Theoretical Investigation of CsSn_1-xGe_xI₃ Perovskite Solar Cells Using Graded Band Gap Profiles

The continuous rise in the global energy demand encouraged research in the manufacture of more efficient cost-effective solar cells. Sn and Ge became a promising alternative to the toxic lead used in conventional perovskite solar cells (PSCs) to reduce environmental hazards. For this work, we studied the effect of band gap grading of CsSnI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>-based PSCs by varying the concentration of Ge atom as replacement of Sn atom in the absorber material utilizing Vegard's law in SCAPS-1D tool. We have analyzed the variation of power conversion efficiency (PCE) while increasing composition x in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{CsSn}_{1-\mathrm{x}}\text{Ge}_{\mathrm{x}}\mathrm{I}_{3}$</tex> from 0 to 1 in the interval of 0.25. It is observed that PSC having an active layer made of CSSn0 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${}_{.75}\text{Ge}_{0.25}\mathrm{I}_{3}$</tex> with a band gap of 1.375 eV shows a better performance (PCE of 17.57<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup>) as compared to pure planar CsSnI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (PCE of 16.05<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup>) and CsGeI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> (PCE of 14.28<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup>) based PSCs. We have done the band gap grading studies of the perovskite layer by using the optimized grading profiles. It is found out that the PCE of the device increased from 16.05% for planar CsSnI<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> to 20.06% for high gradient double graded absorber <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{CsSn}_{1-\mathrm{x}}\text{Ge}_{\mathrm{x}}\mathrm{I}_{3}/\text{CsSnI}_{3}/\text{CsSn}_{1-\mathrm{x}}\text{Ge}_{\mathrm{x}}\mathrm{I}_{3})$</tex>, 19.45 % for high gradient Front Graded <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{CsSn}_{1-\mathrm{x}}\text{Ge}_{\mathrm{x}}\mathrm{I}_{3}/\text{CsSnI}_{3})$</tex> and 17.88<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup> for high gradient Back Graded <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{CsSnI}_{3}/\text{CsSn}_{1}- {}_{\mathrm{x}}\text{Ge}_{\mathrm{x}}\mathrm{I}_{3})$</tex> PSCs. Note that the band gap grading in PSCs improves the performance of the SC significantly.