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 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., 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.

中文翻译：

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从头算激发态方法研究无铅双钙钛矿的光学性质

我们讨论了Cs ₂ AgBiBr ₆（无铅双钙钛矿的原型化合物）的光学激发性质。这种四元材料显示出间接电子带隙，其电子吸收谱宽于2 eV以上。通过从头算激发态方法，我们显示出第一个吸收峰是由于结合的直接激子（在布里渊区的X点附近），而光致发光光谱是通过声子辅助的间接辐射复合来解释的沿L–X和Γ–X方向传递动量的束缚激子。为了解决金属和卤原子在此类材料的电子和光学性质中的作用，我们研究了另外两种三元双钙钛矿，即Cs ₂ In₂ X ₆（X = F，Br）。在准确确定吸收系数和最小间隙的基础上，我们估计了基于此类化合物的太阳能电池光谱受限的最大效率，为它们在光伏中的应用提供了相关信息。