*3.4. Photovoltaic Response of Proposed Solar Cell*

Figure 5 shows the simulated current-voltage responses of the proposed solar cell both in the dark and in the presence of illumination, where the optimized thickness of PCBM (400 nm), NPB (100 nm), and Cs2TiBr6 (350 nm) as the electron-transport layer, hole-transport layer, and absorber layer were already incorporated in the simulation with interface defects states as discussed above. Both curves demonstrate the characteristics behavior of a photovoltaic diode response. In the dark current-voltage response, the proposed solar-cell shows a very low current passing-through the device especially at an earlier stage of the applied voltages. After the threshold voltage (approximately at 1.1 volts) both electrodes Ag and ITO started to inject relatively a large number of electrons and holes as the forward bias current [38,39]. Using the standard technique reported in the literature, the diode ideality factor was estimated as 1.3, which reflects the quality of the proposed diode as a solar cell [40,41].

Similarly, the proposed solar cell is also simulated for the photovoltaic response in the fourth quadrant operating between the open circuit and short circuit current, where a large amount of current flows when the solar-cell is illuminated, but this illuminated current is in the opposite direction compared to the dark current-voltage response, as shown in Figure 5. From the figure, it is clearly observed that the optimized device shows a reasonable photovoltaic response under the AM 1.5 illumination. As a result of this characterization, the photovoltaic parameters such as open-circuit voltage, short-circuit current. and fill-factor are extracted from the figure as 1.29 volts, 16.66 mA·cm<sup>−</sup>2, and 78.11%, respectively. All these photovoltaic parameters lead to the maximum powerconversion efficiency for the optimized proposed solar cell up to 16.85%. This efficiency is better than the previously reported power-conversion efficiency of a doped-free, lead-free perovskite solar cell [42].

**Figure 5.** The simulated dark and AM 1.5 photovoltaic current responses of the proposed Ag/BCP/PCBM/Cs2TiBr6/NPB/ITO solar cell as a function of applied voltage.
