## 3 Results and Discussion
### Optical Properties of Mesoporous TiO\({}_{2}\) Doped with FK209 Cobalt and Lithium  
The transparency of the ETL is another factor that influences the efficiency of perovskite solar cells. High transparency and high conductivity are essential requirements in the function of the ETL. The transparency of undoped mesoporous TiO\({}_{2}\) is round 90% [16]. In this study, the FK209 was used to dope the M-TiO\({}_{2}\) film to increase its conductivity but without reducing its transparency significantly. The conductivity has increased from 22 \(\times\) 10\({}^{3}\) to 30 \(\times\) 10\({}^{3}\) [1/\(\Omega\).cm] while the transparency was maintained its value around 90% at 600-nanometer wavelength. To study the influence of doping on the transparency of the ETL, M-TiO\({}_{2}\) was doped with FK209, lithium and a mixture of FK209/Li\({}^{+}\). An Ultraviolet-Visible spectrometry (Cary 6000i) measurement was utilized to measure the transparency of the ETL films. Generally, it is clear from Figure 3 that 85% transparency have been obtained from all M-TiO\({}_{2}\) films doped with FK209, Li\({}^{+}\) or FK209/Li\({}^{+}\) over the visible wavelength range. There is slight increase in the transparency between 300-500 nm and 650-800 nm for the M-TiO\({}_{2}\) doped with FK209 compared to M-TiO\({}_{2}\) doped with lithium or FK209/Li\({}^{+}\). Figure 4 shows the absorbance of the M-TiO\({}_{2}\) doped with FK209, Li\({}^{+}\) or FK209/Li\({}^{+}\) measured using Cary UV-Vis spectrometer. There is a weak absorption between 350-800 nm, and much higher absorption at a range from 300-350 nm. Figure 2: Left, SEM images of surface topography of the perovskite layer deposited on the (**a**) undoped M-TiO\({}_{2}\) and mesoporous TiO\({}_{2}\) doped with (**b**) FK209, (**c**) FK209/Li\({}^{+}\) and (**d**) Li\({}^{+}\). Right, the histogram shows an average grains size for (**a**) undoped M-TiO\({}_{2}\), 163.9 nm; (**b**) M-TiO\({}_{2}\) doped with FK209, 242 nm; (**c**) FK209/Li\({}^{+}\), 210 nm; (**d**) Li\({}^{+}\), 205 nm. The optical band gap energy of the films can be extrapolated from the absorption data using Tauc plot method [21]. Figure 5 displays the extracted energy band gaps of the un-doped M-TiO\({}_{2}\) and M-TiO\({}_{2}\) doped with FK209, FK209 /Li\({}^{+}\) and Li\({}^{+}\). The Tauc plot was obtained from the absorption spectra shown in Figure 4. An extrapolation technique (dotted lines) was used to determine the optical band gap of the M-TiO\({}_{2}\) doped with different dopants. The energy gaps of the undoped M-TiO\({}_{2}\) and M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) or Li\({}^{+}\) are 3.55 eV, 3.59 eV, 3.56 eV, and 3.58 eV respectively. A number of studies have shown that the energy gap of the un-doped M-TiO\({}_{2}\) is around 3.5 eV in agreement with our results [16; 22; 23]. Figure 4: Optical absorbance properties of mesoporous TiO\({}_{2}\) doped with FK209, FK209 /Li\({}^{+}\), and Li\({}^{+}\). Figure 3: Optical transparency properties of mesoporous TiO\({}_{2}\) doped with FK209, FK209 /Li\({}^{+}\), and Li\({}^{+}\). Figure 6 shows the absorption of the perovskite films deposited on undoped M-TiO\({}_{2}\) and M-TiO\({}_{2}\) doped with FK209, FK209/Li\({}^{+}\) and Li\({}^{+}\) measured over the 440 nm to 800 nm range. It is observed that the perovskite films deposited on M-TiO\({}_{2}\) doped with FK209 has the same absorbance spectrum compared with perovskite deposited on M-TiO\({}_{2}\) doped with FK209/Li\({}^{+}\) or Li\({}^{+}\). However, there is a slight improvement of around 5% in absorption spectrum between 500-800 nm. This could be one of the reasons for the increased current density obtained for the perovskite film deposited on the M-TiO\({}_{2}\) doped with FK209 [6]. As a result, the PSCs prepared with ETL doped with FK209 have produced the highest efficiency. The absorbance spectra of our perovskite samples were in agreement with other reported study [24]. Figure 7 shows the relation between hv (ev) and (Abs hv)\({}^{2}\).