Influence of Thickness on Power Generation

The power generation is determined by the depth of the absorption for the more than 70 % of the blue components (this depth is approximately first 200 nm), and by the distance a photo-generated carrier can travel before it recombines. This distance is given by hx = ixdFt with the drift mobility, F the acting field, and т the carrier lifetime.

The /гт product is larger for electrons than for holes and depends on the structure of the a-Si:H. However, in nc-Si the distance a hole can travel is larger than in a – Si:H by at least a factor 100 (Schiff 2004). The drift mobility of a hole is on the order of 0.01 cm2/Vs and for electrons is ~2 cm2/Vs in a-Si:H, and for holes in nc-Si:H it is, at ~1 cm2/V s, i. e. still smaller than the electron drift mobility. The low drift-mobility is due to trapping of the holes by the valence band-tail states (Scher et al. 1991) and affects the operation of a solar cell markedly (Schiff 2003).

This means that photo-generated holes near the p-layer are more likely to be collected than the electrons in the n-layer. Therefore a pin cell is more efficient than an nip solar cell.

There is a “collection zone” with R ~ 0 that extends from the p-layer interface; carriers generated in this zone contribute to the photocurrent (and power) from the

Fig. 41.5 Calculated profiles for electron level positions and for electric fields in a pin solar cell as a function of the position within the cell (scale in nm). (a) Levels in the dark; note the built-in po­tential Vbi and that the p-layer has a slightly (0.2 eV) larger band-gap than the i – layer. (b) Open-cir­cuit voltage under illumination (uniform photo-generation with g = 3 x 1021 cm-3 s-1); showing the two quasi-Fermi levels and the open-circuit voltage Voc. (c) Short-circuit electric field, which collapses under illumination from near-uniformity in the dark (dashed) to a width dc ~ 400 nm (solid curve)

cell. Beyond the collection zone is a recombination region, where carriers are more likely to recombine instead of contributing to the cell’s power (see Fig. 41.5). The width of the collection zone at which R/g = 0.5, increases from about 200 to 400 nm as the hole band mobility increases from 0.1 to 1.0 cm2/Vs. The open circuit voltage (given at room temperature) increases with the band gap, according to Voc = Eg/e – 0.8, with 0.8 the Voc deficit in a-Si:H. This deficit decreases to ~0.55 in nc-Si:H, because of the shrinking of the band tails.

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