## 3 Results
### Photovoltaic Configuration
In such a way, the distance, \(d\), between fingers increases by reducing the N value. This means that, for each SWCNT film with same resistivity \(\rho_{S}\) (i.e., with the same \(T\) at 550 nm), the resistance lowers, since \(Rs=\rho_{S}\)\(d/t\), where \(t\) is the film thickness. At the same time, the top metal contact resistance \(R_{C}\) increases, due to the greater size of metal electrodes. Nevertheless, in [20], we demonstrated that \(Rs\) reduction is more important than the \(Rc\) increase, resulting in an overall device resistance decrease of about one order of magnitude. This directly reflects on the performances of the PDs. In Figure 2d, responsivity is reported as a function of \(P_{opt}\) for PDs characterized by N = 1, 2, 3, 5, 10, when lit by the calibrated blue LED. For all the samples, we obtained values of \(\mathcal{R}\) in the range 0.3-0.8 A/W, with an increase of \(\mathcal{R}\) with the number of fingers. This is due to \(Rs\) lowering and to the higher probability to collect hole carriers for closer fingers. To investigate the response rate and explore the feasibility of the realized devices for practical applications as ultra-fast PDs, the NSH area was illuminated by a 35-fs pulsed laser used as light source. In Figure 3a, the time dependence of the photocurrent value, \(I\), is reported after the junction is illuminated by a single laser pulse tuned at \(\lambda\) = 800 nm at different energies showing the high sensitivity of our photodetectors. \(I_{ph}\), measured by taking the difference between maximum and minimum in PD response peaks for \(\lambda\) = 800 nm in energy scale is shown in Figure 3b. The data are well approximated by a power law y = \(\chi^{\alpha}\) with \(\alpha\) = 0.82 \(\pm\) 0.4 confirming the possibility to use the device as a linear detector. Similar results are obtained for several incident \(\lambda\) from the UV (240 nm) up to the IR (1600 nm). For a given laser intensity, the measured photocurrent depends on the incident light wavelength. This dependence is shown in Figure 3c where \(I_{ph}\) is reported as a function of \(\lambda\) for E = 100 nJ. \(I_{ph}\) increases up to 800 nm and then decreases following the absorption spectrum of Si (as reported in EQE spectra Figure 2a) but presenting a tail in the NIR region. From these measurements, the ability of the NSH PD in absorbing and responding to a wide spectral range, operating in a large incident optical power range without any need to be powered, was proved. The Si p-n junction PDs do not operate in the IR spectral region because the optical absorption is limited by the Si band gap energy around 1060 nm. On the other hand, SWCNT films were proven to absorb and photogenerate electrical charges ascribed to the presence of van Hove singularities in their electronic density of states of CNT [10; 31; 33; 34]. These optical transitions in SWCNTs are present both for wavelengths in the visible and in the IR range [35]. Therefore, our experimental results highlight that the SWCNT film not only acts as a transparent window, heterojunction constituent, and collector of photogenerated charges in Si, but the SWCNTs actively behave as photocurrent generators. However, given the high transparency of the film, we can note this contribution only in the IR range (in our case for \(\lambda\)= 1300 and 1600 nm), where there is no signal from the optical absorption of Si, predominating the electrical response of the device up to about 1060 nm. Further, normally, a typical Si p-n junction PD also does not operate in UV spectral region despite the Si absorption coefficient in this region being very high. This is because UV radiation is mostly absorbed very close to the surface where charge recombination is very important, thus preventing photo charges from reaching the junction, usually several \(\mu\)m from the surface. In our NSH PD devices, the photogenerated charges can instead safely reach the junction which is very close to the Si surface thanks to the small thickness of CNT film.