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

Predictors and indicators of disability and quality of life 4 years after a severe traumatic brain injury. A Structural Equation Modelling analysis from the PariS-TBI study

ObjectiveTo assess the predictors and indicators of disability and quality of life four years after a severe traumatic brain injury (TBI), using a Structural Equation Modelling (SEM). SEM is a multivariate approach permitting to take into account the complex inter-relationships between individual predictors, in order to disentangle factors which have a direct or indirect relationship with the dependant variable.MethodsThe Paris-TBI study is a longitudinal inception cohort study of 504 patients with severe TBI in the Parisian area [1]. Among 245 survivors, 147 patients were assessed four years post-injury. Two outcome measures were analysed separately using SEM: the Glasgow Outcome Scale-extended (GOS-E) [2], which is a global measure of disability after TBI, and the QOLIBRI, a disease-specific measure of quality of life after TBI [3]. Four groups of variable were entered in the model: demographics; injury severity; psychological and cognitive impairments; somatic impairments.ResultsThe GOS-E was directly significantly related to all four groups of variables (age, gender, severity of injury, psycho-cognitive and somatic impairments). Education duration had an indirect effect, mediated by psycho-cognitive impairments. In contrast, the QOLIBRI was only directly predicted by psycho-cognitive impairments. Age and somatic impairments had an indirect influence on the QOLIBRI, via psycho-cognitive impairments.Discussion/ConclusionDisability and quality of life were directly influenced by different factors. While disability appeared to result from an interaction of a wide range of factors, including demographics, injury severity, psycho-cognitive and somatic deficiencies, quality of life was solely directly related to psycho-cognitive factors. Other factors, such as age and somatic impairments only had an indirect effect

Percolation properties of the 2D Heisenberg model

We analyze the percolation properties of certain clusters defined on configurations of the 2--dimensional Heisenberg model. We find that, given any direction \vec{n} in O(3) space, the spins almost perpendicular to \vec{n} form a percolating cluster. This result gives indications of how the model can avoid a previously conjectured Kosterlitz-Thouless phase transition at finite temperature T.Comment: 4 pages, 3 eps figures. Revised version (more clear abstract, some new references

Two phase transitions in the fully frustrated XYXY model

The fully frustrated $XY$ model on a square lattice is studied by means of Monte Carlo simulations. A Kosterlitz-Thouless transition is found at $T_{\rm KT} \approx 0.446$, followed by an ordinary Ising transition at a slightly higher temperature, $T_c \approx 0.452$. The non-Ising exponents reported by others, are explained as a failure of finite size scaling due to the screening length associated with the nearby Kosterlitz-Thouless transition.Comment: REVTEX file, 8 pages, 5 figures in uuencoded postscrip