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Spray pyrolysis of CuInSe2 and related ternary semiconducting compounds
Thin Solid Films, 60 ( t 979) 141-146 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
141
SPRAY PYROLYSIS OF CuInSe 2 AND RELATED TERNARY SEMICONDUCTING COMPOUNDS BRIAN PAMPLIN AND R. S. FEIGELSON
Center for Materials Research, Stanford University, Stanford, Calif. (U.S.A..) (Received July 13, 1978 ; accepted October 5, 1978)
CulnSe 2 and related compounds and alloys, which are promising for solar cell applications, were produced in thin polycrystalline sphalerite structure layers by spray pyrolysis in both p- and n-type forms with various resistivities. Complete sphalerite structure solid solution is indicated for the entire four-component system CuGaS2-CulnS2-CulnSe2-CuGaSe 2 which suggests the possibility of tailoring properties such as lattice parameter, energy gap and conductivity type to produce heterojunctions suitable for low cost solar cells by spray pyrolysis.
1. INTRODUCTION Two of the principal requirements for a thin film solar cell material for terrestrial applications are high conversion efficiency and low fabrication costs. Several promising materials are currently being investigated at various research laboratories in an effort to improve properties and to reduce process costs: one of these materials is CulnSe2 1-3. Since cell efficiency may not be as high as with competitive materials such as silicon or GaAs, only a very low processing cost would allow it to be economically viable. Chemical spray deposition or spray pyrolysis is a very low cost process for preparing thin films from a wide variety of materials. This process was developed in the early 1960s by Hill and Chamberlin 4 for preparing thin polycrystalline films from binary photoconductors such as CdS and CdSe and their solid solutions and was later studied by a number of other investigatorss-l°. Feigelson et al. 7'8 have prepared solid solution films of the type (ZnCd)S and Cd(S, Se). The spray pyrolysis process involves spraying a solution, usually aqueous, containing soluble salts of the desired compound's constituent atoms (e.g. CdC12 and NH2CSNH 2 for CdS) onto a heated substrate. The equipment is simple and inexpensive to construct and operate and does not require expensive vacuum apparatus or exotic gases. Spray pyrolysis was the subject of a recent conference at Stanford University~ t. The object of this study was to extend the method to ternary compounds and to their quaternary and quinary alloys. Of particular interest are CulnS 2, CulnSe 2 and CuGaSe2 because of their potential as solar cell materials. This family of ternary chalcopyrite structure compounds is the subject of a recent book 12 and review
142
B. PAMPLIN, R. S. FEIGELSON
article 13. They form extensive ranges of solid solutions which are as yet largely unexplored 13. 2. METHOD A clear aqueous solution containing the desired cations, in near-stoichiometric proportions with sulfur in the form of dimethyl thiourea and selenium in the form of dimethyl selenourea, was used. The sources of gallium and indium were their respective trichlorides, which are readily soluble in H 2 0 a s 0.02 molar solutions. Cuprous chloride, however, forms a saturated solution at a concentration of less than 0.01 molar. Because of this, cupric acetate and cupric chloride were also tried as sources of copper. Usually about 100 cm a of a 0.01 molar solution were sprayed onto an area of about 100 cm 2 containing two glass microscope slides. This means that a layer several microns thick should be produced if all the desired elements are completely deposited. This thickness was suitable for evaluation by X-ray diffraction and electron microprobe analysis and by optical absorption measurements. Electrical resistance measurements were made with either the two- or four-point probe method. 3.
RESULTS AND DISCUSSION
Initial experiments on the preparation of CulnS2, CulnSe 2 and Culn(SSe)2 thin films which normally have a chalcopyrite structure showed that sphalerite structure material of the right lattice parameter and absorption edge 14 for each material was deposited using cuprous or cupric salts at temperatures near or below 400°C. Cuprous chloride gave the best results, in spite of its low solubility and tendency to produce cloudy solutions, which were unstable if left overnight. The films deposited using cupric salts always showed some extra lines in the diffractometer powder pattern, and these lines were not identified. This is perhaps to be expected since copper is in the cuprous state in the desired compounds. It was also found that N,N-dimethyl thiourea gave better quality layers than those obtained from thiourea. The film composition as determined by electron probe microanalysis did not always agree with the sprayed composition. A loss of sulfur and selenium was noticed above 450 °C which led to the formation of oxides. All subsequent experiments were performed using freshly prepared saturated cuprous chloride solutions, assumed to be 0.008 molar. The usual substrate temperature for spraying was 350 °C. In the best layers the X-ray powder lines were surprisingly sharp and intense and only sphalerite lines (with no splitting or ordering lines) were seen. In high temperature samples, as well as in samples where cupric salts were used, extra lines occurred. In the sulfur-containing materials, these were clearly due to In2S 3, and the microprobe analysis showed copper deficiency. Electrical measurements using tungsten pressure contacts were made on the samples with a four-probe tester, but the resistivity of the layers, which were about 5 lam thick in most cases, was sometimes too high. Just two probes were then used and the consequent contact resistance problem complicates assessment.
SPRAY PYROLYSIS OF CuInSe2 AND RELATED COMPOUNDS
143
3.1. CuInSe 2
A series of slides were sprayed with the same composition solution--a mixture of freshly prepared saturated CuCl:0.02 molar InC13:0.08 molar N,N-dimethyl selenourea in the ratio 4:1:½. Spray pyrolysis was performed between 600 and 200 °C in 50 °C intervals. The results are presented in Table I. Layers sprayed above 450 °C were very poor, and very little deposit adhered to the slide. Crystallinity as measured by X-ray powder diffractometer patterns improved as the temperature of the slide decreased and good crystallinity was achieved between 450 and 250 °C. The best pattern (a purely sphalerite pattern with no extra lines) was obtained at 350 °C. Below 250 °C the peaks decreased in size, whereas above 400 °C extra lines with d spacings of 2.92 and 2.54 A appeared in the pattern. Some specimens were annealed in air at 500 °C for 1 h. A marked sharpening of the sphalerite lines and a decrease in resistivity by typically two orders of magnitude were observed. No chalcopyrite lines appeared, but weak extra lines (probably due to oxides) appeared or strengthened. Specimens annealed in vacuo in the presence of selenium at 700 °C for 1 h lost all their deposit. There is considerable solid solution between CuInSe 2 and InESea 13 The last specimen in Table I was sprayed at the composition CuInsSe 8 and was also found to have a sphalerite structure. The results of our spray pyrolysis of some defect adamantine materials will be published more fully elsewhere 15, together with results on quaternary compounds normally of the stannite structure. Figure 1 shows the optical absorption curves for specimens prepared at different temperatures. Sharp absorption edges are seen at a value of about 0.95 eV (1300 nm); this is the accepted energy gap ~2 of CuInSe 2. The edges occurred with specimens from the temperature range 300-400 °C. The almost horizontal curve of the 400 °C specimen and the poor edge from the 200 °C curve should be noted. These results were usually qualitatively reproducible. Layer quality is a function of substrate temperature and other variables such as droplet size, spray rate and solution concentration. Some attempt was made to optimize these conditions but this was not studied systematically. Having found a good set of conditions, we preferred not to vary them. When a solution of the sprayed composition was boiled dry and was baked at 400 °C, X-ray analysis showed that no CuInSe 2 was formed. Evidently to form CuInSe 2 the solution must dissociate at an elevated temperature, giving chlorine (or HC1) and the decomposition products of N,N-dimethyl selenourea. Presumably the Cu ÷ and In 3÷ ions react with the selenium in the hot aqueous solution as the drop evaporates, giving a series of chlorinated decomposition products. The specific reactions, however, have not yet been determined. 3.2. CuGaS2, CuGaSe2, CuInS2, CuInSe 2 and their alloys
Table II shows a summary of the results obtained on sprayed layers of CuGaS 2, CuGaSe 2, CuInS2, CuInSe 2 and representative quaternary and quinary solid solutions. All layers had a sphalerite structure (and never a chalcopyrite structure) but often with indications of a second phase especially when microprobe analysis indicated a deviation from the I-III-VI 2 stoichiometry. This was particularly true of compositions containing a high ratio of sulfur to selenium. Complete chalcopyrite solid solution has been found previously for CuGaS 2-
144
8. PAMPLIN, R. S. FEIGELSON
0
"Pa
~S6
n
n
z6
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o
0
0
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.
.
.
ZZO~3ZZO
.
.
o
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~a
SPRAY PYROLYSIS OF C u l n S e 2 AND RELATED COMPOUNDS
145
TABLE II C H A L C O P Y R I T E C O M P O U N D S A N D ALLOYS
Formula
Sphalerite lattice constant (/~)
Type
Resistivity (f~ cm)
Energy gap
(eV) CuInS 2 CulnSe 2 CuGaS z CuGaSe z Culn(So.sSel.s) Culn(SSe) Culn(St.sSeo.s) Cu(Gao.slno.s)S 2 Cu(Gao.~lno.5)Se 2 Cu(Gao.7~Ino.25)Se2 Cu(Gao.251no.Ts)Se 2 Cu(Gao.slno.s)(SSe) Cu(Gao. 2s Ino. 7s)(S 1.5Seo. 5) Cu(Gao.2 flno.75)(So.sSet.~) Cu(Gao: ~Ino.25)(So.~Set.~) Cu(Gao: slno.2~)(S L ~Seo.s)
Observed
Expected
5.51 5.79 5.33 5.60 5.70 5.66 5.61 5.50 5.73 5.70 5.76 5.68 5.59 5.71 5.70 5.56
5.51 5.79 5.32 5.58
0.01 to very high 0.01 to very high 0.01 0.1 to 100 0.01 to very high 0.3 4 to very high 0.02 20 0.1 Very high 20 0.! 20 0.04 0.1
n or p n or p p p p p n and p p p p --
p p p p p
CuGaS2 2.4 eV 5.32 ~,
1.3 0.9 2.1 1.5 1.0 1.2 1.3 1.4 1.1 1.35 1.0 1.2 -1.0 1.1 --
, CulnS2 L54 eV
//
/
5.5~
/
/.-.V /:5.5A /:/
///
~t5eV
I
,
43B
5.7 ~-"
/;///:/ I OeV
40B CuGa5% 17 eV L~
500
I
I
I000
I 1500
NANOMETERS
I
2000
|
2500
~ CulnS% 095 eV
ssa ~
57o ,~ •
COMPOSITIONS SPRAYED
Fig. 1. Optical absorption curves for c u l n s % . Specimens 40, 41, 42, 43 and 44 were sprayed at 400, 350, 300, 250 and 200 °C respectively. The relative absorption scale is arbitrary. Fig. 2. The composition diagram for the quinary adamantine solid solution formed between the fourcomponent system CuGaS2-CuGaSe2-CulnSe2-CulnS2, showing variation of the sphalerite lattice parameter and energy gap: , expected constant lattice parameter lines; , expected constant band gap lines. C u G a S e 2, C u G a S 2 - C u I n S 2 a n d C u I n S 2 - C u I n S e 2 , 3,16,, 7 b u t C u G a S e 2 - C u I n S e 2 has not been studied, nor have any quinary alloys of the form
146
B. PAMPLIN,R. S. FEIGELSON
Cu(Gaxlnl_~)(SrSel_r) 2. This work can be taken as a strong indication that complete solid solution exists also in chalcopyrite form in the entire system, depicted in Fig. 2, even though our samples had a sphalerite structure. Figure 2 shows the expected values of the energy gap and the lattice parameter in the quinary system. The results tabulated in Table II are in agreement with expectations when allowance is made for the blurring of the band gap edges due to band tail states which is expected to reduce the observed energy gap. The lattice parameters are roughly in agreement with Vegard's linear variation law when allowance is made for deviations in composition. The lattice parameter values tabulated for comparison are the weighted mean of the accepted a and c/2 values given in ref. 13, where these are known. 4. CONCLUSION The compounds CuGaS2, CuGaSe2, CulnS 2 and CulnSe 2 were deposited by spray pyrolysis as thin films of sphalerite structure. Representative alloy compositions in the four-component system formed by these compounds with a sphalerite structure are also semiconductors when deposited from aqueous solution. Here is a fertile field of possible p-type materials for the production of solar cells by a low cost and easy process, using n-CdS as the other junction material. ACKNOWLEDGMENTS B.P. is on leave of absence as a Senior N A T O Fellow from Bath University, Avon, Gt. Britain. This work was supported by the N S F - M R L Program through the Center for Materials Research at Stanford University. REFERENCES 1 L.L. Kazmerski, F. R. White and G. K. Morgan, Appl. Phys. Lett., 29 (1976) 268. 2 P.W. Yu, Y.S. ParksandJ. T. Grant, Appl. Phys. Lett.,28(1976)214. 3 B. Tell and P. M. Bridenbaugh,J. App/. Phys., 48 (1977) 2477. 4 U.S. Patent 3,148,084(1964)to J. E. Hill and R. R. Chamberlin. 5 J.S. Skarman, Solid-State Electron., 8 (1965) 17. 6 C. W u a n d R . H. Bube, J. Appl. Phys.,45(1974)648. 7 R.S. Feigelson, A.N'Diaye, S.Y. YinandR. H. Bube, J. Appl. Phys.,48(1977) 3162. 8 R.S. Feigelson,Ph.D. Thesis, Stanford University, 1974. 9 Y.Y. Ma and R. H. Bube, J. Electrochem. Soc., 124 (1977) 1430. 10 Y.Y. Ma, A. L. Fahrenbruch and R. H. Bube, Appl. Phys. Lett., 30 (1977) 423. 11 B.R. Pamplin (ed.), Proc. 1st Conf. on Spray Pyrolysis, Stanford University, 1978, in Prog. Cryst. Growth Characterization, 1 (1979) 389-420. 12 J.L. Shay and J. H. Wernick, Ternary Chalcopyrite Semiconductor Growth, Electronic Properties and Applications, Pergamon, Oxford, 1975. 13 B.R. Pamplin, T. Kiyosawa and K. Masumoto, Prog. Cryst. Growth Characterization, 1 (1979)
331 387. 14 B.R. Pamplin, Tables of properties of semiconductors, Handbook of Chemistry and Physics, 59th edn., Chemical Rubber Co., Cleveland, Ohio, 1978-1979. 15 B.R. Pamplin and R. S. Feigelson, Mater. Res. Bull., 14 (1979) 1. 16 M. Robbinsand V. G. Lambrecht,Jr.,J. Solid State Chem., 6 (1973) 402. 17 N. Yamamotoand T. Miyauchi, Jpn. J. Appl. Phys., 11 (1972) 1383.
141
SPRAY PYROLYSIS OF CuInSe 2 AND RELATED TERNARY SEMICONDUCTING COMPOUNDS BRIAN PAMPLIN AND R. S. FEIGELSON
Center for Materials Research, Stanford University, Stanford, Calif. (U.S.A..) (Received July 13, 1978 ; accepted October 5, 1978)
CulnSe 2 and related compounds and alloys, which are promising for solar cell applications, were produced in thin polycrystalline sphalerite structure layers by spray pyrolysis in both p- and n-type forms with various resistivities. Complete sphalerite structure solid solution is indicated for the entire four-component system CuGaS2-CulnS2-CulnSe2-CuGaSe 2 which suggests the possibility of tailoring properties such as lattice parameter, energy gap and conductivity type to produce heterojunctions suitable for low cost solar cells by spray pyrolysis.
1. INTRODUCTION Two of the principal requirements for a thin film solar cell material for terrestrial applications are high conversion efficiency and low fabrication costs. Several promising materials are currently being investigated at various research laboratories in an effort to improve properties and to reduce process costs: one of these materials is CulnSe2 1-3. Since cell efficiency may not be as high as with competitive materials such as silicon or GaAs, only a very low processing cost would allow it to be economically viable. Chemical spray deposition or spray pyrolysis is a very low cost process for preparing thin films from a wide variety of materials. This process was developed in the early 1960s by Hill and Chamberlin 4 for preparing thin polycrystalline films from binary photoconductors such as CdS and CdSe and their solid solutions and was later studied by a number of other investigatorss-l°. Feigelson et al. 7'8 have prepared solid solution films of the type (ZnCd)S and Cd(S, Se). The spray pyrolysis process involves spraying a solution, usually aqueous, containing soluble salts of the desired compound's constituent atoms (e.g. CdC12 and NH2CSNH 2 for CdS) onto a heated substrate. The equipment is simple and inexpensive to construct and operate and does not require expensive vacuum apparatus or exotic gases. Spray pyrolysis was the subject of a recent conference at Stanford University~ t. The object of this study was to extend the method to ternary compounds and to their quaternary and quinary alloys. Of particular interest are CulnS 2, CulnSe 2 and CuGaSe2 because of their potential as solar cell materials. This family of ternary chalcopyrite structure compounds is the subject of a recent book 12 and review
142
B. PAMPLIN, R. S. FEIGELSON
article 13. They form extensive ranges of solid solutions which are as yet largely unexplored 13. 2. METHOD A clear aqueous solution containing the desired cations, in near-stoichiometric proportions with sulfur in the form of dimethyl thiourea and selenium in the form of dimethyl selenourea, was used. The sources of gallium and indium were their respective trichlorides, which are readily soluble in H 2 0 a s 0.02 molar solutions. Cuprous chloride, however, forms a saturated solution at a concentration of less than 0.01 molar. Because of this, cupric acetate and cupric chloride were also tried as sources of copper. Usually about 100 cm a of a 0.01 molar solution were sprayed onto an area of about 100 cm 2 containing two glass microscope slides. This means that a layer several microns thick should be produced if all the desired elements are completely deposited. This thickness was suitable for evaluation by X-ray diffraction and electron microprobe analysis and by optical absorption measurements. Electrical resistance measurements were made with either the two- or four-point probe method. 3.
RESULTS AND DISCUSSION
Initial experiments on the preparation of CulnS2, CulnSe 2 and Culn(SSe)2 thin films which normally have a chalcopyrite structure showed that sphalerite structure material of the right lattice parameter and absorption edge 14 for each material was deposited using cuprous or cupric salts at temperatures near or below 400°C. Cuprous chloride gave the best results, in spite of its low solubility and tendency to produce cloudy solutions, which were unstable if left overnight. The films deposited using cupric salts always showed some extra lines in the diffractometer powder pattern, and these lines were not identified. This is perhaps to be expected since copper is in the cuprous state in the desired compounds. It was also found that N,N-dimethyl thiourea gave better quality layers than those obtained from thiourea. The film composition as determined by electron probe microanalysis did not always agree with the sprayed composition. A loss of sulfur and selenium was noticed above 450 °C which led to the formation of oxides. All subsequent experiments were performed using freshly prepared saturated cuprous chloride solutions, assumed to be 0.008 molar. The usual substrate temperature for spraying was 350 °C. In the best layers the X-ray powder lines were surprisingly sharp and intense and only sphalerite lines (with no splitting or ordering lines) were seen. In high temperature samples, as well as in samples where cupric salts were used, extra lines occurred. In the sulfur-containing materials, these were clearly due to In2S 3, and the microprobe analysis showed copper deficiency. Electrical measurements using tungsten pressure contacts were made on the samples with a four-probe tester, but the resistivity of the layers, which were about 5 lam thick in most cases, was sometimes too high. Just two probes were then used and the consequent contact resistance problem complicates assessment.
SPRAY PYROLYSIS OF CuInSe2 AND RELATED COMPOUNDS
143
3.1. CuInSe 2
A series of slides were sprayed with the same composition solution--a mixture of freshly prepared saturated CuCl:0.02 molar InC13:0.08 molar N,N-dimethyl selenourea in the ratio 4:1:½. Spray pyrolysis was performed between 600 and 200 °C in 50 °C intervals. The results are presented in Table I. Layers sprayed above 450 °C were very poor, and very little deposit adhered to the slide. Crystallinity as measured by X-ray powder diffractometer patterns improved as the temperature of the slide decreased and good crystallinity was achieved between 450 and 250 °C. The best pattern (a purely sphalerite pattern with no extra lines) was obtained at 350 °C. Below 250 °C the peaks decreased in size, whereas above 400 °C extra lines with d spacings of 2.92 and 2.54 A appeared in the pattern. Some specimens were annealed in air at 500 °C for 1 h. A marked sharpening of the sphalerite lines and a decrease in resistivity by typically two orders of magnitude were observed. No chalcopyrite lines appeared, but weak extra lines (probably due to oxides) appeared or strengthened. Specimens annealed in vacuo in the presence of selenium at 700 °C for 1 h lost all their deposit. There is considerable solid solution between CuInSe 2 and InESea 13 The last specimen in Table I was sprayed at the composition CuInsSe 8 and was also found to have a sphalerite structure. The results of our spray pyrolysis of some defect adamantine materials will be published more fully elsewhere 15, together with results on quaternary compounds normally of the stannite structure. Figure 1 shows the optical absorption curves for specimens prepared at different temperatures. Sharp absorption edges are seen at a value of about 0.95 eV (1300 nm); this is the accepted energy gap ~2 of CuInSe 2. The edges occurred with specimens from the temperature range 300-400 °C. The almost horizontal curve of the 400 °C specimen and the poor edge from the 200 °C curve should be noted. These results were usually qualitatively reproducible. Layer quality is a function of substrate temperature and other variables such as droplet size, spray rate and solution concentration. Some attempt was made to optimize these conditions but this was not studied systematically. Having found a good set of conditions, we preferred not to vary them. When a solution of the sprayed composition was boiled dry and was baked at 400 °C, X-ray analysis showed that no CuInSe 2 was formed. Evidently to form CuInSe 2 the solution must dissociate at an elevated temperature, giving chlorine (or HC1) and the decomposition products of N,N-dimethyl selenourea. Presumably the Cu ÷ and In 3÷ ions react with the selenium in the hot aqueous solution as the drop evaporates, giving a series of chlorinated decomposition products. The specific reactions, however, have not yet been determined. 3.2. CuGaS2, CuGaSe2, CuInS2, CuInSe 2 and their alloys
Table II shows a summary of the results obtained on sprayed layers of CuGaS 2, CuGaSe 2, CuInS2, CuInSe 2 and representative quaternary and quinary solid solutions. All layers had a sphalerite structure (and never a chalcopyrite structure) but often with indications of a second phase especially when microprobe analysis indicated a deviation from the I-III-VI 2 stoichiometry. This was particularly true of compositions containing a high ratio of sulfur to selenium. Complete chalcopyrite solid solution has been found previously for CuGaS 2-
144
8. PAMPLIN, R. S. FEIGELSON
0
"Pa
~S6
n
n
z6
r~ o
o
0
0
K .
.
.
.
ZZO~3ZZO
.
.
o
~0
0
d S ~a e-
~a
SPRAY PYROLYSIS OF C u l n S e 2 AND RELATED COMPOUNDS
145
TABLE II C H A L C O P Y R I T E C O M P O U N D S A N D ALLOYS
Formula
Sphalerite lattice constant (/~)
Type
Resistivity (f~ cm)
Energy gap
(eV) CuInS 2 CulnSe 2 CuGaS z CuGaSe z Culn(So.sSel.s) Culn(SSe) Culn(St.sSeo.s) Cu(Gao.slno.s)S 2 Cu(Gao.~lno.5)Se 2 Cu(Gao.7~Ino.25)Se2 Cu(Gao.251no.Ts)Se 2 Cu(Gao.slno.s)(SSe) Cu(Gao. 2s Ino. 7s)(S 1.5Seo. 5) Cu(Gao.2 flno.75)(So.sSet.~) Cu(Gao: ~Ino.25)(So.~Set.~) Cu(Gao: slno.2~)(S L ~Seo.s)
Observed
Expected
5.51 5.79 5.33 5.60 5.70 5.66 5.61 5.50 5.73 5.70 5.76 5.68 5.59 5.71 5.70 5.56
5.51 5.79 5.32 5.58
0.01 to very high 0.01 to very high 0.01 0.1 to 100 0.01 to very high 0.3 4 to very high 0.02 20 0.1 Very high 20 0.! 20 0.04 0.1
n or p n or p p p p p n and p p p p --
p p p p p
CuGaS2 2.4 eV 5.32 ~,
1.3 0.9 2.1 1.5 1.0 1.2 1.3 1.4 1.1 1.35 1.0 1.2 -1.0 1.1 --
, CulnS2 L54 eV
//
/
5.5~
/
/.-.V /:5.5A /:/
///
~t5eV
I
,
43B
5.7 ~-"
/;///:/ I OeV
40B CuGa5% 17 eV L~
500
I
I
I000
I 1500
NANOMETERS
I
2000
|
2500
~ CulnS% 095 eV
ssa ~
57o ,~ •
COMPOSITIONS SPRAYED
Fig. 1. Optical absorption curves for c u l n s % . Specimens 40, 41, 42, 43 and 44 were sprayed at 400, 350, 300, 250 and 200 °C respectively. The relative absorption scale is arbitrary. Fig. 2. The composition diagram for the quinary adamantine solid solution formed between the fourcomponent system CuGaS2-CuGaSe2-CulnSe2-CulnS2, showing variation of the sphalerite lattice parameter and energy gap: , expected constant lattice parameter lines; , expected constant band gap lines. C u G a S e 2, C u G a S 2 - C u I n S 2 a n d C u I n S 2 - C u I n S e 2 , 3,16,, 7 b u t C u G a S e 2 - C u I n S e 2 has not been studied, nor have any quinary alloys of the form
146
B. PAMPLIN,R. S. FEIGELSON
Cu(Gaxlnl_~)(SrSel_r) 2. This work can be taken as a strong indication that complete solid solution exists also in chalcopyrite form in the entire system, depicted in Fig. 2, even though our samples had a sphalerite structure. Figure 2 shows the expected values of the energy gap and the lattice parameter in the quinary system. The results tabulated in Table II are in agreement with expectations when allowance is made for the blurring of the band gap edges due to band tail states which is expected to reduce the observed energy gap. The lattice parameters are roughly in agreement with Vegard's linear variation law when allowance is made for deviations in composition. The lattice parameter values tabulated for comparison are the weighted mean of the accepted a and c/2 values given in ref. 13, where these are known. 4. CONCLUSION The compounds CuGaS2, CuGaSe2, CulnS 2 and CulnSe 2 were deposited by spray pyrolysis as thin films of sphalerite structure. Representative alloy compositions in the four-component system formed by these compounds with a sphalerite structure are also semiconductors when deposited from aqueous solution. Here is a fertile field of possible p-type materials for the production of solar cells by a low cost and easy process, using n-CdS as the other junction material. ACKNOWLEDGMENTS B.P. is on leave of absence as a Senior N A T O Fellow from Bath University, Avon, Gt. Britain. This work was supported by the N S F - M R L Program through the Center for Materials Research at Stanford University. REFERENCES 1 L.L. Kazmerski, F. R. White and G. K. Morgan, Appl. Phys. Lett., 29 (1976) 268. 2 P.W. Yu, Y.S. ParksandJ. T. Grant, Appl. Phys. Lett.,28(1976)214. 3 B. Tell and P. M. Bridenbaugh,J. App/. Phys., 48 (1977) 2477. 4 U.S. Patent 3,148,084(1964)to J. E. Hill and R. R. Chamberlin. 5 J.S. Skarman, Solid-State Electron., 8 (1965) 17. 6 C. W u a n d R . H. Bube, J. Appl. Phys.,45(1974)648. 7 R.S. Feigelson, A.N'Diaye, S.Y. YinandR. H. Bube, J. Appl. Phys.,48(1977) 3162. 8 R.S. Feigelson,Ph.D. Thesis, Stanford University, 1974. 9 Y.Y. Ma and R. H. Bube, J. Electrochem. Soc., 124 (1977) 1430. 10 Y.Y. Ma, A. L. Fahrenbruch and R. H. Bube, Appl. Phys. Lett., 30 (1977) 423. 11 B.R. Pamplin (ed.), Proc. 1st Conf. on Spray Pyrolysis, Stanford University, 1978, in Prog. Cryst. Growth Characterization, 1 (1979) 389-420. 12 J.L. Shay and J. H. Wernick, Ternary Chalcopyrite Semiconductor Growth, Electronic Properties and Applications, Pergamon, Oxford, 1975. 13 B.R. Pamplin, T. Kiyosawa and K. Masumoto, Prog. Cryst. Growth Characterization, 1 (1979)
331 387. 14 B.R. Pamplin, Tables of properties of semiconductors, Handbook of Chemistry and Physics, 59th edn., Chemical Rubber Co., Cleveland, Ohio, 1978-1979. 15 B.R. Pamplin and R. S. Feigelson, Mater. Res. Bull., 14 (1979) 1. 16 M. Robbinsand V. G. Lambrecht,Jr.,J. Solid State Chem., 6 (1973) 402. 17 N. Yamamotoand T. Miyauchi, Jpn. J. Appl. Phys., 11 (1972) 1383.