The On-Site Nanowire-Shape Graphene Formation on Nanoimprinted Si Nanowires for Radial Schottky Junction Solar Cells

The Schottky junction formed by contacting an n-type semiconductor with a sufficiently high work function conductor has been explored over the years for use as a simple photovoltaic cell. The interface has rectifying diode properties and it can easily be made through various semiconductor-conductor combinations. Graphene (Gr) has been used as an alternative for the conductor material because of its relatively good electrical properties for thin and highly transparent material. Many efforts reported the doping Gr to improve its conductivity and work function, and the engineering at the interface with a passivation layer to improve the power conversion efficiency (PCE) but they have not reached levels necessary for practical use. A limitation to the device performance is the use of planar-type configurations that may be improved with light trapping mechanisms. Another main issue is the Gr usually in form of a planar sheet which cannot conform to the shape of the architectures. The Gr used in most reports must be removed from a catalyst film and onto the target area though very robust for atomically thin material. Therefore, the Gr can be damaged during the transfer process to the final device. This is a tedious procedure in the laboratory, and it is difficult to scale up to industrial levels. Therefore, reliable on-site production of Gr onto the Si substrate is a major step for final application efforts. In this study, Schottky junction photovoltaic devices were successfully assembled by using nanowire (NW) shaped Gr sheets grown on-site onto nano-imprint lithography silicon (Si) NWs. Gr formation was accomplished by rapid chemical vapor deposition synthesis on nickel catalysts deposited on SiNWs. The Gr remains on the SiNWs after the underlying metal catalyst is wetly etched, forming a Gr-Si three-dimensional-junction. The SiNWs substrate contributes light trapping qualities while the NW-shaped Gr increases the carrier collecting area compared to flat Gr resting on SiNWs. Complicated Gr transfer procedures could be avoided, and the process is capable of being scaled up. Simple SiNW-based devices improved the power conversion efficiency (PCE) and ease of fabrication compared to equivalent planar-type controls and could reach a PCE up to 2.19% without doping, or 3.83% after p-type doping with gold chloride. High recombination losses likely limited device performance in this simple system. The photovoltaic characteristics of the best Gr-SiNW sample and planar sample with various optimizations will be discussed. Further necessitating improvements in Gr quality and interface engineering are in progress. [1] S. Wallace, <i>et al.</i>, Nanoscale Adv., <b>2 </b>[12] (2020) 5607-5614. [2] S. Wallace, <i>et al.</i>, Nanoscale Adv., <b>2 </b>[4] (2020) 1718-1725.