83 research outputs found
Simulation of nanostructure-based and ultra-thin film solar cell devices beyond the classical picture
In this paper, an optoelectronic device simulation framework valid for
arbitrary spatial variation of electronic potentials and optical modes, and for
transport regimes ranging from ballistic to diffusive, is used to study
non-local photon absorption, photocurrent generation and carrier extraction in
ultra-thin film and nanostructure-based solar cell devices at the radiative
limit. Among the effects that are revealed by the microscopic approach and
which are inaccessible to macroscopic models is the impact of structure, doping
or bias induced nanoscale potential variations on the local photogeneration
rate and the photocarrier transport regime.Comment: 15 pages, 10 figure
Quantum-kinetic perspective on photovoltaic device operation in nanostructure-based solar cells
The implementation of a wide range of novel concepts for next-generation
high-efficiency solar cells is based on nanostructures with
configuration-tunable optoelectronic properties. On the other hand, effective
nano-optical light-trapping concepts enable the use of ultra-thin absorber
architectures. In both cases, the local density of electronic and optical
states deviates strongly from that in a homogeneous bulk material. At the same
time, non-local and coherent phenomena like tunneling or ballistic transport
become increasingly relevant. As a consequence, the semi-classical, diffusive
bulk picture conventionally assumed may no longer be appropriate to describe
the physical processes of generation, transport, and recombination governing
the photovoltaic operation of such devices. In this review, we provide a
quantum-kinetic perspective on photovoltaic device operation that reaches
beyond the limits of the standard simulation models for bulk solar cells.
Deviations from bulk physics are assessed in ultra-thin film and
nanostructure-based solar cell architectures by comparing the predictions of
the semi-classical models for key physical quantities such as absorption
coefficients, emission spectra, generation and recombination rates as well as
potentials, densities and currents with the corresponding properties as given
by a more fundamental description based on non-equilibrium quantum statistical
mechanics. This advanced approach, while paving the way to a comprehensive
quantum theory of photovoltaics, bridges simulations at microscopic material
and macroscopic device levels by providing the charge carrier dynamics at the
mesoscale.Comment: 22 pages, 8 figures; review article based on an invited talk at the
MRS Spring Meeting 2017 in Phoeni
Theoretical investigation of direct and phonon-assisted tunneling currents in InAlGaAs-InGaAs bulk and quantum well interband tunnel junctions for multi-junction solar cells
Direct and phonon-assisted tunneling currents in InAlGaAs-InGaAs bulk and
double quantum well interband tunnel heterojunctions are simulated rigorously
using the non-equilibrium Green's function formalism for coherent and
dissipative quantum transport in combination with a simple two-band
tight-binding model for the electronic structure. A realistic band profile and
associated built-in electrostatic field is obtained via self-consistent
coupling of the transport formalism to Poisson's equation. The model reproduces
experimentally observed features in the current-voltage characteristics of the
device, such as the structure appearing in the negative differential resistance
regime due to quantization of emitter states. Local maps of density of states
and current spectrum reveal the impact of quasi-bound states, electric fields
and electron-phonon scattering on the interband tunneling current. In this way,
resonances appearing in the current through the double quantum well structure
in the negative differential resistance regime can be related to the alignment
of subbands in the coupled quantum wells.Comment: 4 pages, 5 figure
Simulation of nanostructure-based high-efficiency solar cells: challenges, existing approaches and future directions
Many advanced concepts for high-efficiency photovoltaic devices exploit the
peculiar optoelectronic properties of semiconductor nanostructures such as
quantum wells, wires and dots. While the optics of such devices is only
modestly affected due to the small size of the structures, the optical
transitions and electronic transport can strongly deviate from the simple bulk
picture known from conventional solar cell devices. This review article
discusses the challenges for an adequate theoretical description of the
photovoltaic device operation arising from the introduction of nanostructure
absorber and/or conductor components and gives an overview of existing device
simulation approaches.Comment: Invited paper, accepted for publication in IEEE Journal of Selected
Topics in Quantum Electronic
Impact of nanostructure configuration on the photovoltaic performance of quantum dot arrays
In this work, a mesoscopic model based on the non-equilibrium Green's
function formalism for a tight-binding-like effective Hamiltonian is used to
investigate a selectively contacted quantum dot array designed for operation as
a single junction quantum dot solar cell. By establishing a direct relation
between nanostructure configuration and optoelectronic properties, the
investigation reveals the influence of inter-dot and dot-contact coupling
strengths on the rates of charge carrier photogeneration, radiative
recombination, and extraction at contacts, and consequently on the ultimate
performance of photovoltaic devices with finite quantum dot arrays as the
active medium. For long carrier lifetimes, the dominant configuration effects
originate in the dependence of the joint density of states on the inter-dot
coupling in terms of band width and effective band gap. In the low carrier
lifetime regime, where recombination competes with carrier extraction, the
extraction efficiency shows a critical dependence on the dot-contact coupling.Comment: 11 pages, 15 figures; extensively revised and substantially extended
versio
A Microscopic Perspective on Photovoltaic Reciprocity in Ultrathin Solar Cells
The photovoltaic reciprocity theory relates the electroluminescence spectrum
of a solar cell under applied bias to the external photovoltaic quantum
efficiency of the device as measured at short circuit conditions. Its
derivation is based on detailed balance relations between local absorption and
emission rates in optically isotropic media with non-degenerate
quasi-equilibrium carrier distributions. In many cases, the dependence of
density and spatial variation of electronic and optical device states on the
point of operation is modest and the reciprocity relation holds. In
nanostructure-based photovoltaic devices exploiting confined modes, however,
the underlying assumptions are no longer justifiable. In the case of ultrathin
absorber solar cells, the modification of the electronic structure with applied
bias is significant due to the large variation of the built-in field.
Straightforward use of the external quantum efficiency as measured at short
circuit conditions in the photovoltaic reciprocity theory thus fails to
reproduce the electroluminescence spectrum at large forward bias voltage. This
failure is demonstrated here by numerical simulation of both spectral
quantities at normal incidence and emission for an ultrathin GaAs p-i-n solar
cell using an advanced quantum kinetic formalism based on non-equilibrium
Green's functions of coupled photons and charge carriers. While coinciding with
the semiclassical relations under the conditions of their validity, the theory
provides a consistent microscopic relationship between absorption, emission and
charge carrier transport in photovoltaic devices at arbitrary operating
conditions and for any shape of optical and electronic density of states.Comment: 5 pages, 4 figures, all figures replaced, minor changes and additions
to the tex
Photocarrier extraction in GaAsSb/GaAsN type-II QW superlattice solar cells
Photocarrier transport and extraction in GaAsSb/GaAsN type-II quantum well
superlattices are investigated by means of inelastic quantum transport
calculations based on the non-equilibrium Green's function formalism.
Evaluation of the local density of states and of the spectral current flow
enables the identification of different regimes for carrier localization,
transport, and extraction as a function of configurational parameters. These
include the number of periods, the thicknesses of the individual layers in one
period, the built-in electric field, and the temperature of operation. The
results for the carrier extraction efficiency are related to experimental data
for different symmetric GaAsSb/GaAsN type-II quantum well superlattice solar
cell devices and provide a qualitative explanation for the experimentally
observed dependence of photovoltaic device performance on period thickness.Comment: 5 pages, 7 figures; accepted for publication in Applied Physics
Letter
Impact of built-in fields and contact configuration on the characteristics of ultra-thin GaAs solar cells
We discuss the effects of built-in fields and contact configuration on the
photovoltaic characteristics of ultrathin GaAs solar cells. The investigation
is based on advanced quantum-kinetic simulations reaching beyond the standard
semi-classical bulk picture concerning the consideration of charge carrier
states and dynamics in complex potential profiles. The thickness dependence of
dark and photocurrent in the ultra-scaled regime is related to the
corresponding variation of both, the built-in electric fields and associated
modification of the density of states, and the optical intensity in the films.
Losses in open-circuit voltage and short-circuit current due to leakage of
electronically and optically injected carriers at minority carrier contacts are
investigated for different contact configurations including electron and hole
blocking barrier layers. The microscopic picture of leakage currents is
connected to the effect of finite surface recombination velocities in the
semi-classical description, and the impact of these non-classical contact
regions on carrier generation and extraction is analyzed.Comment: 5 pages, 8 figure
Effect of spin-orbit coupling on zero-conductance resonances in asymmetrically coupled one-dimensional rings
The influence of Rashba spin-orbit coupling on zero conductance resonances
appearing in one dimensional ring asymmetrically coupled to two leads is
investigated. For this purpose, the transmission function of the corresponding
one-electron scattering problem is derived analytically and analyzed in the
complex energy plane with focus on the zero-pole structure characteristic of
transmission (anti)resonances. The lifting of real conductance zeros due to
spin-orbit coupling in the asymmetric Aharonov-Casher (AC) ring is related to
the breaking of spin reversal symmetry in analogy to the time-reversal symmetry
breaking in the asymmetric Aharonov-Bohm (AB) ring.Comment: 10 pages, 11 figure
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