# Presentation: 051922

## Stability of CdS/CdTe Solar Cells with 

- Ag and Ni Back-Contacts
- S. Demtsu1, D. Albin2, T. McMahon2, J. Pankow2 and A. Davies1
-   1 Physics Department, Colorado State University, CO
- 	                      2 National Renewable Energy Laboratory, CO

- With help from J. Sites (CSU)

- May 5, 2005

## Outline

-   J-V, CV and LBIC before and after stress
-   Effects of Ag and Ni diffusion
-   Conclusions

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundametals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## Device Structure

- 500 nm

- 100 nm

- 80 nm

- ~ 50 m

- ~ 50 m

- 10 m

- ~ 250 nm

- Graphite / Ag
- back contact

- Motivation:  To study the effect of Ag diffusion and graphite paste on device stability

- May 5, 2005

## J-V Curves (OC, 1000C, One Sun)

- Graphite/Ag

-   Jsc = 0 mA/cm2
-   Voc = 42 mV
-   FF = 6.5 %
-    =  1.8 %

-   Jsc = 0 mA/cm2
-   Voc = 37 mV
-   FF = 7.5 %
-    =  2.1 %

- Graphite/Ni

- Changes after Stress

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundamentals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## J-V Curves (OC, 1000C, One Sun)

-   Jsc = 1.3 mA/cm2
-   Voc = 39 mV
-   FF = 10 %
-    =  2.8 %

-   Jsc = 2.3 mA/cm2
-   Voc = 52 mV
-   FF = 21 %
-    =  5.1 %

-        Ag only
- (graphite removed)

-         Ni only
- (graphite removed)

- Changes after Stress

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundamentals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## Parameters After Stress (OC, 1000C)

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundamentals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## Temperature Dependence After Stress

- Graphite/Ag

- Graphite/Ni

- Ag only

-  Ni only

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundamentals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## Effect of Large Rs (Simulation)

- Effect of Large Rs (Simulation)

- Modest rollover (b = 0.55 eV)

- Two opposite polarity diodes in Series

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundamentals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## Ag and Ni Diffusion (no graphite)

- 1) E. Belas et al, Phys.stat.sol. 4 (2004) 929-932.
- 2) B. Warlick et al, Mater.Sci.Eng. B 71 (2000) 254-257.
- 3) H. Cordes et al, Z.Metallkd.83 (1992) 656-662.
- 4) A. Abken et al, Sol.Energy Mater. Sol. Cells 73 (2002) 391-409.

- Te
- CuxTe ?
- CdTe:Cu
- Cu+

- May 5, 2005

## Surface Analysis Of The Silver Paste

**1**

**2**

| Area | C | Ag | Cl |
| --- | --- | --- | --- |
| 1 | 2.6 | 82.5 | 15.4 |
| 2 | 100 | 0 | 0 |

- Atomic Concentration

- SEM

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundametals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions

## Conclusions

-  No major difference was seen b/n devices made with **graphite/Ag** and **graphite/ Ni** back contacts.

- Devices degraded faster when the graphite layer is removed.
- 	 a) Diffusion of Ag+
- 	 b) Formation of Ni -Te alloy

- The graphite layer binder (polyacrylic acid ) may function as a diffusion barrier, by trapping metals that diffuse from the back-contact.

- LBIC reveals large non-uniformity when Ag metal is used in the absence of graphite. (due to Ag+ diffusion).

- After stress large increase in series resistance that masked the increase in  back-barrier height was observed when graphite is removed.

- May 5, 2005

**Notes:**
I will start with the photovoltaic basics I.e solar cell fundametals
Then, I will talk about the metal-semiconductor contact
And then the effect of the back-contact which is metal-semiconductor contact on the performance of thin film CdTe solar cells.
Finally I will have the conclusions