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Effect of light soaking on a-Si:H solar cells under reverse biased annealing treatment
JOURNAL OF
ELSEVIER
Journal of Non-Crystalline Solids 204 (1996) 92-94
Letter to the Editor
Effect of light soaking on a-Si:H solar cells under reverse biased annealing treatment R.V.R. Murthy *, V. Dutta Thin Film and Photovoltaic Laborato O, Physics Department and Center for Energy, Studies Indian Institute of Technology New Delhi 110 016, India
Received 1 August 1995; revised 14 May 1996
Abstract
The bulk and interface states and their effect on a-Si:H p - i - n solar cells have been investigated using dark reverse current-voltage (J-V) properties on the basis of thermal generation of carriers from the defect states. In this study, the samples are prepared by plasma chemical vapor deposition CVD (multi-chamber) deposition technique. The open circuit voltage, Voc, short circuit current, Isc, and efficiency of the cells are, respectively, 0.8 V, 12.7 m A / c m 2 and 5.2%. The bulk (i-layer) and interface (p/i) state densities in these cells have been determined. Reverse biased annealing treatment on these cells shows an effect on the interface states only. The interface state density decreases in the cells which are annealed at - 2 V. The observed insignificant effect of light soaking in these cells is explained in terms of hydrogen motion.
With increasing understanding of the a-Si:H materials and a growing number of device applications such as solar cells, methods to characterize interface and junction regions become desirable. It has been known for many years that exposure of a-Si:H to strong band gap light generates defects in the material, widely known as Staebler-Wronski effect (SWE) [1]. Whether SWE is a bulk effect or interface effect is currently being studied [2-5]. A clear knowledge of the distribution of defects in bulk and interface and the effects of light soaking is vital to the interpretation of many of the results on SWE. In this letter, we report the separation of bulk (i-layer) and interface ( p / i ) states using dark reverse
Corresponding author. Tel.: +91-11 685 7654/7321; fax: +91-11 686 2037; e-mail: [email protected].
current as a function of voltage and the effects of light soaking. Samples of area I cm 2 a-Si:H solar ceils (BHEL, India) of structure-glass/indium tin o x i d e / p + (15 n m ) / i (350 n m ) / n + (30 n m ) - a - S i : H / A g have been used in the measurement. The samples were annealed under different reverse biases (0 V, - 1 V and - 2 V) in a chamber at 10 - 6 Torr at 170°C for half an hour in both dark and light conditions and then cooled under bias. A mercury lamp with a light intensity = 100 m W / c m 2 has been used as light source. The samples annealed at 0 V, - 1 V and - 2 V are named sample A, B and C, respectively. Fig. 1 shows the current as a function of voltage of sample A, B and C in absence of light during annealing (solid lines). The reverse current for sample A (reverse biased annealing ( R B A ) = 0 V) is higher than the sample B (RBA = - 1 V) and Sample C (RBA at = - 2 V) in the entire voltage region.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 5 3 2-7
R. V.R. Murthy, V. Dutta / Journal of Non-C~stalline Solids 204 (1996) 92-94
localized states in smaller as well as larger reverse voltage regions can be estimated from the reverse currents. For p - i - n a-Si:H solar cells, the reverse current is given by
-5
10
T = 30014 RBA w i t h o u t ....
E
u
RBA w i t h
tight
93
5ompte
tight
~o-~
Jo = qniod/27", '7,
(1)
where d is the thickness of the i-layer, q is the electric charge, ~- is the lifetime of the carrier (electron) and nio is the electron density in the i-layer which can be expressed as [6]
I0
10-6
/'/io = Arce x p ( - E g / 2 k T ) + Ud exp( -Eg/2Ect ), -9
I.
lO
0
I0
(2)
I
20
3O
Reverse v o l t a g e ( v o l t s ) Fig. I. Reverse current as a function o f voltage in reverse biased
annealed samples (sample A, B and C). Without light ( ~ ) , with light in the annealing process ( - - - - - ) .
The difference in reverse currents in these samples is small in the small voltage region ( < 5 V) and larger at large voltage region ( ~ 24 V). It has recently been demonstrated that analysis of the reverse current as a function of voltage can yield useful information about the mid gap states and interfacial defects in a-Si:H devices if they are controlled by thermal generation of the carriers from the defect states [1,2]. Thus, we expect that, for smaller voltages (Fig. 1), the thermal generation is dominated by the bulk of the device and, with increase of the reverse bias voltage, the generation is spatially spread to the doped layers in the i-layer. In the present studies, the increase in the reverse currents at larger reverse biases can be attributed to the thermal generation of carriers from the defect states at p / i interface. The density of
where Nc and Nd are the density of conduction band states and tail states, respectively, Eg is the energy band gap of the i-layer and E~t is the characteristic energy of the conduction band tail states. And finally the reverse current (Jo) relates to the total number of defect states as
Jo = { qdtr VthNt/2 } {Nc exp( - E J 2 kT) + Nc, exp( - Eg/Z Ect ) } ,
(3)
where Nt is the number of defect states, ~r is the carrier capture cross-section and Uth is the thermal velocity. Hence, Nt can be estimated using Eq. (3) from the measured reverse current and taking the values of different parameters (Eo = 1.77 eV, or= 10 -~5 cm 2, Nc=N~I= 1020 cm 3,~ E~t=28 meV, cth= l07 cm/s). The calculated state density in bulk of the i-layer and at p / i interface of the cell are shown in Table 1. The same experiment has been repeated on four sets of similar samples in order to find out the experimental error in the measurements. The probable
Table 1 Bulk (Nt) and interface (N~0 state densities for reverse biased annealed samples of a-Si:H solar cells without light and with light in the annealing process Sample
A B C
Without light in the annealing process (experimental error is _+0.05 X 1016 c m - 3)
With light in the annealing process (experimental error is _+0.05 X 1016 cm -3)
Nt (crn 3)
Nit (cm-3)
Ni (cm-3)
Nit (crn-3)
1.8 X 1016 1.8 X 1016 1.8 X 1016
5.7 X 1017 2.0 X 1017 6.6 X 1016
2.8 X 1016 2.0 X 1016 1.9 t 1016
1.8 X 1018 4.2 X 1017 8.4 X 1016
94
R. V.R. Murthy, V. Dutta / Journal of Non-Cry'stalline Solids 204 (1996) 92-94
experimental error is observed to be ~ 0.05 X 1016 c m - 3.
It has been observed from Table 1 that defect state density varies from its intrinsic value of 1.8 x 1016 cm -3 in the bulk of the i-layer to 5.7 X 1017 cm -3 at p / i interface. The decrease of Nt at larger absolute voltages from sample A to sample C indicates that the reverse biased annealing treatment reduces the number of defect states at p / i interface only. Neitzert et al. [7] have shown the existence of rich hydrogen content in the i-layer near to the interface. Thus, the decrease of Nt in sample B and C could be due to the movement of hydrogen in the electric field at elevated temperatures towards p / i interface which passivates the interface states [8]. The reverse current as a function of voltage of the same samples (A, B and C) in presence of light in the annealing process is shown in Fig. 1 (dashed lines). A similar behavior is observed as in J - V properties of these samples in the absence of light in the annealing process. The calculated Nts are given in Table 1. We have observed that the defect density is increased by the light soaking throughout i-layer and at p / i interface. This increase is expected since the light soaking produces light induced defects (LIDs) in the cell. However, the bulk region near p / i interface is affected more by light soaking than the regions deep in the bulk of the i-layer. A similar behavior was also observed by Zhou et al. [5]. We also observe from Fig. 1 that the effect of light soaking is reduced from sample A to sample C. Jackson et al. have shown that the hydrogen motion plays a crucial role in the annihilation of these LIDs [9]. The observed difference in sample A and C in our study can be expected that the content of the hydrogen and its motion are enhanced by the existed large fields in sample C and helps to reconstruct dangling bonds more at the p / i interface compared with other two samples. In conclusion, the effect of bulk and interface states in a-Si:H p - i - n solar cells have been differen-
tiated using dark reverse current-voltage characteristics. The reverse biased annealing treatment on these samples shows the effect on the interface states only. The interface state density has been estimated to be 5.7 × 1017 c m -3 in annealed sample biased at 0 V and decreases to a value of 6.6 X 1016 cm -3 in the annealed samples biased at - 2 V. The effect of light soaking is observed more in the bulk regions near the p / i interface as compared to the regions deep in the bulk of the i-layer. The insignificant effect of light soaking in the samples annealed at - 2 V is explained in terms of hydrogen motion towards p / i interface.
Acknowledgements The authors would like to thank staff of the Amorphous Silicon Solar Cell plant (BHEL, India) for supplying the samples for these studies. They are indebted to Professor L.K. Malhotra, Thin Film Lab., liT, New Delhi for providing the experimental facilities. The financial assistance from University Grants Commission, New Delhi is gratefully acknowledged.
References [1] D.L. Staebler and C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292; J. Appl. Phys. 51 (1980) 3262. [2] J.K. Arch and S.J. Fonash, J. Appl. Phys. 72 (1992) 4483. [3l J.K. Arch and S.J. Fonash, Appl. Phys. Lett. 60 (1992) 757. [4] N. Wyrsch, A. Shah and H. Keppner, in: Proc. 1st World Conf. on Photovoltaics Energy Conversion, Dec. 5-9, 1994, Waikoloa, HI. [5] J.-H. Zhou, M. Kumeda and T. Shimizu, Appl. Phys. Lett. 66 (1995) 742. [6] F. Demichelis, A. Tagliaferro and E. Tresso, Solar Cells 14 (1985) 149. [7] H.C. Neitzert, M.A. Briere and P. Lechner, Physica BI70 (1991) 529. [8] A. Rothwarf, in: Proc. 20th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1988 (IEEE, New York, 1988) p. 166. [9] W.B. Jackson and Y. Kakalios, Phys. Rev. B37 (1988) 1020.
ELSEVIER
Journal of Non-Crystalline Solids 204 (1996) 92-94
Letter to the Editor
Effect of light soaking on a-Si:H solar cells under reverse biased annealing treatment R.V.R. Murthy *, V. Dutta Thin Film and Photovoltaic Laborato O, Physics Department and Center for Energy, Studies Indian Institute of Technology New Delhi 110 016, India
Received 1 August 1995; revised 14 May 1996
Abstract
The bulk and interface states and their effect on a-Si:H p - i - n solar cells have been investigated using dark reverse current-voltage (J-V) properties on the basis of thermal generation of carriers from the defect states. In this study, the samples are prepared by plasma chemical vapor deposition CVD (multi-chamber) deposition technique. The open circuit voltage, Voc, short circuit current, Isc, and efficiency of the cells are, respectively, 0.8 V, 12.7 m A / c m 2 and 5.2%. The bulk (i-layer) and interface (p/i) state densities in these cells have been determined. Reverse biased annealing treatment on these cells shows an effect on the interface states only. The interface state density decreases in the cells which are annealed at - 2 V. The observed insignificant effect of light soaking in these cells is explained in terms of hydrogen motion.
With increasing understanding of the a-Si:H materials and a growing number of device applications such as solar cells, methods to characterize interface and junction regions become desirable. It has been known for many years that exposure of a-Si:H to strong band gap light generates defects in the material, widely known as Staebler-Wronski effect (SWE) [1]. Whether SWE is a bulk effect or interface effect is currently being studied [2-5]. A clear knowledge of the distribution of defects in bulk and interface and the effects of light soaking is vital to the interpretation of many of the results on SWE. In this letter, we report the separation of bulk (i-layer) and interface ( p / i ) states using dark reverse
Corresponding author. Tel.: +91-11 685 7654/7321; fax: +91-11 686 2037; e-mail: [email protected].
current as a function of voltage and the effects of light soaking. Samples of area I cm 2 a-Si:H solar ceils (BHEL, India) of structure-glass/indium tin o x i d e / p + (15 n m ) / i (350 n m ) / n + (30 n m ) - a - S i : H / A g have been used in the measurement. The samples were annealed under different reverse biases (0 V, - 1 V and - 2 V) in a chamber at 10 - 6 Torr at 170°C for half an hour in both dark and light conditions and then cooled under bias. A mercury lamp with a light intensity = 100 m W / c m 2 has been used as light source. The samples annealed at 0 V, - 1 V and - 2 V are named sample A, B and C, respectively. Fig. 1 shows the current as a function of voltage of sample A, B and C in absence of light during annealing (solid lines). The reverse current for sample A (reverse biased annealing ( R B A ) = 0 V) is higher than the sample B (RBA = - 1 V) and Sample C (RBA at = - 2 V) in the entire voltage region.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 5 3 2-7
R. V.R. Murthy, V. Dutta / Journal of Non-C~stalline Solids 204 (1996) 92-94
localized states in smaller as well as larger reverse voltage regions can be estimated from the reverse currents. For p - i - n a-Si:H solar cells, the reverse current is given by
-5
10
T = 30014 RBA w i t h o u t ....
E
u
RBA w i t h
tight
93
5ompte
tight
~o-~
Jo = qniod/27", '7,
(1)
where d is the thickness of the i-layer, q is the electric charge, ~- is the lifetime of the carrier (electron) and nio is the electron density in the i-layer which can be expressed as [6]
I0
10-6
/'/io = Arce x p ( - E g / 2 k T ) + Ud exp( -Eg/2Ect ), -9
I.
lO
0
I0
(2)
I
20
3O
Reverse v o l t a g e ( v o l t s ) Fig. I. Reverse current as a function o f voltage in reverse biased
annealed samples (sample A, B and C). Without light ( ~ ) , with light in the annealing process ( - - - - - ) .
The difference in reverse currents in these samples is small in the small voltage region ( < 5 V) and larger at large voltage region ( ~ 24 V). It has recently been demonstrated that analysis of the reverse current as a function of voltage can yield useful information about the mid gap states and interfacial defects in a-Si:H devices if they are controlled by thermal generation of the carriers from the defect states [1,2]. Thus, we expect that, for smaller voltages (Fig. 1), the thermal generation is dominated by the bulk of the device and, with increase of the reverse bias voltage, the generation is spatially spread to the doped layers in the i-layer. In the present studies, the increase in the reverse currents at larger reverse biases can be attributed to the thermal generation of carriers from the defect states at p / i interface. The density of
where Nc and Nd are the density of conduction band states and tail states, respectively, Eg is the energy band gap of the i-layer and E~t is the characteristic energy of the conduction band tail states. And finally the reverse current (Jo) relates to the total number of defect states as
Jo = { qdtr VthNt/2 } {Nc exp( - E J 2 kT) + Nc, exp( - Eg/Z Ect ) } ,
(3)
where Nt is the number of defect states, ~r is the carrier capture cross-section and Uth is the thermal velocity. Hence, Nt can be estimated using Eq. (3) from the measured reverse current and taking the values of different parameters (Eo = 1.77 eV, or= 10 -~5 cm 2, Nc=N~I= 1020 cm 3,~ E~t=28 meV, cth= l07 cm/s). The calculated state density in bulk of the i-layer and at p / i interface of the cell are shown in Table 1. The same experiment has been repeated on four sets of similar samples in order to find out the experimental error in the measurements. The probable
Table 1 Bulk (Nt) and interface (N~0 state densities for reverse biased annealed samples of a-Si:H solar cells without light and with light in the annealing process Sample
A B C
Without light in the annealing process (experimental error is _+0.05 X 1016 c m - 3)
With light in the annealing process (experimental error is _+0.05 X 1016 cm -3)
Nt (crn 3)
Nit (cm-3)
Ni (cm-3)
Nit (crn-3)
1.8 X 1016 1.8 X 1016 1.8 X 1016
5.7 X 1017 2.0 X 1017 6.6 X 1016
2.8 X 1016 2.0 X 1016 1.9 t 1016
1.8 X 1018 4.2 X 1017 8.4 X 1016
94
R. V.R. Murthy, V. Dutta / Journal of Non-Cry'stalline Solids 204 (1996) 92-94
experimental error is observed to be ~ 0.05 X 1016 c m - 3.
It has been observed from Table 1 that defect state density varies from its intrinsic value of 1.8 x 1016 cm -3 in the bulk of the i-layer to 5.7 X 1017 cm -3 at p / i interface. The decrease of Nt at larger absolute voltages from sample A to sample C indicates that the reverse biased annealing treatment reduces the number of defect states at p / i interface only. Neitzert et al. [7] have shown the existence of rich hydrogen content in the i-layer near to the interface. Thus, the decrease of Nt in sample B and C could be due to the movement of hydrogen in the electric field at elevated temperatures towards p / i interface which passivates the interface states [8]. The reverse current as a function of voltage of the same samples (A, B and C) in presence of light in the annealing process is shown in Fig. 1 (dashed lines). A similar behavior is observed as in J - V properties of these samples in the absence of light in the annealing process. The calculated Nts are given in Table 1. We have observed that the defect density is increased by the light soaking throughout i-layer and at p / i interface. This increase is expected since the light soaking produces light induced defects (LIDs) in the cell. However, the bulk region near p / i interface is affected more by light soaking than the regions deep in the bulk of the i-layer. A similar behavior was also observed by Zhou et al. [5]. We also observe from Fig. 1 that the effect of light soaking is reduced from sample A to sample C. Jackson et al. have shown that the hydrogen motion plays a crucial role in the annihilation of these LIDs [9]. The observed difference in sample A and C in our study can be expected that the content of the hydrogen and its motion are enhanced by the existed large fields in sample C and helps to reconstruct dangling bonds more at the p / i interface compared with other two samples. In conclusion, the effect of bulk and interface states in a-Si:H p - i - n solar cells have been differen-
tiated using dark reverse current-voltage characteristics. The reverse biased annealing treatment on these samples shows the effect on the interface states only. The interface state density has been estimated to be 5.7 × 1017 c m -3 in annealed sample biased at 0 V and decreases to a value of 6.6 X 1016 cm -3 in the annealed samples biased at - 2 V. The effect of light soaking is observed more in the bulk regions near the p / i interface as compared to the regions deep in the bulk of the i-layer. The insignificant effect of light soaking in the samples annealed at - 2 V is explained in terms of hydrogen motion towards p / i interface.
Acknowledgements The authors would like to thank staff of the Amorphous Silicon Solar Cell plant (BHEL, India) for supplying the samples for these studies. They are indebted to Professor L.K. Malhotra, Thin Film Lab., liT, New Delhi for providing the experimental facilities. The financial assistance from University Grants Commission, New Delhi is gratefully acknowledged.
References [1] D.L. Staebler and C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292; J. Appl. Phys. 51 (1980) 3262. [2] J.K. Arch and S.J. Fonash, J. Appl. Phys. 72 (1992) 4483. [3l J.K. Arch and S.J. Fonash, Appl. Phys. Lett. 60 (1992) 757. [4] N. Wyrsch, A. Shah and H. Keppner, in: Proc. 1st World Conf. on Photovoltaics Energy Conversion, Dec. 5-9, 1994, Waikoloa, HI. [5] J.-H. Zhou, M. Kumeda and T. Shimizu, Appl. Phys. Lett. 66 (1995) 742. [6] F. Demichelis, A. Tagliaferro and E. Tresso, Solar Cells 14 (1985) 149. [7] H.C. Neitzert, M.A. Briere and P. Lechner, Physica BI70 (1991) 529. [8] A. Rothwarf, in: Proc. 20th IEEE Photovoltaic Specialists Conf., Las Vegas, NV, 1988 (IEEE, New York, 1988) p. 166. [9] W.B. Jackson and Y. Kakalios, Phys. Rev. B37 (1988) 1020.