Microwave Conductivity Research Articles

New Generation Hole Transporting Materials for Perovskite Solar Cells: Amide‐Based Small‐Molecules with Nonconjugated Backbones

AbstractState‐of‐the‐art perovskite‐based solar cells employ expensive, organic hole transporting materials (HTMs) such as Spiro‐OMeTAD that, in turn, limits the commercialization of this promising technology. Herein an HTM (EDOT‐Amide‐TPA) is reported in which a functional amide‐based backbone is introduced, which allows this material to be synthesized in a simple condensation reaction with an estimated cost of <$5 g−1. When employed in perovskite solar cells, EDOT‐Amide‐TPA demonstrates stabilized power conversion efficiencies up to 20.0% and reproducibly outperforms Spiro‐OMeTAD in direct comparisons. Time resolved microwave conductivity measurements indicate that the observed improvement originates from a faster hole injection rate from the perovskite to EDOT‐Amide‐TPA. Additionally, the devices exhibit an improved lifetime, which is assigned to the coordination of the amide bond to the Li‐additive, offering a novel strategy to hamper the migration of additives. It is shown that, despite the lack of a conjugated backbone, the amide‐based HTM can outperform state‐of‐the‐art HTMs at a fraction of the cost, thereby providing a novel set of design strategies to develop new, low‐cost HTMs.

High-Performance Thin Film Transistor with an Neodymium-Doped Indium Zinc Oxide/Al₂O₃ Nanolaminate Structure Processed at Room Temperature.

In this work, a high-performance thin film transistor with an neodymium-doped indium zinc oxide (Nd:IZO) semiconductor via a room temperature approach and adopting the Nd:IZO/Al2O3 nanolaminate structure was investigated. The effects of the ultrathin Al2O3 layer and the thickness of Nd:IZO layer in the nanolaminate structure on the improvement of electrical performance and stability of thin film transistors (TFTs) were systematically studied. Besides the carrier movement confined along the near-channel region, driven by the Al2O3 layer under an electrical field, the high performance of the TFT is also attributed to the high quality of the 8-nm-thick Nd:IZO layer and the corresponding optimal Nd:IZO/Al2O3 interface, which reduce the scattering effect and charge trapping with strong M–O bonds in bulk and the back-channel surface of Nd:IZO, according to the X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and micro-wave photo conductivity decay (μ-PCD) results. As a result, the Nd:IZO/Al2O3 TFT exhibits an outstanding performance, with a high μsat of 32.7 cm2·V−1·s−1, an Ion/Ioff of 1.9 × 108, and a low subthreshold swing (SS) value of 0.33 V·dec−1, which shows great potential for the room temperature fabrication of TFTs in high-resolution or high-frame-rate displays by a scalable, simple, and feasible approach.

Band-Like Charge Transport in Cs2AgBiBr6 and Mixed Antimony-Bismuth Cs2AgBi1-x Sb x Br6 Halide Double Perovskites.

Recently, halide double perovskites (HDPs), such as Cs2AgBiBr6, have been reported as promising nontoxic alternatives to lead halide perovskites. However, it remains unclear whether the charge-transport properties of these materials are as favorable as for lead-based perovskites. In this work, we study the mobilities of charges in Cs2AgBiBr6 and in mixed antimony–bismuth Cs2AgBi1–xSbxBr6, in which the band gap is tunable from 2.0 to 1.6 eV. Using temperature-dependent time-resolved microwave conductivity techniques, we find that the mobility is proportional to T–p (with p ≈ 1.5). Importantly, this indicates that phonon scattering is the dominant scattering mechanism determining the charge carrier mobility in these HDPs similar to the state-of-the-art lead-based perovskites. Finally, we show that wet chemical processing of Cs2AgBi1–xSbxBr6 powders is a successful route to prepare thin films of these materials, which paves the way toward photovoltaic devices based on nontoxic HDPs with tunable band gaps.

Developing Active TiO2 Nanorods by Examining the Influence of Morphological Changes from Nanorods to Nanoparticles on Photocatalytic Activity

We synthesized rutile TiO2 nanorods with high crystallinity, i.e., degree of crystallinity greater than ca. 90%, by using a hydrothermal method and studied the effect of the morphological change from rod shape to subsphere by the subsequent thermal treatment. The photocatalytic activity was estimated for O2 evolution from water oxidation because this reaction was hardly affected by the specific surface area of photocatalysts. Thermal desorption and gas chromatography–mass spectrometry measurements suggested the decomposition of residual glycolic acid in TiO2 nanorods at 272 and 590 °C. Observation with scanning electron microscopy and physical properties such as specific surface area and crystallite size indicated deformation of the rod shape to subsphere by the calcination above 600 °C. Time-resolved microwave conductivity measurements have revealed that the TiO2 nanorods possess the longest lifetime of photogenerated electrons among all samples. However, the O2 evolution rate of the TiO2 nanorods was mu...

Size and shape effect on the photocatalytic efficiency of TiO2 brookite

Thanks to aqueous sol–gel chemistry, it is now possible to prepare several phase pure TiO2 brookite colloidal systems that significantly differ on nanoparticles size and shape. This TiO2 polymorph is more difficult to be obtained as phase pure material than anatase or rutile. Here we have prepared a set of four different sol–gel brookite syntheses with particles size ranging from 10 to 500 nm and significantly different morphologies as demonstrated by X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. We have studied their photocatalytic activities in aqueous solution on phenol and formic acid. The brookite sample with higher specific surface displays better activity for both pollutants abatement than anatase and rutile reference samples and very close to the TiO2 P25 commercial reference. Additional experimental characterization of photogenerated charge carriers and their lifetime is performed using time-resolved microwave conductivity. We could then explain why another efficient brookite material is able to compensate a significantly lower specific surface with a higher photon conversion rate. This study involving a broad set of pure phase brookite samples brings back that phase into the TiO2 polymorphs race for light-enhanced applications. It confirms that size/shape–activity correlation already observed for the anatase polymorph is also valid for the brookite phase.

Charge Mobility and Dynamics in Spin-Crossover Nanoparticles Studied by Time-Resolved Microwave Conductivity.

We use the electrodeless time-resolved microwave conductivity (TRMC) technique to characterize spin-crossover (SCO) nanoparticles. We show that TRMC is a simple and accurate means for simultaneously assessing the magnetic state of SCO compounds and charge transport information on the nanometer length scale. In the low-spin state from liquid nitrogen temperature up to 360 K the TRMC measurements present two well-defined regimes in the mobility and in the half-life times, in which the former transition temperature TR occurs near 225 K. Below TR, we propose that an activationless regime taking place associated with short lifetimes of the charge carriers points at the presence of shallow-trap states. Above TR, these states are thermally released, yielding a thermally activated hopping regime where longer hops increase the mobility and, concomitantly, the barrier energy. The activation energy could originate not only from intricate contributions such as polaronic self-localizations but also from dynamic disorder due to phonons and/or thermal fluctuations of SCO moieties.

Can we use time-resolved measurements to get steady-state transport data for halide perovskites?

Time-resolved, pulsed excitation methods are widely used to deduce optoelectronic properties of semiconductors, including now also Halide Perovskites (HaPs), especially transport properties. However, as yet, no evaluation of their amenability and justification for the use of the results for the above-noted purposes has been reported. To check if we can learn from pulsed measurement results about steady-state phototransport properties, we show here that, although pulsed measurements can be useful to extract information on the recombination kinetics of HaPs, great care should be taken. One issue is that no changes in the material are induced during or as a result of the excitation, and another one concerns in how far pulsed excitation-derived data can be used to find relevant steady-state parameters. To answer the latter question, we revisited pulsed excitation and propose a novel way to compare between pulsed and steady state measurements at different excitation intensities. We performed steady-state photoconductivity and ambipolar diffusion length measurements, as well as pulsed time-resolved microwave conductivity and time-resolved photoluminescence measurements as a function of excitation intensity on the same samples of different MAPbI3 thin films, and found good quasi-quantitative agreement between the results, explaining them with a generalized single level recombination model that describes the basic physics of phototransport of HaP absorbers. Moreover, we find the first experimental manifestation of the boundaries between several effective recombination regimes that exist in HaPs, by analyzing their phototransport behavior as a function of excitation intensity.

Small-Band-Gap Halide Double Perovskites.

Despite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs2 AgTlX6 (X=Cl (1) and Br (2)) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in AI BII X3 perovskites, the band-gap transition in AI2 BB'X6 double perovskites can show substantial metal-to-metal charge-transfer character. This band-edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B- and B'-site metal frontier orbitals. Calculations reveal a shallow, symmetry-forbidden region at the band edges for 1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self-doping reaction in 2 through Br2 loss at ambient conditions.

Small‐Band‐Gap Halide Double Perovskites

AbstractDespite their compositional versatility, most halide double perovskites feature large band gaps. Herein, we describe a strategy for achieving small band gaps in this family of materials. The new double perovskites Cs2AgTlX6 (X=Cl (1) and Br (2)) have direct band gaps of 2.0 and 0.95 eV, respectively, which are approximately 1 eV lower than those of analogous perovskites. To our knowledge, compound 2 displays the lowest band gap for any known halide perovskite. Unlike in AIBIIX3 perovskites, the band‐gap transition in AI2BB′X6 double perovskites can show substantial metal‐to‐metal charge‐transfer character. This band‐edge orbital composition is used to achieve small band gaps through the selection of energetically aligned B‐ and B′‐site metal frontier orbitals. Calculations reveal a shallow, symmetry‐forbidden region at the band edges for 1, which results in long (μs) microwave conductivity lifetimes. We further describe a facile self‐doping reaction in 2 through Br2 loss at ambient conditions.

Flux Synthesis of Layered Oxyhalide Bi4NbO8Cl Photocatalyst for Efficient Z-Scheme Water Splitting Under Visible Light.

An oxyhalide photocatalyst Bi4NbO8Cl has recently been proven to stably oxidize water under visible light, enabling the Z-scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of Bi4NbO8Cl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase Bi4NbO8Cl at as low as 650 °C. Thus, synthesized Bi4NbO8Cl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higherO2 evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO2 or Pt cocatalyst on Bi4NbO8Cl further enhanced the activity for O2 evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H2-evolving photocatalyst Ru/SrTiO3:Rh along with an Fe3+/Fe2+ redox mediator, the RuO2/Bi4NbO8Cl is an excellent O2-evolving photocatalyst, exhibiting highly effective water splitting into H2 and O2 via the Z-scheme.

Microwave conductivity of ferroelectric domains and domain walls in a hexagonal rare-earth ferrite

We report the nanoscale electrical imaging results in hexagonal $Lu_{0.6}Sc_{0.4}FeO_3$ single crystals using conductive atomic force microscopy (C-AFM) and scanning microwave impedance microscopy (MIM). While the dc and ac response of the ferroelectric domains can be explained by the surface band bending, the drastic enhancement of domain wall (DW) ac conductivity is clearly dominated by the dielectric loss due to DW vibration rather than mobile-carrier conduction. Our work provides a unified physical picture to describe the local conductivity of ferroelectric domains and domain walls, which will be important for future incorporation of electrical conduction, structural dynamics, and multiferroicity into high-frequency nano-devices.

Preparation and characterization of doubly substituted microwave absorbing material by sol-gel technique for super high frequency applications

Rare earth Dy3+ and divalent Mn2+ elements substituting W-type hexagonal ferrites Ba1−xDyxZn2Fe16−yMnyO27 (x = 0, 0.02, 0.06, 0.1 and y = 0, 0.1, 0.3, 0.5) were prepared by sol-gel method. The thermo- gravimetric analysis (TGA) and differential scanning calorimetry (DSC) was carried out to find the temperature at which single phase can be obtained. XRD patterns indicate the presence of the single phase for all the synthesized samples with the absence of any extra peak due to unreacted material and secondary phases. The occurrence of absorption bands at low wave numbers (563 and 446 cm−1), can be assigned to the stretching vibration of metal and oxygen ions in FTIR spectra, which also confirms the single hexagonal phase for prepared material. The grains are of platelet like structure, which enhances the microwave absorption properties of hexagonal ferrites. The synthesized material exhibits the minimum reflection loss of −20.9 dB at 11.8 GHz frequency, which reflects the applications of this material in super high frequency devices. The microwave conductivity of the material increases with frequency.

Microwave Conductivity Investigations of AZO Films Deposited by Atomic Layer Deposition Method at a Low Temperature

Microwave Conductivity Investigations of AZO Films Deposited by Atomic Layer Deposition Method at a Low Temperature

Ferric metal-organic framework for microwave absorption

In this study, we have shown for the first time that ferric metal-organic framework (Fe-MOF) has the ability to attenuate gigahertz-range electromagnetic radiation through means of a novel interference mechanism. A large reflection loss value of −54.2 dB can be obtained, equivalent to over 99.999% absorption efficiency. The microwave absorption performance is largely dependent on the dielectric permittivity and magnetic permeability properties. The electrical loss ratio tgδε increases with the frequency from 0.13 at 1.0 GHz to 0.69 at 18.0 GHz, while the magnetic loss ratio tgδμ is only between 0.05 and 0.14. The conductivity σ increases with the frequency from 0.08 S/m at 1.0 GHz to 5.24 S/m at 18.0 GHz, while the skin-depth δ decreases with the frequency. The thickness (d) of the absorber is inversely proportional to the reflection loss peak value (RLpeak), the reflection loss peak frequency (fpeak), and the critical reflection loss peak width (Δf10). The non-zero magnetic susceptibility (χ) accelerates the shift of fpeak to lower values as d grows bigger; however, it improves the RLpeak value and larger microwave absorption efficiency and reduces the quick narrowing of the critical frequency range. The microwave performance of Fe-MOF is likely related to its microwave conductivity, which may originate from the polar rotations in the interface or defects. A new mechanism is proposed for the large reflection loss based on the interference of the reflected microwave from the front and back surface of the microwave absorber, especially when the thickness matches with the odd integer values of the wavelength.

(Invited) Photophysics and Electronic Structure of Metal-Organic Frameworks

The development of porous crystalline materials such as metal–organic frameworks (MOFs) could add a new dimension to a wide number of applications including supercapacitors, electrodes, or thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In the presentation, I will focus on the fundamental insights responsible for the electronic properties of bimetallic systems in which the second metal incorporation occurs through (i) metal replacement in the framework nodes, (ii) metal node extension, and (iii) metal coordination to the organic ligand. Microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling were imployed to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures.

Carbon nanotube sponges as tunable materials for electromagnetic applications

The microwave conductivity and permittivity of both single-walled and multi-walled carbon nanotube (SWCNT and MWCNT) sponges were measured while compressing the samples. Compression leads to a huge variation of the absorptance, reflectance, and transmittance of the samples. The dependence of the microwave conductivity on the sponge density follows a power-law relation with exponents 1.7 ± 0.1 and 2.0 ± 0.2 for MWCNT and SWCNT sponges, respectively. These exponents can be decreased slightly by the addition of a non-conducting component which partly electrically separates adjacent tubes within the samples. The conductivity of MWCNT sponge was measured in the terahertz range while heating in air from 300 to 513 K and it increased due to an increase of a number of conducting channels in MWCNTs.

Plasma Carriers on Graphene: Relaxation of Plasma Carriers in Graphene: An Approach by Frequency‐Dependent Optical Conductivity Measurement (Advanced Optical Materials 14/2018)

Charge carriers injected onto a graphene layer are perturbed by the probing microwave as rippling waves. By tracing these waves the mass and mobility of electrons can be determined experimentally in the technique referred as Field-Induced Time-Resolved Microwave Conductivity (FI-TEMC) measurement, as shown by Shu Seki and co-workers in article number 1701402.

Naphthalenediimide (NDI)‐Conjugated Foldable Polyurethanes: Impact of Chromophoric Location on Hierarchical Supramolecular Assembly and Conductivity

AbstractThe manuscript reports supramolecular assembly and photophysical properties of two polyurethanes (P1, P2) with appended naphthalene‐diimide (NDI) chromophore which serves as a n‐type semiconductor. P1 and P2 are constitutional isomers and they differ in the location of the NDI moiety. In P1, NDI is a part of the polymer backbone (placed between the two urethane groups in each repeating unit) while in P2, it remains as a pendant group. In a non‐polar solvent like tetra‐chloroethylene (TCE), both adopt a pleated structure driven by intra‐chain H‐bonding among the urethane groups. Subsequently the secondary structure of P2 assembles to a meta‐stable spherical structure (similar to polymersome) which after 70–75 h transforms to nanotubes with the folded polymer chains constructing the wall of the tube. In sharp contrast, the intra‐chain folded conformation of P1, owing to its enhanced rigidity contributed by the synergistic π‐stacking and H‐bonding, does not get trapped in the spherical intermediate, but spontaneously grows to fibrils leading to macroscopic gelation within 30 min. Flash‐photolysis time‐resolved microwave conductivity (FP‐TRMC) measurements together with transient absorption spectroscopy studies revealed remarkably high charge carrier mobility (3×10−3 cm2 V−1 s−1) which can be attributed to the long range stacking order of the NDI chromophores in the fibrillar assembly of P1 while for the nanotubular structure of P2, the value was almost an order of magnitude less reflecting importance of the mesoscopic structure on generation of percolated pathway for efficient charge transport.

Landscape of Charge Carrier Transport in Doped Poly(3-hexylthiophene): Noncontact Approach Using Ternary Combined Dielectric, Paramagnetic, and Optical Spectroscopies.

We report on a comprehensive measurement system for mobility and energy states of charge carriers in matter under dynamic chemical doping. The temporal evolution of the iodine doping process of poly(3-hexylthiophene) (P3HT) was monitored directly through electron paramagnetic resonance (EPR) and optical absorption spectroscopy, as well as differential electrical conductivity by the microwave conductivity measurement. The increase in conductivity was observed after the EPR intensity reached a maximum and declined thereafter, and the conductivity finally reached ∼80 S cm-1. The carrier species changed from a paramagnetic polaron with an estimated mobility of μP+ ≈ 2 × 10-3 cm2 V-1 s-1 to an antiferromagnetic polaron pair with μPP+ ≈ 0.6 cm2 V-1 s-1. The technique presented here can be a ubiquitous method for rapid and direct observation of charge carrier mobility and energy states in p-type semiconducting materials as a completely noncontact, experimental, and quantitative technique.

Noble metal-modified faceted anatase titania photocatalysts: Octahedron versus decahedron

Octahedral anatase particles (OAP, with eight equivalent {101} facets) and decahedral anatase particles (DAP, with two additional {001} facets) were modified with nanoparticles of noble metals (Au, Ag, Cu). The titania morphology, expressed by the presence of different arrangements of exposed crystal facets, played a key role in the photocatalytic properties of metal-modified faceted titania. In the UV/vis systems, two-faceted configuration of DAP was more favorable for the reaction efficiency than single-faceted OAP because of an efficient charge separation described by the transfer of electrons to {101} facets and holes to {001} facets. Time-resolved microwave conductivity (TRMC) and reversed double-beam photoacoustic spectroscopy (RDB-PAS) confirmed that distribution of electron traps (ET) and mobility of electrons were key-factors of photocatalytic activity. In contrast, metal-modified OAP samples had higher photocatalytic activity than metal-modified DAP and metal-modified commercial titania samples under visible light irradiation. This indicates that the presence of single type of facets ({101}) is favorable for efficient electron transfer via shallow ET, whereas intrinsic properties of DAP result in fast charge carriers’ recombination when gold is deposited on {101} facets (migration of “hot” electrons: Au→{101}→Au).