Flexible Solar Cell Stacked subcells

Organic heterojunction

Ag

Subcell compositionTransparent,nanopatterned electrodes Metallic nanostructures

Nanostructured Metal-Organic Composite Solar CellsPIs: Mark Brongersma (MSE), Peter Peumans (EE) and Shanhui Fan (EE)

• Peumans group (Electrical Engineering):Organic Photovoltaics; Nanofabrication

• Brongersma group (Material Science and Engineering): Plasmon optics; Near-Field Characterization.

• Fan group (Electrical Engineering): Theory and simulations of nanophotonic devices.

The

Sci

ence

The

Team

Combining Photovoltaics with “Plasmonics”

Silicon and Compound Semiconductor Cells• High efficiency (~24-36%)• High cost.

Organic Solar Cells• Low efficiency (~ 5%)• Low cost

Photos from DOE and B. Kippelen, Georgia Tech

Plasmonics• Metals enable manipulation

of light at the nanoscale !

Exciting New Opportunity !• Combine the fields of plasmonics

and photovoltaics• Organics and metals exhibit a good

materials & processing compatibility • Organic cells offer a very high

possible increase in efficiency

Operational Principle of Organic Solar CellsUnderstanding the operation of a solar cell is key to designing improvements

Transparent contact

Donor Acceptor Back contact

Photon

• A typical cell operation

E

x

• Band diagram

1. Photon absorption2. Exciton diffusion3. Charge transfer4. Charge separation5. Carrier collection

Operational Principle of Organic Solar CellsTwo key problems:

Donor Acceptor

Photon 1

• Short exciton diffusion length (∼ 5-10 nm)

• Transparent contacts need to be high conductivity and low cost

Solution: Realize virtually transparent metal contacts with

High quality (\conductivity) ITO is expensive and typically a lower grade is used

the conductivity of metalsthe transparency of glass or ITO

Photon 2

Transparent contact

Useful region∼ 10 nm

Solution: Create excitons near the D\A interface

Metal (Plasmonic) Nanostructures offer Solutions!

• Metals can be used in unexpected ways…….

• Colorful Czech glass vase• Ag nanoparticles cause yellow coloration• Au nanoparticles cause red coloration

The unique optical properties of metals have been exploited for centuries !

E-field+++++-- - --

Surface plasmon=

Collective electron motion

Plasmon Excitations in Metallic Particles

Excitation of a Single Metallic Nanoparticle

( ) ( )( ) ( )

'3/ 2'

0 2 2, ' ,,9

2M

ext H

M H M

Vc

ε ωωσ ω εε ω ε ε ω

=⎡ ⎤+ +⎣ ⎦

2 Hn

εM = ε’M + iε’’M

εH = ε’H =

Volume = V0

Particle

Host matrix

E-field+++++

-- - --

300 400 500 600 700 800 9000

100

200

300

400

500

n=3.3n=1.5

1

R = 5 nm

σex

t (nm

2 )

λ (nm)

4 3.5 3 2.5 2 1.5

Energy (eV)

Ag clusterD = 10 nm

Properties of a single metallic nanoparticle

Origin Enhanced Absorption Cross-section

Energy flux (Poynting vector) for a plane wave incident on a metallic nanoparticle

Poynting vector

Off resonance On resonance

σabs

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

Enhanced Absorption Due to Metal Nanoparticles

Ag nanoclusters enhance absorption of organic thin films

B. Rand, P. Peumans and S. R. Forrest, Journal of Applied Physics, (2004)

Ag

CuPc

• Reason: field concentration and energy storage by the Ag particles

• This effect can be exploited to preferentially create excitons near the D/A interface

CuPc + Ag

Metal Nanostructure Enhanced Solar Cell

n(x,y)

• Zig-zag configuration concentrates electromagnetic power at active junction

Transparent Contact

Transparent Contact

10nm20nm

Geometry Exciton DensityAbsorbed PowerDensity

|E|

PTCBI CuPc

• Finite-element models for electromagnetic waves + exciton diffusion

Peumans group

Metal Nanostructure Enhanced Solar Cell

300 400 500 600 700 800 900Wavelength [nm]

Gen

erat

ed P

hoto

curr

ent [

arbs

.]

Air cylinders

no metal

Ag cylinders

CuPc

PTCBI

nano-cylindersMetal antenna effects

Metal Nanostructure Enable Spectral Tuning

Selection of metal allows for spectral tuning of organic solar cell

Ag

Au

no metal

300 400 500 600 700 800 900Wavelength [nm]

Cu

Pho

tocu

rren

t [ar

bs.] Al

• Spectral tuning can be employed in multi-junction stacked cells

Importance of Coating the Metallic Particles

400 450 500 550 600 6500

2

4

6

8

10

12

14x 105

emis

sion

w/o Agw/ Ag+SiO2w/ bare Ag

Wavelength [nm]

Pho

tolu

min

esce

nce

[arb

s.]

Ag coreSiO2 shell

TEM Image

No Ag nanoparticles

With Ag/SiO2 nanoparticles

With Ag nanoparticles

Charge transfer and separation should only occur at the D/A interface

• SiO2 shell is essential (bare Ag particles cause exciton recombination as seen in PL)

• Ag/SiO2 particles embedded in organic thin films enhance absorption

100nm

20nm

0

0.5

1

1.5

2

300 400 500 600 700 800 900Wavelength（nm）

Abs

orpt

ion

(a.u

.)

AuNPs AuNRs

Spectral Tuning by structure

20nm Au spheres 8nm spheres

Au nanorods

Au Au Au

Au

Electroless silver growth solution

Sonication removes goldPolycarbonate dissolved

in chloroform

Evaporated Gold Seed layer

Porous Polycarbonate Membrane

Many free wires

Ethanol

Chloroform

Synthesis of metallic nano-antennas

Characterization of the Nanorod Antennas

• Nanorods with a few 10s of nm diameter and up to several µm have been synthesized

• Darkfield microscopy shows resonant light matter interaction

• A spectrometer is currently built onto the microscope to allow measurement of scattering spectra on single particles

5 µm

5 µm200 nm

SEM Overview SEM Close up Dark-field scattering image

Goal: fabricate rods of about λ/2 with a strong ligh-matter interaction

Ed Bernard, Brongersma Group

What is a surface plasmon (polariton) ?

• Compare electron gas in a metal to real gas of molecules

• Metals are expected to allow for electron density waves: plasmons

• This resembles a light wave propagating along a metal surface !

Can be subwavelength! (< 30 nm)

Metal

Dielectric

z

I

Potential to have a strong interaction of light with adjacent organic layers !

D

H

Surface Plasmons at Metal-Dielectric Interface

50 µm

CCDLASER

Photon Scanning Tunneling Microscope (PSTM)

Cantilever

Au film

High NA objective

λ/2 Waveplate

= 50 nm

Light Detector

More info: www.WITec.de

2 µmAu film

Strong Analogy between Light and Surface Plasmons

• Diffraction limit for lateral confinement in metal stripe waveguides !

1773 - 1829

Glass substrate

Au

SPP excitedby ATR

Cantilever

Photodetector

PSTM Image EM Simulation

Paper submitted

Young’s double slit experiment for surface plasmons

Enhanced Transmission through Sub-λ Apertures

T.Thio et al., Optics Letters 26, 1972-1974 (2001).

• Ag film with a 440 nm diameter hole surrounded by circular grooves

• 3x more light than directly impingent on hole !

• Reason: Excitation of surface plasmons

• Transmission enhancement of 10 x compared to a bare hole

Work we were inspired by…

Nano-patterned Transparent Metal Contacts

H. Shin, P. Catrysse, S. Fan, Physical Review B, 72, 085406 (2005)

R. Pala, M. L. Brongersma et al (unpublished)

Fabrication and analysis of transparent electrical contacts

?

2 µmAg/SiO2

Tran

smitt

ance

Wavelength (nm)200 400 600 800 1000

1.0

0.5

0

Full field simulation

Exciting Surface Plasmons Inside an Organic Cell

Surface Plasmon excitation can be exploited to further improve efficiency

• Trick: surface plasmons increase interaction length !

D

A

Patterned metal contact

• Can we demonstrate plasmons increase absorption ?

D

A

Transparent contact

Transparent contact Patterned metal contact

Light propagates normal to A/D interface SPs propagate along the A/D interface

100X OBJ

50X OBJ

SP

Al film

CCD

SiO2

Laser

inout

10 µm

• Determine the propagation length l

• Investigated the change in l

• Fits the theory

in

out

Grating Transmission and Plasmon Excitation

Al film

PMMA with R6G dye

Light detector

Nanopatterned metal film

Free space light beam

Tip

Objective

200nm slits, 1.25µm pitch

Transmission Grating

• The near-field intensity determines the actual absorption near the A/D interface

• What do we measure with our near-field optical microscope (PSTM)

We have started investigating a number of calibration samples

Measuring Grating Transmission in the Near-field

Determination of the near-field intensity is key !

Rashid Zia

Near-field Image Resembles Hz !

E

x

y

zx

10 µm

0 10 µm0

10 µm

5 µm0.0 µm 7.5 µm

-7.5 µm

|Hz|

I (a.u)

I (a.u)

1

0

1

0y

xz

x

y

Experimental cross-section

FDTD simulation of HZ

0.0 µm

Combined EM and Exciton Diffusion Simulations

Peumans Group

Electric Field Exciton Density

Wavelength (nm)400 1000

Pho

to c

urre

nt

• Finite-element models for electromagnetic waves + exciton diffusion

Initial simulations of the entire system indicate possible improvements

• Higher improvements can be obtained for a carefully engineered grating

Summary and Outlook

• The realization of highly conductive, yet transparent, nano-patterned metal film contacts.

• The use of metal nanostructures as efficient antennas for capturing and concentrating solar energy.

• Future idea: Integration of various organic photovoltaic subcells into highly efficient multi-junction stacks (pull current out the side)

Flexible Solar CellStacked subcells Organic

heterojunction

Subcell compositionTransparent,nanopatterned electrodes Metallic nanostructures

Ag

Never underestimate

the Power of Plasmonics !

Summary Continued

• Great potential of enhancing efficiency of organic photovoltaic cell through plasmonics.

• We are beginning to understand the complex interplay between metallic nanostructures and organic photovoltaic cells.

Thanks to GCEP for the Support!

DONOR

ACCEPTOR

IP

EA

D: CuPc (Copper Phthalocyanine)

A: PTCBI (3,4,9,10 perylenetetracarboxylicbisbenzimidazole)

LUMO

HOMO5.

2eV

3.5e

V

6.1e

V

4.6e

V

0.9e

V

1.7e

V

1.7e

V

VacuumLevel

Organic Solar Cell Operation