# Presentation: 686933

## Estimation of Acceleration

- Factors by
- Comparison of Performance
- During Accelerated Stress
- and in Outdoor Conditions

- R. A. Enzenroth, K.L. Barth, and W.S. Sampath

**National CdTe R&D Team Meeting**
**May 5 – 6 2005 NREL**

## Introduction

**using a thermal activation energy model [1,2]. In the present work an **
**empirical comparison of indoor to outdoor data was used. **
** ****A number of Cds/CdTe solar cells have been exposed to accelerated**
**indoor lightsoak stress and outdoor conditions. In both cases the**
**cells were at OC bias and in a desiccated environment. **
** ****It has been found that during accelerated indoor lightsoak stress that**
**the loss of efficiency saturates at long stress times. There is also an **
**efficiency loss under outdoor conditions [3].**
** ****In order to quantify the decay behavior a comparison of JV data **
**from indoor accelerated stress and outdoor conditions is made. **
** ****By comparison of the indoor to outdoor data an estimate of the **
**acceleration factors for indoor accelerated stress is made. Using**
**these factors predictions of outdoor performance at OC bias is made.**

**Introduction**

## References

- [1] T. J. Mcmahon and G.J. Jorgensen, ”Progress towards a CdTe cell lifetime
- Prediction”, AIP Conf. Series **462** , 1998, 54
- [2] J.F. Hiltner and J.R. Sites, "Stability of CdTe Solar-Cells at Elevated Temperatures:
- Voltage, Temperature, and Cu Dependence," AIP Conf. Series **462**, 1998, 170
- [3] R. A. Enzenroth, K. L. Barth, W. S. Sampath, ”Continuous, In-line processing of
- CdS/CdTe devices: progress towards consistent stability”, proceedings of the 19th
- European PV solar energy conference, 2004, 1980-1983
- [4] K.L. Barth, R. A. Enzenroth, W. S. Sampath, ”Consistent Processing and Long Term
- Stability of CdTe Devices”, proceedings of the 31st IEEE PVSC, 2005
- [5] A.W. Czanderna and G.J. Jorgensen, “Accelerated Life Testing and Service
- Lifetime Prediction for PV Technologies in the Twenty First Century”, NREL/CP-520-26710
- 1999

## Indoor Line Fit η Decay Equation

**(single exponential decay with endpoint constant)**

**Where t is total time in hours**

**[1]**

## Indoor Accelerated Stress Data

**Indoor Accelerated Stress Data**

**[4]**

## Slope in Outdoor Data 

** ****There is a change in average efficiency at times of more than 350 days**
** ****Use this slope to estimate outdoor stability  **

## Comparison of Outdoor Data to

**Indoor Data with Acceleration Factor**

** ****Indoor data (time x axis) was multiplied by an acceleration  factor and compared to outdoor data**
** ****Goodness of fit was quantified by student t- test to see if**
**accelerated indoor and outdoor data have the same slope**

**Line fit equation:**

**[2]**

##  Data smoothed with Savitzky-Golay 4th degree polynomial routine

-  Acceleration factor ψ = 5 found by linear regression of both data sets
-   over time = 6400 ~ 47000 hrs; t-test: t = 0.407 < 1.995 indicates same slope
-  Equation [2] used to accelerate data
-  PSI-Plot 7.5 software used

-  Runs
-   65 and 66

- Blue: outdoor
- Green: prediction
- from indoor

## Blue: outdoor

- Green: prediction
- from indoor

-  Run 72

-  Data smoothed with Savitzky-Golay 4th degree polynomial routine
-  Acceleration factor ψ = 8 found by linear regression of both data sets
-   over time = 872.5 ~ 29679 hrs; t-test: t = 0.46 < 2.06 indicates same slope
-  Equation [2] used to accelerate data
-  PSI-Plot 7.5 software used

## Accelerated and Outdoor Efficiency 

**Accelerated and Outdoor Efficiency **
**Comparison**

-  **Indoor data time is stretched by an acceleration factor ****ψ**
** ****Line fit slopes compared where data overlaps (circled in figures)**
** ****Student t-test done to compare slopes of indoor and outdoor data**
** ****JV data on following slides taken at beginning and end of time spans**
**of circled data**

- Ψ = 5

- Ψ = 8

## JV Changes With Accelerated Stress

**And Outdoor Conditions Runs 65, 66**

-  **7 outdoor cells, 21 indoor cells**

**Voc**

**Jsc**

**ff**

**eff**

**Rs**

**Rsh**

## JV Changes With Accelerated Stress

**And Outdoor Conditions Run 72**

-  **3 outdoor cells, 15 indoor cells**

**Voc**

**Jsc**

**ff**

**eff**

**Rs**

**Rsh**

## Tabular JV data

|  | Voc | Jsc | ff | Eff | Rs | Rsh |
| --- | --- | --- | --- | --- | --- | --- |
| 7233 | -3 | -1.61 | -4.55 | -8.89 | 242 | 63.6 |
| 7236 | -0.19 | -2.39 | -4.93 | -7.48 | 60.8 | 44.3 |
| outdoor | -2.51 | -0.80 | -5.21 | -8.96 | 106 | -41.2 |

|  | Voc | Jsc | ff | Eff | Rs | Rsh |
| --- | --- | --- | --- | --- | --- | --- |
| 6552 | -0.13 | -1.35 | -4.73 | -4.73 | 107 | 80.9 |
| 6652 | -0.49 | -1.49 | -7.14 | -8.99 | 105 | 179 |
| 6649 | -0.83 | -0.31 | -7.85 | -8.21 | 153 | 87.8 |
| outdoor
all | -4.25 | 0.53 | -4.54 | -8.21 | 57 | 5.17 |

**Tabular JV data**
**% change**

-  **Runs 65,66:** **7 outdoor cells, 21 indoor cells**

-  **Run 72: 3 outdoor cells, 15 indoor cells**

- Largest
- divergence

- Increase in
- Rs does
- not have
- associated
- roll-over

## Run 65, 66

- Run 72

-  Efficiency changes are generally largest at OC bias [2,3]
-  It is expected that performance at max power would be better

## Tabular Decay Equation Constants

**Accelerated Indoor Data**

| Run | α | λ | Ψ | Δη | Δt | ηsat |
| --- | --- | --- | --- | --- | --- | --- |
|  |  | [t in hrs] |  | [% of
Initial] | [yrs] | [%] |
| 
7233 and
7236
 | 
3.53 | 
7484 | 
8 | 
-29 | 
~ 17 | 
8.34 |
| 
6552,6652
and 6649 | 
1.64 | 
8836 | 
5 | 
-15.8 | 
~ 25
 | 
9.6 |

**Estimated constants for equation [2]**

## Conclusions

** ****An initial estimation of stress acceleration factors for cells has **
**been developed. This is a necessary step in developing a service **
**life prediction (SLP) and an accelerated life test (ALT) for modules [5].**
** ****JV data comparison of the indoor and outdoor JV data show that**
**the changes to Jsc, ff and efficiency are similar. In both cases**
**there is an increase in Rs, without roll-over. The largest divergence**
**in JV changes is in the Rsh changes for run 72 cells. Overall, JV changes **
**indicate that the prediction of outdoor behavior from accelerated stress **
**is valid. (It would be preferable to have larger data sets of cells outdoors)**
** *****Future work would be the more rigorous comparison of accelerated***
***stress and outdoor JV parameters particularly Rsh values***
** ****Using comparison of indoor to outdoor data, acceleration factors **
**for indoor stress of 5 and 8 were found for two sets of devices.**
** ****With these acceleration factors an estimate of outdoor performance**
**at OC bias was made. These initial estimates predict satisfactory**
**stability of cells for time frames of 17 ~ 25 yrs or more outdoors. **