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Refractive index of electrodeposited CuInSe2 films
Materials Letters 17 (1993) 190-191 North-Holland
Refractive index of electrodeposited
CuInSe2 films
S.N. Qiu, C.X. Qiu and I. Shih Electrical Engineering Department, McGill University, 3480 University St., Montreal, P.Q., H3A 2A7 Canada Received 9 May 1993; accepted 14 May 1993
Optical reflection measurements have been carried out in the wavelength range from 2 to 10 urn on polycrystalline CuInSer thin fdms. The CuInSe2 films, having a thickness from 1.5 to 2 urn, were electrodeposited on smooth Mo-coated glass substrates. From reflection interference, the opticalrefractive index was calculated to be 4.7.
1. Introduction
The ternary compound semiconductor CuInSez (Eg= 1.04 eV) is one of the leading materials for photovoltaic solar cell applications. Polycrystalline p-type thin films of CuInSe* have been prepared in our laboratory by an electrodeposition method for CdS/CuInSez cell fabrication [ 1,2]. Electrical properties such as carrier concentration and diffusion length have been determined using the fabricated cells and these have been used for the improvement of thin film fabrication processes. Results obtained from the previous studies showed that the electrodeposited CuInSe2 films, with further improvement, are appropriate for large area solar cell applications. In order to have a better understanding of the electrodeposited CuInSe2, it is also necessary to obtain optical parameters of this material. Accurate optical parameter measurements on the electrodeposited CuInSet thin films have not been obtained, partly due to surface morphology of the material prepared by this method. In this Letter, we report results of refractive index measurements carried out on the electrodeposited CuInSez films in the infrared wavelength range.
Se. The deposition was carried out on MO-coated glass substrate (thickness of Mo about 0.8 pm). The samples used are p-type with carrier concentrations greater than lOi cme3. Reflection measurements were made using a Perkin-Elmer model 13U spectrophotometer with the monochromatic light chopped at 86 Hz. An InSb photovoltaic detector operated at 77 K and a lock-in amplifier were used to measure both the incident and reflected light in the wavelength range from 2 to 5 urn. For the wavelength range from 5 to 10 urn, a HgCdTe detector was used. During the measurements, positions of the sample and the detector were adjusted so that the angle of incidence and that for reflection were about 15 ‘. Samples of CuInSez with the thickness ranging
0.6
_--F _.-.-.-.
9ooC16nh mc
Mdcness
1 .spll
,(jm
2. Experimental results Wavahmgth(pm)
The CuInSe, thin films used in this work were prepared by the electrodeposition method from an electrolyte containing ions and complexes of Cu, In and 190
Fig. 1. Reflectance curves measured on an electrodeposited CuInSe2sample (thickness 1.9 urn), showing two peaks and two valleys in the wavelength range from 5 to 10 urn for each curve.
0167-577x/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 17, number Table 1 Calculated
3,4
refractive
MATERIALS
indices for sample R-3 ‘) at different
Heat-treatment conditions
LETTERS
heat-treatment
1993
stages
2nd minimum (1=2)
1st minimum (1=3)
August
1st maximum (/=4)
2nd maximum (1=3)
T(“C)
t (min)
A (um)
n
1 (pm)
n
I (pm)
n
1 (km)
n
200 250 300 350
15 15 15 15
5.8 5.8 5.8 5.6
4.6 4.6 4.6 4.4
9.0 8.8 9.0 9.2
4.7 4.6 4.7 4.8
4.8 5.0 5.0 5.0
4.4 4.6 4.6 4.6
7.0 7.2
4.6 4.7
5.8
4.6
9.0
4.7
5.0
4.6
7.1
4.7
average ‘) Film thickness= Table 2 Refractive
1.9 pm.
index values for the as-deposited
Heat-treatment conditions
and heat-treated
1st minimum (1=3)
sample R-2 ‘) 2nd minimum (1=2)
1st maximum (1=4)
2nd maximum (1=3)
T(“C)
t (min)
1 (pm)
n
1 (pm)
n
1 (w)
n
A (pm)
n
untreated 250 300
15 15
4.6 4.6 4.6
4.6 4.6 4.6
6.8 7.0 6.6
4.5 4.7 4.4
4.2 4.3 4.2
4.9 5.0 4.9
5.4 5.8 5.6
4.5 4.8 4.7
4.6
4.6
6.8
4.5
4.2
4.9
5.6
4.7
average ‘) Film thickness=
I .5 pm.
from 1.5 to 2 pm, area about 1 x 1 cm’, were used for the measurements. In the device fabrication involving electrodeposited CuInSez, it is often required to heat treat the CuInSez in vacuum in order to improve its crystalline and electronic properties. Therefore, the reflection measurements were carried out on samples both immediately after the deposition and after a heat treatment. Fig. 1 shows results of reflection measurements on one of the samples, R-3, in the wavelength range from 2 to 10 urn. The thickness of the sample is about 1.9 urn, which was determined from an SEM micrograph. For each reflectance curve, taken after the treatment, there are two clear peaks and two valleys in the wavelength range from 5 to 10 urn. From the maxima and minima on each curve, the refractive index is calculated. Table 1 lists the wavelength values of the reflectance maxima and minima for sample R-3. Also given in table 1 are the calculated refractive index value for each maximum or minimum and the average values. It is noted that the post-deposition vacuum heat treatment does not have a discernible effect on the
refractive index value. In table 2, results for sample R-2 (thickness 1.5 urn) are shown. Here, again the effect of the heat treatment on the measured refractive index is small. Furthermore, there is no clear relation between the refractive index and the measurement wavelength. From the above measurement results, the average refractive index in the wavelength range studied for sample R-3 is 4.65 and 4.68 for sample R-2, yielding an average refractive index value of about 4.7.
Acknowledgement This work is supported by Natural Sciences and Engineering Research Council of Canada under the Strategic Grant Program. References [l]C.X.QiuandI.Shih,Can. J.Phys. 65 (1987) 1011. [2] C.X. Qiu, S.N. Qiu and I. Shih, Proceedings of the 6th International Photovoltaic Science and Engineering Conference, New Delhi, India, (1992) pp. 1021-1026.
191
Refractive index of electrodeposited
CuInSe2 films
S.N. Qiu, C.X. Qiu and I. Shih Electrical Engineering Department, McGill University, 3480 University St., Montreal, P.Q., H3A 2A7 Canada Received 9 May 1993; accepted 14 May 1993
Optical reflection measurements have been carried out in the wavelength range from 2 to 10 urn on polycrystalline CuInSer thin fdms. The CuInSe2 films, having a thickness from 1.5 to 2 urn, were electrodeposited on smooth Mo-coated glass substrates. From reflection interference, the opticalrefractive index was calculated to be 4.7.
1. Introduction
The ternary compound semiconductor CuInSez (Eg= 1.04 eV) is one of the leading materials for photovoltaic solar cell applications. Polycrystalline p-type thin films of CuInSe* have been prepared in our laboratory by an electrodeposition method for CdS/CuInSez cell fabrication [ 1,2]. Electrical properties such as carrier concentration and diffusion length have been determined using the fabricated cells and these have been used for the improvement of thin film fabrication processes. Results obtained from the previous studies showed that the electrodeposited CuInSe2 films, with further improvement, are appropriate for large area solar cell applications. In order to have a better understanding of the electrodeposited CuInSe2, it is also necessary to obtain optical parameters of this material. Accurate optical parameter measurements on the electrodeposited CuInSet thin films have not been obtained, partly due to surface morphology of the material prepared by this method. In this Letter, we report results of refractive index measurements carried out on the electrodeposited CuInSez films in the infrared wavelength range.
Se. The deposition was carried out on MO-coated glass substrate (thickness of Mo about 0.8 pm). The samples used are p-type with carrier concentrations greater than lOi cme3. Reflection measurements were made using a Perkin-Elmer model 13U spectrophotometer with the monochromatic light chopped at 86 Hz. An InSb photovoltaic detector operated at 77 K and a lock-in amplifier were used to measure both the incident and reflected light in the wavelength range from 2 to 5 urn. For the wavelength range from 5 to 10 urn, a HgCdTe detector was used. During the measurements, positions of the sample and the detector were adjusted so that the angle of incidence and that for reflection were about 15 ‘. Samples of CuInSez with the thickness ranging
0.6
_--F _.-.-.-.
9ooC16nh mc
Mdcness
1 .spll
,(jm
2. Experimental results Wavahmgth(pm)
The CuInSe, thin films used in this work were prepared by the electrodeposition method from an electrolyte containing ions and complexes of Cu, In and 190
Fig. 1. Reflectance curves measured on an electrodeposited CuInSe2sample (thickness 1.9 urn), showing two peaks and two valleys in the wavelength range from 5 to 10 urn for each curve.
0167-577x/93/$06.00
0 1993 Elsevier Science Publishers B.V. All rights reserved.
Volume 17, number Table 1 Calculated
3,4
refractive
MATERIALS
indices for sample R-3 ‘) at different
Heat-treatment conditions
LETTERS
heat-treatment
1993
stages
2nd minimum (1=2)
1st minimum (1=3)
August
1st maximum (/=4)
2nd maximum (1=3)
T(“C)
t (min)
A (um)
n
1 (pm)
n
I (pm)
n
1 (km)
n
200 250 300 350
15 15 15 15
5.8 5.8 5.8 5.6
4.6 4.6 4.6 4.4
9.0 8.8 9.0 9.2
4.7 4.6 4.7 4.8
4.8 5.0 5.0 5.0
4.4 4.6 4.6 4.6
7.0 7.2
4.6 4.7
5.8
4.6
9.0
4.7
5.0
4.6
7.1
4.7
average ‘) Film thickness= Table 2 Refractive
1.9 pm.
index values for the as-deposited
Heat-treatment conditions
and heat-treated
1st minimum (1=3)
sample R-2 ‘) 2nd minimum (1=2)
1st maximum (1=4)
2nd maximum (1=3)
T(“C)
t (min)
1 (pm)
n
1 (pm)
n
1 (w)
n
A (pm)
n
untreated 250 300
15 15
4.6 4.6 4.6
4.6 4.6 4.6
6.8 7.0 6.6
4.5 4.7 4.4
4.2 4.3 4.2
4.9 5.0 4.9
5.4 5.8 5.6
4.5 4.8 4.7
4.6
4.6
6.8
4.5
4.2
4.9
5.6
4.7
average ‘) Film thickness=
I .5 pm.
from 1.5 to 2 pm, area about 1 x 1 cm’, were used for the measurements. In the device fabrication involving electrodeposited CuInSez, it is often required to heat treat the CuInSez in vacuum in order to improve its crystalline and electronic properties. Therefore, the reflection measurements were carried out on samples both immediately after the deposition and after a heat treatment. Fig. 1 shows results of reflection measurements on one of the samples, R-3, in the wavelength range from 2 to 10 urn. The thickness of the sample is about 1.9 urn, which was determined from an SEM micrograph. For each reflectance curve, taken after the treatment, there are two clear peaks and two valleys in the wavelength range from 5 to 10 urn. From the maxima and minima on each curve, the refractive index is calculated. Table 1 lists the wavelength values of the reflectance maxima and minima for sample R-3. Also given in table 1 are the calculated refractive index value for each maximum or minimum and the average values. It is noted that the post-deposition vacuum heat treatment does not have a discernible effect on the
refractive index value. In table 2, results for sample R-2 (thickness 1.5 urn) are shown. Here, again the effect of the heat treatment on the measured refractive index is small. Furthermore, there is no clear relation between the refractive index and the measurement wavelength. From the above measurement results, the average refractive index in the wavelength range studied for sample R-3 is 4.65 and 4.68 for sample R-2, yielding an average refractive index value of about 4.7.
Acknowledgement This work is supported by Natural Sciences and Engineering Research Council of Canada under the Strategic Grant Program. References [l]C.X.QiuandI.Shih,Can. J.Phys. 65 (1987) 1011. [2] C.X. Qiu, S.N. Qiu and I. Shih, Proceedings of the 6th International Photovoltaic Science and Engineering Conference, New Delhi, India, (1992) pp. 1021-1026.
191