Equilibrium Charge Carriers Research Articles

The effects of humidity on the electrical properties and carrier mobility of semiconducting polymers anion-exchange doped with hygroscopic salts

To improve their electrical conductivity for various applications, semiconducting polymer films are often chemically doped to increase their equilibrium charge carrier density. Recently, a novel doping method involving anion exchange has provided control over the identity of the counterions that reside in such films, leading to increased stability under ambient conditions. In this work, however, we show that by ion-exchanging 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-doped poly(3-hexylthiophene-2,5-diyl) films with hygroscopic salts like bis(trifluoromethane)sulfonimide lithium or LiPF6, the doped film's electrical conductivity drops significantly when exposed to ambient humidity. The change in electrical conductivity depends directly on the degree of hygroscopicity of the counterion and can be over 50% with relatively modest changes in relative humidity (RH), and up to a factor of four between ambient and completely dry conditions. The film's humidity response is entirely reversible when adsorbed water is removed, potentially allowing the doped semiconducting polymer films to function as humidity sensors. Hall effect measurements show that the cause of the drop in conductivity with increasing RH is due to a decrease in carrier mobility and not due to de-doping. Our results emphasize that it is important to control the sample environment when making electrical measurements on anion-exchange doped semiconducting polymer films.

SЕNSORS BASED ON POLYCRYSTALLINE 3C-SiC: IMPACT OF BORON DOPING

As a chemically inert wide bandgap semiconductor material with high hardness and thermal conductivity, stable electrical characteristics, silicon carbide SiC is attractive for harsh environment electronics and sensors applications. The concept of harsh environment includes extremes of temperature, pressure, shock loads, radiation and chemical attacks those arise in aircraft and automotive engines, industrial gas turbines, during oil and gas exploration, etc.
 Lower growing temperatures of polycrystalline cubic silicon carbide, pc-3C-SiC, compared to monocrystalline allow to significantly reduce its cost and expand the possibilities of application. It follows from previous works that the thermal sensitivity of pc -3C-SiC can be significantly increased by doping with an acceptor impurity of boron during the growth of the material. The purpose of this work is to determine the properties of pc-3C-SiC doped with boron for the creation of photosensors and thermosensors, as well as thermal anemometers for extreme operating conditions.
 It is shown that pс-3C-SiC doping with a boron impurity in the growth process causes the formation of acceptor-type centers in the band gap and the appearance of features in the photosensitivity spectrum, which may be of practical interest for photovoltaics. For temperatures T > 150K, the conductivity of the doped sample increases almost exponentially with an activation energy of 0.28 eV, which is close to the activation energy of the photoconductivity of the same sample. This indicates that the ionization process of equilibrium and non-equilibrium charge carriers occurs from the same impurity centers. Boron doping causes the appearance of a broad photoconductivity band with a maximum at 1.7 eV in the impurity absorption region of pc-3C-SiC, similar to the situation in single crystal 3C-SiC.
 It was determined that the temperature coefficient of resistance for boron-doped pc-3C-SiC is 3.0´10-2 K-1 at T=300K and 1.1´10-2 K-1 at T=700K, which is almost an order of magnitude higher than for thermocouples, as well as for metals from which anemometer threads are made. The discussion of the obtained results allows to associate the value of the activation energy E=0.28 eV with the level of shallow boron in pc-3C-SiC and to assume this center is a point defect containing a boron atom that replaces a silicon atom in the 3C-SiC lattice, i.e. BSi.
 Photosensors that can be used as solar cells in the near-IR range of 0.6 - 1.8 μm, and as photocells in the visible range of 0.4 - 0.6 μm are proposed. The ability of pc-3C-SiC to work in extreme operating conditions, as well as the low cost of device manufacturing technology based on it, compared to other SiC polytypes, allow it to be considered a suitable material for creating temperature sensors, thermo-anemometers and photosensors, as well as detectors for monitoring nuclear facilities.

Equilibrium point defect and charge carrier concentrations in a material determined through calculation of the self-consistent Fermi energy

A concise procedure to determine the self-consistent Fermi energy and defect and carrier concentrations in an extended crystalline system is presented. It is assumed that the formation enthalpies of a set of variously charged point defects in thermodynamic equilibrium are known, as well as the density of electronic states in the defect-free system. By applying the constraint of overall charge neutrality, the self-consistent Fermi energy is determined using an iterative searching routine. The procedure is incorporated within a Fortran code ‘SC-FERMI’: the input consists of the defect formation energies, density of sites where they can form, and the degeneracy of each charge state; the material band gap; and the calculated density of states of the pristine system. The output is the self-consistent Fermi energy, the total concentrations of each defect as well as the concentration of its individual charge states, and the free carrier concentrations. Furthermore, the procedure facilitates fixing the concentration of one or more defects and determining the resulting self-consistent Fermi energy and concentrations of other defects (performed using the related code ‘FROZEN-SC-FERMI’), thus modelling ‘frozen-in’ defects which may form by kinetic, rather than thermodynamic, processes. One can fix the total concentration or the concentration of a particular charge state; it is also possible to introduce new defects with a fixed concentration, but here the charge state must be specified. The background theory is discussed in some detail, and the operation of the program is demonstrated by some examples. Program summaryProgram Title:SC-FERMIProgram Files doi:http://dx.doi.org/10.17632/dh3hjdf4fc.1Licensing provisions: MIT licenseProgramming language:FORTRAN 90Nature of problem: To determine the self-consistent Fermi energy and equilibrium defect and carrier concentrations given a set of point defect formation energies in a crystalline system, assuming the constraint of charge neutrality.Solution method: The concentrations of each defect in each charge state are calculated, as are the free carrier concentrations. These concentrations are functions of the Fermi energy. The code, using an iterative search algorithm, determines the Fermi energy that satisfies the charge neutrality constraint (the self-consistent Fermi energy). The defect and carrier concentrations at that Fermi energy are then reported, as well as the Fermi energy itself.Restrictions: Thermodynamic equilibrium is assumed. The defect formation enthalpies and electronic density of states of the pristine system must be known.Additional comments: The concentrations of defects can be fixed to a particular value, thus modelling ‘frozen-in’ defects formed by e.g. kinetic processes. This procedure is facilitated by the related program, FROZEN-SC-FERMI, which is identical to SC-FERMI apart from the additional defect concentration fixing routine.

A technique for controlling the course of surface processes involving adsorbed metal clusters in luminescent condensed media

Despite the huge amount of work devoted to the study of photostimulated processes (PSP) in crystals with a mixed type of communication, the problem of the development and improvement of information and measurement methods for monitoring the physical parameters of objects of several nanometers and the development of evaluation criteria for the use of such methods today remains relevant. From a practical point of view, relevant is the problem of developing information-measuring methods on the basis of kinetic models of the processes that allows us to offer algorithms for the application of new methods to control the processes of nucleation and modification of centers involving ions or metal atoms in crystals with a mixed type of communication, how it relates to a broad class of semiconductors. The problem of control of parameters of technological processes at early stages of new phase origin on the surface of condensed media is described. The possibility of applying the photostimulated luminescence flash (PSLF) method for luminescent crystals with ion-covalent bond is analyzed. The basic formulas allowing to estimate change of concentration of the metal particles adsorbed on a surface are resulted. The dependences of the photostimulated luminescence flash parameters on the concentration of processing solutions, which leads to the formation of clusters of varying degrees of dispersion, are considered. The model of description of process of formation of clusters from adsorbed atoms of metal is resulted. The estimation of the parameters of the centers of localization of non equilibrium charge carriers showed that within the framework of the proposed model it becomes possible to estimate the size of unstable adsorbed metal clusters. For the sample CDs: Ag, the size of such clusters was 5-7 atoms. It is proposed to use the correlation and regression model to describe the parameters of FSVL as a tool for assessing the applicability of the photostimulated luminescence flash method for monitoring surface processes on ion-covalent crystals of AgCl, ZnS, CDs. The best value of the criterion of applicability of the model FSWL was obtained for the crystal of AgCl (R2?1). For the crystals CDs the proposed method of control of technological processes must be developed according to the measurement parameters FSVL.

The Effect of Phosphoryl–Substituted Porphyrins on Mobility of Charge Carriers in P3HT Polymer Photoconductor

The effect of additives of sensitizing Zn(II) and Co(II) complexes with 5,15-(diethoxyphosphoryl)-10,20-diphenylporphyrin on the mobility of holes in thin poly3-hexylthiophene films is studied. The charge carrier mobility is measured using the CELIV and photo-CELIV methods. It is found that the sensitizing additive does not affect the mobility of the equilibrium charge carriers, but causes a decrease in the mobility of the nonequilibrium charge carriers.

Charge carrier chemistry in methylammonium lead iodide

Both electronic and ionic transport properties are key issues in the performance of perovskite solar cells. In order to understand the electrical behavior of organic-inorganic halide perovskites, and the possibilities to influence them, the elucidation of their equilibrium charge carrier chemistry as a function of stoichiometry and dopant concentration is a necessary and fundamental prerequisite. We provide here insight into the point defect chemistry of methylammonium lead iodide, whereby the decisive carriers are identified and their concentrations discussed as a function of stoichiometry (iodine partial pressure) and dopant content (oxygen partial pressure or Na addition). The experimental results indicate how ionic conductivity, which can be attributed to iodine vacancies, and electronic conductivity, which can be attributed to electron holes (or conduction electrons under reducing conditions), can be significantly and systematically altered by such treatments. Experimental results are discussed in the context of simple defect chemical models.

Charge-carrier density collapse in layered crystals of the family [(Ge,Sn,Pb)(Te,Se)] m [(Bi,Sb)2(Te,Se)3] n (m, n = 0, 1, 2…)

The causes of the “charge-carrier density collapse”, i.e., a sharp increase in the equilibrium charge-carrier density n, p = 1 × 1019 → (2–5) × 1020 cm–3 in going from binary alloys such as GeTe and Bi2Te3 to the family of ternary alloys [(Ge,Sn,Pb)(Te,Se)] m [(Bi,Sb)2(Te,Se)3] n [m, n = 0, 1, 2…) are discussed. The phenomenon is associated with the positional disordering of heterovalent cations (Ge+2, Sn+2, Pb+2 ↔ Bi+3, Sb+3) in the cation sublattice of ternary alloys. The phenomenon is not observed during the disordering of isovalent cations (Bi+3 ↔ Sb+3) or anions (Te–2 ↔ Se–2).

Ultralow Self-Doping in Two-dimensional Hybrid Perovskite Single Crystals.

Unintentional self-doping in semiconductors through shallow defects is detrimental to optoelectronic device performance. It adversely affects junction properties and it introduces electronic noise. This is especially acute for solution-processed semiconductors, including hybrid perovskites, which are usually high in defects due to rapid crystallization. Here, we uncover extremely low self-doping concentrations in single crystals of the two-dimensional perovskites (C6H5C2H4NH3)2PbI4·(CH3NH3PbI3)n-1 (n = 1, 2, and 3), over three orders of magnitude lower than those of typical three-dimensional hybrid perovskites, by analyzing their conductivity behavior. We propose that crystallization of hybrid perovskites containing large organic cations suppresses defect formation and thus favors a low self-doping level. To exemplify the benefits of this effect, we demonstrate extraordinarily high light-detectivity (1013 Jones) in (C6H5C2H4NH3)2PbI4·(CH3NH3PbI3)n-1 photoconductors due to the reduced electronic noise, which makes them particularly attractive for the detection of weak light signals. Furthermore, the low self-doping concentration reduces the equilibrium charge carrier concentration in (C6H5C2H4NH3)2PbI4·(CH3NH3PbI3)n-1, advantageous in the design of p-i-n heterojunction solar cells by optimizing band alignment and promoting carrier depletion in the intrinsic perovskite layer, thereby enhancing charge extraction.

Kinetic tendencies of thermal transformations in nanosized Ni–MoO3 systems

The transformations in nanosized Ni–MoO3 systems were studied by optical spectroscopy, microscopy, and gravimetry depending on the thickness of the Ni (d = 1–40 nm) and MoO3 (d = 3–50 nm) films, temperature (473–773 K), and thermal treatment time. The contact potential difference was measured for Ni and MoO3 films; photovoltage, for Ni–MoO3 systems. An energy band diagram of the Ni–MoO3 systems was constructed. A model of the thermal transformation of MoO3 films in Ni–MoO3 systems was suggested, which involves a redistribution of equilibrium charge carriers at the contact, formation of a [(Vа)++е] center during the preparation of the MoO3 film, the transformation of this center into an [е(Vа)++е] center during the formation of Ni–MoO3 systems, and the thermal transition of an electron to the level of the [(Vа)++е] center to form an [е(Vа)++е] center.

Charge carrier transport and recombination in disordered materials

In this brief review the methods for investigation of charge carrier transport and recombination in thin layers of disordered materials and the obtained results are discussed. The method of charge carrier extraction by linearly increasing voltage (CELIV) is useful for the determination of mobility, bulk conductivity and density of equilibrium charge carriers. The extraction of photogenerated charge carriers (photo-CELIV) allows one to independently investigate relaxation of both the mobility and density of photogenerated charge carriers. The extraction of injected charge carriers (i-CELIV) is effective for the independent investigation of transport peculiarities of both injected holes and electrons in bulk heterojunctions. For the investigation of charge carrier recombination we proposed integral time-of-flight (TOF) and double-injection (DI) current transient methods. The methods allowed us to obtain the following significant results: to determine the reason of the conductivity dependence on electric field strength and temperature in the amorphous and microcrystalline hydrogenated silicon and π-conjugated polymers, the time dependent Langevin recombination, the impact of morphology on charge carrier mobility, the reason of reduced Langevin recombination in RR-PHT (regioregular poly(3-hexylthiophene))/PCBM (1-(3-methoxycarbonyl)propyl-1phenyl-[6,6]-methanofullerene) bulk heterojunction structures – 2D Langevin recombination; and to evaluate that the mobility of holes is predetermined by off-diagonal dispersion in poly-PbO.

Structural and photoluminescent properties of nanowires formed by the metal-assisted chemical etching of monocrystalline silicon with different doping level

Silicon-nanowire layers grown by the metal-assisted chemical etching of (100)-oriented p-type monocrystalline silicon substrates with a resistivity of 10 and 0.001 Ω · cm are studied by electron microscopy, Raman scattering, and photoluminescence measurements. It is established that nanowires grown on lightly doped substrates are structurally nonporous and formed as crystalline cores covered by nanocrystals 3–5 nm in dimensions. Nanowires grown on heavily doped substrates are structurally porous and contain both small nanocrystals and coarser crystallites with equilibrium charge carriers that influence interband radiative recombination. It is found that the photoluminescence intensity of nanowires in the spectral range 1.3–2.0 eV depends on the presence of molecular oxygen.

Implications of the band gap problem on oxidation and hydration in acceptor-doped barium zirconate

Charge carrier concentrations in acceptor-doped proton-conducting perovskites are to a large extent determined by the hydration and oxidation of oxygen vacancies, which introduce protons and holes, respectively. First-principles modeling of these reactions involves calculation of formation energies of charged defects, which requires an accurate description of the band gap and the position of the band edges. Since density-functional theory (DFT) with local and semi-local exchange-correlation functionals (LDA and GGA) systematically fails to predict these quantities this can have serious implications on the modeling of defect reactions. In this study we investigate how the description of band gap and band edge positions affects the hydration and oxidation in acceptor-doped BaZrO$_3$. First-principles calculations are performed in combination with thermodynamic modeling in order to obtain equilibrium charge carrier concentrations at different temperatures and partial pressures. Three different methods have been considered: DFT with both semi-local (PBE) and hybrid (PBE0) exchange-correlation functionals, and many-body perturbation theory within the $G_0W_0$-approximation. All three methods yield similar results for the hydration reaction, which are consistent with experimental findings. For the oxidation reaction, on the other hand, there is a qualitative difference. PBE predicts the reaction to be exothermic while the two others predict an endothermic behavior. Results from thermodynamic modeling are compared with available experimental data, such as enthalpies, concentrations and conductivities, and only the results obtained with PBE0 and $G_0W_0$, with an endothermic oxidation behavior, give a satisfactory agreement with experiments.

Thermostimulated transformations in nanosized Bi-MoO3 systems

Transformations in Bi-MoO3 nanosized systems are studied by optical spectroscopy, microscopy, and gravimetry. The contact potential difference for the Bi and MoO3 films and the photovoltage of the Bi-MoO3 systems are measured, depending on the thickness of Bi (d = 3–92 nm) and MoO3 films (d = 5–40 nm) and the temperature (373–673 K) and time of heat treatment. An energy band diagram of the Bi-MoO3 systems is constructed. A model of the thermal transformation of MoO3 films in Bi-MoO3 systems is proposed that involves the redistribution of equilibrium charge carriers on a contact, the formation of a ([(Va)++e]) center during the preparation of a MoO3 film, the transformation of this center into a ([e(Va)++e]) center during the formation of Bi-MoO3 systems, and the thermal transition of an electron to the level of a ([(Va)++e]) center to form a ([e(Va)++e]) center.

Thermal transformations in In-MoO3 nanofilms

Transformations in In-MoO3 nanosystems have been studied by optical spectroscopy, micros-copy, and gravimetry in relation to the thickness of the In and MoO3 layers and heat-treatment temperature and time. We have measured the contact potential difference across the In and MoO3 films and the photo-voltage in the In-MoO3 system and constructed the energy band diagram of the In-MoO3 system. A model has been proposed for the thermal transformation of the MoO3 films in In-MoO3 bilayers, which involves a redistribution of equilibrium charge carriers at the contact, the formation of a [(Va)++e] center during the preparation of the MoO3 film, transformation of the center into a [e(Va)++e] center during the fabrication of the In-MoO3 bilayer, and thermal ionization of the [e(Va)++e] center.

Thermal transformations in nanodimensional Pb—WO3 systems

Transformations in Pb-WO3 nanodimensional systems are investigated in dependence on the thickness of the Pb and WO3 films, temperature, and thermal treatment duration by means of optical spectroscopy, microscopy, and gravimetry. The contact potential difference for Pb and WO3 films and the photoelectric power of Pb-WO3 systems is measured. A diagram of the energy bands of Pb-WO3 systems is constructed. A model of thermal transformation for the WO3 films in Pb-WO3 systems is suggested. The model involves the redistribution of equilibrium charge carriers on the contact, formation of the ([(Va)++e]) center during the preparation of a WO3 film, its transformation into a ([e(Va)++e]) center during the formation of the Pb-WO3 systems, and thermal ionization of the ([e(Va)++e]) center.

Wake effect in doped graphene due to moving external charge

We use the dielectric-response formalism to evaluate the induced density of charge carriers in supported graphene due to an external moving charged particle in terms of its velocity and distance from graphene for several equilibrium charge carrier densities due to graphene doping. We show that, when the particle speed exceeds a threshold value, an oscillatory wake effect develops in the induced charge density trailing the particle. Strong effects are observed in the wake pattern due to finite size of the graphene–substrate gap, as well as due to strong coupling effects, and plasmon damping of grapheneʼs π electrons.

Nonlinear screening of external charge by doped graphene

We solve a nonlinear integral equation for the electrostatic potential in doped graphene due to an external charge, arising from a Thomas-Fermi (TF) model for screening by graphene's $\pi$ electron bands. In particular, we study the effects of a finite equilibrium charge carrier density in graphene, non-zero temperature, non-zero gap between graphene and a dielectric substrate, as well as the nonlinearity in the band density of states. Effects of the exchange and correlation interactions are also briefly discussed for undoped graphene at zero temperature. Nonlinear results are compared with both the linearized TF model and the dielectric screening model within random phase approximation (RPA). In addition, image potential of the external charge is evaluated from the solution of the nonlinear integral equation and compared to the results of linear models. We have found generally good agreement between the results of the nonlinear TF model and the RPA model in doped graphene, apart from Friedel oscillations in the latter model. However, relatively strong nonlinear effects are found in the TF model to persist even at high doping densities and large distances of the external charge.

IR and EPR study of ammonia adsorption effect on silicon nanocrystals

AbstractIR and EPR spectroscopy is used to investigate the effect of adsorption of dry and wet ammonia on the concentration of equilibrium charge carriers in porous silicon layers with various initial types of dopants. It is found by means of IR spectroscopy that only in the presence of water molecules the ammonia adsorption results in an increase in the concentration of free charge carriers in n‐type samples up to a level exceeding 1018 cm–3. In p‐type samples, a nonmonotonic dependence of the charge‐carrier concentration on ammonia pressure is observed. By means of EPR spectroscopy in the atmosphere of wet ammonia the generation of conduction‐band electrons in p‐type silicon nanocrystals and the increase in the electron concentration in n‐type silicon nanocrystals in comparison with the initial level are observed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical study of hydrogen permeation flux in SrCe 0.95Yb 0.05O 3-α and SrCe 0.95Tm 0.05O 3-α

The separation of hydrogen from gas mixtures may be achieved using perovskite membranes doped with rare earth oxides. The separation flux has been studied in such membranes and is based on a model of the proton transfer mechanism. The thermodynamic equilibrium constants ( K w, K i, K ox, K s) and charge carrier mobilities ( μ OH, μ Vo, μ h, μ e) for SrCe 0.95Yb 0.05O 3-α and SrCe 0.95Tm 0.05O 3-α may be estimated by fitting the hydrogen permeation flux model to the actual experimental data for SrCe 0.95Yb 0.05O 3-α and SrCe 0.95Tm 0.05O 3-α. The results indicate that the thermodynamic equilibrium constants K H and K w and the mobility of proton and electrons in SrCe 0.95Tm 0.05O 3-α may be higher than for SrCe 0.95Yb 0.05O 3-α, leading to the higher hydrogen permeation flux in SrCe 0.95Tm 0.05O 3-α. A parametric sensitivity analysis has been performed by using the New Morris Method to investigate the influence of the thermodynamic equilibrium constants and the charge carrier mobilities on the hydrogen permeation flux model. The results indicate that the hydrogen permeation flux model utilised in this study was most sensitive to the internal electronic equilibrium constant ( K i) value and least sensitive to the hole mobility ( μ h).

Effect of ammonia adsorption on charge carriers in mesoporous silicon of n‐ and p‐type conductivity

AbstractInfrared spectroscopy is used to investigate the effect of ammonia adsorption on the concentration of equilibrium charge carriers in porous‐silicon layers with various initial types of dopants at different concentrations. It is found that ammonia adsorption from aqueous solution results in an increase in the concentration of free electrons in n ‐type samples up to a level exceeding 1018 cm–3. In p ‐type samples, a nonmonotonic dependence of the charge‐carrier concentration on ammonia pressure is observed. The obtained results are explained by the appearance of adsorption‐induced shallow donor states that, along with the initial‐dopant and surface‐defect states, determine the charge‐carrier type and concentration in the silicon nanocrystals of the porous layer after ammonia adsorption. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)