Evaluation of combined EBIC/FIB methods for solar cell characterization

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1 Evaluation of combined EBIC/FIB methods for solar cell characterization Frank 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./ in Wien

2 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 Q-Cells, EverQ, CSG Solar, Calyxo, Brilliant 234., Solibro, SSF, City Solar, Solarion TGZ-3 Sovello, Schott Solar, SunWays, PV Silicon, Antec Conergy, First Solar Odersun IWM SolarWorld IWMH, CSP

Valley Q-Cells, EverQ, CSG Solar, Calyxo, Brilliant 234.

3 Solar Cell technologies Si-wafer cells Thin film cells (CdTe, CIS/CIGS,a-Si, CSG ) Source: CdTe CIS reel to reel technology

4 EBIC Principle PE-Beam Primary electrons generate electron-hole pairs in semiconductor material n-type Depletion Zone p-type Electrons and holes were separated within the internal electrical field of a depletion zone generating an EBIC current EBIC Current EBIC signal as input for SEM imaging system depletion zones and electrically defects can be characterized

internal electrical field of a depletion zone generating an EBIC current EBIC Current EBIC

5 EBIC/FIB setup Sample holder with manual prober needles FEI Quanta3D FEG dualbeam FIB Sample holder with mounted Kleindiek manipulator

6 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 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

low-pass-filter adjustable BIAS -10 10 V preconfigured operation modes: EBIC EBIC +BIAS EBIC +Log-In

7 Top down EBIC E-beam 2-10 KeV E-beam 2-10 KeV Metal film P-doped film Depletion layer N- doped film H 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 kv

perpendicular to p/n junction Electron interaction volume has to extend to p/n

8 EBIC at FIB cross sections kv 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 sections SEM/ kv SEM/ kv (mech. polished cross section)

voltage to reduce electron beam interaction volume for high res.

9 Characterization of p/n-junction FIB cross section FIB-Pt to enhance cross section quality Related EBIC image n doped Si-layer SEM/EBIC overlay, EBIC signal was colorized

10 Defect localization and root cause analysis EBIC overview linear recombination centers correlate mostly with grain boundaries inner grain defect localized by EBIC (signal inverted) FIB-cut 3. EBIC at FIB cross section to visualize subsurface defect formation 4. SEM detail shows cracking in Si

11 Characterization of p/n-junction SEM FIB cut FIB Pt EBIC p- layer n+ layer glass beat ARC and glass substrate p and n contacts SEM/EBIC overlay at FIB cross section, EBIC signal in red

12 Defect localization and root cause analysis micro crack BSG is overexposed EBIC/SEM overview shows high density of defects (dark dots) SEM FIB cross section analysis at defect site shows microcracks in Si absorber EBI C depletion zone is cut by the crack

(dark dots) SEM FIB cross section analysis at defect site shows

13 Defect localization and root cause analysis Development of new laser crystallization process EBIC Overview shows high defect density Light optical overview

14 Defect localization and root cause analysis small defect SE + EBIC SE image circular defect areas without EBIC signal small defects at centre FIB preparation of TEM lamella

15 Defect localization and root cause analysis TEM-overview of defect Pt protective layer n+ Si p- Si SiN BSG -> raised absorber layer at defect centre -> disturbed epitaxial grown due to interface contamination between p- and n+ layer

raised absorber layer at defect centre -> disturbed

16 Defect localization and root cause analysis TEM-detail p- Si p- Si n+ Si Interface contamination blocks formation of depletion zone n+ Si BSG SiN

17 Characterization of p/n-junction SE at 5kV EBIC at 5kV SE+EBIC at 5kV TCO CuI CIS CuIn voids Reduced EBIC signal Cu Inhomogeneous EBIC-signal at the CuI/CIS interface Voids CIS layer correlate with reduced EBIC signal Inverted EBIC signal at CuI/TCO interface

Inhomogeneous EBIC-signal at the CuI/CIS interface Voids CIS

18 Defect localization and root cause analysis 10kV EBIC at 10kV Bright dots in EBIC image corresponds to small voids in the TCO-layer SEM EBIC SE+EBIC SEM+EBIC 1μm

19 Characterization of p/n-junction CdTe Irregular formed depletion zone in thin film absorber EBIC signal correlates with grain structure

20 Defect localization and root cause analysis LIT amplitude image EBIC overview CdTe shunt Hot spot shunt localization on a large area by Lock-in-Thermography Top down EBIC to allocate shunts for further FIB cross section analysis SEM/EBIC 50µm EBIC signal was inversed to better visualize the shunted region

Top down EBIC to allocate shunts for further FIB cross section analysis

21 Defect localization and root cause analysis 3D CdTE step 1 step 2 Stepwise FIB milling and SEM/EBIC imaging to investigate 3D structure of shunt Inhomogeneous CdS/CdTe layer with voids could be found

22 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 for solar cell characterization and failure analysis

23 Acknowledgement Ralf Lehmann Laurens F. Tz. Kwakmann Vinh Van Ngo Stacy Stone Marc Tilenschi Uwe Grauel Wolfgang Joachimi PV manufactures for supporting us with samples

The Focused Ion Beam Scanning Electron Microscope: A tool for sample preparation, two and three dimensional imaging. Jacob R.

The Focused Ion Beam Scanning Electron Microscope: A tool for sample preparation, two and three dimensional imaging Jacob R. Bowen Contents Components of a FIB-SEM Ion interactions Deposition & patterns