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Cadmium stannate thin film as a solar energy utilizing element
Solid State Communications, Vol. 88, No. 3, pp. 227-230, 1993. Printed in Great Britain.
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
CADMIUM STANNATE THIN FILM AS A SOLAR ENERGY UTILIZING ELEMENT M.T. Mohammad Department of Physics, College of Science, University of Basrah, Basrah, Iraq and W.A.S. Abdul Ghafor Department of Physics, College of Education, University of Basrah, Basrah, Iraq
(Received 26 October 1992; in revised form 12 March 1993; accepted for publication 19 April 1993 by A.A. Maradudin) Cadmium stannate thin films were prepared by pyrolytic technique. The goodness of the film was studied through obtaining reasonable values of two differently defined figures of merit, namely, Haacks's figure of merit (e~ = TlO/RD) and ( F = ~r/c~). This reflects the high transparency and large electrical conductivity of the film, the main requirements for most practical applications in solar energy utilization. The indirect electronic transition was investigated by considering the derivative data of the absorption coefficient with respect to photon energy and plotting log a a / a h v vs log a depending mainly on Bardeen theory. Films of 1 #m thick furnished direct transition at 2.8 eV and indirect allowed transition at 2.2 eV. These values were confirmed by detecting the corresponding values of the energy at two maxima appeared in the dielectric constant plot. 1. INTRODUCTION THE WORK on transparent oxide semiconductors like Cd2SnO 4 [1, 2], SnO2F [3, 4], In203 : Sn [5, 6] has been of great technological interest during the past few years owing to their potential applications in direct energy conversion devices particularly conductor-insulator semiconductor solar cells. Heterojunctions of transparent conductive oxides on bulk semiconductors [7-9] show considerable promise for use in inexpensive solar cells. The transparent conductive oxide are semiconductors with wide band gap and high carrier concentration. They should have high transmission in visible and low electrical sheet resistivities for applications in solar cell [10]. The refractive indices of these materials are in the right range [11] so as to form an antireflection coating on silicon and GaAs. The coatings also act as transparent heat mirrors because of the high reflectivities in the infrared, hence they have an important application in solar energy collectors and in control of internal environments by application to windows. In terms of resource limitation and economic Cd2SnO4 is the most compatitive compared to the other oxides. For low sheet resistance (1-10) Q/[-] In203 :Sn and Cd2SnO4 are favoured.
Agnihotri et al. [12] prepared polycrystalline and amorphous films of Cd2SnO 4 by spray pyrolysis technique with resistivity in the range (1-100) f~/I-q showing n-type conductivity with oxygen vacancies which provide donor states in the band gap. Avaritsiotis et al. [13] deposited Cd2SnO 4 by ion plating technique on polyster substrates at room temperature with thickness of 0.32#m and conductivity of 6 x 10-6 (Q.cm) -1. High conductivity films were achieved by sputtering from polycrystalline Cd2SnO 4 target in argon [14] with band gap of 2.06 eV shifted to 2.85 eV by doping. Maniv et al. [15] prepared transparent films of Cd2SnO4 by d.c. reactive sputtering in Ar-O2 from a Cd2Sn alloy. The oxygen defect density is adjusted by controlling the oxidation rate at the substrate, obtaining 0.15 #m films with conductivity of 4.4 × 10 - 4 (Q.cm) -1 and transparency of Tvis = 0.85. For optimizing the performance of the film some information concerning the complex dielectric constant are necessary. The real and imaginary parts of the dielectric constant (el and e2) are related to the optical constants n and k (real and imaginary parts of the complex refractive index) by c1 = n2 - k 2 and ~2 = 2nk, where values of n and k were obtained from our previous work [11]. In this study the
227
228
CADMIUM STANNATE THIN FILM AS A SOLAR ENERGY
optoelectronic transitions in the fundamental absorption region were investigated and the results were identified in terms of indirect transition allowed and forbidden by using log Oa/Ohu/loga plots. The performance of the film can be better understood by getting some insight into the figures of merit • and F which represents the goodness of the films.
Vol. 88, No. 3
105
104 2. EXPERIMENTAL PROCEDURE Cd2SnO 4 films of nearly 1 #m thick were prepared by spraying a mixture of two part (0.2M) CdCI2.5H20 solution in ethyl alcohol and one part of (0.2M) SnCI4.3H20 solution in acetic acid on heated glass substrates with spray rate of 15 ml min-i under normal atmospheric conditions. The experimental set up has been previously described [11, 16].
' Eu
,o3
102
3. FIGURE OF MERIT AND DIELECTRIC CONSTANT The main requirements for a transparent oxide to meet the practical applications are high conductivity and high visible transmission giving good impression when two figures of merit are considered; Haacke's figure of merit [17] • = Tl°/Rn and F = tr/a [18] where Rn and tr are the sheet resistance and d.c. dark conductivity of the film. High values of • indicate large T (small a) and small Rn while high values of F indicate small a (large T) and large a. F changes with photon energy in accordance with a-l/hu dependence and changes with photon energy in accordance with T/hv dependence. Large • and F values demonstrate good transparency and conductive properties. Due to their values of ff and F together with their chemical stability these films have been used as electrodes in galvanic cells. 4. RESULTS AND DISCUSSION Figure 1 shows the dependence of absorption coefficient on photon energy at room temperature obtained directly from transmission data. The absorption coefficient show low energy absorption edge which decreases exponentially as photon energy decreases. At high energies there is a deviation in a indicating interband transitions. Figure 2 shows the plot of ~1 = n 2 - k 2 and ~2 = 2nk vs photon energy showing that e2 approaches zero and el approaches n 2 as the energy decreases due to diminishing values of k as shown by Mohammad et al. [11]. The peaks in el curve (7.5, 10.5) correspond to the direct and indirect allowed transition at 2.81 eV and 2.0 eV respectively.
10
I
22
I
I
I
I
2.4 2.6 2.8 3.0 3.2
J
3.4
h~ (eV Fig. 1. Absorption coefficient as a function of photon energy (t = 1 #m).
1.1 0.5
9.0
0.4
W"
(13
7.O
0.2 5.0
/ 0.1
3.0 ~. J~ lL i J
2.0
2.25
2.51 2.76 bY (eV)
3.02
I
3.28
Fig. 2. el and e2 as a function of photon energy
Vol. 88, No. 3
C A D M I U M S T A N N A T E T H I N F I L M AS A SOLAR E N E R G Y
229
4 16 2
5.~ / / /
/
E
-4
tO
/ /
5.0 ,J
,~|~
/ ! I
..1
/..E U..
/
! /¢ /" /
/..2
3.8
20
I
216
3.2
i
318
-10 10
s'0
2:3 2'.s 2'.7 2'9 a?~ 3'.3 ~'.s 3.'7
Log*¢ (cnff 1 )
Fig. 3. The plot of log OalOhv vs log a giving a straight line with slop of (1/2) where r - 2 indicating indirect allowed transition. The optical data for indirect transitions were investigated for evidence of forbidden or allowed according to Bardeen theory [19]. For such transitions:
a = a o ( h v - Eg ± Ep) r. r = 3 for forbidden indirect transition and r = 2 for allowed transition, Eg is the indirect band gap and Ep is the absorbed (+) or emitted ( - ) phonon energy. The value of r was determined by the above equation after differentiating with respect to photon energy. Figure 3 show the plot of log Oa/Ohv vs log c~ according to the equation:
162
h~ ( e V 1
Fig. 4. Figures of merit f~ = Tl°/R m and F = cr/a as a function of photon energy. Z0 = 1/e0c0 = 377 f~ is the free space impedence, e0 is the permittivity of free space, Co is speed of light in the free space and RD----50f~/V1. Substituting these values in the above equation gives R I R = 62% which is smaller than the value reported for S n O a : F [21] due to their smaller sheet resistance indicating that Rm can be raised by doping the film by some suitable dopants to reduce Rm and consequently increase the conductivity. 5. CONCLUSIONS
log~=/3+
1-
loga,
where
{ l/rk /3 = log krao ). The value of r was calculated from the slope of the straight line in Fig. 3 giving r = 1.95 which confirm the r ~- 2 case for allowed transition. Figure 4 shows F/hv and O/hv plots indicating that the film has high transparency and good conductivity which makes it desirable in solar conversion applications bearing in mind the possibility of improving F and • by doping and annealing. The reflectance in the long wavelength region ( I R FIR) is given by [20]: Rm = (1 + 2RD/Zo) -2.
Cadmium stannate films of nearly 1 #m thickness were prepared by spray pyrolysis technique. The main disadvantage of this technique could be in the difficulty of obtaining highly uniform coatings specially for large area sample but the results can be much enhanced by improving the technique such as revolving the sample and keeping it in a vacuum ambient feed with oxygen. The direct and indirect transitions were investigated in terms of the allowed and forbidden transitions showing that direct transition of 2.8eV and indirect allowed transition of 2.2eV are the more probable transitions in the Cd2SnO4 films. High values of • and F demonstrate the good transparent and conductive properties of the film making it attractive in solar energy utilization processes. Moreover values of refractive index shown previously and values of ~1 and e2 reported here
230
CADMIUM STANNATE THIN FILM AS A SOLAR ENERGY
indicate that the film can be used as antireflection surface when applied to Si and GaAs.
10.
REFERENCES
12.
1. 2. 3. 4. 5. 6. 7. 8. 9.
G. Haacke, Appl. Phys. Lett. 28, 622 (1976). C.M. Lampert, Solar Energy Materials 6, 1. North Holland, Amsterdam. (1981). J.C. Manifacier, L. Szepessy, J.F. Bresse, M. Perotin & Stuck, Mat. Res. Bull. 14, 109, 163 (1979). A. Bhardwaj, B.K. Gupta, A. Raza, A.K. Sharma & O.P. Agnihotri, Solar Cells 5, 39 (1981-1982). T. Nagatomo & O. Omoto, Jpn. J. Appl. Phys. 15, 199 (1976). J.B. Dubow, D.E. Burk & J.R. Sites, Appl. Phys. Lett. 29, 404 (1979). J. Dubow, J. Shewchon, C. Wilmsen, R. Singh, D. Burk & J.F. Wager, J. Appl. Phys. 50, 2832 (1979). J. Shewchun, J.B. Dubow, A. Myszkowski & R. Singh, J. Appl. Phys. 49, 855 (1978). R. Singh, M.A. Green & K. Rajkanan, Solar Cells 3, 95 (1988).
11.
13. 14. 15. 16. 17. 18.
19. 20. 21.
Vol. 88, No. 3
V.K. Jain & A.P. Kulshreshta, Solid Energy Materials 4, 151 (1981). M.T. Mohammad & W.A. Abdul-Ghafor, Solid State Commun. 72, 1043 (1989). O.P. Agnihotri, B.K. Gupta & A.K. Sharma, J. Appl. Phys. 4540, 49 (1978). J.N. Avaritsiotis & R.P. Howson, Thin Solid Films 65, 101 (1980). A.J. Nozik, Phys. Rev. B6, 453 (1972). S. Maniv, C. Miner & W. Westwood, Presented at Vac. Soc. Meeting, Detroit, Michigen (1980). O.P. Agnihotri, M.T. Mohammad, A.K. Abasss & K.I. Arshak, Solid State Commun. 47, 195 (1983). G. Haake, J. Appl. Phys. 47, 4086 (1976). O.P. Agnihotri & B.K. Gupta, Solar Energy International Progress, Proc. Int. Symp. Workshop on Solar Energy, Cairo, Egypt, 16-22 June (1978). J. Bardeen, F.J. Blatt & L.H. Hall, Proc. Photoconductivity Conf., Atlantic City, p. 146. Wiley, New York (1956). G. Frank, E. Kouer & H. Kostline, Thin Solid Films 77, 107 (1981). M.T. Mohammad & W.A. Abdul-Ghafor, Phys. Status Solidi (a) 106, 479 (1988).
0038-1098/93 $6.00 + .00 Pergamon Press Ltd
CADMIUM STANNATE THIN FILM AS A SOLAR ENERGY UTILIZING ELEMENT M.T. Mohammad Department of Physics, College of Science, University of Basrah, Basrah, Iraq and W.A.S. Abdul Ghafor Department of Physics, College of Education, University of Basrah, Basrah, Iraq
(Received 26 October 1992; in revised form 12 March 1993; accepted for publication 19 April 1993 by A.A. Maradudin) Cadmium stannate thin films were prepared by pyrolytic technique. The goodness of the film was studied through obtaining reasonable values of two differently defined figures of merit, namely, Haacks's figure of merit (e~ = TlO/RD) and ( F = ~r/c~). This reflects the high transparency and large electrical conductivity of the film, the main requirements for most practical applications in solar energy utilization. The indirect electronic transition was investigated by considering the derivative data of the absorption coefficient with respect to photon energy and plotting log a a / a h v vs log a depending mainly on Bardeen theory. Films of 1 #m thick furnished direct transition at 2.8 eV and indirect allowed transition at 2.2 eV. These values were confirmed by detecting the corresponding values of the energy at two maxima appeared in the dielectric constant plot. 1. INTRODUCTION THE WORK on transparent oxide semiconductors like Cd2SnO 4 [1, 2], SnO2F [3, 4], In203 : Sn [5, 6] has been of great technological interest during the past few years owing to their potential applications in direct energy conversion devices particularly conductor-insulator semiconductor solar cells. Heterojunctions of transparent conductive oxides on bulk semiconductors [7-9] show considerable promise for use in inexpensive solar cells. The transparent conductive oxide are semiconductors with wide band gap and high carrier concentration. They should have high transmission in visible and low electrical sheet resistivities for applications in solar cell [10]. The refractive indices of these materials are in the right range [11] so as to form an antireflection coating on silicon and GaAs. The coatings also act as transparent heat mirrors because of the high reflectivities in the infrared, hence they have an important application in solar energy collectors and in control of internal environments by application to windows. In terms of resource limitation and economic Cd2SnO4 is the most compatitive compared to the other oxides. For low sheet resistance (1-10) Q/[-] In203 :Sn and Cd2SnO4 are favoured.
Agnihotri et al. [12] prepared polycrystalline and amorphous films of Cd2SnO 4 by spray pyrolysis technique with resistivity in the range (1-100) f~/I-q showing n-type conductivity with oxygen vacancies which provide donor states in the band gap. Avaritsiotis et al. [13] deposited Cd2SnO 4 by ion plating technique on polyster substrates at room temperature with thickness of 0.32#m and conductivity of 6 x 10-6 (Q.cm) -1. High conductivity films were achieved by sputtering from polycrystalline Cd2SnO 4 target in argon [14] with band gap of 2.06 eV shifted to 2.85 eV by doping. Maniv et al. [15] prepared transparent films of Cd2SnO4 by d.c. reactive sputtering in Ar-O2 from a Cd2Sn alloy. The oxygen defect density is adjusted by controlling the oxidation rate at the substrate, obtaining 0.15 #m films with conductivity of 4.4 × 10 - 4 (Q.cm) -1 and transparency of Tvis = 0.85. For optimizing the performance of the film some information concerning the complex dielectric constant are necessary. The real and imaginary parts of the dielectric constant (el and e2) are related to the optical constants n and k (real and imaginary parts of the complex refractive index) by c1 = n2 - k 2 and ~2 = 2nk, where values of n and k were obtained from our previous work [11]. In this study the
227
228
CADMIUM STANNATE THIN FILM AS A SOLAR ENERGY
optoelectronic transitions in the fundamental absorption region were investigated and the results were identified in terms of indirect transition allowed and forbidden by using log Oa/Ohu/loga plots. The performance of the film can be better understood by getting some insight into the figures of merit • and F which represents the goodness of the films.
Vol. 88, No. 3
105
104 2. EXPERIMENTAL PROCEDURE Cd2SnO 4 films of nearly 1 #m thick were prepared by spraying a mixture of two part (0.2M) CdCI2.5H20 solution in ethyl alcohol and one part of (0.2M) SnCI4.3H20 solution in acetic acid on heated glass substrates with spray rate of 15 ml min-i under normal atmospheric conditions. The experimental set up has been previously described [11, 16].
' Eu
,o3
102
3. FIGURE OF MERIT AND DIELECTRIC CONSTANT The main requirements for a transparent oxide to meet the practical applications are high conductivity and high visible transmission giving good impression when two figures of merit are considered; Haacke's figure of merit [17] • = Tl°/Rn and F = tr/a [18] where Rn and tr are the sheet resistance and d.c. dark conductivity of the film. High values of • indicate large T (small a) and small Rn while high values of F indicate small a (large T) and large a. F changes with photon energy in accordance with a-l/hu dependence and changes with photon energy in accordance with T/hv dependence. Large • and F values demonstrate good transparency and conductive properties. Due to their values of ff and F together with their chemical stability these films have been used as electrodes in galvanic cells. 4. RESULTS AND DISCUSSION Figure 1 shows the dependence of absorption coefficient on photon energy at room temperature obtained directly from transmission data. The absorption coefficient show low energy absorption edge which decreases exponentially as photon energy decreases. At high energies there is a deviation in a indicating interband transitions. Figure 2 shows the plot of ~1 = n 2 - k 2 and ~2 = 2nk vs photon energy showing that e2 approaches zero and el approaches n 2 as the energy decreases due to diminishing values of k as shown by Mohammad et al. [11]. The peaks in el curve (7.5, 10.5) correspond to the direct and indirect allowed transition at 2.81 eV and 2.0 eV respectively.
10
I
22
I
I
I
I
2.4 2.6 2.8 3.0 3.2
J
3.4
h~ (eV Fig. 1. Absorption coefficient as a function of photon energy (t = 1 #m).
1.1 0.5
9.0
0.4
W"
(13
7.O
0.2 5.0
/ 0.1
3.0 ~. J~ lL i J
2.0
2.25
2.51 2.76 bY (eV)
3.02
I
3.28
Fig. 2. el and e2 as a function of photon energy
Vol. 88, No. 3
C A D M I U M S T A N N A T E T H I N F I L M AS A SOLAR E N E R G Y
229
4 16 2
5.~ / / /
/
E
-4
tO
/ /
5.0 ,J
,~|~
/ ! I
..1
/..E U..
/
! /¢ /" /
/..2
3.8
20
I
216
3.2
i
318
-10 10
s'0
2:3 2'.s 2'.7 2'9 a?~ 3'.3 ~'.s 3.'7
Log*¢ (cnff 1 )
Fig. 3. The plot of log OalOhv vs log a giving a straight line with slop of (1/2) where r - 2 indicating indirect allowed transition. The optical data for indirect transitions were investigated for evidence of forbidden or allowed according to Bardeen theory [19]. For such transitions:
a = a o ( h v - Eg ± Ep) r. r = 3 for forbidden indirect transition and r = 2 for allowed transition, Eg is the indirect band gap and Ep is the absorbed (+) or emitted ( - ) phonon energy. The value of r was determined by the above equation after differentiating with respect to photon energy. Figure 3 show the plot of log Oa/Ohv vs log c~ according to the equation:
162
h~ ( e V 1
Fig. 4. Figures of merit f~ = Tl°/R m and F = cr/a as a function of photon energy. Z0 = 1/e0c0 = 377 f~ is the free space impedence, e0 is the permittivity of free space, Co is speed of light in the free space and RD----50f~/V1. Substituting these values in the above equation gives R I R = 62% which is smaller than the value reported for S n O a : F [21] due to their smaller sheet resistance indicating that Rm can be raised by doping the film by some suitable dopants to reduce Rm and consequently increase the conductivity. 5. CONCLUSIONS
log~=/3+
1-
loga,
where
{ l/rk /3 = log krao ). The value of r was calculated from the slope of the straight line in Fig. 3 giving r = 1.95 which confirm the r ~- 2 case for allowed transition. Figure 4 shows F/hv and O/hv plots indicating that the film has high transparency and good conductivity which makes it desirable in solar conversion applications bearing in mind the possibility of improving F and • by doping and annealing. The reflectance in the long wavelength region ( I R FIR) is given by [20]: Rm = (1 + 2RD/Zo) -2.
Cadmium stannate films of nearly 1 #m thickness were prepared by spray pyrolysis technique. The main disadvantage of this technique could be in the difficulty of obtaining highly uniform coatings specially for large area sample but the results can be much enhanced by improving the technique such as revolving the sample and keeping it in a vacuum ambient feed with oxygen. The direct and indirect transitions were investigated in terms of the allowed and forbidden transitions showing that direct transition of 2.8eV and indirect allowed transition of 2.2eV are the more probable transitions in the Cd2SnO4 films. High values of • and F demonstrate the good transparent and conductive properties of the film making it attractive in solar energy utilization processes. Moreover values of refractive index shown previously and values of ~1 and e2 reported here
230
CADMIUM STANNATE THIN FILM AS A SOLAR ENERGY
indicate that the film can be used as antireflection surface when applied to Si and GaAs.
10.
REFERENCES
12.
1. 2. 3. 4. 5. 6. 7. 8. 9.
G. Haacke, Appl. Phys. Lett. 28, 622 (1976). C.M. Lampert, Solar Energy Materials 6, 1. North Holland, Amsterdam. (1981). J.C. Manifacier, L. Szepessy, J.F. Bresse, M. Perotin & Stuck, Mat. Res. Bull. 14, 109, 163 (1979). A. Bhardwaj, B.K. Gupta, A. Raza, A.K. Sharma & O.P. Agnihotri, Solar Cells 5, 39 (1981-1982). T. Nagatomo & O. Omoto, Jpn. J. Appl. Phys. 15, 199 (1976). J.B. Dubow, D.E. Burk & J.R. Sites, Appl. Phys. Lett. 29, 404 (1979). J. Dubow, J. Shewchon, C. Wilmsen, R. Singh, D. Burk & J.F. Wager, J. Appl. Phys. 50, 2832 (1979). J. Shewchun, J.B. Dubow, A. Myszkowski & R. Singh, J. Appl. Phys. 49, 855 (1978). R. Singh, M.A. Green & K. Rajkanan, Solar Cells 3, 95 (1988).
11.
13. 14. 15. 16. 17. 18.
19. 20. 21.
Vol. 88, No. 3
V.K. Jain & A.P. Kulshreshta, Solid Energy Materials 4, 151 (1981). M.T. Mohammad & W.A. Abdul-Ghafor, Solid State Commun. 72, 1043 (1989). O.P. Agnihotri, B.K. Gupta & A.K. Sharma, J. Appl. Phys. 4540, 49 (1978). J.N. Avaritsiotis & R.P. Howson, Thin Solid Films 65, 101 (1980). A.J. Nozik, Phys. Rev. B6, 453 (1972). S. Maniv, C. Miner & W. Westwood, Presented at Vac. Soc. Meeting, Detroit, Michigen (1980). O.P. Agnihotri, M.T. Mohammad, A.K. Abasss & K.I. Arshak, Solid State Commun. 47, 195 (1983). G. Haake, J. Appl. Phys. 47, 4086 (1976). O.P. Agnihotri & B.K. Gupta, Solar Energy International Progress, Proc. Int. Symp. Workshop on Solar Energy, Cairo, Egypt, 16-22 June (1978). J. Bardeen, F.J. Blatt & L.H. Hall, Proc. Photoconductivity Conf., Atlantic City, p. 146. Wiley, New York (1956). G. Frank, E. Kouer & H. Kostline, Thin Solid Films 77, 107 (1981). M.T. Mohammad & W.A. Abdul-Ghafor, Phys. Status Solidi (a) 106, 479 (1988).