Stretchable and FlexibleStretchable and FlexibleSiliconSilicon--based Solar Cell Arraysbased Solar Cell Arrays

Y. Huang

Depts. of Civil and Environmental Eng. and Mechanical Eng.Northwestern University

Collaborator: J.A. Rogers (University of Illinois)

Challenges and ObjectivesChallenges and ObjectivesChallenges and ObjectivesChallenges and Objectives

90% of solar cells are made of Si90% of solar cells are made of Si.Si has good efficiency and reliability.Si is brittle, and fractures at 1% strain.Si panel can only be placed at roof topSi panel can only be placed at roof topSi panel can only be placed at roof top, …Si panel can only be placed at roof top, …

Objectives:Objectives:Make Si stretchableInvestigate the mechanics issuesSeek innovative applications (e g e-eye cameraSeek innovative applications (e.g., e eye camera, flexible solar cell, cardiac electrophysiology, …)

News agencies:United Press I t ti lInternational, …

Display at the p yTech Museum of Innovation, Silicon Valley, California

Khang, Jiang, Huang, and Rogers, Science 311, 2006

Wavy Silicon StructuresWavy Silicon StructuresWavy Silicon StructuresWavy Silicon Structures

LdL LdLpre =εPDMS

Si PDMS

εmother wafer: Si

Fabricate thin ribbonSi device elements

Bond elements to prestrainedelastomeric substrate PDMS

Peel back PDMS;flip over

preε

Si device elements elastomeric substrate PDMS flip over

Flat Si ribbon becomes buckled due to prestrained PDMS

Spring vs Straight WireSpring vs Straight WireSpring vs. Straight WireSpring vs. Straight Wire

Wavy Silicon: Experimental ImagesWavy Silicon: Experimental ImagesWavy Silicon: Experimental ImagesWavy Silicon: Experimental Images

Scanning electron micrograph Optical image

Periodic wavy structures

Initial Buckling AnalysisInitial Buckling Analysis

1prehAε %9.0=preε

Khang, Jiang, Huang, Rogers, Science, 2006

Sihπλ =

10 −=c

preSihA

ε

1 0

1.5

e (μ

m)

%9.0preε

independent of pre straincε

λ =0

0.5

1.0

Ampl

itude

3231

⎟⎟⎞

⎜⎜⎛ PDMSEε iti l t i f

--- independent of pre-strain

1−= prehAε

0.0

A

60

μm)

4 ⎟⎟⎠

⎜⎜⎝

=Si

PDMSc Eε --- critical strain for

buckling10 −=

cSihA

ε

20

40

elen

gth

(μ

h

0 200 4000

20

Wav

e

c

Sihε

πλ =0

Si thickness (nm)Excellent agreement with experiments without any fitting parameters.

Stretchable Silicon at The Tech San Jose CAStretchable Silicon at The Tech, San Jose, CA

Electronic Eye CameraElectronic Eye CameraElectronic Eye CameraElectronic Eye CameraTV Networks: ABC, BBC, CBC, MSNBC, …

News agencies: AFP (France), Reuters, United Press International, Xinhua, …

Newspapers: Chicago T ib D il T l hTribune, Daily Telegraph (UK), …

Magazines: Discover gMagazine, MIT Technology Review, Nature News & Views, Scientific American, U.S. News and World Report, …

Ko et al., Nature 454, 2008

NSF Press Release

Fabrication StepsFabrication Steps

compressedinterconnect

~1 cm

compressedinterconnect

~1 cm

form hemispherical PDMS transfer element

~1 cm

adhesiveadhesive

radially stretch PDMSform Si focal plane arrayand release from underlyingwafer substrate

cure adhesive; flop over substratecure adhesive; flop over substrate

integrate optics &interconnect to control electronics to complete the device

integrate optics &interconnect to control electronics to complete the device

compressibleinterconnect

transfer focal plane array onto PDMS

hemispherical focal plane arrayhemispherical focal plane arraySi device island(photodetector& pn diode) Ko et al., Nature 454, 2008

Pop Up Si Structure: Experimental ImagesPop Up Si Structure: Experimental ImagesPop Up Si Structure: Experimental ImagesPop Up Si Structure: Experimental Images

Scanning electron micrograph

The interconnects buckle to form “big wave” structure.

Ko et al., Nature 454, 2008

Pop Up Structure: ResultsPop Up Structure: ResultsPop Up Structure: ResultsPop Up Structure: Results-4 10ε ×

8yyε-4 10ε ×

8yyε864

yy 864

yy

( )2 3max

2 0

12 island bridge bridge

island pre

E hE h Lν

ε π ε−

≈20-2xxε20-2xxε

island island bridgeE h L

20 μm

-2

y 20 μm

-2

y

17.5 μmx 17.5 μmx

10 μm10 μm

The island remains almost flat and experiences small strains.The shape of interconnects agrees well with experiments.

Electronics on complex surfacesElectronics on complex surfacesElectronics on complex surfacesElectronics on complex surfaces

Stretchable Circuits

(Science 2008)( )• Stretchability: up to 140%

• TwistabilityTwistabilityTV Networks: BBC, CBC, …

Magazines: Discover Magazine, MIT Technology (PNAS 2008)

g g , gyReview, Newsweek, Scientific American, …

NSF Press Release

Bendable and Transparent Silicon Photovoltaic Devices

(Nature Materials 2008)• User-definable transparency• High degrees of mechanical flexibility• Ultra-thin form factor micro-concentrator design• Excellent reliability• Good efficiency• Inexpensive

TV Networks: ABC, MSNBC, …News agencies: Reuters, Xinhua, …Newspapers: New York Times, People’s Daily…

(Nature Materials 2006)Magazines: MIT Technology Review, Nature News & Views, Scientific American, …Department of Energy Press Release

Microscale, Inorganic Light Emitting Diode (iLED)Microscale, Inorganic Light Emitting Diode (iLED)

•Advantages: flexible high brightness(Science 2009)

TV N t k ABC BBC MSNBC•Advantages: flexible, high brightness, long lifetime, bidirectional emission, inexpensive.

li i bl h l h i

TV Networks: ABC, BBC, MSNBC, …

News agencies: Reuters, …

Newspapers: Boston Globe New York•Applications: wearable health monitor and diagnostics, biomedical imaging devices, ...

Newspapers: Boston Globe, New York Times, …

Magazines: MIT Technology Review, …

Cardiac/Neural ElectrophysiologyCardiac/Neural Electrophysiology(Science, 2010; Science Translational Medicine, 2010; Nature

1 cm 75μm 25μm

( , ; , ;Materials, 2010)

unwrapped unwrapped2.8μmunwrapped

7.0μm2.8μm

unwrapped

wrappedwrapped

0.3 cm 75μm

wrapped

Health monitoringand human-electronics i t tiinteraction

Conclusions and Recent PublicationsConclusions and Recent PublicationsConclusions and Recent PublicationsConclusions and Recent Publications-- Mechanics plays an important role in the

d l f fl ibl d h bl h l iJMPS (2008, 2009, 2010)

development of flexible and stretchable technologies.( )

Nano Letters (2007, 2008, 2009a,b, 2010) Nature (2008)Nature (2008)Nature Materials (2006, 2008, 2010)Nature Nanotechnology (2006)PNAS (2007, 2008, 2009)( )PRL (2010)Science (2006 2008 2009 2010)Science (2006, 2008, 2009, 2010)Science Translational Medicine (2010)

Cover of NSF Budget Request to CongressCover of NSF Budget Request to Congressfor fiscal year 2011for fiscal year 2011for fiscal year 2011for fiscal year 2011

“Researchers YonggangResearchers Yonggang Huang at Northwestern University and John Rogers at the University of Illinois at Urbana-Champaign have developed circuits that candeveloped circuits that can stretch, bend and even twist! …twist! …

Potential uses include flexible sensorsflexible sensors, transmitters, and new photovoltaic and microfludic p oto o ta c a d c o ud cdevices, as well as areas of medicine and athletics.”

Global vs Local BucklingGlobal vs Local Buckling——1D Buckling1D Buckling

Global buckling happens when the substrate is relatively thin and long; otherwise,local buckling happens.

Global vs Local BucklingGlobal vs Local Buckling——2D Buckling2D Buckling

At a certain thickness of the thin film, for the same length and width of plate, localbuckling occurs when the substrate is relatively thicker.

Single Crystal Si and Mother WaferSingle Crystal Si and Mother WaferSiO2

single crystal Si

Single Crystal Si and Mother WaferSingle Crystal Si and Mother Wafer

single crystal Si

etch SiO

single crystal Si

etch SiO2

mother wafer: single mother wafer: single l Sicrystal Si crystal Si

three layers are strongly bonded silicon ribbons sitting on the mother wafer

Electric PerformanceKhang, Jiang, Huang, Rogers, Science, 2006

100

150

m2 )

-7.58% -3.79%

0%

stretch/compr.Al Al 50

100

ity (m

A/cm 0%

3.03% 7.58% 15.56%22.73%

pn

stretch/compr.dark

50

0

rent

Den

s 22.73% model

50 μmlight

-2 -1 0 1

-50

Cur

r

Bias Voltage (V)g ( )

• Wavy Si does not affect the electric performance.

Pixel Distribution: Compare with iexperiment

• The analytical results agree well with experiments• The density change of pixels due to deformation is• The density change of pixels due to deformation is

around 10%, which is fairly small• The distribution of pixels after deformation is quite

if