# Presentation: 081204

## Stability Studies With CSU and UT CdTe Solar Cells

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

## Overview of Studies

**Investigating effects of stressed UT and CSU CdTe Cells**
_**Conditions of Stress**_**:**
**	****	****-  Dry Heat, 60****o****C/90%RH, 85****o****C/85%RH**
**	****	****-  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**
**                                              **

## Background

**PNNL Developed **_**Multilayer Coatings**_** For Organic LEDs**
**PML Coatings Comprised Of **_**Polymer And Oxide Layers**_
**WVTR Values < 2x10****-6**** g/m2/d Demonstrated For OLEDS On Glass**

## Basis For Multilayer Coating

_**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 Approac**_**h:**
**-  First Polymer Layer (P-Layer) **
**   ****Provides **_**Smooth Surface**_
**-  Multilayer Provides **_**Tortuous**_
**   **_**Path For Water Diffusion**_

**Planarizing**
**Polymer**
**Layer**

**Multilayer **
**Coating**

**Cell **
**Surface**

**Polymer**

**Oxide**

**Defect**

## Application To  CSU CdTe Cells

**Top View **

**Cross-Section (Not To Scale)**

_**As-Received CSU Cell**_

## Application of Barrier Coatingto 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**

## CSU Ni-C Back Contact

**CSU Ni-C Back Contact**

**10 ****μ****m**

**   ****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 R****sq**
**  ****Developing Sputtered Contact**
**    ****At PNNL For Comparison**

**10 ****μ****m**

## 
Cell @ 60oC and RH = 0%, 
     Barrier Coating with 5 Dyads

Cell  @ 60oC/90% RH,
     Barrier Coating with 5 Dyads 

Cell  @ 60oC/90% RH,
     Barrier Coating with
     One Dyad and Al Coating

**Cell @ **_**60**__**o**__**C and RH **__***= ***__**0%**_**, **
**     ****Barrier Coating with 5 Dyads**
**Cell  @ **_**60**__**o**__**C/90% RH**_**,**
**     **_**Barrier Coating with 5 Dyads**_** **
**Cell  @ **_**60**__**o**__**C/90% RH**_**,**
**     **_**Barrier Coating with**_
**     **_**One Dyad and Al Coating**_

**Encapsulated CSU Cells**

## I-V Parameters vs Time

_**Cell Stressed at 60**__**o**__**C / Dry**_

_**Coated Cell Stressed at 60**__**o**__**C / 90% RH**_

**J****sc**

**Efficiency**

**V****oc**

**FF**

**Hours @ 60****o****C/ 0 RH**

**Hours @ 60****o****C/ 0 RH**

**J****sc**

**Efficiency**

**V****oc**

**FF**

## CSU Cell With 1 Dyad / Al Coating

**Al Film**

**Polymer/Al****2****O****3**** Dyad **

**Cell**

**CSU Back Contact Surface**

**60****o****C / 90% RH**

**85****o****C / 85%RH**

**Bare Cell (2004)**

## I-V Parameters vs Time for 1 Dyad/ Al Coating

**J****sc**

**Efficiency**

**V****oc**

**FF**

**60C/90RH**

**85C/85RH**

**60C/90RH**

**85C/85RH**

-  **Barrier coating consists of **
**       ****-   Thick polymer layer**
**       ****-   Al****2****O****3**** 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  **
**   ****60****o****C environment**
** ****Effects of 85C/85%RH:**
**       ****All parameters affected, with FF**
**       ****decreasing most rapidly**

## Results for Bare CSU Cells (2005)

**Area of Cells = 0.3 cm****2**
**All Cells Subjected to**
**       ****60****o****C / 90% RH**
**These Results Indicate a Significant Improvement Relative to Behavior in 2004 for Bare Cells**
**	**

**Bare Cells in 60****o****C / 90% RH**

## Universty of Toledo Cells Have Superstrate Configuration

Test Results are for 1 cm2 Cells

Will Begin Working with Toledo
     to Test Coated Cells

**Universty of Toledo Cells Have Superstrate Configuration**
**Test Results are for 1 cm****2 ****Cells**
**Will Begin Working with Toledo**
**     ****to Test Coated Cells**
**      **

_**Bare Cells**_

## Calcium Test for One Dyad Coating

** ****All Samples Developing Holes **
**   ****and Fading at Edge**
** ****Ca Completely Converted to**
**   ****Ca(OH)****2**** in 85****o****C/85%RH**
**   ****(72 hr @ 85/50 and 48 hr @85/85)**
** ****After 550 hours ****:**
**      ****-  Ca Gone for 60/90 and 85/85**
**      ****-  Reacted from Sides for 85/50  **

_**60/90**_

_**85/50**_

- T72 hrs @ 85C/50% RH
-  + T48 hrs @ 85C/85% RH

_**Ca gone**_

**Calcium Test for **_**One Dyad Coating**_
**After **_**120 Hours**_

**Calcium**

_**85/85**_

## Calcium Test for Five Dyad Coating

- Ca / 400A ox /

- 4.5 um polymer

- / (400A ox / 0.5 um polymer)

- 4

- No edge effects

- No edge effects

- Ca / 400A ox /

- 4.5 um polymer

- / (400A ox / 0.5 um polymer)

- 4

- No edge effects

- T72 hrs @ 5C/50% RH

**No edge effects**

**No edge effects**

_**60/90**_

_**85/50**_

_**No edge effects**_

**Calcium Test for **_**Five Dyad Coating**_
**After **_**120 Hours**_

_**Sample Structure**_

**Coating**

**Glass**

** ****No visible effects in any set **
** **_**After 550 hour**_**s ****:**
**      ****-  No Effects for 60/90**
**      ****-  Reacted from**
**         ****Sides for 85/50and 85/85 **

_**85/85**_

**Calcium**

## Key Results

** ****Efficiency of CSU Cells **_**Encapsulated with a 5 Dyad Multilayer **_
**  **_**Coating**_** Decreases only 15 % when Subjected to 60****o****C/90%RH **
**  ****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**

## 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**