| Summary: | Formamidinium tin iodide (FASnI<sub>3</sub>)-based perovskite solar cells (PSCs) have achieved significant progress in the past several years. However, these devices still suffer from low power conversion efficiency (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>PCE</mi><mo>=</mo><mn>6</mn><mo>%</mo></mrow></semantics></math></inline-formula>) and poor stability. Recently, Cesium (Cs)-doped Formamidinium tin iodide (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>FA</mi></mrow><mrow><mn>1</mn><mo>−</mo><mi mathvariant="normal">x</mi></mrow></msub><msub><mrow><mi>Cs</mi></mrow><mi mathvariant="normal">x</mi></msub><msub><mrow><mi>SnI</mi></mrow><mn>3</mn></msub><mo stretchy="false">)</mo></mrow></semantics></math></inline-formula> showed enhanced air, thermal, and illumination stability of PSCs. Hence, in this work, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>FA</mi></mrow><mrow><mn>1</mn><mo>−</mo><mi mathvariant="normal">x</mi></mrow></msub><msub><mrow><mi>Cs</mi></mrow><mi mathvariant="normal">x</mi></msub><msub><mrow><mi>SnI</mi></mrow><mn>3</mn></msub></mrow></semantics></math></inline-formula> PSCs have been rigorously studied and compared to pure FASnI<sub>3</sub> PSCs using a solar cell capacitance simulator (SCAPS) for the first time. The aim was to replace the conventional electron transport layer (ETL) TiO<sub>2</sub> that reduces PSC stability under solar irradiation. Therefore, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>FA</mi></mrow><mrow><mn>1</mn><mo>−</mo><mi mathvariant="normal">x</mi></mrow></msub><msub><mrow><mi>Cs</mi></mrow><mi mathvariant="normal">x</mi></msub><msub><mrow><mi>SnI</mi></mrow><mn>3</mn></msub></mrow></semantics></math></inline-formula> PSCs with different Cs contents were analyzed with TiO<sub>2</sub> and stable ZnOS as the ETLs. Perovskite light absorber parameters including Cs content, defect density, doping concentration and thickness, and the defect density at the interface were tuned to optimize the photovoltaic performance of the PSCs. The simulation results showed that the device efficiency was strongly governed by the ETL material, Cs content in the perovskite and its defect density. All the simulated devices with ZnOS ETL exhibited <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>PCEs</mi></mrow></semantics></math></inline-formula> exceeding <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>20</mn><mo>%</mo></mrow></semantics></math></inline-formula> when the defect density of the absorber layer was below <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mn>10</mn></mrow><mrow><mn>15</mn></mrow></msup><msup><mrow><mrow><mo> </mo><mi>cm</mi></mrow></mrow><mrow><mo>−</mo><mn>3</mn></mrow></msup></mrow></semantics></math></inline-formula>, and deteriorated drastically at higher values. The optimized structure with <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mrow><mi>FA</mi></mrow><mrow><mn>75</mn></mrow></msub><msub><mrow><mi>Cs</mi></mrow><mrow><mn>25</mn></mrow></msub><msub><mrow><mi>SnI</mi></mrow><mn>3</mn></msub></mrow></semantics></math></inline-formula> as light absorber and ZnOS as ETL showed the highest <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>PCE</mi></mrow></semantics></math></inline-formula> of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>22</mn><mo>%</mo></mrow></semantics></math></inline-formula> with an open circuit voltage <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="normal">V</mi><mrow><mi>oc</mi></mrow></msub></mrow></semantics></math></inline-formula> of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>0.89</mn><mrow><mo> </mo><mi mathvariant="normal">V</mi></mrow></mrow></semantics></math></inline-formula>, short-circuit current density <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi mathvariant="normal">J</mi><mrow><mi>sc</mi></mrow></msub></mrow></semantics></math></inline-formula> of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>31.4</mn><mrow><mo> </mo><mi>mA</mi></mrow><mo>·</mo><msup><mrow><mi>cm</mi></mrow><mrow><mo>−</mo><mn>2</mn></mrow></msup></mrow></semantics></math></inline-formula>, and fill factor FF of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>78.7</mn><mo>%</mo></mrow></semantics></math></inline-formula>. Our results obtained from the first numerical simulation on Cs-doped FASnI<sub>3</sub> could greatly increase its potential for practical production.
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