10 research outputs found

    Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution

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    Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting–metallic junction. We image topography, laser absorption at 532 nm, and measure <i>I</i>–<i>V</i> curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting–metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction

    Transparent Metal Films for Detection of Single-Molecule Optical Absorption by Scanning Tunneling Microscopy

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    Atomically flat, conductive, and transparent noble metal films are produced to extend the wavelength range of room-temperature single-molecule optical absorption detected by scanning tunneling microscopy (SMA-STM). Gold films grown on a platinum underlayer to 15 nm total thickness, deposited by electron beam evaporation onto c-plane sapphire substrates, show sufficient light transmission for backside illumination for laser-assisted STM experiments. Low resistance, transparency, and the atomically flat island surfaces make these good substrates for SMA-STM studies. Monte Carlo lattice kinetics were simulated to allow for a better understanding of the growth modes of the Pt–Au films and of the achieved morphologies. SMA-STM is detected for a quantum dot deposited by aerosol spraying onto Pt–Au films, demonstrating the suitability of such films for single-molecule absorption spectroscopy studies

    Imaging Excited Orbitals of Quantum Dots: Experiment and Electronic Structure Theory

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    Electronically excited orbitals play a fundamental role in chemical reactivity and spectroscopy. In nanostructures, orbital shape is diagnostic of defects that control blinking, surface carrier dynamics, and other important optoelectronic properties. We capture nanometer resolution images of electronically excited PbS quantum dots (QDs) by single molecule absorption scanning tunneling microscopy (SMA-STM). Dots with a bandgap of ∌1 eV are deposited on a transparent gold surface and optically excited with red or green light to produce hot carriers. The STM tip-enhanced laser light produces a large excited-state population, and the Stark effect allows transitions to be tuned into resonance by changing the sample voltage. Scanning the QDs under laser excitation, we were able to image electronic excitation to different angular momentum states depending on sample bias. The shapes differ from idealized S- or P-like orbitals due to imperfections of the QDs. Excitation of adjacent QD pairs reveals orbital alignment, evidence for electronic coupling between dots. Electronic structure modeling of a small PbS QD, when scaled for size, reveals Stark tuning and variation in the transition moment of different parity states, supporting the simple one-electron experimental interpretation in the hot carrier limit. The calculations highlight the sensitivity of orbital density to applied field, laser wavelength, and structural fluctuations of the QD

    Speed Limit for Triplet-Exciton Transfer in Solid-State PbS Nanocrystal-Sensitized Photon Upconversion

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    Hybrid interfaces combining inorganic and organic materials underpin the operation of many optoelectronic and photocatalytic systems and allow for innovative approaches to photon up- and down-conversion. However, the mechanism of exchange-mediated energy transfer of spin-triplet excitons across these interfaces remains obscure, particularly when both the macroscopic donor and acceptor are composed of many separately interacting nanoscopic moieties. Here, we study the transfer of excitons from colloidal lead sulfide (PbS) nanocrystals to the spin-triplet state of rubrene molecules. By reducing the length of the carboxylic acid ligands on the nanocrystal surface from 18 to 4 carbon atoms, thinning the effective ligand shell from 13 to 6 Å, we are able to increase the characteristic transfer rate by an order of magnitude. However, we observe that the energy transfer rate asymptotes for shorter separation distances (≀10 Å) which we attribute to the reduced Dexter coupling brought on by the increased effective dielectric constant of these solid-state devices when the aliphatic ligands are short. This implies that the shortest ligands, which hinder long-term colloidal stability, offer little advantage for energy transfer. Indeed, we find that hexanoic acid ligands are already sufficient for near-unity transfer efficiency. Using nanocrystals with these optimal-length ligands in an improved solid-state device structure, we obtain an upconversion efficiency of (7 ± 1)% with excitation at λ = 808 nm

    High Tolerance to Iron Contamination in Lead Halide Perovskite Solar Cells

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    The relationship between charge-carrier lifetime and the tolerance of lead halide perovskite (LHP) solar cells to intrinsic point defects has drawn much attention by helping to explain rapid improvements in device efficiencies. However, little is known about how charge-carrier lifetime and solar cell performance in LHPs are affected by extrinsic defects (<i>i.e.</i>, impurities), including those that are common in manufacturing environments and known to introduce deep levels in other semiconductors. Here, we evaluate the tolerance of LHP solar cells to iron introduced <i>via</i> intentional contamination of the feedstock and examine the root causes of the resulting efficiency losses. We find that comparable efficiency losses occur in LHPs at feedstock iron concentrations approximately 100 times higher than those in p-type silicon devices. Photoluminescence measurements correlate iron concentration with nonradiative recombination, which we attribute to the presence of deep-level iron interstitials, as calculated from first-principles, as well as iron-rich particles detected by synchrotron-based X-ray fluorescence microscopy. At moderate contamination levels, we witness prominent recovery of device efficiencies to near-baseline values after biasing at 1.4 V for 60 s in the dark. We theorize that this temporary effect arises from improved charge-carrier collection enhanced by electric fields strengthened from ion migration toward interfaces. Our results demonstrate that extrinsic defect tolerance contributes to high efficiencies in LHP solar cells, which inspires further investigation into potential large-scale manufacturing cost savings as well as the degree of overlap between intrinsic and extrinsic defect tolerance in LHPs and “perovskite-inspired” lead-free stable alternatives

    <i>A</i>‑Site Cation in Inorganic <i>A</i><sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> Perovskite Influences Structural Dimensionality, Exciton Binding Energy, and Solar Cell Performance

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    Inspired by the rapid rise in efficiencies of lead halide perovskite (LHP) solar cells, lead-free alternatives are attracting increasing attention. In this work, we study the photovoltaic potential of solution-processed antimony (Sb)-based compounds with the formula <i>A</i><sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> (<i>A</i> = Cs, Rb, and K). We experimentally determine bandgap magnitude and type, structure, carrier lifetime, exciton binding energy, film morphology, and photovoltaic device performance. We use density functional theory to compute the equilibrium structures, band structures, carrier effective masses, and phase stability diagrams. We find the <i>A</i>-site cation governs the structural and optoelectronic properties of these compounds. Cs<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has a 0D structure, the largest exciton binding energy (175 ± 9 meV), an indirect bandgap, and, in a solar cell, low photocurrent (0.13 mA cm<sup>–2</sup>). Rb<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has a 2D structure, a direct bandgap, and, among the materials investigated, the lowest exciton binding energy (101 ± 6 meV) and highest photocurrent (1.67 mA cm<sup>–2</sup>). K<sub>3</sub>Sb<sub>2</sub>I<sub>9</sub> has a 2D structure, intermediate exciton binding energies (129 ± 9 meV), and intermediate photocurrents (0.41 mA cm<sup>–2</sup>). Despite remarkably long lifetimes in all compounds (54, 9, and 30 ns for Cs-, Rb-, and K-based materials, respectively), low photocurrents limit performance of all devices. We conclude that carrier collection is limited by large exciton binding energies (experimentally observed) and large carrier effective masses (calculated from density functional theory). The highest photocurrent and efficiency (0.76%) were observed in the Rb-based compound with a direct bandgap, relatively lower exciton binding energy, and lower calculated electron effective mass. To reliably screen for candidate lead-free photovoltaic absorbers, we advise that faster and more accurate computational tools are needed to calculate exciton binding energies and effective masses

    Searching for “Defect-Tolerant” Photovoltaic Materials: Combined Theoretical and Experimental Screening

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    Recently, we and others have proposed screening criteria for “defect-tolerant” photovoltaic (PV) absorbers, identifying several classes of semiconducting compounds with electronic structures similar to those of hybrid lead–halide perovskites. In this work, we reflect on the accuracy and prospects of these new design criteria through a combined experimental and theoretical approach. We construct a model to extract photoluminescence lifetimes of six of these candidate PV absorbers, including four (InI, SbSI, SbSeI, and BiOI) for which time-resolved photoluminescence has not been previously reported. The lifetimes of all six candidate materials exceed 1 ns, a threshold for promising early stage PV device performance. However, there are variations between these materials, and none achieve lifetimes as high as those of the hybrid lead–halide perovskites, suggesting that the heuristics for defect-tolerant semiconductors are incomplete. We explore this through first-principles point defect calculations and Shockley–Read–Hall recombination models to describe the variation between the measured materials. In light of these insights, we discuss the evolution of screening criteria for defect tolerance and high-performance PV materials

    Role of Pressure in the Growth of Hexagonal Boron Nitride Thin Films from Ammonia-Borane

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    We analyze the optical, chemical, and electrical properties of chemical vapor deposition (CVD) grown hexagonal boron nitride (h-BN) using the precursor ammonia-borane (H<sub>3</sub>N–BH<sub>3</sub>) as a function of Ar/H<sub>2</sub> background pressure (<i>P</i><sub>TOT</sub>). Films grown at <i>P</i><sub>TOT</sub> ≀ 2.0 Torr are uniform in thickness, highly crystalline, and consist solely of h-BN. At larger <i>P</i><sub>TOT</sub>, with constant precursor flow, the growth rate increases, but the resulting h-BN is more amorphous, disordered, and sp<sup>3</sup>-bonded. We attribute these changes in h-BN grown at high pressure to incomplete thermolysis of the H<sub>3</sub>N–BH<sub>3</sub> precursor from a passivated Cu catalyst. A similar increase in h-BN growth rate and amorphization is observed even at low <i>P</i><sub>TOT</sub> if the H<sub>3</sub>N–BH<sub>3</sub> partial pressure is initially greater than the background pressure <i>P</i><sub>TOT</sub> at the beginning of growth. h-BN growth using the H<sub>3</sub>N–BH<sub>3</sub> precursor reproducibly can give large-area, crystalline h-BN thin films, provided that the total pressure is under 2.0 Torr and the precursor flux is well-controlled
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