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ZnIn2S4 as a window in heterojunction solar cells
Solar Energy Materials 10 (1984) 139-143 North-Holland, Amsterdam
Znln 2S4 A S A W I N D O W I N H E T E R O J U N C T I O N
139
SOLAR CELLS
O. V I G I L , O. C A L Z A D I L L A , D. S E U R E T , J. V I D A L LIEES, Universidad de La Habana, Cuba
and F. L E C C A B U E MASPEC lnstitute-C.N.R., 43100 Parma, Italy
Received in revised form 19 January 1984 Single crystals of n-type Znln2S4 with a resistivity between 0.8 and 10 ~2cm were prepared by annealing them under a high manganese pressure. These crystals are viewed as potential windows in the preparation of heterojunction solar cells which have chalcopyrite compounds as p-type absorbers. The n-Znln2S4/p-CulnSe2 system has been characterized and discussed.
1. Introduction Several s e m i c o n d u c t o r c o m p o u n d s have b e e n s t u d i e d for use as w i n d o w s in h e t e r o j u n c t i o n solar cells [1-3]. F o r this p u r p o s e these m a t e r i a l s m u s t have a high gap, be very t r a n s p a r e n t to r a d i a t i o n which has an energy b e l o w their b a n d gaps a n d exibit low resistivity, i.e. ~< 1 flcm. T h e Z n I n 2 S 4 c o m p o u n d is a p h o t o c o n d u c t o r m a t e r i a l [4,5] whose p r o p e r t i e s are similar to those of CdS. It is an n - t y p e s e m i c o n d u c t o r with a 2.6 eV direct b a n d gap, b u t single crystals have a high resistivity. In this p a p e r we shall e x a m i n e the p o s s i b i l i t y o f using these m a t e r i a l s as w i n d o w s in the p r e p a r a t i o n of h e t e r o j u n c t i o n solar cells with c h a l c o p y r i t e c o m p o u n d s as p - t y p e absorbers. T h e n - Z n I n E S 4 ( M n ) / p - C u I n S e 2 system was s t u d i e d in o r d e r to e v a l u a t e the use o f Z n I n E S 4 as a window, in spite of the fact that the crystal lattice m a t c h is a p o o r one.
2. Experimental and discussion P o l y c r y s t a l l i n e C u I n S e 2 ingots were p r e p a r e d b y a fusion of the constituents, weighed in s t o i c h i o m e t r i c ratio. Z n I n E S 4 single crystals were grown b y a v a p o r p h a s e c h e m i c a l t r a n s p o r t m e t h o d in a close t u b e system, using i o d i n e as a t r a n s p o r t agent. A l l e l e m e n t s were 99.999% pure. 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
140
O. Vigil et al. / Z n l n 2 S ~ as a window in solar cells
In order to produce n-type single crystals with a low resistivity, ranging between 0.8 and 10 f~cm, an annealing under a maximum Mn pressure at 900 K was performed. Room temperature mobility was about 40 cm2/Vs. X-ray diffraction measurements showed a hexagonal unit cell with a = 3.88 A and c = 37.02 A. Optical transmission spectra were measured from 0.45 to 1.1 rtm. The absorption and transmission spectra for a 63 ~tm thick sample are shown in fig. 1. The band gap is about 2.34 eV. This low band gap value together with the observed absorption tail is thought to be connected with tails of density states resulting from ionized defects, i.e. manganese impurities and vacancies [5]. A heterojunction was prepared by vacuum deposition of CulnSe 2 onto Znln2S 4. Prior to evaporation, the substrates were etched in a 3 HCI: 1HNO 3 solution. CulnSe 2 thin films were deposited by a flash evaporation technique. An excess of selenium was coevaporated in order to make sure that the resulting films had a p-type conductivity. The vacuum deposition took place at 10 -6 Torr with a substrate temperature of 250°C. Film thickness was about 1 rtm. CulnSe 2 films deposited previously onto mica substrate under the same above mentioned conditions had a resistivity of about 10 -2 ~cm. The device area was 7 × 10 -2 cm 2. A typical I - V
6.
T(%~
4_
JO0
"4
b
"8
50
2.
10 0
i
0.4
0.5
i
0.6
=
0.7
!
0.8
I
f
l
0.9
|
I.I
J.2
Xt~ml
Fig. 1. Absorption (a) and transmission (b) spectra of the Znln2S4(Mn ) single crystal (sample thickness of 63 ~m).
O. Vigil et aL / Z n l n 2 S 4 as a window in solar cells
141
characteristic at room temperature is shown in fig. 2. The reverse current is on the order of 10 -6 A and the rectification factor is about 104 at 1 V. The ideality factor is 1.6 and the series resistance is high. When the device is illuminated through the Znln2S 4 window using a light of 70 m W / c m 2, a 200 mV open-circuit voltage and a 6 × 10 -2 m A / c m 2 short-circuit current as seen.
-I io
-2
io
E H
-4
io
-5 I0
•
-g
IO
0.5
'1
1~,5 V (voltg)
Fig. 2. Forward and reverse I - V temperature.
characteristics of n - Z n l n 2 S 4 / p - C u l n S e 2 heterojunction at room
142
o. vigil et aL / Z n l n z S 4 as a window in solar cells
As shown in fig. 3, the spectral response displayed a peak at 2.28 eV and a wide tail at higher wavelengths. We believe that these characteristics are due to the density states at the interface, resulting mainly from the unfavorable lattice mismatch between Z n I n 2S4 and CuInS% compounds. Moreover, the spectral characteristics of the photocurrent is evidently due to the formation of the junction mainly on the ZnIn2S 4 side and the experimental result depends with the resistivity ratio between the ZnIn2S 4 and CuInSe 2 compounds. The peak, seen at 2.28 eV, is probably due to a conduction band level transition. These levels may be explained by the presence of vacancies or by a possible diffusion of the Cu from the CuInSe 2 thin film into the ZnInzS 4 single crystal. It has been shown [6] that in the case of ZnIn2S 4 the (0001) plane is the one which is useful for junction formation, while in the case of CuInSe 2 and the other chalcopyrite compounds the (112) plane is the preferred orientation. Moreover, it would also be possible to calculate [7] the lattice mismatch for these crystallographic directions. In our case, taking the ZnIn2S 4 and CuInS% lattice constants into
351
25-
-
I
0.~
0.6~
i
0.8
i
I
O,~D
l.lO ,~ l y m )
Fig. 3. Photocurrent spectral response of Znln2S,, side).
a Z n l n 2$4/CulnSe2
heterojunction (the light passes through the
O. Vigil et al. / Z n l n 2 S ~ as a window in solar cells
143
consideration, the lattice mismatch is 9.8%. Using this same procedure it is possible to evaluate the lattice mismatch across the (0001) and (112) planes between Znln2S 4 and CuGa(Sel_xSx) 2 for x = 0.58, which is 0.07%. The transmission properties of Znln 2S4 thin films with a low resistivity similar to that of devices based on CdS, was improved when compared with the single crystals. On the other hand Znln2S 4 thin films which were prepared using the flash evaporation had a high resistivity of about 1 × 103 f~cm [8].
3. Conclusion We have demonstrated that it is possible to obtain Znln2S 4 single crystals which are good enough to be used as windows in solar cells. For future potential use it will be necessary to have these materials in the form of thin films with a resistivity of about 10 -1 ~2cm. In line with this, it would be interesting to examine the Znln2S4/CuGa(Se l_xSx)2 system as a component of the stack in thin film tandem solar cells. Analogously, it would be interesting to study the Znln2Sa/CuGaSe 2 system, which has a lattice mismatch of 3% and a CuGaSe2band gap value very close to the best one calculated for the p-type absorber. Heterojunctions with Cdln2S 4 [9] and ZnlnzSe 4 [10] have been made with photovoltaic characteristics. These initial results could make the I I - I I I 2 - V I 4 compounds good candidates in the preparation of the heterojunctions for solar cell applications.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
M. Arienzo and J.J. Loferski, J. Appl. Phys. 51 (1980) 3393. N. Romeo, G. Sberveglieri and L. Tarricone, Appl. Phys. Lett. 32 (1978) 807. J.C. Manifacier, M. de Murcia and J.P. Fillard, Thin Solid Films 41 (1977) 127. N. Romeo and O. Vigil, Phys. Stat. Sol. (a) 10 (1972) 447. L. Hernandez, O. Vigil and F. Gonzalez, Phys. Stat. SoL(a) 36 (1976) 33. A.N.Y. Samaan, N. Abdul-Karim, Abdul-Hussein, R.D. Tomlinson, A.E. Hill and D.G. Harmour, Japan. J. Appl. Phys. 19 Suppl. 19-3 (1980) 15. K.A. Jones, J. Crystal Growth 47 (1979) 235. F. Andreani and N. Romeo, Thin Solid Films 31 (1976) 217. S. Endo and T. Irie, Japan. J. Appl. Phys. 19 Supp. 19-3 (1980) 53. F~J. Carcia and M.S. Tomar, Thin Solid Films 69 (1980) 137.
Znln 2S4 A S A W I N D O W I N H E T E R O J U N C T I O N
139
SOLAR CELLS
O. V I G I L , O. C A L Z A D I L L A , D. S E U R E T , J. V I D A L LIEES, Universidad de La Habana, Cuba
and F. L E C C A B U E MASPEC lnstitute-C.N.R., 43100 Parma, Italy
Received in revised form 19 January 1984 Single crystals of n-type Znln2S4 with a resistivity between 0.8 and 10 ~2cm were prepared by annealing them under a high manganese pressure. These crystals are viewed as potential windows in the preparation of heterojunction solar cells which have chalcopyrite compounds as p-type absorbers. The n-Znln2S4/p-CulnSe2 system has been characterized and discussed.
1. Introduction Several s e m i c o n d u c t o r c o m p o u n d s have b e e n s t u d i e d for use as w i n d o w s in h e t e r o j u n c t i o n solar cells [1-3]. F o r this p u r p o s e these m a t e r i a l s m u s t have a high gap, be very t r a n s p a r e n t to r a d i a t i o n which has an energy b e l o w their b a n d gaps a n d exibit low resistivity, i.e. ~< 1 flcm. T h e Z n I n 2 S 4 c o m p o u n d is a p h o t o c o n d u c t o r m a t e r i a l [4,5] whose p r o p e r t i e s are similar to those of CdS. It is an n - t y p e s e m i c o n d u c t o r with a 2.6 eV direct b a n d gap, b u t single crystals have a high resistivity. In this p a p e r we shall e x a m i n e the p o s s i b i l i t y o f using these m a t e r i a l s as w i n d o w s in the p r e p a r a t i o n of h e t e r o j u n c t i o n solar cells with c h a l c o p y r i t e c o m p o u n d s as p - t y p e absorbers. T h e n - Z n I n E S 4 ( M n ) / p - C u I n S e 2 system was s t u d i e d in o r d e r to e v a l u a t e the use o f Z n I n E S 4 as a window, in spite of the fact that the crystal lattice m a t c h is a p o o r one.
2. Experimental and discussion P o l y c r y s t a l l i n e C u I n S e 2 ingots were p r e p a r e d b y a fusion of the constituents, weighed in s t o i c h i o m e t r i c ratio. Z n I n E S 4 single crystals were grown b y a v a p o r p h a s e c h e m i c a l t r a n s p o r t m e t h o d in a close t u b e system, using i o d i n e as a t r a n s p o r t agent. A l l e l e m e n t s were 99.999% pure. 0 1 6 5 - 1 6 3 3 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
140
O. Vigil et al. / Z n l n 2 S ~ as a window in solar cells
In order to produce n-type single crystals with a low resistivity, ranging between 0.8 and 10 f~cm, an annealing under a maximum Mn pressure at 900 K was performed. Room temperature mobility was about 40 cm2/Vs. X-ray diffraction measurements showed a hexagonal unit cell with a = 3.88 A and c = 37.02 A. Optical transmission spectra were measured from 0.45 to 1.1 rtm. The absorption and transmission spectra for a 63 ~tm thick sample are shown in fig. 1. The band gap is about 2.34 eV. This low band gap value together with the observed absorption tail is thought to be connected with tails of density states resulting from ionized defects, i.e. manganese impurities and vacancies [5]. A heterojunction was prepared by vacuum deposition of CulnSe 2 onto Znln2S 4. Prior to evaporation, the substrates were etched in a 3 HCI: 1HNO 3 solution. CulnSe 2 thin films were deposited by a flash evaporation technique. An excess of selenium was coevaporated in order to make sure that the resulting films had a p-type conductivity. The vacuum deposition took place at 10 -6 Torr with a substrate temperature of 250°C. Film thickness was about 1 rtm. CulnSe 2 films deposited previously onto mica substrate under the same above mentioned conditions had a resistivity of about 10 -2 ~cm. The device area was 7 × 10 -2 cm 2. A typical I - V
6.
T(%~
4_
JO0
"4
b
"8
50
2.
10 0
i
0.4
0.5
i
0.6
=
0.7
!
0.8
I
f
l
0.9
|
I.I
J.2
Xt~ml
Fig. 1. Absorption (a) and transmission (b) spectra of the Znln2S4(Mn ) single crystal (sample thickness of 63 ~m).
O. Vigil et aL / Z n l n 2 S 4 as a window in solar cells
141
characteristic at room temperature is shown in fig. 2. The reverse current is on the order of 10 -6 A and the rectification factor is about 104 at 1 V. The ideality factor is 1.6 and the series resistance is high. When the device is illuminated through the Znln2S 4 window using a light of 70 m W / c m 2, a 200 mV open-circuit voltage and a 6 × 10 -2 m A / c m 2 short-circuit current as seen.
-I io
-2
io
E H
-4
io
-5 I0
•
-g
IO
0.5
'1
1~,5 V (voltg)
Fig. 2. Forward and reverse I - V temperature.
characteristics of n - Z n l n 2 S 4 / p - C u l n S e 2 heterojunction at room
142
o. vigil et aL / Z n l n z S 4 as a window in solar cells
As shown in fig. 3, the spectral response displayed a peak at 2.28 eV and a wide tail at higher wavelengths. We believe that these characteristics are due to the density states at the interface, resulting mainly from the unfavorable lattice mismatch between Z n I n 2S4 and CuInS% compounds. Moreover, the spectral characteristics of the photocurrent is evidently due to the formation of the junction mainly on the ZnIn2S 4 side and the experimental result depends with the resistivity ratio between the ZnIn2S 4 and CuInSe 2 compounds. The peak, seen at 2.28 eV, is probably due to a conduction band level transition. These levels may be explained by the presence of vacancies or by a possible diffusion of the Cu from the CuInSe 2 thin film into the ZnInzS 4 single crystal. It has been shown [6] that in the case of ZnIn2S 4 the (0001) plane is the one which is useful for junction formation, while in the case of CuInSe 2 and the other chalcopyrite compounds the (112) plane is the preferred orientation. Moreover, it would also be possible to calculate [7] the lattice mismatch for these crystallographic directions. In our case, taking the ZnIn2S 4 and CuInS% lattice constants into
351
25-
-
I
0.~
0.6~
i
0.8
i
I
O,~D
l.lO ,~ l y m )
Fig. 3. Photocurrent spectral response of Znln2S,, side).
a Z n l n 2$4/CulnSe2
heterojunction (the light passes through the
O. Vigil et al. / Z n l n 2 S ~ as a window in solar cells
143
consideration, the lattice mismatch is 9.8%. Using this same procedure it is possible to evaluate the lattice mismatch across the (0001) and (112) planes between Znln2S 4 and CuGa(Sel_xSx) 2 for x = 0.58, which is 0.07%. The transmission properties of Znln 2S4 thin films with a low resistivity similar to that of devices based on CdS, was improved when compared with the single crystals. On the other hand Znln2S 4 thin films which were prepared using the flash evaporation had a high resistivity of about 1 × 103 f~cm [8].
3. Conclusion We have demonstrated that it is possible to obtain Znln2S 4 single crystals which are good enough to be used as windows in solar cells. For future potential use it will be necessary to have these materials in the form of thin films with a resistivity of about 10 -1 ~2cm. In line with this, it would be interesting to examine the Znln2S4/CuGa(Se l_xSx)2 system as a component of the stack in thin film tandem solar cells. Analogously, it would be interesting to study the Znln2Sa/CuGaSe 2 system, which has a lattice mismatch of 3% and a CuGaSe2band gap value very close to the best one calculated for the p-type absorber. Heterojunctions with Cdln2S 4 [9] and ZnlnzSe 4 [10] have been made with photovoltaic characteristics. These initial results could make the I I - I I I 2 - V I 4 compounds good candidates in the preparation of the heterojunctions for solar cell applications.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
M. Arienzo and J.J. Loferski, J. Appl. Phys. 51 (1980) 3393. N. Romeo, G. Sberveglieri and L. Tarricone, Appl. Phys. Lett. 32 (1978) 807. J.C. Manifacier, M. de Murcia and J.P. Fillard, Thin Solid Films 41 (1977) 127. N. Romeo and O. Vigil, Phys. Stat. Sol. (a) 10 (1972) 447. L. Hernandez, O. Vigil and F. Gonzalez, Phys. Stat. SoL(a) 36 (1976) 33. A.N.Y. Samaan, N. Abdul-Karim, Abdul-Hussein, R.D. Tomlinson, A.E. Hill and D.G. Harmour, Japan. J. Appl. Phys. 19 Suppl. 19-3 (1980) 15. K.A. Jones, J. Crystal Growth 47 (1979) 235. F. Andreani and N. Romeo, Thin Solid Films 31 (1976) 217. S. Endo and T. Irie, Japan. J. Appl. Phys. 19 Supp. 19-3 (1980) 53. F~J. Carcia and M.S. Tomar, Thin Solid Films 69 (1980) 137.