Fully inorganic lead-free double perovskites (DP) halides have emerged as an important class of environmentally benign phosphors for optoelectronic applications. Studies on them have mostly been limited to the visible spectral region.
Lanthanide (Ln3+) doping is proposed as a valid approach to extend the spectral range of DPs towards the near infrared (NIR) region. Unfortunately, these Ln3+-doped DPs still suffer from low Ln low NIR emission3+ due to the low absorption cross section of Ln3+.
In a study published in Angewandte Chemie International Edition, the research group led by Prof. CHEN Xueyuan of the Fujian Research Institute of Structure of Matter, Chinese Academy of Sciences developed a new class of DPs emitting Ln-based NIR3+-doped Cs2(Na/Ag)BiCl6. Enjoying the Na+– induced rupture of the local symmetry of the site in the Cs2AgBiCl6 DPs, effective NIR emissions from Ln3+(for example Yb3+ and Er3+) are made by Bi3+ sensitization.
The researchers first systematically studied the optical properties of Ln3+-doped Cs2(Na/Ag)BiCl6 phosphors to determine the optimal Na+ content, and then extensively studied the dynamics of the excited state and the process of energy transfer in the optimal Ln3+-doped Cs2AG0.2N / A0.8BiCl6 phosphors using temperature-dependent steady-state and transient photoluminescent spectroscopy.
Raman spectroscopy analysis and first principles Density Functional Theory (DFT) calculations confirmed that the Na/Ag alloy caused the Bi-Cl bond length to change, resulting in the local symmetry breaking of Bi3+ in [BiCl6]3- octahedron.
Compared to that of Cs without Na2AgBiCl6 counterparts, the NIR emission of Yb3+ and Er3+ can be increased by 7.3 times and 362.9 times in Cs2AG0.2N / A0.8BiCl6 DPs, with peak photoluminescence quantum yields (PLQY) of 19.0% and 4.3%, respectively.
Furthermore, the researchers used these Ln3+Cs emitting doped NIRs2AG0.2N / A0.8BiCl6 DP for UV-converted NIR light-emitting diodes (LEDs) at 365 nm, which indicated their potential applications as high-performance optoelectronic devices.
These findings provide new insights into the design of efficient NIR luminescent Ln3+ doped materials based on double perovskite inorganic halides, which could accelerate the development of new NIR optoelectronic devices.