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XPS measurements of sodium in Bridgman-grown CuInSe2+x
bySunyoung Park, C. H. Champness and I. Shih
Electrical and Computer Eng., Dept. McGill University
Sunyoung ParkPh.D. student
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Glass substrate
Earlier cell
Mo
p CuInSe2
n CdSZnO i
ZnO cond
Metal grid evap.hν
J-V characteristics under simulated AM 1.5 conditions of a Cu(In,Ga)Se and a CuInSe2 cell.The cells are AR-coated. Cell temperature is 25 °C (Hedstrom and Ohlsen, 1993)
Glass substrate
Later cell
Mo
p Cu(In,Ga)Se2
n CdSZnO i
ZnO cond
hν
Sputtered
Solution grown(50 nm)
2 µm
Sputtered
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Solar cell efficiency with added Na2Se (Nakada and Ohbo, 1997)
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A quartz ampoule with Cu, In, Se, and Na
Se In Cu Na
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Bridgman Growth Apparatus
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Bridgman-grown ingot
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Bridgman-grown ingot
CuInSe2+x
n-typeIf
[Na]>[Na]crit =2x+δ
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Before abrasion
0 20 40 60 80 100 120 1400
10
20
30
40
50
60
Ar etching (s)
Rela
tive
Perc
enta
ge (%
)
Cu
Se
In
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After abrasion
0 10 20 30 40 50 60 70 80 90 10005
1015202530354045
Ar etching (s)
Rela
tive
Perc
enta
ge (%
)
Cu
Se
In
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XPS survey scans for different locations of an ingot
0200400600800100012001400
cylindrical surface of ingot
interior of ingot
Binding Energy (eV)
Coun
ts/s
Se3d
Se3d
Cu2p3
Cu2p3
In3d
In3d
Na1s O1s
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Na1s scan
106010651070107510801085
cylindrical surface of ingot
interior of ingot
Binding Energy (eV)
Coun
ts/s
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Na1s scan
0 20 40 60 80 100 120 140 160 1800102030405060
HMB56 (3 at. % Na) curved
Ar ion etching time (s)
Atom
ic p
erce
ntag
e (%
)
Cu
In
Se
Na
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[Cu]surf vs. [Na]surf
0 10 20 30 40 50 60 70 80 900
5
10
15
20
25
30
Bulk n-typeBulk p-type
Na at. % (of 4 elements) at surface
At. %
of C
u at
surf
ace
(of 3
el
emen
ts C
u, In
, Se)
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[Cu]surf vs. [Na]melt
0 1 2 3 4 5 60
5
10
15
20
25
30
Bulk n-type Bulk p-type
Na at. % (of CuInSe2) in melt
At. %
of C
u at
surf
ace
(of 3
el
emen
ts C
u, In
, Se)
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[Se]surf vs. [Na]sulf
0 10 20 30 40 50 60 70 80 9001020304050607080
Bulk n-typeBulk p-type
Na at. % (of 4 elements) at surface
At. %
of S
e at
surf
ace
(of 3
el
emen
ts C
u, In
, Se)
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Conclusions• The present results in Bridgman-grown CuInSe2+x demonstrate that:
• (1) The main variations in ternary composition occur in the first 100 nm from the ingot surface, even without Na addition.
• (2) With increase of up to about 1 at. % of added elemental Na to the melt, the relative surface concentration of Cu is decreased and that of Se, in p-type (bulk) material, is increased.
• (3) The Na is mostly present in a 0.2 micron surface layer and none is detected by XPS in the bulk.
• (4) The reduction of Cu proportion at the surface occurs with and without added Na but is accentuated with up to 1 at. % of added Na in the melt.
• (5) Some of the XPS results may have been affected by the abundant carbon present.
• (6) For samples exposed to air, the addition of Na gives rise to the extra compounds CuSe2, Na2SeO4, and Na2SeO3, at least at the surface.
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Speculation
CuInSe2
orCu(InGa)Se2
p
Low CuTernaryn
CdS
hν
CdSinterface
Lower recombinationinterface
Low Cu ternary/CuInSe2
n/p interface
The End
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The End