**4. Conclusions**

Although many investigations have been done on the lead-free CH3NH3SnI3 solar cells, they suffer from low photovoltaic performance. In this paper, a SCAPS device simulation of a hybrid hetero-homojunction CH3NH3SnI3 was performed. The influence of varying the main solar cell parameters on the cell performance was thoroughly investigated. The initial cell was based on an experimental work with a record efficiency of 5.24%. The thickness and doping of both n- and p-side of the cell was investigated to find out the optimum thickness and doping of each layer. It was discovered that the thickness of the p-side should be much more than that of the n-side, taking a fixed overall thickness of the absorber. The optimized doping concentration of the n-side and the p-side was found to be 10<sup>17</sup> and 5 × <sup>10</sup><sup>16</sup> cm−3, respectively. The photovoltaic parameters for the optimized thickness and doping were: *Voc* = 0.7513, *Jsc* = 19.62, *FF* = 58.72% and *η* = 8.66%.

Further, the structure was HTL-free to overcome the organic issues usually encountered with the HTL materials. The HTL-free cell is optimized in terms of doping, work function of the back contact and bulk trap density to obtain the maximum efficiency. Moreover, the impact of different ETL materials was explored. It was observed that positive CBO up to 0.17 eV is adequate to obtain low recombination rates and, consequently, larger open circuit voltage. On the other hand, a negative CBO or a high positive CBO was not beneficial. Optimized photovoltaic parameters could be obtained for the proposed hybrid hetero-homojunction HTL-free cell using a bulk trap density of 1 × <sup>10</sup><sup>15</sup> cm−<sup>3</sup> and utilizing ZnO as an ETL, giving *Voc* = 0.948 V, *Jsc* = 26.48 mA/cm2, *FF* = 77.20 and an efficiency of 19.37%.

The presented study highlights some design rules regarding the hybrid heterohomojunction lead-free PSCs. For the selected initial cell, whose energy gap was 1.3 eV, the p-layer should be higher than the n-layer thickness. The doping levels could be selected in order to boost the cell performance. Moreover, the appropriate choice of the ETL material for the HTL-free configuration is crucial. Further, optimization of the different technological parameters of the HTL-free cell could lead to a low-cost, high-efficiency lead-free PSC.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/en14185741/s1, Figure S1: Electron and hole concentration distribution for three case studies, Figure S1: Impact of doping on the efficiency of the single n-layer and single p-layer, Figure S3: Electric field distribution inside the absorber region for the four cases of absorbers, the filed is calculated at (a) short-circuit condition and (b) *V* = 0.5 V, Figure S4: Impact of bulk trap density on the recombination rate of the HTL-free configuration, Table S1: The parameters of the front and back metal contacts, Table S2: Reverse saturation and ideality factor of the single diode model for three different values of p-side doping concentration, Table S3: Main parameters of selected materials for ETL.

**Author Contributions:** Conceptualization, M.S.S. and A.S.; methodology, M.S.S., A.S. and A.Z.; validation and formal analysis, M.A., M.S.S. and M.T.A.; Funding acquisition, M.S.S.; Visualization, A.S., M.T.A. and A.A.; investigation, all authors; writing—original draft preparation, M.S.S., A.S. and A.A.; writing—review and editing, All authors; supervision, M.S.S., A.S., A.Z., M.A., A.A. and M.T.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Deanship of Scientific Research at the University of Ha'il, project number RG-20 047.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Acknowledgments:** The authors would like to thank the Deanship of Scientific Research at the University of Ha'il for their sponsorship to the project number RG-20 047.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
