Transient Behavior of Phoioxcurrent in a-Si:H Solar Cells
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TRANSIENT BEHAVIOR OF PHOIOXCURRENT IN a-Si :H SOLAR CELLS
GAD CUANG, MIAO QING-IAI, CAO BAOCHENG, GUO XIAC)QIN AND DAI G(JCAI Department of Physics, Shandong University, Jinan, China
ABSTRCT Time-dependent photocurrent response by the monochromatic square-pulsed excitation light of different wavelenths has been studied on the PIN a-Si:H solar cells. The experiments show that the shape of the photocurrent transient is distinctly changed by varying the photon energy of the excitation light and the applied biases. Our results can be explained by the relaxation of space charge distribution in the i-layer.
INTtODUCTICtN In the PIN struture a-Si:H solar cells, the carrier generation and collection processes are strongly dependent on the built-in potential in the i-layer. The changes of the field caused by the light illumination or biases have direct effect on the characters of the solar cell. In principle, if the space charge distribution is given, the built-in potential in the i-layer of the PIN solar cell can be obtained by solving the Poisson equation. However, it is not easy to get a detailed steady space charge distribution from the mere 0.5 pin thickness of the i-layer. It is even more difficult to obtain the transient behavior related built-in potential under illumination or biases. In this paper, we report a square-pulsed excitation light method to study the transient behavior related potential in the i-layer. This study is helpful in understanding the photovoltaic performances and the fabrication conditions of the a-Si:H solar cells.
EXPERIMENTS
Sample fabrication and measurement arrangemants The samples employed in the experiments were prepared by rf glow discharge decomposition of SiH4 ( diluted to 10% with H2 ), with admixtures 4 of diborane and phosphine to fabricate the n* and p layers respectively. The sample stucture is of stainless substrate/thin n-layer/~0.5 um i-layer/ thin p-layer/indium tin oxide(ITO). The substrate temperature was 250°C. 2 The cell area was 0.4 cm . The standard experimental arrangement used in our experiments is shown schematically in Figure 1.
Gen. FIG.l Experimental arrangements for studying the photocurrent responses.
r I I
Oscilloscope
Mat- Res. Soc. Symp. Proc. Vol. 70. c 1986 Materials Research Society
Gen.
L
:_1
_rLLE
RF shield sanple
550
PRotocurrent (arb. unit)
,A
..
trrA
5850 A'
. . .. .J t (ns) FIG.2 The unbised photocurrent transient response in a-Si:H PIN solar cells. The duration time of the excitation light pulses
is
1.1 ms.
6100 A'
7000 A'
.
E,
D
kC
t(ris)
B
0
0.4
0.8
1.2 Tirre(rs)
We have used two pulse generators. One was to trigger the LED to emit the rectangular light pulses, the other was to provide the square-pulsed bias voltages and to synchronously trigger the oscilloscope and the LED pulse generator. The rise time of the LED ( of different wavelengths ) has been detected to be several hundred microseconds, which, in our case, has no effect on the analyses. In order to eliminate the possibility of heating and the built u
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