Batteries: thermoelectric and photoelectric – Photoelectric – Cells
Patent
1996-05-23
1997-12-02
Weisstuch, Aaron
Batteries: thermoelectric and photoelectric
Photoelectric
Cells
136261, 257431, 257438, 257461, H01L 3104
Patent
active
056931514
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This application relates to photovoltaic devices specifically adapted for conversion of visible radiation energy, for example for solar cells.
BACKGROUND
The efficiency of the photovoltaic conversion of energy is of decisive significance for the practical application of photovoltaic means. In such classical photovoltaic means as solar cells the generation quantum efficiency (GQE), i.e. the number of electron/hole pairs generated per quantum of radiation, maximally equals 1, thus resulting in the measurable internal quantum efficiency (IQE) also being maximally equal to 1. It has hitherto been assumed that the value 1 cannot be exceeded and that the proportion of photon energy depending on the band gap of the semiconductor and exceeding the energy necessary for generating an electron hole pair is lost as heat.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a device which uses the proportion of radiation quantum energy exceeding the band gap energy in a photovoltaic semiconductor means, particularly to provide improved and efficient solar cells.
Briefly, according to the invention, the photovoltaic device contains a semiconductor material having such a band structure that an additional internal generation of carrier pairs is able to take place by optical impact ionization, that is, by Auger generation or carrier generation by the Auger effect.
A preferred semiconductor material is germanium and silicon, e.g. GeSi.
As a result of additional charge carriers being generated by impact ionization, the internal quantum efficiency of the carrier generation can be made to exceed the value 1.
For photon energies exceeding twice the fundamental energy gap E.sub.G of the semiconductor of the photovoltaic means, an internal quantum efficiency IQE=2 and more is possible by impact ionization (Auger generation).
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be explained in more detail, whereby further features and advantages of the invention will become evident, with reference to the drawings, in which:
FIG. 1 is a schematic sectional view of a photovoltaic means in the form of a solar cell in which the invention may find application;
FIG. 2 plots the ultimate (theoretically maximum) efficiency n.sub.ult as a percentage dependent on the fundamental gap energy E.sub.G in eV assuming an illumination by radiation having the spectrum of a black radiator;
FIG. 3 plots the ultimate efficiency n.sub.ult for the AM0 spectrum (smooth curves) and the AM1.5G spectrum (curves having several relative maxima);
FIG. 4 is an example of a band structure E(k) with which a quantum efficiency of more than 1 may be achieved by Auger generation with radiation quantum starting energy of 2 E.sub.G ; and
FIG. 5 graphically represents the band structure of the zinc blende type semiconductor material GeSi.
DETAILED DESCRIPTION.
FIG. 1 shows, greatly simplified, the structure of a typical solar cell in section. The solar cell 10 represented as a typical example includes a wafer-like monocrystalline semiconductor body 12 of silicon or preferably main part 14 and an e.g. 0.4 .mu.m thick phosphorous-doped zone 16. The main part 14 is arranged on a substrate electrode 18, and on the surface zone 16 a transparent or structured electrode (not shown) and an e.g. 100 nm thick, thermally generated protective layer of SiO.sub.2 20 are provided.
FIG. 2 shows the maximum possible efficiency of solar cell 10 as a function of band gap energy for the illumination spectrum of a black body having a temperature of 6000 K for a maximum internal quantum efficiency IQE.sub.max =1 for h.nu..gtoreq.E.sub.G, furthermore for IQE.sub.max =1.5 for h.nu..gtoreq.2E.sub.G and for IQE.sub.max =2 for h.nu..gtoreq.2E.sub.G. For the last case, the primary band gap as an optimum for the photovoltaic energy conversion shifts from approx. 1 eV to values below 1 eV whilst at the same time the maximum possible efficiency of 44.7% for the case IQE.sub.max =1 increases to 60% for IQE.sub.max =2. Semiconductor materials in
REFERENCES:
"Journal of Applied Physics", vol. 74, No. 2, Jul. 15, 1993 New York, pp. 51-1452; P.T. Landsberg et al, Band-impact ionization and solar cell efficiency.
"Applied Physics Letters", vol. 63, No. 17, Oct. 25, 1993 New York, pp. 2405-2407, S. Kolodinski et al, Quantum efficiencies exceeding unity due to impact ionization . . . .
"Solar Cells", vol. 25, No. 2, Nov. 1988, Lausanne, Switzerland pp. 163-168, J.R. Sites, Calculation of impact ionization enhanced photovoltaic efficiency.
"Solar Energy Materials and Solar Cells", vol. 28, No. 3, Dec. 1992, Amsterdam, Netherlands, pp. 273-284, S.A. Healy, Efficiency enhancements . . . .
"Tenth E.C. Photovoltaic Solar Energy Conference", Apr. 8, 1991, Lisbon, Portugal, pp. 73-74, S.E. Healy et al, Enhancement of sub-bandgap absorption in X-Si . . . .
"Solar Energy Materials and Solar Cells", vol. 33, No. 3, Jul. 1994, Amsterdam, Netherlands, pp. 275-285, S. Kolodinski et al, Quantum efficiencies exceeding . . . .
"Physical Review Letters", vol. 72, No. 24, Jun. 13, 1994 New York, pp. 3851-3854, J.H. Werner et al, Novel optimization principles and efficiency limits for semiconductor solar cells.
"Phys. Rev. B.", vol. 41, 1990, p. 5919 et seq.
Kolodinski Sabine
Queisser Hans J.
Werner Jurgen H.
Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.v.
Weisstuch Aaron
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