Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1987-06-10
1989-05-09
Weisstuch, Aaron
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
136256, 357 30, H01L 3106
Patent
active
048286281
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a solar cell of a semiconductive material such as silicon, in whose semiconductive substrate charge carriers are generated by radiation energy, said charge carriers being separatable and so dischargable, with an ohmic contact being arranged on the semiconductive substrate.
The price for a solar cell module is made up in differing proportions of the costs for the solar cell itself and of those for encapsulating and framing the cell array to form modules. Manufacturing the solar cell entails in addition to the actual process costs the price for the semiconductive material as a prime factor. For this reason, major efforts are being undertaken worldwide to develop cheaper semiconductive material, with a deterioration of the electrical properties and a reduction in the efficiency of the solar cell being the usual consequences.
A further possibility for cutting the costs of solar cells, particularly when single-crystal and polycrystalline silicon is used, consists of drastically reducing the thickness of the semiconductive substrate and thus the material expenditure. In addition to this reduction in the semiconductive material, which has a very considerable effect on the overall price of the cell, there are further advantages for thin solar cells:
increase in the flexibility of solar cells, so permitting them to be fitted to curved surfaces;
increase of the performance/weight ratio, a particular advantage for space travel applications, but also for earth-based cells;
increased tolerance to high-energy radiation in space;
for example, thin silicon ribbons can be used, the manufacture of which becomes more economical as the ribbons become thinner;
reduced heating up of the cell, permitting higher operating voltage, provided the infra-red radiation is not absorbed in the cell;
possibility of structuring the cell to permit exploitation of the radiation striking from the rear surface of the solar cell, so achieving a considerably higher electrical output.
However, the reduction of the solar cell thickness also involves severe and fundamental problems: if the thickness of the semiconductive substrate drops below the respective diffusion length of the minority charge carriers, the result of the increased recombination of the charge carriers on the back surface of the solar cell is a considerable deterioration of the electrical properties (no-load voltage, short-circuit current) and thus of the efficiency of the solar cell.
To date, this problem has been solved, particularly in space travel cells, by generating an electrical field on the back surface of the solar cell. A potential barrier for minority charge carriers is built in at the back surface, so that said carriers cannot penetrate as far as the ohmic rear-surface contact and recombine there, because an ohmic contact is distinguished by an extremely high recombination velocity.
The electrical field on the back surface can be generated by incorporating impurity atoms in the rear side by means of diffusion from the gas phase, ion implantation, or very frequently by an alloying process. In an n.sup.+ p silicon solar cell, boron or aluminum, for example, are incorporated into the p-doped silicon substrate to generate a pp.sup.+ junction.
These methods are always expensive high-temperature processes which may reduce the carrier lifetime in the semiconductor volume in addition to creating numerous defects at the back surface, greatly reducing the efficiency of the electrical field and so leading to a considerable spread in the solar cell data in large-scale production. In addition, defects occur at the edges of the solar cell, and incomplete applying and poor diffusion result from "diffusion pipes". Furthermore, uneven penetration of the rear-surface metal and precipitation of impurities can be detected.
The object of the present invention is to design a solar cell of the type mentioned at the outset such that the recombination velocity in the area of the ohmic contact is considerably reduced without the usual provision of a potential barrier fo
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J. Eguren et al., Conference Record, 15th IEEE Photovoltare Specialists Cont. (1981), pp. 1343-1348.
Hackstein Karl G.
Hezel Rudolf
Nukem GmbH
Weisstuch Aaron
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