Tuning Defects in a Halide Double Perovskite with Pressure

Area of Science:

• Materials Science
• Solid-State Physics
• Semiconductor Physics

Background:

• Dopant defects critically influence semiconductor electronic transport by trapping or producing charge carriers.
• Halide perovskites are technologically significant semiconductors known for their substantial pressure response.
• The impact of high pressure on defects within halide perovskites remains experimentally unexplored.

Purpose of the Study:

• Investigate the structural, optical, and electronic effects of high pressure on small-bandgap double perovskites Cs2AgTlX6 (X = Cl or Br).
• Determine the role of pressure-induced defect tuning in the observed optoelectronic behavior of these materials.

Main Methods:

• Compression of Cs2AgTlX6 (X = Cl or Br) single crystals up to 56 GPa.
• Measurement of structural, optical, and electronic properties under varying pressures.
• Analysis of orbital interactions to understand defect behavior.

Main Results:

• Mild compression (1.7 GPa) increased Cs2AgTlBr6 conductivity by an order of magnitude and reduced its bandgap (0.94 to 0.7 eV).
• Complex optoelectronic behavior observed at higher pressures, with bandgap variation of 1.2 eV and conductivity changes up to 10^4.
• Conductivity changes correlated with a pressure-induced transition of bromine vacancy defects from shallow to deep states near 1.5 GPa.

Conclusions:

• Pressure-induced tuning of bromine vacancy defects significantly alters halide perovskite conductivity, independent of bandgap changes.
• The shallow-to-deep defect transition is driven by pressure weakening of Tl s-Br p antibonding interactions.
• Halogen vacancies are likely shallow donors in halide double perovskites with s-orbital-derived conduction bands.