Novel low temperature pulsed d.c.
magnetron sputtering of single phase
β
In2S3 buffer layers for CIGS solar cell
application
Karthikeyan, Sreejith, Hill, AE and Pilkington, RD
Title Novel low temperature pulsed d.c. magnetron sputtering of single phase β
In2S3 buffer layers for CIGS solar cell application Authors Karthikeyan, Sreejith, Hill, AE and Pilkington, RD
Type Conference or Workshop Item
URL This version is available at: http://usir.salford.ac.uk/19231/ Published Date 2011
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Novel low temperature pulsed d c magnetron sputtering
Novel low temperature pulsed d.c. magnetron sputtering
of single phase β-In
2S
3buffer layers for CIGS solar cell
application
Sreejith Karthikeyan, Arthur Hill and Richard Pilkington
Introduction
CIS/CIGS thin film based solar cells are the most promising renewable energy source because of their relatively high solar efficiency and stability
because of their relatively high solar efficiency and 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 i l ll CIS/CIGS i i h f f h j i A typical cell CIS/CIGS structure is in the form of a heterojunction.
i Z O 70nm ZnO ~ 1?m
i Z O 70nm Al-ZnO ~ 1 µm
Intrinsic ZnO layer to reduce leakage current Transparent Conductive Oxide layer
Mo layer~ 0.8? m CuInSe2~ 1.0? m
CdS~ 50nm i-ZnO~ 70nm
Mo layer~ 0.8 CdS~ 50nm i-ZnO~ 70nm
µm CuInSe2~ 2µm
N- type buffer layer
Intrinsic ZnO layer to reduce leakage current
P- type absorber layer Back Contact
Substrate Mo layer 0.8? m
Substrate Mo layer 0.8µm
A typical CIGS solar cell
Back Contact
1
References
Introduction
CdS layers are deposited using a chemical bath deposition process
CdS buffer layer - main functions
¾ The optimum thickness (60 nm to 80 nm) CdS layer builds a sufficiently p ( ) y 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
¾ The chemical bath deposited CdS layer coats the absorber CIGS 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.
Why Do We Need An Alternative Buffer Layer ?
¾ i i f Cd
¾ Toxicity of 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 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
3
( ) 2 3
Pulsed D.C. Magnetron Sputtering System
0 5 10 15 20 25 30 35 40 45 50 55 0
100
T (μs) Pulse off Substrate (Anode) K-type Thermocouple Substrate (Anode) K-type Thermocouple
0 5 10 15 20 25 30 35 40 45 50 55
-300 -200 -100 V o lt age ( V ) Argon
S/S*Cover
Camera MFC 100 sccm p Argon
S/S*Cover
Camera MFC 100 sccm p -500 -400 Pulse on
Ideal pulsed d.c signal for 100kHz
N S N
Cu -Target Holder Magnets Water Channel PTFE Insulator
S/S*Base
Window
N S N
Cu -Target Holder Magnets Water Channel PTFE Insulator
S/S*Base
Window Water cooling Turbo & Rotary pump Substrate Heater + Ni-Cr Ni-Al Water cooling Turbo & Rotary pump Substrate Heater + Ni-Cr Ni-Al
AE Pinnacle Plus Power Supply
Rotary pump
+
-AE Pinnacle Plus Power Supply
Rotary pump
+
-4
Reference
S. Karthikeyan et al, Vacuum, 2010, 85; pp.634-638.
Why Pulsed D.C Sputtering From Powder Targets
¾ Long term arc free sputtering ¾ Long term arc free 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.
Bradley JW et al, Plasma Sources Science and Technology, 11, 2002, 165p
Reference
Materials Deposited
1 Molybdenum (back contact) [1]
1. Molybdenum (back contact) [1]
2. Copper indium diselenide (absorber layer)[2]
3. Indium sulphide (buffer layer)
4. Indium oxide (Transparent Conductive Oxide layer)[3]
6
[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
Sputtering in argon atmosphere from commercial In2S3 powder.
Films sputtered at different Films sputtered at different substrate temperatures
I di
S l hid Fil
Indium Sulphide Films
XRD f I S Fil
XRD of In
2S
3Films
Deposition Parameters
Pressure : 7.3x10-3mbar
Mode : Constant Power (25 W) Frequency : 100 kHz
(2 2 12) (1 0 15) (0 0 12) (206) (1 0 9 )
250 0C
(103)
(116
)
Frequency : 100 kHz Pulse off Time : 0.5 µs Distance : 8 cm
200 0C
ty (
a
rb
.u
)
150 0C
100 0C
In te n s it Preferred (109) orientation
Tetragonal – β In2S3
10 20 30 40 50 60
2θ (degrees)
No Heating formed with
No heating !!
2θ (degrees)
SEM f I S Fil
SEM of In
2S
3Films
An SEM image of the sample deposited at 200 0C
An SEM image of the sample deposited at 200 C
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.
Optical studies of In
2S
3Films
No heating 100 0C
⎛ ⎞
0.8 1.0
200 nm 200 nm
( )2
1 .ln
1
T
d R
α = − ⎛⎜ ⎞⎟
⎜ − ⎟
⎝ ⎠
0.6
m
itta
nce 150
0C
0.2 0.4
Trans
m
No heating 100 0C 150 0C 2000C
300 400 500 600 700 800 900 1000 1100 0.0
200 C 250 0C
Wavelength (nm)
1.0
Optical and AFM studies of In
2S
3Films
17.52 nm
No heating
19.49 nm
100 0C
0.6 0.8 m ittanc e 200nm 200nm
200 nm 200 nm
0.0 0.2 0.4 Tra n s m No heating 100 0C 150 0C 200 0C 250 0C
0.00 nm 0.00 nm
17.94 nm 28.62 nm
150 0C 200 0C
300 400 500 600 700 800 900 1000 1100 Wavelength (nm)
Deposition % In %S [% ]S Band Gap
0.00 nm 200nm
0.00 nm 200nm
24.27 nm
200 nm 200 nm
Temperature eV
Non heated 41.26 58.74 1.42 2.768
1000C 41.84 58.16 1.39 2.735
1500C 42.86 57.14 1.33 2.708
[% ]In
250 0C
150 C 42.86 57.14 1.33 2.708
2000C 43.5 56.5 1.29 2.667
2500C 48.51 51.49 1.20 2.526 0.00 nm
200nm
B
d
[I ]/[S]
i
Band gap vs [In]/[S] ratio
1 40 1.45
2.75 2.80
Bandgap [In]%/[S]%
1 30 1.35 1.40
2.65 2.70
g
ap (eV)
%
/[S
]%
1 20 1.25 1.30
2.55 2.60
Band
g
[In
]%
1.20
50 100 150 200 250 2.50
Temperature (oC)
Band gap reduced with reduction in sulphur content
Conclusions
¾ The possibilities of pulsed d c magnetron sputtering for the deposition of In S and
¾ The possibilities of pulsed 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 d i h l ll ffi i
and can increase the solar cell efficiency.
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
Acknowledgments
¾Prof . A. E. Hill , Materials and Physics Research Centre , University of Salford
¾Dr. R. D. Pilkington, Materials and Physics Research Centre , University of Salford
¾Dr. H. Yates, Materials and Physics Research Centre , University of Salford
¾Mr. G. Parr, Salford Analytical Services, University of Salford.
¾Ms. M. Hardacre, Salford Analytical Services, University of Salford.
¾Mr. J. Smith, Physics Technician , University of Salford.
¾Mr. M. Clegg, Physics Technician , University of Salford.
¾Finally , thanks to everybody who has indirectly helped to bring this work to fruition.
