Doing the Work: Farzaneh Afshinmanesh, Ed Barnard, Linyou Cao, Pengyu Fan,

Kevin Huang, Ragip Pala, Jon Schuller, Min-Kyo Seo, Isabel

Thomann, David Schoen, Alok Vasudev,.

Speaking: Mark Brongersma @ Stanford University

Thank you: McGehee Group

Yi Cui Group

Bruce Clemens group

Funding: GCEP, KAUST, DOE, AFOSR

Plasmonics and Mie Scattering

for Solar Energy Harvesting

Electronics

1st transistor and IC

Current technology

Linyou Cao et al., Nano Lett., 2649, 10 (2010)

Semiconductor nanostructures

Semiconductor wafers

Optical Properties Semiconductors

30 nm 185 nm

Optical Properties Metals

Halas Group @ Rice

Metallic nanostructures

Metal wire

E

New Optical Properties Naturally Emerge at the Nanoscale

Solar

?

Measurement of the light absorption in a germanium nanowire

Optical Properties of a High Index Semiconductor Nanowires

The measurement

Challenge: Wires are small compared to the diffraction limit……

L. Cao, J.S. White, J-S Park, J.A. Schuller, B.M. Clemens, and M.L. Brongersma, Nature Mat. 8, 643-647 (2009).

Solution: Light absorption in designed semiconductor nanostructures is naturally enhanced

(With Bruce Clemens Group)

P

ho

tocu

rrent (

a.u

.)

Wavelength (nm)

Photocurrent shows strong enhancement peaks in the photocurrent

Spectral Photocurrent Response of Ge nanowires

Spectral photocurrent measurements on Ge nanowires of different radius

L. Cao, J.S. White, J-S Park, J.A. Schuller, B.M. Clemens, and M.L. Brongersma, Nature Mat. 8, 643-647 (2009).

R=10 nm R=25 nm R=110 nm

Optical resonances give rise to absorption enhancements

Spectral Photocurrent Response of Ge Nanowires

Mie theory can quantify the absorption efficiency

10 nm radius

25 nm radius

110 nm radius

Simulated

Experiment

L. Cao et al., Nature Mat. 8, 643-647 (2009).

Qabs = σabs/σgeom = optical size/physical size

Intuitive resonance condition:

mλeff = 2πr

Simple optimization procedure

Engineering Better NW Photodetectors and Solar Cells

10 nm radius

25 nm radius

110 nm radius

Simulated

Experiment

1

2

3

4

1

2

3

4

Tunable Optical Properties of Si Nanostructure Arrays

Si nanowire arrays can be used to produce structural colors

1µm

170 nm width

200 nm width

240 nm width

300 nm width

Linyou Cao et al., Nano Lett., 2649–2654, 10, 2010.

Every High-index Semiconductor Nanostructure Exhibits Optical Resonances

L. Cao et al., Nano Lett., 10, 439–445 (2010).

Resonant enhancements in light absorption occur in a wide variety of sub- shapes

Example: one-wavelength or dipolar resonances in Si nanowires of different cross sectional shape

Similar behavior was observed for CIGS, Ge, CdTe, aSi, and GaAs.

130 nm

t=130nm

Patterned a-Si with 130 nm beams

Unpatterned a-Si film

• Simple patterning step can improve efficiency by 25%..after removing 50% of the material !

Application to Solar Cells

A simple thought experiment….

t=130nm

Performance of a “cell”

Linyou Cao et al., Nano Lett., 10, 439–445 (2010)

s

Patterning Transparent Conductive Oxides (TCOs)

Alok Vasudev et al, Optics Express 20, A385 (2012)

TCO electrodes are present on many types of solar cells

ZnO Nanobeam Array Outperforms Optimized Planar ZnO layer

300 nm aSi solar cell with either a 240 nm ZnO thick film or a 240nm x 240 nm beams

ZnO

Transparent Electrodes: A Great Use of Metallic Nanostructures

Lee, Peumans et al. Nano Letters 8, 689 (2008)

Wu et al. Nano Letters 10, 4242 (2010)

Plasmonic Nanowelding of a Ag Nanowire Mesh (Led by Erik Garnett in McGehee, Cui, and Brongersma Groups)

Ag nanowires synthesized

by polyol, chemical process

Regular furnace anneal

New approach: plasmonic nanowelding

500 nm

200 nm

Tungsten halogen lamp

I = 30W/cm2

Erik Garnett et al., Nature Materials (2012 )

500 nm

TEM Before and After Plasmonic Nanowelding

50 nm

Conclusion: Bottom nanowire always recrystalizes onto top nanowire at the junction

After welding: Twinning defects continue through the junction only for top nanowire

50 nm

Before welding After welding

As synthesized: Ag nanowires feature a pentagonally twinned crystal structure

Erik Garnett et al., Nature Materials (2012 )

Before melting : 2 nm gap

After melting: – 2 nm overlap

Self-limited Plasmonic Nanowelding Simulations

10

0 n

m

Erik Garnett et al., Nature Materials (2012) and consistent w work by Nordlander, Aizpurua, Garcia de Abajo,…

Heat generation focused in bottom wire near junction

+

+ + - - -

E

Heat generation is self-limited !

gap

k

+ + - - + - - - + - -

Junction resistance is similar to the single wire resistance

Resistance of Welded Wires

Erik Garnett et al., Nature Materials (2012 )

Determining the resistance of two welded Ag nanowires

We can measure single on resistance is similar to the wire resistance

2 µm

Electrical and Optical Properties of Welded Wires

IV curves demonstrate cell survives

welding and improves upon welding

Nanowire meshes can be welded onto

low thermal budget organic solar cells

Erik Garnett et al., Nature Materials (2012)

Ag nanowire mesh on organic cell IV Characteristics semitransparent

Electrical and Optical Properties of Welded Wires

NW meshes with reasonable sheet-conductivities can be welded on heat sensitive saran wrap

Erik Garnett et al., Nature Materials (2012 )

RS = 580 /

Welding nanowire meshes on flexible substrates

Summary and Conclusions

New plasmon science facilitates self-limited welding of Ag NW transparent electrodes

From fundamental process to single NW conduction measurements to first solar cells

50 nm 2 µm

High index semiconductor nanostructures exhibit strong light matter inteaction

From fundamental process to single NW photocurrent measurements to new solar cells

Optical Nanowelding: A Plasmonic Effect

Heating is most effective when E-field is polarized to enable surface plasmon excitation

Heating occurs near the surface plasmon resonance frequency of an individual Ag nanowire

Can Metallic Structures be Used as Transparent Electrodes ?

Lee, Peumans et al. Nano Letters 8, 689 (2008)

Wu et al. Nano Letters 10, 4242 (2010)

Plasmonic Nanowelding of a Ag Nanowire Mesh (Erik Garnett in McGehee, Cui, and Brongersma Groups)

Ag nanowires synthesized

by polyol, chemical process

Regular furnace anneal

New approach: plasmonic nanowelding

500 nm

200 nm

Tungsten halogen lamp

I = 30W/cm2

Erik Garnett et al., Nature Materials (2012 )

500 nm