Intensity dependence of the electron diffusion length in dye-sensitised nanocrystalline TiO2 photovoltaic cells

The efficiency of dye-sensitised nanocrystalline solar cells is limited in part by the back reaction of photo-injected electrons with tri-iodide ions present in the electrolyte. Competition between this back reaction and the collection of electrons by diffusion to the substrate contact can be described in terms of the electron diffusion length L_n=(D_nτ_n)^1/2, where D_n is the electron diffusion coefficient and τ_n is the electron lifetime determined by the rate of reaction of electrons with tri-iodide. D_n and τ_n have been determined over five orders of magnitude of illumination intensity using intensity-modulated photocurrent and intensity-modulated photovoltage spectroscopy. It has been found that τ_n decreases with light intensity, whereas D_n increases. As a consequence, the electron diffusion coefficient L_n is only weakly intensity dependent, and the incident photon to current conversion efficiency (IPCE) is predicted to be almost independent of intensity. The experimental IPCE agrees well with the predicted values. The results suggest that the kinetics of the back reaction of electrons with tri-iodide couple may be second order in electron density.     

A novel charge extraction method for the study of electron transport and interfacial transfer in dye sensitised nanocrystalline solar cells

A novel charge extraction method has been developed to study the transport, trapping and back reaction of photogenerated electrons in dye sensitised nanocrystalline cells (Grätzel cells). The cell is illuminated at open circuit until a steady state is reached in which the rates of photogeneration and of back reaction of electrons with tri-iodide are equal. The illumination is then interrupted, and the electron density is allowed to decay for a given time in the dark before short circuiting the cell using a solid state switch. For high efficiency cells, the integrated current measured at short circuit corresponds closely to the remaining electronic charge in the film. Small corrections are required to account for back reaction and substrate charging. The delay time between interruption of the illumination and short circuit charge extraction is varied systematically to follow the decay of electron concentration. Analysis of the time dependence of the electron charge indicates that the back reaction of electrons with I₃⁻ is second order in electron density, which is consistent with the formation of I₂^−. as an intermediate. Simultaneous measurement of the charge and photovoltage decay curves shows that the density of trap states decreases exponentially with trap depth.     

Stereochemically Active Lone Pair Induced Polar Tri-layered Perovskite for Record-Performance Polarized Photodetection

Bulk photovoltaic effect, a promising optoelectronic phenomenon for generating polarized dependent steady-state photocurrent, has been widely applied in various photodetectors. However, incorporating stereochemically active lone pair to construct bulk photovoltage in organic-inorganic hybrid perovskite (OIHP) is still elusive and challenging. Herein, bulk photovoltage (1.2 V) has been successfully achieved by introducing the stereo-chemically active lone pair perovskitizer to construct a polar tri-layered hybrid perovskite, namely, (IBA)₂MHy₂Pb₃Br₁₀ ( 1 , IBA=iso-butylamine, MHy=methylhydrazine). Strikingly, owning to the promising bulk photovoltage, 1 -based detectors exhibit an ultra-highly sensitive polarized photodetection (polarization ratio of up to 24.6) under self-powered mode. This ratio surpasses all the reported two-dimension OIHP single-crystal photodetectors. In addition, detectors exhibit outstanding responsivity (≈200 mA W⁻¹) and detectivity (≈2.4×10¹³ Jones). More excitingly, further investigation confirms that lone pair electrons in MHy⁺ result in the separation of positive and negative charges to produce directional dipoles, which further directional alignment to generate bulk photovoltage, thereby resulting in polarization-dependent photocurrent. Our findings provide a new demonstration for polar multilayer materials’ construction and may open opportunities for a host of high-sensitive polarized photodetection.     

Optoelectronic and nonlinear optical properties of tBu4PcTiO/polymer composite materials

The optoelectronic and nonlinear optical (NLO) properties of a soluble 2,(3)-(tetra-tert-butylphthalocyaninato)titanium(IV) oxide (tBu₄PcTiO) in solutions and in the solid states have been described. The nonlinear response demonstrated that tBu₄PcTiO exhibited strong RSA at 532 nm for both solution and solid-state based experiments. The decrease in the effective intensity dependent nonlinear absorption coefficient with increasing input intensities possibly results from high order triple state transitions of the excited-state population. No evidence of film fatigue or degradation was observed in the PMMA/tBu₄PcTiO film, after numerous scans at varying laser intensity. The doping of tBu₄PcTiO into poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV) results in the apparent increases of the open circuit voltage (Voc) and the short circuit photocurrent density under illumination with 40 mW cm⁻² white-light. The light absorption of tBu₄PcTiO incorporated into polymer represents the dominant contribution to the enhancement of the photocurrent. The dependence of the short circuit photocurrent in an ITO/tBu₄PcTiO-doped MEH-PPV/Al cell on the incident light intensity (I_in) between 30 and 200 mW cm⁻² was also investigated.     

Effect of small additions of carbon nanotubes on the electrical conductivity of polyurethane elastomer

The effect of small (0.002–0.018 wt %) additions of single-walled carbon nanotubes on the dielectric properties and electrical conductivity of crosslinked polyurethane elastomer is studied in the temperature range of 133–453 K and the 10^?3 to 10⁵ Hz range of electric field frequencies. It is shown that the dependence of direct current conductivity σ _dc on temperature deviates significantly from the Arrhenius dependence and is described by the Vogel-Fulcher-Tamman equation σ _dc = σ _dc0exp{?DT ₀/(T ? T ₀)}, where T ₀ is the Vogel temperature and D is the strength parameter. A correlation is found between the nonmonotonic dependences of the glass transition temperature (T _g), D parameter, and σ _dc and the concentration of nanotubes with earlier results for their effects on the physicomechanical characteristics (strength and Young’s modulus) of these systems.     

Dendritic Hematite Nanoarray Photoanode Modified with a Conformal Titanium Dioxide Interlayer for Effective Charge Collection

This paper describes the introduction of a thin titanium dioxide interlayer that serves as passivation layer and dopant source for hematite (α‐Fe₂O₃) nanoarray photoanodes. This interlayer is demonstrated to boost the photocurrent by suppressing the substrate/hematite interfacial charge recombination, and to increase the electrical conductivity by enabling Ti⁴⁺ incorporation. The dendritic nanostructure of this photoanode with an increased solid–liquid junction area further improves the surface charge collection efficiency, generating a photocurrent of about 2.5 mA cm⁻² at 1.23 V versus the reversible hydrogen electrode (vs. RHE) under air mass 1.5G illumination. A photocurrent of approximately 3.1 mA cm⁻² at 1.23 V vs. RHE could be achieved by addition of an iron oxide hydroxide cocatalyst.     

Photosensitization of TiO2 nanoparticulate thin film electrodes by CdS nanoparticles

Surface photovoltage spectra (SPS) measurements of TiO₂ show that a large surface state density is present on the TiO₂ nanoparticles and these surface states can be efficiently decreased by sensitization using CdS nanoparticles as well as by suitable heat treatment. The photoelectrochemical behavior of the bare TiO₂ thin film indicates that the mechanism of photoelectron transport is controlled by the trapping/detrapping properties of surface states within the thin films. The slow photocurrent response upon the illumination can be explained by the trap saturation effect. For a TiO₂ nanoparticulate thin film sensitized using CdS nanoparticles, the slow photocurrent response disappears and the steady-state photocurrent increases drastically, which suggests that photosensitization can decrease the effect of surface states on photocurrent response. Electronic Publication     

Langmuir probe study of the charged particle characteristics in an analytical radio frequency-glow discharge. Roles of discharge conditions and sample conductivity

The application of a tuned Langmuir probe to the measurement of the charged particle characteristics of electron number density, ion number density, electron energy distribution function, average electron energy and electron temperature, in an analytical radio frequency (r.f.)-glow discharge is described. Studies focus on the roles of discharge operating conditions and plasma sampling position for conductive (copper) and nonconductive (Macor) samples. Based on the data obtained here, apparent differences in plasma characteristics between conductive and nonconductive samples can be reasonably explained. For example, the sputtering of conductive samples results in plasmas with obviously higher electron and ion number densities than the sputtering of nonconductive samples (e.g. n_i = 1.8 × 10¹⁰ cm⁻³ and n_e = 1.5 × 10⁹ cm⁻³ for copper, and n_i = 8 × 10⁹ cm⁻³ and n_e = 5 × 10⁸ cm⁻³ for Macor under the conditions of argon pressure = 4 Torr, r.f. power = 30 W and sampling distance = 4.5 mm). Conversely, nonconductive samples yield electrons with higher energies (average electron energies of 15 and 7.5 eV and temperatures of 6.5 and 3.5 eV respectively for the Macor and copper samples). Lower d.c. bias potentials for the case of sputtering nonconductive samples yield reduced sputtering rates and charged particle densities, though the electrons in the latter case have higher energies and thus improved excitation capabilities. The differences between r.f.- and d.c.-glow discharge optical emission spectra are also discussed relative to reported electron energy characteristics. Studies such as these will lay the ground-work for extensive evaluation of inter-matrix type standardization for r.f.-glow discharge atomic emission spectrometry.     

Shallow and deep trap states of solvated electrons in methanol and their formation,electronic excitation,and relaxation dynamics

We present condensed-phase first-principles molecular dynamics simulations to elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons (e_met⁻) in methanol. Excess electrons injected into liquid methanol are most likely trapped by methyl groups, but rapidly diffuse to more stable trapping sites with dangling OH bonds. After localization at the sites with one free OH bond (1OH trapping sites), reorientation of other methanol molecules increases the OH coordination number and the trap depth, and ultimately four OH bonds become coordinated with the excess electrons under thermal conditions. The simulation identified four distinct trapping states with different OH coordination numbers. The simulation results also revealed that electronic transitions of e_met⁻ are primarily due to charge transfer between electron trapping sites (cavities) formed by OH and methyl groups, and that these transitions differ from hydrogenic electronic transitions involving aqueous solvated electrons (e_aq⁻). Such charge transfer also explains the alkyl-chain-length dependence of the photoabsorption peak wavelength and the excited-state lifetime of solvated electrons in primary alcohols.

Condensed-phase first-principles molecular dynamics simulations elucidate the presence of different electron trapping sites in liquid methanol and their roles in the formation, electronic transitions, and relaxation of solvated electrons.     

Interfacial compositions of cationic and mixed non-ionic micelles by chemical trapping: a new method for characterizing the properties of amphiphilic aggregates

A specially synthesized arenediazonium ion bound to amphiphilic aggregates decomposes spontaneously via rate determining loss of N₂ to give a highly reactive, unselective, aryl cation intermediate. This intermediate is trapped competitively by weakly basic nucleophiles in the interfacial region of aggregates such as micelles and other association colloids. Product yields, analyzed by HPLC with UV detection, are used to estimate, simultaneously, the interfacial concentrations of a number of different nucleophiles, including water, that are commonly found at the surfaces of biomembranes and in many commercial products. Two applications of the method are discussed. First, we show that the interfacial concentrations of X⁻ (X=Br, Cl) increase steadily with increasing cetyltrimethylammonium halide (CTAX) and tetramethylammonium halide (TMAX) concentrations and that the interfacial concentrations of these counterions increase continuously with their aqueous phase concentrations at a constant degree of micelle ionization. Interfacial Br⁻ and Cl⁻ concentrations also show marked increases at their respective sphere-to-rod transitions. This steady increase in interfacial counterion concentration with increasing aqueous counterion concentration contradicts a basic assumption of the pseudophase ion exchange (PIE) model of chemical reactivity in aggregates, i.e. that the total concentrations of ions at aggregate interfaces is constant and independent of the amphiphile and salt concentrations. The consequences for the PIE model are discussed. Second, the chemical trapping reaction is used to estimate: (a) distributions of terminal OH groups of non-ionic amphiphiles in mixed non-ionic micelles composed of amphiphiles with different lengths of oligoethylene oxide chains and (b) hydration numbers of the inner layers of interfacial region next to the hydrocarbon core in these mixed micelles. Terminal OH groups distributions are well fitted by a radial one-dimensional random walk model. The average hydration number for the inner layers at 40°C is about 3, in agreement with estimates from NMR water (D₂O) self-diffusion measurements and with the hydration number of 3 for aqueous solutions of polyethylene oxide. The results suggest that the hydration states of the ethylene oxide (EO) units near the micellar core are near their minimum value. Recent and potential applications of the chemical trapping method are briefly discussed.     

Determination of the density and energetic distribution of electron traps in dye-sensitized nanocrystalline solar cells

Electron transport and recombination in dye-sensitized nanocrystalline solar cells (DSCs) are strongly influenced by the presence of trapping states in the titanium dioxide particles, and collection of photoinjected electrons at the contact can require times ranging from milliseconds to seconds, depending on the illumination intensity. A direct method of determining the density and energetic distribution of the trapping states responsible for slowing electron transport has been developed. It involves extraction of trapped electrons by switching the cell from an open circuit to a short circuit after a period of illumination. An advantage of this charge extraction method is that it is less sensitive than other methods to shunting of the DSC by electron transfer at the conducting glass substrate. Results derived from charge extraction measurements on DSCs (with and without compact TiO(2) blocking layers) are compared with those obtained by analysis of the open circuit photovoltage decay.     

Properties of semiconductor electrodes coated with living films of cyanobacteria

IntactPhormidium sp. cells, immobilized on a SnO₂ semiconductor electrode, are capable of transferring electrons to SnO₂ in a light-dependent reaction. Drying a “wet” algal electrode at 50°C for 60 min increases photocurrent output capacity by 100-fold. We have studied the effect of various parameters on photocurrent generation. The magnitude of the photocurrent increased with increasing light intensity and depended on the nature of the electrolyte solution. The output, about 8 μA 10 μg Chl^?1 cm⁺², was obtained using 50 mM H₃BO₃?Na₂CO₃?KCl buffer as an electrolyte, an irradiance (>460 nm) of 250 J/m², and potentiostatic conditions (the algal working electrode was poised at +0.6 V vs a saturated calomel electrode). The yield was more than doubled upon addition of an electron carrier, such as methyl viologen, benzyl viologen, or Vitamin K₃, to the electrolyte solution. Maximum photocurrent was obtained at around pH 8 and 45°C, which are optimal conditions for growth of the cyanobacterium. Furthermore, DCMU, an inhibitor of photosynthetic electron flow, drastically decreased the yield, as did heat treatment of the electrode at 110°C for 15 min. The photocurrent action spectrum peak coincided well with the absorption peak of the light-harvesting pigment, phycocyanin. These results support the idea that electron transfer can occur across algal cell walls from the source of the light-induced reactions located within the lamellar membranes to the semiconductor electrode.     

Preparation and photoelectrocatalytic activity of a nano-structured WO3 platelet film

A tungsten trioxide (WO₃) film was prepared by calcination from a precursor paste including suspended ammonium tungstate and polyethylene glycol (PEG). The ammonium tungstate suspension was yielded by an acid-base reaction of tungstic acid and an ammonium solution followed by deposition with ethanol addition. Thermogravimetric (TG) analysis showed that the TG profile of PEG is significantly influenced by deposited ammonium tungstate, suggesting that PEG is interacting strongly with deposited ammonium tungstate in the suspension paste. X-ray diffraction (XRD) data indicated that the WO₃ film is crystallized by sintering over 400 °C. The scanning electron microscopic (SEM) measurement showed that the film is composed of the nano-structured WO₃ platelets. The semiconductor properties of the film were examined by Mott-Schottky analysis to give flat band potential E_FB=0.30 V vs. saturated calomel reference electrode (SCE) and donor carrier density N_D=2.5×10²² cm⁻³, latter of which is higher than previous WO₃ films by two orders of magnitude. The higher N_D was explained by the large interfacial heterojunction area caused by the nano-platelet structure, which apparently increases capacitance per a unit electrode area. The WO₃ film sintered at 550 °C produced 3.7 mA cm⁻² of a photoanodic current at 1.2 V vs. SCE under illumination with a 500 W xenon lamp due to catalytic water oxidation. This photocurrent was 4.5-12.8 times higher than those for the other control WO₃ films prepared by similar but different procedures. The high catalytic activity could be explained by the nano-platelet structure. The photocurrent was generated on illumination of UV and visible light below 470 nm, and the maximum incident photon-to-current conversion efficiency (IPCE) was 47% at 320 nm at 1.2 V. Technically important procedures for preparation of nano-structured platelets were discussed.     

Photovoltaic properties of Pt/BiFeO3 thin film/fluorine-doped tin oxide capacitor

We study the photovoltaic properties of the Pt/BiFeO₃ (BFO) thin film/fluorine-doped tin oxide capacitor and obtain the open circuit voltage (V _oc) of 0.44 V and short circuit photocurrent (J _sc) of 0.14 mA/cm² under purple laser illumination. As compared to the BFO film with random orientations, the BFO film with a strong preferred orientation exhibits larger photovoltaic output as a result of its larger spontaneous polarization in the unpoled state. We further demonstrate that there is remarkable influence of external electric field poling on the photovoltage in the two polycrystalline BFO films, and the larger net change of the photovoltage after positive and negative poling in the highly preferred oriented BFO film is believed to be due to its improved crystalline structure. Our experiment results indicate that the responsiveness to external electric field and photovoltaic performance of the narrow-band-gap BFO film can be optimized by structural modification, making it possible to apply in the photosensitive and other optoelectronic devices.     

Highly sensitive fluorescence detection of chloride ion in aqueous solution with Ag-modified porous g-C3N4 nanosheets

The porous g-C₃N₄ (PCN) nanosheets are successfully synthesized and further modified with nano-sized Ag by a simple wet-chemical process. Interestingly, the Ag-modified porous g-C₃N₄ (Ag-PCN) nanosheets exhibit competitive fluorescence detection performance of chloride ion (Cl⁻) in aqueous solution. Under the optimized conditions, the concentration of Cl⁻ could be quantitative analyzed with the Ag-PCN in a wide detection range from 0.5 mmol/L to 0.1 mol/L, with a low detection limitation of 0.06 mmol/L. It is confirmed that the fluorescence of PCN could be effectively decayed by the photoinduced charge transfer via the adsorbed Cl⁻ for trapping holes, mainly by means of the time-resolved fluorescence and surface photovoltage spectra. The porous structure and modified Ag promote the adsorption of Cl⁻ on resulting Ag-PCN, leading to excellent fluorescence detection for Cl⁻. This work provides a feasible route to develop a fluorescence detection of Cl⁻ with g-C₃N₄ nanosheets in environment water.     

利用强度调制光电流/光电压谱研究碳点/KOH电解液界面的动力学行为

通过强度调制光电流谱(IMPS)和强度调制光电压谱(IMVS)技术研究在光电分解水制氢体系中碳点光阳极与KOH电解液界面的动力学行为。结果表明,光强在30–90 m W·cm~(-2)范围内,界面的电子传输时间(τ_d)、电子寿命(τ_n)、电子扩散系数(D_n)、电子扩散长度(L_n)均没有变化;当光强增加到110和130 m W·cm~(-2)时,τ_d和τ_n延长,而D_n减小。实验表明,不同于Ti O_2/电解液等界面,碳点光电极/电解液界面中碳点电极存在的缺陷少,因此电子主要以无陷阱限制扩散方式传输为主。且在30–130 m W·cm~(-2)的光强范围内,与τ_d和τ_n相关的载流子收集效率(η_(cc))相近。     

Gas phase ion chemistry of molybdenum hexacarbonyl

In an ion cyclotron resonance (ICR) cell, Mo(CO)_n⁺ ions (n = 0–6), generated by electron ionization (EI) with 70 eV electrons, on collisions with Mo(CO)₆ undergo charge exchange (confirmed by isotopic experiments), collision-induced dissociation (CID), and association reactions to produce Mo_m(CO)_n⁺ ions (m = 1–6). Reactions are essentially complete within 9 s at a pressure of 3 × 10⁻⁹ Torr, as recorded by the manifold ion gauge (uncalibrated); Mo(CO)_n⁺ ions with n = 0–5 have been consumed within this time whereas Mo(CO)₆⁺ ions have achieved a steady concentration. All Mo₂(CO)_n⁺ ions (n = 0–11) were observed: the abundances of dimolybdenum-containing ions with n < 7 decrease at extended reaction times, whereas those with n ≥ 7 remain steady or increase slowly, implying that reactivity decreases with increasing CO content. The major dimers have n = 7, 9, and 10. When subjected to CID the Mo₂(CO)₇⁺ ion yields Mo₂(CO)_n⁺ ions (n = 0–6). Most Mo₃(CO)_n⁺ ions (n = 0–13) were observed, those with n = 9 being formed most readily. Similar observations apply to larger clusters, the most abundant ions being those with CO:Mo ratios of 2–3:1. Mo(CO)_n⁺ ions (n = 0, 3–6) formed by EI with 15 eV electrons are unreactive for reaction times of at least 5 s at the same pressure. General reaction sequences are proposed. Negative ions generated with 70 eV electrons (∼ 90% Mo(CO)₅⁻) are much less reactive but also lead to cluster ion formation on reaction with Mo(CO)₆.     

Ultra-Narrow Depletion Layers in a Hematite Mesocrystal-Based Photoanode for Boosting Multihole Water Oxidation

Significant charge recombination that is difficult to suppress limits the practical applications of hematite (α-Fe₂O₃) for photoelectrochemical water splitting. In this study, Ti-modified hematite mesocrystal superstructures assembled from highly oriented tiny nanoparticle (NP) subunits with sizes of ca. 5 nm were developed to achieve the highest photocurrent density (4.3 mA cm⁻² at 1.23 V vs. RHE) ever reported for hematite-based photoanodes under back illumination. Owing to rich interfacial oxygen vacancies yielding an exceedingly high carrier density of 4.1×10²¹ cm⁻³ for super bulk conductivity in the electrode and a large proportion of ultra-narrow depletion layers (<1 nm) inside the mesoporous film for significantly improved hole collection efficiency, a boosting of multihole water oxidation with very low activation energy (E_a=44 meV) was realized.