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Light-induced effects in amorphous silicon material and devices
Solar Cells, 9 ( 1 9 8 3 ) 19 - 23
19
LIGHT-INDUCED EFFECTS IN AMORPHOUS SILICON MATERIAL AND DEVICES* D. E, C A R L S O N , A. R. M O O R E , D. J. S Z O S T A K , B. G O L D S T E I N , R. W. SMITH, P. J. Z A N Z U C C H I and W. R. F R E N C H U
R C A Laboratories, Princeton, N J 08540 (U.S.A.) (Received January 3, 1983; accepted January 10, 1983)
Summary We have studied the stability of hydrogenated amorphous silicon (a-Si:H) using several new techniques such as diffusion length measurements, photovoltage profiling and IR absorption via multiple internal reflections. We find that prolonged illumination generally causes a decrease in the diffusion length and an increase in the space charge density of undoped a-Si:H films.
1. Introduction • Light-induced effects in hydrogenated amorphous silicon (a-Si:H)were first reported by Staebler and Wronski in 1977 [1]. They showed that the photoconductivity and dark conductivity of a-Si:H could be reduced significantly by prolonged exposure to illumination and that these effects could be reversed by annealing the films at temperatures of about 150 °C or above. In the last few years, light-induced effects have been observed in many of the properties of a-Si:H films by investigators using a variety of techniques (see for example ref. 2). In this paper we present experimental results showing that light-induced changes can also be observed in the diffusion length, the space charge density and the IR absorption spectrum of a-Si:H films.
2. Diffusion length measurements The diffusion length is measured by varying the wavelength of incident light and adjusting the photon flux to assure a constant surface photovoltage * P a p e r presented at the Solar Energy Research Institute W o r k s h o p o n Light-induced Change in a-Si:H and its E f f e c t o n Solar Cell S t a b i l i t y , San Diego, CA, U.S.A., September 24 - 25, 1 9 8 2 .
0379-6787/83/$3.00
© ElsevierSequoia/Printed in The Netherlands
20
[3, 4]. Using this technique, Dresner et al. [3] showed that the diffusion length of some a-Si:H films decreased after exposure to prolonged illumination. We have continued these measurements on a-Si:H films made in several discharge systems under a wide variety of conditions and find similar behavior in most films. However, in some samples the diffusion length is not significantly affected by prolonged illumination and in a few samples it has actually increased. Two characteristic lengths can be extracted from the constant photovoltage technique: L, the diffusion length in the presence of 1 sun illumination, and W0, the collection length at very low light levels. W0 is roughly the sum of the diffusion length and the space charge width under low light level conditions (about 10 _6 suns). In Fig. 1 we show a plot of L versus Wo for several a-Si:H films made under varying deposition conditions; the arrows show the change in these lengths after exposure to an illumination of 2 suns (about 200 mW cm -2) for 1 h. Most samples show a decrease in both L and W0 after prolonged illumination, b u t samples 2 and 3 show anomalous behavior. The mechanisms responsible for the differences in behavior have not yet been identified.
3. Surface photovoltage profiling We have also used surface photovoltage profiling [5] to determine the field distribution in the i layer of a-Si:H p - i - n cells before and after exposure to prolonged illumination. Figure 2 shows such distributions for a typical unstable cell in the annealed state and after prolonged illumination. Several features are n o t e w o r t h y . First, there is negligible change in Voc, in 1.0
0.8--
0.6
_J
0.=--
0.:'--
0 0
I
I
L0
2D
3.0
w0 (/~m) Fig. 1. Diffusion length as a function of the collection length at low light leve|s; the BL~ZOWS denote the change induced by exposure to 2 suns for 1 h.
21 Ti
n÷
X \\ X X X
m
0
V - 6 4 0 mV
,, • ~------------------~ o ~ ~X
\
--150
Nb= 8.1 x 1015 cm"5
~\/A (ANNEALED)
X X X
-- 300
\ \
-- 450
E
j\
~
X
~
-- 6 0 0
-,-.--. X - - _ _ - - :'L., ,..,
'tt-
(DEGRADED) '~'e,~ •
O----e
V = 700 rnV w= 1500~
-- 750
Nb= 4.2 x tO16 cm 3 !
-- 900
I
750
I
1500
I
2250
I
3000
l
3750
l
4500
15250
6000
DISTANCE X (~)
Fig. 2. A surface p h o t o v o l t a g e profile o f a p - i - n cell in t h e a n n e a l e d s t a t e a n d in degraded state (after exposure to prolonged illumination).
the
accord with cell measurements. Second, both potential profiles for the p - i junction can be well fitted in terms of a one-sided step junction with a constant space charge density as given in the figure. (Similar analyses cannot be made for the n - i junction because the n - i interface is removed by the sputter etching before the i layer is reached.) The most important feature of Fig. 2 is that the profile taken after prolonged illumination exhibits a much larger quasi-neutral or field-free region in the i layer than that found after annealing. This can be traced directly to a fivefold increase in the net positive space charge density. The physical origin of this space charge is c o m m o n l y attributed to dangling bond d o n o r states near midgap and in the b o t t o m half of the gap. Whether the increase in positive space charge is due to an actual increase in dangling bond states or merely to an increased depopulation of these states (e.g. by Fermi level motion) is difficult to say at present and requires further study.
4. IR absorption In earlier work we showed that bleeding air into the Sill4 discharge increased the light-induced degradation of solar cells [6]. Crandall [7] showed that the density of metastable centers increased as the oxygen and nitrogen concentrations in the films increased.
22
We have recently grown films of a-Si:H on germanium prisms using Sill4 discharges containing a b o u t 2% of air or H20. The light was coupled into the films so as to obtain multiple passes via total internal reflection. The IR spectra were scanned 1500 times between 400 and 4000 cm -1 to obtain a good signal-to-noise ratio. The data were stored in a c o m p u t e r , and comparisons were made between spectra for films in annealed and lightsoaked states. As shown in Fig. 3, we observe partially reversible light-induced changes in the Si--H vibrational modes at 2000 and 2090 cm -1 for a film grown in the presence of H20. Irreversible changes in the Si--O modes are also observed on annealing and are at t r i but e d to the f o r m a t i o n of SiO2 clusters. F o r a film grown in the presence of a b o u t 2% air, we also observed partially reversible light-induced changes in the Si - H modes at 2000 and 2090 cm -1. In this case the initial spectrum was similar to t hat for the film grown in the presence o f H2 0 but the Si--O m o d e at 950 cm -1 was n o t affected by annealing [ 8 ]. lfi
6
#
!
t
f
-*tR WAI~..#WUHB~
Fig. 3. Relative c h a n g e s o n a logarithmic scale of the IR absorption of an a-Si:H film g r o w n in the presence of w a t e r v a p o r : spectrum A, log(light-soaked state/as-prepared state); spectrum B, log(annealed state/light-soaked state); spectrum C, log(second lightsoaked state/annealed state); spectrum D, l o g ( s e c o n d a n n e a l e d s t a t e / s e c o n d light-soaked state ).
23
In some samples we observed a small increase in the Si--O mode at about 1050 cm -1 after these samples had been left in air overnight, and a concurrent small decrease was observed in the Si--H mode at 2000 cm -1. Some of the a-Si:H films are porous since compositional analyses using secondary ion mass spectroscopy indicate the penetration of oxygen in some cases [9]. The magnitude of the changes in the Si--H modes appears to be too large to be directly correlated with the density of metastable centers determined by other techniques (e.g. diffusion length measurements, electron spin resonance, deep level transient spectroscopy).
5. Conclusions Both the diffusion length measurements and the surface photovoltage profiling show that prolonged illumination generally increases the net space charge in undoped a-Si:H. The diffusion length measurements also indicate that the density of recombination centers is usually increased by light soaking. The changes observed in the IR absorption spectra appear to be associated with the presence of oxygen in the a-Si:H films, but the mechanism is not understood at this time.
Acknowledgments The research reported herein was supported by the Solar Energy Research Institute under Contract ZE-2-02044-01 and by the RCA Laboratories, Princeton, NJ.
References 1 2 3 4 5 6 7 8 9
D. L. Staebler and C. R. Wronski, Appl. Phys. Lett., 31 (1977) 292. D. E. Carlson, Solar Energy Mater., 8 (1982) 129. J. Dresner, B. Goldstein and D. Szostak, Appl. Phys. Lett., 38 (1980). A. R. Moore, Appl. Phys. Lett., 40 (1982) 403. B. Goldstein, D. Redfield, D. J. Szostak and L. A. Carr, Appl. Phys. Lett., 39 (1981) 258. D. E. Carlson, J. Vac. Sci. Technol., 20 (1982) 290. R. S. Crandall, Phys. Rev. B, 24 (1981) 7457. D. E. Carlson, R. W. Smith, P. J. Zanzucchi and W. R. Frenchu, Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, September 1982, IEEE, New York, to be published. J. Dresner, personal communication, 1982.
19
LIGHT-INDUCED EFFECTS IN AMORPHOUS SILICON MATERIAL AND DEVICES* D. E, C A R L S O N , A. R. M O O R E , D. J. S Z O S T A K , B. G O L D S T E I N , R. W. SMITH, P. J. Z A N Z U C C H I and W. R. F R E N C H U
R C A Laboratories, Princeton, N J 08540 (U.S.A.) (Received January 3, 1983; accepted January 10, 1983)
Summary We have studied the stability of hydrogenated amorphous silicon (a-Si:H) using several new techniques such as diffusion length measurements, photovoltage profiling and IR absorption via multiple internal reflections. We find that prolonged illumination generally causes a decrease in the diffusion length and an increase in the space charge density of undoped a-Si:H films.
1. Introduction • Light-induced effects in hydrogenated amorphous silicon (a-Si:H)were first reported by Staebler and Wronski in 1977 [1]. They showed that the photoconductivity and dark conductivity of a-Si:H could be reduced significantly by prolonged exposure to illumination and that these effects could be reversed by annealing the films at temperatures of about 150 °C or above. In the last few years, light-induced effects have been observed in many of the properties of a-Si:H films by investigators using a variety of techniques (see for example ref. 2). In this paper we present experimental results showing that light-induced changes can also be observed in the diffusion length, the space charge density and the IR absorption spectrum of a-Si:H films.
2. Diffusion length measurements The diffusion length is measured by varying the wavelength of incident light and adjusting the photon flux to assure a constant surface photovoltage * P a p e r presented at the Solar Energy Research Institute W o r k s h o p o n Light-induced Change in a-Si:H and its E f f e c t o n Solar Cell S t a b i l i t y , San Diego, CA, U.S.A., September 24 - 25, 1 9 8 2 .
0379-6787/83/$3.00
© ElsevierSequoia/Printed in The Netherlands
20
[3, 4]. Using this technique, Dresner et al. [3] showed that the diffusion length of some a-Si:H films decreased after exposure to prolonged illumination. We have continued these measurements on a-Si:H films made in several discharge systems under a wide variety of conditions and find similar behavior in most films. However, in some samples the diffusion length is not significantly affected by prolonged illumination and in a few samples it has actually increased. Two characteristic lengths can be extracted from the constant photovoltage technique: L, the diffusion length in the presence of 1 sun illumination, and W0, the collection length at very low light levels. W0 is roughly the sum of the diffusion length and the space charge width under low light level conditions (about 10 _6 suns). In Fig. 1 we show a plot of L versus Wo for several a-Si:H films made under varying deposition conditions; the arrows show the change in these lengths after exposure to an illumination of 2 suns (about 200 mW cm -2) for 1 h. Most samples show a decrease in both L and W0 after prolonged illumination, b u t samples 2 and 3 show anomalous behavior. The mechanisms responsible for the differences in behavior have not yet been identified.
3. Surface photovoltage profiling We have also used surface photovoltage profiling [5] to determine the field distribution in the i layer of a-Si:H p - i - n cells before and after exposure to prolonged illumination. Figure 2 shows such distributions for a typical unstable cell in the annealed state and after prolonged illumination. Several features are n o t e w o r t h y . First, there is negligible change in Voc, in 1.0
0.8--
0.6
_J
0.=--
0.:'--
0 0
I
I
L0
2D
3.0
w0 (/~m) Fig. 1. Diffusion length as a function of the collection length at low light leve|s; the BL~ZOWS denote the change induced by exposure to 2 suns for 1 h.
21 Ti
n÷
X \\ X X X
m
0
V - 6 4 0 mV
,, • ~------------------~ o ~ ~X
\
--150
Nb= 8.1 x 1015 cm"5
~\/A (ANNEALED)
X X X
-- 300
\ \
-- 450
E
j\
~
X
~
-- 6 0 0
-,-.--. X - - _ _ - - :'L., ,..,
'tt-
(DEGRADED) '~'e,~ •
O----e
V = 700 rnV w= 1500~
-- 750
Nb= 4.2 x tO16 cm 3 !
-- 900
I
750
I
1500
I
2250
I
3000
l
3750
l
4500
15250
6000
DISTANCE X (~)
Fig. 2. A surface p h o t o v o l t a g e profile o f a p - i - n cell in t h e a n n e a l e d s t a t e a n d in degraded state (after exposure to prolonged illumination).
the
accord with cell measurements. Second, both potential profiles for the p - i junction can be well fitted in terms of a one-sided step junction with a constant space charge density as given in the figure. (Similar analyses cannot be made for the n - i junction because the n - i interface is removed by the sputter etching before the i layer is reached.) The most important feature of Fig. 2 is that the profile taken after prolonged illumination exhibits a much larger quasi-neutral or field-free region in the i layer than that found after annealing. This can be traced directly to a fivefold increase in the net positive space charge density. The physical origin of this space charge is c o m m o n l y attributed to dangling bond d o n o r states near midgap and in the b o t t o m half of the gap. Whether the increase in positive space charge is due to an actual increase in dangling bond states or merely to an increased depopulation of these states (e.g. by Fermi level motion) is difficult to say at present and requires further study.
4. IR absorption In earlier work we showed that bleeding air into the Sill4 discharge increased the light-induced degradation of solar cells [6]. Crandall [7] showed that the density of metastable centers increased as the oxygen and nitrogen concentrations in the films increased.
22
We have recently grown films of a-Si:H on germanium prisms using Sill4 discharges containing a b o u t 2% of air or H20. The light was coupled into the films so as to obtain multiple passes via total internal reflection. The IR spectra were scanned 1500 times between 400 and 4000 cm -1 to obtain a good signal-to-noise ratio. The data were stored in a c o m p u t e r , and comparisons were made between spectra for films in annealed and lightsoaked states. As shown in Fig. 3, we observe partially reversible light-induced changes in the Si--H vibrational modes at 2000 and 2090 cm -1 for a film grown in the presence of H20. Irreversible changes in the Si--O modes are also observed on annealing and are at t r i but e d to the f o r m a t i o n of SiO2 clusters. F o r a film grown in the presence of a b o u t 2% air, we also observed partially reversible light-induced changes in the Si - H modes at 2000 and 2090 cm -1. In this case the initial spectrum was similar to t hat for the film grown in the presence o f H2 0 but the Si--O m o d e at 950 cm -1 was n o t affected by annealing [ 8 ]. lfi
6
#
!
t
f
-*tR WAI~..#WUHB~
Fig. 3. Relative c h a n g e s o n a logarithmic scale of the IR absorption of an a-Si:H film g r o w n in the presence of w a t e r v a p o r : spectrum A, log(light-soaked state/as-prepared state); spectrum B, log(annealed state/light-soaked state); spectrum C, log(second lightsoaked state/annealed state); spectrum D, l o g ( s e c o n d a n n e a l e d s t a t e / s e c o n d light-soaked state ).
23
In some samples we observed a small increase in the Si--O mode at about 1050 cm -1 after these samples had been left in air overnight, and a concurrent small decrease was observed in the Si--H mode at 2000 cm -1. Some of the a-Si:H films are porous since compositional analyses using secondary ion mass spectroscopy indicate the penetration of oxygen in some cases [9]. The magnitude of the changes in the Si--H modes appears to be too large to be directly correlated with the density of metastable centers determined by other techniques (e.g. diffusion length measurements, electron spin resonance, deep level transient spectroscopy).
5. Conclusions Both the diffusion length measurements and the surface photovoltage profiling show that prolonged illumination generally increases the net space charge in undoped a-Si:H. The diffusion length measurements also indicate that the density of recombination centers is usually increased by light soaking. The changes observed in the IR absorption spectra appear to be associated with the presence of oxygen in the a-Si:H films, but the mechanism is not understood at this time.
Acknowledgments The research reported herein was supported by the Solar Energy Research Institute under Contract ZE-2-02044-01 and by the RCA Laboratories, Princeton, NJ.
References 1 2 3 4 5 6 7 8 9
D. L. Staebler and C. R. Wronski, Appl. Phys. Lett., 31 (1977) 292. D. E. Carlson, Solar Energy Mater., 8 (1982) 129. J. Dresner, B. Goldstein and D. Szostak, Appl. Phys. Lett., 38 (1980). A. R. Moore, Appl. Phys. Lett., 40 (1982) 403. B. Goldstein, D. Redfield, D. J. Szostak and L. A. Carr, Appl. Phys. Lett., 39 (1981) 258. D. E. Carlson, J. Vac. Sci. Technol., 20 (1982) 290. R. S. Crandall, Phys. Rev. B, 24 (1981) 7457. D. E. Carlson, R. W. Smith, P. J. Zanzucchi and W. R. Frenchu, Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, September 1982, IEEE, New York, to be published. J. Dresner, personal communication, 1982.