Black single crystals of lead-free perovskite Cs2Ag(Bi:Ru)Br-6 with an intermediate band

The toxicity of lead halide perovskites hinders their application in optoelectronics. Lead-free Cs2AgBiBr6 is regarded as a promising candidate due to its excellent photoelectric properties. However, its excessively wide band gap limits its absorption in the visible region. Herein, we incorporated the transition metal Ru3+ into Cs2AgBiBr6 and obtained black single crystals by a hydrothermal method. The absorption edge was extended from similar to 660 nm to the near-infrared region (similar to 1200 nm). When 1.8% of Ru was doped, the double perovskite structure was still maintained and the lattice shrank since some Bi3+ was replaced by smaller Ru3+. Theoretical calculation indicates that after Ru-doping, a new intermediate band was generated inside the pristine band gap of Cs2AgBiBr6 as experimentally confirmed by Valence Band X-ray Photoelectron Spectroscopy. The intermediate band lies below the Fermi surface and is mainly dominated by the Ru-d orbital. Moreover, with the fabrication of a near-infrared (NIR) photodetector, a NIR response from the Ru-doped Cs2AgBiBr6 was realized. This work provides an effective way of regulating the energy band structure of Cs2AgBiBr6 and extends the application of lead-free perovskites for optoelectronic devices, such as NIR detectors and intermediate band solar cells.