*3.1. Thickness Optimization of Electron Transport Layer*

For the non-inverted perovskite solar cell, the PCBM film-thickness depends on many factors such as charge collection from the perovskite layer, perovskite surface traps density, electron/hole recombination, carrier transport process, and electrode surface roughness. Therefore, in the first stage, the thickness of PCBM (as an electron-transport layer) was optimized for the proposed solar cell. In order to determine the optimize PCBM thickness for the proposed solar cell, we performed a series of photo current-voltage simulations and calculated the photovoltaic parameters such as open-circuit voltage, short-circuit current, fill-factor, and power-conversion efficiency by varying the PCBM thickness from 10 to 1000 nm, as shown in Figure 2. From the figure, it is observed that the open-circuit voltage sharply decreases from 2.5 volts and then saturates at close to 1.5 volts. While the short-circuit current, fill-factor, and most importantly power-conversion efficiency follow different trends with respect to the open-circuit voltage, where these responses sharply rise and then saturate at nearly 400 nm of PCBM thickness, as shown in Figure 2. Thus, it can be inferred from the above trends that the 400 nm is the optimum thickness of PCBM as an electron-transport layer for the proposed solar cell.

Generally, it is believed that the thinner PCBM layer may lead towards an improved charge transport process for the solar cell [29]. On the other hand, the solar cell is inherently a diode in nature and its photovoltaic response also depends on diode parameters such as the diode ideality factor, barrier-height, threshold-voltage, series resistance, shunt resistance, reverse saturation current, etc. and these parameters become further complex for the space-charge limited behavior of polymer diode [30–34]. Our simulation result shows that the thicker PCBM improves the device performance, which can be justified due to the better diode properties as it is also reported by many other researchers for the PCBM layer [35,36].

**Figure 2.** (**a**) Open-circuit voltage, (**b**) short-circuit current, (**c**) fill-factor, and (**d**) power-conversion efficiency of proposed solar cell Ag/BCP/PCBM/Cs2TiBr6/NPB/ITO as a function of PCBM layer thickness from 10 to 1000 nm.
