Hot carrier solar cell (HCSC) requires a slow cooling rate of carriers in the absorber, which can potentially be fullled by semiconductor superlattices. In this paper the energy relaxation time of electrons in InN InxGa1-xN superlattices are computed with Monte Carlo simulations considering the multi-stage energy loss of electrons. As a result the effect of each stage in the relaxation process is revealed for superlattice absorbers. The energy relaxation rate figures are obtained for different material systems of the absorber, i.e. for different combinations of Indium compositions and the thicknesses of well and barrier layers in the superlattices. The optimum material system for the absorber has been suggested, with the potential to realize HCSCs with high efficiency.
Ge nanocrystals (Ge NCs) were grown in a multilayered superlattice using magnetron co-sputtering and subsequent
thermal annealing. The purpose is to produce a material in which the band gap can be controlled by controlling the Ge
NC size and to investigate the potential of this material for use in tandem solar cells. The presence of size-controlled Ge
NCs was revealed by Raman spectroscopy, glancing incidence X-ray diffraction (GIXRD) and Transmission Electron
Microscope (TEM), and this was supplemented by the observation of blue shifts in the absorption and
photoluminescence (PL) properties. Raman spectra showed Ge-Ge active phonon modes at around 300 cm-1 implying the
formation of high quality Ge NCs. With increasing annealing temperature and duration, more Ge precipitate changed
from a non-crystalline phase to a crystalline phase. However, calculation of degree of crystallinity indicated that a
considerable amount of non-crystalline Ge remained at our chosen annealing conditions. GIXRD measurements
exhibited three Bragg peaks associated with crystalline Ge. TEM images showed direct evidence of the crystal lattice of
the Ge NCs. The size of nanocrystals increased with annealing duration indicating nanocrystal growth by diffusion. The
growth of nanocrystals was found to be confined by the GeO2/SiO2 spacing layers, and the average crystallite size was
determined by the thickness of the GeRO layers. However, enhanced interdiffusion at elevated annealing temperature
weakened the size confinement effect of the multilayer structure. Hence an optimum annealing condition is needed to
produce high quality and reproducible Ge NCs. Our preliminary work indicates that it may be promising to use Ge NCs
as absorber materials in tandem solar cells..
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