Lead-based halide perovskites are emerging as the most promising class of materials for next generation optoelectronics. However, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double-perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs 2 BiAgCl 6 , we perform structural refinement using single-crystal X-ray diffraction, and we characterize its optical properties via optical absorption and photoluminescence measurements.This new perovskite belongs to the Fm3m space group, and consists of BiCl 6 and AgCl 6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-Vis and PL measurements we obtain an indirect gap of 2.2 eV. The new compound is very stable under ambient conditions. Table of Contents ImageKeywords: Noble-metal halide double perovskites, Lead-free perovskites, Computational design, materials synthesis, structure refinement, UV-Vis spectra, Photoluminescence spectra 2 Perovskites are among the most fascinating crystals, and play important roles in a variety of applications, including ferroelectricity, piezzoelectricity, high-T c superconductivity, ferromagnetism, giant magnetoresistance, photocatalysis and photovoltaics. [1][2][3][4][5][6][7][8] The majority of perovskites are oxides and are very stable under ambient temperature and pressure conditions. 4,9 However, this stability is usually accompanied by very large band gaps, therefore most oxide perovskites are not suitable candidates for optoelectronic applications. The most noteworthy exceptions are the ferroelectric perovskite oxides related to LiNbO 3 , BaTiO 3 , Pb(Zr, Ti)O 3 and BiFeO 3 , which are being actively investigated for photovoltaic applications, reaching power conversion efficiencies of up to 8%. 9 The past five years witnessed a revolution in optoelectronic research with the discovery of the organic-inorganic lead-halide perovskite family. These solution-processable perovskites are fast becoming the most promising materials for the next generation of solar cells, achieving efficiencies above 20%. [10][11][12][13] Despite this breakthrough, hybrid lead-halide perovskites are known to degrade due to moisture and heat, 14 upon prolonged exposure to light, 15 and are prone to ion or halide vacancy migration, leading to unstable operation of photovoltaic devices. 16,17 At the same time the presence of lead raises concerns about the potential environmental impact of these materials. 18,19 Given these limitations, identifying a stable, non-toxic halide perovskite optoelectronic material is one of the key challenges to be ad...

ABB'X halide double perovskites based on bismuth and silver have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. In particular, CsBiAgX (X = Cl, Br) have been synthesized and found to exhibit band gaps in the visible range. However, the band gaps of these compounds are indirect, which is not ideal for applications in thin film photovoltaics. Here, we propose a new class of halide double perovskites, where the B and B cations are In and Ag, respectively. Our first-principles calculations indicate that the hypothetical compounds CsInAgX (X = Cl, Br, I) should exhibit direct band gaps between the visible (I) and the ultraviolet (Cl). Based on these predictions, we attempt to synthesize CsInAgCl and CsInAgBr, and we succeed to form the hitherto unknown double perovskite CsInAgCl. X-ray diffraction yields a double perovskite structure with space group Fm3̅m. The measured band gap is 3.3 eV, and the compound is found to be photosensitive and turns reversibly from white to orange under ultraviolet illumination. We also perform an empirical analysis of the stability of CsInAgX and their mixed halides based on Goldschmidt's rules, and we find that it should also be possible to form CsInAg(ClBr) for x < 1. The synthesis of mixed halides will open the way to the development of lead-free double perovskites with direct and tunable band gaps.

Room-temperature films of black-phase caesium lead iodide (CsPbI3) are widely thought to be trapped in a cubic perovskite polymorph. Here, we challenge this assumption. We present structural refinement of room temperature black-phase CsPbI3 in an orthorhombic polymorph. We demonstrate that this polymorph is adopted by both powders and thin-films of black-phase CsPbI3, fabricated either by high-or low-temperature processes. We perform electronic band structure calculations for the orthorhombic polymorph and find agreement with experimental data and close similarities with orthorhombic methylammonium lead iodide. We investigate the structural transitions and thermodynamic stability of the various polymorphs of CsPbI3, and show that the orthorhombic polymorph is the most

Two novel heteroleptic sensitizers, Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-hexyloxystyryl)-2,2-bipyridine)(NCS)2 and Ru((4,4-dicarboxylic acid-2,2'-bipyridine)(4,4'-bis(p-methoxystyryl)-2,2'-bipyridine) (NCS)2, coded as K-19 and K-73, respectively, have been synthesized and characterized by 1H NMR, FTIR, UV-vis absorption, and emission spectroscopy and excited-state lifetime and spectroelectrochemical measurements. The introduction of the alkoxystyryl group extends the conjugation of the bipyridine donor ligand increasing markedly their molar extinction coefficient and solar light harvesting capacity. The dynamics of photoinduced charge separation following electronic excitation of the K-19 dye was scrutinized by time-resolved laser spectroscopy. The electron transfer from K-19 to the conduction band of TiO2 is completed within 20 fs while charge recombination has a half-life time of 800 s. The high extinction coefficients of these sensitizers enable realization of a new generation of a thin film dye sensitized solar cell (DSC) yielding high conversion efficiency at full sunlight even with viscous electrolytes based on ionic liquids or nonvolatile solvents. An unprecedented yield of over 9% was obtained under standard reporting conditions (simulated global air mass 1.5 sunlight at 1000 W/m2 intensity) when the K-73 sensitizer was combined with a nonvolatile "robust" electrolyte. The K-19 dye gave a conversion yield of 7.1% when used in conjunction with the binary ionic liquid electrolyte. These devices exhibit excellent stability under light soaking at 60 degrees C. The effect of the mesoscopic TiO2 film thickness on photovoltaic performance has been analyzed by electrochemical impedance spectroscopy (EIS).

Longevity has been a long-standing concern for hybrid perovskite photovoltaics. We demonstrate high-resilience positive-intrinsic-negative perovskite solar cells by incorporating a piperidinium-based ionic compound into the formamidinium-cesium lead-trihalide perovskite absorber. With the bandgap tuned to be well suited for perovskite-on-silicon tandem cells, this piperidinium additive enhances the open-circuit voltage and cell efficiency. This additive also retards compositional segregation into impurity phases and pinhole formation in the perovskite absorber layer during aggressive aging. Under full-spectrum simulated sunlight in ambient atmosphere, our unencapsulated and encapsulated cells retain 80 and 95% of their peak and post-burn-in efficiencies for 1010 and 1200 hours at 60° and 85°C, respectively. Our analysis reveals detailed degradation routes that contribute to the failure of aged cells.