Stretchable and Flexible
Silicon--based Solar Cell Arrays
Silicon
Y. Huang
Depts. of Civil and Environmental Eng. and Mechanical Eng.
Northwestern University
Collaborator: J.A. Rogers (University of Illinois)
Challenges and Objectives
„
„
„
„
90% of solar cells are made of Si.
Si
Si has good efficiency and reliability.
Si is brittle, and fractures at 1% strain.
Si panel can only be placed at roof top,
top …
Objectives:
„ Make Si stretchable
„ Investigate the mechanics issues
„ Seek innovative applications (e.g.,
(e g ee-eye
eye camera
camera,
flexible solar cell, cardiac electrophysiology, …)
News
agencies:
United Press
I t
International,
ti
l
…
p y at the
Display
Tech Museum
of Innovation,
Silicon Valley,
California
Khang, Jiang, Huang, and Rogers, Science 311, 2006
Wavy Silicon Structures
PDMS
Si
PDMS
mother wafer: Si
Fabricate thin ribbon
Si device elements
„
ε pre = dL L
ε pre
Bond elements to prestrained
elastomeric substrate PDMS
Peel back PDMS;
flip over
Flat Si ribbon becomes buckled due to prestrained PDMS
Spring vs
vs. Straight Wire
Wavy Silicon: Experimental Images
Scanning electron micrograph
„
Periodic wavy structures
Optical image
Initial Buckling Analysis
Khang, Jiang, Huang, Rogers, Science, 2006
λ0 =
πhSi
--- independent of pre-strain
pre strain
εc
23
⎞
⎟⎟ --- critical
iti l strain
t i for
f
⎠ buckling
Wave
elength (μm)
1 ⎛ 3EPDMS
ε c = ⎜⎜
4 ⎝ ESi
ε pre = 0.9%
A
Amplitude
e (μm)
A0 = hSi
ε pre
−1
εc
„
Excellent agreement with experiments
without any fitting parameters.
1.5
10
1.0
0.5
0.0
60
A0 = hSi
ε pre
−1
εc
40
20
0
0
λ0 =
πhSi
εc
200
400
Si thickness (nm)
Stretchable Silicon at The Tech,
Tech San Jose,
Jose CA
Electronic Eye Camera
TV Networks: ABC, BBC,
CBC, MSNBC, …
News agencies: AFP
(France), Reuters, United
Press International,
Xinhua, …
Newspapers: Chicago
Tribune,
T
ib
Daily
D il Telegraph
Tl
h
(UK), …
g
Discover
Magazines:
Magazine, MIT
Technology Review,
Nature News & Views,
Scientific American, U.S.
News and World
Report, …
NSF Press Release
Ko et al., Nature 454, 2008
Fabrication Steps
form hemispherical PDMS transfer element
compressed
interconnect
~1 cm
~1 cm
adhesive
radially stretch PDMS
form Si focal plane array
and release from underlying
wafer substrate
cure adhesive; flop over substrate
integrate optics &
interconnect to control
electronics to complete
the device
compressible
interconnect
transfer focal plane array onto PDMS
hemispherical focal plane array
Si device island
(photodetector
& pn diode)
Ko et al., Nature 454, 2008
Pop 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: Results
ε yy
ε xx
y
ε ×10
-4
8
6
4
2
0
-2
max
ε island
1 −ν
(
≈ 2π
2
island
)E
3
h
bridge bridge
2
island island
E
h
0
bridge
L
20 μm
x
17.5 μm
10 μm
„
„
The island remains almost flat and experiences small strains.
The shape of interconnects agrees well with experiments.
ε pre
Electronics on complex surfaces
Stretchable Circuits
((Science 2008))
• Stretchability: up to 140%
• Twistability
TV Networks: BBC, CBC, …
g
Discover Magazine,
g
, MIT Technology
gy
Magazines:
Review, Newsweek, Scientific American, …
NSF Press Release
(PNAS 2008)
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…
Magazines: MIT Technology Review, Nature News &
Views, Scientific American, …
(Nature Materials 2006) Department of Energy Press Release
Microscale, Inorganic Light Emitting Diode (iLED)
(Science 2009)
•Advantages: flexible,
flexible high brightness
brightness,
TV Networks:
N t
k ABC,
ABC BBC,
BBC MSNBC
MSNBC, …
long lifetime, bidirectional emission,
inexpensive.
•Applications:
li
i
wearable
bl health
h l h monitor
i
and diagnostics, biomedical imaging
devices, ...
News agencies: Reuters, …
Newspapers: Boston Globe,
Globe New York
Times, …
Magazines: MIT Technology Review, …
Cardiac/Neural Electrophysiology
((Science,, 2010;; Science Translational Medicine,, 2010;; Nature
Materials, 2010)
1 cm
75μm
25μm
2.8μm
unwrapped
2.8μm
unwrapped
7.0μm
wrapped
wrapped
wrapped
0.3 cm
Health
monitoring
and
humanelectronics
i t
interaction
ti
75μm
Conclusions and Recent Publications
-- Mechanics plays an important role in the
d l
development
off flexible
fl ibl andd stretchable
h bl technologies.
h l i
„
„
„
„
„
„
„
„
„
JMPS ((2008, 2009, 2010))
Nano Letters (2007, 2008, 2009a,b, 2010)
Nature (2008)
Nature Materials (2006, 2008, 2010)
Nature Nanotechnology (2006)
PNAS ((2007, 2008, 2009))
PRL (2010)
Science (2006,
(2006 2008,
2008 2009,
2009 2010)
Science Translational Medicine (2010)
Cover of NSF Budget Request to Congress
for fiscal year 2011
“Researchers
Researchers Yonggang
Huang at Northwestern
University and John Rogers
at the University of Illinois
at Urbana-Champaign have
developed circuits that can
stretch, bend and even
twist! …
Potential uses include
flexible sensors,
sensors
transmitters, and new
photovoltaic
p
oto o ta c a
and
d microfludic
c o ud c
devices, as well as areas of
medicine and athletics.”
Global vs Local Buckling—
Buckling—1D Buckling
„ Global buckling happens when the substrate is relatively thin and long; otherwise,
local buckling happens.
Global vs Local Buckling—
Buckling—2D Buckling
„ At a certain thickness of the thin film, for the same length and width of plate, local
buckling occurs when the substrate is relatively thicker.
Single Crystal Si and Mother Wafer
SiO2
single crystal Si
single crystal Si
etch SiO2
mother wafer: single
crystal Si
three layers are strongly bonded
mother wafer: single
crystall Si
silicon ribbons sitting on the mother wafer
Electric Performance
Khang, Jiang, Huang, Rogers, Science, 2006
2
Currrent Density (mA/cm
m)
150
stretch/compr.
Al
Al
n
p
stretch/compr.
50 μm
100
50
-7.58%
-3.79%
0%
3.03%
7.58%
15.56%
22.73%
model
dark
0
-50
50
-2
light
-1
0
Bias Voltage
g ((V))
• Wavy Si does not affect the electric
performance.
1
Pixel Distribution: Compare with
experiment
i
• The analytical results agree well with experiments
• The density change of pixels due to deformation is
around 10%, which is fairly small
• The distribution of pixels after deformation is quite
if