## 3 Results and Discussion
Additionally, although this is a simplified approach that can lead to errors in the determination of the optical bandgap, it is the same approach that was used to obtain the bandgap in the NPs. The above offers an idea about the increase or decrease in the bandgap when the NPs are introduced into the polymer. In this case, the Tauc bandgap obtained with a value of 2.1 eV is significantly greater than that of the NPs previously observed. It is important to mention that in the case of composite films with a heterojunction, the Tauc bandgap is calculated for indirect transitions [52]. In this amorphous semiconductor, optical transitions are described as a first approximation by non-direct transitions with no conservation of electronic momentum, for allowed indirect transitions [53]. Although the optical bandgap is higher for the film, it is in the range of semiconductors. The above is explained by the presence of PEDOT:PSS and the heterojunction which is formed between this polymer and the NPs. According to the SEM and AFM results, the heterojunction involves self-assembly of nano-scale heterojunctions by phase separation of PEDOT:PSS and the CuBi\({}_{2}\)O\({}_{4}\) NPs. Due to the phase separation, charge-separating heterojunctions are formed throughout the bulk of the material [54]. This type of heterojunction may have advantages in the electric charge transport, due to the greater contact surface that promotes charge carrier transfer and exciton diffusion in devices, especially those of the photovoltaic type [54; 26; 55]. The electric characterization of the structure of PEDOT:PSS-CuBi\({}_{2}\)O\({}_{4}\) NPs is important to verify the above-mentioned aspects. According to the scheme for the device: glass/ITO/PEDOT:PSS-CuBi\({}_{2}\)O\({}_{4}\) NP/Ag, in Figure 1, when an external voltage is applied  
Figure 6: Spectra of (**a**) FTIR and (**b**) UV–Vis of PEDOT:PSS-CuBi\({}_{2}\)O\({}_{4}\) NP films. to the electrodes ITO and Ag, electrons are injected from ITO to PEDOT:PSS, and from the polymer, they then flow to the NPs or to the Ag contact. However, under illumination, the absorbed photons within the CuBi\({}_{2}\)O\({}_{4}\) NPs generate an electron/hole pair that must travel to the interphase to induce charge separation where electrons migrate to the Ag. The evaluation of the electric behavior in the device was conducted both in natural-light and in dark conditions, using the four-point probe method. Figure 7a shows the current-voltage (I-V) curves obtained. It can be observed that the device presents a marked change under illuminated conditions compared to the dark conditions, indicating a photoelectric behavior and that the device presents an appealing photosensitivity. It is important to note that only a slight change is observed in reverse bias. Moreover, the I-V characteristic curve behavior of the device is similar to that of a Schottky diode, where a small leakage current in reverse bias is observed. The resulting curve presents a threshold voltage of 0.067 V, a saturation current of 4.578 \(\times\) 10\({}^{-9}\) A and an ideality factor of 0.945. Additionally, a generated photocurrent of 1.448 \(\times\) 10\({}^{-9}\) A is observed at 0 V under light conditions. The latter indicates that the device starts operating at a very low voltage with an exponential output, and that also the device operation is close to the ideal diode where the ideality factor equals 1, which is evidence of its proper function as a photovoltaic device, as a consequence of an efficient charge separation and transport. On the other hand, the device characteristic presents three different conduction mechanisms: ohmic behavior, trapped charge limited current (T-CLC) and space charge limited current (SCLC) [56], which affect the resulting conductivity at different applied voltages. The resulting curve shows a current variation between 10\({}^{-9}\) and 10\({}^{-5}\) A (0 to 1.5 V), as it can be observed in the semi-logarithmic curve inset.