Effect of Excitation Frequency on the Performance of Amorphous Silicon Alloy Solar Cells
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11.1%. It was deposited onto a stainless steel substrate precoated with a textured Ag/ZnO back reflector. The J-V characteristic of the cell is shown in Fig. 1, and the corresponding 157 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society
1.0 0.8 0.6
0 0.4 0.2 0.0 350
V (Volts)
450
650 550 Wavelength (nm)
750
850
Fig. 2. Quantum efficiency of the device shown in Fig. 1.
Fig. 1. J-V characteristic of an a-Si alloy n i p cell made by MVHF at -6 A/s.
quantum efficiency data are plotted in Fig. 2. The cell characteristic shows Voc = 0.965 V, FF = 0.688, and Jsc = 16.7 mA/cm2 ; the Jsc value is consistent with the quantum efficiency data. The achievement of an 11.1% initial cell efficiency demonstrates that high efficiency solar cells can be obtained via MVHF at 6 A/s. We next compared the stability of a-Si alloy n ip cells made by MVHF at -6 A/s with those made from RF at -1 A/s and -3 A/s. Since the best stabilized efficiency is obtained for cells with an intrinsic layer thickness of 2000-2500 A, we prepared ss/AgZnO/n i p/ITO structures using the three deposition rates with an intrinsic layer thickness fixed at 2200 A. The initial cell characteristics are given in Table I. We then light soaked the three samples under one-sun, opencircuit conditions at 50 0 C. After -200 hours of light soaking, the efficiencies are substantially Table I. Initial and stabilized J-V characteristics of RF and MVHF a-Si alloy cells deposited on Ag/ZnO back reflector. The intrinsic layers have the same thickness of 2200 A.
Sample
Status
Jsc
Voc
(mA/c)
FF
(V)(%)
{
11 ]Degradation
L9588
initial
14.65
0.992
0.730
10.6
(-1 A/s RF)
stabilized
14.36
0.965
0.672
9.3
B3461
initial
15.54
0.944
0.713
10.5
(-3 A/s RF)
stabilized
14.95
0.901
0.637
8.6
R6840
initial
14.54
0.994
0.683
9.9
(-6 A/s MVHF)
stabilized
13.85
0.957
0.639
8.5
158
12.3
18.1
14.1
stabilized. The characteristics after 1440 hours of light soaking are also listed in Table I. It should be pointed out that the stabilized active-area efficiency of 9.3% for the low-rate RF cell is the highest reported to date for this structure. It is also interesting to note that the stabilized efficiency for the MVHF cell is essentially the same as that of the 3 A/s RF sample. In other words, one can gain a factor of two in the deposition rate by using MVHF instead of RF without compromising the stable cell performance. In our high-efficiency, triple-junction cell design, the top cell used a-Si alloy with a Jsc value of~8.6 mA/cm2 [1]. To see if we could replace the lower rate RF deposition by MVHF for the top cell, we prepared a-Si alloy n i p cells on a stainless steel substrate, but first made certain that the Jsc values were similar (-8.6 mA/cm). Table II lists the initial J-V characteristics of the three cells. It is noted that the initial efficiency is the highest for the low rate RF cell and the lowest for the high rate MVHF cell. However, after 1000 hours of light soaking at 50 0 C, the high rate MVHF cell shows a
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