Defect healing and doping of CVD graphene by thermal sulfurization.

CVD graphene layers are intrinsically polycrystalline; depending on grain size, their structure at the atomic level is scarcely free of defects, which affects the properties of graphene. On the one hand, atomic-scale defects act as scattering centers and lead to a loss of carrier mobility. On the other hand, structural disorder at grain boundaries provides additional resistance in series that affects material conductivity. Graphene chemical functionalization has been demonstrated to be an effective way to improve its conductivity mainly by increasing carrier concentration. The present study reports the healing effects of sulfur doping on the electrical transport properties of single-layer CVD graphene. A post-growth thermal sulfurization process operating at 250 °C is applied on single layers of graphene on Corning-glass and Si/SiO2 substrates. XPS and Raman analyses reveal the covalent attachment of sulfur atoms in graphene carbon lattice without creating new C-sp3 defects. Measurements of transport properties show a significant improvement in hole mobility as revealed by Hall measurements and related material conductivity. Typically, Hall mobility values as high as 2500 cm2 V-1 s-1 and sheet resistance as low as 400 Ohm per square are measured on single-layer sulfurized graphene.

Effective hole conductivity in nitrogen-doped CVD-graphene by singlet oxygen treatment under photoactivation conditions.

Nitrogen substitutional doping in the π-basal plane of graphene has been used to modulate the material properties and in particular the transition from hole to electron conduction, thus enlarging the field of potential applications. Depending on the doping procedure, nitrogen moieties mainly include graphitic-N, combined with pyrrolic-N and pyridinic-N. However, pyridine and pyrrole configurations of nitrogen are predominantly introduced in monolayer graphene:N lattice as prepared by CVD. In this study, we investigate the possibility of employing pyridinic-nitrogen as a reactive site as well as activate a reactive center at the adjacent carbon atoms in the functionalized C-N bonds, for additional post reaction like oxidation. Furthermore, the photocatalytic activity of the graphene:N surface in the production of singlet oxygen (1O2) is fully exploited for the oxidation of the graphene basal plane with the formation of pyridine N-oxide and pyridone structures, both having zwitterion forms with a strong p-doping effect. A sheet resistance value as low as 100 Ω/□ is reported for a 3-layer stacked graphene:N film.

Laser-patterned functionalized CVD-graphene as highly transparent conductive electrodes for polymer solar cells.

A five-layer (5L) graphene on a glass substrate has been demonstrated as a transparent conductive electrode to replace indium tin oxide (ITO) in organic photovoltaic devices. The required low sheet resistance, while maintaining high transparency, and the need of a wettable surface are the main issues. To overcome these, two strategies have been applied: (i) the p-doping of the multilayer graphene, thus reaching 25 Ω□-1 or (ii) the O2-plasma oxidation of the last layer of the 5L graphene that results in a contact angle of 58° and a sheet resistance of 134 Ω□-1. A Nd:YVO4 laser patterning has been implemented to realize the desired layout of graphene through an easy and scalable way. Inverted Polymer Solar Cells (PSCs) have been fabricated onto the patterned and modified graphene. The use of PEDOT:PSS has facilitated the deposition of the electron transport layer and a non-chlorinated solvent (ortho-xylene) has been used in the processing of the active layer. It has been found that the two distinct functionalization strategies of graphene have beneficial effects on the overall performance of the devices, leading to an efficiency of 4.2%. Notably, this performance has been achieved with an active area of 10 mm2, the largest area reported in the literature for graphene-based inverted PSCs.