Time-resolved Microwave Conductivity Signal Research Articles

Heterojunction of CuO nanoclusters with TiO2 for photo-oxidation of organic compounds and for hydrogen production.

Heterojunctions of small CuO nanoclusters (synthesized by radiolysis) with TiO2 (commercial P25) induced a photocatalytic activity under visible light irradiation in a wide range of wavelengths due to the narrow bandgap of CuO nanoclusters of around 1.7 eV. The optical, chemical, and electrical properties of these composite nanomaterials were studied. The photocatalytic properties of bare and modified TiO2-P25 were studied for water purification (photooxidation of organic compounds such as phenol and 2-propanol) and for hydrogen generation under visible light irradiation. Time resolved microwave conductivity signals showed activation of TiO2 under visible light, proving the injection of electrons from CuO nanoclusters to the conduction band of TiO2-P25. The modified materials showed high photocatalytic activity under visible light. The important role of charge-carriers was demonstrated for both photoreduction and photooxidation reactions.

Silver-modified octahedral anatase particles as plasmonic photocatalyst

Octahedral anatase particles (OAPs) were modified with silver nanoparticles (NPs) by photodeposition method. The properties of OAPs influenced the properties of silver deposits, and thus the photocatalytic activity of the obtained silver-modified OAPs. Photocatalytic activities were tested under UV and vis irradiation for oxidative decomposition of acetic acid and oxidation of 2-propanol, respectively. The properties of silver-modified OAPs were investigated by XRD, STEM, DRS, XPS and time-resolved microwave conductivity (TRMC) method. It was found that electron traps (ETs) worked as nucleation sites for silver, resulting in formation of smaller silver NPs on smaller OAPs with larger content of ETs. The modification with silver resulted in enhanced photocatalytic activity under both UV and vis irradiation. It was found that larger crystallite size of silver NPs, and thus larger polydispersity of silver deposits resulted in broad and intense plasmon resonance peak causing enhanced visible activity. The correlation between photocatalytic activity and TRMC data, e.g., slower decay of TRMC signal for more active samples, allowed discussion on property-governed photocatalytic activities of silver-modified titania.

Charge Generation and Recombination in Diketopyrrolopyrrole Polymer: Fullerene Bulk Heterojunctions Studied by Transient Absorption and Time-Resolved Microwave Conductivity

Charge generation and recombination dynamics in organic photovoltaic bulk heterojunction films comprising the donor polymer, PDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}), blended with the fullerene acceptor, PC71BM ([6,6]-phenyl C71-butyric acid methyl ester), have been studied. The charge-carrier generation process was studied using femtosecond transient absorption, and it was found that the efficiency of charge generation is not dominated by geminate recombination but rather is limited by exciton diffusion in the films. Highly sensitive nanosecond transient absorption (ns-TA) and time-resolved microwave conductivity (TRMC) were used to study charge recombination. From ns-TA measurements, we obtained a recombination rate constant of 1 × 10–9 cm3 s–1 and found that charge recombination is limited by the diffusion of charge carriers (Langevin-type recombination). TRMC signals were comparable with ns-TA on shorter time scales. However, in contr...

Surface Modification of TiO2 with Au Nanoclusters for Efficient Water Treatment and Hydrogen Generation under Visible Light

Small gold nanoparticles (Au-NPs) were used to modify the surface of titanium dioxide as visible-light absorbers and thermal redox active centers. Au-NPs were synthesized on commercial TiO2 (P25) by reduction with tetrakis(hydroxymethyl)phosphonium chloride. The Au/P25 composites were characterized by different techniques including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). Time-resolved microwave conductivity (TRMC) was used to study the charge-carrier dynamics. The photocatalytic activity of Au/TiO2 was evaluated for the degradations of phenol, 2-propanol, and acetic acid and for H2 production from aqueous methanol solution. The modification of TiO2-P25 with Au-NPs with preferential localization on the anatase phase led to an increase in its photocatalytic activity under UV and visible-light. TRMC signals showed the injection of electrons from Au-NPs into the conduction band of TiO...

Enhanced photocatalytic activity of octahedral anatase particles prepared by hydrothermal reaction

Octahedral anatase particles (OAPs) were prepared by hydrothermal reaction (HT) with various experimental conditions, including different amounts of titanate nanowires (TNWs), different water volumes and different pH values, to obtain products with high contents of OAPs. The properties of photocatalysts were investigated by XRD, SEM, TEM, XPS and time-resolved microwave conductivity (TRMC). Photocatalytic activities for oxidative decomposition of acetic acid (CO2 system) and anaerobic dehydrogenation of methanol (H2 system) were tested. It was found that a larger amount and concentration of TNWs, as well as higher pressure during HT, resulted in the formation of smaller crystallites with higher density of mobile electrons. Enhanced photocatalytic activity, achieved for samples with the best morphology (higher content of OAPs), correlated with slower TRMC signal decay, i.e., slower recombination of charge carriers (e−/h+) probably due to lower content of deep electron traps. It was found that properties of platinum nanoparticles, deposited in-situ in the H2 system in the absence or presence of pre-sparged oxygen, and their connection with OAPs were decisive factors for photocatalytic activity.

Morphology-dependent photocatalytic activity of octahedral anatase particles prepared by ultrasonication-hydrothermal reaction of titanates.

Octahedral anatase particles (OAPs) were prepared by an ultrasonication (US)-hydrothermal (HT) reaction of partially proton-exchanged potassium titanate nanowires (TNWs). The structural/physical properties of OAP-containing samples, including specific surface area, crystallinity, crystallite size, particle aspect ratio, composition and total OAP content, were analyzed. Photocatalytic activities of samples were measured under irradiation (>290 nm) for oxidative decomposition of acetic acid (CO2 system) and dehydrogenation of methanol (H2 system) under aerobic and deaerated conditions, respectively. Total density of electron traps (ETs) was measured by double-beam photoacoustic spectroscopy (DB-PAS). Mobility and lifetime of charge carriers (electrons) were investigated by the time-resolved microwave conductivity (TRMC) method. The effects of synthesis parameters, i.e., HT duration, HT temperature and US duration, on properties and photocatalytic activities of final products were examined in detail. The sample prepared with 1 h US duration and 6 h HT duration at 433 K using 267 mg of TNWs in 80 mL of Milli-Q water exhibited the highest photocatalytic activity. It was found that change in HT duration or HT temperature while keeping the other conditions the same resulted in changes in all properties and photocatalytic activity. On the other hand, duration of US treatment, before HT reaction, influenced the morphology of both the reagent (by TNWs breaking) and final products (change in total OAP content); samples prepared with various US durations exhibited almost the same structural/physical properties evaluated in this study but were different in morphology and photocatalytic activity. This enabled clarification of the correlation between morphology and photocatalytic activity, i.e., the higher the total OAP content was, the higher was the level of photocatalytic activity, especially in the CO2 system. Although the decay after maximum TRMC signal intensity (Imax) was almost constant for all samples used in this study, photocatalytic activities were roughly proportional to Imax, which tended to be proportional to total OAP content. Assuming that Imax corresponds to the product of density of electrons in mobile shallow ETs and their mobility, the results suggest that OAP particles have beneficial shallow ETs in higher density and thereby the OAP content governs the photocatalytic activities. Thus, morphology-dependent photocatalytic activity of OAP-containing particles was reasonably interpreted by density of ETs presumably located on the exposed {101} facets.

Contactless photoconductance study on undoped and doped nanocrystalline diamond films.

Hydrogen and oxygen surface-terminated nanocrystalline diamond (NCD) films are studied by the contactless time-resolved microwave conductivity (TRMC) technique and X-ray photoelectron spectroscopy (XPS). The optoelectronic properties of undoped NCD films are strongly affected by the type of surface termination. Upon changing the surface termination from oxygen to hydrogen, the TRMC signal rises dramatically. For an estimated quantum yield of 1 for sub-bandgap optical excitation the hole mobility of the hydrogen-terminated undoped NCD was found to be ∼0.27 cm(2)/(V s) with a lifetime exceeding 1 μs. Assuming a similar mobility for the oxygen-terminated undoped NCD a lifetime of ∼100 ps was derived. Analysis of the valence band spectra obtained by XPS suggests that upon oxidation of undoped NCD the surface Fermi level shifts (toward an increased work function). This shift originates from the size and direction of the electronic dipole moment of the surface atoms, and leads to different types of band bending at the diamond/air interface in the presence of a water film. In the case of boron-doped NCD no shift of the work function is observed, which can be rationalized by pinning of the Fermi level. This is confirmed by TRMC results of boron-doped NCD, which show no dependency on the surface termination. We suggest that photoexcited electrons in boron-doped NCD occupy nonionized boron dopants, leaving relatively long-lived mobile holes in the valence band.

Charge separation and recombination in radial ZnO/In2S3/CuSCN heterojunction structures

A ZnO-nanorod/In2S3/CuSCN radial heterostructure has recently shown promising photovoltaic conversion efficiencies. In this work, the charge separation and recombination in single ZnO/In2S3 and In2S3/CuSCN interfaces as well as the complete ZnO/In2S3/CuSCN structure were studied by time resolved microwave photoconductivity. Photoconductivity transients were measured for different thicknesses of the In2S3 light absorbing layer, under variation in the exciting light flux and before and after annealing of the ZnO nanorods at 450 °C. Upon excitation with 532 nm light, a long lived (millisecond) charge separation at the In2S3/ZnO interface was found, whereas no charge separation was present at the In2S3/CuSCN interface. The presence of the CuSCN hole conductor increased the initial amplitude of the time resolved microwave conductivity signal of the In2S3/ZnO interface by a factor of 8 for a 6 nm thick In2S3 layer, but the enhancement in amplitude dropped strongly for thicker films. The measurements show that the primary charge separation is located at the In2S3/ZnO interface but the charge injection yield into ZnO depends critically on the presence of CuSCN.

The Locus of Free Charge‐Carrier Generation in Solution‐Cast Zn1–xMgxO/Poly(3‐hexylthiophene) Bilayers for Photovoltaic Applications

AbstractThe photoconductivity of solution‐cast Zn1–xMgxO (x=0‐0.4) and poly(3‐hexylthiophene) (P3HT) thin films, and Zn1‐xMgxO/P3HT bilayers is investigated using Time‐Resolved Microwave Conductivity (TRMC) with the aim of determining the locus of free charge carrier generation in the bilayer system. The photoconductivity of Zn1–xMgxO thin films, under illumination with 300 nm laser pulses, is limited by the formation of stable excitons and by scattering of the carriers at grain boundaries. The electron mobility in Zn1–xMgxO films decreases exponentially with Mg concentration, up to x=0.4. In agreement with previous work, free carriers are observed in the P3HT film under illumination with 500 nm pulses in the absence of an acceptor. Under illumination with 500 nm pulses, where only the polymer absorbs, the TRMC signal for the Zn1–xMgxO/P3HT bilayers for x≥0.2 is the same as that of pure P3HT, indicating that free carrier generation in these bilayers occurs predominately by exciton dissociation in the polymer bulk, and not at the interface between the polymer and the solution‐cast oxide. At lower Mg concentrations (x<0.2) the TRMC signal increases with decreasing x following the dependence of the electron mobility in the oxide but its light intensity dependence remains consistent with free carrier generation in the polymer bulk. To explain these results and previously published photovoltaic device data (Adv. Funct. Mater. 2007, 17, 264) we propose that free carrier generation in the bilayers predominantly occurs in the bulk of P3HT, and is followed by electron injection to the oxide to yield photocurrent in photovoltaic cells. The dependence of the TRMC signal of the bilayers on Mg concentration is explained in terms of the yield for free carrier generation in the polymer and the relative contributions of electrons in the oxide and holes in the polymer.

Effect of the Particle Size on the Electron Injection Efficiency in Dye-Sensitized Nanocrystalline TiO2 Films Studied by Time-Resolved Microwave Conductivity (TRMC) Measurements

The efficiency of electron injection from photoexcited dye molecules into TiO 2 nanoparticles with two different sizes (20 or 300 nm) was studied using the time-resolved microwave conductivity (TRMC) technique. For the ruthenium dye (N719) adsorbed on 20 nm TiO2 particles, we have found a near 100% injection efficiency, which is consistent with previous transient optical absorption studies. In contrast, the electron injection efficiency for N719 sensitized on 300 nm TiO2 particles was found to be less than 50%. A similar behavior was found for coumarine-sensitized TiO2 nanoparticle films. The difference is tentatively attributed to differences in the adsorption of dye molecules on the TiO2 surface. Interestingly, only for 20 nm TiO2 particles, a trap filling effect has been observed, that is, at low illumination intensities, the TRMC signal increases more than proportional with the illumination intensity. This is attributed to an increase in electron mobility due to occupation of deep traps. The absence of a significant trap filling effect for 300 nm particles points toward a considerably lower trap density.

A time-resolved microwave conductivity study of the optoelectronic processes in TiO2∣In2S3∣CuInS2 heterojunctions

Photoinduced interfacial charge carrier generation, separation, trapping, and recombination in TiO2∣In2S3∣CuInS2 cells have been studied with time-resolved microwave conductivity (TRMC). Single layer, double layer, and complete triple layer configurations have been studied. Selective electronic excitation in one of the components is accomplished by using monochromatic pulsed laser excitation. In bare CuInS2 films and in TiO2∣CuInS2 double layers, photoinduced charge carriers recombine on a subnanosecond time scale. This fast recombination slows down significantly when an In2S3 buffer layer is applied between TiO2 and CuInS2. In that case, the charge separation lifetime increases by more than one order of magnitude. A superlinear dependence of the TRMC signals on the incident laser intensity is observed for the triple layer configuration, which indicates saturation of electron traps in In2S3 or hole traps in CuInS2. Furthermore, TRMC signals from TiO2∣In2S3∣CuInS2 triple junctions and those from In2S3∣CuInS2 double layers are identical, which shows that charge carrier separation exclusively takes place at the In2S3∣CuInS2 interface.

Increase in the Mobility of Photogenerated Positive Charge Carriers in Polythiophene

We report the increase in the mobility of charge carriers in regioregular poly 3-hexyl thiophene (RR-P3HT) films by mixing them with tetracyanoethylene (TCNE), which is examined by in situ time-resolved microwave conductivity (TRMC) and transient optical spectroscopy (TOS). TCNE acts not only as an electron acceptor which increases the number of charge carriers on photoexposure but also as a functional additive which enhances the mobility of the charge carriers. This conclusion was deduced from the results of fluorescence quenching, transient optical absorption and photobleaching, and comparison of the TRMC signal with the TOS signal. The combination of the TRMC and TOS techniques represents a comprehensive and fully experimental approach to the determination of the intrinsic carrier mobility in conjugated polymers.

Study of transport, trapping and recombination mechanisms in microcrystalline silicon by transient photoconductivity

We have studied different sets of microcrystalline silicon films deposited by PECVD and plasma assisted HWCVD at different thicknesses and microstructures. Measurements by time-resolved microwave conductivity (TRMC) give a mobility μ TRMC, diffusion-induced mobility μ DTRMC and field effect mobility μ FE. At high carrier density, the dependence of the TRMC signal on carrier density is sublinear, with slope γ, indicating dominance of shallow trap recombinations. The decay of the signal shows two slopes τ 1 and τ 2. From the analysis of μ TRMC, μ DTRMC, μ FE, γ, τ 1 and τ 2 for different samples, we discuss the contribution of grains, grain boundaries, and column boundaries to the mobility, to the fast recombination process, and to the lifetime.

Microwave mobility in profiled poly-Si thin films deposited on glass by hot-wire CVD

To study the mobility and lifetime of charge carriers in thin film polycrystalline silicon deposited by hot-wire chemical vapor deposition (HWCVD), time-resolved microwave conductivity (TRMC) measurements have been performed. With this technique it is possible to monitor the change in conductivity on pulsed laser excitation on a nanosecond time-scale, without contacting the layer with electrodes. Pulsed laser excitation has been performed at a laser wavelength of 320 nm. The samples studied are Poly1 (highly defective), Poly2 (device quality) and profiled layers of Poly1 and Poly2. The results for the Poly1 film show that lifetimes of the charge carriers generated on front and back side illumination are similar and below a nanosecond. For Poly2, the mobility in the substrate region (μ=0.17 cm 2/Vs) is more than one order of magnitude lower than in the top region (μ=3.8 cm 2/Vs). The introduction of a thin Poly1 layer, acting as a seed layer for the Poly2, results in a relatively small increase of the mobility in the substrate region compared to standard Poly2, while TRMC signals upon front illumination remain approximately constant.

Transient photoconductivity measurements of ultrasonic spray pyrolyzed tungsten oxide thin films

Tungsten oxide (WO 3) thin films were deposited onto amorphous glass substrates by using the ultrasonic spray pyrolysis (USP) technique. A 0.02 M aqueous ammonium metatungstate solution was pulverized, by means of an ultrasonic metallic nozzle, onto the substrates maintained at 250°C. The films were further annealed at 400°C for various time periods (1, 2, 4, and 6 h). H xWO 3 films were formed by intercalating the as-prepared WO 3 films with H + ions from the H 2SO 4 electrolyte electrochemically. The crystallinity and structure of these films were determined by X-ray diffraction. Transient photoconductivity measurements of these films in the microwave frequency range were performed using the time-resolved microwave conductivity (TRMC) method. From TRMC measurements, the decay time of the photogenerated charge carriers was estimated and the mechanism of the decay processes was correlated with the film composition and structure. The dependence of the amplitude of TRMC signal on room temperature electrical resistivity of the sample is described.

In-situ quality monitoring during the deposition of a-Si:H films

A qualitative correlation between time resolved microwave conductivity (TRMC) and the quality of a-Si:H films is discussed. Aim of this work is to find a more quantitative correlation between TRMC signal features and proposed kinetic mechanisms in a-Si:H. Initial results concerning electron-hole recombination reactions are presented and discussed.

Transient microwave detected photoconductivity as a tool for in-situ evaluation of intrinsic a-Si:H films deposited and annealed at different temperatures

The optoelectronic properties of a-Si:H films grown at different substrate temperatures between 30°C and 250°C were evaluated in-situ by contactless Time Resolved Microwave Conductivity (TRMC) measurements. The transient photoconductivity induced by 532nm laser pulses during the growth of multilayer systems composed of a-Si:H layers deposited at different temperatures made a direct comparison of these films possible. The maximum height of the TRMC signal was observed to decrease strongly with decreasing deposition temperature. Additionally, the annealing of layers deposited at low temperatures was studied by TRMC, stationary photoconductivity, IR and ESR measurements. Whereas a considerable increase of the photoconductivity and the TRMC signal and a decrease in the spin density was observed during annealing at 250°C, the IR transmission spectra remained unchanged.

Material Characterization by Contactless Photoconductivity Measurements

AbstractTransient photoconductivity measurements with the time-resolved microwave conductivity (TRMC) method have been applied to three different types of semiconductors: hydrogenated amorphous silicon, single crystalline silicon and GaAs/GaAlAs multiple quantum well structures. Analysis of the signals in terms of charge carrier-lattice interaction points to an intimate relation between TRMC signals and intrinsic material properties. It is shown that the TRMC-technique is an excellent tool to characterize photosensitive materials.