## 3 Results  
SEM images of the DAD films on device substrates were collected. In many cases, the images simply showed a flat topography not unlike that of the bare substrate. As such, to prove the presence of the film, the layer was intentionally damaged and imaged again. In this case, the laser approach to damage the films was used to ablate small holes in the layers. These images are provided in Figure 2. This clearly demonstrates that the film has been successfully deposited. A fuller set of both optical and SEM images are provided in the Supplemental Materials in Figures S2 and S3. From the SEM images in Figure 3, it is apparent that the volume of solution spun on the surface has an effect on the film morphology. Lighter coloured regions are visible on the films. They are larger and more frequent in the 100 \(\mu\)L film than in the 200 \(\mu\)L film and there are none visible at all in the 300 \(\mu\)L sample. It is apparent that a volume of at least 300 \(\mu\)L is required to produce a smooth homogenous film. The light-coloured regions are likely thinner areas of film. Figure 4 shows the images used to determine the thickness of the DAD layers. A fuller set of data is provided in the supplementary information for films made with different solution volumes and different spin speeds (Figures S4-S7). Across the range of volumes used (100 mL to 300 mL) and spin speeds (500 rpm to 7500 rpm), the change in film thickness observed is relatively small. All films appear to have thicknesses in the range of 10-20 nm. Our previous work has shown that this thickness range is ideal as an interlayer. If the layer is thinner, it will typically not be complete which will lower the ability to help with the charge transfer while thicker layers will push the CNTs too far away from the Si and hence the establishment of the heterojunction will be compromised. From the PESA measurements shown in Figure 1b, the oxidation potential was measured and the HOMO value of DPA-ANT-DPA was estimated to be \(-5.46\) eV [9]. The optical properties of the layers made using different spin speeds were measured with uv-vis spectroscopy and the data is provided in Figure S8. At higher spin speed the films do have less optical density indicating that they are getting thinner. The changes in absorbance from 500 rpm to 7500 rpm are on the order of 40% again suggesting that the thickness of the layers is not changing dramatically which is in line with the observation from the AFM measurements. Figure 5 provides representative J/V curves for the control cells and cells with the DAD interlayer while Table 1 provides a summary of the device performance prepared with various suspension volumes. The addition of DAD has a significant effect on the performance of the devices. The data shows that a volume of 200 \(\mu\)L is optimal to produce the best performing devices as they outperformed the other sets across all parameters. The 150 \(\mu\)L set was the worst performing. However, when the J\({}_{\rm sc}\) trend is observed, it is clear that there is still a general trend of improvement with volume up until the 200 \(\mu\)L point. The drop-in performance for the 150 \(\mu\)L devices is due to concurrent drops in V\({}_{\rm oc}\) and FF. The J\({}_{\rm sc}\) displays the clearest trend, with a peak at 200 \(\mu\)L. Additionally, the 200 \(\mu\)L samples displayed the highest consistency within the device set. Figure 2: (**a**) Optical microscope image of DAD film spun from a 100 \(\mu\)L volume. The blue areas are the DAD film and the small white dots are air bubble holes in the film. The dark dots in the red ellipse are the burnt holes; (**b**) SEM image of holes in the DAD film spun from a 100 \(\mu\)L volume. In both cases a spin speed of 5000 rpm was used. Figure 3: Larger area SEM images of DAD films spun from different solution volumes. A spin speed of 5000 rpm was used for all films. Table 1 shows that the optimal volume of 200 \(\mu\)L outperforms devices with no interlayer by almost 0.50% PCE (14% relative improvement).