20 research outputs found
Efficient first-principles calculation of phonon assisted photocurrent in large-scale solar cell devices
We present a straightforward and computationally cheap method to obtain the
phonon-assisted photocurrent in large-scale devices from first-principles
transport calculations. The photocurrent is calculated using nonequilibrium
Green's function with light-matter interaction from the first-order Born
approximation while electron-phonon coupling (EPC) is included through special
thermal displacements (STD). We apply the method to a silicon solar cell device
and demonstrate the impact of including EPC in order to properly describe the
current due to the indirect band-to-band transitions. The first-principles
results are successfully compared to experimental measurements of the
temperature and light intensity dependence of the open-circuit voltage of a
silicon photovoltaic module. Our calculations illustrate the pivotal role
played by EPC in photocurrent modelling to avoid underestimation of the
open-circuit voltage, short-circuit current and maximum power. This work
represents a recipe for computational characterization of future photovoltaic
devices including the combined effects of light-matter interaction,
phonon-assisted tunneling and the device potential at finite bias from the
level of first-principles simulations
Electron-phonon scattering from Green's function transport combined with Molecular Dynamics: Applications to mobility predictions
We present a conceptually simple method for treating electron-phonon
scattering and phonon limited mobilities. By combining Green's function based
transport calculations and molecular dynamics (MD), we obtain a temperature
dependent transmission from which we evaluate the mobility. We validate our
approach by comparing to mobilities and conductivies obtained by the Boltzmann
transport equation (BTE) for different bulk and one-dimensional systems. For
bulk silicon and gold we successfully compare against experimental values. We
discuss limitations and advantages of each of the computational approaches.Comment: 8 pages, 8 figure
Unravelling the role of inelastic tunneling into pristine and defected graphene
We present a first principles method for calculating the inelastic electron
tunneling spectroscopy (IETS) on gated graphene. We reproduce experiments on
pristine graphene and point out the importance of including several phonon
modes to correctly estimate the local doping from IETS. We demonstrate how the
IETS of typical imperfections in graphene can yield characteristic fingerprints
revealing e.g. adsorbate species or local buckling. Our results show how care
is needed when interpreting STM images of defects due to suppression of the
elastic tunneling on graphene
First-principles electron transport with phonon coupling: Large scale at low cost
Phonon-assisted tunneling plays a crucial role for electronic device
performance and even more so with future size down-scaling. We show how one can
include this effect in large-scale first-principles calculations using a single
"special thermal displacement" (STD) of the atomic coordinates at almost the
same cost as elastic transport calculations. We apply the method to
ultra-scaled silicon devices and demonstrate the importance of phonon-assisted
band-to-band and source-to-drain tunneling. In a diode the phonons lead to a
rectification ratio suppression in good agreement with experiments, while in an
ultra-thin body transistor the phonons increase off-currents by four orders of
magnitude, and the subthreshold swing by a factor of four, in agreement with
perturbation theory
Interface band gap narrowing behind open circuit voltage losses in Cu<sub>2</sub>ZnSnS<sub>4</sub> solar cells
We present evidence that band gap narrowing at the heterointerface may be a
major cause of the large open circuit voltage deficit of CuZnSnS/CdS
solar cells. Band gap narrowing is caused by surface states that extend the
CuZnSnS valence band into the forbidden gap. Those surface states are
consistently found in CuZnSnS, but not in CuZnSnSe, by
first-principles calculations. They do not simply arise from defects at
surfaces but are an intrinsic feature of CuZnSnS surfaces. By including
those states in a device model, the outcome of previously published
temperature-dependent open circuit voltage measurements on CuZnSnS
solar cells can be reproduced quantitatively without necessarily assuming a
cliff-like conduction band offset with the CdS buffer layer. Our
first-principles calculations indicate that Zn-based alternative buffer layers
are advantageous due to the ability of Zn to passivate those surface states.
Focusing future research on Zn-based buffers is expected to significantly
improve the open circuit voltage and efficiency of pure-sulfide CuZnSnS
solar cells.Comment: Accepted at Applied Physics Letter
Stacked Janus Device Concepts: Abrupt pn-Junctions and Cross-Plane Channels
Janus transition
metal dichalcogenides with a built-in structural
cross-plane (cp) asymmetry have recently emerged as a new class of
two-dimensional materials with a large cp dipole. Using first-principles
calculations, and a tailored transport method, we demonstrate that
stacking graphene and MoSSe Janus structures result in record high
homogeneous doping of graphene and abrupt, atomically thin, cross-plane
pn-junctions. We show how graphene in contrast to metals can act as
electrodes to Janus stacks without screening the cp dipole and predict
a large photocurrent response dominated by a cp transport channel
in a few-layer stacked device. The photocurrent is above that of a
corresponding thin-film silicon device illustrating the great potential
of Janus stacks, for example, in photovoltaic devices
QuantumATK: An integrated platform of electronic and atomic-scale modelling tools
QuantumATK is an integrated set of atomic-scale modelling tools developed
since 2003 by professional software engineers in collaboration with academic
researchers. While different aspects and individual modules of the platform
have been previously presented, the purpose of this paper is to give a general
overview of the platform. The QuantumATK simulation engines enable
electronic-structure calculations using density functional theory or
tight-binding model Hamiltonians, and also offers bonded or reactive empirical
force fields in many different parametrizations. Density functional theory is
implemented using either a plane-wave basis or expansion of electronic states
in a linear combination of atomic orbitals. The platform includes a long list
of advanced modules, including Green's-function methods for electron transport
simulations and surface calculations, first-principles electron-phonon and
electron-photon couplings, simulation of atomic-scale heat transport, ion
dynamics, spintronics, optical properties of materials, static polarization,
and more. Seamless integration of the different simulation engines into a
common platform allows for easy combination of different simulation methods
into complex workflows. Besides giving a general overview and presenting a
number of implementation details not previously published, we also present four
different application examples. These are calculations of the phonon-limited
mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model
simulation of lithium ion drift through a battery cathode in an external
electric field, and electronic-structure calculations of the
composition-dependent band gap of SiGe alloys.Comment: Submitted to Journal of Physics: Condensed Matte