# Presentation: 361036

## Correlation of Material Structure and 

- G. Yue, B. Yan, G. Ganguly, J. Yang, S. Guha
*United Solar Ovonic Corporation, Troy, Michigan*
- C. W. Teplin
*National Renewable Energy Laboratory, Golden, Colorado*
- D. L. Williamson
*Department of Physics, Colorado School of Mines, Golden, Colorado*

**Correlation of Material Structure and **
**Light-induced Stability in nc-Si:H Solar cells**

## Outline

- Background
- Summary of nc-Si:H stability from the literature
- Summary of what we reported previously
- Motivation of this study
- New results
- Experimental
- Stability improvement by hydrogen dilution profiling technique
- Raman analysis of nc-Si:H made under various deposition conditions
- Correlation between cell stability and Raman results
- XRD analysis of nc-Si:H made under various depositon conditions
- Discussion
- Summary

***UNITED SOLAR SYSTEMS CORP.***

## Background 1 - results from the literature

- nc-Si:H is a mixed material of amorphous phase and grains;
- Reported stability of nc-Si:H solar cells:
- No light-induced degradation;
- Degradation rate increases with increasing amorphous volume fraction;

***UNITED SOLAR SYSTEMS CORP.***

## Background 2- summary of our previous reports             (G. Yue will give an invited talk on Tuesday)

** **

**TABLE II.** Summary of light-soaking results for a-SiGe:H bottom cells.

- Background 2- summary of our previous reports             (_G. Yue will give an invited talk on Tuesday_)
- Light soaking under various color filters shows that the metastable changes are in the amorphous and grain boundary regions
- No degradation is observed by forward current injection in the nc-Si:H solar cells
- Reverses bias during light soaking enhances the light-induced degradation in nc-Si:H cells
- Background 3 – motivation of the current study
- To have a better understanding of the correlation of material structure and stability
- To improve the stability by modifying the material structure

***UNITED SOLAR SYSTEMS CORP.***

## New results 1 – Experimental 

- nc-Si:H solar cells were made using RF and MVHF on Ag/ZnO back reflectors with various hydrogen dilution profiles
- nc-Si:H solar cells were light soaked under one sun at 50 °C for over 1000 hours
- Raman measurements were carried out directly on solar cells at NREL using red (633 nm) and green (532 nm) laser excitations
- XRD measurements were carried out directly on solar cells at Colorado School of Mines

## New Results 2 – Solar cell stability

- Table I. Initial (A) and stable (B) performance of nc-Si:H cells. (∆) refers to the percentage of light induced change.

| Run
# | Dep. 
Method | H
Dilution | State | Eff
(%) | Jsc
(mA/cm2) | Voc
(V) | FF |
| --- | --- | --- | --- | --- | --- | --- | --- |
| 10521 | RF | Constant | A | 7.85 | 23.06 | 0.499 | 0.682 |
|  |  |  | B | 6.73 | 22.44 | 0.470 | 0.638 |
|  |  |  | ∆ | -14.3% | -2.7% | -5.8% | -6.5% |
| 10505 | RF | Dynamic
Profiling | A | 7.56 | 22.76 | 0.520 | 0.638 |
|  |  |  | B | 7.30 | 22.91 | 0.517 | 0.616 |
|  |  |  | ∆ | -3.5% | +0.7% | -0.6% | -3.4% |
| 12085 | MVHF | Step
Profiling | A | 6.62 | 21.76 | 0.479 | 0.635 |
|  |  |  | B | 6.06 | 21.65 | 0.470 | 0.596 |
|  |  |  | ∆ | -8.5% | -0.5% | -1.9% | -6.1% |
| 13348 | MVHF | Dynamic
Profiling | A | 7.82 | 22.72 | 0.524 | 0.657 |
|  |  |  | B | 7.72 | 21.85 | 0.527 | 0.670 |
|  |  |  | ∆ | -1.3% | -3.9% | +0.6% | +2.0% |

## New Results 2 – Solar cell stability

- Degradation of the optimized cell is in the short wavelength region.

- _New Results 2_ – Solar cell stability

## New Results 3 - Raman 

- Raman spectra for the cell made with constant H-dilution ratio.
-  More amorphous volume fraction in the bulk than in the top layer;
-  The cell shows a large light-induced degradation.

## New Results 3 - Raman 

- Raman spectra of the cell made
- with H-dilution profiling.
-  More amorphous fraction in the top layer than in the bulk;
-  The cell shows a small light induced degradation.

- _New Results 3_ - Raman

## New Results 4 – Correlation of light-induced degradation to Raman Results 

- _New Results 4_ – Correlation of light-induced degradation to Raman Results

- Blue: initial efficiency
**Grey**: **stable efficiency**

- Top: crystalline
- Middle: intermedium
- Bottom: amorphous

- H dilution: constant, constant, dynamic, step, dynamic, dynamic

- G: green laser
- R: red laser

## New Results 5 – XRD Results 

- _New Results 5_ – XRD Results

- XRD patterns for nc-Si:H cells made with RF glow discharge.  2B10521 and 2B10514 showed a large light-induced degradation, while 2B10505 was relatively stable. The stable cell shows a weaker scattering intensity.

## New Results 5 – XRD Results 

- _New Results 5_ – XRD Results

## New Results 5 – XRD Results 

- _New Results 5_ – XRD Results

- Comments by Don Williamson:
**Lattice parameters**: Only the P(220) was used since it was the most precise. One should keep in mind that the a may be influenced slightly by the unresolved Ag/Zn peak contributing to the nc-Si (220) peak. However, the relative changes should be meaningful. Variations are likely due to differences in residual stress. Larger a implies more compressive stress while lower a implies more tensile stress. Thus, the RF deposited samples seem to be under more compressive stress than the VHF samples, while among the VHF samples, 13348 seems to be more tensile. The VHF samples tend to be more tensile (or less compressive) near edge or corner regions.
**Grain sizes**: There is relatively little variation in the (111) grain size for all samples, about 10 nm. The (220) sizes are consistently larger (12-18 nm) and there is a clear systematic trend for the RF deposited samples – larger sizes for smaller RAP fraction. The (311) sizes in the RF deposited samples seem to be smaller than the VHF deposited samples.
**Relative intensities**: Comparison with the random powder values indicates only slightly preferred orientations (*either* (220) or (111)) for the MVHF samples, with sample 13324 near 62 showing the strongest (220). The RF deposited sample all showed significantly stronger (220) preferred orientations than the MVHF samples. The relative intensity of the RAP peak may be a good indication of the amount of residual amorphous phase. Among the MVHF samples it is highest in sample 13348 (stable) and smallest in the sample 12085. Among the RF deposited samples it is highest in the sample 10505 (stable) and smallest in the sample 10521. Also, for the MVHF samples, the regions away from the center tend to have a larger RAP fraction. Comparison of these tendencies with the Raman data shown in the figures provided indicates excellent qualitative agreement, supporting the correlation of the RAP with the residual amorphous phase.

## New Results 6 – Summary of the observations

- nc-Si:H cells made with constant hydrogen dilution show a larger light induced degradation than those made with the optimized hydrogen dilution profile.
- Raman results show that cells made with constant hydrogen dilution exhibit crystalline evolution. This evolution is modified by hydrogen dilution profiling.
- The overall crystalline volume fraction, especially at the i/p interface region,  is lower in the nc-Si:H cell made with the optimized hydrogen dilution profile than those made with constant hydrogen dilution.
- XRD results show that the nc-Si:H cells made with the optimized hydrogen dilution profile have a large residual amorphous component than those made with constant hydrogen dilution.

## New Results 6 – Discussions

- _New Results 6_ – Discussions

- The amorphous volume fraction may not be the main factor for determining the stability of nc-Si:H solar cells.
- The amorphous structures in the optimized nc-Si:H cell may be different from the a-Si:H structure that we normally think. The amorphous structure changes with the deposition condition.
- The grains with a size smaller than the Raman detection limit or intermediate orders may play an important role in determining the stability of nc-Si:H solar cells.
- The distribution of the amorphous component along the growth direction may be another factor for determining the light-induced degradation.

## Summary

- We have improved the stability of nc-Si:H cells by optimizing the hydrogen dilution profiling
- Our previous results show that the light-induced defect generation are mainly in amorphous phase or grain boundary region
- However, the amorphous volume fraction is not the main factor for determining the degradation. The degradation is also related to the structure and distribution of the amorphous phase, as well as the properties of the grain boundaries
- Small grains or intermediate orders may play important roles. Further investigation is necessary

***UNITED SOLAR SYSTEMS CORP.***

## Acknowledgments

- _Acknowledgments_
- The continuous scientific and technical supports from the team members are greatly acknowledged
- We appreciate the financial support from NREL and DOE