  Large area graphene sheets grown by chemical vapor deposition can potentially
be employed as a transparent electrode in photovoltaics if their sheet
resistance can be significantly lowered, without any loss in transparency.
Here, we report the fabrication of a graphene-conducting-carbon-nanotube (CCNT)
hybrid material with a sheet resistance considerably lower than neat graphene,
and with the requisite small reduction in transparency. Graphene is deposited
on top of a a self-assembled CCNT monolayer which creates parallel conducting
paths on the graphene surface. The hybrid thereby circumvents electron
scattering due to defects in the graphene sheet, and reduces the sheet
resistance by a factor of two. The resistance can be further reduced by
chemically doping the hybrid. Moreover, the chemically doped hybrid is more
stable than a standalone chemically doped graphene sheet, as the CCNT network
enhances the dopant binding. In order to understand the results, we develop a
2D resistance network model in which we couple the CCNT layer to the graphene
sheet and demonstrate the model accounts quantitatively for the resistance
decrease. Our results show that a graphene-CCNT hybrid system has high
potential for use as a transparent electrode with high transparency and low
sheet resistance.
