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Metal halide perovskites (MHPs) are solution-processed materials with exceptional photoconversion efficiencies that have brought a paradigm shift in photovoltaics. The nature of the peculiar optoelectronic properties underlying such astounding performance is still controversial. The existence of ferroelectricity in MHPs and its alleged impact on photovoltaic activity have fueled an intense debate, in which unanimous consensus is still far from being reached. Here we critically review recent experimental and theoretical results with a two-fold objective: we argue that the occurrence of ferroelectric domains is incompatible with the A-site cation dynamics in MHPs and propose an alternative interpretation of the experiments based on the concept of ferroelasticity. We further underline that ferroic behavior in MHPs would not be relevant at room temperature or higher for the physics of photogenerated charge carriers, since it would be overshadowed by competing effects like polaron formation and ion migration., A.R.G. gratefully acknowledges fruitful discussions with M. Coll from ICMAB-CSIC, Spain, and L. M. Garten from NREL, USA. The Spanish Ministerio de Ciencia e Innovación (MICINN) is gratefully acknowledged for its support through grants nos. SEV-2015-0496 (FUNMAT) and CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program and the AEI/FEDER(UE) grant PGC2018-095411-B-100 (RAINBOW). Thanks also to the Catalan agency AGAUR for grant 2017-SGR-00488 and the National Network “Red Perovskitas” (MICINN funded)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

In this study, the Gadolinium metal based organic framework (Gd-MOF) is synthesized using the hydrothermal route. The as-synthesized material is dissolved in solvent and used to fabricate organic light emitting diodes (OLEDs) in the device structure ITO/PEDOT:PSS/poly-TPD/Gd-MOF/TPBi/LiF/Al. The luminance of Gd-MOF based OLED is found to be of 35.9 cd/ $\text{m}^{2}$ at 8 V. In the emission spectra, three peaks viz. blue, green and red are obtained at 425 nm, 480 nm and 600 nm respectively with excitation at 325 nm. By tuning the operating voltage, we have been able to achieve a white emission from our device in which the peak at 425 nm arises due to poly-TPD however the other two peaks at 480 nm and 600 nm come from Gd-MOF respectively. The voltage dependent study of the electroluminescence spectra of our device indicates that poly-TPD peak intensity increases significantly with the increase in voltage and exceeds the intensity of Gd-MOF peak (488 nm) after 7.5 V. It is also observed that the luminance intensities of Gd-MOF peaks tend to be saturated after 7 V. This is due to the fact that the electron-hole recombination region of our device expands with increasing operating voltage. At lower voltage, the radiative recombination zone is mainly confined within the Gd-MOF region however with increasing voltage the exciton accumulation gradually expands to the Poly-TPD region resulting in a gradual increase in the poly-TPD emission keeping the Gd-MOF emission as to be saturated.

Congo red is a hazardous material in the environment. The present work describes the synthesis of TiO2/CoC@SiO2-bipy (1) and TiO2/CoC@SiO2-phen (2) nanocomposites for the photodegradation of azo-dye Congo red (CR) dye in aqueous solution, by combining pure TiO2 with CoC@SiO2-bipy (s1) and CoC@SiO2-phen (s2) nanoparticles. The prepared nanocomposites were evaluated in term of photocatalytic activity rates in aqueous solution using CR. The nanocomposites TiO2/CoC@SiO2-bipy (1) and TiO2/CoC@SiO2-phen (2) were prepared from TiO2 (75%) and CoC@SiO2-bipy (s1) or CoC@SiO2-phen (s2) (25%) (weight ratio). Ultra-sonication and milling were used to prepare the heterogeneous nano catalysts. The pH, initial dye concentration, and catalyst dosage appeared to have a significant impact on the photocatalytic degradation performance. Molecular oxygen and other active species played a significant role in the photocatalyst degradation of CR with sunlight energy (UV-index 5.0). The chemical reactions were accelerated depending upon electrophilicity (ω) and energy gap (Eg) of azo dye species CR-N=N, CR-N=NH and CR=N-NH species which were calculated by density function theory (DFT). It can be concluded that the rate of electron–hole recombination of the TiO2 catalyst, when adding CoC@SiO2-bipy (s1) or CoC@SiO2-phen (s2), not only enhances the degradation but also effectively removes toxic dye molecules and their by-products. The newly prepared TiO2/CoC@SiO2-bipy (1) nanocomposites showed increased photocatalytic efficiency at low catalyst dose and faster rate of degradation of Congo red compared to TiO2/CoC@SiO2-phen (2) and TiO2 catalysts. The novel catalysts (1) and (2) can be easily separated by centrifugation and filtration, from the reaction mixture compared to TiO2.

Noise spectroscopy is essentially focused on the investigation of electric fluctuations produced by physical mechanisms intrinsic to conductor materials. Very complex electrical transport phenomena can be interpreted through the study of the fluctuation properties, which provide interesting information both from the point of view of basic research and of applications. In this respect, low-frequency electric noise analysis was proposed more than twenty years ago to determine the quality of solar cells and photovoltaic modules, and, more recently, for the reliability estimation of heterojunction solar cells. This spectroscopic tool is able to unravel specific aspects related to radiation damage. Moreover, it can be used for a detailed temperature-dependent electrical characterization of the charge carrier capture/emission and recombination kinetics. This gives the possibility to directly evaluate the system health state. Real-time monitoring of the intrinsic noise response is also very important for the identification of the microscopic sources of fluctuations and their dynamic processes. This allows for identifying possible strategies to improve efficiency and performance, especially for emerging photovoltaic devices. In this work are the reported results of detailed electrical transport and noise characterizations referring to three different types of solar cells (silicon-based, organic, and perovskite-based) and they are interpreted in terms of specific physical models.

This work investigated the relation between direct band-gap conversion and excitation wavelength towards catalysis efficiency in red, green, and blue (RGB) light-emitting diode (LED) reactors. An integrating sphere and spectroradiometer system obtained the emission wavelengths of the operating modes spectra of the RGB-LED reactors. The effects of pH, catalyst, and H2O2 dosage were investigated, and the optimal photocatalysis conditions were found to be at pH 3, catalyst loading of 0.25 g L−1, 0.25 mmol L−1 of H2O2(aq) (30% v/v) for an initial model pollutant concentration of 75 mg L−1 and reaction time of 60 min. Under the higher intensity red mode (R1), the highest color removal rate was reached (88.1%), while in the conventional white light mode (WL), the decolorization efficiency remained 64.3%. Furthermore, the R1 mode showed a superior TOC removal than the WL mode, reaching the final removal efficiencies of 91.86% and 61.06%, respectively. Contrary to what has been reported, as the dominant wavelength of the irradiation source decreased, the efficiency also tended to decrease. The electron-hole recombination increased as the irradiation mode decreased, and a work function (φ) representing this phenomenon was obtained by the deduction of the relation between energy (E) and frequency (f) of the photons involved. Therefore, the insights presented in this work are valuable tools in increasing LED photocatalysis efficiency.

“Concentrate-and-degrade” is an effective strategy to promote mass transfer and degradation of pollutants in photocatalytic systems, yet suitable and cost-effective photocatalysts are required to practice the new concept. In this study, we doped a post-transition metal of Indium (In) on a novel composite adsorptive photocatalyst, activated carbon-supported titanate nanotubes (TNTs@AC), to effectively degrade perfluorooctanoic acid (PFOA). In/TNTs@AC exhibited both excellent PFOA adsorption (>99% in 30 min) and photodegradation (>99% in 4 h) under optimal conditions (25 °C, pH 7, 1 atm, 1 g/L catalyst, 0.1 mg/L PFOA, 254 nm). The heterojunction structure of the composite facilitated a cooperative adsorption mode of PFOA, i.e., binding of the carboxylic head group of PFOA to the metal oxide and attachment of the hydrophobic tail to AC. The resulting side-on adsorption mode facilitates the electron (e‒) transfer from the carboxylic head to the photogenerated hole (h+), which was the major oxidant verified by scavenger tests. Furthermore, the presence of In enables direct electron transfer and facilitates the subsequent stepwise defluorination. Finally, In/TNTs@AC was amenable to repeated uses in four consecutive adsorption-photodegradation runs. The findings showed that adsorptive photocatalysts can be prepared by hybridization of carbon and photoactive semiconductors and the enabled “concentrate-and-degrade” strategy is promising for the removal and degradation of trace levels of PFOA from polluted waters.

Cobalt ferrite-graphene nanoplatelets ((CFO)1-x(GNPs)x) nanocomposites are promising for efficient photocatalysis and high-performance supercapacitors. Multifunctional (CFO)1-x(GNPs)x nanocomposites prepared via facile chemical method have been investigated for their physio-chemical characteristics like crystal structure, morphology, chemical composition, optical properties, infrared vibrational modes, photocatalytic and supercapacitor applications. Interestingly, the photocatalytic activity of CFO nanostructures has been improved significantly from 38.3% to 98.7% with the addition of graphene which can be attributed to control over recombination of charge carriers. It is also found that the specific capacitance of the prepared (CFO)1-x(GNPs)x nanocomposite electrode at 0.5 Ag-1 is three times higher than that of only CFO based electrode which could be due to the conducting nature of graphene nanoplatelets (GNPs). The enhanced photocatalytic and improved electrochemical characteristics suggest the effective use of prepared nanocomposites in water purification and supercapacitor nanodevices., This work was funded by Higher Education Commission of Pakistan (HEC) via NRPU (Grant No: 20–4861/R & D/HEC/14) and HEC-IRSIP (Grant No: 1–8/HEC/HRD/2017/8412 PIN: RSIP 39PSc15).

One dimensional (1D) metal sulfide nanostructures are one of the most promising materials for photocatalytic water splitting reactions to produce hydrogen (H2). However, tuning the nanostructural, optical, electrical and chemical properties of metal sulfides is a challenging task for the fabrication of highly efficient photocatalysts. Herein, 1D CdS nanorods (NRs) were synthesized by a facile and low-cost solvothermal method, in which reaction time played a significant role for increasing the length of CdS NRs from 100 nm to several micrometers. It is confirmed that as the length of CdS NR increases, the visible photocatalytic H2 evolution activity also increases and the CdS NR sample obtained at 18 hr. reaction time exhibited the highest H2 evolution activity of 206.07 &mu, mol.g&minus, 1.h&minus, 1. The higher H2 evolution activity is explained by the improved optical absorption properties, enhanced electronic bandstructure and decreased electron-hole recombination rate.

Electron–hole recombination always competes with desired reactions on semiconductor photocatalysts. Reducing recombination probability is essential for increasing the quantum efficiency of the reactions. Previous studies demonstrated that doping with lanthanoid or alkaline-earth metal cations reduced recombination probability in NaTaO3 photocatalysts for artificial photosynthesis. The motivation behind this study was to reveal how the guest metal cations reduced recombination probability. NaTaO3 photocatalysts were doped with Sr cations through crystallization in molten NaCl flux to produce 50–100 nm sized particles of NaTaO3–Sr(Sr1/3Ta2/3)O3 solid solution. Intraparticle distribution of Sr cations was sensitive to immersion time in the hot flux with a fixed Sr concentration of 2 mol % relative to Ta. Extended immersion for 60 h resulted in a homogeneous Sr distribution. Curtailed immersion for 1 h yielded particles capped with a 3 nm thick Sr-accumulated layer. The population of electrons bandgap-excited under Hg–Xe lamp irradiation was enhanced in the 1-h immersed photocatalyst by 160 times relative to that in a Sr-free NaTaO3 photocatalyst. In the 60-h immersed photocatalyst, population enhancement was not more than 9 times. We interpreted the large population enhancement in the 1-h immersed photocatalyst with a concentration gradient of Sr cations from the surface to bulk. The concentration gradient induced an energy gradient of conduction-band minimum. Photoexcited electrons were driven on the energy gradient to be separated from holes. The overall water splitting reaction rate was evaluated on the photocatalysts to show a 4-times enhancement on the 1-h immersed photocatalyst relative to the rate on the Sr-free photocatalyst. The reaction-rate enhancement less than the electron population enhancement was ascribed to a limited fraction of electrons overriding the energy gradient and returning back to the surface.

To address the slow recombination of photogenerated charges in tetragonal CH3NH3PbI3, the evolution of extra electrons and holes is simulated through advanced ab initio molecular dynamics. We show that the localization of the charge carriers and their hopping from one polaronic state to another occur on a subpicosecond time scale. The localization, attended by weak bond contractions and elongations in the inorganic sublattice, is induced by thermal vibrations and only moderately perturbed by the disordered field generated by the organic cations. The simultaneous simulation of extra electrons and holes shows that they preferentially localize in spatially distinct regions. As determined by the overlap between the electron and hole wave functions, the probability of radiative bimolecular recombination is lowered by two orders of magnitude compared with that of optical generation. The separate polaronic localization of electrons and holes emerges as the key feature for achieving exceptional photovoltaic properties.

Metal halide perovskites, of general formula ABX3, show a complex interplay of the inorganic BX3 sub-lattice and the organic/inorganic A-site cations, which likely determines some of their peculiar optoelectronic properties. Comprehension of the physics underlying this interaction may reveal further means of fine-tuning their optoelectronic response. Here, we investigate in depth charge/lattice interactions associated to the formation of polarons in different models of the prototypical CH3NH3PbI3 perovskite through advanced electronic-structure calculations. We demonstrate that charge localization, while induced by the disordered dipolar field of the organic cations, is only stabilized by distortions in the inorganic sub-lattice. Polaron hopping between neighbouring minima is related to the random reorientation of the organic cations and occurs via a delocalized transition state. Our simulations highlight a struggle between thermally induced disorder, driven by the motion of A-site cations, and polaron stabilization within the BX3 sub-lattice, which explains the simultaneous low mobility and high diffusion length of charge carriers in lead-halide perovskites.

The origin of the long carrier-lifetime in Lead halide perovskites is still under debate and, among different hypothesis, the formation of large polarons preventing the recombination of charge couples is one of the most fascinating. In this work, using state of the art ab initio calculations, we report a systematic study of the polaron formation process in metal halides perovskites focusing on the influence of the chemical composition of the perovskite on the polaron properties. We examine variations in A-site cations (FA, MA, Cs and Cs:MA), B-site cations (Pb, Sn and Pb:Sn) and X-site anions (Br, I). Our study confirms the stronger structural distortions occur for Cs than for MA and FA, with the effect of different A-site cations being almost additive. For the same A-cation, bromide features stronger distortions than iodide perovskites. The pure Sn phase has an almost double polaron stabilization energy than the pure Pb phase. Surprisingly, the trend of polaron stabilization energy is non monotonic in mixed Sn:Pb perovskites, with a maximum for small Sn percentages. Polaron formation is found to be promoted by bond asymmetry, ranging from small to large polarons in mixed Sn:Pb perovskites depending on the relative Sn percentage.

CuS and CuGaS2 heterojunction catalysts were used to improve hydrogen production performance by photo splitting of methanol aqueous solution in the visible region in this study. CuGaS2, which is a chalcogenide structure, can form structural defects to promote separation of electrons and holes and improve visible light absorbing ability. The optimum catalytic activity of CuGaS2 was investigated by varying the heterojunction ratio of CuGaS2 with CuS. Physicochemical properties of CuS, CuGaS2 and CuS@CuGaS2 nanoparticles were confirmed by X-ray diffraction, ultraviolet visible spectroscopy, high-resolution transmission electron microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Compared with pure CuS, the hydrogen production performance of CuGaS2 doped with Ga dopant was improved by methanol photolysis, and the photoactivity of the heterogeneous CuS@CuGaS2 catalyst was increased remarkably. Moreover, the 0.5CuS@1.5CuGaS2 catalyst produced 3250 &mu, mol of hydrogen through photolysis of aqueous methanol solution under 10 h UV light irradiation. According to the intensity modulated photovoltage spectroscopy (IMVS) results, the high photoactivity of the CuS@CuGaS2 catalyst is attributed to the inhibition of recombination between electron-hole pairs, accelerating electron-transfer by acting as a trap site at the interface between CuGaS2 structural defects and the heterojunction.

We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta2O5-overlayer and compare them with samples uniformly doped with the same amount of tantalum. We observe a three-fold improvement in photocurrent with the same onset voltage using Ta-overlayer hematite photoanodes, while electrochemical impedance spectroscopy under visible light irradiation shows a decreased amount of surface states under water splitting conditions. The Tadoped samples have an even higher increase in photocurrent along with a 0.15 V cathodic shift in the onset voltage and decreased resistivity. However, the surface state capacitance for the Ta-doped sample is twice that of the reference photoanode, which implies a larger amount of surface hole accumulation. We further utilize transient absorption spectroscopy in the sub-millisecond to second timescale under operating conditions to show that electron trapping in both Ta2O5-passivated and Ta-doped samples is markedly reduced. Ultrafast transient absorption spectroscopy in the sub-picosecond to nanosecond timescale shows faster charge carrier dynamics and reduced recombination in the Ta-doped hematite photoanode resulting in the increased photoelectrochemical performance when compared with the Ta2O5-overlayer sample. Our results show that passivation does not affect the poor charge carrier dynamics intrinsic to hematite based photoanodes. The Ta-doping strategy results in more efficient electron extraction, solving the electron trapping issue and leading to increased performance over the surface passivation strategy. Funding Agencies|Finnish Cultural Foundation; Jenny and Antti Wihuri Foundation; Academy of Finland [141481, 286713, 309920]; Finnish Centre of Excellence in Atomic Layer Deposition

Mixed iodide-bromide (I-Br) organolead perovskites are of great interest for both single junction and tandem solar cells since the optical bandgap of the materials can be tuned by varying the bromine to iodine ratio. Yet, it remains unclear how bromine incorporation modifies the properties of the perovskite solar cells. Here we use methylammonium lead iodide-bromide [MAPb(I1-xBrx)(3) (x = 0-0.11)] as a model system to study the question. Through elemental analysis we found that bromine exhibits an increased concentration towards the interface between perovskite and TiO2, and that such interface bromine accumulation is more pronounced when a HI acid additive is used to produce the perovskite films. Opto-electrical characterization results further suggest that the carrier lifetime in the perovskite and the charge extraction at the perovskite/TiO2 interface are improved in the mixed halide perovskite samples. Accordingly, the optimal thickness of the perovskite active layer is increased from 330 nm for MAPbI(3) to 510 nm for MAPb(I0.89Br0.11)(3), resulting in an average efficiency increase of 1.6%. The study highlights the role of bromine in passivating defect states at grain boundaries and interfaces in mixed halide perovskite solar cells and demonstrates the use of solvent additives in tuning the electronic properties of the mixed halide perovskite materials.

In the present study, random current fluctuations measured at different temperatures and for different illumination levels are used to understand the charge carrier kinetics in methylammonium lead iodide CH3NH3PbI3‐based perovskite solar cells. A model, combining trapping/detrapping, recombination mechanisms, and electron–phonon scattering, is formulated evidencing how the presence of shallow and deeper band tail states influences the solar cell recombination losses. At low temperatures, the observed cascade capture process indicates that the trapping of the charge carriers by shallow defects is phonon assisted directly followed by their recombination. By increasing the temperature, a phase modification of the CH3NH3PbI3 absorber layer occurs and for temperatures above the phase transition at about 160 K the capture of the charge carrier takes place in two steps. The electron is first captured by a shallow defect and then it can be either emitted or thermalize down to a deeper band tail state and recombines subsequently. This result reveals that in perovskite solar cells the recombination kinetics is strongly influenced by the electron–phonon interactions. A clear correlation between the morphological structure of the perovskite grains, the energy disorder of the defect states, and the device performance is demonstrated.

This review has been written in order to clarify fundamental aspects of photocatalysis, an important subject in inorganic and material chemistry, not to present a list of studies on photocatalysis reported so far, since it seems rather difficult to make a complete review by introducing all or a large part of the reported studies on photocatalysis of relatively long history. This review is based on the author's experience in studies on photocatalysis and topics are limited to so-called semiconductor photocatalysis; definition and examples of photocatalysis, its principle and kinetics, visible light-induced photocatalysis, and design of active photocatalysts are discussed in detail.

Mixed phase rutile/anatase catalysts show increased reactivity compared with the pure phases alone. However, the mechanism causing this effect is not fully understood. The electronic properties of the interface and the relative energy of the electron in each phase play a key role in lowering the rate of recombination of electron hole pairs. Using density functional theory and the +U correction, we calculated the bands offsets between the phases taking into account the effect of the interface. Our model included several thousands atoms, and thus is a good representation of an interface between actual nanoparticles. We found rutile to have both higher conduction and valence band offsets than rutile, leading to an accumulation of electrons in the anatase phase accompanied by hole accumulation in the rutile phase. We also probed the electronic structure of our heterostructure and found a gap state caused by electrons localized in undercoordinated Ti atoms which were present within the interfacial region. Interfaces between bulk materials and between exposed surfaces both showed electron trapping at undercoordinated sites. These undercoordinated (typically four) atoms present localized electrons that could enable reduction reactions in the interfacial region, and could explain the increased reactivity of mixed-phase TiO2 photocatalyst materials.

We present some typical applications of our device simulation model "MOLED" which has proven to be a very useful tool for obtaining insight into the functioning of multi-layer organic light-emitting devices (OLED's). Our general approach consists of combining experimental measurement and numerical device modelling in a deliberate and coordinated way. Taking a step-by-step, evolutive approach, starting with simple single layer devices and gradually moving up to more complex multi-layer architectures with a partially doped emission layer, we could progressively determine unknown parameters in the model and at the same time shed light on specific aspects of OLED functioning. For example, electrode characterization was done by measuring and modeling single layer devices with different electrode materials, while the simulation of time of flight signals gave us insight into the transient state and the effects of correlated disorder on the macroscopic mobility. The internal electric field in multi-layer OLED's could also be investigated by varying the thicknesses of the hole transport and electron transport layers. Applying this method to the standard OLED device structure that has received broad attention in the literature, we have found a number of surprising results. From our experiments, we have demonstrated that the average electric field inside the hole transport layer is larger than or equal to the average field in the emission layer over the entire current range. The device simulations fully clarify the situation, giving insight into the space charge effects as well as the hole and the electron current distributions in the device. In particular, we found that there is a leakage of unrecombined holes towards the cathode at low voltages. We also found a strong variation of the electric field in the Alq3 layer due to space charge effects. By using the laser dye derivatives DCM-TPA with electron trapping capabilities and DCM-II with both electron and hole trapping capabilities as dopants in a standard OLED architecture, we could study the effect on transport and emission characteristics. In the case of the exclusively electron trapping dopant, a blue-shift of the emission color with increasing bias is observed which we find is due to a splitting of the recombination zone

By using pyran-containing donor-acceptor dyes as doping molecules in organic light-emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro-active host material. Laser dyes 4-(dicyanomethylene)-2-methyl-6-[2-[(julolidin-9-yl)phenyl]ethenyl]-4H-pyran (DCM2) and the novel 4-(dicyanomethylene)-2-methyl-6-[2-[(4-diphenylamino)phenyl]ethenyl]-4H-pyran (DCM-TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8-hydroxyquinoline)aluminum (Alq3) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM-TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq3 region. To impede charge-recombination processes taking place in the undoped host matrix, a charge-blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.-% we obtain a maximum luminous efficiency of 10.4 cd A-1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices

Excitonic effects are observed in the optical absorption and photoluminescence of strongly confined CdS quantum dots embedded in the polymer Nafion. The three bands identified in the optical absorption spectra could be attributed to 1s e-1s h, 1p e-1p h, and 2s e-2s h transitions of the noninteracting particle model. Photoluminescence spectra show a strong emission band corresponding to electron-hole recombination and a weak band due to emission from defect states. The strength of electron-phonon coupling is small in the regime of strong confinement and decreases with decreasing particle size. ? 2002 American Institute of Physics.

We have found that controlled Ar ion bombardment enhances the degradation of a-Si: H based surface passivation of c-Si surfaces. The decrease in the level of surface passivation is found to be independent on the ion kinetic energy (7-70 eV), but linearly proportional to the ion flux (6 X 10(14)-6 X 10(15) ions cm(-2) s(-1)). This result suggests that the ion flux determines the generation rate of electron-hole pairs in a-Si: H films, by which metastable defects are created at the H/a-Si:c-Si interface. Possible mechanisms for the ion induced generation of electron-hole pairs are discussed. (C) 2011 American Institute of Physics.

We consider a simple model for the geminate electron-hole separation process in organic photovoltaic cells, in order to illustrate the influence of dimensionality of conducting channels on the efficiency of the process. The Miller-Abrahams expression for the transition rates between nearest neighbor sites was used for simulating random walks of the electron in the Coulomb field of the hole. The non-equilibrium kinetic Monte Carlo simulation results qualitatively confirm the equilibrium estimations, although quantitatively the efficiency of the higher dimensional systems is less pronounced. The lifetime of the electron prior to recombination is approximately equal to the lifetime prior to dissociation. Their values indicate that electrons perform long stochastic walks before they are captured by the collector or recombined. The non-equilibrium free energy considerably differs from the equilibrium one. The efficiency of the separation process decreases with increasing the distance to the collector, and this decrease is considerably less pronounced for the three dimensional system. The simulation results are in good agreement with the extension of the continuum Onsager theory that accounts for the finite recombination rate at nonzero reaction radius and non-exponential kinetics of the charge separation process.

We present a theory of the low-frequency noise gain gn and photocurrent gain gp in quantum well infrared photodetectors (QWIPs). Expressions for gn and gp in terms of QW capture probability pc and number of QWs N are obtained. These expressions are valid for any number of QWs N>=1 and capture probability 01) to 1 (for pc-->0). QWIP is well described by a conventional photoconductor theory in the case of low capture probability pc-->1, which corresponds to practical QWIPs. The assumptions of the model are discussed in detail, and a comparison with previously published results is made.

A systematic study of exciton dynamics is presented in quantum boxes formed naturally along the axis of a V-shaped quantum wire, by means of time and spatially resolved resonant photoluminescence. The dependence of radiative lifetimes and relaxation mechanisms of excitons is determined versus the size of the boxes. The radiative recombination rate varies linearly with the length of the box, showing that the exciton has a coherence volume equal to the volume of the box. In a low excitation regime, emission from excited states has not been observed, which would be a consequence of relaxation bottleneck, but there is clear evidence that relaxation via emission of LA phonons depends strongly on the energy separation between the different quantum box levels.

This paper addresses the problem of the space charge region Shockley-Read-Hall (SRH) recombination currents in heterojunctions with one noncrystalline side. A formulation which generalizes previous works is discussed. The approach is based on the drift-diffusion model with a thermionic-field emission boundary condition. The main physical parameters which determine the relative contribution of each zone of the space charge region (SCR) to the total recombination current are identified. The general analysis is applied for the first time to amorphous/crystalline heterojunctions and design criteria are established to minimize the total recombination current.

Visible-light-induced antibacterial activity of carbon-doped anatase-brookite titania nano-heterojunction photocatalysts are reported for the first time. These heterostructures were prepared using a novel low temperature (100 °C) non-hydrothermal low power microwave (300 W) assisted method. Formation of interband C 2p states was found to be responsible for the band gap narrowing of the carbon doped heterojunctions. The most active photocatalyst obtained after 60 minutes of microwave irradiation exhibits a 2-fold higher visible-light induced photocatalytic activity in contrast to the standard commercial photocatalyst Evonik-Degussa P-25. Staphylococcus aureus inactivation rate constant for carbon-doped nano-heterojunctions and the standard photocatalyst was 0.0023 and -0.0081 min-1 respectively. It is proposed that the photo-excited electrons (from the C 2p level) are effectively transferred from the conduction band of brookite to that of anatase causing efficient electron-hole separation, which is found to be responsible for the superior visible-light induced photocatalytic and antibacterial activities of carbon-doped anatase-brookite nano-heterojunctions. KEYWORDS: Titanium dioxide; Microwave synthesis; Carbon-doping; Visible-light induced photocatalysis; Antibacterial activity; Heterojunctions

Zinc oxide (ZnO), with the unique chemical and physical properties of high chemical stability, broad radiation absorption range, high electrochemical coupling coefficient, and high photo-stability, is an attractive multifunctional material which has promoted great interest in many fields. What is more, its properties can be tuned by controllable synthesized morphologies. Therefore, after the success of the abundant morphology controllable synthesis, both the morphology-dependent ZnO properties and their related applications have been extensively investigated. This review concentrates on the properties of morphology-dependent ZnO and their applications in catalysis, mainly involved reactions on green energy and environmental issues, such as CO2 hydrogenation to fuels, methanol steam reforming to generate H2, bio-diesel production, pollutant photo-degradation, etc. The impressive catalytic properties of ZnO are associated with morphology tuned specific microstructures, defects or abilities of electron transportation, etc. The main morphology-dependent promotion mechanisms are discussed and summarized.

An equivalent circuit model has been developed for quantum-well (QW) lasers from the electrical and optical rate equations and thermal conduction equation. This model and its SPICE implementation allow self-consistent calculation of the electrical, optical, and thermal interactions using lumped elements, and make possible the simulation of multiple components in optoelectronic integrated circuits in the presence of these interactions. Physical effects which are strongly temperature dependent, such as gain variation, leakage current, and Auger recombination, have been incorporated. The model has been validated with measured laser characteristics. >

We used numerical simulation to compare the temperature sensitivity of an InGaAsP MQW laser emitting at 1.55 /spl mu/m and an AlGaAs MQW laser at 0.82 /spl mu/m. By artificially changing the InGaAsP laser gradually into a structure similar to the AlGaAs laser, we gained quantitative insight into how each material or structural parameter causes the relatively low T/sub 0/ of the InGaAsP MQW laser. Using a typical MQW structure we demonstrated the relative importance of parameters involving Auger recombination, current leakage over the quantum barrier, optical confinement and band offset. We found that if these parameters were made the same as the AlGaAs laser, the T/sub 0/ of the InGaAsP laser was even better than that of the AlGaAs laser. Our numerical simulation confirmed that the Auger recombination is the main cause of low T/sub 0/ in MQW InGaAsP lasers. We also discovered that thermal current leakage over the barrier and Auger recombinations are correlated with each other and both factors must be improved to increase the T/sub 0/ of InGaAsP lasers to that of AlGaAs lasers. >

The dynamical behavior of photoexcited states of TiO2 governs the activities of TiO2-based solar cells and photocatalysts. We determined the lifetimes of photoexcited electrons and holes in rutile and anatase TiO2 single crystals by combining advantages of time-resolved photoluminescence, photoconductance, and transient absorption spectroscopy. Electrons and holes in rutile show exponential decays with the lifetime of a few tens of nanoseconds, while non-exponential decays are observed in anatase, indicating the presence of multiple carrier trapping processes. We revealed the generic features of the carrier recombination processes in rutile and anatase TiO2., 二酸化チタン中の光キャリアの振る舞いを解明 -高効率太陽電池の実現に期待-. 京都大学プレスリリース. 2012-09-28.

We propose a first-principle method for evaluating the rate of radiative electron-hole recombination and electron trapping on impurities in semiconductors. The method is based on the Einstein-Planck theory of photon modes and the perturbation theory with dipole approximation for the electron-field interaction. The calculations employing the LMTO-TB band structure method have been done for anatase doped with boron, carbon, and nitrogen. We find that the doping cannot induce radiative processes of electron trapping or electron-hole recombination. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim., The financial support of the Presidium of the Urals Branch of the Russian Academy of Sciences and of the Ikerbasque Fellowship Foundation, Basque Country, Spain, are greatly acknowledged.

We present a density fitted local configuration interaction singles (CIS) method for calculating optical bandgaps in 3D-periodic systems. We employ an Ewald technique to carry out infinite lattice summations for the exciton-exciton interaction, and robust product-density specific local density fitting in direct space for the electron-hole interaction. Moreover, we propose an alternative to the usual cyclic model with Born-von Karman periodic boundary conditions, the so called Wigner-Seitz supercell truncated infinite model, which exhibits much improved convergence of the CIS excitation energy with respect to the size of the supercell. Test calculations on a series of prototypical systems demonstrate that the method at the present stage can be used to calculate the excitonic bandgaps of 3D periodic systems with up to a dozen atoms in the unit cell, ranging from wide-gap insulators to semiconductors.

We studied photoluminescence (PL) spectrum and dynamics of SrTiO3 nanoparticles. A broad blue PL band is observed at around 2.9 eV in nanoparticle thin films. The excitation-density dependence of the blue PL intensity indicates that nonradiative Auger recombination dominates the carrier recombination process under strong photoexcitation. The intrinsic Auger recombination rate in SrTiO3 nanoparticles is much larger than that of SrTiO3 bulk crystals. The origin of enhanced Auger recombination in SrTiO3 nanoparticles is discussed.

The time evolution of interface-state (Dit) buildup following radiation and high-field stressing in MOS capacitors was compared for 45-nm-thick conventional `dry' oxide (SiO2) and reoxidized nitrided oxide (RNO). While the oxide shows the expected postirradiation increase of Dit with time under positive bias, the RNO shows no time-dependent buildup. This indicates that hydrogen transport, widely held responsible for the slow evolution of Dit after radiation, does not play a role in Dit generation in RNO. It is suggested that this is due to a blocking effect of the nitrogen-rich oxynitride layer which is known to exist in RNO near the silicon/silicon-dioxide interface, and which inhibits the drift of hydrogen ions to the interface. Exposure of the capacitors to a hydrogen ambient after irradiation confirms that for RNO, unlike the case of oxide, there is no increase of Dit due to hydrogen effects. Postirradiation electron injection in RNO suggests that trapped-hole recombination may be responsible for the small Dit generation seen in RNO., IEEE

Date du colloque: 12/2009; International audience; We have investigated electrical, charge transport and optical properties of functionalised DNA:PEDT-PSS thin films. Current-voltage dependencies of the samples were linear and symmetrical down to 77 K temperature. Material conductivity at room temperature was about (1-5)×10-10 ¿-1 cm-1. The thermal activation energy of the conductivity measured in the dark was about 0.033 eV near the room temperature independently on the applied bias. The weakly expressed carrier trapping was identified by the thermally stimulated current method, evidencing the fast recombination of light-generated carriers. Though, at constant light excitation a "bistable" photoconduction below the room temperature was identified. I.e., upon excitation by light from the spectral region 500 800 nm a notable increase of the photocurrent could be observed below 140 160 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 230 240 K. Most probably such phenomenon could be attributed to the light-induced morphology changes of the samples.

The carrier recombination processes and the relaxation dynamics of spin-polarized excitons in cubic epitaxial GaN have been investigated by time-resolved photoluminescence and pump-probe spectroscopy experiments. We evidence a very fast exciton spin relaxation tau(S)similar to 0.3 ps that is influenced by both the high defect concentration and the strong electron-hole exchange interaction. Spectral oscillatory features of the transient reflectivity are observed for negative delays that allow us to determine the exciton optical dephasing time T-2.

Optical gain and stimulated emission processes in Si nanostructures are controlled by the dynamics of high-density carriers. Here, we report photoluminescence (PL) dynamics and multiexciton recombination in Si1−xGex/Si superlattices (SLs) under high-density excitation. Saturation of the PL intensity and rapid PL decay are observed as the excitation laser intensity is increased. These phenomena occur due to nonradiative Auger recombination of the electron-hole pairs. We find that the Auger process in Si1−xGex/Si SLs is less pronounced than that in the Si1−xGex/Si single quantum wells. Our results show that coupled nanostructures have an advantage in efficient light emission and the control of many-body carrier dynamics.

In a recently published paper by [Odaka, Appl. Phys. Lett. 88, 123501 (2006)], the authors explain their experimental observations of a weak magnetic field effect on electroluminescence from Alq3 -based light emitting diodes by the magnetic field-induced evolution of singlet-to-triplet spin conversion of loosely bound electron-hole pairs, suggesting the originality of their explanation. ©

A generalized solar cell model for excitonic and classical bipolar solar cells describes the combined transport and interaction of electrons, holes, and excitons in accordance with the principle of detailed balance. Conventional inorganic solar cells, single-phase organic solar cells and bulk heterojunction solar cells, i.e., nanoscale mixtures of two organic materials, are special cases of this model. For high mobilities, the compatibility with the principle of detailed balance ensures that our model reproduces the Shockley-Queisser limit irrespective of how the energy transport is achieved. For less ideal devices distinct differences become visible between devices that are described by linear differential equations and those with nonlinear effects, such as a voltage-dependent collection in bipolar $p\text{\ensuremath{-}}i\text{\ensuremath{-}}n$-type devices. These differences in current-voltage characteristics are also decisive for the validity of the reciprocity theorem between photovoltaic quantum efficiency and electroluminescent emission. Finally, we discuss the effect of band offset at the heterointerface in a bulk heterojunction cell and the effect of the average distances between these heterointerfaces on the performance of a solar cell in order to show how our detailed balance model includes also these empirically important quantities.

We report on the optical determination of the carrier-density profiles near surfaces in SrTiO3 crystals by means of photoluminescence (PL) dynamics measurements. The PL dynamics under band-to-band excitation depends strongly on the excitation photon energy for different optical penetration depths. In nondoped and Ar+-irradiated SrTiO3 crystals, we evaluate the depth profile of carriers near the surface based on the lifetime of the Auger recombination of electrons originating from oxygen vacancies with photocarriers. Our PL spectroscopy clarifies that in nondoped SrTiO3, the near-surface oxygen-deficient region is a few tens of nanometers in depth.

We report that the blue photoluminescence (PL) dynamics are dependent on both the photogenerated and the chemically doped carrier densities in SrTiO3. We found that the PL decay dynamics are well explained using a simple model that includes nonradiative single-carrier trapping, radiative bimolecular recombination, and nonradiative Auger recombination processes. In highly photoexcited SrTiO3 and heavily electron-doped SrTiO3, the rapid Auger recombination of the polarons determines the PL properties.

Received 29 January 2008; revised manuscript received 20 June 2008; published 20 August 2008We present a detailed study of the dependence of the spectral characteristics of quasiresonantly excitedInAs/GaAs quantum dots on electronic structure, temperature, and hydrogen content. Multiphonon resonancesdominate the resonantly excited emission spectra. Such multiphonon resonances are attributed to the compe-tition between two different channels ruling carrier thermalization in an ensemble of quantum dots: decaybetween polaronic states and efficient nonradiative intradot decay. This supports an effective “phonon bottle-neck” in quantum dots, which is only partially relaxed by the presence of a fast polaron thermalization channel.DOI: 10.1103/PhysRevB.78.085313 PACS number s : 78.67.Hc, 73.21.La, 63.20.kd

3 páginas, 3 figuras., In this work we report the stacking of 50 InAs/GaAs quantum dot layers with a GaAs spacer thickness of 18 nm using GaP monolayers for strain compensation. We find a good structural and optical quality of the fabricated samples including a planar growth front across the whole structure, a reduction in the quantum dot size inhomogeneity, and an enhanced thermal stability of the emission. The optimized quantum dot stack has been embedded in a solar cell structure and we discuss the benefits and disadvantages of this approach for high efficiency photovoltaic applications., The authors gratefully acknowledge financial support by the Spanish MEC and CAM through Projects 200560M089, S-05050/ENE-0310, TEC-2005-05781-C03-01, and -02, Consolider-Ingenio 2010 CSD2006-0004, the Junta de Andalucia (Project TEP383, Group TEP120), and by the European Commission through the SANDIE Network of Excellence (NMP4-CT-2004-500101).

Multilayer organic electroluminescent devices derive their advantages over their single-layer counterparts from the processes occurring at heterojunctions in organic media. These processes significantly differ from those in the bulk of the material. This paper presents three-dimensional modeling, numerical simulations, and a discussion of transport and recombination in a system with a heterojunction. We consider partial cross sections for the creation of excitons and exciplexes, and probabilities for recombination in the respective channels. We examine the influence of the energy barrier, electric field, site-energy disorder, and structural disorder at an organic-organic interface on transport and recombination. In particular, we investigate optimal parameter domains for recombination in the exciton channel. The interface roughness, unlike the site-energy disorder, is found to strongly affect the partial cross sections.

The performance of organic light emitting field effect transistors is strongly influenced by the width of the recombination zone. We present an analytical model for the recombination profile. By assuming Langevin recombination, the recombination zone width W is found to be given by W = root 4.34d delta, with d and delta the gate dielectric and accumulation layer thicknesses, respectively. The model compares favorably to both numerical calculations and measured surface potential profiles of an actual ambipolar device. (C) 2008 American Institute of Physics.

We have carried out continuous wave and time resolved photoluminescence experiments in self-assembled In(Ga)As quantum dots and quantum rings embedded in field effect structure devices. In both kinds of nanostructures, we find a noticeable increase of the exciton radiative lifetime with the external voltage bias that must be attributed to the field-induced polarizability of the confined electron hole pair. The interplay between the exciton radiative recombination and the electronic carrier tunneling in the presence of a stationary electric field is therefore investigated and compared with a numerical calculation based on the effective mass approximation., The authors gratefully acknowledge financial support by the Spanish MEC and CAM through Projects Nos.TEC-2005-05781-C03-01/03, NAN2004-09109-C04-01/03,CSD2006-0019, and S-505/ESP/000200, and by the European Commission through the SANDIE Network of Excellence (Contract No. NMP4-CT-2004-500101).

The current and electroluminescence transient responses of standard poly phenylene vinylene -based light-emitting devices have been investigated. The electroluminescence time response is longer milliseconds scale than the current switch-off time by more than one order of magnitude, in the case of small area devices 0.1 cm2 . For large area devices 6 cm2 the electroluminescence decay time decreases from 1.45 ms to 100 s with increasing bias voltage. The fast current decay limits the electroluminescence decay at higher voltages. Several approaches are discussed to interpret the observed slow decrease of electroluminescence after turning off the bias. One relies upon the Langevin-type bimolecular recombination kinetics which is governed by the minority carriers electrons , and another focuses on the slow release of trapped electrons as possible explanations. Additionally, we show that the device current density is mainly determined by the transport of the fastest carriers (holes). Ministerio de Educación y Ciencia de España MAT2004-038499

The present work is a photoluminescence study of Si-embedded Stranski-Krastanov Ge quantum dots. The value of the conduction band offset is a result of the magnitude of the tensile strain in the Si surrounding the compressive strained Ge dot. Due to the increased Si/Ge intermixing and reduced strain in the Si barrier, a reduction of the conduction band offset is observed at increased growth temperatures. The optical properties as derived from photoluminescence spectroscopy are correlated with structural properties obtained as a function of the growth temperature. High growth temperatures result in large Ge dots with low density due to the pronounced surface diffusion and Si/Ge intermixing. As confirmed by photoluminescence, the band gap of the Ge dots increases with increased growth temperature due to the higher degree of Si/Ge intermixing. The band alignment is of type II in these structures, but the occurrence of both spatially indirect and spatially direct transitions are confirmed in temperature-dependent photoluminescence measurements with varied excitation power conditions. An increasing temperature results in a gradual transition from the spatially indirect to the spatially direct recombination in the type-II band lineup, due to higher oscillator strength for the spatially direct transition combined with a higher population factor at higher temperatures. Original Publication:Mats Larsson, Anders Elfving, Wei-Xin Ni, Göran V. Hansson and Per-Olof Holtz, Growth-temperature-dependent band alignment in Si/Ge quantum dots from photoluminescence spectroscopy, 2006, Physical Review B, (73), 195319.http://dx.doi.org/10.1103/PhysRevB.73.195319Copyright: American Physical Societyhttp://www.aps.org/