Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs AgTlX (X=Cl (1) and Br (2)) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in A B X perovskites, the band-gap transition in A BB'X double perovskites can show substantial metal-to-metal charge-transfer character. This band-edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B- and B'-site metal frontier orbitals. Calculations reveal a shallow, symmetry-forbidden region at the band edges for 1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self-doping reaction in 2 through Br loss at ambient conditions.

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Despite their compositional versatility,m ost halide double perovskites feature large band gaps.H erein, we describe as trategy for achieving small band gaps in this family of materials.T he new double perovskites Cs 2 AgTlX 6 (X = Cl (1)a nd Br (2)) have direct band gaps of 2.0 and 0.95 eV,respectively,whichare approximately 1eVlower than those of analogous perovskites.T oour knowledge,compound 2 displays the lowest band gap for any knownh alide perovskite.U nlike in A I B II X 3 perovskites,t he band-gap transition in A I 2 BB'X 6 double perovskites can showsubstantial metal-to-metal charge-transfer character.This band-edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B-and B'-site metal frontier orbitals.C alculations reveal as hallow, symmetry-forbidden region at the band edges for 1,w hich results in long (ms) microwave conductivity lifetimes.W ef urther describe af acile self-doping reaction in 2 through Br 2 loss at ambient conditions.The exceptional optoelectronic properties of lead-halide perovskite solar-cell absorbers [1] have instigated ar enewed interest in this well-known family of materials.A lthough optimization of these absorbers have afforded record solarcell efficiencies, [2] there is now growing appreciation for the need for new perovskite compositions. [3] Studying analogues is at ime-tested method for understanding the origins of achampion materialsremarkable properties and establishing design rules for realizing these properties in alternative compositions.T oa ccess greater compositional variation within the perovskite lattice,w e [4] and others [5] recently explored the photophysical properties of the A I 2 BB'X 6 (X = halide) double perovskite platform. Unlike A I B II X 3 perovskites,w hich are restricted to 2 + B-site cations,d ouble perovskites can support oxidation states from 1 + to 4 + for the B-site metals.This compositional flexibility should afford diverse electronic structures,asobserved in oxide perovskites, which range from insulators to metals. [6] However,todate,the vast majority of halide double perovskites [4,5,7] are either chlorides or bromides with band gaps over approximately 2eV. Indeed, halide substitution has been the most explored method for reducing the band gap in halide perovskites,with iodides affording the lowest band gaps. [8] Herein, we demonstrate that double perovskites enable ad ifferent method for creating small band gaps-through the energetic alignment of the frontier orbitals of the Band B'-site metals.T his strategy allows us to form chloride and bromide double perovskites with band gaps that are about 1eVs maller than those of analogous perovskites with the same halide.T oo ur knowledge,t he bromide perovskite features the smallest band gap reported for ah alide perovskite since their discovery in 1883. [9] Crystalline powders of Cs 2 AgTlCl 6 (1)a nd Cs 2 AgTlBr 6 (2)w ere precipitated from solutions containing Tl 2 O 3 ,C sX, and AgX (X = Cl À ,Br À )in6m HCl and 9 m HBr, respectively. Impor...