## 3 Results  
To probe surface morphology of PFB thin film, deposited on p-Si substrate, SEM and AFM analyses were performed. Figure 2a shows SEM images of PFB thin film. The micrograph reveals the porous sponge-like structure of the film, which improves the sensing mechanism of the film. This type of structure makes it suitable for deposition over flexible substrate. Moreover, such films can be employed in a number of applications such as photodetectors, solar cells, and sensors. The micrograph also exhibits some agglomeration that produces discontinuity in the film and affects the flow of current. The agglomerates and discontinuity in the film are believed to be responsible for lowering the current in the diode and thus the mobility. Further investigations on the morphology of the film can be carried out to improve the conductivity of the film. Energy dispersive X-ray spectroscopy (EDX) is a very important technique employed for elemental analysis of the sample. Figure 2b shows the EDX spectrum of the PFB polymer thin film. The peaks in the spectrum provide information about the elements and their percentage existence. The higher peak intensity in the spectrum shows higher concentration of Si. It is confirmed from the spectra that the PFB thin film/p-Si comprises carbon (C), oxygen, and Si. As from the molecular structure of PFB--shown in Figure 1a--it does not contain Si and oxygen; however, these elements appear in the EDX spectrum with highest intensities. The source of Si and oxygen peaks in the spectrum is the Si substrate and the possible oxide layer on Si, respectively. It is due the fact that  
Figure 1: (**a**) Molecular structure of PFB and (**b**) schematic diagram of Ag/PFB/p-Si heterojunction. the EDX beam has large penetration depth and is able to go through the PFB thin film to reach the Si substrate. Whereas, the peak of C arises from the PFB film. Since the SEM micrograph reveals some roughness, the AFM analysis is performed in a noncontact mode to estimate the film roughness of PFB thin film. The 2D and 3D AFM images of PFB polymer on p-Si substrate are shown in Figure 2c,d, respectively. The micrographs exhibit rough and nonuniform film. Moreover, the 3D image shows fairly high spikes. From the AFM analysis, root-mean square (RMS) roughness of the PFB film was found to be \(25\pm 10\) nm, which is 13.9% of the total film thickness (180 nm). The grains are randomly oriented with distinct sizes and shapes, which is in agreement with the SEM analysis. The nonuniform surface profile due to random distribution and size of grains produce discontinuity in the film that act as potential wells and are responsible for limiting the current by trapping charge carriers. The absorption spectrum of PFB polymer on p-Si substrate is shown in Figure 3a. There are two distinct sharp peaks in the spectrum, which correspond to the Soret band and the Q-band. The first peak appearing at 220 nm lies in the ultraviolet spectral region (Soret band) and the second peak lies at 380 nm and its shoulder at 320 nm lies in the visible region (Q-band). There is a complete agreement of the acquired results with the those reported previously [25]. The optical bandgap is calculated from the plot of \((\alpha E)^{2}\) versus \(E\), shown in Figure 3b, by extrapolating the linear part of \((\alpha E)^{2}\) to zero. Here, \(E\) denotes the energy of absorbed light and \(\alpha\) is the absorption coefficient. The optical bandgaps were estimated for visible region and infrared regions as 2.88 and 5.22 eV, respectively. The X-ray diffraction pattern of PFB film, at room temperature, is shown in Figure 3c. It can be clearly seen from the figure that there is no significant peak in the curve, which verifies the noncrystalline nature of PFB. Instead a  
Figure 2: Surface morphology of PFB thin film: (**a**) SEM micrograph, (**b**) EDX spectrum, (**c**) 2D atomic force microscope (AFM) micrograph, and (**d**) 3D AFM image of PFB thin film.