Self-Crack-Filled Graphene Films by Metallic Nanoparticles for High-Performance Graphene Heterojunction Solar Cells

摘要

Graphene exhibits excellent carrier transport because of its unique 2D energy dispersion and also shows a high transparency, with a transmittance of 97.7% in a single layer, making it a promising candidate for transparent electrode applications. High-quality graphene films have been grown on SiC substrates or on transition metal substrates, such as Cu, Ni, Pd, Ru, and Ir by using the chemical vapor deposition (CVD) method. Graphene that can be deposited in a single layer on a Cu foil over a large area and transferred to targeted substrates has been developed, providing access to high-quality graphene for industrial applications such as replacing indium tin oxide as the material for transparent conductive films. However, because the large-area graphene film grown through CVD on a Cu substrate is typically a polycrystalline material, the electrical properties of CVD-grown graphene are strongly influenced by topological defects in graphene, such as dislocations and grain boundaries. Much effort has been focused on growing graphene with larger grain sizes to improve the transport properties of graphene because of the lower density of grain boundaries. In addition to grain boundaries, the presence of wrinkles and cracks in CVD graphene films during growth is generally inevitable, potentially causing detrimental effects on the mobility and conductivity of graphene because of the disruption of transport paths. Because of the atomically thin nature of graphene, such topological imperfections may considerably limit the applications of CVD-grown graphene in large-area electronics with increased resistance. In this study, we demonstrated a novel approach for enhancing the electrical properties of large-area CVD-grown graphene films by selectively filling the topological cracks in graphene with metallic nanoparticles. Through a simple reduction-oxidation reaction between a Cu substrate and a metal precursor solution, the crack sites of graphene grown on the Cu substrate can be selectively filled with Au nanoparticles (NPs). Crack-filled graphene (CFG) films with Au nanoparticles showed excellent electrical properties as transparent electrodes; they exhibited a substantial improvement in sheet resistance in the lateral direction and also a markedly decreased contact series resistance (R_s) in the vertical direction at the heterojunction between the graphene and a semiconductor. A graphene/Si Schottky junction solar cell based on the CFG film demonstrated a nearly 30% enhancement in power conversion efficiency (PCE) compared with the device in which the graphene was not subjected to the crack-filling treatment, and this enhancement is mainly attributed to a significant improvement in the fill factor (FF) of the graphene/Si Schottky junction solar cell device as a result of superior electrical properties of CFG films. The CFG films of which the cracks in graphene were filled with Au nanoparticles provide an effective route for producing large-area, high-quality, graphene-based transparent conductive films for future electronic applications.