Optical Properties of Lead-Free Double Perovskites by Ab Initio Excited-State Methods

We discuss the nature of the optical excitations of Cs2AgBiBr6, the archetypal compound of lead-free double perovskites. Such quaternary material shows an indirect electronic band gap with a broad optical absorption spectrum above 2 eV. By means of ab initio excited-state methods we show that the first absorption peak is due to a bound direct exciton (near the X point of the Brillouin zone), while the photoluminescence spectrum is explained in...