213 research outputs found
Linear Algebraic Calculation of Green's function for Large-Scale Electronic Structure Theory
A linear algebraic method named the shifted
conjugate-orthogonal-conjugate-gradient method is introduced for large-scale
electronic structure calculation. The method gives an iterative solver
algorithm of the Green's function and the density matrix without calculating
eigenstates.The problem is reduced to independent linear equations at many
energy points and the calculation is actually carried out only for a single
energy point. The method is robust against the round-off error and the
calculation can reach the machine accuracy. With the observation of residual
vectors, the accuracy can be controlled, microscopically, independently for
each element of the Green's function, and dynamically, at each step in
dynamical simulations. The method is applied to both semiconductor and metal.Comment: 10 pages, 9 figures. To appear in Phys. Rev. B. A PDF file with
better graphics is available at http://fujimac.t.u-tokyo.ac.jp/lses
Theoretical analysis of GaAs/AlGaAs quantum dots in quantum wire array for intermediate band solar cell
A GaAs quantum dot (QD) array embedded in a AlGaAs host material was
fabricated using a strain-free approach, through combination of neutral beam etching
and atomic hydrogen-assisted molecular beam epitaxy regrowth. In this work, we
performed theoretical simulations on a GaAs/AlGaAs quantum well, GaAs QD and
QD array based intermediated band solar cell (IBSC) using a combined multiband
k·p and drift-diffusion transportation method. The electronic structure, IB band
dispersion, and optical transitions, including absorption and spontaneous emission
among the valence band, intermediate band, and conduction band, were calculated.
Based on these results, maximum conversion efficiency of GaAs/AlGaAs QD
array based IBSC devices were calculated by a drift-diffusion model adapted to
IBSC under the radiative recombination limit
Intermediate Band Solar Cell with Extreme Broadband Spectrum Quantum Efficiency
We report, for the first time, about an intermediate band solar cell implemented with InAs/AlGaAs quantum dots whose photoresponse expands from 250 to ~ 6000 nm. To our knowledge, this is the broadest quantum efficiency reported to date for a solar cell and demonstrates that the intermediate band solar cell is capable of producing photocurrent when illuminated with photons whose energy equals the energy of the lowest band gap. We show experimental evidence indicating that this result is in agreement with the theory of the intermediate band solar cell, according to which the generation recombination between the intermediate band and the valence band makes this photocurrent detectable
Voltage limitation analysis in strain-balanced InAs/GaAsN quantum dot solar cells applied to the intermediate band concept
Several attempts have been carried out to manufacture intermediate band solar cells (IBSC) by means of quantum dot (QD) superlattices. This novel photovoltaic concept allows the collection of a wider range of the sunlight spectrum in order to provide higher cell photocurrent while maintaining the open-circuit voltage (VOC) of the cell. In this work, we analyze InAs/GaAsN QD-IBSCs. In these cells, the dilute nitrogen in the barrier plays an important role for the strain-balance (SB) of the QD layer region that would otherwise create dislocations under the effect of the accumulated strain. The introduction of GaAsN SB layers allows increasing the light absorption in the QD region by multi-stacking more than 100 QD layers. The photo-generated current density (JL) versus VOC was measured under varied concentrated light intensity and temperature. We found that the VOC of the cell at 20 K is limited by the bandgap of the GaAsN barriers, which has important consequences regarding IBSC bandgap engineering that are also discussed in this work
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