Lead halide perovskites show excellent optoelectronic properties but are unsatisfactory in terms of stability and toxicity. Herein, bismuth (Bi)‐doped lead‐free inorganic perovskites Cs2SnCl6:Bi are reported as blue emissive phosphors. Upon Bi doping, the originally nonluminous Cs2SnCl6 exhibits a highly efficient deep‐blue emission at 455 nm, with a Stokes shift of 106 nm and a high photoluminescence quantum yield (PLQY) close to 80%. Hybrid density functional theory calculations suggest the preferred formation of [BiSn+VCl] defect complex, which is believed to be responsible for the optical absorption and the associated blue emission. The Cs2SnCl6:Bi also shows impressive thermal and water stability due to its inorganic nature and the formation of protective BiOCl layer. White light‐emitting diodes (LEDs) are constructed using Cs2SnCl6:Bi and commercial yellow phosphors combined with commercial UV LED chips, giving the Commission Internationale de I'Eclairage (CIE) color coordinates of (0.36, 0.37). This work represents a significant step toward the realization of highly efficient, stable, and environmentally benign next‐generation solid‐state lighting.

Using density-functional theory calculations, we analyze the optical absorption properties of lead (Pb)-free metal halide perovskites (AB 2+ X 3 ) and double perovskites (AB + B 3+ X 6 ) (A = Cs or monovalent organic ion, B 2+ = non-Pb divalent metal, B + = monovalent metal, B 3+ = trivalent metal, X = halogen). We show that, if B 2+ is not Sn or Ge, Pb-free metal halide perovskites exhibit poor optical absorptions because of their indirect bandgap nature. Among the nine possible types of Pb-free metal halide double perovskites, six have direct bandgaps. Of these six types, four show inversion symmetry-induced parity-forbidden or weak transitions between band edges, making them not ideal for thin-film solar cell application. Only one type of Pb-free double perovskite shows optical absorption and electronic properties suitable for solar cell applications, namely those with B + = In, Tl and B 3+ = Sb, Bi. Our results provide important insights for designing new metal halide perovskites and double perovskites for optoelectronic applications. TOC GRAPHICS3 Organic-inorganic lead (Pb) halide perovskite solar cells have attracted significant attention, owing to their rapid improvement in record power conversion efficiency (PCE) over the past few years. [1][2][3][4][5][6] Despite the demonstration of the great potential, this emerging photovoltaic technology is still facing serious challenges, most notably with regards to cell instability against moisture and temperature and the inclusion of toxic Pb. Extensive efforts have been paid to discover nontoxic or low-toxicity and air-stable metal halide perovskite-based solar cell materials. [7][8][9][10][11][12][13][14][15][16][17][18][19][20] There are two general routes for designing Pb-free metal halide perovskite related absorber materials. The first route is to replace Pb by a divalent cation to create a Pb-free AB 2+ X 3 perovskite (A = Cs or monovalent organic ion, B 2+ = non-Pb divalent metal, X = halogen). The divalent cations from group 2, group 12, and group 14 (Ge and Sn) of the periodic table are the choices for the B 2+ site. Among these possible Pb-free metal halide perovskites, so far, only Sn halides have achieved solar cells with reasonable PCEs. [7][8][9][10] Previous studies have revealed the importance of the strong Pb 6s-I 5p antibonding coupling and the high symmetry of the perovskite structure for the superior photovoltaic properties of lead halide perovskite absorbers. [21][22][23][24] Therefore, lone-pair Bi 3+ /Sb 3+ trivalent cations have been employed to replace Pb, resulting in A 3 (Bi 3+ /Sb 3+ ) 2 X 9 low-dimensional perovskite and non-perovskite (also called perovskite alternative) compounds, which show low electronic and structural dimensionalities and associated large bandgaps (> 2 eV), heavy carrier effective masses, detrimental defect properties, and thus poor photovoltaic performances. 11,21,25 To overcome the low structural dimensionality issue, trivalent Bi 3+ /Sb 3+ has been combined with monovalent cations on the Bsites of halide per...

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

Bismuth- or antimony-based lead-free double perovskites represented by Cs AgBiBr have recently been considered promising alternatives to the emerging lead-based perovskites for solar cell applications. These new perovskites belong to the Fm3‾ m space group and consist of two types of octahedra alternating in a rock-salt face-centered cubic structure. We show, by density functional theory calculations, that the stable chemical potential region for pure Cs AgBiBr is narrow. Ag vacancies are a shallow accepters and can easily form, leading to intrinsic p-type conductivity. Bi vacancies and Ag antisites are deep acceptors and should be the dominant defects under the Br-rich growth conditions. Our results suggest that the growth of Cs AgBiBr under Br-poor/Bi-rich conditions is preferred for suppressing the formation of the deep defects, which is beneficial for maximizing the photovoltaic performance.

Practical application of hybrid Pb-based halide perovskites needs not only to fabricate high-quality film samples on a large scale but also to properly overcome the issues of Pb toxicity and materials instability. Finding new, stable, Pb-free perovskites currently attracts significant research interest. Among various strategies, hetero-substitution of Pb to form quaternary halide double perovskites represents a promising direction to keep the high structural (and likely electronic) dimensionality nature of perovskite lattice and meanwhile offers rich chemical compositions a degree of freedom for discovering new perovskite materials. We herein present a perspective that concisely reviews the progress of rational design of Pb-free halide double perovskites by both theoretical and experimental efforts as well as their current and potential applications in various optoelectronic categories. We also envision the future research directions to realize new materials by exploring broader chemical composition space and to better utilize existing materials by considering optoelectronic properties modulation.