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A Superior Broadband Light Trapping Technology

Plasmonic Applications for Thin Film PV

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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.

Commercial Considerations

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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.

Commercial Considerations

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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

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Plasmonics Enables Unparalleled Light Concentrating

Plasmonics enables unparalleled light concentration

and light trapping capabilities

Plasmonics allows for simultaneous electrical and

optical functions (light and charge management)

C.F. Bohren, D.R. Huffman, Absorption and Scattering of Light by Small Particles, Wiley, New York. 1983.

Light focusing by a 20nm Aluminum particle

Explanation: Electron oscillations/plasmonics

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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

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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).

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Conventional Light Trapping Schemes

• Efficiencies > 20% have been realized

• Careful surface passivation is required

• Increased optical absorption

• Not ideal for thin film cells

Utilizing Wavelength Scale Texturing to Boost Absorption

Examples from Martin Green Group (UNSW, Australia)

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Nanoscale particles are highly effective for light scattering and trapping “without” absorption

Efficiencies, Q, are normalized cross sections:

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Utilizing Sub-Wavelength Metallic NanostructuresExample from Halas Group (Rice University)

• 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

Measurement of local photocurrent change due to particles:

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Utilizing Sub-Wavelength Metallic NanostructuresExample 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

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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

IQE(λ) = Internal quantum efficiency

T(λ) = Transmission coefficient

Where:

Where: SR(λ) = JPh(λ) / I(λ) = photocurrent generated at λ/spectral irradiance

• Often the product of IQE(λ) and T(λ) is stated as spectral response: SR(λ)

• In initial simulations we have assumed perfect electrical quality (IQE = 100%)

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Example of a Plasmon Enhanced Thin Film Solar CellGoal: 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

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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

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Absorption Enhancements in an E- βPlotAbsorption due to waveguide and particle resonances

• 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)

Plot of absorption enhancement with and without 60 x 80 nm particles on 10 nm oxide

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Optimization of TE and TM Absorption EnhancementsOptimizing absorption for randomly polarized sunlight w/equal TM & TE contributions

• 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)

Plots of absorption enhancement with and without 60 x 80 nm particles on 10 nm oxide

TM Enhancement Map TE Enhancement Map

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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

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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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Spectral Contributions to Plasmon Enhanced PhotocurrentStrong enhancements from light trapping and particle resonances

• Short circuit current vs. wavelength for bare Si slab with and without metallic structures

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Design, Fabrication and Optimization Strategy

• Initial Computational Design

• Deposition on Cells

• Structural and Optical Studies

• Revised Optical Simulations

• Performance Verification

• Cell Fabrication and Test

Optimization Loop

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• 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

Computational Design & Optimization

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• Coatings perform excellent passivation

• Coatings enable light concentration and trapping

• Scalable deposition technology

Coating of Solar Cells

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• Structure: SEM, RBS, and AFM

• Optical: Reflection and elipsometry

• Parameters are extracted: particle size, spacing, shape, metal volume

Structural and Optical Studies

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• Experimental data can be understood with simulations

Simulation Optical Data Using Structural Information

• Simulations provide: reflection, transmission, and absorption data

• Simulations provide suggestions for optimizing particle size, shape, spacing,etc.

Example: Coating with 32 nm average diameter particles

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• Verify that enhancement meets required performance criteria

• Continue to refine design process with new data

Performance Verification

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Flexible Solar Collector Courtesy of Global Solar

Cell Fabrication and Testing