# Preparation of Hybrid Molybdenum Disulfide/Single Wall Carbon Nanotube-n-Type Silicon Solar Cells
## 3 Results
### Layered Structure  
A final set of cells was made by first putting a MoS\({}_{2}\) layer on the Si and then placing a SWCNT layer over the top of the MoS\({}_{2}\) layer. The SWCNT electrode was made using 300 \(\upmu\)L of the SWCNT dispersion for all cells to keep the thickness of the SWCNT layer constant. The summary of the results are shown in Figure 5 while the detailed parameters are provided in Table S5 from the representative JV curves given in Figure S7. The cell with only a layer of MoS\({}_{2}\) flakes gave no current. This suggests the MoS\({}_{2}\) is n-type, [38] the same as the Si, and hence no heterojunction was created meaning there is no ability to separate charges. The observations are very similar to the hybrid system where there is little change in J\({}_{\text{sc}}\) but fill factor and open circuit voltage are enhanced. Average cell efficiencies of 10.04% are observed for devices made using 500 \(\upmu\)L of MoS\({}_{2}\) dispersion to form the first layer. The small changes in transmittance are consistent with the constant J\({}_{\text{sc}}\). In this case, the presence of an n-type layer on the Si will help the transfer of holes to the SWCNT layer, which will improve the fill factor and hence efficiency. Similar enhanced hole transport has been reported in a graphene/MoS\({}_{2}\)/Si system [44] where the energy levels due to the insertion of the MoS\({}_{2}\) also lead to effective electron blocking by the MoS\({}_{2}\) layer. For the SWCNT system, the observed V\({}_{\text{oc}}\) does shift from 0.515 V for SWCNTs only to 0.575 V for the system with the MoS\({}_{2}\) layered under the SWCNTs, which is similar to the increases observed for the graphene system suggesting the MoS\({}_{2}\) will also play a role in blocking electrons. The decrease in efficiency at higher loadings in this case will be due to the fact that thicker coverages of MoS\({}_{2}\) will be pushing the SWCNTs further from the Si and hence the quality of the heterojunction that is established will be decreasing. It is possible that junctions between the MoS\({}_{2}\) and SWCNTs could be established but these will be much smaller in area than the junction made with the substrate. The results clearly show that the layered system performs better than the hybrid system. The efficiencies reported in this work are among the highest reported for systems using 2D materials  
Figure 5: (a) Electrode properties for films made with a layer of MoS\({}_{2}\) covered by a film of SWCNTs; (b) Efficiencies of solar cells with films made with a layer of MoS\({}_{2}\) covered by a film of SWCNTs. (shown in Table 1). Our previous work using the layered MoS\({}_{2}\) system [35] reported slightly higher efficiencies, which may be due to some degradation of the MoS\({}_{2}\) dispersion. The better performance of the layered system points to the fact that modification at the interface of the heterojunction is more effective at improving performance than addition of material within the film. Given that photon absorption occurs in the Si substrate, the importance of transferring the charge carriers across the heterojunction is paramount to performance. The MoS\({}_{2}\) layer assists this transfer across the junction. The higher performance of the layered system means that the facilitated charge transfer is more effective at improving efficiencies than the additional material in the SWCNT films (hybrid system) even though this extra material in the film will help charge carrier lifetime. Additionally, of course, in the hybrid the distribution of the 2D material is completely random and hence there is no guarantee that all the material will be used effectively to help performance. The results in this current paper are similar to those reported by [PERSON]'s group using grapheme [45]. In this case, the graphene between the SWCNTs and the Si provided a dramatic increase in efficiency from 1.4% to 8.00%. With the addition of the antireflection layers and doping of the SWCNTs, impressive efficiencies of 14.9% are obtained.