We demonstrate four- and two-terminal perovskite-perovskite tandem solar cells with ideally matched band gaps. We develop an infrared-absorbing 1.2-electron volt band-gap perovskite, FACsSnPbI, that can deliver 14.8% efficiency. By combining this material with a wider-band gap FACsPb(IBr) material, we achieve monolithic two-terminal tandem efficiencies of 17.0% with >1.65-volt open-circuit voltage. We also make mechanically stacked four-terminal tandem cells and obtain 20.3% efficiency. Notably, we find that our infrared-absorbing perovskite cells exhibit excellent thermal and atmospheric stability, not previously achieved for Sn-based perovskites. This device architecture and materials set will enable "all-perovskite" thin-film solar cells to reach the highest efficiencies in the long term at the lowest costs.

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.

During the past year, halide double perovskites attracted attention as potential lead-free alternatives to Pb-based halide perovskites. However, none of the compounds discovered so far can match the optoelectronic properties of MAPbI (MA = CHNH). Here we argue that, from the electronic structure viewpoint, the only option to make Pb-free double perovskites retaining the remarkable properties of MAPbI is to combine In and Bi as B and B cations, respectively. While inorganic double perovskites such as CsInBiX were found to be unstable due to In oxidizing into In, we show that the +1 oxidation state of In becomes progressively more stable as the A-site cation changes from K to Cs. Hence, we propose the use of MA and FA [FA = CH(NH)] to stabilize AInBiBr double perovskites. We show that the optoelectronic properties of MAInBiBr are remarkably similar to those of MAPbI and explore the mixed-cation (Cs/MA/FA)InBiBr halide double perovskites.

Owing to their record-breaking energy conversion efficiencies, hybrid organometallic perovskites have emerged as the most promising light absorbers and ambipolar carrier transporters for solution-processable solar cells. Simultaneously, due to its exceptional electron mobility, graphene represents a prominent candidate for replacing transparent conducting oxides. Thus, it is possible that combining these wonder materials may propel the efficiency toward the Schokley-Queisser limit. Here, using first-principles calculations on graphene-CH3NH3PbI3 interfaces, we find that graphene suppresses the octahedral tilt in the very first perovskite monolayer, leading to a nanoscale ferroelectric distortion with a permanent polarization of 3 mC/m(2). This interfacial ferroelectricity drives electron extraction from the perovskite and hinders electron-hole recombination by keeping the electrons and holes separated. The interfacial ferroelectricity identified here simply results from the interplay between graphene's planar structure and CH3NH3PbI3's octahedral connectivity; therefore, this mechanism may be effective in a much broader class of perovskites, with potential applications in photovoltaics and photocatalysis.

Understanding the electronic energy landscape in metal halide perovskites is essential for further improvements in their promising performance in thin-film photovoltaics. Here, we uncover the presence of above-bandgap oscillatory features in the absorption spectra of formamidinium lead triiodide thin films. We attribute

Here we report on technology developments implemented into the Graphene Flagship European project for the integration of graphene and graphene-related materials (GRMs) into energy application devices. Many of the technologies investigated so far aim at producing composite materials associating graphene or GRMs with either metal or semiconducting nanocrystals or other carbon nanostructures (e.g., CNT, graphite). These composites can be used favourably as hydrogen storage materials or solar cell absorbers. They can also provide better performing electrodes for fuel cells, batteries, or supercapacitors. For photovoltaic (PV) electrodes, where thin layers and interface engineering are required, surface technologies are preferred. We are using conventional vacuum processes to integrate graphene as well as radically new approaches based on laser irradiation strategies. For each application, the potential of implemented technologies is then presented on the basis of selected experimental and modelling results. It is shown in particular how some of these technologies can maximize the benefit taken from GRM integration. The technical challenges still to be addressed are highlighted and perspectives derived from the running works emphasized.

The past few years witnessed the rise of halide perovskites as prominent materials for a wide range of optoelectronic applications. However, oxide perovskites have a much longer history and are pivotal in many technological applications. As of today, a rational connection between these important materials is missing. Here, we explore this missing link and develop a novel concept of perovskite analogs, which led us to identify a new semiconductor, Ba2AgIO6. It exhibits an electronic band structure remarkably similar to that of our recently discovered halide double perovskite Cs2AgInCl6, but with a band gap in the visible range at 1.9 eV. We show that Ba2AgIO6 and Cs2AgInCl6 are analogs of the well-known transparent conductor BaSnO3. We synthesize Ba2AgIO6 following a low-temperature solution process, and we perform crystallographic and optical characterizations. Ba2AgIO6 is a cubic oxide double perovskite with a direct low gap, opening new opportunities in perovskite-based electronics optoelectronics and energy applications.