Introduction. CIS/CIGS thin film based solar cells are the most promising renewable energy source because of their relatively high solar efficiency and stability.
Novel low temperature pulsed dd.c. c magnetron sputtering of single phase β-In2S3 buffer layers for CIGS solar cell application
Sreejith Karthikeyan, Arthur Hill and Richard Pilkington Materials and Physics Research Centre University of Salford, UK
Introduction CIS/CIGS thin film based solar cells are the most promising renewable energy source because of their relatively high solar efficiency and stability. stability Single crystal Si cells need more material to absorb light due to its indirect band gap. Presently CIGS cells have achieved a maximum efficiency of 20.3% [1]. A typical i l cell ll CIS/CIGS structure is i in i the h form f off a heterojunction. h j i Al-ZnO ZnO ~ 1?m 1 µm
Transparent Conductive Oxide layer
i Z O ~ 70nm i-ZnO
Intrinsic ZnO layer to reduce leakage current
CdS ~ 50nm
N- type buffer layer
CuInSe ~ 2µm CuInSe m 2 ~2 1.0?
P- type absorber layer
0.8?µm m Mo layer ~ 0.8
Back Contact
Substrate Substrate
A typical CIGS solar cell References [1] P. Jackson et. al Progress in Photovoltaics: Research and Applications, (2011) EU PVSEC WCPEC-5, Valencia, Spain, 2010.
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Introduction CdS layers are deposited using a chemical bath deposition process
CdS buffer layer - main functions ¾ The optimum p thickness ((60 nm to 80 nm)) CdS layer y builds a sufficiently y wide depletion layer that minimises tunnelling and reduces recombination, which in turn increases the efficiency of the solar cell. ¾ The chemical bath deposited CdS layer coats the absorber CIGS surface, surface minimising voids at the metallurgical interface. ¾ The CdS layer provides electronic and metallurgical junction protection against subsequent sputter damage from the TCO layer deposition and acts as a mechanical protective layer.
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Why Do We Need An Alternative Buffer Layer ? ¾
Toxicity i i off Cd
¾ The light absorption in the buffer layer reduces the spectral response of the solar cell in the blue region of the solar spectrum (band gap is 2.4 eV) ¾ Integrating the CBD technique with other vacuum processes in the production line when it comes to the in-line production of CIGS solar cells is difficult.
Solution Replace the CdS buffer with an alternative buffer material with higher band gap energy and optical constants similar to those of CdS ZnS
In(OH)3 ( )
In2S3
ZnSe
ZnMgO
ZnO 3
Pulsed D.C. Magnetron Sputtering System 100
Pulse off T (μs)
0 0
5
10
15
20
25
30
35
40
45
50
55
Voltage (V)
-100
Substrate (Anode) K-type p Thermocouple
-200
-300
Camera
-400
MFC S/S* Cover
Window N
S
N
100 sccm
-500
Pulse on
Argon
Ideal pulsed d.c signal for 100kHz
Cu -Target Holder Magnets Water Channel PTFE Insulator S/S* Base
Ni-Al
Ni-Cr
Substrate Heater
Water cooling
+
-
Turbo & Rotary pump
AE Pinnacle Plus Power Supply
Reference S. Karthikeyan et al, Vacuum, 2010, 85; pp.634-638.
In2S3 powder target
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Why Pulsed D.C Sputtering From Powder Targets ¾ Long term arc free sputtering. sputtering ¾ Insulators and semiconductors can be sputtered. ¾ The enhanced ion flux near the substrate can help to crystallise the compound at low substrate temperatures. ¾ No material wastage associated with the Race Track effect.
Reference Bradley JW et al, Plasma Sources Science and Technology, 11, 2002, 165p
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Materials Deposited
1 Molybdenum (back contact) [1] 1. 2. Copper indium diselenide (absorber layer)[2] 3. Indium sulphide (buffer layer) 4. Indium oxide (Transparent Conductive Oxide layer)[3]
[1] S. Karthikeyan et al, Thin Solid Films, 2011, 250, pp.266-271. [2] S.Karthikeyan et al, Thin Solid Films, 2011, 519; pp.3107 -3112 [3] S.Karthikeyan et al,The effect of oxygen on the properties of pulsed d.c magnetron sputtered In2O3 films. in: 1st CSE
.
Postgraduate Conference 2010, University of Salford, United Kingdom
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Sputtering in argon atmosphere from commercial In2S3 powder. Films sputtered at different substrate temperatures
I di Indium Sulphide S l hid Films Fil
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XRD off IIn2S3Films Fil
(2 2 12)
(1 0 15)
(0 0 12)
(109) (206)
Pressure : 7.3x10-3 mbar Mode : Constant Power (25 W) Frequency : 100 kHz Pulse off Time : 0.5 µs Distance : 8 cm
0
250 C
Intensitty (arb.u)
(116)
(103)
Deposition Parameters
0
200 C
0
150 C 0
100 C No Heating 10
20
30
40
50
Preferred (109) orientation Tetragonal – β In2S3 formed with No heating !!
60
2θ (degrees)
8
SEM off IIn2S3Films Fil An SEM image of the sample deposited at 200 0C
The very thin and smooth nature of the films was a barrier for high resolution SEM images. The films were analysed using AFM to obtain a clear picture. 9
Optical studies of In2S3 Films No heating
100 0C
1 d
⎛
⎞ ⎟ ⎜ (1 − R )2 ⎟ ⎝ ⎠
α = − .ln ⎜
1.0 200 nm
T
200 nm
Transm mittance
0.8 150 0C
0.6
0.4
No heating 0 100 C 0 150 C 0 200 C 0 250 C
0.2
0.0 300
400
500
600
700
800
Wavelength (nm)
900
1000 1100 200 nm
1 0
Optical and AFM studies of In2S3 Films 19.49 nm
17.52 nm
No heating
100 0C
1.0
Transm mittance
0.8
0.6
200 nm 200nm
0.4
0.0 300
400
500
600
700
800
900
% In
%S
28.62 nm
17.94 nm
150 0C
200 0C
200 nm 200nm
200 200nm nm
1000 1100
Wavelength (nm)
Deposition Temperature
0.00 nm
0.00 nm
No heating 0 100 C 0 150 C 0 200 C 0 250 C
0.2
200 200nm nm
[% S ] [% In]
0.00 nm
0.00 nm
Band Gap eV
24.27 nm
250 0C Non heated
41.26
58.74
1.42
2.768
100 0C
41.84
58.16
1.39
2.735
150 0C
42.86
57.14
1.33
2.708
200 0C
43.5
56.5
1.29
2.667
250 0C
48.51
51.49
1.20
2.526
200 nm 200nm 0.00 nm
1 1
B d gap vs [I Band [In]/[S] ]/[S] ratio i
2.80 1.45
Bandgap [In]%/[S]%
2.75 2.70
1.35 2.65 1 30 1.30 2.60
[In]% %/[S]%
Band g gap (eV)
1 40 1.40
1.25 2.55 1 20 1.20 2.50
50
100
150
200 o
Temperature ( C)
250
Band gap reduced with reduction in sulphur content 1 2
Conclusions ¾ The possibilities of pulsed d.c d c magnetron sputtering for the deposition of In2S3 and films from powdered targets were studied. ¾ The XRD analysis revealed that the films are in single phase. ¾ The room temperature sputtered In2S3 films showed a maximum band gap about 2.768 eV which is higher compared to reported single crystal value. ¾The higher band gap reduces the absorption in the blue region of the solar spectrum andd can increase i the h solar l cell ll efficiency. ffi i ¾The band gap of the In2S3 thin films reduced with sputtering temperature possibly due to the reduction in sulphur content. ¾A single process for all the layer can cut down the overall cost of production of the solar cell and use of an In2S3 buffer layer can produce Cd free solar cells
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Acknowledgments
¾ Prof . A. E. Hill , Materials and Physics Research Centre , University of Salford ¾ Dr. R. D. Pilkington, Materials and Physics Research Centre , University of Salford ¾ Dr. H. Yates, Materials and Physics Research Centre , University of Salford ¾ Mr. G. Parr, Salford Analytical Services, University of Salford. ¾ Ms. M. Hardacre, Salford Analytical Services, University of Salford. ¾ Mr. J. Smith, Physics Technician , University of Salford. ¾ Mr. M. Clegg, Physics Technician , University of Salford. ¾ Finally , thanks to everybody who has indirectly helped to bring this work to fruition.
Thanks for your attention
