Electron and hole drift mobility have been measured in n- and p-type amorphous Si Schottky-barrier solar cells. At room temperature μdn= (2–5) ×10−2 cm2/V sec and μdp= (5–6) ×10−4 cm2/V sec. Both mobilities are trap controlled with ΔE=0.19 eV for electrons and ΔE=0.35 eV for holes above 250 °K and ΔE=0.16 and 0.26 eV, respectively, below 250 °K. Majority-carrier lifetimes are estimated to be 1 μsec for electrons and 25 μsec for holes.
The factors affecting the efficiency of a solar cell change when the solar cell is sUbjected to concentrated sunlight. In this paper, we examine the effects of high solar intensities on Si and GaAs solar cells. It is shown that the current-collection efficiency in Si solar cells increases at intermediate levels but may be reduced at very high solar intensities due to plasma recombination. Methods to avoid the degradation of efficiency are suggested. The open-circuit voltage increases with concentration, and the rate of increase is faster in Si than in GaAs. The fill factor also increases with concentration, again at a faster rate in Si than in GaAs. Consequently, the efficiency in a Si solar cell under concentration may increase faster than in a GaAs solar cell. The effect of increased temperature is also examined. It is shown that the increase in temperature degrades the efficiency of Si faster than in GaAs. Thermal analysis shows that it is possible to limit the temperature rise to a low value (25°C) even under 1000 sun concentration by using simple designs of heat sinks. Consequently, there is no significant advantage of GaAs solar cells over Si solar cells for high-intensity operation, except when high temperatures are desired for a complementary thermal system.
In a surface photovoltage determination of the collection length of holes in undoped amorphous Si:H, the ac surface photovoltage has been picked up by the use of a liquid Schottky barrier. The redox couple quinone-hydroquinone proved the best liquid. Simultaneous illumination with a bias light of up to 1 sun removes most of the internal barrier field allowing measurement of the ambipolar diffusion length. Values in the range 0.01–0.8 μm are found depending on the conditions of sample preparation.
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