## 3 Results and Discussion
### Initial CNT/Si Device Comparison  
Slow and fast-filtered films of arc-discharge SWCNTs were produced from 400 \(\upmu\)L aliquots in 10 mL of water and were attached to a CNT/Si device substrate with an active area 0.079 cm\({}^{2}\). For the fast-filtered films, the film areas were homogeneous on the macro scale and thus any area of the films could be used in CNT/Si devices. As discussed above, slow-filtered films are not homogeneous on the macroscale and thus it is important to keep consistent which areas of a film are chosen for solar device usage. All slow-filtered films have a small area of high CNT density and a larger area with a much lower density, for solar device applications, the high SWCNT density area was never used in order to keep the devices as consistent as possible. It is apparent that, on average, the devices with fast-filtered films outperform those with slow-filtered films with an average PCE of 7.21% vs. 6.51% although this includes a low performing 2.26% device in the slow-filtered batch (see Figure 6). Without this outlier device the slow-filtered films average rises to 8.64% albeit from only 2 cells. To address the large offset between devices in the slow-filtered samples a further set of 3 devices were fabricated and tested. This data is shown in Appendix B. The additional set of devices did not serve to improve the stability of the average PCE, with one device failing to produce a photocurrent and two other devices of 6.51% and 2.25%. If the three top performing devices from the slow and fast sample sets are averaged the slow-filtered films outperform the fast-filtered films with an average of 7.93% compared to 7.21%. However, the existence of several poorer performing devices in the slow-filtered film sets highlights the inhomogeneity of the films produced. The full sets of device data after final 2% HF etch can be seen in Table 2. It is noticeable that, for most parameters, the slow-filtered devices best performance was superior to or equal to the best performance of the fast-filtered devices. However, for every parameter (except shunt resistance) the variation between individual data points was greater for the devices with slow-filtered films. The high variability between devices produced with slow-filtered SWCNT films is likely due to the macroscale inhomogeneity of the films (Figure 2c). Figure 5: UV/Visible spectra and sheet resistance comparison for slow vs. fast-filtered SWCNT films. An important observation from Table 2 is that the best performing device with a slow-filtered film produced a PCE of 9.6%, which is higher than the best performing device with a fast-filtered film of 8.1%. However, the large inhomogeneity in the devices with slow-filtered films led to a significantly lower average of PCE (5.7% vs. 7.2%) and a much higher error value of 3.2% vs. 0.8%. Thus, slow-filtered films have the potential to improve the performance of the CNT/Si device but the inhomogeneity in performance, likely due to the macroscale variant in CNT density, makes the improvement difficult to achieve. The slow-filtered films used for solar devices let through 30% more light than the fast-filtered films but have twice the sheet resistance. Together, the sheet resistance and transmittance can be combined to produce a ratio of DC electrical conductivity (\(\sigma_{\text{dc}}\)) and optical conductivity (\(\sigma_{\text{OP}}\)). This ratio is a good way to directly compare thin SWCNT films with differing resistance and transmittance properties [41; 42]. The ratio is defined relative to the transmittance as shown in Equation (1) and this equation can be rearranged to directly give the \(\sigma_{\text{dc}}\):\(\sigma_{\text{OP}}\) ratio with an experimental transmittance at