Matter, Inc. describes a plasmonic technology that can substantially improve the efficiency of thin film solar cells. Simulation results show the technology is capable of increasing short circuit current by almost 50% through enhanced light trapping. The plasmonic structures are robust, scalable to deposit, and enable simultaneous light concentration and charge extraction. An ongoing development cycle aims to optimize coatings and fabricate prototype thin film cells to validate the performance enhancements predicted by simulations.

1. Plasmonic Applications for Thin Film PV
A Superior Broadband Light Trapping Technology
Matter, Inc. © 2008 Confidential

2. Commercial Considerations
Our technology provides substantial efficiency enhancements and performance improvements in thin
film solar cells. First order optimization results demonstrate close to 50% increase in short circuit
current based on an ideal thin cell operating at 100% electrical efficiency. Optimization and
development can substantially improve performance.
Results were derived from testing actual depositions and generating simulations based on
experimental data.
First order metallic nanostructures were deposited on an independent substrate interface using a
sample silicon wafer characterized as 100 mm P<100> 381±15µm, 5-8.5 Ω-cm SSP with 20 nm
thermal oxide.
Matter, Inc. © 2008 Confidential

3. Commercial Considerations
Commercial grade sputtering equipment was used to deposit controlled structures and coatings. Full-
field electromagnetic simulations accurately predict the performance of metallic nano particle
coatings.
We have an ongoing development cycle for optimization and fabrication of operating prototype thin
film cells. These will be expressed in a-Si-ITO with optimized designed thickness of 2-500 nm. They
may be significantly thinner than commercial models allowing substantial material cost savings.
Integration in the fabrication process could incorporate our coatings in a modified ITO deposition
stage.
Since our technology is essentially substrate independent it can be designed and optimized for Cigs /
CdTe or any other thin film cell.
Matter, Inc. © 2008 Confidential

4. An Obvious Need for Photon Management in Thin Film PV
A large mismatch exists between electronic and photonic lengthscales
 Thick cells are desirable from a photonics standpoint to enable efficient light absorption
 Thin cells are desirable from an electronics standpoint to enable efficient charge extraction
A radical new technology is required to match both length scales…
…and to break open the barriers towards substantially higher efficiencies
The rapidly developing field of Plasmonics offers the right ingredients for this task
Matter, Inc. © 2008 Confidential

5. Plasmonics Enables Unparalleled Light Concentrating
Light focusing by a 20nm Aluminum particle

Plasmonics enables unparalleled light concentration
and light trapping capabilities
C.F. Bohren, D.R. Huffman, Absorption and Scattering of
Light by Small Particles, Wiley, New York. 1983.
Explanation: Electron oscillations/plasmonics

Plasmonics allows for simultaneous electrical and
optical functions (light and charge management)
Matter, Inc. © 2008 Confidential

6. Plasmonic Structures are Robust and Scalable

Plasmonic structures and coatings are robust
in harsh environments

Plasmonic structures can be generated using
inexpensive, scalable deposition techniques
Matter, Inc. © 2008 Confidential

7. Plasmonics Offers Simultaneous Electrical & Optical Functions
• Metal dielectric interfaces support
surface plasmons (“light”)
• Metals exhibit high electrical
conductivities
Metals enable simultaneous charge extraction and light concentration / trapping
Rashid Zia, Jon A. Schuller and Mark L. Brongersma, Materials Today 9, 20-27 (2006).
Matter, Inc. © 2008 Confidential

8. Conventional Light Trapping Schemes
Utilizing Wavelength Scale Texturing to Boost Absorption
Examples from Martin Green Group (UNSW, Australia)
• Efficiencies > 20% have been realized
• Careful surface passivation is required
• Increased optical absorption
• Not ideal for thin film cells
Matter, Inc. © 2008 Confidential

9. Nanoscale particles are highly effective for light
scattering and trapping “without” absorption
Efficiencies, Q, are normalized cross sections:
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10. Utilizing Sub-Wavelength Metallic Nanostructures
Example from Halas Group (Rice University)
Measurement of local photocurrent change due to particles:
• Bright spots indicate current enhancement and dark spots indicate a reduction
• Photocurrent is increased at some wavelengths and reduced at others
• It is possible to attain a boost in the overall energy conversion efficiency
Matter, Inc. © 2008 Confidential

11. Utilizing Sub-Wavelength Metallic Nanostructures
Example from Yu Group (UC San Diego)
• Results are very encouraging
• Simulations do not include entire structure (just particle response)
• Measurement not with a Solar Simulator (halogen bulb)
• We can further optimize and scale this technology to large areas
Matter, Inc. © 2008 Confidential

12. Rational Design of a Plasmon Enhanced AR Coating
Simulations to optimize absorption in Si and Jsc
Jsc = Generated short circuit current density
I(λ) = Spectral irradiance
Where: IQE(λ) = Internal quantum efficiency
T(λ) = Transmission coefficient
• Often the product of IQE(λ) and T(λ) is stated as spectral response: SR(λ)
Where: SR(λ) = JPh(λ) / I(λ) = photocurrent generated at λ/spectral irradiance
• In initial simulations we have assumed perfect electrical quality (IQE = 100%)
Matter, Inc. © 2008 Confidential

13. Example of a Plasmon Enhanced Thin Film Solar Cell
Goal: Quantify and Optimize Absorption Enhancement in a thin Si layer
• Incident light is assumed to be randomly polarized (equal TM and TE contributions)
• Metal stripes enhance absorption by a) Coupling to waveguide modes of Si slab; b) Exploiting
plasmonic resonances of metal stripes
• Both effects can operate in unison
• Metal stripes can assist in the current extraction as well
• Metals are separated from the Si layer, enabling good passivation
Matter, Inc. © 2008 Confidential

14. Full-field Simulations Showing Different Coupling Regimes
Periodic array of Ag nano-stripes on top of a 50 nm Si slab
• Illustration of both types of couplings for TM polarization
• Plots show normalized absorption enhancement (80 nm wide x 60 nm thick particles)
• Resonances can be engineered for maximum enhancement
Matter, Inc. © 2008 Confidential

15. Absorption Enhancements in an E- βPlot
Absorption due to waveguide and particle resonances
Plot of absorption enhancement with and without 60 x 80 nm particles on 10 nm oxide
• Enhancement is on a 10 Log scale: Red and Yellow areas provide strong enhancement
Blue area corresponds to absorption reduction
• Highest absorption enhancement occurs for relatively small β (periods around 315 nm)
Matter, Inc. © 2008 Confidential

16. Optimization of TE and TM Absorption Enhancements
Optimizing absorption for randomly polarized sunlight w/equal TM & TE contributions
Plots of absorption enhancement with and without 60 x 80 nm particles on 10 nm oxide
TM Enhancement Map TE Enhancement Map
• Enhancement is on a 10 Log scale: Red and Yellow areas provide strong enhancement
Blue area corresponds to absorption reduction
• Highest absorption enhancement occurs for relatively small β (periods around 315 nm)
Matter, Inc. © 2008 Confidential

17. Spectral Contributions to Total Short Circuit Current
I(λ) is the solar spectral irradiance
SRBare(λ) is the spectral response of the bare Si
slab without metallic nanoparticles
Π(λ) is the absorption enhancement
provided by the metal as calculated in the
previous slide
Matter, Inc. © 2008 Confidential

18. Total Short Circuit Current Enhancement
Calculated from Spectral Contributions
• Enhancements of approximately 45% are obtainable in a first optimization round
• Higher enhancements can be obtained with an optimized development
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19. Spectral Contributions to Plasmon Enhanced Photocurrent
Strong enhancements from light trapping and particle resonances
• Short circuit current vs. wavelength for bare Si slab with and without metallic structures
Matter, Inc. © 2008 Confidential

20. Design, Fabrication and Optimization Strategy
• Initial Computational Design
• Deposition on Cells
• Structural and Optical Studies
Optimization Loop
• Revised Optical Simulations
• Performance Verification
• Cell Fabrication and Test
Matter, Inc. © 2008 Confidential

21. Computational Design & Optimization
• State of the art full-field simulations (FDTD and FDFD)
• Coatings designed for specific thin film cells
• Any semiconductor material/PV cell can be modeled
Matter, Inc. © 2008 Confidential

22. Coating of Solar Cells
• Coatings perform excellent passivation
• Coatings enable light concentration and trapping
• Scalable deposition technology
Matter, Inc. © 2008 Confidential

23. Structural and Optical Studies
• Structure: SEM, RBS, and AFM
• Optical: Reflection and elipsometry
• Parameters are extracted: particle size, spacing, shape, metal volume
Matter, Inc. © 2008 Confidential

24. Simulation Optical Data Using Structural Information
• Experimental data can be understood with simulations
Example: Coating with 32 nm average diameter particles
• Simulations provide: reflection, transmission, and absorption data
• Simulations provide suggestions for optimizing particle size, shape, spacing,etc.
Matter, Inc. © 2008 Confidential

25. Performance Verification
• Verify that enhancement meets required performance criteria
• Continue to refine design process with new data
Matter, Inc. © 2008 Confidential

26. Cell Fabrication and Testing
Flexible Solar Collector Courtesy of Global Solar
Matter, Inc. © 2008 Confidential