. While LIBs already represent the technology of choice for portable electronics (smartphones, laptop), key improvements are still required in order to promote electrical storage systems in the mobility sector that can compete with internal combustion engines. Power and energy density efficiencies, lifetime, fast recharging times and cost, together with additional functionalities such as flexibility, are the challenges to be tackled for the future and the majority of these challenges require the development of novel materials for anodes and cathodes. Additionally, different technologies such as solid-state Li-ion batteries, Li-Sulphur batteries, metal-air batteries and hybrid supercapacitors promise to significantly surpass LIBs in the medium-long term.

Most of the commercial Li-ion batteries are based on a LiCoO2 cathode on aluminum substrate and a graphite anode on Cu, which yield a theoretical specific energy density of 387 Wh kg–1 and an energy density of commercial systems that has reached today values of 200–250 Wh kg–1 (a target of at least 400Wh kg–1 for applications in the automotive sector is set by several roadmaps for 2025.) Graphite, in particular, has a theoretical capacity limited to 372 mAh g–1 and its replacement represents one of the major challenging strategies to increase the performances of Li-ion batteries. It is now widely accepted that silicon (Si) is the contender for next-generation anode materials owing to its high specific capacity (3579 mAh g–1), availability, cost, and environmental benignity. However, to bring Si in the Li-ion battery market, there is a need to re-design and modify Si-based anodes to overcome the physicochemical degradation of the electrode. The latter is the result of the electrochemically driven volume expansion (> 300%) from the lithium alloying reactions rapidly leading, upon just a few cycling, to degradation of the battery performance. Additionally, Si is a semiconductor, which does not favor redox reactions and electron transport.

One effective way to achieve prolonged cycling with Si is to fabricate uniform distributions of active Si nano-particles onto a flexible and conductive matrix, either to accommodate the volume changes in Si and to enhance the electrical conductivity of the electrode. Graphene has emerged as an ideal candidate for this role. Pristine FLG exhibits excellent electrical conductivity mechanical strength and for these reasons, it is becoming the preferential choice among other additives for Si-based anodes. It should be recalled that graphene was initially proposed as the active material to replace graphite in LIBs. However, due to its large specific surface area (SSA) leading to an unstable interface with the electrolyte combined with a relatively small improvement in the theoretical capacity compared to graphite (744 mAh g–1 vs. 372 mAh g–1) has led to the conclusion that graphene alone is not probably suited to accomplish this task.

The incorporation of graphene in Si anodes has made significant advances in the last years towards a possible commercialization of such type of electrodes. As an example of the maturity of the material, fig. 4 reports a prototype LIB coin cell in which a Si-graphene anode is combined with a commercial LiNi1/3Mn1/3Co1/3O2 (NMC111) cathode provided by the company Varta. The operating voltage of the cell is 3.7 V and fig. 4 reports an outstanding stable areal capacity >1.6 mAh cm–2 and a capacity retention exceeding 90% after 300 cycles at $C/2$, corresponding to a calculated energy density of ~395 Wh kg–1. Following these and other similar results, Varta developed a prototype coin cell similar to one of its most advanced products.

Batteries can store high energy densities but display low power densities and therefore they are not appropriate in applications requiring store/release of energy in short times. Typical examples are energy regenerative braking systems, emergency power units in avionics and trains. Capacitors can in principle address these applications since they are able to offer such high power densities (>10 kW kg–1) but their very low energy densities $E$, given by the relation $E=1/2 CV^{2}$ (in which $C$ is the capacitance of the device and $V$ the voltage applied) prevents their commercial exploitation for this or similar uses. The solution is provided by supercapacitor devices. Since $C$ is inversely proportional to the distance between the two plates of the capacitor, the supercapacitors are designed to have such a distance reduced to the nanometer range. This is achieved in supercapacitors based on a reversible accumulation of charges at the interface of the electrodes and electrolyte through ion adsorption, the so-called electrochemical double-layer capacitors (EDLCs). Electrochemical double-layer capacitors represent a promising approach towards innovative energy storage solutions that combine relatively high energy density with high power density. Despite that, the specific energy of commercial EDLCs (< 10 Wh kg–1) is still significantly lower than that of the battery systems and this calls for additional efforts particularly on the material side to fill the gap between supercapacitors and batteries. Activated carbon (AC) is currently used as electrodes in supercapacitors thanks to its high SSA (between 500 and 3500 m2g–1) as well as its low cost (<10 USD kg–1). Graphene can also play a role in this technology, possibly not as a replacement of AC but in combination with this material. The large SSA of graphene combined with electrical mechanical and thermal performances suggests promising routes of development. As an example of the possible impact of graphene in EDLCs, recently we have proposed a sprayable “green” ink of EDLC electrode materials based on a mixture of AC and single/few-layer graphene (SLG/FLG) flakes as active materials. The introduction of graphene leads to spray-coating-based electrodes that efficiently operate in a wide range of temperature (−40 /+100 °C) reaching energy density > 12.5 Wh kg–1 at power densities of 30 kW kg–1 overcoming the specific power limits of graphene-free devices.

The specific examples provided above highlight how graphene can impact the field of energy storage when used as additive in the formulation of electrode composite materials. Despite the promising performances achieved, the commercial exploitation of graphene-based supercapacitors and Li-ion batteries will depend on several factors, including cost and supply of the material, certification, and strategic considerations of battery producers and it is expected to require a few more years. Meanwhile other two-dimensional materials beyond graphene are exploited as additives or active materials in LIB, while graphene is studied in metal-air and Li-S batteries, which promise much higher energy densities compared to LIBs. A portfolio of novel material solutions for both cathodes and anodes are being investigated and this suggests that the coming years will see an expanding research activity in the field of energy storage based on technologies related to two-dimensional crystals.

5 Thinner materials for high efficiency

A photovoltaic (PV) device or solar cell converts the electromagnetic radiation in electrical current. The most important figure of merit (FoM) of a solar cell is the energy conversion efficiency, which is the percentage of the solar energy shining on a PV device that is converted into electricity. The PV sector is classified in 3 technology generations. The 1st generation PV is based on crystalline-Si (c-Si), which dominates the PVs market with ~90% market share and a certified efficiency exceeding 25%. 2nd generation PV is based on thin-film technologies, e.g., CdTe, with an overall market ~5–15%. The aim of 2nd generation PV relies on the reduction of both material and production costs compared to 1st generation one. The 2nd generation PVs are flexible and lightweight appealing solutions for roof-top applications and in building integrated PV (BIPV). The 3rd generation PVs encompass: organic photovoltaics (OPV), dye sensitized solar cells (DSSC), quantum dot photovoltaics (QDPV), and perovskite solar cells (PSCs). The 3rd generation PV is driven by further cost reduction and use of more abundant materials, targeting the design and development of flexible devices as well as indoor and off-grid applications, e.g., in the internet of things. An added value of 3rd generation PVs is the fact that the devices can be (semi-) transparent, thus new BIPV concepts and tandem solar cells might be developed.

Graphene is of particular interest for solar cells due to the high electrical conductivity, coupled with transmittance of ~97.7% for single layer. In all the PV technologies, graphene can be considered for the realization of transparent conductive electrodes (TCE). In 1st generation PV, graphene-based conductive inks can be used to realize power bars/connectors, competing with the current state-of-the art silver-based ink. In 2nd and 3rd generation PVs, due to the high SSA, graphene can also be used as Pt-free counter-electrode (CE). In DSSCs, QDSCs and OPVs graphene quantum dots can play a role as photosensitizers for light harvesting. Graphene with tuneable work function can be used as an interfacial (buffer) layer, charge transport layer or dopant for charge transport layers in OPVs, and PSCs to improve both charge transport and collection. In this context, graphene can be used as electron transport layer (ETL), while graphene-based materials, e.g., GO as well as other inorganic 2D crystals such as MoS2, WSe2, etc., can be exploited as hole transport material (HTM) to replace PEDOT:PSS (for OPV) and Spiro-OMeTAD (for PSCs). Contrary to inorganic HTMs such as metal oxides, graphene-based materials are compatible with roll-to-roll processes, not requiring vacuum techniques for the deposition. In addition, the tuneable work function, coupled with the resistance against corrosion make graphene-based materials effective buffer-layers. In PSCs graphene-based materials have been used for this purpose, increasing stability and efficiency of the cells. The combination of graphene with other 2D materials (i.e., MoS2 quantum dots) allowed PSCs to reach an efficiency >20% in 2018, and ~13.4% and ~15.3% on active areas of 108 cm2 and 82 cm2, respectively, see fig. 5. Graphene-based materials cover also other roles in PV technologies. For example, graphene was used to develop low-temperature (< 150 °C) processed PSCs, reducing the fabrication costs. Moreover, graphene can be used as barrier for moisture, and as encapsulant in PSCs. Graphene-based materials can prevent the entrance of moisture and reduce internal degradation through heating and, thus, increase durability. Regarding the PSCs, this technology can be considered to be at an early stage of development and, as such, major issues for implementation are still under investigation. Current critical issues of PSCs are their Pb content, the stability and the scaling up. Since graphene-based materials address several of the aforementioned issues (e.g., stability and manufacturability) they can represent a great opportunity to boost further the development of this technology.

Graphene-based materials fit standard processes used in 2nd and 3rd generation PV. Here, the engagement with the PV manufactures and test under industrially relevant conditions are mandatory to close the gap towards the commercialization. The fact that the use of cleanroom is not needed and most processes are based on wet chemistry and suitable for roll-to-roll manufacturing techniques make the integration of graphene-based materials compatible with common production techniques. However, there is the need to elaborate the benefit of 2nd and 3rd generation PV solutions compared to 1st generation PV. Here, graphene-based materials, acting as intermediate recombination layer, could enable or support the hybridization of various 3rd generation PVs in tandem structures with 1st generation PV to reach higher efficiencies. It is important to strengthen the unique selling proposition of 3rd generation PV with graphene advancement. The use of graphene-based materials as TCE in all the PV technologies appears still premature and other solutions offer better performances. For 3rd generation PV and in particular for PSCs, graphene-based materials can address some of the current drawbacks and appear particularly interesting, e.g., by interfacial engineering to improve both stability and efficiency.

6 We are just at the beginning

This paper addressed the challenges linked to the industrial/commercial exploitation of graphene and other 2D crystals. Standardization of production methods and material quality and cost/ performance evaluation play the pivotal roles. We have also highlighted some of the most promising applications in the field of composites, coatings and energy. After 16 years from the first experimental studies on graphene, this novel material is finally approaching its last mile before becoming a mature material technology for different applications. The optimization of production capabilities and material quality certifications still represent a gap to be filled for the industrialization.

Acknowledgments

We thank S. Abouali, S. Bellani, A. E. Del Rio Castillo, A. Di Carlo, E. Kymakis, N. Pugno, C. Stangl, S. B. Thorat and P. S. Toth. We acknowledge funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 881603‐GrapheneCore3. This project has received funding from European Union’s MSCA‐ITN ULTIMATE project under grant agreement No. 813036 and SENSIBAT grant agreement No. 957213 and from the Italian Ministry of Foreign Affairs and International Cooperation (MAECI) through Cooperation Project “GINGSENG” (Grant PGR05249) between Italy and China.