Non-single-crystal solar cell

Details Non-single-crystal solar cell Non-single-crystal solar cell

2000-03-15

2002-05-07

Diamond, Alan (Department: 1753)

Batteries: thermoelectric and photoelectric

Photoelectric

Cells

C136S252000, C136S255000, C136S290000, C257S053000, C257S055000, C257S431000, C257S458000, C257S464000, C438S096000, C438S097000, C438S057000

Reexamination Certificate

active

06384319

ABSTRACT:

FIELD OF THE INVENTION
The invention relates in general to thin-film solar cells using a non-single-crystal thin-film. More specifically, the invention provides a non-single-crystal solar cell that is especially efficient in post-irradiation,
BACKGROUND OF THE INVENTION
When compared to monocrystal silicon solar cells, PIN thin-film solar cells, having at least one PIN junction, namely, a lamination of a p-type semiconductor layer of non-single-crystal thin-film (hereafter referred to as “p-layer”) , a substantially intrinsic i-type semiconductor layer (referred to below as “i-layer”), and an n-type semiconductor layer (referred to below as “n-layer”), and especially PIN thin-film solar cells, using non-single-crystal thin-films such as silicon-derived amorphous silicon (referred to below as “a-Si”), microcrystalline silicon (referred to below as “&mgr;c-Si”), and polycrystalline silicon, can be made large in area, at low temperatures, and at low cost, and as such, are promising for use as a large-area thin-film solar cell to generate electric power.
“The substantially intrinsic i-layer” described in this specification is not limited to undoped semiconductor layer. Amorphous semiconductors, even if intentionally undoped, are sometimes automatically doped by diffusion of residual gas or impurity emitted from the inner surface of a chamber wall during deposition, and thus Fermi energy may be shifted from the center of the bandgap. A semiconductor layer compensated such that the Fermi energy is located at the center of the bandgap by microdoping p or n-type impurity also includes “substantially intrinsic i-layer”.
Examples have been reported where wide-gap a-Si alloy and &mgr;c-Si were applied as the material for the player, the window layer of the PIN-type solar cell, for the purpose of increasing the efficiency of the non-single-crystal solar cell. Amorphous silicon carbide (hereafter referred to as “a-SiC”), amorphous silicon oxide (hereafter referred to as “a-SiO”), and amorphous silicon nitride (hereafter referred to as a-SiN), among others, were brought together to form the a-Si alloy. It has been shown that using these materials in the p-layer results in increased short circuit current density (hereafter referred to as “Jsc”) due to a reduction in light absorption loss, and an increase in open-circuit voltage (hereafter referred to as “Voc”) from increased diffusion potential.
It is known, however, that non-single-crystal solar cells, and a-Si solar cells in particular, experience reduced efficiency with illumination. As to the special features of the initial period, following irradiation there is, on the whole, a reduction in Voc, Jsc, and the curved factor (hereafter referred to as FF), respectively, and a decline in efficiency (hereafter referred to as Eff).
Until now, p-layer conditions have mostly been optimized with regard to the peculiar characteristics of the initial period, and what was optimized for the initial period was thought to be equally optimal for post-irradiation, but not much consideration has really ever been given to the peculiar characteristics of post-irradiation.
In contrast, there has recently been reported instances whereby application to the i-layer of an a-Si film made with hydrogen-diluted silane, resulted in an increase in post-irradiation Voc. For example, Siamchai et al., who used the photo-CVD method, report that when you raise the hydrogen dilution levels for the i-layer in an a-Si solar cell with no p-i interface layer, post-irradiation Voc increases against the initial period, and that the cause of this is attributable to a disparity in defect energy within the i-layer due to hydrogen dilution [Pavan Siamchai and Makoto Konagai, Proc. IEEE 25th Photovoltaic Specialists Conference (1996) pp. 1093; Pavan Siamchai and Makoto Konogai, Appl. Phys. Lett., Vol.67 (1995) pp.3468]. The post-irradiation Eff in a-Si solar cells with increased Voc from illumination, however, did not exceed the Eff in a-Si solar cells which includes the p-i interface layer and exhibits ordinary degradation under illumination. Furthermore, they are of the opinion that increases in Voc by irradiation depends on the conditions under which the i-layer is prepared.
Meanwhile, Isomura et al. report that when they used the plasma CVD method and diluted silane with hydrogen by ten times or more, they achieved an increase in post-irradiation Voc, and believe the reason has nothing to do with either the p-layer or the p-i interface layer [Masao Isomura, Hiroshi Yamamoto, Michio Kondo and Akihisa Matsuda, Proc. 2
nd
World Conference on Photovoltaic Energy Conversion, to be published]. However, even in this case, whatever increased Voc as a result of irradiation did not lead to a rise in Eff. This report also states that the increases in Voc by irradiation depend upon the conditions under which the i-layer is prepared.
Furthermore, Xi et al. report raising Voc through the optimization of the a-SiC p-layer comprised of large quantities of carbon (hereafter referred to as “C”), and p-i interface layer, and that whenever Voc was low in the initial period, post-irradiation Voc increased [Jinping Xi, Tongyu Liu, Vincent Iafelice, Martin Nugent, Kevi Si, Joe del Cueto, Malathi Ghosh, Frank Kampas, Proc. 23
rd
IEEE Photovoltaic Specialists Conference (1993) pp.821]. However, when post-irradiation Voc increased, no increases in Eff were exhibited. Further, nothing regarding any link between the conditions of p-layer or p-i interface layer preparation and the increases in post-irradiation Voc was reported in any concrete fashion.
An increase Eff has been sought using a wide gap non-single-crystal alloy and &mgr;c-Si in the p-layer of our non-single-crystal PIN solar cell,. However, Eff needs to increase to even a greater extent before it can be put to real use.
Moreover, insofar as the conditions for the p-layer were concerned, it had been believed that the elements which had been optimized in accordance with the initial period's peculiar characteristics would also be optimal in the post-irradiation period. The problem, however, is that these conditions were not necessarily optimal given the peculiar characteristics of post-irradiation. For example, as was mentioned above, Voc increased in post-irradiation because of the manner in which the i-layer was prepared, but without resulting in an increased post-irradiation Eff.
In light of this problem, it is an object of the present invention to provide non-single-crystal solar cell that is especially efficient in post-irradiation, and doing so by focusing on the peculiar characteristics of post-irradiation.
SUMMARY OF THE INVENTION
In order to resolve the problem discussed above, the inventors performed their own experiments by taking a p-layer, for example, whose Voc and other elements were optimized in the initial period's peculiar characteristics, and observing closely what was not optimal about it in post-irradiation. By paying particular attention to the specific characteristics of post-irradiation, particularly with regard to efficiency, conditions were established that would maximize its efficiency. More precisely, taking non-single-crystal solar cell having at least one PIN junction, formed by laminating together of a p-type semiconductor layer made from non-single-crystal thin-film, a substantially intrinsic i-type semiconductor layer and an n-type semiconductor layer, things like film thickness and acceptor impurity levels of the above-mentioned p-type semiconductor layer were adjusted in an effort to achieve 0.85-0.99 times the maximum pre-irradiation open-circuit voltage value in this non-single-crystal solar cell before performing irradiation.
As described in greater detail below, experiments were conducted to change the material, film thickness and impurity levels in the p-type semiconductor layer. In each instance, the results endeavored to be achieved were the same: 0.85-0.99 times the pre-irradiation open-circuit voltage. In this case, it is possible to achieve a higher efficiency r

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