Figure 1. Illustration of the fabrication process of an AgNW/SWNT/Si solar cell involving the following steps: (top) transferring a SWNT film on a Si wafer with a pre-patterned window and (bottom) spinning cast AgNWs on top of the SWNT film.While research on silicon solar cells has progressed the development of all organic, inorganic, and hybrid materials systems to simultaneously address the diverse set of design criteria for optimal photovoltaic (PV) performance, incorporation of hybrid materials systems has proven to be an effective method to improve some of these issues. With crystalline silicon representing the standard for high efficiency in solar cell designs, cell cost and production capacity remain concerns for the growing emphasis on broad implementation of renewable energy strategies on a global basis, with solar PV being a leading competitor. With recent studies demonstrating that the approach incorporating p-type nano-Carbon with n-type silicon in a hybrid film approach provides excellent diode junction rectification properties, improved collection and transport efficiencies due to the enhanced conductivity of the nano-C film, and superior semiconductor barrier properties at the nano-C/silicon junction. While this has proven effective for small cell design of a few square millimeters, scaling the cell area has proven challenging due to the increase in sheet resistance (Rs) of the nano-C layer as area increases resulting in a reduction in cell efficiency.
Recently, Li, et.al, from the Taylor group in the Chemical and Environmental Engineering Department at Yale University, reported on a approach to significantly improve the performance for scaling up cell area for hybrid single walled carbon nanotubes (SWNT)/Silicon solar cells. In this work, the authors utilized p-type SWNTs cast onto n-type silicon as a dense film approximately 15 nm in thickness. For small cell areas on the order of 1-2 mm2, cell performance was significantly improved in comparison to other hybrid approaches due to the low Rs of the SWNT film. For larger cell areas, the Rs increased substantially to kilo-ohm/square, resulting in decreased cell efficiency. While increasing the SWNT film thickness could potentially lower Rs, the trade-off would be a reduction in optical transparency for the film, which would still reduce cell efficiency during scale-up. Patterning of metal conductor traces over the SWNT film was considered as a means to reduce Rs, but the evaporation of metal over the SWNT film resulted in cell shorting as some of the metal penetrated the pores in the film to the silicon junction. Instead, a strategy of casting silver nanowires (AgNWs) from solution at medium densities was investigated as a means to lower Rs while maintaining reasonable optical transparency during cell area scale-up.
Figure 2. J–V curve for 49 mm2 SWNT/Si solar cell before and after the spin coating of AgNWs under 1 sun illumination.Reported results showed that casting of the AgNW films over the SWNT film reduced Rs for the scaled cell structures, and that even with the slight increase in optical absorption with the additive bilayer film, the overall performance of the scaled cells was significantly improved in comparison to the SWNT/Silicon hybrid cell design. The cells exhibited improved fill-factors which were most predominant in enhancing the efficiency, even with slight reductions in open circuit voltage and short circuit current observed for the scaled cell areas. To further improve the optical absorption for the cell, the authors cast titania (TiO2) nanoparticles over the AgNW/SWNT surfaces to reduce reflection and increase forward scattering of incident solar radiation, resulting in a marginal improvement which was further increased via post process steps. This work has developed a solution-based approach to mitigate the total resistive power loss that typically hinders the area scale-up of hybrid nano-C/Si solar cells. A nearly twofold increase of photovoltaic efficiency is observed upon the coating of AgNWs onto SWNT/Si junctions, resulting from the significant reduction in the Rs enabled by the AgNW/SWNT bilayer. The SWNT thin film with high optical transparency and extremely small thickness also allows for the direct solution deposition of antireflective TiO2 nanoparticles. A final efficiency of >10% was realized in 49 mm2 cells, with implications for complete solution processed solar cell manufacturing and ultimately cell cost reduction. The work further illustrates the role and versatility that additive nanostructured films can contribute to performance improvements for cell area scale-up.
