## 3 Results and Discussion
### Optical Properties of Mesoporous TiO\({}_{2}\) Doped with FK209 Cobalt and Lithium
Where, hv is the incident photon energy and Abs is absorption of film. The energy gap of the perovskite film deposited on M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) and Li\({}^{+}\) is measured around 1.6 eV. Hence, there is no large shift in energy gap of perovskite active layer with changing the ETL dopant as expected. These results confirm that the active perovskite layer deposited on the M-TiO\({}_{2}\) doped with the studied dopants is not affected by the dopant type of the ETL [25]. Figure 5: Extrapolation of the energy gap of M-TiO\({}_{2}\) doped with FK209, FK209 /Li\({}^{+}\), and Li\({}^{+}\), using Tauc plot. Figure 6: Absorption of perovskite film deposited on M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) and lithium. The photoluminescence (PL) spectra of perovskite films deposited on M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) or lithium are shown in Figure 8. It can be observed that the perovskite films deposited on undoped M-TiO\({}_{2}\) or on M-TiO\({}_{2}\) doped with FK209/Li\({}^{+}\) or on Li\({}^{+}\) have a higher PL intensity, indicating the existence of strong recombination in the perovskite active layer [16; 26]. A peak emission at 775 nm have been obtained for all perovskite films deposited on doped or undoped M-TiO\({}_{2}\). The PL spectra showed a lower intensity for perovskite film deposited on M-TiO\({}_{2}\) doped with FK209, indicating that faster charge extraction at the interface layer occurs before their recombination [9]. This implies that the M-TiO\({}_{2}\) doped with FK209 improves the interface layer and increases charge extraction from the active perovskite layer. Consequently, this prevents carrier charge recombination, resulting in improved short circuit current Jsc, open circuit voltage Voc and fill factor FF of the PSC because of the passivation of oxygen defects at the interface layer [9]. Figure 9 shows the PL decay time of the perovskite films deposited on various ETL coated substrates; M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) and Li\({}^{+}\). In principle, electrons generated in the active perovskite layer transfer to the ETL before recombination occurs in the perovskite film [16; 26]. This is due to the fact that long carrier lifetime lead to an enhancement in charge transfer and extraction of electrons. The charge carrier lifetime (\(\tau\)) of the PL decay signals can be estimated by fitting the dynamic exponential curve expressed as:  
\[\mathrm{Y}=\mathrm{Y0}+A1\times\exp\left(\frac{-t}{\tau}\right) \tag{1}\]  
where, \(A1\) is the time independent coefficient of the amplitude fraction, \(\tau\) is decay time, Y0 is a constant. The measured lifetime of the perovskite films deposited on the M-TiO\({}_{2}\) doped with FK209 was 115.8 ns as compared to 113.4 ns for perovskite deposited on the M-TiO\({}_{2}\) doped with lithium and 120.9 ns for M-TiO\({}_{2}\) doped with FK209/Li\({}^{+}\). This results indicating more efficient charge transfer and better electrons extraction can be obtained with a mixture of FK209/Li\({}^{+}\). The PL measurements also demonstrate that FK209 cobalt is more suitable dopant to the M-TiO\({}_{2}\) compared with Li\({}^{+}\). Figure 7: Energy gap of perovskite films deposited on M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) and lithium, using Tauc plot.