P. Fiorini

Gap states in a-Si:H by photoconductivity and absorption

Abstract The spectral dependence of photoconductivity in a series of a-Si:H samples has been analyzed in order to derive the absorption coefficient α. It has been found that the exponent β which relates photoconductivity and generation rate varies with photon energy and chopping frequency. Its critical influence upon the value of α is established. The Urbach tail in the region 1.4–1.8 eV has been studied and found to be independent of extrinsic gap states and hydrogen content. Finally an inverse correlation has been determined between the photoconductivity lifetime and the extrinsic gap-state density, as measured from the corresponding integrated area of the absorption coefficient.

Investigation of amorphous a-Si:H/a-Si1−xCx:H multi-quantum-well structures

a-Si:H/a-Si1−xCx:H multi-quantum-well structures have been grown and investigated. Optical transmission and electroabsorption measurements show that the optical gap increases with decreasing a-Si:H layer thickness. This behaviour is explained in terms of quantum confinement effects. An increase of the Urbach tail and a decrease of the photoconductivity were also observed with decreasing silicon thickness. Both effects have been attributed to the observed increase of the gap state density.

Fine structure of indirect exciton in GaP

Abstract Wavelenght modulation spectra of the 1s indirect exciton in GaP are reported. Four lines are found which can be easily explained taking into account the “camels back” structure of the conduction band. An exciton splitting of 0.9 meV - ±0.2 has been measured.

DISTRIBUTION OF OCCUPIED STATES IN A-C-H AND A-SI1-XCX-H ALLOYS AS DETERMINED BY TOTAL YIELD SPECTROSCOPY

The density of occupied states at the valence band edge and in the pseudo-gap is investigated by using Total yield spectroscopy. It is found that close to the valence band edge the yield spectra follow a (hυ-E υ ) 3 /(hυ) 2 dependence. A large increase of gap state density with increasing carbon content is also found

A-SI-H BASED PARTICLE DETECTORS WITH LOW DEPLETION VOLTAGE

Abstract We have prepared p-i-n particle detectors using for the active layer both undoped and slightly boron doped amorphous silicon. The doped devices exhibit a much lower depletion voltage. The efficiency improvements over the undoped ones are limited by the decrease of the mobility-lifetime product of the electrons.

Optimum band gap for amorphous silicon based solar cells

The problem of the optimum bandgap for amorphous solar cells is considered. The transport equations governing the behavior of p-i-n solar cells have been numerically solved as a function of the bandgap energy and the thickness of the active layer. It is found that very small benefits in the conversion efficiency of single-gap amorphous solar cells can be obtained with the use of a-Si/sub x/Ge/sub 1-x/:H alloys, the forbidden band of a-Si:H appearing to be nearly optimum. The results obtained lead to the conclusion that a-Si:H is presently the best material for single junction solar cells.<<ETX>>

a-Si:H p-i-n junctions as ionizing particle detectors

Abstract We have measured the response of a-Si:H p-i-n detectors with different thicknesses and applied voltage upon exposition to sources of α and β particles. A simple model suggested by a numerical simulation is used to explain the experimental results and to determine the effective density of states.

DETECTION OF STRONGLY IONIZING PARTICLES WITH A HYDROGENATED AMORPHOUS-SILICON DETECTOR

Abstract We report here the first results obtained with an ultrarelativistic (200 A GeV/ c ) sulphur beam impinging on a hydrogenated amorphous silicon (a-Si:H) detector with p-i-n structure. In addition, measurements with α-particles ( 241 Am) are reported in order to compare our detectors with other detectors already constructed by different laboratories. This is a novel type of device; it is presently under development and shows promising characteristics as a particle detector. In this article our results are discussed mainly in terms of the specific ionization rate. For this purpose a comparison is made with the published data obtained with low-energy protons and alphas.

Photoconductivity and interface defects in a-Si:H/a-Si1−xCxH multi-quantum-well structures

Abstract a-Si:H/a-Si 1− x C x multi-quantum-well structures have been grown and investigated. The density of defects at the interface is measured by photothermal deflection spectroscopy and correlated to the decrease of photoconductivity observed with decreasing silicon layer thickness.

Sub-band gap photocurrent in reverse biased p-i-n a-Si:H solar cells

The spectra of the primary photocurrent excited by subband-gap radiation in p-i-n solar cells were simulated by solving the full set of transport equations. The expressions of the distribution function and of the recombination rate were modified to take into account the effects of subband-gap generation. The relationship between these spectra and the absorption coefficient was analyzed for different sets of the parameters involved in the simulation. It was concluded that the absorption coefficient as derived from the primary photocurrent is a good estimate of the absorption coefficient in the i-layer. In order to verify these concepts, measurements of the primary photocurrent on different kinds of p-i-n solar cells were performed.<<ETX>>