Trap-limited Mobility Research Articles

The determination of carrier lifetimes and associated mobility magnitudes using photoconductivity recovery dynamics in thin-film amorphous semiconductors

The extraction of trap-limited mobility information using photoconductivity relaxation dynamics in thin-film amorphous semiconductors is evaluated. Simulated photoconductivity dynamics suggest that free carrier decay is characterised by a spectrum of recombination lifetimes, where the spectra are sensitive to the underlying distribution of shallow trap densities and the experimental photoconductivity generation conditions employed. Weighted averaging of the spectral lifetimes is found to return trap-limited lifetime estimates which agree to within 5% of the lifetime values expected from free-to-trapped carrier ratios established under steady-state photoconductivity conditions. The averaged spectral lifetimes may consequently be used with mobility-lifetime magnitudes to determine the associated trap-limited mobility. The analysis procedure is evaluated using photoconductivity data obtained from a set of a-Si:H films deposited over a range of substrate temperatures.

Interpretation of trap-limited mobility in space-charge limited current in organic layers with exponential density of traps

Charge carrier transport in disordered organic semiconductors, performed in electronic devices such as optoelectronic and photovoltaic ones, is usually affected by an exponential distribution of localized states in the band-gap (traps) under space-charge limited current. In this paper, we provide a full analysis for the trap-controlled transport of the single-carrier device in the frequency domain. Trap-limited mobility is interpreted in terms of the classical multiple-trapping picture with one transport state and the trapping-detrapping dynamics of the exponential density of traps. This allows us to provide a suitable explanation of the usual experimental features of the mobility dependence on voltage as along with the capacitance spectra.

Trap-limited mobility in space-charge limited current in organic layers

Space-charge limited current transport in organic devices, relevant to the operation of a range of organic optoelectronic devices, is analyzed in the frequency domain. The classical multiple trapping picture with one transport state and one trap level is used as the basis for the descriptions. By varying the energetic and kinetic properties of the traps, we show that the admittance and the capacitance spectra are considerably modified depending on the interplay between the trap-limited mobility and the trap kinetics. We point out that capacitance steps at low-frequency, usually found in experiments, are observed only for slow traps.

Beyond the quasistatic approximation: Impedance and capacitance of an exponential distribution of traps

Electronic carriers in disordered organic and inorganic semiconductor materials, used in electronic, optoelectronic, and photovoltaic devices, are usually affected by an exponential distribution of localized states in the band gap traps. In this paper we provide a full solution of the relaxation of carriers in traps of such distribution, as a function of frequency and steady-state Fermi level. This includes in a unified treatment both the quasistatic limit, in which the traps modify time constants such as the trap-limited mobility, and the power-law relaxation at high frequency due to detrapping kinetics. We also analyze the combination of trapping with diffusion transport and recombination dynamics in photovoltaic devices. The different features of impedance and capacitance spectra are interpreted and also the analysis of the spectra in order to derive the main material parameters.

The trap-limited diffusivity of electrons in nanoporous semiconductor networks permeated with a conductive phase

The transport of photogenerated electrons in nanocrystalline semiconductor networks permeated with a conducting phase is studied, with a particular emphasis on dye-sensitized nanoporous TiO2 solar cells. We extend the classical approach to the trap-limited mobility according to specific features of the nanoporous configuration: electron transport by diffusion, the capacitive behavior of the nanoporous film and the possible bandshifts due to the charging of surface states. We show that the trap-limited diffusivity, as measured by small-signal techniques, is proportional to the ratio of the conduction-band capacitance and the trap capacitance. These capacitances are defined in terms of a pseudopotential related to the chemical energy of the free electrons, in order to account for possible band unpinning. Several specific distributions of bandgap states are investigated. The dependence of the trap capacitance on the number of free electrons takes the general form Ctrap=An1-a, where 0≤a≤1 depends on the distribution of the traps. The trap-limited diffusivity depends on the number of free electrons as Dn=Bna, and Dn also shows a power-law dependence with the light intensity. We describe the correlation of the electron conductivity with the photovoltage in the solar cell and the photon irradiation intensity.

Decoupling of Transport, Charge Storage, and Interfacial Charge Transfer in the Nanocrystalline TiO2/Electrolyte System by Impedance Methods

Processes in the dark of electron transport and recombination in several nanoporous titanium dioxide films have been studied as a function of the applied potential, using the electrochemical impedance technique. Contact and bulk characteristics have been identified, decoupled, and interpreted, applying a transmission line model that identifies the following elements: (i) the capacitance of the interface between the exposed surface of the substrate and the electrolyte, (ii) the electron transport resistance, (iii) the charge-transfer resistance distributed in the TiO2/electrolyte interface, and (iv) a distributed capacitive element related to charging the porous matrix. The model provides a satisfactory description of the spectra in widely different conditions of conductivity of the TiO2 phase. The electron conductivity has been determined as a function of applied potential and coincides for the different samples under study. Classical electrochemical frameworks of transport and interfacial charge transfer used in porous electrodes do not explain the obtained parameters completely. A more complete framework is suggested to explain the system on the basis of the following characteristic features: (i) existence of a large density of electron traps, giving rise both to a trap-limited mobility and to an exponentially increasing capacitance, and (ii) band-edge shifts under external polarization potential.

Influence on the transport properties of the deposition temperature of a-Si:H films deposited from mixtures of silane in helium at high deposition rates

We study the electronic properties of hydrogenated amorphous silicon (a-Si:H) deposited at high rates (up to 11 Å/s) in a silane-helium mixture containing 60% of helium and compare them with those of “standard” a-Si:H deposited at 250°C in pure silane. For the films obtained under helium dilution in silane, increasing the deposition temperature from 250°C to 350°C leads to improved electronic properties (an increase of the trap-limited mobility along with a decrease in the density of states (DOS) of the conduction band tail), while the characteristic features deduced from photothermal deflection spectroscopy absorption spectra (Urbach tail, deep defect density), which are mainly related to the DOS below midgap, remain unchanged. The changes in the electronic properties are correlated to changes in the hydrogen-related structure of the material.

Influence of hydrogen on electronic transport in dc triode sputtered amorphous silicon

The electron drift mobility μn in dc triode sputtered amorphous hydrogenated silicon (a-Si:H) has been measured using a time of flight experiment. Moreover the product of the mobility by the lifetime (μn τn) has been determined from the steady state photoconductivity. Thus it has been possible to study the influence of hydrogen on both mobility and lifetime. The results indicate that the incorporation of hydrogen in a-Si:H greatly increases the electron lifetime but has little effect on the trap limited mobility. This behavior can be explained in the following way: the H incorporation reduces the number of deep lying localized states, corresponding to dangling bonds and acting as recombination centers, but has little influence on the states in the conduction band tail, which control the trap limited mobility.

Galvanomagnetic Properties of n-Type InSb at Low Temperatures. III. Transport in the Band Tailing and Hopping between Donors in the Magnetic Freeze-Out Regime

The temperature dependence of the Hall coefficient and transeverse magnetoconductivity of n-InSb containing 2–5×10 14 cm -3 donors has been systematically investigated in the magnetic freeze-out regime. Anomalous behavior in the low temperature conduction band transport seperated from the impurity conduction is presented. Evidence is observed for the existence of mobility edge in the conduction band tail states. At high temperatures transport is predominated there by trap-limited mobility, while at lower temperatures by hopping between localized states. Hall mobility at the mobility edge well agrees with Friedman's theory. As for dependence of the impurity conduction on the temperature, magnetic field and donor concentration, a good agreement has been obtained between our experimental result and a recent theoretical prediction on the hopping conduction.