Promising photovoltaic and solid-state-lighting materials: two-dimensional Ruddlesden-Popper type lead-free halide double perovskites Csn+1Inn/2Sbn/2I3n+1 (n=3) and Csn+1Inn/2Sbn/2Cl3n+1/Csm+1Cum/2Bim/2Cl3m+1 (n=3, m=1)

As a newly-rising member in the perovskite family of solar cell absorbers, the lead-free halide double perovskites Cs2M+M3+X6 (X = Cl, Br, I), with suitable bandgaps in the visible-light region and intrinsic thermodynamic stability, pave a new way in designing environmentally friendly perovskite solar cell devices. Here, for the first time, we optimize 24 kinds of layered Ruddlesden-Popper (RP) type n = 1-3 Csn+1Mn/2+Mn/23+X3n+1 (M+: In+, Cu+, Ag+, Au+; M3+: Bi3+, Sb3+) and investigate their electronic, optical and transport properties based on powerful first-principles and GW-BSE calculations. Surprisingly, we find that Cs4In3/2Sb3/2I10 (n = 3) has a direct bandgap of 1.29 eV, a very suitable value for visible-light absorption and emission. In order to modulate the bandgap and charge distribution, we further construct 7 two-dimensional halide double perovskite heterostructures with direct bandgaps and type-II band alignment, leading to a remarkable spatial separation of photo-generated carriers and a noticeable reduction of the carrier recombination rate, which is beneficial for photovoltaic devices. Typically, the Cs4In3/2Sb3/2Cl10/Cs2Cu1/2Bi1/2Cl4 heterostructure has a direct bandgap of 1.65 eV with a great electron-hole spatial separation. Both the different layered CsInSbI perovskites and CsInSbCl/CsCuBiCl-based heterostructures have an ultrahigh absorption coefficient of approximate to 10(5) cm(-1) in the visible-light region, which is comparable with bulk MAPbI(3). Moreover, their electron and hole mobilities are three/four orders of magnitude larger than MAPbI(3) up to 10(2)-10(3) cm(2) V-1 s(-1). The novel properties of the suitable bandgap, spatial separation of carriers, excellent absorption coefficient and extremely high carrier mobility suggest that the 2D layered RP-type CsInSbI- and CsInSbCl/CsCuBiCl-based perovskites have great potential as promising lead-free solar cell absorbers and solid-state-lighting materials.