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.
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.
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.
In the first part of the present contribution, we will report on transport calculations of nanoscaled devices based on Carbon Nanotubes obtained via self-consistent density-functional method coupled with non-equilibrium Green's function approaches. In particular, density functional tight-binding techniques are very promising due to their intrinsic efficiency. This scheme allows treatment of systems comprising a large number of atoms and enables the computation of the current flowing between two or more contacts in a fully self-consistent manner with the open boundary conditions that naturally arise in transport problems. We will give a description of this methodology and application to field effect transistor based on Carbon nanotubes.
The advances in manufacturing technology are allowing new opportunities even for vacuum electron devices producing radio-frequency radiation. Modern micro and nano-technologies can overcome the typical severe limitations of vacuum tube devices. As an example, Carbon Nanotubes used as cold emitters in micron-scaled triodes allow for frequency generation up to THz region. The purpose of the second part of this contribution will be a description of the modelling of Carbon Nanotube based vacuum devices such as triodes. We will present the calculation of important figures of merit and possible realizations.
In the present work we investigate the influence of molecular vibrations on the tunneling of electrons through a molecule sandwiched between two metal contacts. The study is confined to the elastic scattering only, but beyond the harmonic approximation. The problem is tackled both from a classical and a quantum-mechanical point of view. The classical approach consists in the computation of the time-dependent current uctuations calculated at each step of a molecular dynamics (MD) simulation. On the other hand, the vibrational modes are treated quantum-mechanically and the tunneling current is computed as an ensemble average over the distribution of
the atomic configurations obtained by a suitable approximation of the density matrix for the normal mode oscillators. We show that the lattice fluctuations modify the electron transmission. At low temperatures the quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. However, for temperatures higher than few hundreds Kelvin the simple harmonic approximation which leads to the phonon modes breaks because the oscillation amplitudes of the lowest energy modes become large.
Density Functional theory calculations combined with non-equilibrium Green's function technique have been used to compute electronic transport in organic molecules. In our approach the system Hamiltonian is obtained by means of a self-consistent density-functional tight-binding (DFTB) method. This approach allows a first-
principle treatment of systems comprising a large number of atoms. The implementation of the non-equilibrium Green's function technique on the DFTB code allows us to perform computations of the electronic transport properties of organic and inorganic molecular-scale devices. The non-equilibrium Green's functions are used to compute the electronic density self-consistently with the the open-boundary conditions naturally encountered in transport problems and the boundary conditions imposed by the potentials at the contacts. The Hartree potential of the density-functional Hamiltonian is obtained by solving the three-dimensional Poisson's equation involving the non-equilibrium charge density.
The recently published data on phototransit signals in smectic and discotic liquid crystals, have led us to reconsider the old problem of the weak temperature dependence of the mobility in ordered narrow band systems and inthe liquid crystalline phases. We argue that one has to distinguish between currents which are due to light-generated carriers and currents due to band conduction in equilibrium and which can be described using the Kubo-Greenwood formula. We use a first principle band model in an electric field and show how a T-independent mobility can be derived for a single particle which obeys band transport, includes joule energy relaxation, elastic disorder, and agrees with the few carrrier limit of the Kubo formula. The result is essentially a generalized Drude velocity applicable to describe "single particle currents". We also discuss alternative explanations for the observed temperature independent mobilities which are based on hopping with weak disorder and polaron theories.
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