Photoluminescence lifetimes and optical absorption cross sections of Si nanocrystals embedded in SiO2 have been studied as a function of their average size and emission energy. The lifetimes span from 20 μs for the smallest sizes (2.5 nm) to more than 200 μs for the largest ones (7 nm). The passivation of nonradiative interface states by hydrogenation increases the lifetime for a given size. In contrast with porous Si, the cross section per nanocrystal shows a nonmonotonic behavior with emission energy. In fact, although the density of states above the gap increases for larger nanocrystals, this trend is compensated by a stronger reduction of the oscillator strength, providing an overall reduction of the absorption cross section per nanocrystal for increasing size.
We present back‐contacted amorphous/crystalline silicon heterojunction solar cells (IBC‐SHJ) on n‐type substrates with fill factors exceeding 78% and high current densities, the latter enabled by a SiNx /SiO2 passivated phosphorus‐diffused front surface field. Voc calculations based on carrier lifetime data of reference samples indicate that for the IBC architecture and the given amorphous silicon layer qualities an emitter buffer layer is crucial to reach a high Voc, as known for both‐side contacted silicon heterojunction solar cells. A back surface field buffer layer has a minor influence. We observe a boost in solar cell Voc of 40 mV and a simultaneous fill factor reduction introducing the buffer layer. The aperture‐area efficiency increases from 19.8 ± 0.4% to 20.2 ± 0.4%. Both, efficiencies and fill factors constitute a significant improvement over previously reported values. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
In this article, the microstructure and photoluminescence (PL) properties of Nd-doped silicon-rich silicon oxide (SRSO) are reported as a function of the annealing temperature and the Nd concentration. The thin films, which were grown on Si substrates by reactive magnetron co-sputtering, contain the same Si excess as determined by Rutherford backscattering spectrometry. Fourier transform infrared (FTIR) spectra show that a phase separation occurs during the annealing because of the condensation of the Si excess resulting in the formation of silicon nanoparticles (Si-np) as detected by high-resolution transmission electron microscopy and X-ray diffraction (XRD) measurements. Under non-resonant excitation at 488 nm, our Nd-doped SRSO films simultaneously exhibited PL from Si-np and Nd 3+ demonstrating the efficient energy transfer between Si-np and Nd 3+ and the sensitizing effect of Si-np. Upon increasing the Nd concentration from 0.08 to 4.9 at.%, our samples revealed a progressive quenching of the Nd 3+ PL which can be correlated with the concomitant increase of disorder within the host matrix as shown by FTIR experiments. Moreover, the presence of Nd-oxide nanocrystals in the highest Nd-doped sample was established by XRD. It is, therefore, suggested that the Nd clustering, as well as disorder, are responsible for the concentration quenching of the PL of Nd 3+ .
ContentsFull text on our homepage at www.pss-rapid.com FRONT COVERIt has been well known that a nanometer-or micrometerscale surface roughness affects the heterogeneous nucleation and controls the growth kinetics of nanocrystals. In their Letter on pp. 202-204, Wi, Shrestha and Nagao demonstrate that the preferred nucleation of silver nanoparticles inside lithographically defined silicon nanowells allows to locate the silver nanoclusters in predetermined positions, and thus to form ordered arrays of surface enhanced Raman scattering (SERS)-active sensors. Highly enhanced surface plasmon fields from the Ag nanoparticle clusters were successfully confirmed by three-dimensional electromagnetic simulation and by confocal measurements of increased Raman signals. The proposed approach, which enables to combine a top-down nanoscale templating and a bottom-up nanoparticles synthesis, can be utilized for the growth and assembly of functional nanomaterials in a broad range of materials, array designs, and applications. s BACK COVERThe crystalline silicon wafer currently accounts for 40% of the photovoltaic module cost. Kerf-less wafering technologies for generating thin wafers could significantly reduce the silicon material consumption. The electrochemical separation of textured thin macroporous silicon layers from crystalline silicon wafers is a candidate for cost-effective thin wafer production. In their Letter on pp. 187-189, Ernst et al. report on the first successful demonstration of a 33 µm thick freestanding macroporous silicon solar cell. This cell achieves an energy conversion efficiency of 7.2%. One key challenge solved in this work is the avoidance of shunting despite the through-going pores.
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