Evelyne Vallat-Sauvain

Evolution of the Microstructure in Microcrystalline Silicon Prepared by Very High Frequency Glow-Discharge using Hydrogen Dilution

A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH4/(SiH4+H2) varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy (TEM) and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a (220) crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherr...

High Rate Growth of Microcrystalline Silicon by VHF-GD at High Pressure

Microcrystalline silicon growth using very high frequency-glow discharge PECVD has been studied under conditions of high pressure and high VHF-power conditions. Hereby, the influence of the total gas flow and the silane concentration on the deposition rate has been investigated. Deposition rates of over 25 A/s have been achieved at relatively low total gas flows of 100 sccm. These high-rate films show device-grade quality with respect to subband gap absorption and microcrystalline structure. Dark conductivity measurements reveal midgap character and transmission electron microscopy investigations confirm a highly crystalline microstructure from the bottom to the top of the μc-Si:H films. These high-rate μc-Si:H layers are very interesting candidates for solar cell and other devices.

Microstructure and open-circuit voltage of n−i−p microcrystalline silicon solar cells

A series of microcrystalline silicon n−i−p solar cells has been deposited by very high frequency plasma enhanced chemical vapor deposition at various values of silane to hydrogen source gas ratio and on two different substrate types. Relationships between microstructure and electrical characteristics of these solar cells are investigated by transmission electron microscopy, atomic force microscopy, and I(V) measurements. A mixed phase (so-called heterophase) layer consisting of amorphous plus microcrystalline material is observed at the bottom of the solar cell and identified here as one of the key microstructural features of the device: the relationship between the crystalline nuclei density and the heterophase layer thickness is presented as well as its relationship with the open-circuit voltage (Voc). The effects of substrate roughness and of silane to hydrogen gas ratio used for the fabrication of the device on the heterophase layer are evidenced. These observations underline the importance of the fir...

Microstructure and surface roughness of microcrystalline silicon prepared by very high frequency-glow discharge using hydrogen dilution

The microstructure of a series of silicon films deposited by very high frequency glow discharge (VHF-GD) with silane concentration in hydrogen varying from 100% down to 1.25% has been observed with transmission electron microscopy (TEM). The surface topography of the layers has been analysed by atomic force microscopy (AFM). At silane concentration below 8.6%, a phase transition between amorphous hydrogenated silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) is observed by TEM. After this transition, the further decrease of silane concentration leads to complex changes of the crystalline microstructure of the layers. AFM observations of the surface reveal that the film rms roughness increases with the decrease of the silane concentration. The surface morphology is not related simply to the microstructure of crystalline grains as observed by TEM.

Light-induced degradation of thin film amorphous and microcrystalline silicon solar cells

Absorption spectra of two dilution series of microcrystalline solar cells deposited by VHF-PECVD were measured by FTPS. The dilution series were composed of pin and of nip cells, with i-layers ranging from highly crystalline to mainly amorphous. This paper evaluates stability of the cells when exposed to white light (AM 1.5-like spectrum). Defect-related absorption is minimum for cells of medium crystallinity (deposited in the transition region); but is increased for theses cells by a factor 2 to 4 under light-soaking. It still remains lower than that of highly crystalline cells which show very little degradation. Variation of electrical parameters is also investigated as a function of light soaking and annealing steps. Cells of medium crystallinity show approximately 10% reversible relative efficiency loss after 1000 hours of light soaking.

Improved interface between front TCO and microcrystalline silicon p-i-n solar cells.

Note: IMT-NE Number: 335 Reference PV-LAB-CONF-2001-002 Record created on 2009-02-10, modified on 2017-05-10

Hydrogenated Microcrystalline Silicon: From Material to Solar Cells

Undoped hydrogenated microcrystalline silicon (µc-Si:H) layers and solar cells have been deposited by plasma-enhanced chemical vapour at low temperature and at different values of VHF plasma power and silane to hydrogen dilution ratios. Transport and defect density measurements on layers suggest that structural properties (e.g. crystallite shape and size) only marginally influence the electronic transport properties. The latter are influenced strongly by the Fermi level, which depends on the oxygen impurity content. Furthermore, they are best described by the quality parameter μ 0 τ 0 (deduced from photoconductivity and ambipolar diffusion length). Cell efficiency correlates better with μ 0 τ 0 than with the defect density as determined from subbandgap absorption. Anisotropy of the transport properties in some µc-Si:H is also demonstrated but does not seem to play a major role in μc-Si:H cells deposited at high rates under VHF glow discharge conditions.

Numerical Simulation of Microcrystalline Silicon Growth on Structured Substrate

Note: IMT-NE Number: 434 Reference PV-LAB-CONF-2006-012 Record created on 2009-02-10, modified on 2017-05-10

Phase diagram and microstructure of microcrystalline and amorphous silicon: a numerical growth simulation

Growth dynamics and microstructure of thin-film silicon simulated by a 3D dynamical numerical model are investigated. The model, recently introduced, is characterized here with its phase diagram. It reproduces the main features of the growth and microstructure of thin film silicon: amorphous to crystalline phase transition, conical/columnar shape of the conglomerates of nanocrystals, surface roughness evolution of the layer. It is observed that preferential etching of the amorphous silicon is sufficient to reproduce qualitatively the surface evolution observed experimentally. In the presence of preferential etching, nucleation of the microcrystalline phase in the simulated layers always coincides with a surface roughness increase as observed experimentally. This model opens new perspectives for the simulation of thin-film microstructure and surface morphology.