## 3 Results and Discussion
### Current Density-Voltage (J\(-\)V) Characteristics  
All the illuminated current density-voltage characteristics of the PSCs were measured using (ABET Sun3000) with AM 1.5 G sunlight simulator with an average light intensity of 100 mw/cm\({}^{2}\). An Abet technology model 15,150 reference cell with KGS filter traceable to NIST, NREL, Fraunhofer ISE and ISPRA standard artefacts were employed to certify our efficiency measurements. In this work, the overall samples size was 2.5 \(\times\) 2.5 cm\({}^{2}\) with an active device area of 0.36 cm\({}^{2}\). The active area is defined by the area of the window opening on the otherwise masked backside of the glass substrate. The cells were exposed to sunlight simulator though this window. Measured value of our active device area is 0.36 cm\({}^{2}\), which is larger than most of the reported areas in the literatures [8,16,17]. This indicates the potential of scaling up attributed to the use of CsI\({}_{0.05}\) ([FAPbI\({}_{3}\)]\({}_{0.85}\) (MAPbBr\({}_{3}\))\({}_{0.15}\)]\({}_{0.95}\) instead of MAI (CH\({}_{3}\)NH\({}_{3}\)PbI\({}_{3}\)) and the processes followed in this work [9]. To ensure reproducibility, 15 samples were fabricated using M-TiO\({}_{2}\) doped with FK209 cobalt and 10 samples using un-doped M-TiO\({}_{2}\) and M-TiO\({}_{2}\) doped FK209/Li\({}^{+}\) or lithium. Figure 12 shows the (J\(-\)V) characteristic curves of the perovskite solar cells. All key solar cells electrical parameters are shown in Table 2. The solar cell prepared with perovskite deposited on M-TiO\({}_{2}\) doped with FK209 yielded the highest efficiency of 15% in the forward sweep and 16.3% backward sweep. There is a noticeable increase of 14%  
Figure 11: An SEM image of the cross-section of the perovskite solar cell device fabricated in this study showing the different layers. Figure 10: Left, schematic structure of the perovskite solar cell. Right, photograph of 2.5 \(\times\) 2.5 cm\({}^{2}\) PSC fabricated in this study. in Voc in the perovskite film deposited on M-TiO\({}_{2}\) doped with FK209 as compared to the perovskite deposited on M-TiO\({}_{2}\) doped with Li\({}^{+}\). The fill factor (FF) have significantly increased by 18.5% in the perovskite deposited on M-TiO\({}_{2}\) doped with FK209 compared to perovskite deposited on un-doped M-TiO\({}_{2}\) control. The significant improvement of both the FF and Voc is attributed to the increase in conductivity of the ETL and reduction of the device series resistance. The FF have increased by 13.5% in the perovskite films deposited on M-TiO\({}_{2}\) doped with FK209 compared to perovskite deposited on M-TiO\({}_{2}\) doped with FK209/Li\({}^{+}\) or Li\({}^{+}\). The improvement in the FF is attributed to the drop in the series resistance of the solar cell from 12 to 9.3 \(\Omega\).cm\({}^{2}\)[27]. The combined resistance of the transparent conductive oxides used for the electrodes and the active perovskite film results in a series resistance Rs that significantly influence the fill factor and the efficiency of the solar cell. In perovskite solar cells, the conductivity of the transparent conductive oxide can be enhanced by increasing the film thickness, but this will lead to decrease in the transparency, which impact the cell performance. This has limited our fill factor to 56%. Our results showed an average Rs, of 9 \(\Omega\).cm\({}^{2}\) for cobalt doped TiO\({}_{2}\), and 15 \(\Omega\).cm\({}^{2}\) for un-doped M-TiO\({}_{2}\). A compromise has to be reached between transparency and conductivity to achieve higher fill factor, this is beyond the scope of this study. Moreover, we are using FTO glass substrate with sheet resistance of 12-15 \(\Omega\)/sq while the reported FTO coated glass substrate has a sheet resistance of 8 \(\Omega\)/sq [17]. This might also added to the increased value of Rs found in this study. When we compare our average series resistance with others reported, it is in agreement with other published work [7; 28].