Model for a-Si:H/c-Si interface recombination based on the amphoteric nature of silicon dangling bonds

The performance of many silicon devices is limited by electronic recombination losses at the crystalline silicon (c-Si) surface. A proper surface passivation scheme is needed to allow minimizing these losses. The surface passivation properties of amorphous hydrogenated silicon (a-Si:H) on monocrystalline Si wafers are investigated here. We introduce a simple model for the description of the surface recombination mechanism based on recombination through amphoteric defects, i.e. dangling bonds, already established for bulk a-Si:H. In this model, the injection-dependent recombination at the a-Si:H/c-Si interface is governed by the density and the average state of charge of the amphoteric recombination centers. We show that with our surface recombination model, we can discriminate between the respective contribution of the two main mechanisms leading to improved surface passivation, which is achieved by (a) the minimization of the density of recombination centers and (b) the strong reduction of the density of one carrier type near the interface by field effect. We can thereafter reproduce the behaviors experimentally observed for the dependence of the surface recombination on the injection level on different wafers, i.e., of both p and n doping type as well as intrinsic. Finally, we are able to exploit the good surface passivation properties of our a-Si:H layers by fabricating flat heterojunction solar cells with open-circuit voltages exceeding 700 mV. © 2007 The American Physical Society

Kinetics of creation and of thermal annealing of light-induced defects in microcrystalline silicon solar cells

Single-junction microcrystalline silicon (mu c-Si:H) solar cells of selected i-layer crystalline volume fractions were light soaked (AM1.5, 1000 h at 50 degrees C) and subsequently annealed at increasing temperatures. The variations of subbandgap absorption during light soaking and during thermal annealing were monitored by Fourier transform photocurrent spectroscopy. The kinetics were shown to follow stretched exponential functions over long times such as 1000 h. The effective time constants appearing in the stretched exponential function decrease with decreasing crystalline volume fraction as well with increasing annealing temperature. Their Arrhenius-like dependence on temperature is characterized by a unique value of the activation energy. Furthermore, we demonstrate that the configuration of the solar cells (p-i-n or n-i-p) does not influence the degradation kinetics, as long as the average crystallinity of the intrinsic layer is of comparable value. (C) 2008 American Institute of Physics

Influence of the substrate's surface morphology and chemical nature on the nucleation and growth of microcrystalline silicon

Hydrogenated microcrystalline silicon (μc-Si:H) layers about 500 nm thick were deposited in the same run on flat and rough substrates (rms = 60 nm) of various chemical nature. This study reveals that the spatial distribution of the microcrystalline/amorphous phases within the layer depends on the substrate's topography. The influence of the chemical nature of the substrate is shown to be preponderant on the layers nucleation. In particular, this study shows that nucleation density is the highest on plasma enhanced chemical vapor deposited silicon dioxide, whereas it is independent of the substrate's surface topography. Finally, the interpretation of Micro-Raman experiments for the evaluation of the respective volume fractions of amorphous/microcrystalline phases in the layers is discussed in relation with their spatial distribution. © 2005 Elsevier B.V. All rights reserved

Determination of Raman emission cross-section ratio in microcrystalline silicon

The determination of the crystalline volume fraction from the Raman spectra of microcrystalline silicon involves the knowledge of a material parameter called the Raman emission cross-section ratio y. This value is still debated in the literature. In the present work, the determination of y has been carried out on the basis of quantitative analysis of medium-resolution transmission electron microscopy (TEM) micrographs performed on one layer deposited by very high frequency plasma enhanced chemical vapor deposition (VHF- PECVD) close to the amorphous/microcrystalline transition. Subsequent comparison of these data with the crystallinity as evaluated from measured Raman spectra yields a surprisingly high value of y = 1.7. This result is discussed in relation to previously published values (that range from 0.1 to 0.9). © 2006 Elsevier B.V. All rights reserved

Proton-induced degradation of Thin-Film Microcrystalline Silicon Solar Cells

This paper investigates the stability of dilution series of pin and nip microcrystalline silicon solar cells under low-energy proton irradiation (E = 405 keV). Variation of electrical parameters, defect-related absorption and Urbach parameter are investigated as a function of irradiation and annealing steps. Highly microcrystalline cells show a relative efficiency loss of up to 80% after proton irradiation. The efficiency loss is observed not to be completely reversible under thermal annealing. Increase of defect-related absorption and Urbach parameter is also only partially reversible. The electrical parameters (Jsc, Voc, FF) show proton-induced reductions which increase with crystallinity for both pin and nip series; short-circuit current density suffers the largest variations with relative losses of up to 65%. Defect-related absorption is shown to be low for cells of medium crystallinity, before and after irradiation. © 2006 Elsevier B.V. All rights reserved