Photoinduced Defects Research Articles

Factors that Limit Continuous‐Wave Lasing in Hybrid Perovskite Semiconductors

AbstractMetal halide perovskites are a promising platform for solution‐processed semiconductor lasers but have so far only achieved continuous‐wave (cw) operation at low temperature. Here, the origin of this limitation is investigated in prototypical methylammonium lead iodide (MAPbI3) distributed feedback (DFB) lasers and is found to stem from a combination of pump‐induced heating and the accumulation of photoinduced defects. A temperature rise of more than 30 K in the first few hundred ns following pump turn‐on is observed, which leads to a dynamic increase in threshold owing to the strong temperature dependence of stimulated emission in MAPbI3 thin films. In parallel, it is found that the Shockley–Read–Hall rate coefficient increases by an order of magnitude on a similar timescale, suppressing the carrier density generated by the pump. The combined impact of both effects is understood using a comprehensive model that quantitatively explains the duration of lasing as a function of pump intensity and predicts that the threshold of current DFB lasers must be decreased by an order of magnitude to achieve cw operation at room temperature.

Intrinsic five-photon non-linear absorption of two-dimensional BN and its conversion to two-photon absorption in the presence of photo-induced defects

Unlike most two-dimensional materials, h-BN nanoplatelets (BNNPs) exhibit multi-photon absorption. This was previously attributed to a two-photon absorption (2PA) process at 1.16 eV by Kumbhakar et al., Advanced Optical Materials, 3, 828, 2015. This is counter-intuitive because a 2PA process at 1.16 eV cannot excite electrons across the wide band gap of BNNPs (∼5.75 eV). Here, through systematic experimental and theoretical work, we provide compelling evidence that BNNPs exhibit five-photon absorption (5PA) at a low-laser fluence (<0.6 J/cm2). Furthermore, we show that a high laser fluence (>0.6 J/cm2) generates photo-induced defects in BNNPs that support a 2PA process. Accordingly, we assert that BNNPs inherently exhibit 5PA at a low laser fluence. In the previous study reported by Kumbhakar et al. a high laser fluence was used, which led to the observation of 2PA process.

Highly efficient light responsive BiOCl/AgI composites for photocatalytic degradation of 3-CP under visible and UV light irradiations

Novel visible light sensitive BiOCl/AgI composites were synthesized by a facile precipitation technique. Samples were characterized by SEM (scanning electron microscope), XRD (X-ray diffraction), UV-DRS (UV-diffuse reflectance spectroscopy) and XPS (X-ray photoelectron spectroscopy). Photocatalytic activity of the samples was evaluated using 3-CP under UV–Visible irradiation. Compared to pure BiOCl, AgI and other BiOCl/AgI, the composites exhibited more catalytic activity under both UV and visible lights within 120 min. Optimal content of the BiOCl in the composite system was found as 40%. The excellent degradation of 3-CP under light was attributed to the efficient separation of photo-induced charge carriers, defect levels, iodide and chlorine $$({{\text{I}}^ \cdot },{\text{C}}{{\text{l}}^ \cdot })$$ radicals. The radical scavenging activities also illustrate that holes and superoxide radicals $$({{\text{h}}^+})$$ $$~{\text{and}}\,({\text{O}}_{2}^{{ - \cdot }})$$ are dominant agents in the photocatalytic degradation process. These results demonstrated that BiOCl/AgI systems are very useful to decompose persistent organic pollutants.

Photoconductive behavior of binary porphyrin crystalline assemblies

The mechanism of photoconductivity in a crystalline photoconductor synthesized from 1:1 ratio of meso-tetra(4-pyridyl)porphyrin (TPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) ionic tectons was examined. The rod-like crystals of TPyP:TSPP insulate in the dark but become photoconducting on illumination and a portion of the photoinduced current persists after the laser light is turned off. This persistent photoconductivity (PPC) is investigated as a function of laser illumination wavelength, laser power, and sample temperature. The primary charge carriers in the TPyP:TSPP upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. The number of electrons contributing to the photocurrent is directly proportional to the number of photons absorbed thus, the mechanisms of the photoconductivity resulting from excitations within the Soret band and the Q-band are the same. The PPC is interpreted to be the result of the formation of photoinduced metastable defects that allow for Miller–Abrahams-like hopping conductivity. The TPyP:TSPP has an incommensurately modulated crystal lattice and its proposed model structure is based on both ionic and neutral porphyrin tectons. The thermogravimetric analysis shows that the porphyrin crystals undergo dehydration on heating (˜50 ∘C) by losing water molecules located in the crystalline channels. Temperature dependent XRD indicates that dehydration causes irreversible changes to the crystal structure. The loss of crystallinity observed with heating the TPyP:TSPP crystals above 90 ∘C causes approximately 25% loss in photoconductivity but has little effect on the lifetime associated with the persistent photoconductivity.

Ultrafast photoinduced anisotropy in GeSe2thin film.

In this Letter, we demonstrate for the first time that anisotropy can be induced at ultrafast time scales in an otherwise isotropic a-GeSe2 thin film using polarized femtosecond light. This photoinduced anisotropy (PA) spans the bandgap to the sub-bandgap region and self-annihilates over picosecond time scales. The ultrafast decay rate of PA is a clear indication that the observed effect is due to photoinduced transient defects in the sub-bandgap region and associated structural rearrangement in the near-bandgap region.

Modification of the thermal relaxation kinetics of the photoinduced (at T = 425 K) metastable dark conductivity of a-Si:H films by weak illumination during the initial stage of relaxation

The effect of weak illumination during the initial stage of relaxation of the dark metastable conductivity of an undoped a-Si:H film, photoinduced at T = 425 K, on the rate of its subsequent thermal relaxation is studied. It is found that the kinetics of relaxation upon illumination or in the absence of illumination is described by stretched exponents with the parameters τ0 and β, which are smaller in the case of illumination. It is shown that a decrease in these parameters increases the rate of thermal relaxation of the dark conductivity of the film. Because the temperature and the illumination intensities at which the study is carried out are low, the changes in the relaxation rate of the metastable conductivity are unlikely associated with significant restructuring of the amorphous network. This may be due to changes in the system of hydrogen bonds, which can result, in particular, from the generation and relaxation of slow photoinduced defects under the influence of illumination.

Pathway for recovery of photo-degraded polymer solar cells by post degradation thermal anneal

The photo-degradation of polymer solar cells is a critical challenge preventing its commercial deployment. We experimentally fabricate organic solar cells and characterize their degradation under solar simulators in an environmental chamber under nitrogen flow, without exposure to oxygen and moisture. We have developed a thermally stable inverted organic solar cell architecture in which light induced degradation of device characteristics can be reversibly annealed to the pristine values. The stable inverted cells utilized MoOx layers that are thermally treated immediately after their deposition on the organic layer, and before metal cathode deposition. Organic solar cells that are photo-degraded in the presence of oxygen, however show irreversible degradation that cannot be thermally recovered. The decrease of organic solar cell characteristics correlates with increases in mid-gap electronic states, measured using capacitance spectroscopy and dark current. It is likely the photo-induced defect states caused by local H motion from the alkyl chains to the aromatic backbone, can be reversibly annealed at elevated temperatures after photo-degradation. Our results provide a pathway for improving the stability of organic photovoltaics.

Direct surface relief formation in nanomultilayers based on chalcogenide glasses: A review

In this review the investigations of direct surface relief formations in nanomultilayers from chalcogenide glasses are summarized. Chalcogenide glasses are known to exhibit several photoinduced phenomena, both scalar and vectorial, which are connected with photoinduced structural transformations, defects creation, and atoms diffusion. Surface relief formation in chalcogenide glasses films has been intensively studied due to its applicability to reversibly form versatile patterns and diffractive optical elements. Both intensity and polarization holography have been employed to generate surface relief structures in chalcogenide glasses materials, including monolayers and multilayer structures. The research outlined here has not only led to better understanding of the material properties that affect the optical performance of chalcogenides structures, but also illustrated the momentum in the field that has led to the development of high-performance nanostructured devices.

Photoluminescence fatigue and inhomogeneous line broadening in semi-insulating Tl6SeI4 single crystals

Photoluminescence (PL) properties of semi-insulating Tl6SeI4 have been investigated. A broad emission band centered at 1.63 ± 0.02 eV was observed in all samples. The PL emission band is excitonic in nature and is tentatively attributed to a bound exciton emission. PL fatigue (a reduction in PL intensity under prolonged laser excitation) was always observed. The amount of PL fatigue depended on excitation power and temperature. PL fatigue kinetics are described by a stretched exponential with nominal lifetimes in the 10–265 s range. The recovery of the PL occurred within a few seconds of light cessation. The magnitude of PL fatigue in different samples correlated with inhomogeneous line broadening of the 1.63 eV emission band, such that broader bands exhibited more fatigue. An additional luminescence band centered at 1.78 eV was observed which increased in intensity under prolonged laser irradiation. The fatigue phenomenon is tentatively attributed to two mechanisms—the formation of photo-induced defects and the formation of quasi-stable particles. Both of these mechanisms introduce additional radiative and non-radiative recombination channels that lead to a decrease in the PL intensity under prolonged laser irradiation. Since inhomogeneous line broadening and PL fatigue are related to the concentration of defects or impurities, the measurement of these two parameters is an effective method to screen sample quality.

Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon

In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser micromachining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study femtosecond (800 nm, 110 fs, 0.1-1 mJ/pulse) micromachining of a material with well-defined initial defect concentrations: doped Si across a range of dopant types and concentrations. The single-pulse ablation threshold (Fth,1) was observed to decrease with increasing dopant concentration, from a maximum of 0.70 J/cm2 (+/-0.02) for undoped Si to 0.51 J/cm2 (+/-0.01) for highly N-type doped Si. The effect was greater for N-type doped Si than for P-type, consistent with the higher carrier mobility of electrons compared to holes. In contrast, the infinite-pulse ablation threshold (Fth,inf) was the same for all doping levels and types. We attribute this asymptotic behaviour to a maximum defect concentration that is independent of the initial defect concentration and type. These results lend insight into the mechanism of multipulse, femtosecond laser ablation.

Radiation damage in X-ray crystallography: a quantum-mechanical study of photoinduced defect formation in beeswax-analogue n-eicosane crystals

We study the nuclear dynamics of n-eicosane ( $$\hbox {C}_{20}\hbox {H}_{42}$$ ) in the crystalline state after photoirradation at room temperature using adiabatic ab initio excited-state dynamics based on hybrid time-dependent density-functional theory. We consider the weak perturbation (absorption) limit, in which an excited electron and a hole are simultaneously created in the system, and the strong perturbation (photoemission) regime, in which one electron is removed. We examine the changes in the carbon chain conformation occurring over timescales of the order of ca. 5 ps relative to the unperturbed (ground state) crystal structure at room temperature, which we simulate using standard ab initio molecular dynamics based on hybrid density-functional theory. Whereas the system retains its ground-state structure in the photoemission limit, the formation of structural defects, in the form of local distortions of the chain geometry, is observed in the absorption limit. We attribute the formation of these defects to the nuclear screening of the electron–hole pair created by photoexcitation. We discuss these findings in the context of radiation damage in organic/biological macromolecules and X-ray diffraction techniques.

Photoinduced defects in a-Si:H Films and InGaN/GaN multiple quantum well structures doped with Eu, Sm, and Eu + Sm

Photoinduced defects in a-Si:H films arise due to an increase in the density of midgap states when weak strained silicon–hydrogen (Si–H) bonds transform to dangling Si—Si bonds and also due to the presence of separate regions with different densities and types of Si—H bonds. In rare-earth-doped InGaN/GaN multiple quantum well structures, defects are induced as a result of increasing the luminescence excitation intensity in taking microphotoluminescence spectra. This complicates the spatial relief of the random potential mainly in the lateral plane and may result in clustering, with In content in the clusters differing from the mean value, and even precipitation of the InN and GaN phases.

Effects of photo-induced defects on the performance of PBDTTT-C/PC70 BM solar cells

In this work, the effects of photo-induced defects on polymer solar cells (PSCs) based on the blends of poly (4,8-bis-alkyloxybenzo (1,2-b:4,5-b′) dithiophene-2,6-diyl-alt-(alkyl thieno(3,4-b) thiophene-2-carboxylate)-2,6-diyl) (PBDTTT-C) and [6,6]-phenyl C70-butyric acid methyl ester (PC70BM) are investigated. In addition to conventional PSC characterization approaches, the capacitance–voltage (C–V) response and the ideality factor are used to analyze the changes in the built-in potential and defect-induced recombination at the internal donor/acceptor interface and the electrode interfaces in illuminated PSCs. The results show that the dominant trap states increase with increasing illumination time, which indicates that photo-induced defects play an important role in the photodegradation of inverted PSCs. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Phenomenological model of photoluminescence degradation and photoinduced defect formation in silicon nanocrystal ensembles under singlet oxygen generation

We propose a phenomenological model to explain photoluminescence degradation of silicon nanocrystals under singlet oxygen generation in gaseous and liquid systems. The model considers coupled rate equations, which take into account the exciton radiative recombination in silicon nanocrystals, photosensitization of singlet oxygen generation, defect formation on the surface of silicon nanocrystals as well as quenching processes for both excitons and singlet oxygen molecules. The model describes well the experimentally observed power law dependences of the photoluminescence intensity, singlet oxygen concentration, and lifetime versus photoexcitation time. The defect concentration in silicon nanocrystals increases by power law with a fractional exponent, which depends on the singlet oxygen concentration and ambient conditions. The obtained results are discussed in a view of optimization of the photosensitized singlet oxygen generation for biomedical applications.

Photoreduction of CO2 on BiOCl nanoplates with the assistance of photoinduced oxygen vacancies

CO2 photoreduction by semiconductors is of growing interest. Fabrication of oxygen-deficient surfaces is an important strategy for enhancing CO2 photoreduction activity. However, regeneration of the oxygen vacancies in photocatalysts is still a problem since an oxygen vacancy will be filled up by the O atom from CO2 after the dissociation process. Herein, we have fabricated highly efficient BiOCl nanoplates with photoinduced oxygen vacancies. Oxygen vacancies were easily regenerated by light irradiation due to the high oxygen atom density and low Bi-O bond energy even when the oxygen vacancies had been filled up by the O atom in the photocatalytic reactions. These oxygen vacancies not only enhanced the trapping capability for CO2, but also enhanced the efficiency of separation of electron-hole pairs, which resulted in the photocatalytic CO2 reduction under simulated solar light. Furthermore, the generation and recovery of the defects in the BiOCl could be realized during the photocatalytic reduction of CO2 in water. The existence of photoinduced defects in thin BiOCl nanoplates undoubtedly leads to new possibilities for the design of solar-driven bismuth based photocatalysts.

Controlling Photoinduced Electron Transfer Via Defects Self-Organization for Novel Functional Macromolecular Systems

The electrons transfer (ET) from an atom or a molecule, donor (D), to another, acceptor (A) is the basis of many fundamental chemical and physical processes. The ET mechanism is controlled by spatial arrangements of donor and acceptors: it’s the particular spatial arrangement and thus the particular distance and the orientation between the electron donors and acceptors that controls the efficiency in charge separation processes in nature. Here, we stress the importance of this concept reviewing how spatial distribution of atomic and molecular self-assembly can determine the quality and physical features of ET process from biology to material science. In this context, we propose novel lab-on-chip techniques to be used to control spatial distribution of molecules at nanoscale. Synchrotron source brightness jointly to focusing optics fabrication allows one nowadays to monitor and visualize structures with sub-micrometric spatial resolution. This can give us a new powerful tool to set up sophisticated X-ray imaging techniques as well as spectroscopic elemental and chemical mapping to investigate the structure-function relationship controlling the spatial arrangement of the molecules at nanoscale. Finally, we report intriguing recent case studies on the possibility to manipulate and control this spatial distribution and material functionality at nanoscale by using X ray illumination.

Improvement of the proton exchange membrane fuel cell (PEMFC) performance at low-humidity conditions by exposing anode in Ultraviolet light

External humidifying system is generally needed for the fully hydration of membrane and catalyst layer, but it is considered as a barrier to PEMFC all the time. In this study, ultraviolet (UV) light was served as external stimulus to increase the wettability of anode which consists of photo-sensitive hygroscopic materials titanium oxide (TiO2). Under UV irradiation, water adsorbed on the photo-induced oxygen defects of TiO2 and transformed into surface hydroxyl groups. The generated hydroxyl groups furthered the back diffusion and decreased the charge resistance, which improved the performance at the low-humidity condition. The problems on the introduction of excessive hygroscopic materials are avoided by the new method.

The dynamics of photoinduced defect creation in amorphous chalcogenides: The origin of the stretched exponential function

The article discusses the dynamics of photoinduced defect creations (PDC) in amorphous chalcogenides, which is described by the stretched exponential function (SEF), while the well known photodarkening (PD) and photoinduced volume expansion (PVE) are governed only by the exponential function. It is shown that the exponential distribution of the thermal activation barrier produces the SEF in PDC, suggesting that thermal energy, as well as photon energy, is incorporated in PDC mechanisms. The differences in dynamics among three major photoinduced effects (PD, PVE, and PDC) in amorphous chalcogenides are now well understood.

Improved Sensitization of Zinc Oxide Nanorods by Cadmium Telluride Quantum Dots through Charge Induced Hydrophilic Surface Generation

This paper reports on UV‐mediated enhancement in the sensitization of semiconductor quantum dots (QDs) on zinc oxide (ZnO) nanorods, improving the charge transfer efficiency across the QD‐ZnO interface. The improvement was primarily due to the reduction in the interfacial resistance achieved via the incorporation of UV light induced surface defects on zinc oxide nanorods. The photoinduced defects were characterized by XPS, FTIR, and water contact angle measurements, which demonstrated an increase in the surface defects (oxygen vacancies) in the ZnO crystal, leading to an increase in the active sites available for the QD attachment. As a proof of concept, a model cadmium telluride (CdTe) QD solar cell was fabricated using the defect engineered ZnO photoelectrodes, which showed ∼10% increase in photovoltage and ∼66% improvement in the photocurrent compared to the defect‐free photoelectrodes. The improvement in the photocurrent was mainly attributed to the enhancement in the charge transfer efficiency across the defect rich QD‐ZnO interface, which was indicated by the higher quenching of the CdTe QD photoluminescence upon sensitization.

Suppression of Photoinduced BBO Defects Generation on TiO2(110) by Water

We have investigated creation of variable concentrations of defects on TiO2(110)−(1×1) surface by 266 nm laser using temperature programmed desorption technique. Oxygen-vacancy defects can be easily induced by ultraviolet light, the defects concentration has a linear dependence on power density higher than 50 mW/cm2 for 90 s irradiation. No observation of O2 molecule and Ti atom desorption suggests that UV induced defects creation on TiO2(110)−(1×1) is an effective and gentle method. With pre-dosage of thin films of water, the rate of defects creation on TiO2(110)−(1×1) is slower at least by two orders of magnitude than bare TiO2(110)−(1×1) surface. Further investigations show that water can be more easily desorbed by UV light, and thus desorption of bridging oxygen is depressed.