Solar Energy Materials and Solar Cells 29 (1993) 415 North-Holland

SolarEnergy Materials and Solar Cells

Letter to the Editor

Last year, a paper has been presented by Li and coworkers (Appl. Phys. Lett. 60 (18) (4 May, 1992)) at the Chinese Academy of Sciences, concerning the achieve- ment of 35% solar conversion efficiency with a silicon cell in which an implant of defects had been implemented near the junction. The paper claimed for an increased current of 69.1 m A / c m 2, without substantial reduction of the voltage, of 0.62 V, leading to an efficiency of 35%.

The paper has stirred a lot of attention. The result, if true, is very important, both scientifically and economically.

Unfortunately, several groups have tried to reproduce these results without success. Other researchers have objected to the results on theoretical bases.

For instance, according to the data on photo-response, it seems likely that these imply quantum efficiencies close to 2 for certain wavelengths, and this seems surprising.

Other researchers have objected to the voltage on the basis of the Shockeley and Queisser theory of detailed balance, suggesting that they violate the second law of thermodynamics.

This is due to the fact that if a cell has its band gap reduced so to collect more photons then the radiative recombination, which is the reverse process of light absorption, becomes increased. For the photons that are absorbed in the Li cell, the voltage is higher than that expected from the radiative recombination associ- ated with this absorption, and thus, if the results of the cell were true, work could be obtained from the thermal radiation, without a second colder reservoir.

However, tandem cells in series give rather high voltages. This is possible because the voltage in any individual cell is small and produces less radiative recombination, the total radiative recombination smaller than that obtained if all the voltage is developed in a single band gap. A hidden tandem cell might explain theoretically the results of the Li cell, but the procedure described does not seem to explain the formation of a tandem cell.

Thus, most probably, the results presented are due to an experimental mistake. It is not difficult to include currents generated by unaccounted light when the cell is small.

Antonio Luque Instituto de Energia Solar,

ETSI Telecomunicacidn, Uniuersidad Polit~cnica, Madrid,

Spain

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