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Cs2AgBiBr6, as a benchmark lead-free double perovskite, has emerged as a promising alternative to lead-based perovskites because of its high stability, non-toxicity, exceptional optoelectronic properties, and multifunctionality. To encourage further research on Cs2AgBiBr6 and its broad applications, in this review, its fundamental properties including the structure-property relation, synthesis, stability, origin of absorption and photoluminescence, electron-phonon coupling, role of defects, charge carrier dynamics, and bandgap modulation are comprehensively emphasized. The recent progress on the wide applications including solar cells, light/X-ray detectors, and ferroelectric/magnetic devices are highlighted. Moreover, the challenges of Cs2AgBiBr6 materials and related applications are discussed and perspectives are provided for guiding the future development of this research area. ; Funding Agencies|National Natural Science Foundation of ChinaNational Natural Science Foundation of China (NSFC) [11904115]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Energy AgencySwedish Energy Agency [46564-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

Lead-free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature-derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs2AgBiBr6 double perovskites is demonstrated. Along with the thermochromism, temperature-dependent changes in the bond lengths of Ag Symbol of the Klingon Empire Br (R-Ag Symbol of the Klingon Empire Br) and Bi Symbol of the Klingon Empire Br (R-Bi Symbol of the Klingon Empire Br) are observed. The first-principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R-Ag Symbol of the Klingon Empire Br and R-Bi Symbol of the Klingon Empire Br at high temperatures. The synergy of anharmonic fluctuations and associated electron-phonon coupling, and the peculiar spin-orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs2AgBiBr6 shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead-free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation-induced bandgap narrowing of Cs2AgBiBr6 provide new insights for the further development of optoelectronic devices based on the lead-free halide double perovskites. ; Funding Agencies|Knut and Alice Wallenberg Foundation; National Natural Science Foundation of China [61704078, 61722403]; Natural Science Foundation of Jiangsu Province of China [BK20160990]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [200900971]; European Commission Marie Skodowska-Curie actions [691210]; European Commission SOLAR-ERA.NET; Swedish Energy Agency (Energimyndigheten); Swedish Research Council (FORMAS); China Scholarship Council; Recruitment Program of Global Youth Experts in China; National Key Research and Development Program of China [2016YFB0201204]; Program for JLU Science and Technology Innovative Research Team; Swedish Research Council (VR) [2014-4750]; Ministry of Education and Science of the Russian Federation [074-02-2018-327]

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs2AgBiBr6, shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal-engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs(2)AgBiBr(6)under ambient conditions. The band-gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first-principles calculations indicate that enhanced Ag-Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band-gap narrowing effect. This work provides new insights for achieving lead-free double perovskites with suitable band gaps for optoelectronic applications. ; Funding Agencies|Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation; Swedish Energy AgencySwedish Energy Agency [2018-004357]; VR Starting Grant [2019-05279]; Carl Tryggers Stiftelse; Olle Engkvist Byggmastare Stiftelse; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish Energy AgencySwedish Energy Agency; SSFSwedish Foundation for Strategic Research; China Scholarship Council (CSC)China Scholarship Council; Ministry of Science and High Education of the Russian Federation [075-15-2019-872 (14.Y26.31.0027/074-02-2018-327)]; Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Research CouncilSwedish Research Council [2016-07213]

We show by electron spin resonance (ESR) and Raman spectroscopies that the crystal phase transition of the lead-free double-perovskite Cs2AgBiBr6 has a profound symmetry-breaking effect on the high spin states of, for example, a transition-metal ion Fe3+ and the vibrational modes. It lifts their degeneracy when the crystal undergoes the cubic-tetragonal phase transition, splitting the six-fold degenerate S = 5/2 state of Fe3+ to three Kramer doublets and the enharmonic breathing mode T-g of the MBr6 octahedra (M = Ag, Bi, Fe) into E-g + A(g). The magnitudes of both spin and Raman line splitting are shown to directly correlate with the strength of the tetragonal strain field. This work, in turn, demonstrates the power of the ESR and Raman spectroscopies in probing structural phase transitions and in providing in-depth information on the interplay between the structural, spin, and vibrational properties of lead-free double perovskites, a newly emerging and promising class of materials for low-cost and high-efficiency photovoltaics and optoelectronics. ; Funding Agencies|Swedish Research CouncilSwedish Research Council [VR-2017-05285]; Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]

Spintronics holds great potential for next-generation high-speed and low-power consumption information technology. Recently, lead halide perovskites (LHPs), which have gained great success in optoelectronics, also show interesting magnetic properties. However, the spin-related properties in LHPs originate from the spin-orbit coupling of Pb, limiting further development of these materials in spintronics. Here, we demonstrate a new generation of halide perovskites, by alloying magnetic elements into optoelectronic double perovskites, which provide rich chemical and structural diversities to host different magnetic elements. In our iron-alloyed double perovskite, Cs2Ag(Bi:Fe)Br-6, Fe3+ replaces Bi3+ and forms FeBr6 clusters that homogenously distribute throughout the double perovskite crystals. We observe a strong temperature-dependent magnetic response at temperatures below 30 K, which is tentatively attributed to a weak ferromagnetic or antiferromagnetic response from localized regions. We anticipate that this work will stimulate future efforts in exploring this simple yet efficient approach to develop new spintronic materials based on lead-free double perovskites. ; Funding Agencies|Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Energy AgencySwedish Energy Agency [2018004357, P43288-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [GA19-05259S]; China Scholarship Council (CSC)China Scholarship Council; U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering DivisionUnited States Department of Energy (DOE); European CommunityEuropean Community (EC) [312284]; Czech Ministry of EducationMinistry of Education, Youth & Sports - Czech Republic [LM2015057]; CERIC users grant; International Synchrotron Access Program of the Australian Synchrotron

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs2AgBiBr6, the prototypical compound in this class and show that the lattice dynamics of Cs2AgBiBr6 is highly anharmonic, largely in regards to tilting of AgBr6 and BiBr6 octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs2AgBiBr6 yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures. ; Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish e-science Research Centre (SeRC); Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Ministry of Science and High Education of the Russian Federation of National University of Science and Technology (NUST) "MISIS" [K2-2019-001, 211]; Swedish Research CouncilSwedish Research Council [2016-07213]

Solution-processed metal halide perovskites (MHPs) have demonstrated great advances on achieving high-performance photodetectors. However, the intrinsic material instability and the toxicity of lead still hinder the practical applications of MHPs-based photodetectors. In this work, the first highly sensitive and fast-response lead-free perovskite photodetectors based on Cs2AgBiBr6 double perovskite films are demonstrated. A convenient solution method is developed to deposit high-quality Cs2AgBiBr6 film with large grain sizes, low trap densities, and long charge carrier lifetimes. Incorporated within a photodiode device architecture comprised of optimized hole- and electron-transporting layers, lead-free perovskite photodetectors are achieved exhibiting a high detectivity of 3.29 x 10(12) Jones, a large linear dynamic range of 193 dB, and a fast response time of approximate to 17 ns. All the key figures of merit of the devices are comparable with the reported best-performing photodetectors based on lead halide perovskites. In addition, the resulting devices exhibit excellent thermal and environmental stability. The nonencapsulated devices show negligible degradation after thermal stressing at 150 degrees C and less than 5% degradation in the photoresponsivity after storage in ambient air for approximate to 2300 h. The results demonstrate the great potential of the lead-free Cs2AgBiBr6 double perovskite in applications for environmentally friendly and high-performance photodetectors. ; Funding Agencies|National Natural Science Foundation of China [51472164]; 1000 Talents Program for Young Scientists of China, Shenzhen Peacock Plan [KQTD2016053112042971]; Educational Commission of Guangdong Province [2015KGJHZ006, 2016KCXTD006]; Science and Technology Planning Project of Guangdong Province [2016B050501005]; China Postdoctoral Science Foundation [2017M622744, 2018T110886]; Swedish Research Council FORMAS [942-2015-1253]; European Commission Marie Sklodowska-Curie Actions [691210]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Following the emergence of lead halide perovskites as record-breaking materials for emerging photovoltaic applications, lead-free perovskites are gaining interest as novel materials which are compatible with inkjet-printing based technologies1,2,3. Bulk properties are of the prime importance, but charge transport can be rapidly altered across the interface of perovskite/ (hole or electron) transport layer (HTL or ETL). Yet, due to the size and complexity of the interface systems, a thorough ab initio modeling of the interfaces remains elusive to-date. Within this work, to explore the atomic-scale details of the structural and electronic properties of these materials, we employ state-of-the-art density functional theory (DFT) based calculations of model tin-based heterostructures. We investigate in particular three-dimensional corner-sharing Sn-based perovskites, their surface and interface properties and calculate the interaction of the materials with charge transport layers (CTLs). Our approach reveals the electronic band alignments and dielectric mismatches between perovskite layers and the most important hole and electron transport layers that have been used in prototype thin film device architectures4,5. In an effort to understand the fundamental physico-chemical mechanisms that govern the interactions between the photo-active absorber material and CTLs, we also probe the influence of surface termination on structural and electronic properties at FASnI3/C60 (as ETL) interfaces and FASnI3/NiOx (as ETL) interfaces. Our findings give a detailed insight into the interface engineering necessary for optimizing the next generation of environmental-friendly high-performing opto-electronic devices. This DROP-IT project1 has received funding from the European Union's Horizon 2020 research and innovation Program under the grant agreement No 862656. The information and views set out in the abstracts and presentations are those of the authors and do not necessarily reflect the official opinion of the European Union. ...

Lead-free halide double perovskites (A(2)B(I)B(III)X(6)) with attractive optical and electronic features are considered to be a promising candidate to overcome the toxicity and stability issues of lead halide perovskites (APbX(3)). However, their poor absorption profiles limit device performance. Here the absorption band edge of Cs(2)AgBiBr(6)double perovskite to the near-infrared range is significantly broadened by developing doped double perovskites, Cs-2(Ag:Cu)BiBr6. The partial replacement of Ag ions by Cu ions in the crystal lattice is confirmed by the X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (ssNMR) measurements. Cu doping barely affects the bandgap of Cs2AgBiBr6; instead it introduces subbandgap states with strong absorption to the near-infrared range. More interestingly, the near-infrared absorption can generate band carriers upon excitation, as indicated by the photoconductivity measurement. This work sheds new light on the absorption modulation of halide double perovskites for future efficient optoelectronic devices. ; Funding Agencies|Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Energy AgencySwedish Energy Agency [2018-004357]; Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [GA19-05259S]; VR Starting Grant [2019-05279]; Carl Tryggers Stiftelse; Olle Engkvist Byggmastare Stiftelse; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; China Scholarship Council (CSC)China Scholarship Council; Ministry of Science and Higher Education of the Russian Federation of NUST "MISIS" [K2-2019-001, 211]; Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Research CouncilSwedish Research Council [2016-07213]

It remains a central challenge to the information display community to develop red light-emitting diodes (LEDs) that meet demanding color coordinate requirements for wide color gamut displays. Here, we report high-efficiency, lead-free (PEA)2SnI4 perovskite LEDs (PeLEDs) with color coordinates (0.708, 0.292) that fulfill the Rec. 2100 specification for red emitters. Using valeric acid (VA)—which we show to be strongly coordinated to Sn$^{2+}$—we slow the crystallization rate of the perovskite, improving the film morphology. The incorporation of VA also protects tin from undesired oxidation during the film-forming process. The improved films and the reduced Sn$^{4+}$ content enable PeLEDs with an external quantum efficiency of 5% and an operating half-life exceeding 15 hours at an initial brightness of 20 cd/m$^{2}$. This work illustrates the potential of Cd- and Pb-free PeLEDs for display technology. ; E.H.S. and all coauthors from the Department of Electrical and Computer Engineering at the University of Toronto acknowledge the financial support from the Ontario Research Fund–Research Excellence Program, the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Global Research Outreach program of Samsung Advanced Institute of Technology. Z.-H.L. and all coauthors from the Department of Materials Science and Engineering at the University of Toronto acknowledge the financial support from the NSERC (grant number 216956-12) and the National Natural Science Foundation of China (grant number 11774304). Computations were performed on the Niagara supercomputer at the SciNet HPC Consortium. SciNet is funded by the Canada Foundation for Innovation, the Government of Ontario, Ontario Research Fund–Research Excellence, and the University of Toronto.

Poster on Electronic properties of vacancy ordered double perovskites. This project has received funding from the European Union's Horizon 2020 research and in- novation Programme under the grant agreement No 862656. The information and views set out in the abstracts and presentations are those of the authors and do not necessarily reflect the official opinion of the European Union. Neither the European Union institutions and bodies nor any person acting on their behalf may be held responsible for the use which may be made of the information contained herein.