Computational Design Approximation of Efficient Low-Toxic Rb₂TiI₆/RbSn_0.5Ge_0.5I₃ Hetero-Bilayer Perovskite Solar Cell

Thermalization loss and transmission loss restrict the efficiency of a single-junction solar cell. Owing to the limitations of single p-n junctions, bilayer solar cells have proven to be the most effective approach for increasing photovoltaic (PV) device efficiency, enabling more efficient use of the solar spectrum. To address the toxicity concern of lead ($\mathbf{P b}$) in the active materials, this work proposes $\mathrm{Rb}_{2} \mathrm{TiI}_{6} / \mathrm{RbSn}_{0.5} \mathrm{Ge}_{0.5} \mathrm{I}_{3}$ hetero-bilayer solar cell of two different direct bandgap semiconductors, where both main absorbers belong to the perovskite structural family, and exhibit low toxicity, stability, and cost-effectiveness in the pursuit of long-term commercial viability. The proposed $\mathrm{Rb}_{2} \mathrm{TiI}_{6} / \mathrm{RbSn}_{0.5} \mathrm{Ge}_{0.5} \mathrm{I}_{3}$ hetero-bilayer PV device is modeled and numerically analyzed in the wxAMPS-1D simulation environment, which consists of the following layer sequence: $\quad \mathbf{I n}_{2} \mathbf{O}_{3}: \mathbf{H} / \mathbf{T i}_{3} \mathbf{C}_{2} \mathbf{T}_{\mathbf{x}} - \mathrm{TiO}_{2} / \mathrm{Rb}_{2} \mathrm{TiI}_{6} / \mathrm{RbSn}_{0.5} \mathrm{Ge}_{0.5} \mathrm{I}_{3}$/PEDOT: PSS/Ag. Absorber layer thickness, radiative (band-to-band transition) and non-radiative (trap-assisted transition via defects) recombination effects, acceptor concentration, front and bottom electrode work function, and temperature sensitivity are all thoroughly analyzed. Combining a high-bandgap $1.62 \mathrm{eV} \mathrm{Rb}_{2} \mathrm{TiI}_{6}$ absorber at $0.3 \mu \mathrm{~m}$ with a low-bandgap $1.21 \mathrm{eV} \mathrm{RbSn}_{0.5} \mathrm{Ge}_{0.5} \mathrm{I}_{3}$ absorber at $0.3 \mu \mathrm{~m}$ allows for an optimal power conversion efficiency (PCE) of $\pm \mathbf{2 8 \%}$; furthermore, it is found that optimizing the structural parameters increases the PCE significantly. Therefore, the theoretical model described here may aid in concentrating efforts to seek appropriate perovskite-solar energy materials and to enhance device manufacturing via the regulation of physical attributes.