Tim Gessert, DOE Solar Energy

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Advances in the In-House CdTe Research
Activities at NREL
T. Gessert, X. Wu, R.G. Dhere, H. Moutinho, S.
Smith M. Romero, C. Corwine, J. Zhou, and A.
Duda National Renewable Energy
Laboratory Colorado State University
Acknowledgements First Solar, LLC U.S. DOE
Contract No. DE-AC36-99GO10337
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Areas Highlighted

• High Efficiency and Manufacturability
• Advanced Nano-Probe Techniques
• Back Contact Investigations
• Radiative Recombination Studies

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High Efficiency and Manufacturability
Use of NREL Processes with Soda-Lime Glass (X. Wu
and J. Zhou, 19th European PVSEC, June 2004)

• Compare Substrates
• Commercial Soda-lime glass/SnO2
• Borosilicate glass/CTO
• Integrate Novel Layers
• Zinc-Tin-Oxide (ZTO) buffer layer
• Oxygenated Nanocrystalline CdS (CdSO) layer
• Optimize Device at Lower Temp
• 625C (NREL World Record Cell)
• lt570ºC (Soda-Lime Glass)
• Reduce CdTe thickness
• Typical NREL 8-12 µm
• Target 5 µm

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High Efficiency and Manufacturability
Incorporate One heat-up step Process
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High Efficiency and Manufacturability
Device Results(Commercial Soda-Lime Glass/SnO2
Substrates)
NREL confirmed total-area efficiency Cell
1-5 on Tek15 Cell 6 on Asahi textured SnO2/SL
glass substrate
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High Efficiency and Manufacturability
Uniformity Results(Commercial Soda-Lime
Glass/SnO2 Substrates)
NREL confirmed total-area efficiency
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High Efficiency and Manufacturability
Soda-Lime Glass Devices Summary

• Demonstrate a process to produce high-efficiency,
  thin-film CdTe solar cells on commercial SL-
  glass /SnO2 substrates
• Incorporated ZTO and CdSO into commercial
  SL-glass devices.
• NREL-confirmed total-area efficiency of more than
  14.
• Transferred related technologies for preparing
  NREL-developed materials to industrial partner
• Recipes, Quality Criteria, Characterization
  Procedures

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Advanced Nano-probe Techniques
Conductive AFM (C-AFM) Analysis of Br/Meth
Pre-contact Etch (H. Moutinho, R. Dhere et. al.,
19th PVSC, June 2004)
AFM
Simultaneously Acquired C-AFM
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Advanced Nano-probe Techniques
Near-Field Scanning Optical Microscopy (NSOM)
Analysis (S. Smith, et. al., Appl. Phys. Lett.
85 (17) 2004)
Simultaneously-Acquired Near Field Optical Beam
Induced Current n-OBEC Image
AFM
Intensity
Photocurrent Collected near Grain Boundaries up
to 5 Times Higher than Photocurrent Collected on
Grain!!
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Back Contact Investigations
Controlled Cu Incorporation During ZnTeCu
Contacting
ZnTeCu Deposition Temperature
ZnTeCu Thickness
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Back Contact Investigations
Controlled Cu Incorporation During ZnTeCu
Contacting
CV study of ZnTeCu thickness
CV study of effect of CdTe thickness
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Back Contact Investigations
Very Thick ZnTeCu Contacts (i.e., Lots and Lots
of Cu!) (To Be Pub. 31 IEEE PVSC)
Contact deposition Temperatures less than optimum
Contact Deposition Temperatures greater than
optimum
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Back Contact Investigations
CdS/CdTe Evolution During Back Contacting (To Be
Pub. 31 IEEE PVSC)
Contact with ZnTeCu that is too thick (too much
Cu into CdTe)
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CdTe Radiative Recombination Studies
Cross-Sectional Cathodoluminescence (T. Gessert.
M. Romero, et. al., 3rd WCPEC, Osaka, 2003)
NREL
First Solar
Junction Region
TCO
CdS
4 mm
CdTe
Back Contact Region
200 meV
ZnTeCu/Ti contacting
4 mm
CL Temperature 77K
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CdTe Radiative Recombination Studies
Controlled Diffusion of Cu into CdTe (C.
Corwine, T. Gessert, et. al., Submitted Appl.
Phys. Lett,)
Temperature-Dependent PL of Crystalline and CdTe
Films
Low-Temperature CL (19.7 K)
Cu Deposited Surface
Colorado State University
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Conclusions

• Conclusions
• Significant progress has been made in
  understanding many aspects of polycrystalline
  CdTe device technology.
• Reproducible devices using ZTO and CdSO on SL
  glass gt14.
• Nano-probe analysis indicates some insight for
  high collection.
• ZnTeCu contact analysis reveals insight to
  junction evolution
• One key to significant future improvements in
  performance and/or stability will be
  understanding and controlling defect formation
  during various process steps.