The project GLADIATOR (Graphene Layers: Production, Characterization and Integration) is driven by the vision of developing an approach to the production of high quality graphene (transmittance ≤ 90%, sheet resistance ≥ 10 Ohm/sq.) both on a large scale and at a low cost. This will make this type of graphene (produced by chemical vapour deposition - CVD) a competitive alternative to indium tin oxide (ITO) as the electrode material used in (flexible) organic electronics.

The project is now just over half-way through its 42 month duration and has already achieved very promising and interesting results, in accord with its ambitious schedule. These first re¬sults are outlined below:

Doping to improve conductivity

Figure 1. Graphene doped with gold by CEA.

By applying external dopants to graphene the conductivity can be improved (by increasing the charge carrier density). The dopant HAuCl4 (chloroauric acid) gives the best doping effi-cien¬cy of the materials tested by the project so far, and a method has been developed to improve the stability of the doping by using a layer of porous aluminium oxide to prevent dopant ag¬glomeration. This seems to result in stability good enough for commercial use, but tests are continuing. Unfortunately, the solvents used in the trans¬fer process (to move graphene from one sub-strate to another), also remove some of the dopant and so the resistance could not be re¬duced to its theoretical limit. Nevertheless, a good value of 126 Ω/sq. with a transmittance of 93%, has been achieved. and further im¬provement is expected. In addition, a dopant material class has been identified which can be placed between sub-strate and graphene, thus is not affected du¬ring transfer. The efficiency of doping with such a material is not yet as high as for the chloroauric acid, but demonstrated much better long term stability. Therefore, it is planned to try a combination of several dopants.

Process control

For future CVD graphene manu¬facture and the integration of graphene in products at low cost, it is essential to monitor and/ or control the quality as soon as possible during the synthesis process. Accordingly, a spectral ellipsometer has been adapted to a CVD graphene lab tool and first results show that differentiating between mono- and multilayers is possible in situ. Although the spectral ellipsometry is very powerful, the project continues to work on the more difficult adaptation of a Raman spectrometer to acquire complementary information.

The application of UV absorption spectroscopy to the working gas of the CVD chamber has been shown to allow a direct correlation between the gas composition and the subsequent graphene quality to be drawn.

Characterisation of sheet resistance

Eddy current based measurements require a minimum conductivity to produce useful measurements, and to measure graphene by this useful non-contact technique the project has had to develop new equipment. By improving hardware components and modifying the drift compensation algorithm, the sensitivity has been enhanced to 500 Ω/sq. With this performance level, eddy current measurements (cf. Figure 1) have routinely been used for graphene quality characterisation after doping and/ or prior to OLED integration.

Figure 2. Sheet resistance mapping of a graphene film on SiO2 wafer with defects performed by Suragus.

Large volume production of undoped graphene

Figure 3. Different areas of the investigated graphene monolayer film from Graphenea. The average amount of multilayer areas is only ~3%.

The reliable production of (CVD) graphene is the key to its future success. Based on the de-veloped recipe for 150 mm wafer scale graphene production, the growth parameter set (pres-sure, temperature, gas composition) has been transferred to the 300 mm waferscale production line (with four times the graphene area per wafer). Twenty-five graphene wafers were pro¬du¬ced one after another with no manual intervention to the process, and Raman spectroscopy then used to demonstrate the high quality of the monolayer graphene. These large area graphene samples have also been success¬ful¬ly transferred onto both flexible polymer foils and glass substrates.

With regard to monolayer coverage graphene samples with 100 mm in diameter have been manufactured with a coverage >97% (cf. Fi¬gure 2). Although it has only been realized on a smaller scale, it is considered as a signi¬ficant breakthrough and to be applicable on larger scale, as well.

Hazard assessment

The gladiators from the National Research Centre for Working Environment have continued with the physicochemical characterization and analysis of data of the graphene powders used both for the in vitro and in vivo study. Main results of the powder characterization is summar¬ized Table 1 and 2.

Table 1. Morphology of GO, rGO-s and rGO-l.

In vitro: The Assessment of prolonged genotoxicity (DNA damage) has previously been performed after 24 hours of ex¬posure. The additional assess¬ment has been continued after 3 hours of exposure to estimate tran¬sient genotoxicity. GO and rGO did not induce genotoxicity at 3 or 24 hours after exposure.

Table 1. Morphology of GO, rGO-s and rGO-l.

In vivo: The pulmonary inflam¬mation has been determined and the genotoxicity in the lung fluid has been assessed as well as the lung and liver tissue. The pulmo¬nary inflammation and geno¬toxi¬city were assessed at 4 time points after exposure – day 1 and 3 (short term), day 28 and 90 (long term). GO and rGO induced lung inflammation and genotoxicity in lung fluid across doses and time points. This study provides evidence that pulmonary expo¬sure to both GO and rGO induces long lasting inflammation and are genotoxic.

Large area graphene transfer on flexible foils and glass

Figure 4. 300 mm graphene which is transferred per¬formed by DTU on the GLADIATOR foil from

Large graphene sizes are essential for the successful commercialization of graphene, the pro¬duction and transfer of large area sheets is one direct approach to address this need. Within the last 12 months much progress has been achieved to provide a second generation of transfer technique which allows the reuse of the copper catalyst and the utilisation of a water soluble transfer polymer (i.e. poly (vinyl alcohol)) preventing any potential damage when the target substrates are poly¬mer-based. In addition, it has also been shown that this transfer works effectively both on small (100 mm) and large wafer-scale (300 mm) by achieving high-quality monolayer graphene. Moreover, parameters have been identified to further increase the transfer speed which is essential for later automation and production.

This transfer technique has also been applied to the GLADIATOR barrier foil with modified surface and barrier properties, which will later be essential to realize fully working flexible OLEDs.

Integration into organic light emitting diodes

Figure 5. Orange emitting OLEDs built by Fraunhofer FEP- the graphene was provi¬ded by Graphenea and structured by laser ablation.

Organic light emitting diodes (OLED) are very sensitive to defects of any origin which have direct contact with the OLED materials. Thus, such devices are a very effective way of monitoring the graphene quality relevant to a real-world application. Although challenging, massive progress has been made in (i) removing impurities on the graphene by careful rinsing and heating and (ii) reducing the defects implemented during transfer, which have been identified as the main source of device failures (defects are locked between graphene and substrates and can hardly be removed). Materials and techniques have also been found which can be used to structure the active area of the OLED, and the project has successfully made some of the first functioning OLED's on graphene (see figure).

Although the electro-optical performance with graphene based OLEDs is still lower than for reference devices using ITO electrodes, devices could be built with no initial defects. This will be used as starting point to continue with the development of fully flexible, transparent, and large-area devices on graphene.

Ellipsometry for Organic Webinar