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 terms of phonon-assisted radiative recombination of indirect-bound excitons with transferred momenta along the L-X and ?-X directions. To address the role of metal and halide atoms on the electronic and optical properties of this materials class, we investigate two additional ternary double perovskites, i.e., Cs2In2X6 (X = F, Br). On the basis of the accurate determination of the absorption coefficients and minimum gaps, we estimate the spectroscopic limited maximum efficiency of solar cells based on such compounds, providing relevant information for their application in photovoltaics.