# Presentation: 762413

## 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, University of Delaware***
_**Introduction**_
- Apply bias from -0.5V to +2V during stress
- Previous stability primarily on FS CdTe with IEC contacts
- New: IEC VT CdTe with IEC contact process
- BDH then Cu/Ni, Ni, CuI/Ni, ZnTe:Cu/ITO
- Focus on contacts with or without Cu
- •If Cu2Te decomposition and ion motion responsible for degradation, should be bias dependent

## 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**

## 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@85°C, SC or OC, 15 days, dry air**

| contact | condition | Voc 
(V) | Jsc (mA/cm2) | FF 
(%) | Eff
(%) |
| --- | --- | --- | --- | --- | --- |
| Ni | Initial | 0.77 | 20.1 | 58.2 | 9.0 |
|  | Stress@OC | 0.73 | 19.9 | 57.8 | 8.4 |
| Cu/Ni | Initial | 0.81 | 20.7 | 69.5 | 11.6 |
|  | Stress@OC | 0.77 | 20.2 | 60.0 | 9.4 |

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

**IEC VT128 with Ni or Cu (15 nm)/Ni contact stressed 15 d @ 85°C,  OC **

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

**IEC VT154: Ni or Cu (15 nm)/Ni contact stressed 10 d @ 100°C,  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? **** **

## Effect of stress (2): variation in stability Ni contact 

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

| VT#
contact | condition | Voc 
(V) | Jsc (mA/cm2) | FF 
(%) | Eff
(%) |
| --- | --- | --- | --- | --- | --- |
| VT128 | Initial | 0.77 | 20.1 | 58.2 | 9.0 |
| Ni | 85°C/15 day/OC | 0.73 | 19.9 | 57.8 | 8.4 |
| VT154 | Initial | 0.81 | 22.0 | 57.7 | 10.3 |
| Ni | 100°C/10 day/OC | 0.64 | 21.1 | 47.1 | 6.4 |

## Change with stress: ZnTe transparent contact

**IEC VT154 with ZnTe:Cu/ITO stressed in light at 100°C for 10 days**

**• ****Back contact becomes blocking after stress, but not when illuminated**
**• ****Back contact “barrier” is photoconductive**
**• ****Consistent with IEC recontacting study: back contact and Voc independent**

## Monitoring degradation during stress: Voc@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 R****S**** or  ********BC **** ********Voc********monitors junction**

## 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**

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

**Stressed IEC’s 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)**

## Effect of bias during stress: different contacts

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

## 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 ZnTe:Cu 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 **

## 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 100°C**
**Right: 6 devices from FS with different Cu and Te thickness, stressed dark at 60°C: two studies; thicker Te is more stable at +2V**