# Presentation: 177778

## Optical Characterization of ZnO/Ag Structures for Applications as Back-Reflectors in Thin Film Si Solar Cells 

- Contributors
- Deepak Sainju, Nikolas Podraza, Jie Chen, Robert Collins, and Xunming Deng
-  University of Toledo, Ohio
- Maarij Syed, Rose-Hulman Institute of Technology, Indiana

- Research supported by NREL TFPPP Subcontract No. ZXL-5-44205-06

## Outline

- Introduction and motivation
- Experimental details:
- 	Real time spectroscopic ellipsometry (RTSE) of back reflector
- Results and discussion:
-     RTSE study of  Ag and ZnO growth
- 	RTSE study of Ag/ZnO interface
- 	Corroboration by reflectance
- 	ECD back-reflector
- Conclusion and future work

## Introduction

- The a-Si:H-based n-i-p solar cell is deposited on a "back-reflector" consisting of a metal/oxide two-layer structure
- Ag or Al is chosen as the metal whose purpose is to reflect the light not absorbed in the cell in the first pass so that it can be absorbed in successive passes. The metal layer incorporates macroscopic roughness to allow for oblique scattering and longer optical path lengths.
- The ZnO layer on top of the metal prevents diffusion of metal into the cell and acts as an optical spacer.
- With this back-reflector, the multijunction cell will gain ~25-30% of its current from near-ir (~1.2–1.7 eV) absorption.
- These back-reflectors are not perfect, however. Our goal is to identify the origins of optical absorption losses in Ag/ZnO and Al/ZnO back-reflectors that reduce their reflectance and then mitigate these losses through metal/ZnO deposition and processing.

## Origin of Optical Absorption Losses in Back-Reflectors 

- The intrinsic optical absorption in the ZnO and metal
- Physical intermixing due to microscopic surface roughness at the metal/ZnO interface modeled as an effective medium theory
- Chemical intermixing at the metal/ZnO interface formed by magnetron sputtering deposition
- Isolation of Ag "particles" at the Ag/ZnO interface leading to plasmon resonances
-     Hence an understanding of the structure and optical properties of the interface is crucial in determining the optical losses in back reflectors.

## Experimental Strategy 

- Start with the "ideal situation" of ZnO growth on an ultra-smooth Ag surface; characterize the observed interface structure, optical properties, and losses
- Gradually add microscopic surface roughness to the Ag and explore the changes that occur in the interface structure, optical properties, and losses
- Extend into the macroscopic surface roughness regime with the same analysis goals
- Reproduce the ECD/Unisolar back-reflector structure as assessed through ex situ comparisons; perform a complete real time optics analysis of such a structure
-     Apply understanding achieved to reduce optical losses in back-reflectors.

##   Experimental details: real time monitoring of metal/ZnO sputtering with spectroscopic ellipsometer

-   Experimental details: real time monitoring of metal/ZnO sputtering with spectroscopic ellipsometer

- Multichannel spectroscopy: 0.75 ~ 6.5 eV; 706 spectral points
- Fast spectral acquisition:  tacq ~ 32 ms (minimum)

## Structural evolution of Ag films sputtered onto oxidized Si at two temperatures 

- Range of Ag surface roughness: 4 - 55 Å

- control
- of final
- surface
- roughness

## Dielectric functions of Ag and ZnO (grown on c-Si) extracted from spectroscopic ellipsometry 

## Evolution of bulk ZnO and Ag/ZnO interface thickness

- ds(Ag)
- = 12 Å

## Interface dielectric function fitted by sum of Drude, Lorentz, and Tauc + Lorentz components

## Interface properties 

**Particle plasmons**

## Measurement and simulation of reflectance

**Parameters **
**and dielectric**
**functions for **
**reflectance**
**simulation **
**deduced by**
**real time SE**

- ZnO bulk 1500 Å

## Simulation of reflectance for various interface thickness for a thicker ZnO layer

## Initial Analysis of ECD BR

- Ellipsometric spectra for an ECD Ag/ZnO back reflector
- deposited on flexible stainless steel.  The solid lines represent the results of a best fit model shown above.

- Model used to fit the experimental data for the ECD BR, including semi-infinite Ag at the base, two layers representing the roughness between Ag and ZnO, a bulk ZnO layer with some density deficit relative to the reference, and a surface roughness layer on the ZnO.

- surf.
- rough

- Fits need improvement

## Refined Analysis of ECD BRSignificant improvement by incorporating resonance features into the optical functions of the interface region

|  |
| --- |
| SR: ~150-175 A (fixed) |
| Bulk ZnO: void (8 %)
~3275 - 3300 A
 |
| 

Interface: 2 Lorentz + 1 CPPB
~100 A |
| Silver (semi-infinite) |

- Refined Analysis of ECD BRSignificant improvement by incorporating resonance features into the optical functions of the interface region

- ambient

- Significant improvement in fit

## Interface Losses and 

- Interface Losses and
- Reflectance of ECD BR

- Interface dielectric function shows
- evidence of lower energy absorption
- feature possibly due to larger
- isolated Ag structures

- Simulated reflectance deduced from
- the model established by ex situ SE
- shows considerable losses due to
- absorption even in the low energy
- range.  These losses can be ascribed
- to the absorption feature shown
- above.
- The lower measured reflectance
- is due to scattering losses, with the
- difference Rsim  Rexp a measure
- of the scattering losses integrated
- over all solid angles.

## Total Integrated Scattering for ECD BRResults are obtained as the difference between the predicted reflectance from an ex situ SE analysis that is immune to scattering and a measurementof specular reflectance.

- Total Integrated Scattering for ECD BRResults are obtained as the difference between the predicted reflectance from an ex situ SE analysis that is immune to scattering and a measurementof specular reflectance.

## Conclusions and Future Work

- Strong evidence of physical intermixing of Ag and ZnO is seen in the Ag/ZnO interface even starting from ultra-smooth Ag (4 Å roughness).
- Particle plasmon absorption is observed for the interface over the wavelength range where it does not affect solar cell performance. However, when we increase silver surface roughness, the red shift of plasmon peak may fall into the region of interest for solar cell (~1.2 – 1.7 eV), as indicated by ex situ SE studies of ECD BR's.
- In future, Ag depositions with higher surface roughness comparable to that of ECD BR's will be investigated in real time.
- Similar studies of the Al/ZnO back-reflector will be carried out.