Nanometer- and Micrometer-scale LED arrays are useful not only for display applications, but also for specialized applications like lens-less microscopy, mask-less lithography or optogenetics. In these contexts, the spatial resolution of the optical field and precise control over the illumination pattern at the object plane is of special importance. We have studied numerically different GaN LED array designs, calculating light extraction, optical near field and crosstalk between pixels. We find that 3D-patterning can help in shaping the light emission, while optical crosstalk becomes a critical issue for small LEDs and pitches below 300 nm.
This work is focused on understanding the role that ferroelectric domains in methylammonium lead halide perovskite (MAPbI3) on the one hand and grain boundaries on the other can have on the performance of solar cells built from this material. We study 2D and 3D systems considering different polarization domain patterns, inspired by measurement data, by proposing a polarization model based on the knowledge of the crystalline structure, symmetry considerations and electrical simulations. Structures with grains are constructed from SEM data. We compute charge carrier transport by solving a drift-diffusion model, in which the Poisson equation for the electrostatic potential calculation explicitly includes the polarization field. The effects of grain boundaries are simulated by considering different types of trap states at the boundaries.
We show that the presence of polarization domains has a strong impact on charge separation, thus leading to a decrease of recombination losses and formation of current pathways at domain interfaces. Specifically, the decrease of Shockley-Read Hall recombination losses improves the open-circuit voltage, while the low resistivity current pathways lead to improved transport and an increase of the short-circuit current. The achieved results demonstrate that the presence of ordered ferroelectric domains, even with weak magnitude of polarization, can actually affect the performance of the solar cell in terms of enhanced power conversion. Moreover, from the comparison between our results and experimental IV characteristics of MAPb(I,Cl)3 devices we conclude that the polarization model proposed can effectively reproduce the solar cell operation.
In this work, we study the optical emission from arrays of InGaN/GaN MQW nanofin and nanorod arrays with sizes ranging from a few micrometers down to sub-wavelength dimensions (i.e., nanometers). Such systems are of interest for developing arrays of single addressable nanoLEDs, which could be used to obtain a visible wavelength super-resolution microscope where the resolution is due to highly localized light spots with sub-wavelength LED-to-LED pitch.
We have used commercial full-wave Maxwell solvers (COMSOL, CST) to calculate the optical field emitted from a single nanoLED in a periodic array for a wavelength of 450 nm. Simulations on 11×11 nanoLED arrays with pitches of 200 nm up to 800 nm and diameters of down to 50 nm have been conducted, in which the dependency of the emission pattern on different structural parameters is studied. In case of small nanoLED array with very narrow pitch, a large optical cross-talk between the activated LED and its neighboring pixels was found. Moreover, in presence of cross-talks, test objects smaller than the LED pitch placed on its surface with influence of near field could potentially be resolved by evaluating the varied emission patterns obtained by different pixel activations. Routes to achieve higher localized optical fields and reduce optical cross-talk have been also investigated by modifying the nanoLED array structures (e.g., by introducing filling material among the LED pixels).
During the last decade a number of both theoretical and experimental studies have shown the importance and the possible effects of random alloy fluctuations in InGaN. Interesting results have been obtained in particular with atomistic simulation models. Based on experimental evidence, most theoretical studies so far concentrated on a uniform random alloy, i.e. where the probability of finding an indium instead of a gallium atom is spatially constant.
In this work, we calculated the density of states, the spontaneous emission spectrum and the radiative coefficient for InGaN/GaN single quantum wells and for bulk InGaN in presence of alloy non-uniformity, using an empirical tight binding approach. We considered an indium concentration of 20%, and 10 nm large supercells. The non-uniform indium distribution has been obtained by distributing a certain percentage of all indium atoms with uniform probability, and the rest with a probability that depends on the number of indium atoms already present locally. This allows to produce structures ranging from random alloy up to strong clustering.
We find that non-uniformity reduces the band gap and the peak energy of the optical emission spectrum. Moreover, increasing degree of clustering decreases the average value of the ground state transition matrix element, which can be explained by the carriers’ spatial localization, combined with quantum confined Stark effect in quantum wells. The radiative coefficient on the other hand is not substantially influenced by light non-uniformity, while it increases for stronger degree of clustering, compatible with a transition to a quantum dot system.
Dye Sensitized solar cells (DSC) are an interesting alternative to conventional silicon based solar cells. Although
DSCs are very close to be commercialized, still many issues need to be addressed. Part of the problem is related
to the lack of a reliable and consistent simulator able to catch the physics and the chemistry underlining the
functioning of the cell. The need of a reliable simulator and modelling is particularly important for the engineering
of the cell and to define trends not only in the component characteristics, but also in the building of the device.
Among the different parts which compone a DSC the relevance of semiconductor titanium oxide substrate can
hardly be underestimated. TiO2 is where the dye molecule is chemisorbed and where the recombination occurs.
Moreover, changes in the topology of the semiconductor paste can lead to other smaller effects in the total
efficiency. In this paper we investigate the effects of changing working parameters for the titanium oxide and
varying its topology. The simulations are performed using a finite element code based on TiberCAD software1 to
describe in details the electrical properties of the cell. The CAD allows to calculate steady-state properties and
ideal I-V characteristics of the cell solving a set of differential equations on meshes in 1, 2 and 3 dimensions.
In this work we use the multi-scale software tool TiberCAD to study the transport and optical properties of InGaN
quantum disk (QD) - based GaN nanocolumn p-i-n diode structures. IV characteristics have been calculated for
several values of In concentration in the QD and of nanocolumn width. Strain maps show a clear relaxation effect
close to the column boundaries, which tends to vanish for the larger columns. Effects of strain and polarization
fields on the electron and hole states in the QD are shown, together with the dependence of optical emission
spectra on geometrical and material parameters.
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