Graphene Interface Engineering for Perovskite Solar Modules: 12.6

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Letter

Graphene Interface Engineering for Perovskite Solar Module: A Power Conversion Efficiency Exceeding 12.5% over 50 cm2 Active Area. Antonio Agresti, Sara Pescetelli, Alessandro Lorenzo Palma, Antonio Esau Del Rio Castillo, Dimitrios Konios, George Kakavelakis, Stefano Razza, Lucio Cinà, Emmanuel Kymakis, Francesco Bonaccorso, and Aldo Di Carlo ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00672 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 27, 2016

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Graphene Interface Engineering for Perovskite Solar Module: a Power Conversion Efficiency Exceeding 12.5% over 50 cm2 Active Area. §

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Antonio Agresti , Sara Pescetelli , Alessandro L. Palma , Antonio E. Del Rio Castillo , ¥

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Dimitrios Konios , George Kakavelakis , Stefano Razza , Lucio Cinà , Emmanuel Kymakis , ǂ

Francesco Bonaccorso (*), Aldo Di Carlo

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§ (*)

C.H.O.S.E. (Centre for Hybrid and Organic Solar Energy), Department of Electronic

Engineering, University of Rome Tor Vergata, via del Politecnico 1, 00133 Rome, Italy ǂ

¥

Istituto Italiano di Tecnologia, Graphene Labs, Via Morego 30, 16163 Genova, Italy Center of Materials Technology and Photonics & Electrical Engineering Department School of

Applied Technology Technological Educational Institute (T.E.I) of Crete Heraklion, 71 004 Crete, Greece. AUTHOR INFORMATION Corresponding Author *Aldo Di Carlo Tel.: +39 6 7259 7456. E-mail address: [email protected]; *Francesco Bonaccorso Tel.: +39 010 71781795. E-mail address: [email protected]

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ABSTRACT

Interfaces between the perovskite solar cells (PSCs) layer components play a pivotal role in obtaining high performance premium cells and large area modules. Graphene and related two dimensional (2D) materials (GRMs) can be used to “on-demand” tune the interface properties of PSCs. We successfully used GRMs to realize large area (active area 50.6 cm2) perovskite-based solar modules (PSMs), achieving record high power conversion efficiency of 12.6%. We ondemand modulated the photo-electrode charge dynamic by doping the mesoporous TiO2 (mTiO2) layer with graphene flakes. Moreover, we exploited lithium-neutralized graphene oxide flakes as interlayer at mTiO2/perovskite interface to improve charge injection. Notably, prolonged aging tests have shown the long-term stability for both small and large area devices using graphenedoped mTiO2. Furthermore, the possibility to produce and process graphene in the form of inks open a promising route to further scale-up and stabilize the PSM, the gateway for the commercialization of this technology.

TOC GRAPHICS


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The recent development of perovskite solar cell (PSC) technology gave rise to an unprecedented power conversion efficiency (PCE) improvement from η=3.8%1 up to 22.1%2 in less than 7 years. The demonstrated PCE values make the PSC technology competitive with second generation thin-film photovoltaics such as copper indium gallium selenide (CIGS) or cadmium telluride (CdTe).3 The PSC success is strictly linked with the remarkable efforts made to improve the device’s structure, to fine control the growth and the morphology of the active perovskite layer, and to engineer the interfaces between the cell’s constituent layers.4–10 The PCE record of PSC11 has been achieved with the archetypal mesoscopic device configuration using a n type mesoporous TiO2 (mTiO2) layer as bottom electron transport layer (ETL)12,13. For this structure,11 a mixture of formamidinium and methylammonium as the monovalent cations with the addition of inorganic cesium has been used to grow perovskite crystals, with the layer morphology finely controlled by exploiting a solvent-engineering technique.14 The latter consists in spin-coating the perovskite layer by using a mixture of γ-butyrolactone (GBL) and dimethyl sulfoxide (DMSO) as the main solvents, followed by a toluene or chlorobenzene (used as antisolvents) treatments during the spinning process.15–18 However, although uniform and pinholefree perovskite layers are produced, this approach is hardly scalable to large, module size substrates due to the difficulty to uniformly deposit by spin coating the anti-solvent on large area substrates. In order to overcome such limitations, several alternative techniques for the perovskite deposition have been proposed,19 with the solution-process methods being the most promising ones, e.g., enabling cost-effective20,21 roll-to-roll production.22 In particular, the onestep deposition allows a pinhole-free perovskite deposition directly on compact TiO2 layer (cTiO2), in the so-called planar configuration, leading to the most efficient fabrication procedure (i.e. PCE approaching 18%).23 However, planar PSC usually suffers from large current-voltage


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(I-V) hysteresis phenomena.24–26 The insertion of a mTiO2 scaffold25 combined with a two-step perovskite deposition,27 with the deposition of PbI2 layer prior to dip the substrate in a methylammonium (MAI)-EtOH solution,28–30 reduces the I−V hysteresis.25 In fact, the presence of mesoporous metal oxide scaffolds such as Al2O331 or TiO232 aids the perovskite crystal formation, hindering short-circuit between photo (PE) and counter (CE) electrode.25,33 The twostep procedure is the preferred perovskite deposition method for large area mesoscopic PSC and perovskite-based solar modules (PSMs), ensuring a deeper perovskite infiltration into the mTiO2 scaffolds compared to the single step process.

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This determines a fine control of the

morphology of the perovskite capping layer over interpenetrated TiO2/perovskite substrate, beneficial to improve the perovskite film uniformity,35 boosting the performance of the final devices.27 Despite the remarkable PCE value achieved by using mesoscopic PSC,36–38 record efficiency (22.1%)11 is yet far from the predicted efficiency limit (~31%).39 Notably, losses due to interfacial recombination40 negatively affect the charge injection at perovskite/transporting layer interface. Similarly, poor charge transport in electron (ETL)41 and hole (HTL)42,43 transporting layers severely limits charge collection at the electrodes. These phenomena lead to a reduction of both device short circuit current (ISC) and fill factor (FF),44–46 thus reducing the PCE. Amongst the recently identified recombination mechanisms for methylammonium lead triiodide (MAPI) based PSCs,47 those involving TiO2/MAPI and MAPI/HTM interfaces play a crucial role in limiting the PCE.40 In particular, i) interfacial electron transfer from the MAPI conduction band (CB) to the HTM and/or to TiO2 surface states47 and ii) interfacial electron transfer from TiO2 CB to the HTM and/or to MAPI need to be prevented.48,49 A fine tuning of interface/interlayer properties is mandatory to enhance the charge transport properties and extraction at


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mTiO2/MAPI interface by: i) chemical treatment of the mTiO2 layer prior to the perovskite deposition32,50 and/or TiO2 doping,51,52 ii) the use of different TiO2 nanostructures53–55 and heterostructures56 and iii) the modification of energy-levels by the addition of interface layers.57– 59

Moreover, the potential to tune the perovskite/mTiO2 interface60 allows to reduce the I-V

curve hysteresis,61 improving, at the same time, the charge collection at the photo-electrode (PE).32 Graphene and related 2D materials (GRMs) are emerging as the paradigm shift of interface engineering to boost both photovoltaic performance19,62,63 and stability of PSC.58,64 In fact, owing to their 2D nature and the large variety of 2D crystals possessing complementary (opto)electronic properties,65 which can be on-demand tuned by chemical functionalization and edge modification,66 GRM can be considered ideal materials for the PSCs interface engineering. The first experiments have demonstrated the use of graphene oxide (GO) or reduced graphene oxide (RGO) as dopants in transport layers67–69 and as interlayers between perovskite and transporting layers70,71 with the aim to improve the charge collection mechanism at the electrodes.72,73

Moreover, GRM-based inks can be produced by cheap and high-yield

manufacturing processes using non-toxic solvents such as ethanol (EtOH) or 2-propanol (IPA).74–76 This allows the integration of GRMs in an in-line production process for large area perovskite devices and modules, with the aim to reduce performance losses experienced by PSCs scaling-up.77,78 In fact, scaling-up process amplify typical problems79 undergone in perovskite films deposition such as nanoscale pinholes,80 crystal grain boundaries81 and perovskite film roughness,82 which can severely affect the film quality and consequently the module PCE.23,83–87 The aforementioned problems have so far limited the maximum delivered power (MDP) of PSMs. The record PSM efficiency, i.e. PCE=14.9 %, has been achieved for an active area of 4


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cm2 with a MDP of 58 mW,88,89 while increasing the active areas up to 60 cm2 the PCE reduces down to 8.7%,14 with a MDP of 552 mW. In this work, we demonstrate that GRMs can indeed be the key elements for an efficient strategy of PSCs scaling up. Graphene interface engineering (GIE) is proposed as an effective way to boost PCE of both PSCs and PSMs by limiting the charge losses occurring at perovskite/mTiO2 interface, improving at the same time the stability. In particular, lithiumneutralized graphene oxide (GO-Li) flakes have been introduced as interlayer at the mTiO2/perovskite interface72 with the aim to improve the charge injection from the perovskite to the mTiO2, while graphene flakes have been dispersed into the mTiO2 layer58 in order to speedup the charge dynamic at the PE.58 This allowed us to realize a GRMs-based PSC module having a PCE of 12.6% on over 50 cm2 active area and a MDP of 638 mW at 1 SUN illumination conditions. Small area cells. Small area solar cells (0.1 cm2) are realized to assess the influence of GIE on PSCs with respect to the reference device (Fig. 1a). We exploit graphene flakes and GO-Li produced by solution processing,75,90 see supplementary information (S.I.) for both technical details and morphological characterization. In addition, Raman characterization91–93 of graphenebased mesoscopic substrates is provided in figure S3. By using the standard two-step production procedure detailed in S.I, the small area reference PSCs (indicated in the following as PSC-A) have shown an averaged PCE of 13.5% calculated on 12 cells. In addition, 3 PSC sets are realized by using the GIE strategy. In particular, in sample B (PSC-B) a GO-Li interlayer is spin coated onto the mTiO2 layer before the perovskite deposition (Fig.1b), while in samples C (PSCC), see Fig. 1c graphene ink is dispersed into the mTiO2 paste to form the graphene-doped cell scaffold (G+mTiO2), see S.I. for experimental details and Raman characterization (see Fig. S3).


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Finally, in device D (PSC-D) the GO-Li interlayer is deposited onto the G+mTiO2 scaffold (Fig.1d), realizing a combined structure. The different PSC configurations are reported in Fig. 1 (see Fig. S4 in the S.I. for the corresponding energy band diagram), while the statistical photovoltaic parameters measured for each set of PSCs are reported in Fig. 2.

Figure 1. Structures of small area PSCs. a) PSC-A with the reference mesoscopic structure FTO/cTiO2/mTiO2/perovskite/spiroOMeTAD/Au, b) PSC-B with GO-Li as interlayer between perovskite and mTiO2, c) PSC-C using graphene-doped mTiO2 layer, d) combined PSC-D structure FTO/cTiO2/G+mTiO2/GO-Li/perovskite/spiro-OMeTAD/Au.


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Figure 2. Photovoltaic parameters measured at 1 SUN and relative standard deviation on 12 PSCs (open circuit voltage - VOC, short circuit current density - JSC, fill factor - FF, and PCE reported in panels a, b, c, d), respectively, for each investigated device typology: type A reference cells, type B GO-Li onto mTiO2, type C graphene-doped mTiO2, type D GO-Li onto G+mTiO2.

The GRM-based PSCs (i.e., PSC-B, PSC-C and PSC-D) show higher PCEs (+14.8%, +13.6%, +6.1%, respectively) with respect to the PSC-A, mainly due to an increase of the short circuit current density (JSC), see Fig. 2. In particular, the insertion of GO-Li, as interlayer between perovskite and mTiO2 (PSC-B), leads to a significant improvement of the average JSC values (+18%), while a 4.3% loss in the averaged open circuit voltage (VOC) is observed. We do hypothesize that the VOC reduction is linked with the presence of the GO-Li, which induces a downward displacement of the TiO2 conduction band (CB) with a consequent reduction of VOC72,94 (see Fig. 2 panel a). With respect to the use of graphene flakes, the GO-Li insertion


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within the mesoporous layer resulted in an increased JSC (+9.6%) compared to the PSC-A (Fig. 2 panel b). Contrary to the GO-Li case, the graphene flakes addition does not lead to a reduction in the VOC value. Finally, the type D structure shows an overall PCE improvement of about 7% with respect to the reference one, which is linked with an increase of 7% of the JSC value, retaining, at the same time, satisfying VOC values, when compared to the PSC-A. We point out that the reduction of the PCE standard deviation achieved in the case of PSC-D is highly desirable for large area PSCs, where local inhomogeneity of the active layer can affect the devices PCE.34,95 In order to get a deeper understanding onto the effect of GIE on the PSCs performance, electro-optical characterizations and transient measurements are carried out on encapsulated PSCs, see S.I. for details. The increase in JSC for PSC-B and PSC-C (see Fig. 2b), with respect to PSC-A, is confirmed by the incident photon to current conversion efficiency (IPCE) spectra and by the extracted integrated JSC values reported in Fig. S5a. In particular, the GO-Li interlayer enhances the IPCE in the spectral range between 400-600 nm, with respect to the PSC-A,72 up to a maximum of +7% at 440 nm (see Fig. S5b). An IPCE increase of 8.6% is observed at ~760nm. The long-wavelength IPCE rise with respect to reference PSC is more significant for PSC having the G+mTiO2 scaffold (PSC-B), see Fig. S5b. This phenomenon could be linked with two different processes, i.e. (i) efficient electron injection from perovskite to mTiO2 and/or (ii) increased charge transport/collection at the PE. In order to get an insight onto the physical mechanism responsible for the IPCE increase, discriminating the charge injection process from transport/collection phenomena, we carried out transient measurements. We recorded JSC (Fig. 3a) and VOC (Fig. S6a) over 3 decades of incident optical power (i.e. from 0.002 to 2 SUN) by using a white LED, see S.I. for details.


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Figure 3. a) JSC and b) Normalized VOC rise profile acquired by retaining the tested devices in open circuit condition, in the dark and by suddenly (t=0) switching on the light at 1 SUN irradiation condition.

The JSC vs Pinc plots of Fig. 3a show linear trends for all the investigated PSCs, an indication of energy level matching (see Fig. S4) of the device component layers. In fact, a nonlinear trend of JSC versus light intensity is linked with the existence of energy barriers within the device, negatively affecting the charge extraction process.96 Moreover, a nonlinear shape is indicative of geminate recombination mechanism involving the hole/electron exciton and/or space charge limitation at the heterojunction associated with unbalanced electron and hole mobilities.97 For our PSCs, both the insertion of graphene into the mTiO2 layer (PSC-C) and the GO-Li interlayer between perovskite and mTiO2 (PSC-B) led to an improvement of the electrons injection at perovskite/TiO2 interface, as confirmed by the increase of JSC vs Pinc slope,96 i.e., up to 18.4% (Fig. 3a), with respect to the reference PSC-A. In fact, for PSC-B the slope of the linear fitting is 224 mA/W, for PSC-C is 217 mA/W, PSC-D is 225 mA/W, while for the reference PSC-A a value of only 190 mA/W was obtained. The optimization of charge extraction has been experimentally demonstrated recently for both graphene flakes/perovskite58 and GOLi/perovskite72 interfaces. In particular, G. Volonakis et al.98 predicted, by means of firstprinciples calculations on graphene−CH3NH3PbI3 interfaces, that pristine graphene suppresses


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the octahedral tilt in the first perovskite monolayer, leading to a nanoscale ferroelectric distortion with a permanent polarization. This interfacial ferroelectricity drives electron extraction from the perovskite, hindering electron−hole recombination,98 a phenomenon that could explain the electron injection mechanism at the perovskite/graphene-doped mTiO2 interface experimentally demonstrated in this work. Additionally, the energy level matching between GO-Li and mTiO2 layer together with TiO2 trap passivation72 facilitate electron injection in PSCs with GO-Li interlayer. A detailed study of the VOC dependence on Pinc (see Fig. S6a) further confirms that neither graphene flakes nor GO-Li introduce trap states in PSCs. In order to investigate the influence of the GRM on the electron injection and collection at the PE of the devices, we tested the dynamic performance under pulsed light condition with transient photovoltage (TPV) measurements.99 TPV tests have been already used in dye-sensitized solar cell (DSC) technology and recently adopted also for PSCs.99,100 The VOC rise test is carried out by taking the device from steady state operating conditions under dark at open circuit, switching on the light source and monitoring the subsequent rise in photovoltage.101 The concentration of the photo-injected electrons in the TiO2 film primarily determines the photovoltage transient profile. Moreover, the build-up of electrons is in competition with the electron recombination processes, which is detrimental for the device performance. As reported in Fig. 3b, both PSC-B and PSC-D show faster VOC rise profile, which can be associated to a better charge injection at the perovskite/mTiO2 interface, with respect to the reference one. This result is correlated to the increase of JSC vs Pinc slope already reported in Fig. 3a. Remarkably, PSC-C and PSC-D have shown the fastest dynamic due to the presence of graphene flakes within the mTiO2 layer. Thus, the obtained PCE values reported in Fig. 2d (i.e., PCE=16.4% for top PCE using G+mTiO2 layer), are linked with optimized charge injection processes.


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The influence of GIE on the stability of the PSCs is assessed by both prolonged illumination at maximum power point (MPP) and shelf life tests at open circuit conditions (see Fig.S6 and Fig.S7). Amongst the tested PSCs, the PSC-C and PSC-D retain ~88% of the initial PCE after 16 hours of endurance test, showing longer lifetime with respect to the one obtained by the other types of PSCs. Stationary and transient electro-optical analyses, reported in S.I. (Figs. S6 and S8), show that PSC-C and PSC-D have a moderated PE degradation with respect to PSC-A, suggesting a reduced occurrence of trap sites and charge recombination paths following the light soaking test. On the contrary, the GO-Li interlayer dramatically affects the device’s long-term stability due to a significant reduction of the JSC (-84% after 16.5 hours of light soaking test at 1 SUN). Lithium atoms degrade the perovskite layer,69,102–104 thus compromising the perovskite/GO-Li/mTiO2 interface and, consequently, the electron injection process. Notably, the combined use of graphene in mTiO2 and GO-Li interlayer results in a considerably improved stability with respect to the PSC-B, especially for what concern light stress at MPP. In fact, PSC-D (Fig.S6d, curve D) retains more than 70% of the initial PCE after 16 hours of prolonged 1SUN stress test. Large area modules. To test the effectiveness of our proposed GIE approach, we developed large area modules, fabricated on a 10x10 cm2 substrate area, consisting in 8 series-connected PSCs (active area 6.32 cm2), with an overall active area of 50.56 cm2. The module aperture ratio, i.e. the ratio between the active area and the aperture area, is approximately 73% (see the schematic representation of module layout reported in Fig. S10). The I-V characteristics of the as-produced PSMs are reported in Fig.4b for each tested device structures named as the respective PSCs (reference module PSM-A, module with GO-Li interlayer PSM-B, module with graphene-doped mTiO2 based PSM-C, module with GO-li


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interlayer and graphene in mTiO2 PSM-D), with the photovoltaic performance parameters reported in Tab.1.

Figure 4. a) Photograph of the large area perovskite solar module (50 cm2 active area); b) Current-Voltage (I-V) characteristics of tested modules (reference module PSM-A, module with GO-Li interlayer PSM-B, module with graphene-doped mTiO2 based PSM-C, module with GO-li interlayer and graphene in mTiO2 PSM-D). All the I-V measurements refer to the encapsulated devices. Table 1. Electrical parameters for tested module structure, extracted by 1 SUN current-voltage characteristics. Module type

VOC (V)

I (mA)

FF (%)

PCE (%)

∆PCE (%)

PSM-A: Ref

8.72

-112.8

59.4

11.6

-

PSM-B: mTiO2/GO-Li

8.23

-118.0

62.4

11.9

+3%

PSM-C: G+mTiO2

8.46

-121.6

61.4

12.5

+8%

PSM-D: G+mTiO2/GO-Li

8.57

-114.8

64.6

12.6

+9%


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In particular, high PCE values are obtained by using G+mTiO2 (PCE=12.5%) and G+mTiO2/GO-Li (PCE=12.6%) with a MDP of 632 mW and 638 mW, respectively. Indeed, when GO-Li interlayer is used (PSM-B) the gain in ISC (+4.7%) is counter-balanced by the reduction of VOC (-5.6%), as already observed also for the case of PSCs, see Fig. 2. On the contrary, when graphene flakes are used as dopant in mTiO2 layer (PSM-C), the module PCE overcome by 8% the value achieved with the reference one, which mainly reflects the remarkable increase in ISC (+7.8%). The combination of G+mTiO2 and GO-Li interlayer in PSMD results in the most performant PSM, i.e., exceeding by 9% the PCE of the reference one. The as-obtained PCE values are linked with the increase of FF (+8.8%), still maintaining a VOC of 8.57V. Differently from the results obtained with PSCs, the PSM-D has shown the best PCE performance (Tab.1), confirming the crucial role of GIE in retaining high PCE uniformity passing from small area, i.e. >10cm2. The fabricated PSMs are tested with the ISOS-D-1 shelf life ageing protocol.105 The test is carried out over a 1630 hour time-frame, until the reference module has shown PCE reduction of 20% of the initial value (T80 lifetime), according to the stability test protocol105 (see Fig. 5, series resistance RS trends are reported in S.I., Fig. S11).


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Figure 5. Normalized a) VOC, b) ISC, c) FF, d) PCE trends vs time extracted by 1 SUN I-V characteristics, periodically acquired during the shelf-life test (ISOS-D-1) for the realized modules.

The shelf-life experiment gave indication on the long-term stability of the PSMs (Figure 5d). In particular, the PSM-C has shown the best stability by retaining ~91% of the initial PCE value after 1630 hours of endurance test. Similar to the results obtained with PSCs, the PSM-B has shown the fastest PCE decreasing trend due to the mTiO2/GO-Li/perovskite interface degradation that strongly affects the ISC value (curve PSMB and PSMA, Figure 5b). The use of G+mTiO2 as electron transporting layer (curve PSMD) allows maintaining a higher ISC values with respect to the ones achieved with the other PSMs. This suggests reduced interface degradation at the perovskite/mTiO2 interface with respect to, for example, PSM-B. In fact,


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PSM-D retained more than 82% of the initial PCE value at the end of shelf-life test while the PCE of PSM-B significantly decreases (∆PCE -26%). These results are consistent with those reported for PSCs under prolonged illumination stress test. In particular, the optimized electron injection rate of the device containing G+mTiO2 mitigates the degradation at the perovskite/mTiO2 interface, by eventually extending the devices T80 lifetime. The reported stability test demonstrated that the embedding of graphene flakes into the mTiO2 layer can significantly improve the long-term stability of high efficient GO-Li based PSMs by opening a viable and cheap route to further stabilize this emerging photovoltaic technology. In this work, we introduced the concept of graphene interface engineering (GIE) as a viable route for mastering the interface properties of large area perovskite module (PSM), having an active area of 50.56 cm2, paving the way for the industrial exploitation of graphene/perovskite solar cells. Graphene flakes have been dispersed into the mesoscopic TiO2 while lithium neutralized graphene oxide (GO-Li) has been deposited onto mTiO2 as interfacial layer between the perovskite absorber and the electron transporting material. The use of GIE boosted the power conversion efficiency (PCE) from 11.6% of the reference standard module to 12.6% of the module

with

mTiO2+graphene

(G+mTiO2)

and

GO-Li

interlayer.

Electro-optical

characterizations and transient measurements show an increased electron injection from perovskite to mTiO2 and a reduction of back-transfer processes from the mTiO2 to the active layer. Beside the neat efficiency, GIE is also showing a strong impact on the stability of both perovskite solar cells (PSCs) and PSMs. Stability tests carried out on small area PSCs demonstrated a remarkable lifetime improvement following the GIE approach. T80 lifetime of


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PSCs with G+mTiO2 was approximately 5 time longer with respect to the one without graphene, while PSC with G+mTiO2 and GO-Li interlayer has a T80 lifetime 3 times larger than the reference one. Moreover, GIE resulted in prolonged shelf-life stability for PSM that retained more than 90% of the initial PCE after 1630 hours when G+mTiO2 is used as scaffold. The obtained results together with the availability of a wide library of 2D materials coupled with the easy solution process makes the interface engineering with graphene and other 2D materials a new design strategy for perovskite solar cells and in general for the new generation of photovoltaic technologies. ASSOCIATED CONTENT Supporting Information. Experimental details, characterization of materials (TEM, AFM, SEM, Raman Spectroscopy, FT-IR, XPS, UPS), spectro-electrical characterization (IPCE, I-V curves, stress test) of devices. AUTHOR INFORMATION Aldo Di Carlo Tel.: +390672597456. E-mail address: [email protected]. Website:www.chose-uniroma2.it/en/ Francesco Bonaccorso Tel.: +39 010 71781795. E-mail address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 696656 – GrapheneCore1.


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