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

Abstract

We discuss the nature of the optical excitations of Cs 2 AgBiBr 6 , the archetypal compound of lead-free double-perovskites. Such quaternary material shows an indirect electronic bandgap 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 (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 Γ-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. Cs 2 In 2 X 6 (X=F, Br). Based on 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. Bulk Hybrid Organic Inorganic Halide Perovskites (OIHPs) and derived systems have completely changed the scenario of modern, cheap, photovoltaics (PV), 1-7 as testified by the huge conversion efficiencies (PCEs) reached by the perovskite solar cells (PSCs) that nowadays pass 23%. 8 Despite the so high PCEs reached by PSCs based on the archetypal compound MAPbX 3 (MA=CH 3 NH + 3 , methylammonium; X=halide) as light harvester, there remain issues mainly related to device stability and toxicity. For the former, the dimensionality reduction (3D → 2D, via the partial/total replacement of MA with longer-chain, hydrophobic, aliphatic (BA=butylammonium) or aromatic (PEA=phenethylammonium), organic cations), at the price of reduced PCEs, seems one of the best direction to follow. 9-12

Cs2NaGaBr6: a new lead-free and direct band gap halide double perovskite

“…Dual halides of perovskite have stimulated great attention. [3][4][5][6][7][8][9][10][11][12][13][14][15] The double-perovskite halides are a large family of quaternary halides. The vast geometrical range of these materials poses opportunities to nd new solar and optoelectronic materials; however, it also raises challenges in testing a large number of compounds to classify the most promising with ideal energetic, structural and electronic properties for solar cell applications.…”

Lead‐Free Halide Double Perovskites: Structure, Luminescence, and Applications

“…[ 168 ] Palummo et al analyzed the nature of optical properties by means of ab initio excited‐state methods. [ 169 ] DFT results revealed that STE emission from Cs 2 AgBiBr 6 is explained in terms of phonon‐assisted radiative recombination of indirect‐bound excitons with transferred momenta along the L–X and Γ–X directions. In situ kinetics of this radiative recombination is also observed via time‐resolved PL spectra and differential transmission spectroscopy in Cs 2 AgBiBr 6 NCs (Figure 11f).…”

Chemically Localized Resonant Excitons in Silver–Pnictogen Halide Double Perovskites

“… 26 − 29 The exciton binding energies themselves remain unclear: reports of measured binding energies range between 70 meV 26 and 268 meV, 29 and a calculated exciton binding energy of 340 meV obtained with the GW +Bethe–Salpeter equation (BSE) approach falls outside this range. 30 This calls for a systematic study of the nature of excitons and their binding energies in this class of materials.…”