Stability Studies With CSU and UT CdTe Solar Cells

1
Stability Studies With CSU and UT CdTe Solar Cells

• Larry Olsen
• Pacific Northwest National Laboratory (PNNL)
• CdTe Team Meeting, May 5, 2005

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Overview of Studies

• Investigating effects of stressed UT and CSU CdTe
  Cells
• Conditions of Stress
• - Dry Heat, 60ºC/90RH, 85ºC/85RH
• - Dark, Open-Circuit Conditions
• Uncoated and coated (Barrier Coatings) cells
• Measurements Dark and illuminated I-V vs time
• Analyses To date, have concentrated on applying
  barrier
• coatings to protect CdTe
  cells from moisture

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Background

• PNNL Developed Multilayer Coatings For Organic
  LEDs
• PML Coatings Comprised Of Polymer And Oxide
  Layers
• WVTR Values OLEDS On Glass

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Basis For Multilayer Coating
Multilayer Coating
Defect

• Diffusion Rate Of Water And Oxygen Through
  Homogeneous Oxide Layers Is Extremely Low
• Penetration Of Moisture Through Oxide Based
  Coatings Occurs Because Of Defects In Oxide
• PNNL Approach
• - First Polymer Layer (P-Layer)
• Provides Smooth Surface
• - Multilayer Provides Tortuous
• Path For Water Diffusion

Oxide
Polymer
Cell Surface
Planarizing Polymer Layer
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Application To CSU CdTe Cells
As-Received CSU Cell
Top View
Cross-Section (Not To Scale)
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Application of Barrier Coatingto As-Received CSU
Cell
Process Steps

• Vacuum Deposit Ag Contact
• Ring and Leg to Corner In Pad
• 2. Deposit Polymer Bridge
• Vacuum Deposit Ni/Ag Contact to CSU Ni-C Back
• Contact (1000 Å Ni Followed by 2000 Å
  Ag)
• 4. Apply Barrier Coating

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CSU Ni-C Back Contact

• CSU Ni-C Contact Very
• Rough With 10 to 20 µm
• Features
• Contact is 75 µm Thick
• Resulting In Large Step At
• Periphery
• CSU Contact is Very Robust
• and Provides Low Rsq
• Developing Sputtered Contact
• At PNNL For Comparison

10 µm
10 µm
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Encapsulated CSU Cells

• Cell _at_ 60ºC and RH 0,
• Barrier Coating with 5 Dyads
• Cell _at_ 60ºC/90 RH,
• Barrier Coating with 5 Dyads
• Cell _at_ 60ºC/90 RH,
• Barrier Coating with
• One Dyad and Al Coating

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I-V Parameters vs Time
Cell Stressed at 60ºC / Dry
Coated Cell Stressed at 60ºC / 90 RH
Jsc
Jsc
Efficiency
Efficiency
Hours _at_ 60ºC/ 0 RH
Voc
Voc
FF
FF
Hours _at_ 60ºC/ 0 RH
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CSU Cell With 1 Dyad / Al Coating
Al Film
Polymer/Al2O3 Dyad
60ºC / 90 RH
85ºC / 85RH
Cell
Bare Cell (2004)
CSU Back Contact Surface
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I-V Parameters vs Time for 1 Dyad/ Al Coating
60C/90RH
85C/85RH
Jsc

• Barrier coating consists of
• - Thick polymer layer
• - Al2O3 layer
• - Aluminum layer
• Performance under stress
• of 60C / 90 RH appears to be
• determined by inherent properties
• of CSU cell i.e., changes due to
• 60ºC environment
• Effects of 85C/85RH
• All parameters affected, with FF
• decreasing most rapidly

Efficiency
85C/85RH
60C/90RH
Voc
FF
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Results for Bare CSU Cells (2005)

• Area of Cells 0.3 cm2
• All Cells Subjected to
• 60ºC / 90 RH
• These Results Indicate a Significant Improvement
  Relative to Behavior in 2004 for Bare Cells

Bare Cells in 60ºC / 90 RH
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Results for Bare UT Cells (2005)
Bare Cells

• Universty of Toledo Cells Have Superstrate
  Configuration
• Test Results are for 1 cm2 Cells
• Will Begin Working with Toledo
• to Test Coated Cells

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Calcium Test for One Dyad Coating After 120 Hours
Calcium
60/90
Ca gone

• All Samples Developing Holes
• and Fading at Edge
• Ca Completely Converted to
• Ca(OH)2 in 85ºC/85RH
• (72 hr _at_ 85/50 and 48 hr _at_85/85)
• After 550 hours
• - Ca Gone for 60/90 and 85/85
• - Reacted from Sides for 85/50

85/85
85/50
T72 hrs _at_ 85C/50 RH T48 hrs _at_ 85C/85 RH
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Calcium Test for Five Dyad Coating After 120 Hours
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Sample Structure
Ca / 400A ox /
4.5 um polymer
/ (400A ox / 0.5 um polymer)
Coating
Calcium
No edge effects
No edge effects
60/90
Glass
4

• No visible effects in any set
• After 550 hours
• - No Effects for 60/90
• - Reacted from
• Sides for 85/50and 85/85

No edge effects
Ca / 400A ox /
4.5 um polymer
/ (400A ox / 0.5 um polymer)
No edge effects
No edge effects
85/50
85/85
No edge effects
T72 hrs _at_ 5C/50 RH
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Key Results

• Efficiency of CSU Cells Encapsulated with a 5
  Dyad Multilayer
• Coating Decreases only 15 when Subjected to
  60ºC/90RH
• and Dark, Open-Circuit Stress
• Decrease in Efficiency Due Mainly to Reduction
  in FF
• Encouraging Results Were Obtained With a Coating
  Consisting
• of One Dyad and an Aluminum Coating
• Recent Results For Bare CSU Cells Indicate The
  Back Contact
• is Less Sensitive to Moisture Than Cells
  Received in 2004
• Collaboration Was Initiated with Al Compaan at
  UT to Investigate
• Approaches to Encapsulation and Effects of
  humidity on UT Cells

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Future Work

• Initiate Efforts to Focus More on Changes in
  Device Parameters and Physical Changes Due
  Temperature and Humidity Stress
• Investigate Changes in Cells and Encapsulation
  Resulting from 85ºC / 85 RH Stress
• Investigate Approaches to Edge Seals and Continue
  to Develop
• Low Cost Encapsulation Processes
• Collaborate with CSU, UT and Others (as Resources
  
• Permit) to Develop Effective Approach to
  Encapsulation and
• to Identify Effects of Moisture and
  Temperature on CdTe Cells