Optical Transport Properties Research Articles

Electronic, optical, and charge transport properties of A-π-A electron acceptors for organic solar cells: Impact of anti-aromatic π structures

Organic solar cells based on acceptor-π-acceptor (A-π-A) electron acceptors have attracted intensive attention due to their increasing and record power conversion efficiencies. To date, almost all of the reported A-π-A electron acceptors are based on aromatic π structures. Here, we have investigated the impact of anti-aromatization of the π-bridges on the optoelectronic properties of A-π-A electron acceptors by (time-dependent) density functional theory. Our calculations show that besides the frontier molecular orbitals corresponding to the aromatic π-bridge based acceptors (“aromatic” acceptors), additional and unique occupied and unoccupied frontier orbitals are found for the acceptors based on the anti-aromatic π-bridges (“anti-aromatic” acceptors). Moreover, by tuning isomeric structures of the π-bridges (e.g., fusion orientations or linking positions of thiophene moieties), the optical excitation energies for the transition between the additional occupied and unoccupied levels turn to be close to or substantially lower with respect to those for the transition between the “aromatic” frontier orbitals. The optical absorption of the “anti-aromatic” acceptors is thus either stronger or broader than the “aromatic” acceptors. Finally, the reorganization energies for electron transport are tunable and dependent on the π-bridge structures. These results indicate a great potential of “anti-aromatic” electron acceptors in organic photovoltaics.

Tunable Optical and Electrical Transport Properties of Size- and Temperature-Controlled Polymorph MoS2 Nanocrystals

The phase transition of chemically synthesized MoS2 nanocrystals (NCs) from the metallic 1T to the semiconducting 2H phase has been investigated in detail. The metallic 1T phase NCs were prepared by the Li+ intercalation–deintercalation exfoliation techniques followed by prolonged sonication. The effect of ex situ thermal annealing on MoS2 polymorphs and their transformation from the 1T to 2H phase has been extensively monitored by the X-ray photoelectron, Raman, and optical absorption spectroscopy techniques. Electrical conductivity measurements have also been carried out to probe the phase transition of the synthesized NCs. The temperature-dependent (10–350 K) electrical charge transport properties of variable-sized NCs have been investigated to probe the scaling of conductivity and activation energy with size, which are yet to be reported experimentally. The charge transport mechanisms through the NC assembly for different temperature regions have been modeled and it is observed that the electron trans...

Influence of chromium on structural, non-linear optical constants and transport properties of CdO thin films

Transparent and conducting CdO and Cr doped CdO (CdO:Cr) crystalline films were prepared on glass substrate at 350 °C by cost effective spray pyrolysis technique. Structural analysis indicates CdO:Cr films are polycrystalline cubic structure comprises with spherical or semi-spherical nano-scale particles. The direct band gap energy of CdO was found to change with Cr doping. The photoluminescence (PL) spectra of CdO show extended band edge emissions accompanied by red emission originated from different impurity states. The optical dispersion parameters calculated using Wemple–DiDomenico single-oscillator model were found to vary with Cr concentration in CdO. The dispersion energy of oscillator found to decrease with increasing Cr concentration. Simultaneously, the variation of third-order nonlinear susceptibility with photon energy was found to decrease with increasing Cr concentrations. Hall study confirms that CdO and CdO:Cr films are n-type semiconductor having carrier concentration of the order of ∼1019 cm−3. The obtained physical properties of CdO film were improved by Cr doping which make them suitable for optoelectronic applications.

On the diversity in the thermal transport properties of graphene: A first-principles-benchmark study testing different exchange-correlation functionals

Due to the extraordinary mechanical, electronic, optical, and thermal transport properties, graphene had been one of the most fascinating and extensively studied two-dimensional materials in recent years. Benefiting from the extremely high thermal conductivity, the thermal transport in graphene has received exceptional attention in both experimental and theoretical studies. In this paper, based on first-principles calculations, we performed systematic study on the thermal transport properties of graphene using 10 different exchange-correlation (XC) functionals combined with ultrasoft or projector augmented wave (PAW) pseudopotentials. The thermal transport properties of graphene show distinct diversity for different XC functionals despite their overall good agreement with experimental measurements. It is found that the thermal conductivity of graphene could range from 1936 to 4376 W m−1 K−1 with different XC functionals employed. The reason for the diversity in the thermal transport properties of graphene is analyzed based on the insight into the mode level phonon behaviors. Our study executed a comprehensive investigation of the thermal transport properties of graphene with different XC functionals employed, which would shed light on future researches related to graphene and other novel materials.

Low field magneto-tunable photocurrent in CoFe2O4 nanostructure films for enhanced photoelectrochemical properties

Efficient solar to hydrogen conversion using photoelectrochemical (PEC) process requires semiconducting photoelectrodes with advanced functionalities, while exhibiting high optical absorption and charge transport properties. Herein, we demonstrate magneto-tunable photocurrent in CoFe2O4 nanostructure film under low applied magnetic fields for efficient PEC properties. Photocurrent is enhanced from ~1.55 mA/cm2 to ~3.47 mA/cm2 upon the application of external magnetic field of 600 Oe leading to ~123% enhancement. This enhancement in the photocurrent is attributed to the reduction of optical bandgap and increase in the depletion width at CoFe2O4/electrolyte interface resulting in an enhanced generation and separation of the photoexcited charge carriers. The reduction of optical bandgap in the presence of magnetic field is correlated to the shifting of Co2+ ions from octahedral to tetrahedral sites which is supported by the Raman spectroscopy results. Electrochemical impedance spectroscopy results confirm a decrease in the charge transfer resistance at the CoFe2O4/electrolyte interface in the presence of magnetic field. This work evidences a coupling of photoexcitation properties with magnetic properties of a ferromagnetic-semiconductor and the effect can be termed as magnetophototronic effect.

Semitransparent Blue, Green, and Red Organic Solar Cells Using Color Filtering Electrodes

AbstractColorful, semitransparent organic photovoltaic cells (OPVs) are increasing in demand due to their applicability in aesthetically fashioned power‐generating windows. The traditional method of generating different colors in OPVs has been through employing different active materials exhibiting distinct absorption spectra. This can complicate fabrication processes for production and cause deviations in device performance among differently colored OPVs. Herein, semitransparent and colorful OPVs with a single broadband‐absorbing active material of fixed thickness are demonstrated using Ag–TiOx–Ag color filters (CFs). The CF enables the OPV to transmit spectrally pure colors with peak transmission efficiencies surpassing 25%, and allows the colors to be freely tuned without disturbing the charge transport properties such that the device performance stays consistent. The CF not only functions as an effective electrode but as a mirror for light outside of the spectral transparency window. It is shown that when compared to a transparent OPV consisting of the same active layer material and thickness, CF‐integrated OPVs generate 32.38(±0.03)–34.10(±0.02)% more short‐circuit current. Such results point to a simple and convenient solution to achieving spectrally pure colors, moderate transmission efficiencies, decoupled optical and charge transport properties, and enhanced charge generation relative to a transparent OPV in semitransparent, colorful power‐generating windows.

Molecular Clusters: Nanoscale Building Blocks for Solid-State Materials.

The programmed assembly of nanoscale building blocks into multicomponent hierarchical structures is a powerful strategy for the bottom-up construction of functional materials. To develop this concept, our team has explored the use of molecular clusters as superatomic building blocks to fabricate new classes of materials. The library of molecular clusters is rich with exciting properties, including diverse functionalization, redox activity, and magnetic ordering, so the resulting cluster-assembled solids, which we term superatomic crystals (SACs), hold the promise of high tunability, atomic precision, and robust architectures among a diverse range of other material properties. Molecular clusters have only seldom been used as precursors for functional materials. Our team has been at the forefront of new developments in this exciting research area, and this Account focuses on our progress toward designing materials from cluster-based precursors. In particular, this Account discusses (1) the design and synthesis of molecular cluster superatomic building blocks, (2) their self-assembly into SACs, and (3) their resulting collective properties. The set of molecular clusters discussed herein is diverse, with different cluster cores and ligand arrangements to create an impressive array of solids. The cluster cores include octahedral M6E8 and cubane M4E4 (M = metal; E = chalcogen), which are typically passivated by a shell of supporting ligands, a feature upon which we have expanded upon by designing and synthesizing more exotic ligands that can be used to direct solid-state assembly. Building from this library, we have designed whole families of binary SACs where the building blocks are held together through electrostatic, covalent, or van der Waals interactions. Using single-crystal X-ray diffraction (SCXRD) to determine the atomic structure, a remarkable range of compositional variability is accessible. We can also use this technique, in tandem with vibrational spectroscopy, to ascertain features about the constituent superatomic building blocks, such as the charge of the cluster cores, by analysis of bond distances from the SCXRD data. The combination of atomic precision and intercluster interactions in these SACs produces novel collective properties, including tunable electrical transport, crystalline thermal conductivity, and ferromagnetism. In addition, we have developed a synthetic strategy to insert redox-active guests into the superstructure of SACs via single-crystal-to-single-crystal intercalation. This intercalation process allows us to tune the optical and electrical transport properties of the superatomic crystal host. These properties are explored using a host of techniques, including Raman spectroscopy, SQUID magnetometry, electrical transport measurements, electronic absorption spectroscopy, differential scanning calorimetry, and frequency-domain thermoreflectance. Superatomic crystals have proven to be both robust and tunable, representing a new method of materials design and architecture. This Account demonstrates how precisely controlling the structure and properties of nanoscale building blocks is key in developing the next generation of functional materials; several examples are discussed and detailed herein.

Significant suppression of Rayleigh scattering loss in silica glass formed by the compression of its melted phase.

We propose the formation of silica glass with improved optical transport properties by compressing its melted phase with a hot isostatic pressure machine at high pressure and temperature. The lowest Rayleigh scattering loss was obtained for the glass held at 200 MPa and 2073 K for 4 h. The observed loss corresponds to 0.07 dB/Km at 1.55 μm, which is about half of the loss in conventional silica glass fiber. The decrease in the loss was well explained in terms of the decrease in the size of the sub-nanometer-sized structural voids observed by positron annihilation lifetime spectroscopy in silica glass. The achievement of high transparency and strong confinement of light represents a promising result for the development of future fiber-core media.

Optimization of sputtering NiOx films for perovskite solar cell applications

In this work, we systematically investigated the properties of three kinds of NiOx films prepared by magnetron sputtering either NiO or Ni target under direct current (DC) or radio frequency (RF) power without any preheating. Those NiOx films were found to be different in composition ratios, lattice strain and energy levels, thus showing different optical and carrier transport properties. By RF power source, sputtering Ni target in pure O2 atmosphere led to NiOx film with relatively good crystallization and surface properties compared with other two types of NiOx films. These NiOx films were further introduced into CH3NH3PbI3 based planar p-i-n heterojunction perovskite solar cells (PSCs) as the hole transport layers (HTL). It is revealed that PSCs based on NiOx HTL prepared by sputtering Ni target in pure O2 atmosphere achieved a highest power conversion efficiency of 16.3% with negligible hysteresis. Compared with devices with PEDOT:PSS HTL, PSCs based on magnetron sputtered NiOx HTL presented substantially improved stability in air. The results provide theoretical and experimental guidance on selecting NiOx HTLs fabricated by magnetron sputtering techniques.

A comparative device performance assesment of CVD grown MoS2 and WS2 monolayers

In this study, optical and electronic transport properties of chemical vapor deposition (CVD) grown 2D WS2 and MoS2 based transistors and photodetectors are investigated and compared in ambient air by using 2D flakes grown with the same CVD system. To assess the performance variations between these two materials and understand the underlying mechanisms, it is essential to utilize identical growth methods (i.e. using the same CVD system), identical substrate and dielectric materials with the identical device fabrication methods and geometries. Transistor devices fabricated out of these flakes are examined in terms of their field effective mobility, current ON/OFF ratio, and photoresponsivity. Our results show that the MoS2 based devices have higher mobility and photoresponsivity than the WS2 based devices. However, the hysteresis curve of WS2 based transistors is smaller when compared to that of MoS2 based transistors. The mobilities of MoS2 and WS2 are estimated from measurements as 1.45 and 0.98 cm2 V−1 s−1, respectively. The electronic transport performance of MoS2 based devices (FETs and photodetectors) are found to be unexpectedly better than the WS2 based devices in terms of effective carrier mobility and photoresponsivity at ambient atmosphere and temperature. Our results suggest that WS2 is more sensitive to ambient conditions in comparison to MoS2, in spite of its theoretically estimated superior performance.

Microstructural, optical and electrical transport properties of Cd-doped SnO2 nanoparticles

We have successfully investigated the structural, optical and dielectric properties of Cd assimilated SnO2 nanoparticles synthesized via very convenient precipitation route. The structural properties were studied by x-ray diffraction method (XRD) and Fourier Transform Infrared (FTIR) Spectroscopy. As-synthesized samples in the form of powder were examined for its morphology and average particle size by Transmission electron microscopy (TEM). The optical properties were studied by diffuse reflectance spectroscopy. Dielectric properties such that complex dielectric constant and ac conductivity were investigated by LCR meter. Average crystallite size calculated by XRD and average particle size obtained from TEM were found to be consistent and below 50 nm for all samples. The optical band gap of as-synthesized powder samples from absorption study was found in the range of 3.76 to 3.97 eV. The grain boundary parameters such that Rgb, Cgb and were evaluated using impedance spectroscopy.

Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates

Understanding the excitonic nature of excited states in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) is of key importance to make use of their optical and charge transport properties in optoelectronic applications. We contribute to this by the direct experimental determination of the exciton binding energy (Eb,exc) of monolayer MoS2 and WSe2 on two fundamentally different substrates, i.e. the insulator sapphire and the metal gold. By combining angle-resolved direct and inverse photoelectron spectroscopy we measure the electronic band gap (Eg), and by reflectance measurements the optical excitonic band gap (Eexc). The difference of these two energies is Eb,exc. The values of Eg and Eb,exc are 2.11 eV and 240 meV for MoS2 on sapphire, and 1.89 eV and 240 meV for WSe2 on sapphire. On Au Eb,exc is decreased to 90 meV and 140 meV for MoS2 and WSe2, respectively. The significant Eb,exc reduction is primarily due to a reduction of Eg resulting from enhanced screening by the metal, while Eexc is barely decreased for the metal support. Energy level diagrams determined at the K-point of the 2D TMDCs Brillouin zone show that MoS2 has more p-type character on Au as compared to sapphire, while WSe2 appears close to intrinsic on both. These results demonstrate that the impact of the dielectric environment of 2D TMDCs is more pronounced for individual charge carriers than for a correlated electron–hole pair, i.e. the exciton. A proper dielectric surrounding design for such 2D semiconductors can therefore be used to facilitate superior optoelectronic device function.

Optical Parameters and Electrical Transport Properties of Some Barium-Sodium-Borate Glasses Doped Bismuth Oxide

Glasses having the composition, [(70-x) mol% B₂O₃- x mol% Bi₂O₃- 10 mol% BaO- 20 mol% Na₂O, where 0≤ x ≤20], have been prepared by the melt quenching method. Density, molar volume, infrared analysis, optical parameters and Electrical properties have been thoroughly investigated. It was found that, both density and molar volume increased with the gradual replacement of B₂O₃ by Bi₂O₃ and the comparison between their experimental and empirical values confirm the amorphous nature and the random structure of all samples. The obtained infrared results indicated that different structural borate groups appeared such as BO4 units (in di-, tri- and penta-borate groups) and BO₃ units (in meta- and ortho-borate chains). Also, both BiO₃ and BiO₆ are present in all Bi doped glasses, and the BiO₃ / BiO₆ ratio appeared to be approximately stable as Bi₂O₃ was increased to 15 mol%, then it showed a jump increase when BiO₃ reached 20 mol%. The optical band gap energy and cut-off wavelength increased with the increase of Bi₂O₃ while Urbach energy decreased. On the other hand, the electrical conductivity decreased as Bi₂O₃ was gradually replaced by B₂O₃, while the activation energy increased and all samples exhibit semi-conductors behavior and the values of the exponent factor decreased gradually with temperature, which is compatible with the correlated barrier hopping conduction mechanism.

Structural, optoelectronic and charge transport properties of the complexes of indigo encapsulated in carbon nanotubes.

Using the dispersion-corrected density functional B97D and 6-31g(d,p) basis set, the structural, stability, electronic, optical and charge transport properties of the complexes formed by encapsulating indigo inside carbon nanotubes (CNTs) of varying diameters are investigated. Based on the stabilization energy of the complexes indigo@(n,n)CNT (where n = 6, 7 and 8), indigo@(7,7)CNT is shown to be the most stable owing to the ideal diameter of (7,7)CNT for encapsulating indigo. The nature of the interaction between the guest and the host is investigated by means of energy decomposition analysis employing the symmetry adapted perturbation theory. Electronic properties such as the ionization energy, the electron affinity and the energy gap between the highest occupied and lowest unoccupied molecular orbitals (ΔEH-L) of the complexes are determined. The low values of ΔEH-L (<1 eV) for the complexes suggest that they can act as narrow energy gap semiconductors. All the complexes exhibit high hole and electron mobilities which vary inversely with respect to the diameter of the CNT. Using the time-dependent density functional theoretical method, the absorption properties are predicted for the most stable complex indigo@(7,7)CNT. The presence of charge transfer peaks in the visible and near-infrared regions of the electromagnetic spectrum suggests that the complexes are suitable for optoelectronic devices such as solar cells.

Probing into the optical and electrical properties of hybrid Zn1−xCoxSe thin films

Probing into the elemental composition, structural, morphological, optical and electrical transport properties of chemically deposited Zn1−xCoxSe (0 ≤ x ≤ 0.275) thin films with a special emphasis given to the Co2+-concentration is presented in this paper. Elemental and structural analysis confirmed the successful realization of Co(ZnSe) thin films. Addition of Co2+ into ZnSe host lattice caused morphological changes from globule like morphology to the formation of leaf like appearance composing the disc-decked micro-flakes elongated in size. The optical studies done in the range of wavelengths between 350 to 1200 nm showed a slight red shift in the optical spectrum with increased Co2+ concentration in the ZnSe matrix. Effect of increased impurity addition is also reflected in the band gap measurements that a decrease in the bandgap, typically from 2.71 to 1.96 eV, is observed for an increase in Co2+ concentration from x = 0–0.275. The other optical parameters namely, refractive index, extinction coefficient, power factor and dielectric constants were determined from these studies and other variations are adequately explained as a special reference to the Co-concentration. The composition dependence of the electrical transport characteristics were studied using the two-probe and Hall measurement techniques. The effect of Co-concentration on the transport characteristics has been studied and mechanism of an electrical conduction is discussed. A continuous increase in an electrical conductivity with n-type conduction has been observed for these samples.

Electrical and optical transport properties of single layer WSe2

The electronic properties of single layer WSe2 are distinct from the famous graphene due to strong spin orbit coupling, a huge band gap and an anisotropic lifting of the degeneracy of the valley degree of freedom under Zeeman field. In this work, band structure of the monolayer WSe2 is evaluated in the presence of spin and valley Zeeman fields to study the electrical and optical transport properties. Using Kubo formalism, an explicit expression for the electrical Hall conductivity is examined at finite temperatures. The electrical longitudinal conductivity is also evaluated. Further, the longitudinal and Hall optical conductivities are analyzed. It is observed that the contributions of the spin-up and spin-down states to the power absorption spectrum depend on the valley index. The numerical results exhibit absorption peaks as a function of photon energy, ℏω, in the range ∼1.5−2 eV. Also, the optical response lies in the visible frequency range in contrast to the conventional two-dimensional electron gas or graphene where the response is limited to terahertz regime. This ability to isolate carriers in spin-valley coupled structures may make WSe2 a promising candidate for future spintronics, valleytronics and optical devices.

Bulk-Heterojunction Thin-Film Solar Cells Utilizing Miscible Binary Donor Materials of Liquid Crystalline Phthalocyanine and its Analogue

Small-molecule bulk-heterojunction solar cells utilizing a binary-blended donor material composed of a liquid crystalline phthalocyanine and its analogue have been studied. The short-circuit current density (Jsc) and power conversion efficiency (PCE) of the solar cells utilizing the blended donor material with an optimized blend ratio reached 11.6 mA/cm2 and 4.8%, respectively, which were superior to those of organic solar cells utilizing each donor material. The improvement of Jsc and PCE has been discussed from the viewpoints of the crystal structure, optical and carrier transport properties of the blended donor material.

Bad Metals from Fluctuating Density Waves

Bad metals have a large resistivity without being strongly disordered. In many bad metals the Drude peak moves away from zero frequency as the resistivity becomes large at increasing temperatures. We catalogue the position and width of the ‘displaced Drude peak’ in the observed optical conductivity of several families of bad metals, showing that \omega_\text{peak} \sim \Delta \omega \sim k_BT/\hbarωpeak∼Δω∼kBT/ℏ. This is the same quantum critical timescale that underpins the TT-linear dc resistivity of many of these materials. We provide a unified theoretical description of the optical and dc transport properties of bad metals in terms of the hydrodynamics of short range quantum critical fluctuations of incommensurate density wave order. Within hydrodynamics, pinned translational order is essential to obtain the nonzero frequency peak.

Ultralong Radiative States in Hybrid Perovskite Crystals: Compositions for Submillimeter Diffusion Lengths

Organic-inorganic hybrid perovskite materials have recently evolved into the leading candidate solution-processed semiconductor for solar cells due to their combination of desirable optical and charge transport properties. Chief among these properties is the long carrier diffusion length, which is essential to optimizing the device architecture and performance. Herein, we used time-resolved photoluminescence (at low excitation fluence, 10.59 μJ·cm-2 upon two-photon excitation), which is the most accurate and direct approach to measure the radiative charge carrier lifetime and diffusion lengths. Lifetimes of about 72 and 4.3 μs for FAPbBr3 and FAPbI3 perovskite single crystals have been recorded, presenting the longest radiative carrier lifetimes reported to date for perovskite materials. Subsequently, carrier diffusion lengths of 107.2 and 19.7 μm are obtained. In addition, we demonstrate the key role of the organic cation units in modulating the carrier lifetime and its diffusion lengths, in which the defect formation energies for FA cations are much higher than those with the MA ones.

Observation of Reverse Saturable Absorption of an X-ray Laser.

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption. It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast nonradiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10^{16} -10^{17} W/cm^{2}. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in the extreme state of matters, as well as affording the potential to regulate ultrafast x-ray free-electron laser pulses.