# Preparation of Hybrid Molybdenum Disulfide/Single Wall Carbon Nanotube-n-Type Silicon Solar Cells
## 1 Introduction  
It is clear that the sun can provide more energy than the current global daily energy consumption as well as fulfill future requirements for many years. Solar power is the most sustainable renewable energy and will help lower greenhouse gas emissions. Additionally, utilising the sun in a distributed network can eliminate energy lost due to the distribution grid, which accounts for almost 25% of produced electricity being lost [1]. The search for novel solar cell designs as an alternative to standard silicon solar cells is important for the future of renewable energy production. One such alternative design is the carbon nanotube/silicon (CNT/Si) heterojunction solar cell. The first such cells were designed by [PERSON] et al. in 2007 [2; 3] using double wall carbon nanotubes (DWCNTs) deposited via water expansion and aqueous film transfer of an as-grown chemical vapour deposition (CVD) film. While these cells only had a photoconversion efficiency of 1.3% (compared to commercial cells at 20%-30%), many improvements have since been made to the cell design and doping methods, with 15% efficiencies reported in 2012 [4] and 17% efficiencies reported in 2015 [5]. The typical architecture for these cells is much like that of a single-junction n-type crystalline silicon solar cell with a front side emitter layer replaced by a film of \(p\)-doped CNTs [2]. While the initial design used DWCNTs, most designs use single wall carbon nanotube (SWCNT) electrodes; however, multiwall carbon nanotubes (MWCNTs) may also be used [6; 7]. [PERSON] et al. [8] performed the first comparison between SWCNTs, DWCNTs, and MWCNTs for use in CNT/Si cells in terms of the area density of the films. The majority of research performed on nanotube/silicon cells uses amixture of nanotube types to form the film as nanotubes are grown as a mixture of types. [PERSON] et al. investigated the effects of fabricating cells with pure metallic or semiconducting nanotubes versus cells with an as-grown mix of types [9]. Similar experiments have been undertaken by [PERSON] et al. in their comparison of semiconducting and metallic nanotube films on silicon [10]. Overall, the metallicity of the nanotube film plays a significant role in the cell operation. In all cases, the CNT film acts as a component of the heterojunction setting up a built-in potential to enable charge separation, as a highly conductive network for charge collection and transport, and as a transparent electrode to allow good light illumination of, and photocurrent generation in, the silicon base [7]. A variety of approaches have been examined to improve device performance. For example, in 2008, [PERSON] et al. [11] found that treatment of the SWCNT layer with the p-dopant thionyl chloride (SOCl\({}_{2}\)) increased the power conversion efficiency of the cells by over 45% (relative) by lowering the sheet resistance and increasing the short-circuit current density and open-circuit voltage. [PERSON] et al. experimented with doping the nanotube film using nitric acid [12]. Doping using CuCl\({}_{2}\)/Cu(OH)\({}_{2}\) colloidal ethanol solution looks to be a particularly promising approach and demonstrates considerable stability compared to many of the other approaches [13]. Other p-dopants can be used in the treatment of SWCNT films in order to increase the efficiency of the CNT/Si heterojunction cell, including gold chloride (AuCl\({}_{3}\)) [14]. Research was carried out by [PERSON] et al. [15] in 2013 to find an optimal thickness for a film using large diameter arc-discharge SWCNTs with high carboxyl functionality. They determined that a film transmittance of 70% gave an optimal efficiency, although thicker films displayed fairly stable efficiencies. The device efficiency was found to taper significantly with thinner films. This was due to a decrease in the amount of SWCNT/Si contacts for charge separation and an exponential increase in sheet resistance as the film conductivity became dominated by the limited pathways in the very thin SWCNT films.