Back reflector of solar cells

Details Back reflector of solar cells Back reflector of solar cells

2002-06-21

2004-06-15

Diamond, Alan (Department: 1753)

Batteries: thermoelectric and photoelectric

Photoelectric

Cells

C136S259000, C136S246000, C136S261000, C257S432000, C257S436000, C257S437000

Reexamination Certificate

active

06750393

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to the field of solar cells, and in particular to thin crystalline silicon solar cells.
Thin crystalline Si solar cells are attractive because they use small volumes of Si material, and they should prove to be cost effective. However, the short optical path length in crystalline Si solar cells reduces the conversion efficiency of photons to carriers.
FIG. 1
shows the wavelength dependence of internal quantum efficiency (IQE) for thin-film Si cells (shown by line
102
) and the photon number spectrum calculated from the air mass two (AM2) sun spectrum (shown by line
100
). As shown, an IQE reduction starts at 0.8 &mgr;m and goes to zero at ~1.1 &mgr;m, despite the fact that photon wavelengths up to 1.2 &mgr;m can yield carrier generation (optical bandgap defined by absorption coefficient &agr;~10
−1
cm
−1
at 1.2 &mgr;m).
The photon number spectrum
100
consists of various peaks that survive absorption and scattering in the air. Since the Si optical edge is at 1.2 &mgr;m, photons in roughly half of the second peak
104
as well as the third peak
106
are wasted in thin Si solar cells because of the IQE reduction. This results in a low efficiency for current thin Si solar cells (the best reported one is ~15%, as described in R. Brendel, “
Crystalline Thin
-
film Silicon Solar Cells from Layer
-
transfer Processes: a Review
,” Proc. 10
th
Workshop on Crystalline Silicon Solar Cell Materials and Processes, ed. by B. L Sopori, 117, 2000.). The efficiency of thin Si solar cells should at least equal the efficiency of bulk Si solar cells, which is 25%.
To overcome this deficiency in thin Si solar cells, light is typically bounced between the top and bottom surfaces of the solar cell. The current structure to perform this light trapping is the Lambertian top surface and Al backside electrode. The Al electrode has a reflectivity as low as 98%. Assuming that the Lambertian structure has the same internal reflectivity as Al, more than 99% of incident photons escape from cells if they bounce only 100 times between the surfaces. Yet, light with wavelengths near Si's bandgap must be bounced back and forth more than 1000 times to be fully absorbed in current thin Si cells. This is because optical paths 10 cm or longer are required for 1.2 &mgr;m light to be absorbed in Si and generate electron-hole pairs, while current thin Si solar cells are only about 50 &mgr;m thick. Thus, it is very difficult to increase the absorption at near band edge wavelengths (near 1.2 &mgr;m) by using the structure based on the Lambertian surface and Al reflectors.
SUMMARY OF THE INVENTION
The present invention alleviates problems with current light trapping in solar cells by using a photonic crystal as a backside reflector.
Thus, one aspect of the present invention provides for a solar cell that comprises a photoactive region; a Lambertian surface on the topside of the photoactive region; and a photonic crystal on the backside of the photoactive region.
Another aspect of the present invention provides a method of forming a solar cell that comprises forming a Lambertian surface on a topside of a photoactive region; and forming a photonic crystal on a backside of the photoactive region.


REFERENCES:
patent: 4918030 (1990-04-01), Lamb et al.
patent: 5080725 (1992-01-01), Green et al.
patent: 6130780 (2000-10-01), Joannopoulos et al.
patent: 6469682 (2002-10-01), de Maagt et al.
Gee, “Optically Enhanced Absorption in Thin Silicon Layers Using Photonic Crystals,” 29th IEEE Photovoltaic Specialists Conference, pp. 150-153 (2002).*
Kosaka et al, “Superprism Phenomena in Photonic Crystals: Toward Microscale Lightwave Circuits,” Journal of Lightwave Technology, vol. 17, No. 11, pp. 2032-2038, Nov. 1999.*
Baba et al, “Photonic Crystal Light Deflection Devices Using the Superprism Effect,” IEEE Journal of Quantum Electronics, vol. 38, No. 7, pp. 909-914, Jul. 2002.

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