65 research outputs found
Quantum-kinetic theory of steady-state photocurrent generation in thin films: Coherent versus incoherent coupling
The generation of photocurrents due to coupling of electrons to both
classical and quantized electromagnetic fields in thin semiconductor films is
described within the framework of the nonequilibrium Green's function
formalism. For the coherent coupling to classical fields corresponding to
single field operator averages, an effective two-time intraband self-energy is
derived from a band decoupling procedure. The evaluation of coherent
photogeneration is performed self-consistently with the propagation of the
fields by using for the latter a transfer matrix formalism with an extinction
coefficient derived from the electronic Green's functions. For the "incoherent"
coupling to fluctuations of the quantized fields, which need to be considered
for the inclusion of spontaneous emission, the first self-consistent Born
self-energy is used, with full spatial resolution in the photon Green's
functions. These are obtained from the numerical solution of Dyson and Keldysh
equations including a nonlocal photon self-energy based on the same interband
polarization function as used for the coherent case. A comparison of the
spectral and integral photocurrent generation pattern reveals a close agreement
between coherent and incoherent coupling for the case of an ultrathin,
selectively contacted absorber layer at short circuit conditions.Comment: 11 pages, 7 figures, published versio
Nonequilibrium Green's function theory of coherent excitonic effects in the photocurrent response of semiconductor nanostructures
Excitonic contributions to absorption and photocurrent generation in
semiconductor nanostructures are described theoretically and simulated
numerically using steady-state non-equilibrium Green's function theory. In a
first approach, the coherent interband polarization including Coulomb
corrections is determined from a Bethe-Salpeter-type equation for the equal
time interband single-particle charge carrier Green's function. The effects of
excitonic absorption on photocurrent generation are considered on the same
level of approximation via the derivation of the corresponding corrections to
the electron-photon self-energy.Comment: 8 pages, 4 figure
Spectral properties of photogenerated carriers in quantum well solar cells
The use of low-dimensional structures such as quantum wells, wires or dots in
the absorbing regions of solar cells strongly affects the spectral response of
the latter, the spectral properties being drastically modified by quantum
confinement effects. Due to the microscopic nature of these effects, a
microscopic theory of absorption and transport is required for their
quantification. Such a theory can be developed in the framework of the
non-equilibrium Green's function approach to semiconductor quantum transport
and quantum optics. In this paper, the theory is used to numerically
investigate the density of states, non-equilibrium occupation and corresponding
excess concentration of both electrons and holes in single quantum well
structures embedded in the intrinsic region of a p-i-n semiconductor diode,
under illumination with monochromatic light of different energies. Escape of
carriers from the quantum well is considered via the inspection of the spectral
photocurrent at a given excitation energy. The investigation shows that escape
from deep levels may be inefficient even at room temperature.Comment: 11 pages, 6 figure
Microscopic theory of absorption and emission in nanostructured solar cells: Beyond the generalized Planck formula
Absorption and emission in inorganic bipolar solar cells based on low
dimensional structures exhibiting the effects of quantum confinement is
investigated in the framework of a comprehensive microscopic theory of the
optical and electronic degrees of freedom of the photovoltaic system. In a
quantum-statistical treatment based on non-equilibrium Green's functions, the
optical transition rates are related to the conservation of electronic
currents, providing a quantum version of the balance equations describing the
operation of a photovoltaic device. The generalized Planck law used for the
determination of emission from an excited semiconductor in quasi-equilibrium is
replaced by an expression of extended validity, where no assumptions on the
distribution of electrons and photons are made. The theory is illustrated by
the numerical simulation of single quantum well diodes at the radiative limit.Comment: 6 pages, 5 figures, extended LaTeX version of the EUPVSEC09
proceedings articl
Hamilton's Principle as Variational Inequality forMechanical Systems with Impact
The classical form of Hamilton's principle holds for conservative systems with perfect bilateral constraints. Several attempts have been made in literature to generalise Hamilton's principle for mechanical systems with perfect unilateral constraints involving impulsive motion. This has led to a number of different variants of Hamilton's principle, some expressed as variational inequalities. Up to now, the connection between these different principles has been missing. The aim of this paper is to put these different principles of Hamilton in a unified framework by using the concept of weak and strong extrema. The difference between weak and strong variations of the motion is explained in detail. Each type of variation leads to a variant of the principle of Hamilton in the form of a variational inequality. The conclusion of the paper is that each type of variation leads to different necessary and sufficient conditions on the impact law. The principle of Hamilton with strong variations is valid for perfect unilateral constraints with a completely elastic impact law, whereas the weak form of Hamilton's principle only requires perfect unilateral constraints and no condition on the energ
Electronic states of elongated PbSe/PbS Core/shell quantum dots
The optical characteristics of colloidal quantum dots (QDs) are highly dependent on the physical geometry of the QD (size, shape) as well as composition. These dependencies make such systems attractive for application in novel optical devices, notably for solar cell technology. Empirical electronic structure methods, such as kcenterdotp theory, or empirical pseudopotential theories have successfully reproduced experimentally observed transitions in CdSe and PbSe colloidal QDs. Our approach uses the kcenterdotp method to predict such properties as the electronic structure and dipole transitions of ellipsoidal PbSe/PbS core/shell structure colloidal QDs, as a function of eccentricity. Due to the anisotropy between the longitudinal (z) and transverse (x and y) directions, we present results from elongation along both the x and z directions
Quantum-kinetic theory of photocurrent generation via direct and phonon-mediated optical transitions
A quantum-kinetic theory of direct and phonon mediated indirect optical
transitions is developed within the framework of the non-equilibrium Green's
function formalism. After validation against the standard Fermi-Golden-Rule
approach in the bulk case, it is used in the simulation of photocurrent
generation in ultra-thin crystalline silicon p-i-n-junction devices.Comment: 12 pages, 11 figure
Microscopic non-equilibrium theory of quantum well solar cells
We present a microscopic theory of bipolar quantum well structures in the
photovoltaic regime, based on the non-equilibrium Green's function formalism
for a multi band tight binding Hamiltonian. The quantum kinetic equations for
the single particle Green's functions of electrons and holes are
self-consistently coupled to Poisson's equation, including inter-carrier
scattering on the Hartree level. Relaxation and broadening mechanisms are
considered by the inclusion of acoustic and optical electron-phonon interaction
in a self consistent Born approximation of the scattering self energies.
Photogeneration of carriers is described on the same level in terms of a self
energy derived from the standard dipole approximation of the electron-photon
interaction. Results from a simple two band model are shown for the local
density of states, spectral response, current spectrum, and current-voltage
characteristics for generic single quantum well systems.Comment: 10 pages, 6 figures; corrected typos, changed caption Fig. 1,
replaced Fig.
Theory and simulation of photogeneration and transport in Si-SiOx superlattice absorbers
Si-SiOx superlattices are among the candidates that have been proposed as high band gap absorber material in all-Si tandem solar cell devices. Owing to the large potential barriers for photoexited charge carriers, transport in these devices is restricted to quantum-confined superlattice states. As a consequence of the finite number of wells and large built-in fields, the electronic spectrum can deviate considerably from the minibands of a regular superlattice. In this article, a quantum-kinetic theory based on the non-equilibrium Green's function formalism for an effective mass Hamiltonian is used for investigating photogeneration and transport in such devices for arbitrary geometry and operating conditions. By including the coupling of electrons to both photons and phonons, the theory is able to provide a microscopic picture of indirect generation, carrier relaxation, and inter-well transport mechanisms beyond the ballistic regime
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