## 3 Results and Discussion
_Characterization of composite films of PEDOT:PSS-CuBi\({}_{2}\)O\({}_{4}\) NPs._ Composite films with PEDOT:PSS were deposited by the spin coating technique in order to evaluate the use of the CuBi\({}_{2}\)O\({}_{4}\) NPs as semiconductors in optoelectronics. The construction of a polymer-NP heterojunction will have the ability to separate photoexcited electron/hole pairs [26], where the polymer serves as a support for the NPs and as a hole injector layer [26; 45]. Before carrying out the optical and electric characterization, the composite film was characterized in terms of its morphology by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM). Figure 5a shows the microphotograph obtained at 250x, in which it is possible to observe the distribution of NPs along the polymer in the film. The CuBi\({}_{2}\)O\({}_{4}\) NPs are distributed along the whole PEDOT:PSS polymer, meaning a proper charge transport along the PEDOT:PSS-CuBi\({}_{2}\)O\({}_{4}\) NP system is expected. In order to complement the information provided by SEM, AFM measurements were conducted. Figure 5b shows the image for a 2 \(\upmu\)m \(\times\) 2 \(\upmu\)m area. The obtained RMS (root mean square) for this film was 24.76 nm, and the roughness average was 19.83 nm. These results were expected, considering that the size of the CuBi\({}_{2}\)O\({}_{4}\) NPs is 24 \(\pm\) 2 nm. In the zones where the NPs are present, the roughness is greater than in the zones where only PEDOT:PSS is present. It is expected that this bulk heterojunction between the polymer and the metal oxide will promote electric charge transport throughout the film [26; 45; 46]. The interface between PEDOT:PSS and all the NPs generates a large interfacial area due to the NPs' size, which is desirable for photoinduced charge separation and charge transport [26]. Although CuBi\({}_{2}\)O\({}_{4}\) NPs seemed to present thermal and chemical stability, IR spectroscopy was conducted in order to check that no material degradation occurred during the spin coating deposition. In Figure 6a, the spectrum shows signals at 413 and 523 cm\({}^{-1}\)  
Figure 4: (**a**) UV–visible spectrum from DRS and (**b**) optical bandgap energy estimated by [PERSON]’s plot method for CuBi\({}_{2}\)O\({}_{4}\) NPs. Figure 5: (**a**) SEM and (**b**) AFM of CuBi\({}_{2}\)O\({}_{4}\) NPs in PEDOT:PSS. corresponding to the vCu-O bond and the stretching vibration of the vBi-O bond, respectively. In addition, the typical bands in PEDOT:PSS, such as 1172, 1131, 1006, 960, 802 and 690 cm\({}^{-1}\), were identified [47; 48; 49]. The C-O-C bending vibrations in the ethylenedioxy group occur between 1172 and 1131 cm\({}^{-1}\), while C-S-C stretching vibrations in the thiophene ring occur between 960 and 690. The 1006 cm\({}^{-1}\) band is assigned to the O-S-O symmetric stretching mode in PSS, and the C-H angular deformation of the aromatic ring in PSS presents a band at 802 cm\({}^{-1}\)[47; 48]. From the previous results, it is deduced that there was no degradation of the NPs, nor of PEDOT:PSS, during the deposition of the composite films. Regarding the absorbance of the film measured from UV-Vis spectroscopy, shown in Figure 6b, the characteristic absorption band of the NPs centered at 527 nm is not observed, and this is because the NPs are embedded within the polymer matrix, and they form an amorphous film. In amorphous semiconductor films, a tail in the absorption spectrum encroaches into the gap region [50]. This tail in the optical absorption spectrum, as a consequence of the disorder which characterizes these semiconductors, makes the absorption edge of an amorphous semiconductor difficult to define experimentally. As a result, several models have been developed to describe the bandgap [51]. The Tauc model has served as the standard empirical model whereby the optical gap of an amorphous semiconductor may be determined. The Tauc model has been applied to amorphous films, as well [51].