Stretchable and Flexible Stretchable and Flexible Silicon Silicon- -based Solar Cell Arrays based 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 Objectives Challenges and Objectives Challenges and Objectives Challenges and Objectives � 90% of solar cells are made of Si � 90% 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 top, … Si panel can only be placed at roof top, … Si panel can only be placed at roof top Si panel can only be placed at roof top Objectives: Objectives: � Make Si stretchable � Investigate the mechanics issues � Seek innovative applications (e g e-eye camera � Seek innovative applications (e.g., e eye camera, flexible solar cell, cardiac electrophysiology, …)
News agencies: United Press I t International, ti l … Display at the p y Tech Museum of Innovation, Silicon Valley, California Khang, Jiang, Huang, and Rogers, Science 311, 2006
Wavy Silicon Structures Wavy Silicon Structures Wavy Silicon Structures Wavy Silicon Structures ε pre = dL L dL L PDMS Si PDMS mother wafer: Si ε ε pre Fabricate thin ribbon Peel back PDMS; Bond elements to prestrained Si device elements Si device elements flip over flip over elastomeric substrate PDMS elastomeric substrate PDMS � Flat Si ribbon becomes buckled due to prestrained PDMS
Spring vs Straight Wire Spring vs Straight Wire Spring vs. Straight Wire Spring vs. Straight Wire
Wavy Silicon: Experimental Images Wavy Silicon: Experimental Images Wavy Silicon: Experimental Images Wavy Silicon: Experimental Images Optical image Scanning electron micrograph � Periodic wavy structures
Initial Buckling Analysis Initial Buckling Analysis Khang, Jiang, Huang, Rogers, Science , 2006 ε ε ε = 0 0 . . 9 9 % % = − pre pre A A h h 1 1 pre pre ε 0 Si e ( μ m) 1.5 c π h λ = λ = Si --- independent of pre-strain independent of pre strain Amplitude 1.0 1 0 ε 0 c 2 3 ⎛ ⎞ 0.5 ε 1 3 E ε ε = ⎜ ⎜ ⎟ ⎟ = = − − pre PDMS PDMS A A A h h 1 1 ⎜ ⎜ ⎟ ⎟ --- critical strain for iti l t i f ε 0 Si c ⎝ ⎠ 4 E c buckling 0.0 Si μ m) 60 elength ( μ 40 π 20 20 Wave h h λ = Si 0 ε c 0 0 200 400 Si thickness (nm) � Excellent agreement with experiments without any fitting parameters.
Stretchable Silicon at The Tech San Jose CA Stretchable Silicon at The Tech, San Jose, CA
Electronic Eye Camera Electronic Eye Camera Electronic Eye Camera Electronic Eye Camera TV Networks: ABC, BBC, CBC, MSNBC, … News agencies: AFP (France), Reuters, United Press International, Xinhua, … Newspapers: Chicago Tribune, Daily Telegraph T ib D il T l h (UK), … Magazines: Discover g 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 Fabrication Steps form hemispherical PDMS transfer element compressed compressed interconnect interconnect ~1 cm ~1 cm ~1 cm adhesive adhesive form Si focal plane array radially stretch PDMS and release from underlying wafer substrate cure adhesive; flop over substrate cure adhesive; flop over substrate integrate optics & integrate optics & interconnect to control interconnect to control compressible electronics to complete electronics to complete interconnect the device the device transfer focal plane array onto PDMS hemispherical focal plane array hemispherical focal plane array Si device island (photodetector Ko et al., Nature 454, 2008 & pn diode)
Pop Up Si Structure: Experimental Images Pop Up Si Structure: Experimental Images Pop Up Si Structure: Experimental Images 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 Pop Up Structure: Results Pop Up Structure: Results Pop Up Structure: Results ε × ε × -4 -4 10 10 ε ε yy yy yy yy 8 8 8 8 ( ) − ν 2 3 1 E h 6 6 ε ≈ π island bridge bridge ε max 2 4 4 island pre 2 0 E E h h L L island island bridge 2 2 ε ε 0 0 xx xx -2 -2 -2 -2 20 μ m 20 μ m y y 17.5 μ m 17.5 μ m x x 10 μ m 10 μ m � The island remains almost flat and experiences small strains. � The shape of interconnects agrees well with experiments.
Electronics on complex surfaces Electronics on complex surfaces Electronics on complex surfaces Electronics on complex surfaces
Stretchable Circuits (Science 2008) ( ) • Stretchability: up to 140% • Twistability Twistability TV Networks: BBC, CBC, … Magazines: Discover Magazine, MIT Technology g g , gy (PNAS 2008) Review, 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 … Magazines: MIT Technology Review, Nature News & Views, Scientific American, … (Nature Materials 2006) Department of Energy Press Release
Microscale, Inorganic Light Emitting Diode (iLED) Microscale, Inorganic Light Emitting Diode (iLED) (Science 2009) • Advantages: flexible high brightness • Advantages: flexible, high brightness, TV N t TV Networks: ABC, BBC, MSNBC, … k ABC BBC MSNBC long lifetime, bidirectional emission, News agencies: Reuters, … inexpensive. Newspapers: Boston Globe, New York Newspapers: Boston Globe New York • Applications: wearable health monitor li i bl h l h i Times, … and diagnostics, biomedical imaging Magazines: MIT Technology Review, … devices, ...
Cardiac/Neural Electrophysiology Cardiac/Neural Electrophysiology ( (Science, 2010; Science Translational Medicine, 2010; Nature , ; , ; Materials, 2010) 75 μ m 25 μ m 1 cm 2.8 μ m unwrapped unwrapped unwrapped unwrapped 2.8 μ m 7.0 μ m wrapped wrapped wrapped 75 μ m 0.3 cm Health monitoring and human- electronics i t interaction ti
Conclusions and Recent Publications Conclusions and Recent Publications Conclusions and Recent Publications Conclusions and Recent Publications -- Mechanics plays an important role in the development of flexible and stretchable technologies. d l f fl ibl d h bl h l i � JMPS (2008, 2009, 2010) ( ) � 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 Congress Cover of NSF Budget Request to Congress for fiscal year 2011 for fiscal year 2011 for fiscal year 2011 for fiscal year 2011 “ Researchers Yonggang Researchers Yonggang Huang at Northwestern University and John Rogers at the University of Illinois at Urbana-Champaign have developed circuits that can developed circuits that can stretch, bend and even twist! … twist! … Potential uses include flexible sensors flexible sensors, transmitters, and new photovoltaic and microfludic p oto o ta c a d c o ud c devices, as well as areas of medicine and athletics. ”
Global vs Local Buckling Global vs Local Buckling— —1D Buckling 1D Buckling � Global buckling happens when the substrate is relatively thin and long; otherwise, local buckling happens.
Global vs Local Buckling Global vs Local Buckling— —2D 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 Single Crystal Si and Mother Wafer Single Crystal Si and Mother Wafer Single Crystal Si and Mother Wafer SiO 2 single crystal Si single crystal Si single crystal Si etch SiO etch SiO 2 mother wafer: single mother wafer: single crystal Si l Si crystal Si silicon ribbons sitting on the mother wafer three layers are strongly bonded
Electric Performance Khang, Jiang, Huang, Rogers, Science , 2006 150 -7.58% -3.79% 2 ) m ity (mA/cm 100 100 0% 0% 3.03% stretch/compr. 7.58% Al Al 50 15.56% 22.73% 22.73% rent Dens p n dark model 0 stretch/compr. Curr -50 50 light 50 μ m -2 -1 0 1 Bias Voltage (V) g ( ) • Wavy Si does not affect the electric performance.
Pixel Distribution: Compare with experiment i • 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
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