Wave Function Localization Reduces the Bandgap of Disordered Double Perovskite Cs₂AgBiBr₆

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Chemistry

Background:

• Cesium silver bismuth bromide (Cs$_{2}$AgBiBr$_{6}$) is a stable, lead-free perovskite alternative with promising optoelectronic properties.
• A wide bandgap in Cs$_{2}$AgBiBr$_{6}$ hinders its application in solar cells and photodetectors.
• Ag-Bi atomic disorder is known to reduce the bandgap but the underlying mechanism is not fully understood.

Purpose of the Study:

• To elucidate the mechanism by which Ag-Bi disorder affects the bandgap of Cs$_{2}$AgBiBr$_{6}$ using theoretical calculations.
• To investigate the relationship between disorder degree and bandgap reduction.
• To provide insights into order-disorder transitions in double perovskites.

Main Methods:

• Density Functional Theory (DFT) calculations were employed.
• Simulations focused on analyzing the electronic structure and atomic arrangements in disordered Cs$_{2}$AgBiBr$_{6}$.
• Wave function localization and electronic state generation at band edges were examined.

Main Results:

• Ag-Bi disorder generates localized electronic states at the band edges, effectively tuning the bandgap.
• Disordered structures promote segregation of Ag and Bi atoms, forming homoatomic clusters.
• These clusters lead to significant wave function localization.
• The bandgap reduction shows a non-monotonic dependence on the degree of disorder.

Conclusions:

• Ag-Bi disorder is a key factor in reducing the bandgap of Cs$_{2}$AgBiBr$_{6}$ by creating localized electronic states.
• The findings align with experimental observations and clarify the role of atomic disorder.
• This study offers a fundamental understanding of bandgap engineering in double perovskites through controlled disorder.