Slower Recombination Rate Research Articles

Rotational dynamics of organic cations in the CH3NH3PbI3 perovskite.

Methylammonium lead iodide (CH3NH3PbI3) based solar cells have shown impressive power conversion efficiencies of above 20%. However, the microscopic mechanism of the high photovoltaic performance is yet to be fully understood. Particularly, the dynamics of CH3NH3(+) cations and their impact on relevant processes such as charge recombination and exciton dissociation are still poorly understood. Here, using elastic and quasi-elastic neutron scattering techniques and group theoretical analysis, we studied rotational modes of the CH3NH3(+) cation in CH3NH3PbI3. Our results show that, in the cubic (T > 327 K) and tetragonal (165 K < T < 327 K) phases, the CH3NH3(+) ions exhibit four-fold rotational symmetry of the C-N axis (C4) along with three-fold rotation around the C-N axis (C3), while in the orthorhombic phase (T < 165 K) only C3 rotation is present. At around room temperature, the characteristic relaxation times for the C4 rotation are found to be τC4 ≈ 5 ps while for the C3 rotation τC3 ≈ 1 ps. The T-dependent rotational relaxation times were fitted with Arrhenius equations to obtain activation energies. Our data show a close correlation between the C4 rotational mode and the temperature dependent dielectric permittivity. Our findings on the rotational dynamics of CH3NH3(+) and the associated dipole have important implications for understanding the low exciton binding energy and a slow charge recombination rate in CH3NH3PbI3 which are directly relevant for the high solar cell performance.

Ultraslow recombination in AOT-capped TiO2 nanoparticles sensitized by protoporphyrin IX.

Aerosol OT (AOT) capped TiO2 nanoparticles have been prepared by a phase transfer mechanism. The TiO2 nanoparticles have a diameter of 5-10 nm, are highly crystalline (anatase) and show high photoluminescence. They are effectively sensitized by protoporphyrin IX (PPIX) and show high electron injection rates while the rate of back recombination is much slower than those reported previously. Thus the AOT capped TiO2 nanoparticles synthesized in this work are highly effective not only in promoting ultrafast electron injection from PPIX to TiO2 but more importantly they lead to extremely slow back recombination rates. The significance of this work is in the synthesis of highly photoluminescent TiO2 nanoparticles which can be easily sensitized by a porphyrin dye, whereby ultraslow recombination is observed.

Electron Extraction Dynamics in CdSe and CdSe/CdS/ZnS Quantum Dots Adsorbed with Methyl Viologen

Semiconductor quantum dots (QDs) composed of multiple components are playing an important role in solar energy conversion as light harvesting materials. Electron extraction dynamics in CdSe and core/shell CdSe/CdS/ZnS colloidal QDs are studied by femtosecond transient absorption spectroscopy in this Article. Our study demonstrates that, in the presence of the commonly used electron acceptor, methyl viologen (MV2+), electrons in the 1S state of CdSe QDs can be effectively extracted with a time constant less than 150 fs. With regard to type I core/shell CdSe/CdS/ZnS QDs, 400 nm excitation will mainly populate the CdS first, due to its large absorption cross section at around that wavelength. Electrons from the conduction band of CdS then can be directly extracted by MV2+ before transferring to core CdSe. Therefore, MV2+ can serve as an efficient bridge to extract electrons from the shell of type I QDs. As compared to the bare QDs, core/shell QDs have slower charge separation and much slower recombination rates. Thus, the core/shell QDs are beneficial for designing solar cells.

Fast Light-Emitting Silicon-Germanium Nanostructures

Results obtained from photoluminescence (PL) measurements performed on Si/Si <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{1\hbox{-}x}$</tex></formula> Ge <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{x}$</tex> </formula> nanostructures with a single Si <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{1\hbox{-}x}$</tex></formula> Ge <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{x}$</tex></formula> nanometer-thick layer (NL) incorporated into Si/Si <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{0.6}$</tex></formula> Ge <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$_{0.4}$</tex></formula> cluster multilayers (CMs) are reviewed. By employing depth-dependent pulsed laser excitation, the SiGe NL PL decay is found to be nearly a 1000 times faster compared to that in CM PL. A physical model taking into consideration the Si/SiGe hetero-interface composition is proposed to explain the fast and slow time-dependent recombination rates.

Low-threshold stimulated emission using colloidal quantum wells.

The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 μJ/cm(2) and gain as high as 600 cm(-1), both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths.

Carrier recombination in tailored multilayer Si/Si1−xGex nanostructures

Photoluminescence (PL) measurements were performed in Si/Si1−xGex nanostructures with a single Si0.92Ge0.08 nanometer-thick layer incorporated into Si/Si0.6Ge0.4 cluster multilayers. Under pulsed laser excitation, the PL decay associated with the Si0.92Ge0.08 nano-layer is found to be nearly a 1000 times faster compared to that in Si/Si0.6Ge0.4 cluster multilayers. A model considering Si/SiGe hetero-interface composition and explaining the fast and slow time-dependent recombination rates is proposed.

On the origin of dielectric properties of water

The dielectric spectrum of liquid water, $10^{4} - 10^{11}$ Hz, is interpreted in terms of diffusion of charges, formed as a result of self-ionization of H$_{2}$O molecules. This approach explains the Debye relaxation and the dc conductivity as two manifestations of this diffusion. The Debye relaxation is due to the charge diffusion with a fast recombination rate, $1/\tau_{2}$, while the dc conductivity is a manifestation of the diffusion with a much slower recombination rate, $1/\tau_{1}$. Applying a simple model based on Brownian-like diffusion, we find $\tau_{2} \simeq 10^{-11}$ s and $\tau_{1} \simeq 10^{-6}$ s, and the concentrations of the charge carriers, involved in each of the two processes, $N_{2} \simeq 5 \times 10^{26}$ m$^{-3}$ and $N_{1} \simeq 10^{14}$ m$^{-3}$. Further, we relate $N_{2}$ and $N_{1}$ to the total concentration of H$_{3}$O$^{+}$--OH$^{-}$ pairs and to the pH index, respectively, and find the lifetime of a single water molecule, $\tau_{0} \simeq 10^{-9}$ s. Finally, we show that the high permittivity of water results mostly from flickering of separated charges, rather than from reorientations of intact molecular dipoles.

Tripartite Layered Photoanode from Hierarchical Anatase TiO2 Urchin-Like Spheres and P25: A Candidate for Enhanced Efficiency Dye Sensitized Solar Cells

A trilaminar layer photoanode for dye-sensitized solar cells (DSSCs) was constructed using urchin-like TiO2 hierarchical microspheres and P25. The top layer made of hierarchical microspheres enhanced light scattering; the middle layer consisting of P25 and as-prepared microspheres is a multifunctional layer for DSSCs, high adsorption ability to the dye, light scattering ability, and slow recombination rates coexistence; the bottom layer used P25. The DSSCs based on the photoanode with tripartite-layers structure exhibited a much higher short-circuit photocurrent density of 18.97 mA cm–2 and energy conversion efficiency of 8.80%, which indicated a 36% increase in the conversion efficiency compared to those of the P25 electrode (14.51 mA cm–2, 6.50%). The great improvements of photocurrent density and energy conversion efficiency for hierarchical TiO2 microspheres were mainly attributed to a considerable surface area, a higher light scattering ability, and slower electron recombination rates for the former.

An improved semiclassical theory of radical pair recombination reactions

We present a practical semiclassical method for computing the electron spin dynamics of a radical in which the electron spin is hyperfine coupled to a large number of nuclear spins. This can be used to calculate the singlet and triplet survival probabilities and quantum yields of radical recombination reactions in the presence of magnetic fields. Our method differs from the early semiclassical theory of Schulten and Wolynes [J. Chem. Phys. 68, 3292 (1978)] in allowing each individual nuclear spin to precess around the electron spin, rather than assuming that the hyperfine coupling-weighted sum of nuclear spin vectors is fixed in space. The downside of removing this assumption is that one can no longer obtain a simple closed-form expression for the electron spin correlation tensor: our method requires a numerical calculation. However, the computational effort increases only linearly with the number of nuclear spins, rather than exponentially as in an exact quantum mechanical calculation. The method is therefore applicable to arbitrarily large radicals. Moreover, it approaches quantitative agreement with quantum mechanics as the number of nuclear spins increases and the environment of the electron spin becomes more complex, owing to the rapid quantum decoherence in complex systems. Unlike the Schulten-Wolynes theory, the present semiclassical theory predicts the correct long-time behaviour of the electron spin correlation tensor, and it therefore correctly captures the low magnetic field effect in the singlet yield of a radical recombination reaction with a slow recombination rate.

Dynamic Hydrogen Production from Methanol/Water Photo-Splitting Using Core@Shell-Structured CuS@TiO2Catalyst Wrapped by High Concentrated TiO2Particles

This study focused on the dynamic hydrogen production ability of a core@shell-structured CuS@TiO2photocatalyst coated with a high concentration of TiO2particles. The rectangular-shaped CuS particles, 100 nm in length and 60 nm in width, were surrounded by a high concentration of anatase TiO2particles (&gt;4~5 mol). The synthesized core@shell-structured CuS@TiO2particles absorbed a long wavelength (a short band gap) above 700 nm compared to that pure TiO2, which at approximately 300 nm, leading to easier electronic transitions, even at low energy. Hydrogen evolution from methanol/water photo-splitting over the core@shell-structured CuS@TiO2photocatalyst increased approximately 10-fold compared to that over pure CuS. In particular, 1.9 mmol of hydrogen gas was produced after 10 hours when 0.5 g of 1CuS@4TiO2was used at pH = 7. This level of production was increased to more than 4-fold at higher pH. Cyclic voltammetry and UV-visible absorption spectroscopy confirmed that the CuS in CuS@TiO2strongly withdraws the excited electrons from the valence band in TiO2because of the higher reduction potential than TiO2, resulting in a slower recombination rate between the electrons and holes and higher photoactivity.

Modeling of the shape of infrared stimulated luminescence signals in feldspars

This paper presents a new empirical model describing infrared (IR) stimulation phenomena in feldspars. In the model electrons from the ground state of an electron trap are raised by infrared optical stimulation to the excited state, and subsequently recombine with a nearest-neighbor hole via a tunneling process, leading to the emission of light. The model explains the experimentally observed existence of two distinct time intervals in the luminescence intensity; a rapid initial decay of the signal followed by a much slower gradual decay of the signal with time. The initial fast decay region corresponds to a fast rate of recombination processes taking place along the infrared stimulated luminescence (IRSL) curves. The subsequent decay of the simulated IRSL signal is characterized by a much slower recombination rate, which can be described by a power-law type of equation. Several simulations of IRSL experiments are carried out by varying the parameters in the model. It is found that the shape of the IRSL signal is remarkably stable when the kinetic parameters are changed within the model; this is in agreement with several previous studies of these signals on feldspars, which showed that the shape of the IRSL curves does not change significantly under different experimental conditions. The relationship between the simulated IRSL signal and the well-known power-law dependence of relaxation processes in solids is also explored, by fitting the IRSL signal at long times with a power-law type of equation. The exponent in this power-law is found to depend very weakly on the various parameters in the model, in agreement with the results of experimental studies. The results from the model are compared with experimental IRSL curves obtained using different IR stimulating power, and good quantitative agreement is found between the simulation results and experimental data. ► A new empirical model describing infrared stimulation phenomena in feldspars. ► The shape of the IRSL signal is remarkably stable when the parameters are changed. ► The simulated IRSL signal follows the power-law of relaxation processes in solids. ► The simulated exponent of the power-law depends very weakly on the intrinsic properties of the crystal. ► The simulated results are compared with experimental data for different stimulating IR power.

Optical and electrical applications of ZnSxSe1−x nanowires-network with uniform and controllable stoichiometry

Single crystalline ternary ZnS(x)Se(1-x) nanowires with uniform chemical stoichiometry and accurately controllable compositions (0≤x≤ 1) were synthesized through a simple and yet effective one-step approach with a specially designed modification. Energy-gap-tuning via compositional change was achieved for a direct band gap from 2.6 to 3.6 eV. Raman spectroscopy studies revealed typical two-mode behavior indicative of high miscibility in the alloyed compound. Moreover, the enhanced electrical-conductivity and gating effect behavior after the formation of ternary alloy enable their application in nano/micro-field effect transistor devices. In addition, the slow recombination rate in the photo-response process indicates their potential for photoelectric applications.

The Effect of Hole Transport Material Pore Filling on Photovoltaic Performance in Solid‐State Dye‐Sensitized Solar Cells

AbstractA detailed investigation of the effect of hole transport material (HTM) pore filling on the photovoltaic performance of solid‐state dye‐sensitized solar cells (ss‐DSCs) and the specific mechanisms involved is reported. It is demonstrated that the efficiency and photovoltaic characteristics of ss‐DSCs improve with the pore filling fraction (PFF) of the HTM, 2,2’,7,7’‐tetrakis‐(N, N ‐di‐ p ‐methoxyphenylamine)9,9’‐spirobifluorene(spiro‐OMeTAD). The mechanisms through which the improvement of photovoltaic characteristics takes place were studied with transient absorption spectroscopy and transient photovoltage/photocurrent measurements. It is shown that as the spiro‐OMeTAD PFF is increased from 26% to 65%, there is a higher hole injection efficiency from dye cations to spiro‐OMeTAD because more dye molecules are covered with spiro‐OMeTAD, an order‐of‐magnitude slower recombination rate because holes can diffuse further away from the dye/HTM interface, and a 50% higher ambipolar diffusion coefficient due to an improved percolation network. Device simulations predict that if 100% PFF could be achieved for thicker devices, the efficiency of ss‐DSCs using a conventional ruthenium‐dye would increase by 25% beyond its current value.

Photoinduced charge separation and recombination in solution and in gels of a Pt(II) terpyridyl–naphthalene diimide complex

Light induced electron transfer and subsequent recombination in a covalently linked terpyridyl Pt(II)/naphthalene diimide (ND) donor acceptor complex are discussed. The complex consists of the ND moiety covalently linked to a terpyridine ligand through a tolyl linkage (ND-tpy) with chloride serving as the fourth ligand of the Pt(II) center ([(ND-tpy)PtCl]). Photolysis of the complex in acetonitrile solution results electron transfer from the Pt(II) complex to the ND unit. This is followed by a significant structural change in the oxidized Pt complex/reduced ND that leads to formation of a charge separated species that decays by back electron transfer with a rate constant of 8 × 10 4 s −1, a much slower recombination rate than other Pt(II) donor/organic acceptor systems. The complex forms gels when concentrated DMSO solutions are heated and allowed to cool slowly. The gels have long tubular structures and the dried gels exhibit long range order of the Pt centers, as evidenced by X-ray powder diffraction. Photolysis of the gels suspended in solution leads to formation of long lived radical ions, formed presumably from self-exchange electron hopping between stacked donor and acceptor moieties in the gels.

Long-lived photoinduced charge separation in new Ru(bipyridine)32+—phenothiazine dyads

Synthesis and photophysical properties of three Ru(bpy)(3)(2+)-Ptz (bpy = 2,2'-bipyridine and Ptz = phenothiazine) dyads, where the number of Ptz groups increased from one to three, are reported. The MLCT absorption bands of these compounds were slightly red shifted compared to Ru(bpy)(3)(2+). The emission, however, was highly quenched and this is attributed to electron transfer from the Ptz moiety to the excited Ru(bpy)(3)(2+) to generate the charge separated state Ru(bpy)(3)(+)-Ptz (+). Observed electron transfer rates (k(et) > 10(8) s(-1)) were much faster than those previously reported (k(et) < 10(7) s(-1)) for linked Ru(bpy)(3)(2+)-Ptz systems. Compared to the previous systems, back electron transfer rates in these systems were about 100 times slower. This has enabled us to observe the charge separated state in nanosecond flash photolysis experiments. Transient absorptions assignable to Ru(bpy)(3)(+) and Ptz (+), having lifetimes in the range of 10-30 ns were observed. In order to explain the fast charge separation and slow charge recombination rates, formation of a folded conformer where the Ptz group attached to one bpy residue comes closer to and associates with another bpy moiety was invoked. A scheme which explains the fast electron transfer and slow recombination in this pre-associated state is proposed.

Theoretical Study of Adsorption of O(3P) and H2O on the Rutile TiO2(110) Surface

The adsorption of oxygen atoms O(3P) on both ideal and hydrated rutile TiO(2)(110) surfaces is investigated by periodic density functional theory (DFT) calculations within the revised Perdew-Burke-Ernzerhof (RPBE) generalized gradient approximation and a four Ti-layer slab, with (2 x 1) and (3 x 1) surface unit cells. It is shown that upon adsorption on the TiO(2) surface the spin of the O atom is completely lost, leading to stable surface peroxide species on both in-plane and bridging oxygen sites with O-binding energies of about 1.0-1.5 eV, rather than to the kinetically unstable terminal Ti-O and terminal O-O species with smaller binding energies of 0.1-0.7 eV. Changes in O-atom coverage ratios between 1/3 and 1 molecular layer (ML) and coadsorption of H(2)O have only minor effects on the O-binding energies of the stable peroxide configurations. High O-atom diffusion barriers of about 1 eV are found, suggesting a slow recombination rate of adsorbed O atoms on TiO(2)(110). Our results suggest that the TiOOTi peroxide intermediate experimentally observed in photoelectrolysis of water should be interpreted as a single spinless O adatom on TiO(2) surface rather than as two Ti-O* radicals coupled together.

Electrical characteristics due to differences in crystal damage induced by various implant conditions

Small differences in initial implant damage change electrical characteristics on devices in spite of implantation at the same dose and energy. Typically, Rs shifts are easily found if a beam current changes. It can be mentioned specially that this phenomenon tends to become more significant for nodes beyond 90nm than for the larger scale device generations. In some cases, it can cause troubles in the process control. These phenomena can be explained by competition between the quantity of ion cascades and the relatively slow (millisecond order) recombination rate of interstitial silicon atoms and vacancies at a temperature of lower than 80°C. This phenomenon is more common for heavier ions; arsenic and BF2.

The resistance anomaly in the surface layer of Bi 2Sr 2CaCu 2O 8+ x single crystals under radio-frequency irradiation

We observed that radio-frequency (rf) irradiation significantly enhances the c-axis resistance near and below the superconducting transition of the CuO 2 layer in contact with a normal-metal electrode on the surface of Bi 2Sr 2CaCu 2O 8+ x single crystals. We attribute the resistance anomaly to the rf-induced charge imbalance nonequilibrium effect in the surface CuO 2 layer. The relaxation of the charge-imbalance in this highly anisotropic system is impeded by the slow quasiparticle recombination rate, which results in the observed excessive resistance.

Ionization measurements in optically pumped discharges

The kinetics of ionization and electron removal in opticallypumped non-equilibrium plasmas sustained by a CO laser are studied usingnon-self-sustained dc and RF electric discharges. Experiments in opticallypumped CO/Ar/N2 mixtures doped with O2 and NOdemonstrated that associative ionization of CO produces free electrons at arate up to S = 1015 cm-3 s-1. The ionization ratecoefficient, inferred from the CO vibrational population measurements, iskion = (1.1-1.8)×10-13 cm3 s-1. It isshown that excited NO and possibly O2 molecules also contribute tothe vibrationally stimulated ionization process. In a CO/Ar plasma,applying a dc bias to the cell electrodes resulted in the rapidaccumulation of a deposit on the negative electrode due to a large clusterion current. The average mass of an ion in this plasma, estimated bymeasuring the mass of the deposit, is m≅250 amu, which isconsistent with the mass spectrometer analysis of the deposit. The depositdid not accumulate when small amounts of O2 and NO were added tothe CO/Ar plasma, which presumably indicates the destruction of the clusterions.It is demonstrated that adding small amounts of O2 to theoptically pumped CO/Ar plasmas significantly increases the electrondensity, from ne = (4-7)×109 cm-3 tone = (1-2)×1011 cm-3. This effect occurs at anearly constant (within 50%) electron production rate S, indicating asubstantial reduction in the overall electron removal rate. This reductioncan be qualitatively interpreted as the destruction of rapidly recombiningcluster ions in the presence of the O2 additive, and theirreplacement by monomer ions with a slower recombination rate. Furtherstudies of the ion composition in optically pumped plasmas are suggested.

Lyman‐Alpha Absorption Lines from Minipancakes

Recent numerical simulations show that many Lyα absorption lines of column densities NH I 1015 cm-2 are produced in transient minipancakes. Such pancakes are modeled here, approximating the initial perturbation leading to the formation of the pancake as a single sinusoidal wave. The density and temperature profiles of the gas in the pancake are determined in the case where cooling and heating rate of the gas is larger than the Hubble time, which is shown to hold for zc ~ 3, where zc is the collapse redshift. The Lyα absorption-line profiles for a line of sight through the pancake are then calculated. The absorption lines in general have wings signifying bulk motions in the gas. It is shown that the deviation from a single Voigt profile is large for small H I column density lines, in which the effect of bulk motions is large. For lines with NH I > 1013 cm-2, high temperatures tend to wash out the signatures of bulk motion. The analytical modeling of minipancakes associated with Lyα forest lines—with 1013 NH I 1015 cm-2—gives the corresponding mass scales. In structure formation models with cold dark matter and with reheated intergalactic matter (IGM), the gas in the IGM is inhibited from falling into dark matter potential wells with very small velocity dispersions. Structures with masses larger than this limit (the Jeans mass) can have an infall of gas and form minipancakes. It is shown here that, for typical values of cosmological parameters, absorption lines with NH I ~ 1014 cm-2 correspond to structures with baryonic mass Mb ~ 1010 M☉ with an overdensity of ~10 at z ~ 3. The value of NH I can change by a factor of ~3 in the course of evolution of the pancake in time. It is also shown that there is an upper limit to NH I from a pancake due to the slow recombination rate and the importance of collisional ionization at high temperatures. Minipancakes do not give rise to Lyα lines with NH I 1014.5 cm-2, for a temperature of the IGM of =104 K, J-21 = 1, and ΩIGM ~ 0.03.