possess higher prospective to be used in CdS/CdTe thin film solar cells. ... â¢Absorber. â Reported by First Solar @ 33rd IEEE PVSC, San Diego, CA, May 2008.
Properties of CdTe Thin Films Grown by Close Spaced Sublimation (CSS) Technique for Photovoltaic Application K. S. Rahman1, N. A. Khan2, F. M. T. Enam1, M. I. Kamaruzzaman1, M. Akhtaruzzaman2, A. R. M. Alamoud3 and N. Amin1,2,3* 1
Department of Electrical, Electronic and Systems Engineering, Faculty of Engineering and Built Environment, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia 2 Solar Energy Research Institute, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia 3 Department of Electrical Engineering, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia
Cadmium Telluride (CdTe) thin films were deposited by Close Spaced Sublimation (CSS) technique at a pressure of 1.5 Torr under Ar gas ambient on borosilicate glass substrates. The samples were prepared at source temperature of 625ºC and substrate temperature of 595ºC, respectively. The role of various deposition times has been explored with the aim of investigating its impacts on structural, topographical, morphological and electrical properties of CdTe thin films. The crystalline structure, surface topology, surface morphology and electrical properties of the films were determined by using X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Hall Effect measurement, respectively. XRD patterns reveal that, CdTe shows polycrystalline nature with more than one diffraction peaks corresponding to the (111)cub, (220)cub and(311)cub reflection planes at 2θ=23.76º, 2θ=39.30º and 2θ=46.42º, respectively. Variations in the deposition time are responsible for the deviations in the crystallinity of the CSS grown CdTe thin films. Significant changes were also observed in the film’s surface roughness values due to the different deposition times. The FESEM images illustrate that the surface morphology and the average grain size of the films are strongly dependent on the deposition time of CdTe thin films. A particular structure and surface morphology were observed in FESEM images for all films. The carrier concentration, mobility, resistivity and Hall coefficients were calculated for various deposition times. Bulk carrier density was in the order around
1015cm-3. With the increase in deposition time of CdTe thin films, the bulk carrier density was improved. The resistivity was low for higher deposition times. All the films therefore possess higher prospective to be used in CdS/CdTe thin film solar cells. [1] C.S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P. Selvaraj, D.L. Morel, Thin Solid Films 361–362, 520 (2000). [2] J.D. Major, Y.Y. Proskuryakov, K. Durose, S. Green, Thin Solid Films 515, 5828 (2007).
Properties of CdTe Thin Films Grown by Close Spaced Sublimation (CSS) Technique for Photovoltaic Application
K. S. Rahman, N. A. Khan, F. M. T. Enam, M. I. Kamaruzzaman, M. Akhtaruzzaman, A. R. M. Alamoud and N. Amin Department of Electrical, Electronic and Systems Engineering, Faculty of Engineering and Built Environment, The National University of Malaysia (@Universiti Kebangsaan Malaysia, UKM) 43600 Bangi, Selangor, Malaysia
Thin Film Solar Cells 3rd International Conference on Nanotechnology, Nanomaterials & Thin Films for Energy Applications (NANOENERGY 2016), Liverpool, United Kingdom, 27-29 July 2016.
Outline Of The Presentation Introduction: Energy, Environment and Solar Cells General Classification Of Solar Cells Thin Film PV
Why CdTe? Physical, Structural, Thermal & Mechanical Properties of CdTe Advantages of CdTe
CdTe: Devices Structure & Key Issues TCO CdS CdTe Back Contact
Experimental Results XRD Analysis AFM Analysis FESEM Analysis
Conclusion
1
Energy, Environment & Solar Cells demand is ever energy resources, solar energy is Among all the• Energy renewable • Carbon dioxide increasing considered the• Main most consistent and emission abundant renewable energy energy production • Global warming is by fossil fuels source.
Energy versus Environment Solar
Renewable Energy Sources
Wind
Geothermal Biomass Wave
PV System → Solar Cell
A
solar cell is a semiconductor device designed to convert sunlight into electricity. The conversion of light into electricity in a solar cell is called the photovoltaic (PV) effect. Photovoltaic stands for photo, meaning “light”, and voltaic, meaning “electricity”.
2
General Classification of Solar Cells
3
Thin Film PV THIN FILM SOLAR CELLS: 1-10 μm vs. Si 150-200 μm due to a higher optical absorption coefficient
Why thin films? The Promise of Low Cost … (but 1st CdTe/CdS thin film cell was the result of a space program!)
Presently First Solar reports costs at $0.57/W Projected to decrease below $0.4/W; as low as $0.20/W‡
Champion Thin Film Cells: CdTe @ 22.1% (held by First Solar) Commercial Modules
@ approx. 55% of champion cells
Key Issues:
Long term stability (must be proven) Manufacturability (no off-the-shelf equipment) Performance Contacts Buffers Absorber † Reported by First Solar @ 33rd IEEE PVSC, San Diego, CA, May 2008 ‡ From “Technology Choice and Cost Reduction Potential of PV”, K. Zweibel, WCPEC-4, 2006
4
Physical Properties Of CdTe Table below lists the stable crystal structures at atmospheric pressure of 4 binary compounds formed by combining Zn, Cd with O, S, and Te. Compound
Crystal structure (a)
Lattice constant (Å)
Lattice mismatch with CdTe (%)
Energy Conductivity type gap at~2K (eV)
ZnO
W
a=3.250 c=5.207
-29.1
3.435
n
ZnTe CdS
Z W
6.103 a=4.137 c=6.716
-5.8 -9.7
2.391 2.583
p n
CdTe
Z
6.481
-
1.606 (at 2K)
n, p
(a) Z = zincblende, W = wurtzite
Source: Amin, N. 2000. Study of high efficiency CdTe ultra-thin film solar cells by CSS. PhD Thesis. Tokyo Institute of Technology, Tokyo, Japan.
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Properties Of CdTe Structural Properties of CdTe Crystal structure
Zincblende
Molecules per unit cell
4
Molecular weight (g)
240.00
Lattice constant (Å)
6.481
Shortest Cd-Te distance (Å)
2.806
Conc. of Cd sites (cm-3)
1.469 × 1022
Molar volume (cm3)
40.99
Zincblend structure of CdTe
Thermal and Mechanical Properties of CdTe Crystal structure
Zincblende
Linear expansion co-eff. (K-1)
(4.9 ± 0.1) × 10-6
Thermal conductivity (W cm-1 K-1)
0.075
Heat capacity, Cp (cal g-atom-1K-1)
5.9
Isothermal compressibility (kbar-1)
3.69 × 10-3
Micro hardness (kp mm-2)
55 to 140
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Place of CdTe as a Solar Cell Material Candidate Bandgap 1.45 eV is almost optimum for PV The energy gap is ‘direct’- strong light absorption CdTe has a high absorption coefficient >5×105/cm
Polycrystalline materials and glass, cheaper… PV modules seal the cadmium, encapsulate and can be recycled Thus safe, Cd is only 3.27 g/m2 of PV
Simple and variety of low cost deposition techniques
Absorption Coefficient a (cm-1)
1 06
35
Efficiency (%)
B la ck -bo dy Li mi t (A M0) 30
CdTe 25
A M1. 5
T= 30 0 K
G aAs a- Si :H Cu 2 S Si a- Si :H:F
20 A M0 15
Bandgap (eV)
Ge
10
CdS
C uISe 2 1 05
1 04
0 .5
1 .0
1 .5
2 .0
Solar cell efficiency vs. Bandgap
2 .5
C uS 2 x -Si
1 03 Ga A s
a -Si:H
1 02
Zn 3P 2
C dS
Photon Energy (eV) 0 .8
5
Cd T e
In P
1 .1
1 .4
1 .7
2 .0
2 .3
2 .6
Absorption coefficient spectrum of principal semiconductors for solar cells
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CdTe: Device Structure & Key Issues Glass:
Borosilicate, soda lime. Inexpensive, good optical properties, compatible with deposition process (T).
TCO/Buffers: SnO2, ITO, Cd2SnO4, ZnO, ZTO, In2O3 etc. High transparency, low resistivity, Good electro-optical properties; compatible with subsequent processing steps; “buffers” important for thin Cads
CdS: EG=2.42 eV (510 nm); ~ 7 mA/cm2 below 510 nm. Must be thin (600Å) around 100 nm and pinhole free. CdS:O used for record efficiencies, forms excellent heterojunction with CdTe, CdS by CBD makes a very compact film that covers perfectly the TCO layer.
CdTe: versatility in deposition technology;
thickness 3-8 m, The CdTe layer is p-type doped. Active layer, Eg = 1.45 eV suited to the solar spectrum and high absorption coefficient >5×105/cm
8
CdCl2 Heat Treatment – Grain Size Heat Treatment: “activation” process; improves bulk and interface properties; carried out in the presence of CdCl2. CdCl2 Heat Treatment Typical Process: Apply CdCl2 onto CdTe surface Anneal @ 300-500ºC for 15-30 mins Device performance improves in several ways: improved collection improved back contact performance (i.e. reduction in back barrier) higher VOCs
• Grain growth – Mainly in small grain films; typically deposited at low processing temperatures
• “Limited” Grain growth – In large grain films, elimination of smaller grains near the interface and in grain boundaries
• Small grain films from: – IEC and CSU (IEEE PVSC 1997); PVD and Electrodeposition
• Large grain films from: – USF; CSS
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CdTe: Back Contact Back Contact: P-type CdTe is a notoriously difficult material, No low-cost metals available with appropriate high work function (>4.5 eV) to form ohmic contact on CdTe
Usually Au, Cu or Al, the back contact In an alternative approach a pseudo-ohmic contact can be used. There, a highly doped semiconductor is first deposited/formed on the CdTe surface followed by the application of a metal film contact
The metal layer needs only be a few tens of nm in thickness
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CdTe deposition technique
Why CSS?
More than 10 technologies have demonstrated ability to produce small-area solar cell with efficiency >10%, among which 5 technologies have been or are being utilized for commercialization.
It is proved that CSS (close-space sublimation) commercialization. It has merits as followings:
technology
is
best
for
- high deposition rate (5-10 um/min) - depositing high quality CdTe thin film with large grain size (1-5 um); CdTe is also easy to form single phase due to its binary compound structure. - high material utilization > 85%.
First solar Inc. of USA is the largest manufacturer of CdTe thin film solar modules.
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Schematic View of the CdS/CdTe Solar Cell hn
Borosilicate ITO (250-300) nm C dS
M O CV D
Ag
C dTe C a rb o n Ag
(50-100) nm
CS S (2-5) μm Screen Printing ( 2 0 μm )
CdCl2 Treatment p (C u -D o p in g )
＋ 13
Schematic of the CSS Apparatus & Growth Conditions
Tsubstrate: 550 ~ 620°C Tsource : 560 ~ 630°C Ar Pressure: 1.5 ~ 2 Torr Film Thickness: 2 ~ 5 µm
Deposition by the Reversible Dissociation 2 CdTe (s) ⇆ 2 Cd (g) + Te2 (g)
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Cadmium Telluride (CdTe) Thin Films Fabricated by Close Spaced Sublimation (CSS) System
CSS Process Parameters
Value
Substrate temperature
595 ºC
Source temperature
625 ºC
Spacing
2 mm
Pressure
1.5 Torr (Ar gas ambient)
CdTe film thickness
Substrate
2 μm – 4.0 μm
Boro silicate glass
Temperature profile of CSS Deposition time
3,4,5,6 7 min
XRD diffraction patterns of CSS deposited CdTe thin films grown at different times
XRD spectra of CdTe thin film grown by CSS technique for various growth times.
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XRD Analysis
The films showed polycrystalline nature with a preferential orientation along the (111) cubic plane and found at 2θ=23.8º for all the growth times confirming a pure cubic zinc blend structure.
Another two low intensity diffracted peaks were observed belong to CdTe with positions at 2θ= 39.51º and 2θ= 46.51º correspond to (220) and (311) plane, respectively.
All these diffracted peaks were well matched with the JCPDS (00-015-0770) file and in good agreement with the literature of CdTe cubic structure.
The variations in the peak height were obtained for different growth times. The peak intensity is maximum for 6 min deposition time.
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AFM Images : topography (from 3D image) and grains (from 2D image) of CSS grown CdTe thin films
18
AFM Analysis Deposition Time 3 min 4 min 5 min 6 min 7 min
Average Roughness, Sa (nm) 87.673 137.88 91.831 241.01 115.15
RMS Roughness, Sq (nm) 101.38 165.65 109.09 266.79 137.16
Measured values of average roughness (Sa) and RMS roughness (Sq) of CSS deposited CdTe thin films for various deposition times
AFM can provide precise, qualitative and quantitative roughness measurements of surfaces and produces two and three-dimensional images of the surface.
The surface topography and roughness of the films were found by “NANOSURF EASYSCAN 2 AFM” (Atomic Force Microscopy) System.
From the images, it is found that surface morphology is strongly affected by deposition times.
For 3 min, the average and root mean square (RMS) of the roughness is about 87.673 nm and 101.38 nm, respectively.
The roughness values increased for higher deposition times. Therefore, trade off among different deposition times and roughness values is required for better CdTe films.
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FESEM Images : Surface Morphology images of CSS deposited CdTe thin films grown at different times
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FESEM Images : Cross sectional FESEM images showing thickness of CdTe thin films grown at different times
Thickness and grain size of CdTe films for different growth times
Deposition Thickness Time 2.63µm 3 min 2.50µm 4 min 2.32µm 5 min 3.88µm 6 min 3.32µm 7 min
Grain Size 2.04µm 2.23µm 2.35µm 3.62µm 3.21µm
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FESEM Analysis
Field Emission Scanning Electron Microscopy (FESEM) images have been taken for 20µm and 5µm thin CdTe thin films for surface morphology and cross sectional images, respectively.
From these images, it is observed that the morphology is affected by the deposition times.
The films were scattered throughout all the regions and the films have pinholes for lower growth times.
Surface morphology and the average grain size of the films is strongly dependent on the growth times as a function of thickness and grain size.
The variations in the thickness and grain sizes were found for different films. The bigger grain size and higher thickness is found for sample grown at 6 min.
The different morphology and structure (crystalline) of the films were observed may be due to the defects created by different growth temperatures because the rate of particle aggregation during the film preparation is a major factor that controls the morphology and structure (crystalline) of the films.
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Conclusion This study demonstrates the influence of different growth times on the structural, topographical and morphological properties of the CSS deposited CdTe thin films examined thoroughly by XRD, AFM and FESEM.
The variations in the peak height were obtained for different growth times and highest for 6 min deposition times.
Surface topography is strongly affected by deposition times and roughness values increased for higher deposition times.
Therefore, trade off among different deposition times and roughness values is required for better CdTe films.
The bigger grain size and higher thickness was found for sample grown at (Source/Bottom 625ºC, substrate/top 595ºC) for 6 min.
All the films therefore possess higher prospective to be used in CdTe based thin film solar cells.
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5. Lewis, N., 2007. Global energy perspective. URL http://nsl.caltech.edu/energy.htmal 6. M. Hosenuzzaman, N.A. Rahim, J. Selvaraj, M. Hasanuzzaman, A.B.M. A Malek and A. Nahar, Renewable and Sustainable Energy Reviews 41 (2015) 284–297 7. M.M Aman, K.H. Solangi, M.S. Hossain, A. Badarudin, G.B. Jamson, H. Mokhlis, A.H.A. Bakar and S.N. Kazi, Renewable and Sustainable Energy Reviews 41 (2015) 1190–1204 8. Cusano, D.A. 1963. CdTe solar cells and PV heterojunctions in II-VI compounds. Solid State Electronics. 6: 217-218. 9. http://www.firstsolar.com/ 24
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