Energetic Cascades Research Articles

Unraveling Optical and Electrical Gains of Perovskite Solar Cells with an Antireflective and Energetic Cascade Electron Transport Layer

Electron transport layers (ETLs) are imperative in n-i-p structured perovskite solar cells (PSCs) because of their capability to affect light propagation, electron extraction, and perovskite crystallization, and any mismatch of optical constants, band position, and surface potential between the ETLs and the perovskites can cause unintentional optical and electrical losses. Herein, an antireflective and energetic cascade bilayer ETL with ubiquitously used SnO2 and TiO2 was constructed at 150 °C for PSCs, and the in-depth mechanism for performance improvement was systematically unraveled. It was revealed that the construction of an ETL with gradually increasing refractive indices can circumvent light reflection loss, resulting in enhanced photocurrent. The combined ETL forms an energetic cascade to promote electronic conductivity and facilitate electron extraction with reduced energy loss. Moreover, topologic perovskite growth with improved crystallinity and vertical orientation was preferred owing to the relative dewetting behavior, leading to reduced defect states and enhanced carrier mobility in the perovskite layer.

Multi-scale dynamics of Kelvin–Helmholtz instabilities. Part 2. Energy dissipation rates, evolutions and statistics

Fritts et al. (J. Fluid Mech., vol. xx, 2022, xx) describe a direct numerical simulation of interacting Kelvin–Helmholtz instability (KHI) billows arising due to initial billow cores that exhibit variable phases along their axes. Such KHI exhibit strong ‘tube and knot’ dynamics identified in early laboratory studies by Thorpe (Geophys. Astrophys. Fluid Dyn., vol. 34, 1985, pp. 175–199). Thorpe (Q.J.R. Meteorol. Soc., vol. 128, 2002, pp. 1529–1542) noted that these dynamics may be prevalent in the atmosphere, and they were recently identified in atmospheric observations at high altitudes. Tube and knot dynamics were found by Fritts et al. (J. Fluid. Mech., 2022) to drive stronger and faster turbulence transitions than secondary instabilities of individual KH billows. Results presented here reveal that KHI tube and knot dynamics also yield energy dissipation rates $\sim$ 2–4 times larger as turbulence arises and that remain $\sim$ 2–3 times larger to later stages of the flow evolution, compared with those of secondary convective instabilities (CI) and secondary KHI accompanying KH billows without tube and knot influences. Elevated energy dissipation rates occur due to turbulence transitions by tube and knot dynamics arising on much larger scales than secondary CI and KHI where initial KH billows are misaligned. Tube and knot dynamics also excite large-scale Kelvin ‘twist waves’ that cause vortex tube and billow core fragmentation, more energetic cascades of similar interactions to smaller scales and account for the strongest energy dissipation events accompanying such KH billow evolutions.

Investigation of the different possible energy band structure configurations for planar heterojunction organic solar cells

We present a study of performances of organic photovoltaic cells (OPVs) using a wide variety of small molecules. These OPVs use two or three molecules to form binary or ternary cells, i.e. consisting of two or three flat stacked layers. Different ternary OPVs designs for efficient light harvesting and energy band structure combinations are studied. Molecules have been chosen to study all possible energy band structure alignments in ternary configuration. We discuss various parameters limiting their efficiency, such as carrier mobility, organic layer morphology, energy band structure alignment. Special attention has been paid to understand the device behavior. It is shown that, if light absorption domains, carriers mobility, energy band structure alignment are decisive for achieving performing OPVs, the geometry of the molecules is also decisive. In ternary OPVs, the third organic layer, intercalated between the electron donor and the electron acceptor, must serve as a bridge in the energy levels of the outer layers, which permits an energetic cascade effect. For this effect to be positive it is necessary that this intermediate layer facilitates creation and transport of charge carriers, but also that it allows transport of energy, all leading to an improvement of the performance of the OPVs.

Structural stability of titanate pyrochlores undergoing radiation damage

Pyrochlores are the candidate materials for the nuclear storage materials. We use molecular dynamics simulations to compare the properties of titanate pyrochlores Y2Ti2O7 and Gd2Ti2O7 undergoing energetic cascades. An extensive set of data corresponding to different conditions for the initiation of the cascades is generated to obtain good statistics. Study of temperature shows that the thermal conductivity during the cascade decreases more in Y2Ti2O7 than that in Gd2Ti2O7. Both the titanate pyrochlores are quite resistant to amorphization however Gd2Ti2O7 is seen to be more robust in resisting the damage during the whole span of the displacement cascade.

APPLICATION OF ATHERMAL RECOMBINATION CORRECTED DPA MODEL IN NUCLEAR DATA PROCESSING CODE NECP-ATLAS

The Displacement per Atom (DPA) is an important factor used to quantify the irradiation damage of materials. The radiation damage energy production cross section obtained by nuclear data processing code is an essential nuclear data used to calculate the DPA of materials. Nuclear data processing code, such as NJOY, adopts NRT-DPA model to calculate the radiation damage energy production cross section. However, the NRT-DPA model has several well-known limitation. Especially, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-DPA model prediction. To improve the precision accuracy of DPA calculation, the state-of-the-art Athermal Recombination Corrected DPA (ARC-DPA) model is adopted in heat production and radiation damage energy production cross section module of nuclear data processing code NECP-Atlas. ARC-DPA model can be obtained by simply multiplying with the new efficiency function. The parameters in the new efficiency function are material constants that can be determined for a given material from Molecular Dynamics (MD) simulations or experiments. The material constants of some materials are obtained by MD simulations firstly in this paper. Numerical results show that the radiation damage cross section produced by nuclear data processing code NECP-Atlas adopting NRT-DPA model can agree well with the NJOY code. What’s more, the results from NECP-Atlas adopting ARC-DPA model can provide more physically realistic descriptions of primary defect and agree well with the results from MD simulation numerically.

Morphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell.

The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.

Synthesis, photophysical characterization and dye adsorption behavior in unsymmetrical squaraine dyes with varying anchoring groups

Four newly designed unsymmetrical squaraine dyes bearing different functional groups for their anchoring on the surface of mesoporous TiO2 were successfully synthesized aiming towards development of far-red sensitizers for dye-sensitized solar cells (DSSCs). The synthesized dyes were characterized by 1H NMR, fast ion bombardment mass, and subjected to photophysical investigations by electronic absorption/fluorescence emission spectroscopy and cyclic voltammetry. These dyes exhibited excellent solubility in a number of common organic solvents. Adsorption behavior of these dyes on the thin films of mesoporous TiO2 was investigated in detail and results indicated that rate of dye adsorption follows the order −COOH>-PO3H2>OH > SO3H. At the same time, dye desorption studies demonstrated that stability of adsorbed dyes on the mesoporous TiO2 follows the order -PO3H2>>OH>−COOH > –SO3H. Further, implication of the nature of anchoring groups of dye molecules upon the energy levels of their highest occupied molecular orbital and lowest unoccupied molecular orbital was evaluated by both of the combined theoretical and experimental approaches. In spite of enhanced dye loading, very high binding strength and favorable energetic cascade, SQ-143 bearing Phosphonic acid exhibited hampered photovoltaic performance as compared to that of SQ-138 with carboxylic acid anchoring group. Amongst various sensitizers used for present investigation, SQ-138 exhibited best photovoltaic performance having short-circuit current density, open circuit voltage and fill factor of 12.49 mA/cm2, 0.60 V, and 0.53, respectively, leading to power conversion efficiency of 4.07 % after simulated solar irradiation.

Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction

Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination-poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient Stark effect, caused by nanoscale electric fields of ∼487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ∼2000 times less than predicted by Langevin theory. This in turn leads to the buildup of electric charge in donor and acceptor domains-away from the interface-resistant to bimolecular recombination. Interestingly, this signal is only experimentally obvious in thick films due to the different scaling of electroabsorption and photoinduced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of >8% because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies the bulk heterojunction can go beyond its established role in charge photogeneration and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination.

Suppression of Recombination Losses in Polymer:Nonfullerene Acceptor Organic Solar Cells due to Aggregation Dependence of Acceptor Electron Affinity

AbstractHere, it is investigated whether an energetic cascade between mixed and pure regions assists in suppressing recombination losses in non‐fullerene acceptor (NFA)‐based organic solar cells. The impact of polymer‐NFA blend composition upon morphology, energetics, charge carrier recombination kinetics, and photocurrent properties are studied. By changing film composition, morphological structures are varied from consisting of highly intermixed polymer‐NFA phases to consisting of both intermixed and pure phase. Cyclic voltammetry is employed to investigate the impact of blend morphology upon NFA lowest unoccupied molecular orbital (LUMO) level energetics. Transient absorption spectroscopy reveals the importance of an energetic cascade between mixed and pure phases in the electron–hole dynamics in order to well separate spatially localized electron–hole pairs. Raman spectroscopy is used to investigate the origin of energetic shift of NFA LUMO levels. It appears that the increase in NFA electron affinity in pure phases relative to mixed phases is correlated with a transition from a relatively planar backbone structure of NFA in pure, aggregated phases, to a more twisted structure in molecularly mixed phases. The studies focus on addressing whether aggregation‐dependent acceptor LUMO level energetics are a general design requirement for both fullerene and NFAs, and quantifying the magnitude, origin, and impact of such energetic shifts upon device performance.

Improving atomic displacement and replacement calculations with physically realistic damage models

Atomic collision processes are fundamental to numerous advanced materials technologies such as electron microscopy, semiconductor processing and nuclear power generation. Extensive experimental and computer simulation studies over the past several decades provide the physical basis for understanding the atomic-scale processes occurring during primary displacement events. The current international standard for quantifying this energetic particle damage, the Norgett−Robinson−Torrens displacements per atom (NRT-dpa) model, has nowadays several well-known limitations. In particular, the number of radiation defects produced in energetic cascades in metals is only ~1/3 the NRT-dpa prediction, while the number of atoms involved in atomic mixing is about a factor of 30 larger than the dpa value. Here we propose two new complementary displacement production estimators (athermal recombination corrected dpa, arc-dpa) and atomic mixing (replacements per atom, rpa) functions that extend the NRT-dpa by providing more physically realistic descriptions of primary defect creation in materials and may become additional standard measures for radiation damage quantification.

The Role of Exciton Delocalization in the Major Photosynthetic Light-Harvesting Antenna of Plants

In the major peripheral plant light-harvesting complex LHCII, excitation energy is transferred between chlorophylls along an energetic cascade before it is transmitted further into the photosynthetic assembly to be converted into chemical energy. The efficiency of these energy transfer processes involves a complicated interplay of pigment-protein structural reorganization and protein dynamic disorder, and the system must stay robust within the fluctuating protein environment. The final, lowest energy site has been proposed to exist within a trimeric excitonically coupled chlorophyll (Chl) cluster, comprising Chls a610-a611-a612. We studied an LHCII monomer with a site-specific mutation resulting in the loss of Chls a611and a612, and find that this mutant exhibits two predominant overlapping fluorescence bands. From a combination of bulk measurements, single-molecule fluorescence characterization, and modeling, we propose the two fluorescence bands originate from differing conditions of exciton delocalization and localization realized in the mutant. Disruption of the excitonically coupled terminal emitter Chl trimer results in an increased sensitivity of the excited state energy landscape to the disorder induced by the protein conformations. Consequently, the mutant demonstrates a loss of energy transfer efficiency. On the contrary, in the wild-type complex, the strong resonance coupling and correspondingly high degree of excitation delocalization within the Chls a610-a611-a612 cluster dampens the influence of the environment and ensures optimal communication with neighboring pigments. These results indicate that the terminal emitter trimer is thus an essential design principle for maintaining the efficient light-harvesting function of LHCII in the presence of protein disorder.

In-situ TEM observation of nano-void formation in UO2 under irradiation

Transmission electron microscopy (TEM) observations of UO2 polycrystals irradiated in situ with 4MeV Au ions were performed at room temperature (RT) to better understand the mechanisms of cavity and ultimately fission products nucleation in UO2. Experiments were carried out at the JANNuS Orsay facility that enables in situ ion irradiations inside the microscope to be carried out. The majority of 4MeV gold ions were transmitted through the thin foil, and the induced radiation defects were investigated by TEM. Observations showed that nano-void formation occurs at ambient temperature in UO2 thin foils irradiated with energetic heavy ions under an essentially nuclear energy loss regime. The diameter and density of nano-objects were measured as a function of the gold irradiation dose at RT. A previous paper has also revealed a similar nano-object population after a Xe implantation performed at 390keV at 870K. The nano-object density was modelled using simple concepts derived from Classical Molecular Dynamics simulations. The results are in good agreement, which suggests a mechanism of heterogeneous nucleation induced by energetic cascade overlaps. This indicates that nano-void formation mechanism is controlled by radiation damage. Such nanovoids are likely to act as sinks for mobile fission products during reactor operation.

Mechanism for thermal conductivity in energetic displacement cascades

Temperature relaxation inside and outside the energetic displacement cascade in fast neutron-irradiated metals is consistently described. The characteristic time of the energy transfer between phonons and electrons in the damaged area is calculated. The space–time temperature distribution in the cascade damaged area in Fe and Ni is presented. The electron–phonon coupling is shown to play an important role in the evolution of the damaged area due to non-equilibrium between the local phonon and electron systems at the beginning of the cooling phase of the displacement cascade.

A Desirable Hole‐Conducting Coadsorbent for Highly Efficient Dye‐Sensitized Solar Cells through an Organic Redox Cascade Strategy

Over the past decade, dye-sensitized solar cells (DSSCs) based on nanocrystalline mesoporous metal oxides (typically TiO2) have been intensively studied and developed as a promising, low-cost alternative to conventional silicon photovoltaic devices. The components of such DSSCs are a dye sensitizer, a TiO2 metal oxide coated on conductive glass, a redox electrolyte couple, and a counter electrode. As illustrated in Scheme 1, the power-conversion efficiency of DSSCs is strongly dependent upon the minimization of charge recombination losses at the TiO2/dye/electrolyte interface (pathways 3 and 4). There are two recombination pathways of importance in DSSCs, in which electrons photoinjected into the TiO2 electrode can recombine with either dye cations or with redox electrolytes. Moreover, such charge recombination leads to losses in both the short-circuit photocurrent (Jsc) and the open-circuit photovoltage (Voc), resulting in a decrease in overall energy conversion efficiency (h). To reduce the possible charge recombination pathways occurring at the TiO2/dye/electrolyte interface, several kinds of additives, such as decylphosophonic acid (DPA), dineohexyl bis(3,3-dimethylbutyl)phosphinic acid (DINHOP), and chenodeoxycholic acid (CDCA) have been introduced to adsorb onto the TiO2 surface. [3] Among them, for example, cholic acid (CA) derivatives as coadsorbents in DSSCs, based on a Ru–pyridyl complex, courmarin, porphyrin, phthalocyanine, and naphthalocyanine dye, have been investigated well. Deoxycholic acid (DCA) is often used as a coadsorbent to break up organic and Ru–dye aggregates and hence to significantly improve Voc and Jsc. [5,9] Moreover, alternative approaches directed toward the minimization of recombination losses have been extensively studied by not only the insertion of a metal oxide blocking layer, but also by energetic cascades for multistep hole conductors (HC), as well as superior molecular sensitizer dyes and the insertion of a barrier layer between the sensitizer dye and the HC. To date, however, the desired redox intermediate mediators between the dye and I /I3 redox electro-

Experimental and Computational Studies of Ion-Solid Interactions in Silicon Carbide

ABSTRACTExperimental and computational results on ion-beam-induced defect production, damage accumulation, and thermal recovery in SiC are reviewed. The accumulation and recovery of disorder on the Si and C sublattices have been determined experimentally by ion-channeling methods. Atomic-level simulations are used to determine defect production, cascade-overlap effects, and defect migration energies. Energetic Si and C collision cascades, with energies up to 50 keV, primarily produce single interstitials, mono-vacancies, antisite defects, and small defect clusters. Overlapping of Si and C cascades results in the interaction of defects and clusters that stimulates cluster growth and produces long-range structural disorder. For energetic Au cascades, nanoscale amorphous clusters are produced directly within about 25% of the Au cascades, along with point defects and smaller clusters. The disordering behavior and the changes in volume and elastic modulus obtained experimentally and from molecular dynamics simulations are in good agreement, thus providing atomic-level interpretation of experimentally observed features. Simulations of close-pair production and recombination in SiC indicate that the activation energies for recombination of most close pairs range from 0.24 to 0.38 eV. Multiaxial channeling measurements indicate annealing below 300 K results in relaxation of some interstitials to lower-energy configurations. Long-range migration energies for interstitials and vacancies have likewise been determined by computational methods.

Vacancy clustering model for Ti non-steady-state radiation-enhanced diffusion in MgO(100)

Ti diffusion in MgO(100) prebombarded by Ar+, Cl+, and Cr+ has been studied by secondary ion mass spectroscopy under conditions for non-steady-state radiation-enhanced diffusion. The Ti diffusion behaviors of these samples are different, although the ballistic behaviors of the implanting ions are similar. For short annealing periods, the Ti diffusion enhancements follow the order Cr+>Ar+>Cl+, while for long annealing periods the order is Cl+>Ar+>Cr+. A model based on a vacancy clustering mechanism during thermal relaxation of the energetic cascade is proposed to explain these trends.

Compositional variation of microstructure in ion-implanted AlxGa1−xAs

The ion damage produced in alloys of AlxGa1−xAs (x = 0.6, 0.7, 0.8, and 0.85) by implantation at 77 K with Kr ions (500, 700, and 1500 keV) was studied by using Rutherford backscattering channeling and transmission electron microscopy. In addition, the accumulation of ion damage at 50 K was studied by performing the ion implantations in situ in the transmission electron microscope. In Al0.8Ga0.2As, damage accumulation at 77 K was independent of dose rate, indicating that dynamic annealing is not occurring at 77 K. The in situ studies demonstrated that planar defects are produced on warm-up from 50 K to room temperature, indicating that they are not the nucleation site for amorphization. The lower energy implantations revealed that amorphization initiated within the AlxGa1−xAs layer, showing that heterointerfaces are not required for amorphization. These results, along with the similarity of the room-temperature microstructures in the different alloys, imply that the amorphization mechanism is independent of Al content. It is proposed that the observed dependence of the amorphization dose on Al content is related to an increase in the number of cascade overlaps needed to initiate and to produce a continuous amorphous layer. A mechanism explaining the microstructural changes with composition, based on the thermal and physical properties of the alloy and on the distribution of energetic cascade events, is presented.

Behavior and computer simulation of SiC under irradiation with energetic particles

This paper reviews previous works focused on fundamental neutron damage simulations in β-SiC. The reason to propose ceramic composites based on SiC, instead of the monolithic form, is concluded, and the effect of radiation (neutron irradiation) on the macroscopic responses of the material are shown. Molecular dynamics simulations of threshold displacement energy, defect energetics, melting point, and 3–5 keV cascades are presented, together with a criticism of various interatomic potentials such as Pearson, Tersoff, and Modified Embedded Atom Model. Most of the presented results have been performed with Tersoff's potential which has demonstrated a better physics description of the material's parameters. The directional dependence of the threshold energy is mentioned, and the consequences of energetic cascades such as potential amorphization are compared with results of Si. The effect of the accumulation of damage, at a rate higher than expected in real Inertial Confinement Fusion applications is also mentioned.

Electron-phonon coupling in molecular dynamics codes

Abstract Computer simulations using molecular dynamics (MD) have become the standard tool to describe the dynamics of energetic collision cascades. However, several problems concerning the physical description within MD schemes still remain open. One of the most challenging is the fact that MD simulations describe the thermal behavior of an insulator, neglecting the conductivity due to electrons. In this work we review some of the relevant ideas proposed to incorporate this effect.

Molecular dynamics simulation of irradiation damage cascades in copper using a many-body potential

Abstract The evolution of irradiation damage cascades in copper has been simulated by molecular dynamics, using a many-body potential. Nearly 200 cascades have been produced with random knock-on directions and primary knock-on atom (PKA) energies in the range from 60 eV to 10 keV. The starting temperature for these simulations was 100 K and 600 K, this report will confine itself to the data obtained at 100 K. The cascade evolution has been followed for times typically up to ∼10 psec and in some cases up to ∼30 psec. The cascades are characterised by the sudden emission of replacement collision sequences and with shape variations due to local channelling events. At the higher energies the core has been shown to be liquid-like structure with cavitation. The annealing phase leaves loosely clustered vacancies at the cascade centre but collapse to a vacancy loop is not generally observed. A feature of the more energetic cascades is the production by a ballistic mechanism of interstitial atom clusters at the pe...