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.

Theoretically predicted values of the open circuit voltage (V OC ) for a-Si:H or c-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 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 S , monitored with ESR) in a-Si:H and c-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 OC , measured for the same set of a-Si:H and c-Si:H solar cells. We find that the V 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 × 10 16 cm −3 and, therefore, other limiting mechanisms have to be identified to improve voltage in these devices. In contrast, c-Si:H solar cells show nearly classical V OC -N S relation. The bulk defect density in c-Si:H absorber layer is thus likely the key limiting factor for V OC in these devices at present status of material quality (N S of 3-7 × 10 15 cm −3 ). Further optimization of c-Si:H in terms of bulk defect density is highly relevant for V OC improvement in solar cells. PACS Nos.: 88.40.jj, 72.20.Jv, 61.80.Fe.Résumé : Les valeurs théoriques prédites pour le voltage en circuit ouvert (V OC ) de cellules solaires basées sur du a-Si : H ou du c-Si : H sont substantiellement plus élevées que celles que l'on obtient dans les dispositifs à la fine pointe. Fondamentalement, le voltage en circuit ouvert est contrôlé par la cinétique de la génération-recombinaison, où la recombinaison est souvent déterminée par la densité de défauts dans la couche absorbante de la cellule solaire. Ce dernier point est le sujet principal de cette publication. Nous étudions la relation entre V OC et la recombinaison en volume dans la couche absorbante en variant la densité de défauts. Par bombardement avec des électrons de 2 MeV, suivi de recuits successifs, nous varions sur deux ordres de grandeur la densité de défauts dans la couche absorbante (densité de spin, N S , vérifié par ESR) dans des cellules solaires de a-Si : H et de c-Si : H. Les résultats du bombardement électronique sont analysés en fonction de la dépendance de V OC sur l'intensité du bombardement, mesuré pour deux ensembles de cellules solaires a-Si : H et c-Si : H. Nous trouvons que V OC dans les cellules a-Si : H n'est pas limité par les défauts en volume de la couche absorbante, même à relativement grande densité jusqu'à 3-5 × 10 16 cm -3 et, par conséquent, d'autres mécanismes limitatifs doivent être identifiés pour améliorer le voltage dans ces dispositi...