Effect of applied bias during stress: old and new results focusing on Cu contact

1
Effect of applied bias during stress old and
new results focusing on Cu contact
Steven Hegedus, Darshini Desai, Dan Ryan, Brian
McCandless Institute of Energy Conversion, Unive
rsity of Delaware Introduction Apply bias from
-0.5V to 2V during stress Previous stability pr
imarily on FS CdTe with IEC contacts
New IEC VT CdTe with IEC contact process
BDH then Cu/Ni, Ni, CuI/Ni, ZnTeCu/ITO
Focus on contacts with or without Cu
If Cu2Te decomposition and ion motion
responsible for degradation, should be bias
dependent
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Effect of stress light, voltage bias,
temperature, Cu

• Similar bias dependence in light or dark, 60 or
  100 C
  
• SC(or OV) typically most stable bias point
• No Cu much less degradation at forward bias
• Significant variation for nominally similar
  devices

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Effect of stress (1) new VT results Cu/Ni vs Ni
contact

• IEC VT devices with Ni or Cu (15 nm)/Ni contact
• stressed light_at_85C, SC or OC, 15 days, dry air

4
New results (1) effect of stress Cu/Ni vs Ni
contact

• IEC VT128 with Ni or Cu (15 nm)/Ni contact
  stressed 15 d _at_ 85C, OC
  

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New results (2) effect of stress Cu/Ni vs Ni
contact

• IEC VT154 Ni or Cu (15 nm)/Ni contact stressed
  10 d _at_ 100C, OC
  

Comparing VT128 vs 154 Significant differences
in forward bias JV after stress,
much greater loss in FF,for nominally identical
VT runs and contacts. Why?
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Effect of stress (2) variation in stability Ni
contact

• Nominally identical IEC VT devices with Ni
  contact VT128 and VT154
  
• stressed light _at_ 85C/15 days or 100C/10 days,
  SC or OC, dry air
  
• VT128 much better initial and stability (both SC
  or OC bias). Why?
  

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Change with stress ZnTe transparent contact

• IEC VT154 with ZnTeCu/ITO stressed in light at
  100C for 10 days
  

Back contact becomes blocking after stress, but
not when illuminated Back contact barrier is
photoconductive Consistent with IEC recontactin
g study back contact and Voc independent
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Monitoring degradation during stress Voc_at_T

• Focus on in-situ monitoring of Voc during
  stress at temperature
  
• Voc changes after stress related to increased
  recombin. at deep states
  
• Voc changes during stress can dominate stability
• Independent of RS or ?BC ? Voc monitors
  junction
  

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Switching bias degradation and recovery
transients

• Stressed FS/IEC at 100 C, light, air for 38 days
• Cell 1 SC for 24 days then switched to OC
• Cell 2 OC for 1 day then switched to SC
• Cell 3 OC for 24 days then switched to SC

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Switching bias degradation and recovery
transients with Cu/Ni and Ni contacts

• Stressed IECs VT at 85 C, light, air, for 14
  days
  
• Cell 1 SC for 14 days
• Cell 2 OC for 14 days
• Cell 3 OC (4 days), 1.3xOC (4 days), SC (4
  days), OC (2 days)
  

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Effect of bias during stress different contacts

• Left Stressed VT154 pieces with 4 different back
  contacts Cu/Ni, CuI/Ni, Ni, ZnTeCu/ITO
  
• Right Very different bias dependence from
  previous FS/IEC devices
  

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Summary and future work

• IEC VT devices have much different bias
  dependence compared to previous FS/IEC devices
  
• Less difference between SC and OC
• Less difference between contacts with or without
  Cu layer
  
• Different transient directions when switched from
  OC to SC
  
• Transparent ZnTeCu contact shows that roll-over
  due to stress associated photo-active back
  contact barrier
  
• Variability in initial and stressed performance
  of VT needs to be solved
  
• Stress devices with different thickness light
  through ZnTe back contact different forms of Cu
  and surface treatment, careful temperature
  dependence to determine various activation
  energies

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Variability in efficiency change with stress old
results

• Left 10 devices from FS with IEC contact, some
  variation in contact process, stressed in light
  at 100C
  
• Right 6 devices from FS with different Cu and Te
  thickness, stressed dark at 60C two studies
  thicker Te is more stable at 2V