## 3 Results
### Photovoltaic Configuration
The rise time, \(\tau_{r}\), measured as the time difference between the 90% and 10% of the current peak value, is shown in Figure 3d as a function of \(\lambda\) at E = 100 nJ. From this figure emerges the ability of our NSH PD to detect a fast multicolor signal, responding fast (some tens of nanoseconds) to a fs laser pulse, with results comparable to those reported for commercial PDs [22]. Interestingly, \(\tau_{r}\) increases for higher wavelengths due to lower absorption coefficient and different penetration depth and diffusion length inside the device. A detailed analysis and explanation of this mechanism is reported elsewhere [36] for a similar PD device based on graphene/silicon heterojunction. The reported device performances in terms of noise dark current, responsivity, and rise time can be further improved by acting on the quality of the nanotubes/Si interface. For this purpose, a new type of structure for the upper interdigitated contact was conceived and designed. The new generation of substrates is realized with the fingers of the top contact hollowed inside the n-Si region to form a smooth surface without protuberances due to the electrodes (Figure 4a, bottom panel). In the following, this configuration is referred to as the hollowed finger (HF) one, while the usual device design is referred to as the top finger (TF) one (Figure 4a, top panel). In Figure 4b, the I-V features acquired in dark conditions and under 100 mW/cm\({}^{2}\) white light illumination for two different devices are reported. The straight lines correspond to the hollowed fingers device, while the dashed lines correspond to the device with the top electrodes evaporated on top of the Si region. Both devices are realized with the same SWCNT film, with the same optical (\(T\approx 50\%\)) and electrical properties (\(R_{s}\)), and have 10 interdigitated fingers (five per side). This means that the differences in I-V curves are only due to the difference in the geometry (hollowed or not) of the fingers. From the dark characteristic, the decrease of about one order of magnitude in the saturation current \(I_{0}\) can be noticed, decreasing from 25.6 nA to 3.4 nA in the new generation device. Moreover, using the previous reported equation for modelling the NSH junction, the series resistance \(R_{S}\) and the ideality factor \(n\), which reduces by almost 50%, passing from 8.78 \(\Omega\) to 4.96 \(\Omega\) and from 8.2 \(\Omega\) to 4.4 \(\Omega\), respectively, were extrapolated. Besides, when illuminated, the device performances using the new design concept improve; in fact, the short circuit current density (\(I_{\text{SC}}\)) increases by more than 100%, going from 10.4 to 21.0 mA/cm\({}^{2}\), and also the open circuit voltage (\(V_{\text{OC}}\)) grows from 0.35 to 0.51 V. In addition, there is also an increase of the responsivity, the detectivity, and the \(\mathcal{NEP}\), obtained by illuminating with EQE on-off cycles of several power 475 nm LED. All these improvements can be ascribed to the fact that in the devices with the electrode hollowed in Si, when the nanotube film is deposited on the top surface touching both the n-Si region and the electrode metal region, it finds a flat surface and is able to settle more effectively without leaving empty or uneven spaces. In fact, being a few tens of nanometers thick, while the evaporated electrode is 150 nm thick, the SWCNT film in the previous substrates must follow the metal profile on the Si surface, probably creating areas where the intimate contact between the SWCNT film and the Si active area is not reached. Finally, it is important to highlight that in PV mode, the measurements are all carried out in the absence of polarization V = 0, i.e., no energy must be spent to power the device that works without any associated electronics, (signal amplifiers, power supplies, control cards, filters, etc.). Indeed, the electrical signal is directly transmitted by the PD electrodes to coaxial cables that can be connected to any current or voltage reading system.