## 3 Results and Discussion
### Optimization of the FK209 Cobalt TFSI Concentration in Mesoporous TiO\({}_{2}\)
Figure 15 illustrates the relationship between hv (ev) and (Abs hv)\({}^{2}\) for the film. The energy gap of the M-TiO\({}_{2}\) doped with FK209 at 1.5-mg, 2.5-mg and 5-mg concentration deduced from the plot were 3.6 eV, 3.59 eV and 3.56 eV respectively. To evaluate the reproducibility of the ETL deposition and doping process, 30 samples were prepared and tested. Figure 16 illustrates the efficiency statistic histogram for devices fabricated using compact TiO\({}_{2}\) prepared employing DC-sputtering as a first layer in the ETL followed by an M-TiO\({}_{2}\) doped with various concentrations of FK209. The PCE of the PSCs deposited on M-TiO\({}_{2}\) doped with 2.5 mg FK209 is ranging between (12.5-15.5)% and displaying narrower distribution compared to the ones prepared with 1.5 mg and 5 mg concentration. For example, the PCE of the PSCs fabricated using M-TiO\({}_{2}\) doped with 1.5 mg FK209 is ranging between (11-15)%. This indicates that devices fabricated with 2.5-mg concentration of FK209 have higher reproducibility compared with other concen  
Figure 14: Absorbance spectrum of the M-TiO\({}_{2}\) doped with 1.5-mg, 2.5-mg and 5-mg concentrations of FK209 cobalt. Figure 13: Transparency of the M-TiO\({}_{2}\) doped with 1.5-mg, 2.5-mg and 5-mg concentrations of FK209 cobalt. tration. It is worth noticing that the 2.5-mg concentration produced larger grains size and smoother surfaces. Consequently, the 2.5 mg FK209 concentration produced higher conductivity films with slightly better transparency. As a result, the PCE of these devices were higher and more reproducible. One of the main challenges facing the wide spread use of perovskite solar cells is their long time stability and it was reported in a number of articles [16; 32]. The stability of perovskite or the deterioration in performance of perovskite is due to changing in crystal structure, which was noticed when exposing the material to high temperature, moisture, oxygen and prolonged illumination [33]. Solvents and additives used in the HTL and ETL layers can also influence the stability of perovskite cells [33; 34]. Some of the reported efficiency of perovskite exhibited deterioration by up to 20% after 200 h [9] and some deteriorated by 8% after 20 days [34]. In our devices, the stability of the fabricated devices were studied by measuring the solar cell efficiency over an extended period of 40 weeks while the devices were kept under ambient laboratory conditions of 50% humidity and 25 \({}^{\circ}\)C temperature. The efficiency was measured using sunlight simulator as described in Section 3.4. The illuminated J-V characteristics were monitored over 40 weeks by repeating the characterization every 2 weeks period then at every month in an ambient laboratory conditions. Figure 17 illustrates the stability test deduced from the forward and backward J-V scan conducted to measure the PSCs efficiency. The drop in efficiency was  
Figure 16: Statistical histogram of the PCE measured over 30 fabricated solar cells. The ETL was fabricated using C-TiO\({}_{2}\) prepared using DC-sputtering and M-TiO\({}_{2}\) doped with various concentrations of FK209. Figure 15: Energy gap of M-TiO\({}_{2}\) doped with various concentrations of FK209 (1.5 mg, 2.5 mg and 5 mg) using Tauc plot. Dotted lines represent the tangent to the slope for each plot. around 20% for samples prepared using M-TiO\({}_{2}\) doped with FK209, while the drop was 27% for control samples prepared without doping over the same period of 40 weeks. We reported previously that the stability could be improved by using green solvents such as Ethanol rather than using Acetonitrile (ACN) [35]. It is known that ACN is toxic and can accelerate the deterioration of the perovskite films [34]. Ethanol and Isopropyl alcohol (IPA) have been suggested as alternative solvents in dissolving FK209 cobalt and lithium TFSI salt respectively. These solvents have resulted in improve stability, device performance and slowed down the degradation of the perovskite films [34; 35].