Cs 2 AgBiBr 6 is a promising metal halide double perovskite offering the possibility of efficient photovoltaic devices based on lead-free materials. Here, we report on the evolution of photoexcited charge carriers in Cs 2 AgBiBr 6 using a combination of temperature-dependent photoluminescence, absorption and optical pump–terahertz probe spectroscopy. We observe rapid decays in terahertz photoconductivity transients that reveal an ultrafast, barrier-free localization of free carriers on the time scale of 1.0 ps to an intrinsic small polaronic state. While the initially photogenerated delocalized charge carriers show bandlike transport, the self-trapped, small polaronic state exhibits temperature-activated mobilities, allowing the mobilities of both to still exceed 1 cm 2 V –1 s –1 at room temperature. Self-trapped charge carriers subsequently diffuse to color centers, causing broad emission that is strongly red-shifted from a direct band edge whose band gap and associated exciton binding energy shrink with increasing temperature in a correlated manner. Overall, our observations suggest that strong electron–phonon coupling in this material induces rapid charge-carrier localization.

Cs 2 AgBiBr 6 thin film preparation for characterization. The double-perovskite thin films studied in this work were all prepared through sequential vapour deposition. In a vacuumsealed chamber, AgBr (99% Fluka), BiBr 3 (≥98% Sigma Aldrich) and CsBr (99.9% Sigma Aldrich) were placed in separate crucibles and sequentially thermally evaporated onto the substrates. In particular, the standard procedure we optimized evaporated 90 nm of AgBr, 120 nm of BiBr 3 and 150 nm of CsBr to obtain 300 nm of Cs 2 AgBiBr 6 . This basic stack was repeated the necessary number of times to achieve the desired total film thickness. To achieve thicknesses that are not multiples of 300 nm (like the 750 nm reported in the text), we ran the last evaporation cycle depositing half of each precursor thickness, keeping always the same precursors ratio (1:1.3:1.6 AgBr:BiBr 3 :CsBr). After the deposition of the desired thickness, we annealed the samples on a hotplate in air at 250 ºC for 30 minutes. The post-deposition annealing temperature and time were optimised to deliver maximum solar cell performance.Solar cell preparation. FTO or ITO coated glasses were cleaned by sequential sonication in soap, water, acetone and isopropanol. After being dried with a N 2 gun, the substrates were further cleaned by O 2 plasma for 10 minutes. Titanium isopropoxide (140 µl in 1 ml of EtOH) was added to 1 ml of acidic EtOH (10 µl of HCl 2M in 1 ml EtOH), and deposited on the FTO substrates by spincoating at 2000 rpm for 45 sec with 2000 rpm/sec acceleration. Following this, the films were annealed at 150°C for 15 min and 500°C for 30 min. SnO 2 layers were prepared by spincoating at 3000 rpm (200 rpm/sec) for 30 sec of a solution of SnCl 4 ⋅5H 2 O in isopropanol (17.5 mg/ml) on top of the FTO or ITO coated glasses. The so-prepared films were annealed at 100°C for 10 min followed by an annealing at 180°C for 30 min. The SnO 2 and TiO 2 films were placed in the vacuum chamber, and the Cs 2 AgBiBr 6 film was deposited as previously presented. The hole transport material (Spiro-OMeTAD, Lumtec) was dissolved in chlorobenzene (85 mg/ml) and doped with 20 µl of LiTFSI (500 mg/ml in BuOH) and 30 µl of tert-butylpyridine. The solution was then deposited on the active layer by spincoating in air at 2000 rpm (2000 rpm/sec) for 45 sec. The devices were then left overnight in a desiccator in air atmosphere, and then completed by the evaporation of 100 nm silver contacts. All the

Since the emergence of lead halide perovskites for photovoltaic research, there has been mounting effort in the search for alternative compounds with improved or complementary physical, chemical, or optoelectronic properties. Here, we report the discovery of Cu 2 AgBiI 6 : a stable, inorganic, lead-free wide-band-gap semiconductor, well suited for use in lead-free tandem photovoltaics. We measure a very high absorption coefficient of 1.0 × 10 5 cm –1 near the absorption onset, several times that of CH 3 NH 3 PbI 3 . Solution-processed Cu 2 AgBiI 6 thin films show a direct band gap of 2.06(1) eV, an exciton binding energy of 25 meV, a substantial charge-carrier mobility (1.7 cm 2 V –1 s –1 ), a long photoluminescence lifetime (33 ns), and a relatively small Stokes shift between absorption and emission. Crucially, we solve the structure of the first quaternary compound in the phase space among CuI, AgI and BiI 3 . The structure includes both tetrahedral and octahedral species which are open to compositional tuning and chemical substitution to further enhance properties. Since the proposed double-perovskite Cs 2 AgBiI 6 thin films have not been synthesized to date, Cu 2 AgBiI 6 is a valuable example of a stable Ag + /Bi 3+ octahedral motif in a close-packed iodide sublattice that is accessed via the enhanced chemical diversity of the quaternary phase space.

Conformationally flexible “Carbene–Metal–Amide” (CMA) complexes of copper and gold show photoemissions across the visible spectrum, including mechanochromic behavior which led to the first CMA-based white light-emitting OLED.