# Presentation: 143742

## CdS/CdTe Thickness and O2 Effects on Stability

- D. Albin1, S. Demtsu2, and T. McMahon1
- 1National Renewable Energy Laboratory
- 2Colorado State University

- CdTe National Team Meeting
- May 5-6, 2005
- Golden, CO 80401

## Experimental Design

**Experimental**** ****Design**

- 60 nm CdS

- 80 nm CdS

- 100 nm CdS

- no NP

- NP

- no NP

- NP

- no NP

- NP

- VCC No O2

- VCC No O2

- VCC + O2

- VCC + O2

- 2 Replicates of 24 Variable Combinations = 48 devices
- Stress Testing: Voc bias, 1 Sun, 100 ± 2 oC
- Measurements performed at t = 0, 4.25, 30, 119, 195, 408, and 693 hrs.
- Prior to J-V measurements samples stored in dark at 25 oC

- 8 um CdTe

- 11 um CdTe

- VCC no O2 = 500 torr He
- VCC + O2 = 100/400 torr O2/He
- 8 um CdTe = 3.25 min dep time  11 um CdTe = 5.25 min dep time

- VCC = vapor CdCl2 (T=400C)
- NP = nitric phosphoric acid etch
- CdTe = CSS CdTe
- CdS = CBD CdS

**Notes:**
This slide sums up the orthogonal design of the experiment involving 3 levels for the CdS thickness, 2 levels for the CdTe thickness, with and without oxygen during the VCC process, and with and without NP etch etch prior to applying the backcontact paste layers.

We used 2 replicates at each experimental point giving us a total of 48 devices.

These are the conditions of stress-testing.

## compare blue with red – NP (Te) better

- 600 A CdS

- 800 A CdS

- 1000 A CdS

- 8 um CdTe

- 11 um CdTe

**Red** – NP; **Blue** – no Etch
- Solid - O2 during VCC (100/400); dashed - NO O2 (0/500)

- BiLayer TCO
- CdTe thk 8-11 um
- Paste: 12012000 (old paste)
- VCC time= 6 min
- CdTe dep time= 3.25 – 5.25 min
- PO2 = 0.9

- T328

- T329

- T330

- T331

- T332

- T333

- compare blue with red – NP (Te) better

## Look at Shunting (catastrophic failure) obvious problems with thinner films

- 600 A CdS

- 800 A CdS

- 1000 A CdS

- 8 um CdTe

- 11 um CdTe

**Red** – NP; **Blue** – no Etch
- Solid - O2 during VCC (100/400); dashed - NO O2 (0/500)

- BiLayer TCO
- CdTe thk 8-11 um
- Paste: 12012000 (old paste)
- VCC time= 6 min
- CdTe dep time= 3.25 – 5.25 min
- PO2 = 0.9

- T328

- T329

- T330

- T331

- T332

- T333

## 600 A CdS

- 800 A CdS

- 1000 A CdS

- 8 um CdTe

- 11 um CdTe

**Red** – NP; **Blue** – no Etch
- Solid - O2 during VCC (100/400); dashed - NO O2 (0/500)

- BiLayer TCO
- CdTe thk 8-11 um
- Paste: 12012000 (old paste)
- VCC time= 6 min
- CdTe dep time= 3.25 – 5.25 min
- PO2 = 0.9

- T328

- T329

- T330

- T331

- T332

- T333

- Thin CdS and/or Thin CdTe
- increases shunting

## CdTe microstructure after VCC + O2

- CdTe microstructure after VCC + O2

- CdO, TeO2

- CdCl2 is “thermodynamically” unreactive due to the large heat of formation associated with CdCl2.  (Interaction with H2O not studied to date)

- CdTe after VCC with O2 ambient oxidizes CdTe to form CdO and TeO2.  High temperature (400 C) extends this down grain boundaries.

- <solid>  (gas)

*O**2** – grain boundary effect*

## ??

- CdTe microstructure after NP etch + Cu:dag

- Cu

- Surface oxides removed by NP etch, leaving oxide residues primarily located down grain boundaries.
- “CuxTe free Cu” unclear, but we know Cu is free to move somehow.  CuxTe powder is not single phase, could contain free Cu in elemental form.
- CuxTe is not most stable phase in this system

- Te

- CuxTe

- Cu

- CdO, TeO2

- <solid>  (gas)

*O**2** – grain boundary effect*

## CdTe microstructure after thermal processing and/or stress

- Cu

- Te

- Oxides at grain boundary reduce diffusion of Cu, minimize tendency towards forming shunts of Cu at grain boundaries.
- TeO2 more likely to “getter” Cu than CdO.  Cu will not reduce CdO.
- Of course, Cu+TeCuTe will “getter” Cu.

- CdO, TeO2

- <solid>  (gas)

*O**2** – grain boundary effect*

## CdTe microstructure after thermal processing and/or stress (VCC w/o O2)

- CdTe microstructure after thermal processing and/or stress (VCC w/o O2)

- CdTe by itself will not impede the diffusion of Cu either through forming Cu2Te or CuTe.
- Non-oxidized grain boundaries rapid sinks for Cu diffusion.
- Still have “gettering” aspect associated with CuxTe formation

- <solid>  (gas)

*O**2** – grain boundary effect*

## Shunt Mechanism Enhanced by Over-Etching

- Ipeak

- Original thought was that edge shunting was the problem; did see some improvement with “margined” structures
- Still saw hot spots occurring near the edges; particularly with thinner films
- Higher resolution analysis showed hot spots not at edges, rather at edge defined by our liftoff mask
- NP acid “leaks” under mask, is not rinsed off, and effectively over-etches at corners of device mesa

- localized heating due to shunting

- “margined device”

- liftoff mask

- liftoff mask

## Summary

- Device stability as based upon 100 oC light-soak tests at Voc bias and 1-Sun illumination show that thinner CdS and CdTe films are more prone to catastrophic shunting.
- Shunting can be reduced by incorporating O2 during the vapor CdCl2 process
- A thermodynamic model has been proposed suggesting that TeO2 can “getter” Cu through the formation of Cu2O in much the same way that Te can react with Cu to form CuxTe
- Shunting mechanisms seen in our devices may be attributed to poor device fabrication techniques which lead to over-etching of devices near mesa corners.