Wavelength Dependence of the Turn-Off Character in a-Si:H Solar Cells
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WAVELENGTH DEPENDENCE OF THE TURN-OFF CHARACTER IN a-Si:H SOLAR CELLS
MIA OQINGHAI, GAO GUANG, CAO BAOCHENG AND DAI GUOCAI Department of Physics, Shandong University, Jinan, P.R.C.
ABSTRACT The turn-off character in a-Si:H solar cells has been studied by rectangular monochromatic light pulses. It is found that the turn-off time increases obviously with the decrease of the excitation light wavelengths when the turn-off steady photovoltages are the same. This phenomenon can be explained by the wavelength dependence of the photocapacitance and the time constant products lC in the a-Si:H solar cells. INTRO3DL7--TI(CN In recent years, the technology for the fabrication of the a-Si:H solar cells has been widely developed. A remarkable progress has been made in the promotion of the cell efficiency. However, in order to improve the performances and the applications of the amorphous solar cells, one should get to know more about the device physics. From what we have known, in the transient behaviors of the a-Si:H solar cells, the switching characters have not been extensively studied. In this paper, we have focused our attention to this point. The study on the wavelength dependence of the turn-off character in the cells has been carried out.
RESULTS AND EXPLANATICtN In the experiment, we have used the rectangular monochromatic light pulses to study the turn-off character in the PIN a-Si:H solar cells. The samples and measurement arrangemants employed in the experiment have been described in another paper [1]. The light pulses are emitted from the LED, whose turn-off time, according to our detection, is shorter than 100 ns. Figure 1 is the typical results of the turn-off character under the square light pulses of different wavelengths. The steady state photovoltages just before the turn-off of the light pulses are the same.
FIG.1 The turn-off character of the a-Si:H solar cells under different square light pulses. The above curve is the excitation light pulse. The lower curves are the responses of the solar cell. The arrow directs to the increase of the wavelengths.
Light pulses
S
RPhotovoltage resxpnses
A
Mat. Res. Soc. Symp. Proc. Vol. 70. 1986 Materials Research Society
554
0.6 FIG.2 Wavelength dependence of the turn-off time of the a-Si:H solar cell. The load resistance across the cell is 30 kn.
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K
0.5 04
5600
6200
6800
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In fact, the turn-off time can be divided into two parts, the storage and the fall time tf . From the Figure 1, we found that ts is time ts so short, which can be neglected compared with the tf . Thus the turn-off the obtained wavelength is mainly determined by tt . Figure 2 is time dependence of the turn-off time toff ( i.e. tf ) of the PIN solar cell. As the result indicates, with the decrease of the excitation light We found that this time is time increases. the turn-off wavelengths, dependent on the load resistance across the solar cell. When the cell the wavelength discrepancies are is under the short circuit condition, small. While in the open circuit, the discrepancies are
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