Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i>

The organic–inorganic hybrid perovskites methylammonium lead iodide (CH₃NH₃PbI₃) and the partially chlorine-substituted mixed halide CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> emit strong and broad photoluminescence (PL) around their band gap energy of ∼1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> using time- and excitation-energy dependent photoluminescence spectroscopy. We show that the PL spectrum is homogenously broadened with a line width of 103 meV most likely as a consequence of phonon coupling effects. Further analysis reveals that defects or trap states play a minor role in radiative decay channels. In terms of possible lasing applications, the emission spectrum of the perovskite is sufficiently broad to have potential for amplification of light pulses below 100 fs pulse duration

Formation Dynamics of CH₃NH₃PbI₃ Perovskite Following Two-Step Layer Deposition

Hybrid metal-halide perovskites have emerged as a leading class of semiconductors for optoelectronic devices because of their desirable material properties and versatile fabrication methods. However, little is known about the chemical transformations that occur in the initial stages of perovskite crystal formation. Here we follow the real-time formation dynamics of MAPbI₃ from a bilayer of lead iodide (PbI₂) and methylammonium iodide (MAI) deposited through a two-step thermal evaporation process. By lowering the substrate temperature during deposition, we are able to initially inhibit intermixing of the two layers. We subsequently use infrared and visible light transmission, X-ray diffraction, and photoluminescence lifetime measurements to reveal the room-temperature transformations that occur in vacuum and ambient air, as MAI diffuses into the PbI₂ lattice to form MAPbI₃. In vacuum, the transformation to MAPbI₃ is incomplete as unreacted MAI is retained in the film. However, exposure to moist air allows for conversion of the unreacted MAI to MAPbI₃, demonstrating that moisture is essential in making MAI more mobile and thus aiding perovskite crystallization. These dynamic processes are reflected in the observed charge-carrier lifetimes, which strongly fluctuate during periods of large ion migration but steadily increase with improving crystallinity

Photocurrent Spectroscopy of Perovskite Solar Cells Over a Wide Temperature Range from 15 to 350 K

Solar cells based on metal halide perovskite thin films show great promise for energy generation in a range of environments from terrestrial installations to space applications. Here we assess the device characteristics of the prototypical perovskite solar cells based on methylammonium lead triiodide (CH₃NH₃PbI₃) over a broad temperature range from 15 to 350 K (−258 to 77 °C). For these devices, we observe a peak in the short-circuit current density and open-circuit voltage at 200 K (−73 °C) with decent operation maintained up to 350 K. We identify the clear signature of crystalline PbI₂ contributing directly to the low-temperature photocurrent spectra, showing that PbI₂ plays an active role (beyond passivation) in CH₃NH₃PbI₃ solar cells. Finally we observe a blue-shift in the photocurrent spectrum with respect to the absorption spectrum at low temperature (15 K), allowing us to extract a lower limit on the exciton binding energy of 9.1 meV for CH₃NH₃PbI₃

Radiative Monomolecular Recombination Boosts Amplified Spontaneous Emission in HC(NH₂)₂SnI₃ Perovskite Films

Hybrid metal-halide perovskites have potential as cost-effective gain media for laser technology because of their superior optoelectronic properties. Although lead-halide perovskites have been most widely studied to date, tin-based perovskites have been proposed as a less toxic alternative. In this Letter, we show that amplified spontaneous emission (ASE) in formamidinium tin triiodide (FASnI₃) thin films is supported by an observed radiative monomolecular charge recombination pathway deriving from its unintentional doping. Such a radiative component will be active even at the lowest charge-carrier densities, opening a pathway for ultralow light-emission thresholds. Using time-resolved THz photoconductivity analysis, we further show that the material has an unprecedentedly high charge-carrier mobility of 22 cm² V^–1 s^–1 favoring efficient transport. In addition, FASnI₃ exhibits strong radiative bimolecular recombination and Auger rates that are over an order of magnitude lower than for lead-halide perovskites. In combination, these properties reveal that tin-halide perovskites are highly suited to light-emitting devices

High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs_0.65Sb_0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs_0.65Sb_0.35 nanowires to date, exceeding 16,000 cm² V^–1 s^–1 at 10 K

Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI₃ (thin film) and find remarkably different behavior across the β–γ phase transition compared with MAPbI₃. While MAPbI₃ has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI₃ vary smoothly over a range of 100–160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI₃ exhibits a clear <i>T</i>^–0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron–phonon interactions to be governed by the Fröhlich mechanism but in contrast to the <i>T</i>^–1.5 dependence previously observed for MAPbI₃. Finally, we directly observe intraexcitonic transitions in FAPbI₃ at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K

Nanoengineering Coaxial Carbon Nanotube–Dual-Polymer Heterostructures

We describe studies of new nanostructured materials consisting of carbon nanotubes wrapped in sequential coatings of two different semiconducting polymers, namely, poly(3-hexylthiophene) (P3HT) and poly(9,9′-dioctylfluorene-<i>co</i>-benzothiadiazole) (F8BT). Using absorption spectroscopy and steady-state and ultrafast photoluminescence measurements, we demonstrate the role of the different layer structures in controlling energy levels and charge transfer in both solution and film samples. By varying the simple solution processing steps, we can control the ordering and proportions of the wrapping polymers in the solid state. The resulting novel coaxial structures open up a variety of new applications for nanotube blends and are particularly promising for implementation into organic photovoltaic devices. The carbon nanotube template can also be used to optimize both the electronic properties and morphology of polymer composites in a much more controlled fashion than achieved previously, offering a route to producing a new generation of polymer nanostructures

Photon Reabsorption Masks Intrinsic Bimolecular Charge-Carrier Recombination in CH₃NH₃PbI₃ Perovskite

An understanding of charge-carrier recombination processes is essential for the development of hybrid metal halide perovskites for photovoltaic applications. We show that typical measurements of the radiative bimolecular recombination constant in CH₃NH₃PbI₃ are strongly affected by photon reabsorption that masks a much larger intrinsic bimolecular recombination rate constant. By investigating a set of films whose thickness varies between 50 and 533 nm, we find that the bimolecular charge recombination rate appears to slow by an order of magnitude as the film thickness increases. However, by using a dynamical model that accounts for photon reabsorption and charge-carrier diffusion we determine that a single intrinsic bimolecular recombination coefficient of value 6.8 × 10^–10 cm³s^–1 is common to all samples irrespective of film thickness. Hence, we postulate that the wide range of literature values reported for such coefficients is partly to blame on differences in photon out-coupling between samples with crystal grains or mesoporous scaffolds of different sizes influencing light scattering, whereas thinner films or index-matched surrounding layers can reduce the possibility for photon reabsorption. We discuss the critical role of photon confinement on free charge-carrier retention in thin photovoltaic layers and highlight an approach to assess the success of such schemes from transient spectroscopic measurement

Biocomposite for hard tissue replacement design with respect to its osseointegration

The study presented is aimed at development of an advanced porous composite material with suitable mechanical properties for potential use in the bone tissue engineering, hierarchically organized at both micrometer and nanometer scale

Strong Carrier Lifetime Enhancement in GaAs Nanowires Coated with Semiconducting Polymer

The ultrafast charge carrier dynamics in GaAs/conjugated polymer type II heterojunctions are investigated using time-resolved photoluminescence spectroscopy at 10 K. By probing the photoluminescence at the band edge of GaAs, we observe strong carrier lifetime enhancement for nanowires blended with semiconducting polymers. The enhancement is found to depend crucially on the ionization potential of the polymers with respect to the Fermi energy level at the surface of the GaAs nanowires. We attribute these effects to electron doping by the polymer which reduces the unsaturated surface-state density in GaAs. We find that when the surface of nanowires is terminated by native oxide, the electron injection across the interface is greatly reduced and such surface doping is absent. Our results suggest that surface engineering via π-conjugated polymers can substantially improve the carrier lifetime in nanowire hybrid heterojunctions with applications in photovoltaics and nanoscale photodetectors