Extending the absorption of Cs₂AgBiCl₆ double perovskite to the near infra-red region by copper doping

The high toxicity and instability of lead halide perovskites hamper their broad applications in optoelectronics and photocatalysis. Lead-free halide Cs2AgBiCl6 double perovskite is considered a potential candidate due to less toxicity and high stability. However, its relatively large band gap of 2.64 eV absorbs a very narrow range of the solar spectrum. Herein, we synthesized Cu-doped Cs2AgBiCl6 double perovskites using three different methods, namely a hydrothermal, a post synthetic grinding and an in situ mechanochemical grinding method. We show that the absorption band of Cs2AgBiCl6 is significantly extended into the NIR range by doping with transition metal Cu. The extended absorption tail up to similar to 850 nm and broad absorption centred at around 950 nm are attributed to the formation of new sub band gap states and d-d transition of Cu doped in the lattice, respectively. This work provides an effective material that could possibly extend the optical response of lead-free double perovskites for optoelectronic devices.