Solution-processable metal halide perovskites show immense promise for use in photovoltaics and other optoelectronic applications. The ability to tune their bandgap by alloying various halide anions (for example, in CH3NH3Pb­(I1–x Br x )3, 0 < x < 1) is however hampered by the reversible photoinduced formation of sub-bandgap emissive states. We find that ion segregation takes place via halide defects, resulting in iodide-rich low-bandgap regions close to the illuminated surface of the film. This segregation may be driven by the strong gradient in carrier generation rate through the thickness of these strongly absorbing materials. Once returned to the dark, entropically driven intermixing of halides returns the system to a homogeneous condition. We present approaches to suppress this process by controlling either the internal light distribution or the defect density within the film. These results are relevant to stability in both single- and mixed-halide perovskites, leading the way toward tunable and stable perovskite thin films for photovoltaic and light-emitting applications.

] Of relevance to this work is the binary metal perovskite CH 3 NH 3 (Pb x Sn 1-x )I 3 [0 ≤ x ≤ 1]. [30,31] Interestingly, the bandgap bows and becomes lower when Sn 2+ is substituted by Pb 2+ for samples with 80% and 60% Sn content compared to 100% Sn-based perovskite, in line with previous observations. [30,31] While such tin-based perovskites offer tunable bandgaps down to 1.1 eV, the fabrication of efficient optoelectronic devices has been impeded by factors including poor semiconductor quality and low surface coverage. [30] As a consequence, solar cells made using these perovskites often exhibit very low efficiencies, with typical PCEs < 1% obtained for planar heterojunction devices. [30] To overcome this challenge, we have developed a novel elevated temperature processing method (depicted in Figure 1A), [32] for preparing CH 3 NH 3 (Pb x Sn 1-x )I 3 perovskites on a Poly(3,4-ethylenedioxythiophene):poly(styrenesulf onate) (PEDOT:PSS)/nickel oxide (NiO) bilayer, which results in the formation of large micron-sized grains ( Figure 1B) with almost complete substrate coverage. Our semiconductors not only exhibit relatively low energetic and structural disorder but also impart high PCEs when fabricated into a PV device. For PVs prepared using the lowest bandgap perovskites, open circuit voltages (V OC 's) approaching the prediction of the Shockley-Queisser (S-Q) model are demonstrated. Such promising performance metrics are obtained against a backdrop of fast radiative recombination and low photoluminescence quantum efficiencies (PLQEs), pointing toward the crucial role of high intrinsic charge carrier mobility in these low-bandgap semiconductors.To study the optical properties of the CH 3 NH 3 (Pb x Sn 1-x )I 3 [0 ≤ x ≤ 1] perovskite thin films, linear absorption and photoluminescence (PL) were measured as shown in Figure S1 (Supporting Information). It can be observed in Figure 1C that the bandgap bows as we substitute Pb 2+ in place of Sn 2+ (until 40% Sn 2+ ions are replaced by Pb 2+ ) and results in a nonmonotonic bandgap lowering similar to what was observed previously by Hao etal. [31] Briefly, the bandgap of the 60% and 80% Sn content films exhibit a lower bandgap than the 100% Sn-substituted films. A similar trend can also be traced in the PL spectra (see Figure S1B of the Supporting Information) where the PL spectra of 80% and 60% Sn content thin-film samples are red-shifted compared to the 100% Sn content thinfilm sample, which is consistent with the absorption spectra. Such anomalous bandgap bowing and lack of conformity with Vegard's law [31,33] have been attributed to the competition The performance of organometallic halide (hybrid) perovskite solar cells has improved dramatically in just a few years, with photovoltaic (PV) power conversion efficiencies (PCEs) now exceeding 22% for state-of-the-art devices. [1][2][3][4][5] This remarkable result, coupled with their low cost, tunability, and versatile lowtemperature preparation methods, makes hybrid perovskites one of the most promising semiconduct...

TiNb2O7 is a Wadsley–Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in Li x TiNb2O7) is rapid with low activation barriers from NMR and D Li = 10–11 m2 s–1 at the temperature of the observed T 1 minima of 525–650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100–200 meV down the tunnels but ca. 700–1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in Li x TiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1–3 eV, indicating that this structure is specifically suited to Li+ mobility.

We investigate the origin of the broadband visible emission in layered hybrid lead-halide perovskites and its connection with structural and photophysical properties. We study ⟨001⟩ oriented thin films of hexylammonium (HA) lead iodide, (CHN)PbI, and dodecylammonium (DA) lead iodide, (CHN)PbI, by combining first-principles simulations with time-resolved photoluminescence, steady-state absorption and X-ray diffraction measurements on cooling from 300 to 4 K. Ultrafast transient absorption and photoluminescence measurements are used to track the formation and recombination of emissive states. In addition to the excitonic photoluminescence near the absorption edge, we find a red-shifted, broadband (full-width at half-maximum of about 0.4 eV), emission band below 200 K, similar to emission from ⟨110⟩ oriented bromide 2D perovskites at room temperature. The lifetime of this sub-band-gap emission exceeds that of the excitonic transition by orders of magnitude. We use X-ray diffraction measurements to study the changes in crystal lattice with temperature. We report changes in the octahedral tilt and lattice spacing in both materials, together with a phase change around 200 K in DAPbI. DFT simulations of the HAPbI crystal structure indicate that the low-energy emission is due to interstitial iodide and related Frenkel defects. Our results demonstrate that white-light emission is not limited to ⟨110⟩ oriented bromide 2D perovskites but a general property of this class of system, and highlight the importance of defect control for the formation of low-energy emissive sites, which can provide a pathway to design tailored white-light emitters.

Infrared emitters are reasonably rare in solution-processed materials. Recently, research into hybrid organo-lead halide perovskite, originally popular in photovoltaics,1-3 has gained traction in light-emitting diodes (LED) due to their low-cost solution processing and good performance.4-9 The lead-based electroluminescent materials show strong colorful emission in the visible region, but lack emissive variants further in the infrared. The concerns with the toxicity of lead may, additionally, limit their wide-scale applications. Here, we demonstrate tunable near-infrared electroluminescence from a lead-free organo-tin halide perovskite, using an ITO/PEDOT:PSS/CH3NH3Sn(Br1-xIx)3/F8/Ca/Ag device architecture. In our tin iodide (CH3NH3SnI3) LEDs, we achieved a 945 nm near-infrared emission with a radiance of 3.4 W sr(-1) m(-2) and a maximum external quantum efficiency of 0.72%, comparable with earlier lead-based devices. Increasing the bromide content in these tin perovskite devices widens the semiconductor bandgap and leads to shorter wavelength emissions, tunable down to 667 nm. These near-infrared LEDs could find useful applications in a range of optical communication, sensing and medical device applications.