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The photoluminescence (PL) of semiconductors can be used to determine their absorption coefficient (α) using Planck's generalized law. The standard method, suitable only for self-supported thick samples, like wafers, is extended to multilayer thin films by means of the transfer-matrix method to include the effect of the substrate and optional front layers. α values measured on various thin-film solar-cell absorbers by both PL and photothermal deflection spectroscopy (PDS) show good agreement. PL measurements are extremely sensitive to the semiconductor absorption and allow us to advantageously circumvent parasitic absorption from the substrate; thus, α can be accurately determined down to very low values, allowing us to investigate deep band tails with a higher dynamic range than in any other method, including spectrophotometry and PDS.

Theoretically predicted values of the open circuit voltage ([V.sub.OC]) for a-Si:H or μc-Si:H based solar cells are substantially higher than the values achieved in of state-of-the-art devices. Fundamentally, open circuit voltage is determined by generation-recombination kinetics, where recombination is often controlled by the defect density in the absorber layer of a solar cell. The latter aspect is the focus of the paper. The relation between the [V.sub.OC] and the bulk recombination in the absorber layer is addressed in experiment by varying the defect density. The absorber layer defect density (spin density, [N.sub.S], monitored with ESR) in a-Si:H and μc-Si:H solar cells was varied over two orders of magnitude using a 2 MeV electron bombardment and successive stepwise annealing. The results of the electron bombardment experiment are analyzed with respect to the illumination intensity dependency of the [V.sub.OC], measured for the same set of a-Si:H and μc-Si:H solar cells. We find that the [V.sub.OC] of a-Si:H solar cells is not limited by defects in the bulk of the absorber layer, even at relatively high defect density up to 3-5 x 1016 [cm.sup.-3] and, therefore, other limiting mechanisms have to be identified to improve voltage in these devices. In contrast, μc-Si:H solar cells show nearly classical [V.sub.OC]-[N.sub.S] relation. The bulk defect density in μc-Si:H absorber layer is thus likely the key limiting factor for [V.sub.OC] in these devices at present status of material quality ([N.sub.S] of 3-7 x [10.sup.15] [cm.sup.-3]). Further optimization of μc-Si:H in terms of bulk defect density is highly relevant for [V.sub.OC] improvement in solar cells. PACS Nos.: 88.40.jj, 72.20.Jv, 61.80.Fe. Les valeurs theoriques predites pour le voltage en circuit ouvert ([V.sub.OC]) de cellules solaires basees sur du a-Si : H ou du μc-Si : H sont substantiellement plus elevees que celles que l'on obtient dans les dispositifs a la fine pointe. Fondamentalement, le voltage en circuit ouvert est controle par la cinetique de la generation-recombinaison, oU la recombinaison est souvent determinee par la densite de defauts dans la couche absorbante de la cellule solaire. Ce dernier point est le sujet principal de cette publication. Nous etudions la relation entre [V.sub.OC] et la recombinaison en volume dans la couche absorbante en variant la densite de defauts. Par bombardement avec des electrons de 2 MeV, suivi de recuits successifs, nous varions sur deux ordres de grandeur la densite de defauts dans la couche absorbante (densite de spin, [N.sub.S], verifie par ESR) dans des cellules solaires de a-Si : H et de pc-Si : H. Les resultats du bombardement electronique sont analyses en fonction de la dependance de [V.sub.OC] sur l'intensite du bombardement, mesure pour deux ensembles de cellules solaires a-Si : H et μc-Si : H. Nous trouvons que [V.sub.OC] dans les cellules a-Si : H n'est pas limite par les defauts en volume de la couche absorbante, meme a relativement grande densite jusqu'a 3-5 x [10.sup.16] [cm.sup.-3] et, par consequent, d'autres mecanismes limitatifs doivent etre identifies pour ameliorer le voltage dans ces dispositifs. Au contraire, les cellules μc-Si : H montrent une relation [V.sub.OC]-[N.sub.S] pratiquement classique. La densite en volume des defauts dans la couche absorbante μc-Si : H est donc le facteur limitant pour [V.sub.OC] dans ces dispositifs au niveau actuel de qualite des materiaux ([N.sub.S] de l'ordre de 3-7 x [10.sup.15] [cm.sup.-3]). Pousser plus loin l'optimisation de μc-Si : H en terme de densite volumique de defauts est tres pertinent pour ameliorer [V.sub.OC] dans les cellules solaires. [Traduit par la Redaction], Introduction The open circuit voltage achieved in state-of-the-art a-Si:H or μc-Si:H based solar cells is considerably below theoretically predicted limits [1]. The rather high density ([10.sup.15]-[10.sup.16] [cm.sup.-3]) of midgap defects--silicon [...]

The concept of photonic random textures for application as a light-trapping scheme in thin-film solar cells is introduced. Those textures consist of a randomly textured interface, as commonly applied in thin-film solar cells, which is superimposed with a two-dimensional grating structure. The light-scattering properties of those textures are investigated by scalar scattering theory for transmission into the absorber layer and reflection at the back contact. A quantity to describe the light-trapping efficiency is derived and verified by rigorous diffraction theory. The photonic random textures outperform the random textures and the grating structures significantly. PACS Nos.: 42.25.Dd, 42.79.Dj, 88.40.fc. Nous introduisons le concept de texture photonique aleatoire dans le schema de captation de la lumiere dans les cellules minces photovoltaiques. Ces textures nouveau genre consistent en une interface texturee aleatoirement, frequemment utilisee en cellules minces photovoltaiques, superposee a un reseau de diffraction 2D. Nous utilisons la theorie scalaire de la diffraction pour etudier les proprietes de diffusion de la lumiere par ces textures pour la transmission vers la couche absorbante et la reflexion arriere. Nous utilisons la theorie rigoureuse de la diffraction pour evaluer et verifier une quantite qui decrit l'efficacite de captation de la lumiere. Les textures photoniques aleatoires surpassent les textures aleatoires et les reseaux de diffraction de facon significative. [Traduit par la Redaction], Introduction Light trapping in silicon-based thin-film solar cells is an important issue, because the absorption near the band gap is quite low for flat devices [1,2] and the device thickness [...]

The influence of implanted hydrogen (up to a concentration level of 3 at. %) on the microstructure of silicon (Si) materials is investigated by Fourier transform infrared spectroscopy as well as by effusion of hydrogen and of (low dose) implanted helium. Three materials of low original hydrogen concentration, crystalline Si, electron beam evaporated amorphous Si, and plasma-deposited hydrogenated amorphous Si (using high deposition temperature) were investigated. Significant differences between crystalline and amorphous materials were observed. In crystalline Si, implanted hydrogen is found to generate multivacancies that trap diffusing helium while this is not the case in amorphous Si. Accordingly, cavities where hydrogen is located in amorphous Si must be smaller than divacancies. Those cavities in amorphous Si, present from the growth process, that trap helium tend to disappear when the implanted hydrogen concentration increases. Annealing of the materials up to temperatures of 575°C was also studied. No significant change in the density of voids (trapping helium) occur but in case of crystalline Si the bonding sites of hydrogen as well as the diffusion paths of helium change. PACS Nos.: 61.43.Dq, 61.72.uf, 81.40.-z. Nous utilisons la spectroscopie infrarouge a transformee de Fourier, l'effusion d'hydrogene (H) et l'implantation d'helium (He; a basse dose), pour etudier l'influence de l'implantation d'H (jusqu'a des concentration de 3 at. %) sur les microstructures de materiaux de silicium. Nous etudions trois de ces materiaux de faible concentration initiale en H, le silicium cristallin, le silicium amorphe evapore par faisceau electronique et le silicium amorphe hydrogene par depot (a haute temperature). Des differences importantes apparaissent entre materiaux cristallins et amorphes. Dans le silicium cristallin, l'H implante cree des multi-lacunes qui piegent l'He en diffusion, ce qui n'est pas le cas pour le silicium amorphe. Consequemment, les cavites ou se trouve l'H dans le Si amorphe doivent etre plus petites que des bi- lacunes. Ces cavites qui piegent l'He dans le silicium amorphe, presentes lors de l'elaboration du materiau, tendent a disparaitre lorsque la concentration de l'H implante augmente. Nous etudions aussi l'effet d'un recuit des materiaux jusqu'il 575°C. Aucun changement significatif de la densite de pieges n'apparait, mais dans le cas cristallin, les sites de capture de H et les trajectoires de diffusion de He changent. [Traduit par la Redaction], 1. Introduction Void-related microstructure is an important issue in hydrogenated amorphous silicon (a-Si:H) technology as it may limit the performance and the lifetime of a-Si:H based devices. The relation between [...]