evaluation of combined ebic/fib methods for solar cell ... · methods for solar cell...
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© Fraunhofer-Institut für Werkstoffmechanik IWM
Evaluation of combined EBIC/FIB methods for solar cell characterizationFrank Altmann*, Jan Schischka*, Vinh Van Ngo**, Laurens F. Tz. Kwakman**, Ralf Lehmann***Fraunhofer Insitute for Mechanics of Materials Halle** FEI Company
Drei-Länder FIB Workshop, 28./29. 6.2010 in Wien
© Fraunhofer-Institut für Werkstoffmechanik IWM
Fraunhofer-Center for Silicon Pholtovoltaics (CSP)
joint initiative of the Fraunhofer Institute for Mechanics of Materials (IWM) and Solar Energy (ISE)
Location: Halle/Saale, Center of Germany’s Solar Valley IWM
TGZ-3
Q-Cells, EverQ, CSG Solar, Calyxo,Brilliant 234., Solibro, SSF, City Solar, Solarion
Sovello, Schott Solar, SunWays, PV Silicon, Antec
SolarWorld
Conergy, First SolarOdersun
IWMH, CSP
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Solar Cell technologies
Source: http://de.wikipedia.org/wiki/Datei:Solarzelle_Funktionsprinzip2.svg
Si-wafer cells Thin film cells(CdTe, CIS/CIGS,a-Si, CSG …)
CIS reel to reel technologyCdTe
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PE-Beam
EBIC Current
p-typen-typeDepletion
Zone
EBIC Principle
Primary electrons generate electron-hole pairs in semiconductor material
Electrons and holes were separated within the internal electrical field of a depletion zone generating an EBIC current
EBIC signal as input for SEM imaging system
depletion zones and electrically defects can be characterized
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EBIC/FIB setup
FEI Quanta3D FEG dualbeam FIB
Sample holder with manual prober needles
Sample holder with mounted Kleindiek manipulator
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DISS5 EBIC
variable gain 1 E3….1 E10 V/A adjustable input offset -1…1 µA variable contrast 1…100x variable brightness -1…1 V adjustable low-pass-filter adjustable BIAS -10…10 V preconfigured operation modes:
•EBIC•EBIC +BIAS•EBIC +Log-In amplifier•EBIC +Compensation•Calibration•Beam current measurement display of EBIC-current for each
pixel calculation of efficency saveable preamp settings preamp settings and calculations
in the image
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Metal film
N- doped film
Depletion layer
P-doped film
E-beam2-10 KeV
E-beam2-10 KeV
H2
5
102
5
10
Metal film
N- doped film
Depletion layer
P-doped film
E-beam2-10 KeV
E-beam2-10 KeV
H2
5
102
5
10
Top down EBIC
EBIC @8kV
EBIC @24 kV
Primary electron beam perpendicular to p/n junction
Electron interaction volume has to extend to p/n junction to get EBIC signal
Influence of topography and layer above p/n junction
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EBIC at FIB cross sections
EBIC at FIB cross section, sample tilt 52° High acc. voltage to allocate subsurface defects Low acc. voltage to reduce electron beam
interaction volume for high res. EBIC imaging Ga interaction at cross section can distort EBIC
imaging -> 2kV FIB finish, verification at mechanically polished cross sectionsSEM/EBIC @30 kV
SEM/ EBIC @5 kV SEM/ EBIC @5 kV (mech. polished cross section)
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FIB cross section
Related EBIC image
SEM/EBIC overlay, EBIC signal was colorized
n doped Si-layer
FIB-Pt to enhance cross section quality
Characterization of p/n-junction
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Defect localization and root cause analysis
1. EBIC overview linear recombination centers correlate mostly with grain boundaries
1
2. inner grain defect localized by EBIC (signal inverted)
2
4. SEM detail shows cracking in Si
4
3. EBIC at FIB cross section to visualize subsurface defect formation
3
FIB-cut
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Characterization of p/n-junction
SEM
EBIC
SEM/EBIC overlay at FIB cross section, EBIC signal in redp and n contacts
FIB cut
n+ layer
p- layer
ARC and glass substrate
FIB Pt
glass beat
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Defect localization and root cause analysis
EBIC/SEM overview shows high density of defects (dark dots)
SEM
micro crack
BSG is overexposed
FIB cross section analysis at defect site shows microcracks in Si absorber
EBIC
depletion zone is cut by the crack
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EBIC Overview shows high defect density
Defect localization and root cause analysis
Light optical overview
Development of new laser crystallization process
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FIB preparation of TEM lamella
SE imageSE + EBIC
small defect
circular defect areas without EBIC signal small defects at centre
Defect localization and root cause analysis
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-> raised absorber layer at defect centre -> disturbed epitaxial grown due to interface contamination between p- and n+ layer
BSG
SiN
Pt protective layer
p- Si
n+ Si
Defect localization and root cause analysis
TEM-overview of defect
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BSG
n+ Si
p- Si
n+ Si
p- Si
Interface contaminationblocks formation of depletion zone
Defect localization and root cause analysis
SiN
TEM-detail
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SE at 5kV EBIC at 5kV
TCO
SE+EBIC at 5kV
Inhomogeneous EBIC-signal at the CuI/CIS interface Voids CIS layer correlate with reduced EBIC signal Inverted EBIC signal at CuI/TCO interface
CuI
CIS
CuIn
Cu
voids Reduced EBIC signal
Characterization of p/n-junction
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SEM @ 10kV
SEM
EBIC at 10kV
EBIC
Bright dots in EBIC image corresponds to small voids in the TCO-layer
SE+EBIC
Defect localization and root cause analysis
SEM+EBIC
1μm
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Irregular formed depletion zone in thin film absorber EBIC signal correlates with grain structure
Characterization of p/n-junction
CdTe
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Defect localization and root cause analysis
EBIC overview
SEM/EBIC detail @30kV
shunt
shunt localization on a large area by Lock-in-Thermography
Top down EBIC to allocate shunts for further FIB cross section analysis
LIT amplitude image
Hot spot
CdTe
EBIC signal was inversed to better visualize the shunted region
50µm
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Defect localization and root cause analysis
Stepwise FIB milling and SEM/EBIC imaging to investigate 3D structure of shunt
Inhomogeneous CdS/CdTe layer with voids could be found
CdTE3D FIB//EBIC @30kV FIB/EBIC @2kV
step 1
step 2
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Summary and Conclusion
FIB/EBIC was successfully applied to investigate varies solar cell samples
Depletion zones could be visualize and correlated to the thin film layer stack at FIB cross sections
Electrically active defects could be found and allocated by top down EBIC
Following EBIC at FIB cross sections visualize the subsurface defect structure
It could be demonstrated, that FIB/EBIC is an effective tool forsolar cell characterization and failure analysis
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Acknowledgement
Ralf LehmannLaurens F. Tz. KwakmannVinh Van NgoStacy StoneMarc Tilenschi
PV manufactures for supporting us with samples
Uwe GrauelWolfgang Joachimi