16 research outputs found
Exchange coupling in CaMnO and LaMnO: configuration interaction and the coupling mechanism
The equilibrium structure and exchange constants of CaMnO and LaMnO
have been investigated using total energy unrestricted Hartree-Fock (UHF) and
localised orbital configuration interaction (CI) calculations on the bulk
compounds and MnO and MnO clusters. The
predicted structure and exchange constants for CaMnO are in reasonable
agreement with estimates based on its N\'eel temperature. A series of
calculations on LaMnO in the cubic perovskite structure shows that a
Hamiltonian with independent orbital ordering and exchange terms accounts for
the total energies of cubic LaMnO with various spin and orbital orderings.
Computed exchange constants depend on orbital ordering. UHF calculations tend
to underestimate exchange constants in LaMnO, but have the correct sign
when compared with values obtained by neutron scattering; exchange constants
obtained from CI calculations are in good agreement with neutron scattering
data provided the Madelung potential of the cluster is appropriate. Cluster CI
calculations reveal a strong dependence of exchange constants on Mn d e
orbital populations in both compounds. CI wave functions are analysed in order
to determine which exchange processes are important in exchange coupling in
CaMnO and LaMnO.Comment: 25 pages and 9 postscript figure
Specific heat and magnetic order in LaMnO_{3+\delta}
Magnetic and specific-heat measurements are performed in three different
samples of LaMnO_{3+\delta}, with \delta=0.11, 0.15 and 0.26, presenting
important disorder effects, such as carrier localization, due to high amounts
of La and Mn vacancies. For the samples with \delta =0.11 and 0.15, magnetic
measurements show signatures of a two-step transition: as the temperature is
lowered, the system enters a ferromagnetic phase followed by a disorder-induced
cluster-glass state. Spin-wave-like contributions and an unexpected large
linear term are observed in the specific heat as a function of temperature. In
the sample with the highest vacancy content, \delta=0.26, the disorder is
sufficient to suppress even short-range ferromagnetic order and yield a
spin-glass-like state.Comment: RevTeX 2-col, 8 pages, 5 ps figures included, submitted to PR
The AM 1.5 absorption factor of thin-film solar cells
Both for photovoltaic and photovoltaic/thermal applications insight is required in the mechanisms that determine the effective absorption factor Aeff. Aeff is the part of the incident irradiation that is converted into heat, taking into account that part of the energy is withdrawn as electricity. Aeff was studied for five different solar cell technologies using an optical simulation model and ranges from 74% for single junction amorphous silicon solar cells to 82% for CIGS solar cells. The simulations also show that the longer wavelength part of the spectrum is hardly absorbed by the active semiconductors, but mostly by free carrier absorption in the transparent conductive oxide film present in these devices
The luminescent concentrator illuminated
Luminescent concentrator (LC) plates with different dyes were combined with standard multicrystalline silicon solar cells. External quantum efficiency measurements were performed, showing an increase in electrical current of the silicon cell (under AM1.5, 1 sun conditions, at normal incidence) compared to a bare cell. The influence of dye concentration and plate dimensions are addressed. The best results show a 1.7 times increase in the current from the LC/silicon cell compared to the silicon cell alone. To broaden the absorption spectrum of the LC, a second dye was incorporated in the LC plates. This results in a relative increase in current of 5-8% with respect to the one dye LC, giving. Using a ray-tracing model, transmission, reflection and external quantum efficiency spectra were simulated and compared with the measured spectra. The simulations deliver the luminescent quantum efficiencies of the two dyes as well as the background absorption by the polymer host. It is found that the luminescent quantum efficiency of the red emitting dye is 87%, which is one of the major loss factors in the measured LC. Using ray-tracing simulations it is predicted that increasing the luminescent quantum efficiency to 98% would substantially reduce this loss, resulting in an increase in overall power conversion efficiency of the LC from 1.8 to 2.6%
An Architectural Approach for Improving Aesthetics of PV
In order to improve the social acceptance of photovoltaic modules, the choice in panel color and size should be enlarged. Although various approaches have been reported to change the appearance of PV modules, it often adds complexity to the manufacturing process. Here an approach is presented in which a design module can be manufactured on standard module lines, by adding a print interlayer to the module. First results are shown on small PV panels, including performance and stability tests. Also full size panels are shown with an aluminum back panel including mounting structures for easy mounting on roofs and facades. The results show that although there is a drop in conversion efficiency by applying a print, the overall drop is lower than expected based on the print coverage. The stability tests show promising results after thermal cycling, damp heat, UV degradation and outdoor exposure