Charles Feldman

Evaporated polycrystalline silicon films for photovoltaic applications - grain size effects

Vacuum deposited, polycrystalline silicon films were fabricated into planar photovoltaic diodes by double diffusion techniques. Scanning electron microscopy showed that the crystallites are columnar in shape, with grain lengths several times larger than grain diameters. The dependency of average grain diameter on deposition conditions is discussed. Secondary ion mass spectrometry was used to obtain doping profiles and junction depths. Dark and illuminated I-V curves, spectral responses, and minority carrier diffusion lengths are presented for photovoltaic devices having grain sizes in the range 0.2 to 5 μm. Samples formed on sapphire and on a special alkaline-earth aluminosillcate glass processed under the same conditions had similar photovoltaic characteristics. Data on open-circuit voltage, short-circuit current, and solar cell efficiency are presented as functions of average grain diameter.

The behavior of TiB2 thin film electrodes in polycrystalline silicon thin film solar cells

Abstract Stoichiometric TiB 2 films 1–2 μm thick on Al 2 O 3 substrates were examined as base electrodes in vacuum-deposited silicon photovoltaic cells. The TiB 2 films were formed by sequential vacuum deposition of titanium and boron, and this was followed by a diffusion reaction at high temperature (950 °C). Silicon layers were deposited on top of the TiB 2 layers at substrate temperatures up to 1250 °C. The interactions between the layers were examined by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffractometry. A TiB 2 ue5f8Si interaction resulting in the formation of TiSi 2 was identified. The TiSi 2 at the Siue5f8TiB 2 interface stimulates the growth of large silicon crystallites. Evidence of a reaction between the TiB 2 and the Al 2 O 3 substrate was observed, but the reaction products could not be identified.

Mössbauer study of amorphous and crystalline tellurium

Abstract Comparison of the 125Te Mossbauer spectra in amorphous and crystalline Te films indicates that in the amorphous phase the quadrupole splitting is slightly greater and the recoil-free fraction about one-third as great as in the crystalline phase. These changes are interpreted as indicating a decrease in the length of the covalent bond between the nearest neighbor Te atoms in the amorphous state, and furthermore that dangling bonds at the ends of the Te chains are responsible for a change in the density of phonon states in the system.

Electrical properties of carbon-doped amorphous boron films

Abstract Resistivity, EPR, and Poole-Frenkel Effect were investigated in amorphous boron films containing up to 16 at.% carbon. The resistivity increases with carbon content while the EPR signal remains essentially unchanged. The increase in resistivity is discussed in terms of a compensation model.

Vacuum-deposited polycrystalline silicon solar cells on foreign substrates☆

Abstract The problem of fabricating low cost solar cells on foreign substrates has been addressed through the use of vacuum-deposited polycrystalline silicon films. The experimental cells consisted of the following layers on an Al 2 O 3 substrate: TiB 2 bottom electrode/p-type polycrystalline silicon film/n-type silicon region/Tiue5f8Ag electrode. The formation and properties of each layer are described. Interfacial reactions and purity were examined by secondary ion mass spectrometry, X-ray diffraction and scanning electron microscopy. A reactionbettween silicon and TiB 2 resulting in large silicon crystallite growth has been identified. The n-type region was formed by standard phosphorus diffusion techniques. Typical photovoltaic responses without an antireflection coating were V oc = 0.28 V, J sc = 18 mA cm -2 , an efficiency of 2.7% and a fill factor of 0.55. The factors limiting the cell efficiency were primarily the grain size and the purity of the p-type silicon layer.

The study of amorphous and crystalline silicon thin films by sputter-ion source mass spectrometry

Silicon films, vacuum deposited onto fused silica substrates, were examined by sputter-ion source mass spectrometry. The Si+n cluster and the positive ion impurity species in the mass spectra of the amorphous and crystalline phases are related to sample preparation.

ANTIMONY DOPING IN VACUUM DEPOSITED THIN FILM SILICON PHOTOVOLTAIC CELLS

A method for antimony doping silicon polycrystalline thin films and single crystals has been investigated. The method is compatible with the concept of forming photovoltaic cells completely in a vacuum system. Layers of Sb2O3- and Si were deposited either simultaneously or sequentially through masks onto the silicon surface. Heating (e.g. 1100° C, 1 hr) in either an inert atmosphere or oxygen brought about the formation of an SiOx -Sb glassy layer and caused Sb to diffuse into the base silicon surface. The oxide complex is then etched off leaving n-type regions on the surface. Reactions of the layers were examined by secondary ion mass spectrometry and X-ray diffraction.