Interaction Of Charge Carriers Research Articles

Unveiling the Nature of Charge Carrier Interactions by Electroabsorption Spectroscopy: An Illustration with Lead-Halide Perovskites.

Unravelling the nature of the interactions between photogenerated charge carriers in solar energy conversion devices is key to enhance performance. In this perspective, we discuss electroabsorption spectroscopy (EAS), as the spectral bandshape of the electroabsorption (EA) signal directly depends on the strength of the charge carrier interactions. For instance, the electroabsorption response in molecular or confined excitonic systems can be modelled perturbatively yielding the Stark effect. In contrast, most solids exhibit weaker interactions, and a perturbative approach cannot be taken in general. For solids with negligible charge carrier interactions, one resorts to the Franz-Keldysh theory of a continuum in a field, that, in the low-field limit, simplifies to the low-field FKA effect. Alternatively, when the continuum approximation breaks down, the problem of a Wannier exciton in a field has to be solved, and numerical methods emerged as the best solution. We illustrate our discussion with two examples involving lead-halide perovskites, a new, high-stake solar cell material. In the first example, we discuss the lineshape of the electroabsorption response for thin-films of lead-iodide perovskite, that sustains the photogeneration of free carriers. In the second example, we address a confined excitonic case with lead-bromide perovskite nanoparticles, and demonstrate the presence of so-called charge-transfer excitons.

Electrical Properties of the Multilayered Graphene in Terms of Carrier Density and Mobility

Due to the graphene superior electrical mechanical and optical properties, graphene is particularly advantageous for flexible transparent conductive electrode applications. However, its sheet resistance still higher than indium tin oxide has been a critical problem. Increasing of graphene layers is well known and basic method for sheet resistance reduction. But, this enhancement was not fully explained in terms of carrier density and mobility which are the elements of the sheet resistance. Because of the graphene electrical property is strongly influenced by surface condition, each graphene layer in the multilayer might be has different electrical property. Therefore, simple measuring the sheet resistance gives insufficient information about the origin of sheet resistance decreasing. By understanding the number of layer and surface condition effect on carrier density and mobility, we can get the potential pathway to reduce the sheet resistance. So, separated measurement of electrical property is required. In this work, enhancement of electrical properties with increasing the number of graphene layer is systemically investigated by Hall measurement which can divide sheet resistance into the carrier mobility and density. In order to get multilayer graphene with the different interface condition, two different layer stacking methods were introduced. Single layer graphenes were synthesized by chemical vapor deposition and stacked the graphene with and without benzimidazole which strongly p-dopes the graphene. 1 to 4 layers of graphene were stacked and analyzed. The sheet resistance of inter-layer undoped and doped graphene are reduced from ~700 to ~220 Ω/sq and ~280 to ~80 Ω/sq, respectively. Both sheet resistances are inversely proportional to the number of layers. However, carrier mobility and density show clearly different characteristics. Regardless of the number of graphene layers, carrier density of inter-layer undoped graphene is maintained in the range of ~8 x 1012 /cm2, but it is linearly increased from ~1.8 x 1013 to ~6.2 x 1013 /cm2 when inter-layer was chemically doped. The carrier mobility of inter-layer doped graphene remained same in 1200 cm2/Vs but, that of undoped graphene is increased from 1000 to 3000 cm2/Vs. Because the major carrier generation of undoped graphene occurred only on the top of the layer, it has constant carrier density regardless of the number of graphene layer. Compare to the top layer, the layers inside undoped graphene have a much smaller carrier density it leads less charge carrier interaction and high carrier mobility. On the other hands, inter-layer doped graphene has uniform carrier density in inside each layer. So, it result the uniform carrier mobility and linearly increasing carrier density depending on the number of graphene layers. In addition, by stacking the various coverage of graphene which are the 16, 38, 84 and 100 % onto the full coverage monolayer graphene, we checked the partial coverage additional layer effect even without the second layer conducting path. The sheet resistance is gradually decreases from ~700 to ~350 Ω/sq by increasing the second layer coverage. The carrier density show uniform value of ~8 x 1012 /cm2 in all coverage, but mobility linearly increases from 1040 to 2288 cm2/Vs. Increased mobility leads the sheet resistance decreases. We find out that the origin of sheet resistance reduction in the multilayer graphene is different depending on the surface condition. This demonstration would be very useful for the use of graphene as a flexible transparent conductive electrode, and in other electrical applications.

Role of Adsorbed Water on Charge Carrier Dynamics in Photoexcited TiO2.

Overall photocatalytic water splitting is one of the most sought after processes for sustainable solar-to-chemical energy conversion. The efficiency of this process strongly depends on charge carrier recombination and interaction with surface adsorbates at different time scales. Here, we investigated how hydration of TiO2 P25 affects dynamics of photogenerated electrons at the millisecond to minute time scale characteristic for chemical reactions. We used rapid scan diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). The decay of photogenerated electron absorption was substantially slower in the presence of associated water. For hydrated samples, the charge carrier recombination rates followed an Arrhenius-type behavior in the temperature range of 273–423 K; these became temperature-independent when the material was dehydrated at temperatures above 423 K or cooled below 273 K. A DFT+U analysis revealed that hydrogen bonding with adsorbed water stabilizes surface-trapped holes at anatase TiO2(101) facet and lowers the barriers for hole migration. Hence, hole mobility should be higher in the hydrated material than in the dehydrated system. This demonstrates that adsorbed associated water can efficiently stabilize photogenerated charge carriers in nanocrystalline TiO2 and suppress their recombination at the time scale up to minutes.

The impact of long-range electron-hole interaction on the charge separation yield of molecular photocells.

We discuss the effects of charge carrier interaction and recombination on the operation of molecular photocells. Molecular photocells are devices where the energy conversion process takes place in a single molecular donor-acceptor complex attached to electrodes. Our investigation is based on the quantum scattering theory, in particular on the Lippmann-Schwinger equation; this minimizes the complexity of the problem while providing useful and non-trivial insight into the mechanism governing photocell operation. In this study, both exciton pair creation and dissociation are treated in the energy domain, and therefore there is access to detailed spectral information, which can be used as a framework to interpret the charge separation yield. We demonstrate that the charge carrier separation is a complex process that is affected by different parameters, such as the strength of the electron-hole interaction and the non-radiative recombination rate. Our analysis helps to optimize the charge separation process and the energy transfer in organic solar cells and in molecular photocells.

Nitride Dielectric Environments to Suppress Surface Optical Phonon Dominated Scattering in High‐Performance Multilayer MoS2 FETs

The ultrathin channel in 2D semiconductors, although playing host to several interesting properties, also renders strong interactions (scattering) of charge carriers with the surrounding medium. The over‐arching dominance of surface (interfacial) optical phonons in 2D charge transport and engineering ideal nitride‐based dielectric environments for large performance gains is reported. Charge transport in MoS2 field effect transistors (FETs) fabricated on three conventional substrates, SiO2, Al2O3, and HfO2, is contrasted with a newly introduced, CMOS‐compatible nitride‐based dielectric: aluminum nitride (AlN) by employing semi‐classical models which account for charged impurity, surface optical phonon, and intrinsic phonon scattering. Unlike previous reports focused on charge impurity scattering, this work presents a new paradigm of utilizing high optical phonon energies intrinsic to “stiff” nitride bonds. This results in substantially lower surface optical phonon scattering in 2D FETs which directly influences peak field effect (FE) mobility, high field mobility degradation, and temperature‐dependent mobility. Leveraging on these insights, high‐performance sulfur‐passivated MoS2 FETs with an optimum all‐nitride environment (hexagonal boron nitride/MoS2/AlN) are demonstrated with FE mobility up to 72.8 cm2 V−1 s−1. This work is envisioned to address important issues in design of dielectric environments for a host of applications based on 2D materials.

Can Exciton-Delocalizing Ligands Facilitate Hot Hole Transfer from Semiconductor Nanocrystals?

Exciton-delocalizing ligands (EDLs) are of interest to researchers due to their ability to allow charge carriers to spread into the ligand shell of semiconductor nanocrystals (NCs). By increasing charge carrier surface accessibility, EDLs may facilitate the extraction of highly photoexcited carriers from NCs prior to their relaxation to the band edge, a process that can boost the performance of NC-based photocatalysts and light harvesting systems. However, hot carrier extraction must compete with carrier cooling, which could be accelerated by the stronger interaction of charge carriers and EDLs. This report describes the influence of the EDL phenyldithiocarbamate (PTC) on the electron and hole cooling rates of CdSe NCs. Using state-resolved transient absorption spectroscopy, we find that PTC treatment accelerates hole cooling by a factor of 1.7. However, upon further treatment of these NCs with cadmium(II) acetate, the hole cooling rate reverts to the value measured prior to PTC treatment, yet these NCs m...

Experimental Discovery of Magnetoresistance and Its Memory Effect in Methylimidazolium-Type Iron-Containing Ionic Liquids

The ordering and interactions of charge carriers play a critical role in many physicochemical properties. It is, therefore, interesting to study how a magnetic field affects these physicochemical processes and the consequent behavior of the charge carriers. Here, we report the observation of positive magnetoresistance and its memory effect in methylimidazolium-type iron-containing ionic liquids (ILs). Both the electrical transport and magnetic properties of ILs were measured to understand the mechanism of magnetoresistance behavior and its memory effect. The magnetoresistance effect of [BMIM][FeCl4] was found to increase with increasing applied currents. This observed memory effect can be ascribed to the slow order and disorder processes in these ILs due to the large viscosity caused by the interactions among ions.

Charge carrier extraction study of hole mobility and dynamics in poly(3-hexylthiophene) films subjected to photothermal aging

The evolution of charge carriers mobility μ and their concentration p in the diode devices with the architecture: indium tin oxide (ITO)/poly(3-hexylthiophene) (P3HT)/Al, subjected to photothermal aging in argon atmosphere was investigated using Current Extracted by the non-Linearly Increasing Voltage (CEnLIV) technique based on measuring charge carrier extraction under applied voltage pulses of arbitrary shape and limited amplitude. The kinetics of accumulation of radicals in the P3HT films was explored by the electron spin resonance (ESR) spectroscopy under similar conditions. A remarkable dynamics of μ and p in the system was observed after applying bias voltage to the photothermally aged P3HT-based diode devices. This process was shown to be fully reversible and the system undergoes relaxation to the original state after removal of the bias voltage. The experimental data reflecting the photothermal degradation of P3HT in the presence of trace amounts of oxygen trapped in the films were explained by the proposed kinetic scheme describing the interactions of free charge carriers with the donor-acceptor pairs.

Fine structure of the emission spectrum of a two-dimensional electron–hole liquid in SiGe/Si quantum wells

The fine structure of the emission spectrum of a quasi-two-dimensional electron–hole liquid in shallow SiGe/Si quantum wells is observed experimentally. This fine structure is explained by the occurrence of steps in the density of states resulting from the coexistence of light and heavy holes in the liquid phase and by the interaction of charge carriers with charge-density oscillations in the liquid.

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

Understanding how the complex intermolecular- and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than $\pm10\%$ as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.

Dielectric relaxation of vanadium–molybdenum tellurite glasses modified by alkaline-earth oxides

Electrical response changes due to the incorporation of alkaline-earth oxides in the vanadium–molybdenum tellurite glassy matrix have been studied. The results are explained by analyzing the electric formalisms representations. A non-straightforward relationship to the modifier oxide ionic radius was found and the results suggest poor charge carrier interactions even at high alkaline-earth concentrations. The electrical behavior of the studied materials gives strong evidence that alkaline-earth modified tellurite glasses are poor candidates to become good ionic glassy conductors.

Polaronic effects in monolayer black phosphorus on polar substrates

We investigate the effect of charge carrier interaction with surface optical phonons on the band properties of monolayer black phosphorus induced by polar substrates. We develop an analytical method based on the Lee-Low-Pines theory to calculate the spectrum of Fr\"ohlich type continuum Hamiltonian in the long-wavelength limit. We examine the modification of a band gap and renormalization of effective masses due to the substrate-related polaronic effect. Our results show that an energy gap in supported monolayer black phosphorus is enlarged depending on a particular substrate and the interlayer distance $z$. Among the substrate considered, the largest gap broadening at $z=2.5\phantom{\rule{0.16em}{0ex}}\AA{}$ is observed for the ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrate, which is found to be $\ensuremath{\sim}50$ meV. Carrier-phonon coupling also renormalizes the effective masses which is more pronounced along the zigzag direction. Anisotropy of the effective masses becomes stronger by the influence of the polaronic effect corresponding to direction-dependent carrier-phonon coupling. We conclude that substrate phonons have a non-negligible effect on the static band properties of monolayer black phosphorus, which may be further exploited in its experimental and theoretical studies.

Interference effects in silicon-germanium heterostructures with quantum wells of different widths

Weak localization effects and the interactions of charge carriers are studied in two Si0.7Ge0.3/Si0.2Ge0.8/Si0.7Ge0.3 p-type heterostructures, where one or two quantum levels are filled, respectively. A weak localization effect for two-dimensional charge carriers is found to occur in weak magnetic fields when the spin-orbital and inelastic scattering times are close, which is indicative of splitting of the spin states under the influence of a perturbing potential related to the formation of a two-dimensional potential well (Rashba mechanism). In higher magnetic fields when one quantum level is occupied, interaction effects appear that are caused by Coulomb interactions with a scatterer. When the two quantum levels are occupied, the dominant mechanism is scattering on Friedel oscillations of the charge carrier density induced by an impurity electric field. In all regions, the quantum corrections are in good agreement with modern theoretical predictions.

High-Performance Single-Walled Carbon Nanotube-Based Thin-Film Transistors by Reducing Charge Transfer

We demonstrate high-performance single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) using the simple and robust method of an interacting fluoropolymer encapsulation with thermal annealing in vacuum to reduce the charge transfer. With an optimized combination of the fabrication process and the device configuration, drop-casted SWCNT-TFTs exhibit a field-effect mobility of $\sim 18$ cm2/ $\text{V}\,\cdot \, \text{s}$ in the linear regime, an ON–OFF-current ratio of $\sim 10^{6}$ , a threshold voltage of around 0 V, and a subthreshold swing of 97 mV/decade, which is among the best results that have been achieved in 2-D random networks of the SWCNTs. We also investigate the instability characteristics in the SWCNT-TFTs encapsulated with fluoropolymer during the prolonged electrical bias-stress. Interestingly, the drop in drain current of the SWCNT-TFTs was partially recovered with a second phenomenon that increases the drain current after a certain time. The hydrophobic surface that the fluoropolymer film possesses, reduced the chemical interaction of charge carriers with the hydroxyl (–OH) functional groups which act as defects in the SWCNT-TFTs.

Polaron mass of charge carriers in semiconductor quantum wells

A theory of the interaction of charge carriers with optical phonons in a quantum well is developed with consideration for interface optical phonons. The dependence of the polaron effective mass on the quantum-well dimensions and dielectric characteristics of barriers is analyzed in detail. It is shown that, in narrow quantum wells, a quasi-two-dimensional polaron can be formed. In this case, however, the interaction parameters are defined by the charge-carrier effective mass in the quantum well and by the frequencies of interface optical phonons. If barriers are made of a nonpolar material, the polaron effective mass depends on the quantum-well width. As the quantum-well width is increased, a new mechanism of enhancement of the electron–phonon interaction develops. The mechanism is implemented, if the optical phonon energy is equal to the energy of one of the electronic transitions. This condition yields an unsteady dependence of the polaron effective mass on the quantum-well width.

Kinetic-energy-driven superconductivity in cuprate superconductors

Superconductivity in cuprate superconductors occurs upon charge-carrier doping Mott insulators, where a central question is what mechanism causes the loss of electrical resistance below the superconducting (SC) transition temperature? In this paper, we attempt to summarize the basic idea of the kinetic-energy-driven SC mechanism in the description of superconductivity in cuprate superconductors. The mechanism of the kinetic-energy-driven superconductivity is purely electronic without phonons, where the charge-carrier pairing interaction in the particle–particle channel arises directly from the kinetic energy by the exchange of spin excitations in the higher powers of the doping concentration. This kinetic-energy-driven d-wave SC-state is controlled by both the SC gap and quasiparticle coherence, which leads to that the maximal SC transition temperature occurs around the optimal doping, and then decreases in both the underdoped and overdoped regimes. In particular, the same charge-carrier interaction mediated by spin excitations that induces the SC-state in the particle–particle channel also generates the normal-state pseudogap state in the particle–hole channel. The normal-state pseudogap crossover temperature is much larger than the SC transition temperature in the underdoped and optimally doped regimes, and then monotonically decreases upon the increase of doping, eventually disappearing together with superconductivity at the end of the SC dome. This kinetic-energy-driven SC mechanism also indicates that the strong electron correlation favors superconductivity, since the main ingredient is identified into a charge-carrier pairing mechanism not from the external degree of freedom such as the phonon but rather solely from the internal spin degree of freedom of the electron. The typical properties of cuprate superconductors discussed within the framework of the kinetic-energy-driven SC mechanism are also reviewed.

(Invited) Thin Films of Eumelanin Pigment: Charge Carrier Transport, Ion Storage, and Interaction with Metal Electrodes

The ubiquitous natural pigment eumelanin is widely studied for its photoprotective, thermoregulating, free-radical scavenging, and anti-oxidant functions in the human body. Recently, eumelanin received increased attention for potential applications in organic bioelectronics due to its unique set of physicochemical properties including strong broad-band UV-Vis absorption, metal chelation properties, and potentially mixed electronic-ionic conduction in a hydrated state [1,2]. To further explore the use of eumelanin in organic bioelectronic devices, the properties of eumelanin thin films interfaced with device components such as metal electrodes need to be investigated in presence of water and ionic species. A good understanding of the interaction of eumelanin with different metal electrodes is also essential to study the intrinsic charge transport properties of eumelanin films, which are still largely undiscovered. We characterized eumelanin films grown on substrates patterned with gold, platinum, and palladium hydride electrodes using hydration-dependent transient electrical current and atomic force microscopy measurements. We discovered an electrochemical interaction among metal-chelating catechol groups of hydrated eumelanin, Cl- traces in the eumelanin material and Au electrodes, assumed to be electrochemically stable in previously published works on the electrical properties of eumelanin. This interaction leads to the growth of highly conductive Au-eumelanin dendrites between the electrodes, ultimately leading to sudden resistive changes of the sample. This phenomenon suggests new possibilities for biocompatible memory devices and has to be taken into account when integrating eumelanin-like materials in electronic devices [3]. Electrical characterization of eumelanin films interfaced with ion-blocking Pt and proton-transparent electrodes [4] complemented by cyclic voltammetry and impedance spectroscopy measurements give new insights into the charge transport properties of eumelanin films. Our results indicate the presence of protonic currents in eumelanin thin films, increasing with sample hydration, and cannot exclude the presence of electronic currents. These results support recently published data obtained by spectroscopic measurements on eumelanin pellets [2] and underline the attractiveness of eumelanin films for applications at the interface of electronics and biology. [1] M. D’Ischia et al., Angew. Chem. Int. Edit. 48, 3914 (2009). [2] A. B. Mostert et al., Proc. Natl. Acad. Sci. 109, 8943 (2012). [3] J. Wünsche et al., Adv. Funct. Mater. 23, 5569 (2013). [4] C. Zhong, Y. Deng, A. F. Roudsari, A. Kapetanovic, M. P. Anantram, M. Rolandi, Nat. Commun. 2, 476 (2011).

Snapshots of the retarded interaction of charge carriers with ultrafast fluctuations in cuprates

One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive since electronic correlations are expected to dramatically speed up the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and the frequency range of transient reflectivity measurements up to an unprecedented level that enables us to directly observe the 16 fs build-up of the effective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations.

Low Exciton-Phonon Coupling, High Charge Carrier Mobilities, and Multiexciton Properties in Two-Dimensional Lead, Silver, Cadmium, and Copper Chalcogenide Nanostructures.

The development of two-dimensional (2D) nanomaterials has revealed novel physical properties, like high carrier mobilities and the tunable coupling of charge carriers with phonons, which can enable wide-ranging applications in optoelectronic and thermoelectric devices. While mechanical exfoliation of graphene and some transition metal dichalcogenides (e.g., MoS2, WSe2) has enabled their fabrication as 2D semiconductors and integration into devices, lack of similar syntheses for other 2D semiconductor materials has hindered further progress. Here, we report measurements of fundamental charge carrier interactions and optoelectronic properties of 2D nanomaterials made from two-monolayers-thick PbX, CdX, Cu2X, and Ag2X (X = S, Se) using colloidal syntheses. Extremely low coupling of charge carriers with phonons (2-6-fold lower than bulk and other low-dimensional semiconductors), high carrier mobilities (0.2-1.2 cm(2) V(-1) s(-1), without dielectric screening), observation of infrared surface plasmons in ultrathin 2D semiconductor nanostructures, strong quantum-confinement, and other multiexcitonic properties (different phonon coupling and photon-to-charge collection efficiencies for band-edge and higher-energy excitons) can pave the way for efficient solution-processed devices made from these 2D nanostructured semiconductors.

Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling

We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS measurements in single crystals. The values of the two gaps and , once plotted as a function of the local critical temperature , turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.