214 research outputs found
Electron-phonon interaction in quantum-dot/quantum-well semiconductor heterostructures
Polar optical phonons are studied in the framework of the dielectric
continuum approach for a prototypical quantum-dot/quantum-well (QD/QW)
heterostructure, including the derivation of the electron-phonon interaction
Hamiltonian and a discussion of the effects of this interaction on the
electronic energy levels. The particular example of the CdS/HgS QD/QW is
addressed and the system is modelled according to the spherical geometry,
considering a core sphere of material "1" surrounded by a spherically
concentric layer of material "2", while the whole structure is embedded in a
host matrix assumed as an infinite dielectric medium. The strength of the
electron-LO phonon coupling is discussed in details and the polaronic
corrections to both ground state and excited state electron energy levels are
calculated. Interesting results concerning the dependence of polaronic
corrections with the QD/QW structure size are analyzed.Comment: 8 pages, 5 figure
Aluminium metallisation for interdigitated back contact silicon heterojunction solar cells
Back contact silicon heterojunction solar cells with an efficiency of 22 were manufactured, featuring a simple aluminium metallisation directly on the doped amorphous silicon films. Both the open circuit voltage and the fill factor heavily depend on the parameters of the annealing step after aluminium layer deposition. Using numerical device simulations and in accordance with the literature, we demonstrate that the changes in solar cell parameters with annealing can be explained by the formation of an aluminium silicide layer at temperatures as low as 150 C, improving the contact resistance and thus enhancing the fill factor. Further annealing at higher temperatures initialises the crystallisation of the amorphous silicon layers, yielding even lower contact resistances, but also introduces more defects, diminishing the open circuit voltag
Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures
Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot
quantum well nanocrystals are determined by use of a multiband theory that
includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian,
and nonparabolicity of the conduction band. The multiband theory correctly
describes the recently observed dark-exciton ground state and the lowest,
optically active, bright-exciton states. Charge separation in pair states is
identified. Previous single-band theories could not describe these states or
account for charge separation.Comment: 10 pages of ReVTex, 6 ps figures, submitted to Phys. Rev.
Interdigitated back contact silicon heterojunction solar cells Towards an industrially applicable structuring method
We report on the investigation and comparison of two different processing approaches for interdigitated back contacted silicon heterojunction solar cells our photolithography based reference procedure and our newly developed shadow mask process. To this end, we analyse fill factor losses in different stages of the fabrication process. We find that although comparably high minority carrier lifetimes of about 4 ms can be observed for both concepts, the shadow masked solar cells suffer yet from poorly passivated emitter regions and significantly higher series resistance. Approaches for addressing the observed issues are outlined and first solar cell results with efficiencies of about 17 and 23 for shadow masked and photolithographically structured solar cells, respectively, are presente
A nonextensive approach to Bose-Einstein condensation of trapped interacting boson gas
In the Bose-Einstein condensation of interacting atoms or molecules such as
87Rb, 23Na and 7Li, the theoretical understanding of the transition temperature
is not always obvious due to the interactions or zero point energy which cannot
be exactly taken into account. The S-wave collision model fails sometimes to
account for the condensation temperatures. In this work, we look at the problem
within the nonextensive statistics which is considered as a possible theory
describing interacting systems. The generalized energy Uq and the particle
number Nq of boson gas are given in terms of the nonextensive parameter q. q>1
(q<1) implies repulsive (attractive) interaction with respect to the perfect
gas. The generalized condensation temperature Tcq is derived versus Tc given by
the perfect gas theory. Thanks to the observed condensation temperatures, we
find q ~ 0.1 for 87Rb atomic gas, q ~ 0.95 for 7Li and q ~ 0.62 for 23Na. It is
concluded that the effective interactions are essentially attractive for the
three considered atoms, which is consistent with the observed temperatures
higher than those predicted by the conventional theory
Toward Annealing Stable Molybdenum Oxide Based Hole Selective Contacts For Silicon Photovoltaics
Molybdenum oxide MoOX combines a high work function with broadband optical transparency. Sandwiched between a hydrogenated intrinsic amorphous silicon passivation layer and a transparent conductive oxide, this material allows a highly efficient hole selective front contact stack for crystalline silicon solar cells. However, hole extraction from the Si wafer and transport through this stack degrades upon annealing at 190 C, which is needed to cure the screen printed Ag metallization applied to typical Si solar cells. Here, we show that effusion of hydrogen from the adjacent layers is a likely cause for this degradation, highlighting the need for hydrogen lean passivation layers when using such metal oxide based carrier selective contacts. Pre MoOX deposition annealing of the passivating a Si H layer is shown to be a straightforward approach to manufacturing MoOX based devices with high fill factors using screen printed metallization cured at 190
Three Terminal Perovskite Silicon Tandem Solar Cells with Top and Interdigitated Rear Contacts
We present a three terminal 3T tandem approachfor the interconnection of a perovskite top cell with aninterdigitated back contact IBC silicon heterojunction SHJ bottom cell. The general viability of our cell design is verified withdrift diffusion simulations indicating efficient charge carriertransport throughout the whole device and an efficiency potentialof amp; 8776;27 by using readily available absorber and contact materials.Our experimental proof of concept device reaches a combinedPCE of 17.1 when both subcells are operating at their individualmaximum power point. To emulate different operation conditions,the current amp; 8722;voltage characteristics of both cells were obtained bymeasuring one subcell while the other cell was set to afixed biasvoltage. Only a slight mutual dependence of both subcells wasfound. As determined by electrical simulations, this dependence likely stems from the resistance of the electron contact on the cell srear side, which is shared by both subcells. The optimization of this contact turns out to be a major design criterion for IBC 3Ttandems. We demonstrate that our current proof of concept cells are limited by this series resistance as well as by optical losses, andwe discuss pathways to approach the simulated efficiency potential by an optimized device desig
Synthesis of CdS and CdSe nanocrystallites using a novel single-molecule precursors approach
The synthesis of CdS and CdSe nanocrystallites using the thermolysis of several dithioor
diselenocarbamato complexes of cadmium in trioctylphosphine oxide (TOPO) is reported.
The nanodispersed materials obtained show quantum size effects in their optical spectra
and exhibit near band-edge luminescence. The influence of experimental parameters on
the properties of the nanocrystallites is discussed. HRTEM images of these materials show
well-defined, crystalline nanosized particles. Standard size fractionation procedures can
be performed in order to narrow the size dispersion of the samples. The TOPO-capped CdS
and CdSe nanocrystallites and simple organic bridging ligands, such as 2,2¢-bipyrimidine,
are used as the starting materials for the preparation of novel nanocomposites. The optical
properties shown by these new nanocomposites are compared with those of the starting
nanodispersed materials
Electronic structure and optical properties of ZnS/CdS nanoheterostructures
The electronic and optical properties of spherical nanoheterostructures are
studied within the semi-empirical tight-binding model including
the spin-orbit interaction. We use a symmetry-based approach previously applied
to CdSe and CdTe quantum dots. The complete one-particle spectrum is obtained
by using group-theoretical methods. The excitonic eigenstates are then deduced
in the configuration-interaction approach by fully taking into account the
Coulomb direct and exchange interactions. Here we focus on ZnS/CdS, ZnS/CdS/ZnS
and CdS/ZnS nanocrystals with particular emphasis on recently reported
experimental data. The degree of carrier localization in the CdS well layer is
analyzed as a function of its thickness. We compute the excitonic fine
structure, i.e., the relative intensities of low-energy optical transitions.
The calculated values of the absorption gap show a good agreement with the
experimental ones. Enhanced resonant photoluminescence Stokes shifts are
predicted.Comment: 6 pages, 4 Figures, revtex
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