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Polycrystalline Si thin-film solar cell prepared by solid phase crystallization (SPC) method
mtl .$0a¢c a ~ ELSEVIER
Solar Energy Materials and Solar Cells 34 (1994) 285-289
Polycrystalline Si thin-film solar cell prepared by solid phase crystallization (SPC) method T a k a o M a t s u y a m a *, T o s h i a k i Baba, T s u y o s h i T a k a h a m a , Shinya T s u d a , Shoichi N a k a n o SANYO Electric Co., Ltd. New Materials Research Center, 1-18-13 Hashiridani, Hirakata, Osaka 573, Japan
Abstract
The solid phase crystallization (SPC) method has been studied for fabricating polycrystalline (poly) Si thin films for solar cells. The approach was to optimize the "partial doping structure" (nondoped a-Si/phosphorus(P)-doped a-Si) which we proposed as a starting structure before SPC. A conversion efficiency of 6.3% was obtained by using nondoped a-Si with a large structural disorder. This cell showed a collection efficiency of 51% at a wavelength of 900 nm. In order to significantly reduce the incubation time which is the important factor for the enlargement of the grain size, P doping of more than 1020 cm -3 was required for the P-doped layer.
1. Introduction
a-Si/poly-Si thin film tandem solar cells are considered to be very promising [1] in terms of their performance and cost, and from an ecological point of view. We have firstly adopted the solid phase crystallization method for preparing thin poly-Si films as a solar cell material because this method offers several prominent features, as follows: (1) a simple and low-cost process, (2) in-situ P doping, (3) easy enlargement of the cell size. We have developed several techniques for fabricating high quality poly-Si films, such as a partial doping method and use of textured substrates, and have demon-
* Corresponding author.
0927-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 0 2 4 8 ( 9 4 ) 0 0 0 4 4 - S
286
7~ Matsuyama et al. / Solar Energy Materials and Solar Celia" 34 (1994) 285-289
1 Nondoped
½
/
a-si
[] ICrystallization] • , ~'~ P-doped Substrate ] a-Si Fig. 1. Schematicdiagram of the sample structure. strated the feasibility of these methods for fabricating high-quality poly-Si films for solar cell use [2]. In order to improve the film quality, many things still require improvement. Among them we will discuss the optimization of the partial doping method.
2. Experimental The starting material, a-Si, was deposited by a glow discharge of Sill 4 or Si2H 6 gas on flat or textured substrates. The films were doped by mixing PH 3 gas. Fig. 1 shows a sample structure before SPC. We call this the "partial doping structure", in which the P-doped layer has the role of enhancing the crystal nucleation and makes the grain size larger. The typical thickness of the P-doped layer is 170 nm. We changed the structure of the nondoped a-Si by changing the deposition conditions, such as pressure, RF power, and so on. Table 1 summarizes the deposition conditions. After deposition, annealing was performed in a vacuum at 600°C. In order to investigate the relationship between the structure of the nondoped a-Si and the quality of the poly-Si film, the structure of the a-Si was examined by Raman spectroscopy. Raman measurements were carried out with the exciting light of the 488 nm line from an Ar ion laser, typically with 40 mW of incident power. We characterized the structure of the a-Si by the T A / T O in the Raman spectrum since this value represents the randomness of an a-Si network [3]. The grain size and its density were measured from scanning electron microscopy (SEM) images after Secco's etching which reveals the grain boundaries.
Table 1 Deposition conditions for a-Si Substrate temperature (°C) Gas flow Sill 4 rate (sccm) SizH~, PH ~ RF power (mW/cm 2) Pressure (Pa)
Undoped a-Si 500-600 30-50 10-20 100-300 30-70
P-doped a-Si 500-600 40 0.001-0.1 100-300 27
T. Matsuyama et al. / Solar Energy Materialsand Solar Cells 34 (1994)285-289
(a) TA/TO=0.28
287
(b) TA/TO=0.37
Fig. 2. Surface SEM images of the SPC films. Next we examined the dependence of the crystallization process of thin P-doped films on P concentration to optimize the nucleation layer. We estimated the crystal fraction from the dark conductivity m e a s u r e m e n t [4]. We confirmed the validity of this method by comparing the estimated fractions with the surface SEM images. Thin poly-Si solar cells were fabricated using thus-prepared poly-Si thin films. The output characteristics of the cell were measured under A M 1.5, 100 m W / c m z irradiation, and the collection efficiency was measured at a constant photon density m o d e of 2 × 1014 p h o t o n s / c m 2. s under the white bias light of 100 m W / c m 2.
3. R e s u l t s a n d D i s c u s s i o n
3.1. Optimization o f the crystal growth (nondoped) layer Figs. 2a and 2b show SEM images of thin poly-Si films after Secco's etching with the T A / T O of 0.28 and 0.37, respectively. The grain size after SPC clearly
°sir
"~ 1.0
<
, , ,. 0.30 0.35 0.40 TAfrO Fig. 3. Relationship between the TA/TO of a-Si and the average grain size of poly-Si films. 015¢-
.
b2 5
.
.
.
.
.
.
.
.
.
.
.
Metal ITO (~70nm) p-type a-Si ~ 10nm i-type a-Si n-type poly-Si (~10/zm) Metal Fig. 4. Cell structure of the poly-Si thin-film solar cell. A tungsten metal was used as a substrate.
288
T. Matsuyama et al. / Solar Energy Materials and Solar Cells 34 (1994) 285-289
2.5 1.0
~'~
•~
0.8
~ :I
. . . . . . Si cell ".,. ~%.,-- poly-Sicell
.:"
o.6_.i j
. . . . . . . . . . . . .
.........
~02t
'...
\ ,\ :
a5
1.5-.9
500
700
900
1100
Wavelength (nm) Fig. 5. Collection efficiencyof the poly-Si thinfilm solar cell.
//~
, 0.5
k a~
~00
I
2
/ /
°15-]0-5
0 .... 5
10
-1 (Absorption coefficient) (lain) Fig. 6. SPV plot for the poly-Si thin-film solar cell.
becomes large, using a-Si films with a large T A / T O . Fig. 3 shows the relationship between the T A / T O of the a-Si and the average grain size. This shows that the grain size has a good correlation with the T A / T O . Applying this knowledge to the active layer of thin poly-Si solar cells, we obtained a conversion efficiency of 6.3% with the size of 1 cm 2. The cell structure is shown in Fig. 4 [5]. A high I~c (28.4 m A / c m 2) has been achieved even if the cell thickness is only 10 txm. To examine the reason, the collection efficiency was measured as shown in Fig. 5. This cell has a collection efficiency of up to 51% at 900 nm. Fig. 6 shows the SPV (surface photo-voltage) plot of this data [6]. This figure shows that the minority carrier diffusion length is about 11 t~m, which is the reason for the high Isc.
3.2. Optimization of the nucleation (P-doped) layer In the case of the P-doped layer, we could not find out the T A / T O relationship which held in the nondoped case. P atoms might have a stronger effect on the crystallization process than the structural disorder. Thus, we changed the P concentration to optimize the layer. In order to achieve large grain poly-Si, the nucleation layer must have a short total crystallization time so that the whole nucleation layer crystallizes before the nucleation in the crystal growth layer takes place. It has been reported that P ion implantation reduces the incubation time, the nucleation rate and the total crystallization time when the P concentration is more than about 102o cm -3 [7,8]. Here we examined the dependence of the crystallization process of the P-doped layer on the P concentration because there have been no systematic studies about the gas phase doping case. Fig. 7 shows the time dependence of the crystal fraction of P-doped layer for various P concentrations. From this figure, it can be seen that P doping of more than 102° cm -3 significantly reduces the incubation time. This result shows that the P concentration must be larger than 102o cm -3 for the nucleation layer. A higher conversion efficiency can be expected by increasing the
T. Matsuyama et al. /Solar Energy Materials and Solar Cells 34 (1994) 285-289
o=
•
289
d
0.5
• •
2 X l O ~rn s I X I O ~na 3
o ra
I X I O lc~m-S 5 X I O ~m -s
L) 0 0
50 annealing time (min)
Fig. 7. Annealing time dependence of the crystal fraction of P-doped layer.
P concentration from the present condition (less than 10 2o cm-3). Further optimization is now in progress.
4. Conclusion We optimized the partial doping structure, and found that a-Si with large disorder was suited for use as the nondoped layer. Applying this result, we obtained a conversion efficiency of 6.3%. We also found that P doping of more than 10 20 c m - 3 w a s required for the P-doped layer.
Acknowledgement This work is supported by NEDO (New Energy and Industrial Technology Development Organization) as a part of the New Sunshine Program under the Ministry of International Trade and Industry.
References [1] [2] [3] [4] [5] [6] [7] [8]
H. Takakura et al., Proc 4th Int. Photovoltaic Science and Engineering Conf., 1989, p. 403. T. Matsuyama et al., Jpn. J. Appl. Phys. 29 (1990) 2690. T. Shimada et al., J. Non-Cryst. Solids. 59 and 60 (1983) 783. K. Zellama et al., J. Appl. Phys. 50(11) (1979) 6995. M. Taguchi et al., Proc 5th Int. Photovolatic Science and Engineering Conf., 1990, p. 26. E.D. Stokes et al., Appl. Phys. Lett. 30(8) (1977) 425. L. Csepregi et al., J. Appl. Phys. 48 (1977) 4234. M. Moniwa et al., Jpn. J. Appl. Phys. 32 (1993) 312.
Solar Energy Materials and Solar Cells 34 (1994) 285-289
Polycrystalline Si thin-film solar cell prepared by solid phase crystallization (SPC) method T a k a o M a t s u y a m a *, T o s h i a k i Baba, T s u y o s h i T a k a h a m a , Shinya T s u d a , Shoichi N a k a n o SANYO Electric Co., Ltd. New Materials Research Center, 1-18-13 Hashiridani, Hirakata, Osaka 573, Japan
Abstract
The solid phase crystallization (SPC) method has been studied for fabricating polycrystalline (poly) Si thin films for solar cells. The approach was to optimize the "partial doping structure" (nondoped a-Si/phosphorus(P)-doped a-Si) which we proposed as a starting structure before SPC. A conversion efficiency of 6.3% was obtained by using nondoped a-Si with a large structural disorder. This cell showed a collection efficiency of 51% at a wavelength of 900 nm. In order to significantly reduce the incubation time which is the important factor for the enlargement of the grain size, P doping of more than 1020 cm -3 was required for the P-doped layer.
1. Introduction
a-Si/poly-Si thin film tandem solar cells are considered to be very promising [1] in terms of their performance and cost, and from an ecological point of view. We have firstly adopted the solid phase crystallization method for preparing thin poly-Si films as a solar cell material because this method offers several prominent features, as follows: (1) a simple and low-cost process, (2) in-situ P doping, (3) easy enlargement of the cell size. We have developed several techniques for fabricating high quality poly-Si films, such as a partial doping method and use of textured substrates, and have demon-
* Corresponding author.
0927-0248/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0 9 2 7 - 0 2 4 8 ( 9 4 ) 0 0 0 4 4 - S
286
7~ Matsuyama et al. / Solar Energy Materials and Solar Celia" 34 (1994) 285-289
1 Nondoped
½
/
a-si
[] ICrystallization] • , ~'~ P-doped Substrate ] a-Si Fig. 1. Schematicdiagram of the sample structure. strated the feasibility of these methods for fabricating high-quality poly-Si films for solar cell use [2]. In order to improve the film quality, many things still require improvement. Among them we will discuss the optimization of the partial doping method.
2. Experimental The starting material, a-Si, was deposited by a glow discharge of Sill 4 or Si2H 6 gas on flat or textured substrates. The films were doped by mixing PH 3 gas. Fig. 1 shows a sample structure before SPC. We call this the "partial doping structure", in which the P-doped layer has the role of enhancing the crystal nucleation and makes the grain size larger. The typical thickness of the P-doped layer is 170 nm. We changed the structure of the nondoped a-Si by changing the deposition conditions, such as pressure, RF power, and so on. Table 1 summarizes the deposition conditions. After deposition, annealing was performed in a vacuum at 600°C. In order to investigate the relationship between the structure of the nondoped a-Si and the quality of the poly-Si film, the structure of the a-Si was examined by Raman spectroscopy. Raman measurements were carried out with the exciting light of the 488 nm line from an Ar ion laser, typically with 40 mW of incident power. We characterized the structure of the a-Si by the T A / T O in the Raman spectrum since this value represents the randomness of an a-Si network [3]. The grain size and its density were measured from scanning electron microscopy (SEM) images after Secco's etching which reveals the grain boundaries.
Table 1 Deposition conditions for a-Si Substrate temperature (°C) Gas flow Sill 4 rate (sccm) SizH~, PH ~ RF power (mW/cm 2) Pressure (Pa)
Undoped a-Si 500-600 30-50 10-20 100-300 30-70
P-doped a-Si 500-600 40 0.001-0.1 100-300 27
T. Matsuyama et al. / Solar Energy Materialsand Solar Cells 34 (1994)285-289
(a) TA/TO=0.28
287
(b) TA/TO=0.37
Fig. 2. Surface SEM images of the SPC films. Next we examined the dependence of the crystallization process of thin P-doped films on P concentration to optimize the nucleation layer. We estimated the crystal fraction from the dark conductivity m e a s u r e m e n t [4]. We confirmed the validity of this method by comparing the estimated fractions with the surface SEM images. Thin poly-Si solar cells were fabricated using thus-prepared poly-Si thin films. The output characteristics of the cell were measured under A M 1.5, 100 m W / c m z irradiation, and the collection efficiency was measured at a constant photon density m o d e of 2 × 1014 p h o t o n s / c m 2. s under the white bias light of 100 m W / c m 2.
3. R e s u l t s a n d D i s c u s s i o n
3.1. Optimization o f the crystal growth (nondoped) layer Figs. 2a and 2b show SEM images of thin poly-Si films after Secco's etching with the T A / T O of 0.28 and 0.37, respectively. The grain size after SPC clearly
°sir
"~ 1.0
<
, , ,. 0.30 0.35 0.40 TAfrO Fig. 3. Relationship between the TA/TO of a-Si and the average grain size of poly-Si films. 015¢-
.
b2 5
.
.
.
.
.
.
.
.
.
.
.
Metal ITO (~70nm) p-type a-Si ~ 10nm i-type a-Si n-type poly-Si (~10/zm) Metal Fig. 4. Cell structure of the poly-Si thin-film solar cell. A tungsten metal was used as a substrate.
288
T. Matsuyama et al. / Solar Energy Materials and Solar Cells 34 (1994) 285-289
2.5 1.0
~'~
•~
0.8
~ :I
. . . . . . Si cell ".,. ~%.,-- poly-Sicell
.:"
o.6_.i j
. . . . . . . . . . . . .
.........
~02t
'...
\ ,\ :
a5
1.5-.9
500
700
900
1100
Wavelength (nm) Fig. 5. Collection efficiencyof the poly-Si thinfilm solar cell.
//~
, 0.5
k a~
~00
I
2
/ /
°15-]0-5
0 .... 5
10
-1 (Absorption coefficient) (lain) Fig. 6. SPV plot for the poly-Si thin-film solar cell.
becomes large, using a-Si films with a large T A / T O . Fig. 3 shows the relationship between the T A / T O of the a-Si and the average grain size. This shows that the grain size has a good correlation with the T A / T O . Applying this knowledge to the active layer of thin poly-Si solar cells, we obtained a conversion efficiency of 6.3% with the size of 1 cm 2. The cell structure is shown in Fig. 4 [5]. A high I~c (28.4 m A / c m 2) has been achieved even if the cell thickness is only 10 txm. To examine the reason, the collection efficiency was measured as shown in Fig. 5. This cell has a collection efficiency of up to 51% at 900 nm. Fig. 6 shows the SPV (surface photo-voltage) plot of this data [6]. This figure shows that the minority carrier diffusion length is about 11 t~m, which is the reason for the high Isc.
3.2. Optimization of the nucleation (P-doped) layer In the case of the P-doped layer, we could not find out the T A / T O relationship which held in the nondoped case. P atoms might have a stronger effect on the crystallization process than the structural disorder. Thus, we changed the P concentration to optimize the layer. In order to achieve large grain poly-Si, the nucleation layer must have a short total crystallization time so that the whole nucleation layer crystallizes before the nucleation in the crystal growth layer takes place. It has been reported that P ion implantation reduces the incubation time, the nucleation rate and the total crystallization time when the P concentration is more than about 102o cm -3 [7,8]. Here we examined the dependence of the crystallization process of the P-doped layer on the P concentration because there have been no systematic studies about the gas phase doping case. Fig. 7 shows the time dependence of the crystal fraction of P-doped layer for various P concentrations. From this figure, it can be seen that P doping of more than 102° cm -3 significantly reduces the incubation time. This result shows that the P concentration must be larger than 102o cm -3 for the nucleation layer. A higher conversion efficiency can be expected by increasing the
T. Matsuyama et al. /Solar Energy Materials and Solar Cells 34 (1994) 285-289
o=
•
289
d
0.5
• •
2 X l O ~rn s I X I O ~na 3
o ra
I X I O lc~m-S 5 X I O ~m -s
L) 0 0
50 annealing time (min)
Fig. 7. Annealing time dependence of the crystal fraction of P-doped layer.
P concentration from the present condition (less than 10 2o cm-3). Further optimization is now in progress.
4. Conclusion We optimized the partial doping structure, and found that a-Si with large disorder was suited for use as the nondoped layer. Applying this result, we obtained a conversion efficiency of 6.3%. We also found that P doping of more than 10 20 c m - 3 w a s required for the P-doped layer.
Acknowledgement This work is supported by NEDO (New Energy and Industrial Technology Development Organization) as a part of the New Sunshine Program under the Ministry of International Trade and Industry.
References [1] [2] [3] [4] [5] [6] [7] [8]
H. Takakura et al., Proc 4th Int. Photovoltaic Science and Engineering Conf., 1989, p. 403. T. Matsuyama et al., Jpn. J. Appl. Phys. 29 (1990) 2690. T. Shimada et al., J. Non-Cryst. Solids. 59 and 60 (1983) 783. K. Zellama et al., J. Appl. Phys. 50(11) (1979) 6995. M. Taguchi et al., Proc 5th Int. Photovolatic Science and Engineering Conf., 1990, p. 26. E.D. Stokes et al., Appl. Phys. Lett. 30(8) (1977) 425. L. Csepregi et al., J. Appl. Phys. 48 (1977) 4234. M. Moniwa et al., Jpn. J. Appl. Phys. 32 (1993) 312.