755 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 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
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
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
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
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
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
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