A Novel P-Type Nanocrystalline Si Buffer at the P/I Interface of A-SI Solar Cells for High Stabilized Efficiency
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RESULTS Preparation of p-type nanocrystalline Si film Figure 1 shows the dark and photo conductivities of the p-nc-Si:H films prepared onto glass and 35 A-thick p-a-SiC as a function of film thickness. With decreasing the thickness of the p-nc-Si film, the dark conductivity is reduced exponentially because electrons transport through a larger fraction of amorphous tissue layer with a high photosensitive and resistive characteristics among the crystallites. 10
100
• . 101 10.-•...... 2
-----
E..
?lO 10
.
r-
. Fig. I Dark conductivity (ad) and photo conductivity (Gd) of p-nc-Si
-"3
"
-
10 10-
U
film grown on glass (0 0
S10
-0-
10. 108
ad,
0
aph)
and on p-a-SiC(E ad, ED Op,5 ) as a function of film thickness.
glass/p-nc-Si (ad)
-0- glass/p-nc-Si (ca) -- 0--p-a-SiC/p-nc-Si (a)
, M,
9 I I II 10- t 100 200 300 400 500 600 700 800 900 1000
thickness (A) When the electrical conductivity of the film deposited onto p-a-SiC is compared with that of the film onto glass, it can be known that the presence of an initial p-a-SiC layer hinders the formation of crystallites and enhances the formation of a void-free denser amorphous matrix than that of the p-nc-Si film prepared onto glass. Thereby the p-nc-Si film has higher photosensitivity and lower dark conductivity when it is deposited onto p-a-SiC. It has been known that crystallization process proceeds after a hydrogen-rich porous amorphous layer, which is called an incubation layer, is formed within -100A thickness when microcrystalline Si is grown onto a-Si substrate [6]. According to the fact, it is not certain if thin -100A-thick p-jc-Si layer, as used as a window layer in solar cell, has microcrystalline phase or fully
Fig. 2 Raman spectra of the p-nc-Si films grown on p-a-SiC with a "structureof (a) (52 A-thick p-a-SiC/80 A-thick p-nc-Si)5 multilayer
(C)
CC__
(b) (17 A-thick p-a-SiC/61 A-thick
p-nc-Si),o multilayer
(b)
E
cc
(c) (35 A-thick p-a-SiC/910 A-thick p-nc-Si),.
(a)
400
L
. .....
.
.
.
j
.
450 500 550 Wavenumber (cm-')
600
508
amorphous phase. To investigate the structural properties of the ultrathin p-nc-Si grown onto p-a-SiC, the multilayers of p-a-SiC and p-nc-Si were prepared as shown in Fig. 2. The existence of nanometer-size Si crystallites in the p-nc-Si film grown onto p-a-SiC at an initial growth regime below a thickness of -100A is confirmed by the 520 cm' crystal peak from Raman spectrum of the (52A-thick p-a-SiC/80 A-thick p-nc-Si) 5 multilayer and the (17Athick p-a-SiC/61 A-thick p-nc-Si)1 0 multilayer as displayed in Fig. 2. The larger crystal peak intensity in the (17A-thick p-a-SiC/61 A-thick p-nc-Si)10 multilayer than that in the (52Athick p-a-SiC/80A-thick p-nc-Si)5 multilayer is attributed to the reduced amorphous peak originating from a thinner p-a-SiC. Therefore, the incubation layer formed at the interface between the p-a-SiC and the p-nc-Si, is demonstrated to consist of nanocrystal seeds and amorphous phase. In the FTIR spectrum of the p-nc-Si as shown in Fig. 3, we find the dominant peak indi
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