*3.3. Optimization of Cs2TiBr6 as the Absorber Layer*

The thickness optimization of the perovskite absorber layer is considered as one of the most crucial parts for the overall design of the proposed device for efficient performance. Either a very thin or very thick perovskite layer, is problematic in nature for the photovoltaic application. The thin perovskite layer offers poor photon absorption, while the thick perovskite layer compromises higher electron-holes recombination and hence degrades the photovoltaic response. Therefore, the optimum absorber layer thickness is the balance trade-off between these two factors. For the determination of design thickness of absorber layer, the optimized thickness of both PCBM (400 nm) and NPB (100 nm) layers and interface defect states were used in the proposed device and then a series of photo currentvoltage simulations were carried-out. The photovoltaic parameters such as open-circuit voltage, short-circuit current, fill-factor, and power-conversion efficiency were calculated by varying the Cs2TiBr6 perovskite layer thickness from 10 to 1000 nm, as shown in Figure 4. From the figure, it is observed that photovoltaic parameters such as the fill-factor, shortcircuit current, and power-conversion efficiency follow one trend, while the open-circuit

voltage follows different trends from the others. The fill-factor, short-circuit current, and power-conversion efficiency sharply rise and then saturate at nearly 350 nm of the absorber layer thickness, as shown in Figure 4. However, the open-circuit voltage shows a little different behavior, it sharply increases at 50 nm and then gradually decreases and becomes stable at nearly 200 nm of the absorber layer thickness. As the proposed device reaches the maximum power-conversion efficiency at 350 nm of the absorber layer thickness, therefore, it can be inferred from the above discussion that the 350 nm can be estimated as the optimized thickness (gives maximum power-conversion efficiency ~16.5%) for the perovskite absorber layer for the proposed solar cell.

**Figure 4.** (**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 Cs2TiBr6 thickness from 10 to 1000 nm.
