On the Analytical Determination of the Seebeck Coefficient Using the Fermi–Dirac Approximation

A theoretical model describing the electron transport in vertical conductivity quantum dot infrared photodetectors is presented. The carrier wave functions and energy levels were evaluated using the strain dependent eight-band k∙p Hamiltonian and used to calculate all intra- and interperiod transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi’s golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. A system of rate equations was then formed, from which the macroscopic device output parameters such as dark current and responsivity, as well as microscopic information about carrier distribution in quantum dots and continuum states, could be extracted. The model has been applied to simulate the dark current, as well as the midinfrared photoresponse in an experimentally realized device [Chen et al., J. Appl. Phys. 89, 4558 (2001)], and a good agreement with experiment has been obtained. Being free from any fitting or phenomenological parameters, the model should be a useful tool in the design and prediction of the characteristics of the existing or other types of quantum dot infrared photodetectors.

The indium selenium (InSe) bilayer thin films of various thickness ratios, InxSe(1-x) (x = 0.25, 0.50, 0.75), were deposited on a glass substrate keeping overall the same thickness of 2500 Ǻ using thermal evaporation method under high vacuum atmosphere. Electrical, optical, and structural properties of these bilayer thin films have been compared before and after thermal annealing at different temperatures. The structural and morphological characterization was done using XRD and SEM, respectively. The optical bandgap of these thin films has been calculated by Tauc’s relation that varies within the range of 1.99 to 2.05 eV. A simple low-cost thermoelectrical power measurement setup is designed which can measure the Seebeck coefficient “S” in the vacuum with temperature variation. The setup temperature variation is up to 70°C. This setup contains a Peltier device TEC1-12715 which is kept between two copper plates that act as a reference metal. Also, in the present work, the thermoelectric power of indium selenide (InSe) and aluminum selenide (AlSe) bilayer thin films prepared and annealed in the same way is calculated. The thermoelectric power has been measured by estimating the Seebeck coefficient for InSe and AlSe bilayer thin films. It was observed that the Seebeck coefficient is negative for InSe and AlSe thin films.

P-Type Chemical Doping-Induced High Bipolar Electrical Conductivities in a Thermoelectric Donor–Acceptor Copolymer

Host engineering for improving the performance of blue phosphorescent organic light-emitting devices

Transport in organic semiconductors has traditionally been investigated using measurements of the temperature and gate voltage dependent mobility of charge carriers within the channel of organic field-effect transistors (OFETs). In such measurements, the behavior of charge carrier mobility with temperature and gate voltage, studied together with carrier activation energies, provide a metric to quantify the extent of disorder within these van der Waals bonded materials. In addition to the mobility and activation energy, another potent but often-overlooked transport coefficient useful in understanding disorder is the Seebeck coefficient (also known as thermoelectric power). Fundamentally, the Seebeck coefficient represents the entropy per charge carrier in the solid state, and thus proves powerful in distinguishing materials in which charge carriers move freely from those where a high degree of disorder causes the induced carriers to remain trapped. This paper briefly covers the recent highlights in the field of organic thermoelectrics, showing how significant strides have been made both from an applied standpoint as well as from a viewpoint of fundamental thermoelectric transport physics. It shall be illustrated how thermoelectric transport parameters in organic semiconductors can be tuned over a significant range, and how this tunability facilitates an enhanced performance for heat-to-electricity conversion as well as quantifies energetic disorder and the nature of the density of states (DOS). The work of the authors shall be spotlighted in this context, illustrating how Seebeck coefficient measurements in the polymer indacenodithiophene-co-benzothiadiazole (IDTBT) known for its ultra-low degree of torsion within the polymer backbone, has a trend consistent with low disorder. 1 Finally, using examples of the small molecules C8-BTBT and C10-DNTT, it shall be discussed how the Seebeck coefficient can aid the estimation of the density and distribution of trap states within these materials. 2, 3

Recently it was shown [A. Bentien, S. Johnsen, G.K.H. Madsen, B.B. Iversen, F. Steglich, Europhys. Lett. 80, 17008 (2007)], that the strongly correlated Kondo insulator FeSb2 exhibits a colossal Seebeck coefficient; however, due to its large lattice thermal conductivity, the dimensionless thermoelectric figure of merit (zT) is only 0.005 at 12 K. This experimental result motivate us to perform a theoretical study of the thermoelectric properties of Kondo insulators, within the framework of the X-boson approach [R. Franco, M.S. Figueira, M.E. Foglio, Phys. Rev. B 66, 045112 (2002)] for the periodic Anderson model. We consider a set of parameters adequate for describing the compound FeSb2, and we calculate the resistivity, the thermoelectric power (Seebeck coefficient), the charge carrier thermal conductance, the thermoelectric factor power (zT), the Wiedemann-Franz law, the specific heat, and the Sommerfeld’s γ coefficient. The result of the colossal maximum of the Seebeck coefficient, at low temperatures, has the same order of magnitude as the experimental result [A. Bentien, S. Johnsen, G.K.H. Madsen, B.B. Iversen, F. Steglich, Europhys. Lett. 80, 17008 (2007)]. We also show that those temperature region presents an intermediate valence behavior, characterized by a moderate increase of the Sommerfeld’s γ coefficient.

ConspectusLocalized surface plasmon resonance (LSPR), a distinctive optoelectronic property of plasmonic nanocrystals, arises from the collective oscillation of conduction electrons in resonance with incident light. The excitation of LSPR confines incident light near the surface of plasmonic nanocrystals and amplifies the local electric field. Moreover, the frequency of LSPR is highly tunable in the visible and near-IR regions, allowing plasmonic nanocrystals to efficiently absorb and scatter light across the solar spectrum. Such a property makes plasmonic nanocrystals promising candidates for utilizing solar irradiation to drive chemical reactions, a process known as plasmonic photocatalysis. Upon the resonant excitation of LSPR, energetic hot electrons and holes are generated via the nonradiative decay of LSPR in plasmonic nanocrystals. Those hot carriers can be transferred into the molecular orbitals of adsorbed reactants, enabling chemical transformations at the surface of nanocrystals. However, during the charge transport within plasmonic nanocrystals, hot carriers rapidly relax into lower-energy states. As a result, their energy is often dissipated to the lattice as heat, increasing the local temperature rather than directly contributing to chemical reactions─posing a fundamental challenge to achieving efficient solar-to-chemical energy conversion using plasmonic nanocrystals.To address this challenge, our group has developed multiple strategies to control the lifetime, energy level, and spatial distribution of plasmon-generated hot carriers to enhance the photocatalytic activity of Au nanocrystals. To extend the lifetime of hot carriers to match the slow kinetics of chemical reactions, Au nanocrystals were attached to an n-type semiconductor to form a heterojunction. This structure was found to prolong the lifetime of hot electrons through efficient spatial separation of hot electrons and holes, facilitated by the Schottky barrier at the metal/semiconductor interface. In parallel, decorating Au nanocrystals with redox-active molecules was shown to extend the lifetime of hot holes. Those hot holes were chemically stabilized and trapped within the bonds of the redox-active species, allowing them to participate in subsequent chemical reactions. Furthermore, a direct correlation between the activity of hot-electron-driven reduction reactions and the size of plasmonic nanocrystals, as well as between hot-hole-driven oxidation reactions and the wavelength of incident light, was established. Those observations demonstrated that energy levels of hot carriers involved in chemical reactions can be manipulated by tuning the size of nanocrystals and the wavelength of light. Moreover, positively charged molecules with facet-selective adsorption on Au nanocrystals were found to stabilize the plasmon-generated hot electrons, enabling control over the spatial distribution of hot carriers. Manipulating plasmon-generated hot carriers not only enhances the kinetics of plasmon-driven chemical reactions─such as oxygen evolution, hydrogen evolution, and nanocrystal growth─but also introduces new reaction pathways in those chemical processes, paving the way for highly efficient plasmonic photocatalysis.

The strong and counterproductive interrelationship of thermoelectric parameters remains a bottleneck to improving thermoelectric performance, especially in polymer-based materials. In this paper, we decouple the electrical conductivity and Seebeck coefficient, achieving increases in at least one of these two parameters while the other is maintained or slightly increased as well. This is done using an alkylthio-substituted polythiophene (PQTS12) as additive in poly(bisdodecylquaterthiophene) (PQT12) with tetrafluorotetracyanoquinodimethane (F4TCNQ) and nitrosyl tetrafluoroborate (NOBF4) as dopants. The power factor increased two orders of magnitude with the PQTS12 additive at constant doping level. Using a second pair of polymers, poly(2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTTC12) and poly(2,5-bis(3-dodecylthiothiophen-2-yl)thieno[3,2-b]thiophene, (PBTTTSC12), with higher mobilities, we also observed the decoupling of the Seebeck coefficient and electrical conductivity, and achieved higher power factor. Distinguished from recently reported works, these two sets of polymers possess very closely offset carrier energy levels (0.05 ~ 0.07 eV), and the microstructure, assessed using grazing incidence X-ray scattering, and mobility evaluated in field-effect transistors, was not adversely affected by the blending. Experiments, calculations and simulations are consistent with the idea that blending and doping polymers with closely-spaced energy levels and compatible morphologies to promote carrier mobility favors increased power factors.

The thermoelectric power, the dark electrical resistivity and the grain boundary potential barrier in CdIn 2Se 4 thin films

A quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons and valence band holes in all three spatial directions, thus creating fully discrete energy levels. The confinement in the InAs/GaAs material system is generated by the bandgap difference for the two materials (∼ 0.4 eV for InAs, and ∼ 1.4 eV for GaAs) which provides a minimum of potential energy for both electrons and holes inside the InAs nanoparticle. The appearance of new nanoscale effects modifies the electro-optical properties of such a system which makes possible the improvement of existing device performance or even fascinating novel device applications. For the creation of coherent structures with very high purity and high density, as it is required for application in light emitting devices, self-assembling is a very important pathway. The quantum dot creation mostly relies on the relaxation of the strain accumulated during the growth of lattice mismatched materials, which happens in a non-equlibrium condition. This results in a statistical distribution of all the parameters characterizing the InAs nanocrystals as size, composition and strain, whose direct manifestation is the inhomogeneously broadened light emission deriving from quantum dot ensembles. At the dawning of the quantum dot development the broadening was considered as a limiting factor for application in semiconductor lasers. Today, it is considered favorably for all the applications where a broad gain spectrum is required as optical amplifiers, monolithic and external-cavity tunable lasers, and superluminescent diodes. The objective of this thesis is the application of InAs/GaAs self-assembled quantum dots to the active region of high power and broad-band superluminescent diodes emitting in the 1.3 μm wavelength region. Superluminescent diodes are edge-emitting semiconductor light sources based on the amplification of spontaneous emission along a waveguide where optical gain is achieved through current injection. They combine the high power and brightness of laser diodes with the low coherence of LEDs. The latter is a fundamental feature for the application in optical coherence tomography medical imaging, where the image resolution is related to the spectral bandwidth of the light source used. The achievement of bandwidths larger than 100 nm are demonstrated in this work through the optimization of molecular beam epitaxy growth conditions. This may be done optimizing the statistical size-dispersion of the dot ensemble, or through the use of different dot layers emitting at slightly different wavelengths. The large bandwidth emission is obtained also due to the peculiar energy structure of this material, for which a multi-state emission appears under particular injection conditions. As a term of comparison, best commercial single-chip superluminescent diodes for the imaging application at 1.3 μm show bandwidths around 60 nm. Moreover, the discrete nature of the energy levels in InAs quantum dots together with the implementation of a GaAs-based technology can be beneficial for the device temperature stability (the competing technology, based on InP, suffers from strong thermal degradation). The analysis and understanding of the device temperature characteristics will be detailed in the last chapter of the manuscript. Even though standard devices show an important temperature dependence, a better stability may be achieved through the use of p-doped active regions. In the last few years quantum dot devices have shown outstanding properties if compared to the competing quantum well technology. Low threshold currents, high temperature stability and low chirp in lasers, large bandwidths and low gain saturation in amplifiers have been demonstrated. However, in spite of the large interest among the scientific community, many of the microscopic processes at the origin of the electro-optical characteristics of this material are not completely understood yet. The physics of the quantum dot active material, will be modeled in this work through the use of systems of rate equations with different complexity depending on the system they are applied to. We will provide a mean-field rate equation model allowing to get insights about carrier dynamics in QD lasers. Also, we will develop two different traveling-wave rate equation models, one for the modeling of the L-I characteristics and the other for the modeling of the spectral characteristics of quantum dot superluminescent diodes. Considering the carrier and photon density distributions across the device cavity is essential in superluminescent diodes, where the high single pass gain results in large non-homogeneities of photon and carrier distributions. The work is motivated by an industrial cooperation with EXALOS AG, a leading company in the development, manufacturing, and sales of broadband superluminescent diodes.

Enormous enhancement of thermoelectric properties via piezo-gating effect

Electrical conductivity and thermoelectric power studies of aluminium-substituted lithium ferrites

Four series of ternary compounds $R\mathrm{Pd}\mathrm{Sb}$ $(R=\mathrm{Y},\mathrm{Ho},\mathrm{Er})$, $R\mathrm{Pd}\mathrm{Bi}$ $(R=\mathrm{Nd},\mathrm{Y},\mathrm{Dy},\mathrm{Ho},\mathrm{Er})$, $R{\mathrm{Pd}}_{2}\mathrm{Sb}$ $(R=\mathrm{Y},\mathrm{Gd}\ensuremath{-}\mathrm{Er})$, and $R{\mathrm{Pd}}_{2}\mathrm{Bi}$ $(R=\mathrm{Y},\mathrm{Dy}\ensuremath{-}\mathrm{Er})$ were studied by means of magnetization, magnetic susceptibility, electrical resistivity, magnetoresistivity, thermoelectric power, and Hall effect measurements, performed in the temperature range $1.5--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and in magnetic fields up to $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. All these ternaries, except for diamagnetic Y-based phases, exhibit localized magnetism of ${R}^{3+}$ ions, and a few of them order antiferromagnetically at low temperatures $({T}_{N}=2--14\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. The equiatomic compounds show half-metallic conductivity due to the formation of narrow gaps in their electronic band structures near the Fermi energy. Their Seebeck coefficient at room temperature is exceptionally high (S up to $200\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{V}∕\mathrm{K}$), being promising for thermoelectric applications. In contrast, all the 1:2:1 phases are semimetals and their thermoelectric power is much lower (maximum S of $10--25\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{V}∕\mathrm{K}$). The Hall effect in the compounds studied corroborates complex character of their electronic structure with multiple electron and hole bands with different temperature and magnetic field variations of carrier concentrations and their mobilities.

Realizing ultrahigh room-temperature seebeck coefficient and thermoelectric properties in SnTe-based alloys through carrier modulation and band convergence

PbS quantum dot (QD) solar cells harvest near-infrared solar radiation. Their conventional hole transport layer has limited hole collection efficiency due to energy level mismatch and poor film quality. Here, how to resolve these two issues by using Ag-doped PbS QDs are demonstrated. On the one hand, Ag doping relieves the compressive stress during layer deposition and thus improves film compactness and homogeneity to suppress leakage currents. On the other hand, Ag doping increases hole concentration, which aligns energy levels and increases hole mobility to boost hole collection. Increased hole concentration also broadens the depletion region of the active layer, decreasing interface charge accumulation and promoting carrier extraction efficiency. A champion power conversion efficiency of 12.42% is achieved by optimizing the hole transport layer in PbS QD solar cells, compared to 9.38% for control devices. Doping can be combined with compressive strain relief to optimize carrier concentration and energy levels in QDs, and even introduce other novel phenomena such as improved film quality.