Transparent conductive electrodes based on graphene have been previously proposed as an attractive candidate for optoelectronic devices. While graphene alone lacks the antireflectance properties needed in many applications, it can still be coupled with traditional transparent conductive oxides, further enhancing their electrical performance. In this work, the effect of combining indium tin oxide with between one and three graphene monolayers as the top electrode in silicon heterojunction solar cells is analyzed. Prior to the metal grid deposition, the electrical conductance of the hybrid electrodes was evaluated through reflection-mode terahertz time-domain spectroscopy. The obtained conductance maps showed a clear electrical improvement with each additional graphene sheet. In the electrical characterization of the finished solar cells, this translated to a meaningful reduction in the series resistance and an increase in the devices' fill factor. On the other hand, each additional sheet absorbs part of the incoming radiation, causing the short circuit current to simultaneously decrease. Consequently, additional graphene monolayers past the first one did not further enhance the efficiency of the reference cells. Ultimately, the increase obtained in the fill factor endorses graphene-based hybrid electrodes as a potential concept for improving solar cells' efficiency in future novel designs.
Keywords: ITO; graphene; silicon heterojunction; solar cells; terahertz time-domain spectroscopy; transparent conductive electrodes.