Noble Design of metallic semi-hierarchical nanostructures for overcoming absorption limitations in lead-free Cs₂AgBiBr₆ perovskite photovoltaic devices

Lead-free Cs₂AgBiBr₆ double perovskites are stable compared to other perovskites, but have a wide indirect bandgap, resulting in a low absorption coefficient and consequently a low short-circuit current density (J_sc). In this study, we computationally designed metallic semi-hierarchical nanostructures (SHNs) and incorporated them into environmentally friendly, lead-free Cs₂AgBiBr₆ double perovskite solar cells to achieve ultra-high optical absorptance. The effects of SHNs on photovoltaic performance were systematically investigated through numerical simulations, considering both intrinsic material properties and structural dimensions. SHN materials were selected based on their interfacial reflectance with Cs₂AgBiBr₆ and subsequently categorized into two distinct groups. Group 1, represented by Ti and Al, did not exhibit total internal reflection (TIR) in the long-wavelength region; instead, a portion of the incident light was transmitted into the SHNs bulk. This spectral behavior allows these materials to absorb incident light that is otherwise unabsorbed by Cs₂AgBiBr₆, owing to its inherently deep absorption depth (DAD) at longer wavelengths. Conversely, Group 2, comprising materials such as Au and Ag, demonstrated TIR in the long-wavelength region, enhancing light trapping and absorption within the Cs₂AgBiBr₆ rather than within the SHNs. Among the investigated materials, Ti (Group 1) exhibited the highest optical absorptance, maintaining a stable value of approximately 90 % over the full solar spectrum and increasing the J_sc from 15.02 mA/cm² (without SHNs) to 29.26 mA/cm². Overall, this study demonstrates that leveraging the structural advantages of SHNs and strategically selecting SHN materials can effectively overcome the DAD of Cs₂AgBiBr₆ and significantly enhance the light-harvesting performance.