## 3 Results
### Photovoltaic Configuration
**Inset**: typical external quantum efficiency of n-silicon heterojunction (NSH) photodetector (PD) sample; (**b**) PD response under dark and light conditions (frequency 10 Hz) when the junction is lightened with different LED light power (green curve: 1.4 mW/cm\({}^{2}\); turquoise curve: 1.8 mW/cm\({}^{2}\); violet curve: 2.2 mW/cm\({}^{2}\); yellow curve: 2.5 mW/cm\({}^{2}\); red curve: 2.8 mW/cm\({}^{2}\)); (**c**) responsivity (closed symbols, left axis) and detectivity (open symbols, right axis) of the three different samples with different optical transmittance at \(\lambda\) = 550 nm \(T\) = 63% for red symbols, \(T\) = 44% for green symbols, and \(T\) = 42% for blue symbols; (**d**) responsivity as a function of \(P_{opt}\) for PDs with different numbers of fingers N. Here the SWCNT film optical transmittance at \(\lambda\) = 550 nm is \(T\) = 49%. Data reported in (**a**–**c**) have been adapted from [19] and those in (**d**) from [20]. where they are absorbed, generating _e-h_ pairs. The built-in electric field formed at the carbon nanotube/Si interface separates the _e-h_ pairs, driving the electrons in the silicon towards the G electrode and injecting the holes into the CNT film where they are collected by the interdigitated electrodes. The CNT film therefore assumes not only the role of an optical window, but it is also necessary as junction constituent material: without it, the photogenerated _e-h_ pairs in the Si substrate would not be separated. Moreover, it takes the part of hole extracting and the transporting layer up to the interdigitated electrodes, thanks to its good electrical conductivity characteristics [19]. This means that the higher the CNT film transparency, the higher the number of photons reaching the Si substrate, and as a consequence, generating charge carriers that can be collected at the electrodes. At the same time, the device series resistance plays another fundamental role. I-V dependence is well approximated by the expression valid for both metallic/semiconductor (M/S) and semiconductor/semiconductor (S/S) junctions [32]:  
\[I=I_{0}e^{e(V-RI)/mk_{B}T}-I_{sc}\]  
where \(k_{B}\) is the Boltzmann constant, T is the temperature, \(n\) is the diode ideality factor and \(R\) is the diode series resistance, which can be defined as \(R=R_{S}+R_{C}+R_{Si}\), where \(R_{C}\) is the film top contact resistance, \(R_{Si}\) is the Si bottom contact resistance, and \(R_{S}\) is the SWCNT film resistance. \(I_{0}\) and \(I_{SC}\) are the diode inverse saturation current and the short circuit photocurrent, respectively. Through a detailed analysis of \(I_{0}\), \(n\) and the height of the potential barrier \(\varphi_{B}\), on a large series of samples characterized by different film thicknesses of CNT, we demonstrated that the interface Schottky junctions formed by metallic SWCNT/Si govern the device conduction mechanism in dark conditions while under radiation, all the SWCNTs in contact with the n-Si surface take part in the hole collection and transport [19]. In this case, the photogenerated hole current passes through a larger number of junctions with respect to the dark case, resulting in high values of \(\mathcal{R}\) and \(\mathcal{D}\). Moreover, as shown in the \(\mathcal{R}\) and \(\mathcal{D}\) measurements, the PD samples realized with thinner CNT films allow a greater absorption of radiation and charge generation; however, as seen in Figure 1a, the thinner films with a higher \(T\) are also characterized by a greater electrical resistance \(R_{S}\), and consequently, the collection of the photogenerated charges is affected. However, we can play with \(R_{S}\) by suitably designing the upper contacts, thus decreasing its value while keeping the transparency of the SWCNT film as high as possible. Indeed, we designed and fabricated a number of samples with the same characteristics as regards the CNT film but which differ only in the upper interdigitated electrode, which varied in the number of fingers N from 1 to 10, leaving the size of the active area of the device unchanged.