## 4 Conclusions  
In summary, FK209 cobalt TFSI and lithium TFSI salt were investigated as dopant for the mesoporous TiO\({}_{2}\) in the electron transport layer. The concentration of the FK209 cobalt was varied from 1.5 mg to 5 mg. It was found that the ETL with M-TiO\({}_{2}\) doped with FK209 cobalt at a 2.5-mg concentration achieved highest efficiency due to its transparency, good conductivity, high mobility and low surface roughness compared with M-TiO\({}_{2}\) doped with lithium. From the optical characterization, it was found that the transparency of the M-TiO\({}_{2}\) doped with FK209 is around 90% and the energy gap was 3.59 eV. The M-TiO\({}_{2}\) doped with FK209 has a minimal surface roughness and low contact angle (hydrophilic) as compared with undoped control sample. The perovskite film deposited on M-TiO\({}_{2}\) doped with FK209 has a lower PL intensity indicating faster charge extraction. The measured lifetime of the perovskite films deposited on the M-TiO\({}_{2}\) doped with FK209 was 115.8 ns, which is about 2.5% longer than the perovskite films deposited on the M-TiO\({}_{2}\) doped with lithium. The average efficiency of the perovskite solar cell deposited on the C-TiO\({}_{2}\) and M-TiO\({}_{2}\) doped with FK209 electron transport layer is 15.6%. This is 44% higher than the PSCs deposited on undoped M-TiO\({}_{2}\). The FK209 cobalt doping also offers improved stability compared to lithium doping or undoped M-TiO\({}_{2}\) control device. The stability of the devices was monitored over 40 weeks period under ambient laboratory conditions of 50% humidity and 25 \({}^{\circ}\)C temperature. The perovskite solar cells exhibit a 20% drop in efficiency over the 40 weeks testing period when using M-TiO\({}_{2}\) ETL doped with FK209 and a drop of 27% for the undoped M-TiO\({}_{2}\). This study demonstrated that using FK209 cobalt with 2.5-mg concentration to dope the M-TiO\({}_{2}\) provides high efficiency PSC devices with improved stability and reproducibility.  
Figure 17: Device stability testing by measuring variations in the efficiency of the perovskite solar cells measured over 40 weeks period in weekly intervals. Devices were kept in ambient laboratory and tested under AM1.5, 100 mW/cm\({}^{2}\) illumination using ABET sunlight simulators with reference cell as a control.  
**Author Contributions:** [PERSON] conceived the original idea, performed the experimental work and wrote draft of the manuscript; [PERSON] supervised the photoluminescence and spectroscopy measurements; [PERSON] supervised the work, performed analysis and contributed to the discussion of the results and revised the manuscript. All authors have read and agreed to the published version of the manuscript.  
**Funding:** [PERSON], acknowledge the UC Doctoral Scholarship funding, University of Canterbury, New Zealand. [PERSON] and [PERSON] acknowledge the support of the MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors would like to thank [PERSON], [PERSON] and [PERSON] from the Nanofabrication Laboratory, University of Canterbury, New Zealand for providing technical assistance.  
**Conflicts of Interest:** The authors declare no conflict of interest.