| 1. |
- Anttu, Nicklas, et al.
(författare)
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Tailored emission to boost open-circuit voltage in solar cells
- 2019
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Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 3:5
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Tidskriftsartikel (refereegranskat)abstract
- Recently, a lot of research focus has been on how to make solar cells more efficient. One direction is to enhance the open-circuit voltage Voc by optimizing the emission of photons in the cell, where emission is a necessary loss process due to the reciprocity between absorption and emission of light. Here, we performed a Shockley-Queisser detailed balance analysis to predict the benefit of managing emitted photons in a single-junction solar cell. First, at low internal luminescence efficiency ηint, non-radiative recombination dominates, and management of emitted photons plays negligible role for Voc . Similarly, for an external luminescence efficiency ηext<10%, externally emitted photons play negligible role, and Voc is set either by non-radiative recombination; or parasitic absorption of internally emitted photons. For higher ηext, the Voc can be boosted, maximally by 15%, by restricting the external emission to match the incidence cone of the AM1.5D sun light spectrum. Such emission restriction corresponds to lower escape probability of internally emitted photons, enhances photon recycling, drops ηext, and actually makes the solar cell into a worse LED. Finally, for partly diffuse incident light, by restricting the angular emission for photons in a 130 nm wavelength range around the bandgap, we predict a maximum 14% relative boost in solar cell efficiency. The results of this paper are intended to serve as a general guideline on how to utilize emission-tuning possibilities to develop highly efficient photovoltaic devices.
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| 2. |
- Kalaee, Alex Arash Sand, et al.
(författare)
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Electron extraction from excited quantum dots with higher order coulomb scattering
- 2020
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Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 4:3
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Tidskriftsartikel (refereegranskat)abstract
- The electron kinetics in nanowire-based hot-carrier solar cells is studied, where both relaxation and extraction are considered concurrently. Our kinetics is formulated in the many-particle basis of the interacting system. Detailed comparison with simplified calculations based on product states shows that this includes the Coulomb interaction both in lowest and higher orders. While relaxation rates of 1 ps are obtained, if lowest order processes are available, timescales of tens of ps arise if these are not allowed for particular designs and initial conditions. Based on these calculations we quantify the second order effects and discuss the extraction efficiency, which remains low unless an energy filter by resonant tunnelling is applied.
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| 3. |
- Kumar, Pushpendra, et al.
(författare)
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Two-photon excitation spectroscopy of 1,5–Diphenyl-1,3,5-hexatriene using phase modulation
- 2019
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Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 3
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Tidskriftsartikel (refereegranskat)abstract
- We have used two-photon Fourier transform spectroscopy to investigate the first singlet excited state (S1) of a prototypical polyene molecule 1,5—Diphenyl-1,3,5-hexatriene. As the S1 state in the polyenes is a one-photon forbidden transition, the structure of its vibrational levels cannot be studied using resonant linear excitation. Although this level is accessible with two-photon excitation, previous studies done by using wavelength tunable pulsed lasers did not have enough resolution to investigate the details of the vibrational levels. In Fourier transform spectroscopy, one uses a pair of laser beams to excite the sample. The measurements are done by varying the time delay between the pulses. The spectral resolution is given by the inverse of the maximum time delay rather than the spectral width of the pulses. We have used the method to investigate the vibrational levels of the S1 state. In our implementation, we have used phase modulation to carry out the measurements in the rotating frame, which requires less data points along the time delay thereby significantly reducing the measurement time.
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| 4. |
- Lindgren, Georg, et al.
(författare)
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Effective persistency evaluation via exact excursion distributions for random processes and fields
- 2022
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Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Finding the probability that a stochastic system stays in a certain region of its state space over a specified time—a long-standing problem both in computational physics and in applied and theoretical mathematics—is approached through the extended and multivariate Rice formula. In principle, it applies to any smooth process multivariate both in argument and in value given that efficient numerical implementations of the high-dimensional integration are available. The computational method offers an exact integral representation yielding remarkably accurate results and provides an alternative method of computing persistency probability and exponent for a physical system. It can be viewed as an implementation of path integration for a smooth Gaussian process with an arbitrary covariance. Its high accuracy is due to efficient computation of expectations with respect to high-dimensional nearly singular Gaussian distributions. For Gaussian processes, the computations are effective and more precise than those based on the Rice series expansions and the independent interval approximation. For the benchmark diffusion process, it produces the persistency exponent that is essentially the same as the recently obtained analytical value and surpasses accuracy, interpretability as well as control of the error, previous methods including the independent or Markovian approximation. The method solves the two-step excursion dependence for a stationary differentiable Gaussian process, in both theoretical and numerical sense. The solution is based on exact expressions for the probability density for one and two successive excursion lengths. The numerical routine RIND computes the densities using recent advances in scientific computing and is easily accessible for a general covariance function, via a simple numerical interface. The work offers also analytical results that explain the effectiveness of the implemented methodology and elaborates its utilization for non-Gaussian processes.
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| 5. |
- Aljure, Mauricio, et al.
(författare)
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On the injection and generation of charge carriers in mineral oil under high electric fields
- 2019
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Ingår i: JOURNAL OF PHYSICS COMMUNICATIONS. - : IOP PUBLISHING LTD. - 2399-6528. ; 3:3
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Tidskriftsartikel (refereegranskat)abstract
- Charge injection and generation mechanisms under intense electric fields (up to 10(9)Vm(-1)) in mineral oil are assessed experimentally and numerically. For this, current-voltage characteristics under positive and negative polarities are measured in a needle-plane configuration using sharp needles (with tip radius R-tip <= 1.1 mu m). In addition, a state of the art electro-hydrodynamic (EHD) model is implemented to calculate the contribution of the different mechanisms on the high-field conduction currents in the liquid. In order to evaluate exclusively the contribution of field emission, experiments are also performed in vacuum. It is found that neither field emission nor field ionisation can explain the conduction currents measured in mineral oil. It is proposed that field molecular ionisation, as described by Zener tunnelling model for solids, and electron impact ionisation are the processes dominating the generation of excess electron-ion pairs in mineral oil under positive and negative polarity, respectively. It is also shown that Zener molecular ionisation alone grossly overestimates the measured currents when parameters previously suggested in the literature for mineral oil are used. Preliminary model parameters for these mechanisms that best fit the conduction currents measured in mineral oil are presented and discussed.
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| 6. |
- Hansson, Johan
(författare)
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The Klein-Alfvén cosmology revisited
- 2019
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Ingår i: Journal of Physics Communication. - : Institute of Physics (IOP). - 2399-6528. ; 3:11
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Tidskriftsartikel (refereegranskat)abstract
- The Klein-Alfvén model is based on the pragmatic belief that also cosmology, just like all other fields of physics, should be based on physical laws independently tested in the laboratory. It actually has a number of attractive features, described in this article. As almost all matter in the known universe is in the plasma state, the model is by necessity based on both gravity and electromagnetism, and as most cosmic plasmas are inhomogeneous and magnetized, it is automatically inhomogeneous (as is the real universe). It is not perfect (no models are), but many of the outstanding unsolved 'problems' of the contemporary standard big bang-model of cosmology are either solved/sidestepped by, or non-existent in, the Klein-Alfvén model. One should remember that the standard model of cosmology also is just that—a model, and highly idealized at that, with many ad hoc ingredients and a large number of free parameters and hypothetical ingredients that are fixed only through comparison with cosmological data in a global best-fit fashion. It is not, and should never be considered to be, sacrosanct. If a comparable number of man-hours had been invested in the direction of the Klein-Alfvén model it is plausible that it would describe the real observed universe as good as, or even better than, the big bang-model—with much fewer speculative additions to known physics.
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| 7. |
- Ariza, David, et al.
(författare)
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Positive streamers : inception and propagation along mineral-oil/solid interfaces
- 2020
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Ingår i: Journal of Physics Communications. - : Institute of Physics (IOP). - 2399-6528. ; 4:2
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents an experimental characterization of the prebreakdown phenomena in liquid/solid interfaces. The characterization is devoted to the 2nd mode positive streamers initiated and propagated along interfaces of mineral-oil and solids with different chemical composition and physical properties. Polymers of low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) polyvinylidene fluoride (PVDF) and papers made of kraft paper and a kraft fibril paper (made from cellulosic micro and nano fibrils), lignin-free paper and paper with high lignin content (referred to as k107 kraft paper) are used as the solid to study their influence on the streamer inception and propagation. The streamers are initiated at the interface by applying steps of voltage to a point-plane electrode arrangement with a solid (dielectric barrier) into the gap. The solid is placed diagonal to the oil gap and near to the point electrode. Shadowgraphs, charge and light intensity recordings are obtained during the inception and propagation of the streamers. Thus, estimations of the streamer length, velocity, current and average charge, are also presented. A time delay has been observed before the initiation of the streamer. This delay is probably correlated to the initiation process and formation of the gaseous phase of the streamer near to the interface. The threshold propagation voltage of the 2nd mode streamers at mineral-oil/solid interfaces is shown to be independent of the interface. However, the inception voltage is highly influenced by the interface. Additionally, the observed characteristics of streamers propagation (e.g. current, length, velocity, etc) along the tested interfaces cannot be fully explained by a capacitive coupling effect (permittivity mismatch). This open a discussion for the possibility that properties of the solid such as chemical composition, wettability and surface roughness can influence the streamer propagation.
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| 8. |
- Ayoub, M. W. Ben, et al.
(författare)
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Quantification of free and bound water in selected materials using dielectric and thermo-coulometric measurement methods
- 2018
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Ingår i: Journal of physics communications. - : Institute of Physics Publishing (IOPP). - 2399-6528. ; 2:3
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Tidskriftsartikel (refereegranskat)abstract
- Broadband electromagnetic characterization offers useful tools for the quantitative estimation of water content in a insulating solid materials. However, the correlation between electromagnetic characteristics and the amount of water in solids needs to be characterized using precise direct measurement methods. In this study, a two-port coaxial transmission cell has been characterized for water content estimation based on a study with the thermo-coulometric water detection method. The latter allows us to determine the proportions of the different water bonding forms that can exist in the material. The purpose of this paper is to provide a dielectric relaxation behavior study of the kaolinite clay from 10 MHz to 1.5 GHz, which was preceded by an experimental analysis of alpha-D-lactose monohydrate and the calcium oxalate monohydrate, which have stable water content under various humidity conditions.
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| 9. |
- Becerra Garcia, Marley, 1979-, et al.
(författare)
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High field conduction in mineral oil based zno nanofluids prior to negative streamer inception
- 2021
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Ingår i: Journal of Physics Communications. - : IOP Publishing. - 2399-6528. ; 5:4
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Tidskriftsartikel (refereegranskat)abstract
- The electric conduction under intense electric fields (up to ∼ 10 V/m) in nanofluids using surface-modified ZnO–C nanoparticles dispersed in mineral oil as host, is investigated with both experiments and numerical simulations. The measurements are used to estimate unknown parameters necessary to represent the generation and loss of electrons in an electrohydrodynamic model for mineral oil with and without ZnO–C nanoparticles in a needle-plane configuration. The model suggests that ZnO–C nanoparticles induce an enhanced field emission from negative needles, explaining the significantly larger conduction currents measured in the nanofluid compared with those in the host liquid. It is also found that the scavenging of electrons by ZnO–C nanoparticles is a process which is negligible compared with the loss of electrons due to attachment in mineral oil. It is shown that ZnO–C nanoparticles hinder the streamer initiation process by reducing the effective electric field at the tip of the needle. This electric field reduction is caused by the combined effect of enhanced electron injection through ZnO–C nanoparticles and strong electron attachment in mineral oil. Thus, the electric field on the needle tip reaches the same threshold value when the streamer is incepted in the nanofluid as in mineral oil, although at a larger voltage. Solid evidence indicating that the additional electron scavenging and the reduced electron mobility introduced by nanoparticles has no effect in the conduction currents and in the negative streamer inception in the tested ZnO–C nanofluids is shown.
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| 10. |
- Johansson, Magnus, 1965-, et al.
(författare)
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Nonlinear gap modes and compactons in a lattice model for spin-orbit coupled exciton-polaritons in zigzag chains
- 2019
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Ingår i: Journal of Physics Communications. - : Institute of Physics Publishing (IOPP). - 2399-6528. ; 3:1, s. 1-17
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Tidskriftsartikel (refereegranskat)abstract
- We consider a system of generalized coupled Discrete Nonlinear Schrödinger (DNLS) equations, derived as a tight-binding model from the Gross-Pitaevskii-type equations describing a zigzag chain of weakly coupled condensates of exciton-polaritons with spin-orbit (TE-TM) coupling. We focus on the simplest case when the angles for the links in the zigzag chain are ±π/4 with respect to the chain axis, and the basis (Wannier) functions are cylindrically symmetric (zero orbital angular momenta). We analyze the properties of the fundamental nonlinear localized solutions, with particular interest in the discrete gap solitons appearing due to the simultaneous presence of spin–orbit coupling and zigzag geometry, opening a gap in the linear dispersion relation. In particular, their linear stability is analyzed. We also find that the linear dispersion relation becomes exactly flat at particular parameter values, and obtain corresponding compact solutions localized on two neighboring sites, with spin-up and spin-down parts π/2 out of phase at each site. The continuation of these compact modes into exponentially decaying gap modes for generic parameter values is studied numerically, and regions of stability are found to exist in the lower or upper half of the gap, depending on the type of gap modes.
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