Charge Carriers In Thin Films Research Articles

Application of Pump-Probe Technique for Tracking of Charge Carrier Relaxation In Nanostructured Semiconductors

The pump-probe technique is a powerful tool for probing and characterizing the electronic and structural properties of short-lived excited states of materials. Upon the absorption of photons of the pump, excited states of the materials are established. Relaxation of these states reflects many physical aspects of the materials which can be tracked by a consequent beam – the probe. In this study, we present a conventional pump-probe technique at the University of Amsterdam and its application for tracking relaxation of charge carriers in thin films containing Si and Ge nanocrystals embedded in SiO2 matrix. The pump beam is obtained from a 150-fs laser pulse with the photon wavelength at 340 nm. The probe beam is constituted from a white-light beam in the wavelength range from approximately 900 - 1300 nm (0.9 - 1.4 eV). The photon-generated charge carriers feature with multi-exponent decay dynamics, involving to different physical characteristics. The fast decay components of about few ps time scale arise from defect-related trapping or Auger processes, while the slow decay components of about few hundred ps come from relaxation of the exciton left in the semiconductor nanocrystals. The deep-insight characterization of the materials involving to individual relaxation processes are presented and discussed.

Structural Characteristics and Photoluminescence of Thin Films of Cu(In1–xgax)(Syse1–y)2 Solid Solutions

The phase composition and structural and optical characteristics of thin films of Cu(In1–xGax)(SySe1–y)2 solid solutions with the chalcopyrite structure were investigated. The unit-cell constants according to x-ray diffraction analysis were a ~ 5.720 A and c ~ 11.52 A. The elemental compositions were x = Ga/(Ga + In) ~ 0.14 and y = S/(S + Se) ~ 0.11 for Cu(In1–xGax)(S ySe1–y)2 thin films. The optical band gap of Cu(In1–xGax)(S ySe1–y)2 solid solutions determined from measurements of photoluminescence spectra and luminescence excitation spectra at ~4.2 K was Eg ~ 1.122 eV. The mechanism of radiative recombination of nonequilibrium charge carriers in thin films of Cu(In1–xGax) (S ySe1–y)2 solid solutions is discussed.

Scattering of Charge Carriers in Thin Films PbTe:Bi

The calculations of the scattering mechanisms of charge carriers in thin films of PbTe:Bi with 1 at. % of bismuth is carried out. The dominant mechanisms of scattering in the investigated samples are determined. The thickness dependence of the ratio msurf/mbulk is considered.

Characterizing Ion Profiles in Dynamic Junction Light-Emitting Electrochemical Cells

Organic semiconductors have the unique ability to conduct both ionic and electronic charge carriers in thin films, an emerging advantage in applications such as light-emitting devices, transistors, and electrochromic devices, among others. Evidence suggests that the profiles of ions and electrochemical doping in the polymer film during operation significantly impact the performance and stability of the device. However, few studies have directly characterized ion profiles within LECs. Here, we present an in-depth study of the profiles of ion distributions in LECs following application of voltage, via time-of-flight secondary ion mass spectrometry. Ion distributions were characterized with regard to film thickness, salt concentration, applied voltage, and relaxation over time. Results provide insight into the correlation between ion profiles and device performance, as well as potential approaches to tuning the electrochemical doping processes in LECs.

Tuning the Temperature Domain of Phonon Drag in Thin Films by the Choice of Substrate

At low temperatures, in reasonably pure conductors subjected to a thermal gradient, charge carriers (electrons and holes) are swept (dragged) by out of equilibrium phonons, giving rise to a large contribution to the Seebeck coefficient called phonon drag. We demonstrate a spectacular influence of substrate phonons on charge carriers in thin films of Bi2Te3. We show that one can control and tune the position and magnitude of the phonon-drag peak over a wide range of temperatures by depositing thin films on substrates with vastly different Debye temperatures. Our experiments also provide a way to study the nature of the phonon spectrum in thin films, which is rarely probed but clearly important for a complete understanding of thin film properties and the interplay of the substrate and films.

Optical generation of free charge carriers in thin films of tin oxide

The methods of infrared absorption spectroscopy and Raman spectroscopy are used to study nanocrystalline SnOx films (1 ≤ x ≤ 2) prepared by thermal oxidation of metallic tin layers. A monotonic decrease in the transmittance of films in the infrared region has been observed as a result of exposure of the films to light with the wavelength of 380 nm at room temperature. The effect is at a maximum for the samples with x ≈ 2 and is observed for ∼10 min after switching off of illumination. The mentioned variations in optical properties, similarly to those observed in the case of heating of the samples in the dark, are accounted for by an increase in the concentration of free charge carriers (electrons) in nanocrystals of tin dioxide. The data of infrared spectroscopy and the Drude model are used to calculate the concentrations of photogenerated charge carriers (∼1019 cm−3); variations in these concentrations in the course of illumination and after switching off of illumination are determined. Mechanisms of observed photogeneration of charge carriers in SnOx films and possible applications of this effect to gas sensors are discussed.

10.1007/s11451-008-3029-1

The temperature dependences of the parameters of charge carriers in thin films of an electroactive poly(diphenylenephthalide) polymer are investigated in the range preceding the thermally stimulated electronic switching (110–400 K). The thermally stimulated current spectra and current-voltage characteristics are measured in the same temperature range. The parameters of charge carriers are estimated in the framework of the model of injection currents limited by the space charge. It is revealed that the charge carrier mobility decreases with an increase in the temperature in the range from 110 to 400 K. A correlation between the temperature behavior of the current-voltage characteristics and the thermally stimulated current spectra is established. The possible contribution of the Pool-Frenkel effect is considered, and the inference is made that the electric field plays an important role in the thermally stimulated electronic switching.

Tetrafullerene Conjugates for All-Organic Photovoltaics

The synthesis of two new tetrafullerene nanoconjugates in which four C60 units are covalently connected through different pi-conjugated oligomers (oligo(p-phenylene ethynylene) and oligo(p-phenylene vinylene)) is described. The photovoltaic (PV) response of these C60-based conjugates was evaluated by using them as the only active material in organic solar cells, showing a low PV performance. Photophysical studies in solution demonstrated a very fast ( approximately 10 ps) deactivation of the singlet excited state of the central core unit to produce both charge-separated species (i.e., C60*--oligomer+*-(C60)3 and C60 centered singlet excited states). The charge-separated state recombines partly to the C60 centered singlet state that undergoes subsequent intersystem crossing. Photophysical studies carried out in films support these data, exhibiting long-lived triplet excited states. For both tetrafullerene arrays, the low yield of long-lived charge carriers in thin films accounts for the limited PV response. On the contrary, utilizing the oligo(p-phenylene vinylene) centered precursor aldehyde as an electron donor and antennae unit and mixing with the well-known C60 derivative PCBM, the photophysical studies in films show the formation of long-lived charges. The PV devices constructed from these mixtures showed a relatively high photocurrent of 2 mA cm(-2). The sharp contrast between the nanoconjugates and the physical blends tentatively was attributed to improved charge dissociation and the collection of more favorable energy levels in the blends as a result of partial aggregation of both of the components.

Estimation of the parameters of charge carriers in polymers in the vicinity of the threshold of thermally stimulated switching

The temperature dependences of the parameters of charge carriers in thin films of an electroactive poly(diphenylenephthalide) polymer are investigated in the range preceding the thermally stimulated electronic switching (110–400 K). The thermally stimulated current spectra and current-voltage characteristics are measured in the same temperature range. The parameters of charge carriers are estimated in the framework of the model of injection currents limited by the space charge. It is revealed that the charge carrier mobility decreases with an increase in the temperature in the range from 110 to 400 K. A correlation between the temperature behavior of the current-voltage characteristics and the thermally stimulated current spectra is established. The possible contribution of the Pool-Frenkel effect is considered, and the inference is made that the electric field plays an important role in the thermally stimulated electronic switching.

Comparison of Charge‐Carrier Transport in Thin Films of Spiro‐Linked Compounds and Their Corresponding Parent Compounds

AbstractThe charge‐transport properties of the spiro‐linked compounds 2,2′,7,7′‐tetrakis(diphenylamino)‐9,9′‐spirobifluorene, 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methylphenylamino)‐9,9′‐spirobifluorene, 2,2′,7,7′‐tetra(m‐tolyl‐phenylamino)‐9,9′‐spirobifluorene, and 2,2′,7,7′‐tetra(N‐phenyl‐1‐naphthylamine)‐9,9′‐spirobifluorene, and their corresponding parent compounds, N,N,N′,N′‐tetraphenylbenzidine, N,N,N′,N′‐tetrakis(4‐methylphenyl)benzidine, and N,N′‐bis(3‐methylphenyl)‐(1,1′‐biphenyl)‐4,4′‐diamine, N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)‐1,1′‐biphenyl‐4,4′‐diamine, are investigated. The field‐effect mobilities of charge carriers in thin films of the parent compounds are slightly higher than those of the spiro‐linked compounds. However, the transistor action of the parent‐compound thin films vanishes because the films crystallize after being stored in ambient atmosphere for a few days. In contrast, the hole mobilities in thin films of the spiro‐linked compounds do not change significantly after the samples are stored in ambient atmosphere for up to nine months. Also discussed is the temperature dependency of the mobilities of charge carriers, which is presented using two models, namely the Arrhenius and the Gaussian disorder models.

Time-dependent Langevin-type bimolecular charge carrier recombination in regiorandom poly(3-hexylthiophene)

We have studied transport and recombination of photogenerated charge carriers in thin films of regiorandom poly(3-hexylthiophene) (RRaPHT). Time-of-Flight studies show space charge perturbed current transients with a saturation of current level at high light intensities as a result of diffusion controlled Langevin-type bimolecular charge carrier recombination. We have been able to measure the time-dependent charge carrier mobility by using the carrier extraction by linearly increasing voltage technique. We found that the hole mobility follows a power law dependence over a few orders of magnitude in time as μ = μ 0 t −0.42. By using the carrier extraction by pulsed voltage technique we measured the photogenerated charge carrier concentration decay as a function of delay time and found that the decay can be fitted using a dispersive time-dependent bimolecular recombination coefficient governed by the time-dependent hole mobility.

Anisotropy in the Mobility and Photogeneration of Charge Carriers in Thin Films of Discotic Hexabenzocoronenes, Columnarly Self‐Assembled on Friction‐Deposited Poly(tetrafluoroethylene)

Discotic derivatives of hexabenzocoronene macroscopically self‐align on friction‐deposited poly(tetrafluoroethylene) (see Figure for polarization microscopy image of a partially coated sample). Such films display highly anisotropic photoconductivity with charge transport along the columnar director favored by a factor of at least 20. The anisotropy in the optical absorption depends on the tilt angle of the aromatic core in the columns.

Influence of π-conjugation length on mobilities of charge carriers in conducting polymers

Mobilities of charge carriers in thin films of a series of copolymers with repeat units consisting of oligothiophenes bridged by Si atoms are measured over a range of oxidation stages in order to study the influence of π-conjugation length on the mobility. The number of thiophene units in oligothiophenes ranges from five to nine, the polymers being designated accordingly by PS5T to PS9T. Mobilities of charge carriers in these polymer films, where intrachain hopping transport is restricted within each oligothienylene unit, are constant in the low doping regions below 1%. As the doping level increases, mobilities for polymers with longer π-conjugation lengths start to rise at lower doping levels. The mobility enhancement follows also an increasing order of the π-conjugation length, although the 30 times enhancement of mobility for PS9T is still smaller than the four orders-of-magnitude increase of mobility observed earlier for electrochemically synthesized poly(3-methylthiophene). The results are discussed with electrochemical and spectroelectrochemical measurements from the viewpoint of the involvement of polarons, π-dimers, and bipolarons as possible charge carriers in the conduction mechanisms.

Effect of fullerene on the photogeneration and transport of charge carriers in triphenylamine-containing polyimides

Based on studies of the quantum yield and drift mobility of charge carriers in thin films of fullerene-containing dye-sensitized polyimides performed in wide ranges of electric-field strengths ((1–10)×105 V cm−1) and temperatures (77–350 K), a mechanism of the interaction of fullerene molecules with aromatic fragments of polyimides and dye anions is suggested. It was found that fullerene, being an efficient sensitizer of the photoeffect in aromatic triphenylamine-containing polyimides, reduces by more than an order of magnitude the photosensitivity and mobility of charge carriers in dye-sensitized polyimides, restoring these parameters to values typical of unsensitized polyimides.

Study of scattering of charge carriers in thin films of (Bi 0.25Sb 0.75) 2Te 3 alloy with 2% excess Te

Thin films of (Bi 0.25Sb 0.75) 2Te 3 with 2% excess Te of different thicknesses were prepared by the flash evaporation technique. Electrical resistivity and thermoelectric power were measured for different thickness films and at different temperatures. Applying Jain–Verma theory of carrier energy dependent relaxation time, thermoelectric data of thin films were analysed to understand the nature of scattering mechanisms in this thermoelectric material. The scattering parameter b was calculated from the thermoelectric and resistivity data. This gives us an indication about the nature of scattering processes in the before mentioned composition thin films. It is found from such an analysis of the thermoelectric data of thin films of the alloy with 2% excess Te that the scattering index parameter b varies in this material thin films from −0.2 to −0.1 and to positive values at higher temperatures.

Charge transport in hopping systems under open-circuit conditions: computersimulation

Discharge under open-circuit conditions may be often used to find the driftmobility of charge carriers in thin films of low-mobility materials. Theexisting theories of the discharge assume domination of charge transport inthe conduction band. In order to find whether the theories may be used forinterpretation of experimental results in hopping systems a computersimulation was carried out. It has been shown that direct usage of theexisting theories may lead to wrong values of drift mobility found from thedischarge. The method of estimation of real value of mobility in thetemperature range 100-500 K for various values of the decay parameter of theelectron localized wavefunction and for various standard deviations of theenergetic distribution of the localized states is presented in this paper.

Properties of CdS nanocrystallites embedded in to thin ZrO 2 waveguides

In this paper we present results on CdS nanocrystal (NC) formation in ZrO 2 thin film grown by sol–gel process. Raman and waveguiding Raman scattering spectroscopies were applied to determine CdS nanocrystal precipitation in our films. RBS (Rutherford back scattering) and optical absorption spectroscopy were applied to determine CdS distribution homogeneity through the layer depth, NC size and dispersion. Thus we obtained low size dispersed CdS nanocrystals with preparation temperature controlled size. Moreover, Raman and high-resolution transmission electron microscopy (HRTEM) evidenced simultaneous correlated crystallisation of hexagonal CdS and tetragonal/monoclinic ZrO 2 phases. We applied also light induced dynamic grating technique under high excitation conditions in order to analyse dynamic properties of nonequilibrium charge carriers in thin films.

Diffusion behaviour of charge carriers in thin films of phthalocyanines

Phthalocyanine (Pc) thin films were prepared by the vacuum-evaporating method. The diffusion behaviour of the charge carriers in these films was investigated by means of transient photovoltage (Dember effect) measurements. The experiments imply that the dominant charge carriers in phthalocyanines are electrons which diffuse from the surface closest to the light to the internal bulk. As a comparative system, photovoltic cells of Pc were also investigated. There was a close relationship between the short-circuit photocurrent ( I sc ) of the devices and the maximum transient Dember photovoltage ( V max ).

Charge carrier photogeneration in poly(methylphenylsilylene) and its derivatives with π-conjugated side groups

The photogeneration of charge carriers in thin films of poly(methylphenylsilylene) (PMPSi) and its derivatives with attached pendant π-conjugated chromophores was investigated as a function of the electric field and temperature by the photoinduced discharge current method. Six new substituted polysilylenes were used in this study. The electric field dependence of photogeneration efficiency suggests that the Onsager theory of geminate recombination is a suitable model for the description of the electron-hole pair dissociation during the photogeneration process in these polymers. The most effective separation distances of electron-hole pairs participating in the photogeneration process were found to be 1.8–3.5 nm and 0.9–1.75 nm for low and high electric fields, respectively.