The imaging techniques enable the possibility of higher-level quality control and defect analysis of solar cell materials in in-line production processes

Ahrenkiel, R.

Berman, G.

Call, N.

S., Johnston.

UNT Digital Library

2008 Fall MRS Steve Johnston Greg Berman Nathan Call Richard Ahrenkiel Sponsored by Material Research Society December 3, 2008 Boston, MA  USA Evaluation of Four Imaging Techniques for the Electrical Characterization of Solar Cells NREL/PR-520-44763 NREL is a national laboratory of the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy operated by the Alliance for Sustainable Energy, LLC National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Outline •? Introduction – development of techniques •? Minority-Carrier Lifetime •? Photoconductive Decay 1)? Photoluminescence (PL) Imaging 2)? Carrier Density Imaging (CDI) •? Diffusion Length – Finished Cells 3)? Electroluminescence (EL) Imaging •? Shunt Detection – Finished Cells 4)? Dark Lock-In Thermography (DLIT) •? Summary Development of mapping & imaging Microwave Conductivity Decay (lifetime in Ge; 1959)  Semiconductor Physics Laboratory, Inc. (Semilab) 1989, >700 systems. Quasi-Steady-State Photoconductance  Sinton Consulting, Inc. 1992, >200 systems. Photoluminescence in Si  First mapping: S. Ostapenko, I. Tarasov, J. Kalejs, et al., 1999; M. Tajima et al. 2004;  Then imaging, T. Trupke, R. Bardos, et al. ~2006.  BT Imaging, 2008 Carrier Density Imaging:  S. Glunz, W. Warta, et al. first mapping with detector since mid-1990s, then camera imaging, M. Bail et al. in 2000.  AESCUSOFT GmbH Automation, 2002; with alliance partner:  Fraunhofer Institute for Solar Energy Systems Electroluminescence Imaging: T. Fuyuki et al. 2005 Shunt Imaging:  Breitenstein, O. since mid-1990s  AESCUSOFT GmbH Automation, 2002 National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Lifetime Measurements by μ-PCD •?A pulse of laser light creates excess carriers •?The increased carriers change the conductivity of the semiconductor •?Microwave reflection is dependent on the conductivity of the semiconductor •?Measure the time constant of the decay in conductivity Light pulse Conductivity t t National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Photoconductive decay transients on silicon sample Excitation wavelength ? 1000 nm Injection level ? 1016 cm-3 Semilab Lifetime Mapping National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Photoluminescence (PL) Imaging National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future •? Excite excess carriers with light, h? > Eg (810 nm), use filters to block reflections. •?Measure PL, i.e. portion of recombination that is radiative recombination, h? ~ Eg (~1150 nm). •? Si CCD camera cooled to -75°C. •? 1024 x 1024 array of 13 μm square pixels. National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future  p-type CZ Si, 200 ?-cm, ~6x1013 cm-3, 310 μm thick With oxide No oxide Image acquired in as little as  1 second, longer for shorter lifetimes and poor surface passivation National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future mc-Si with thermal oxide Semilab map PL image n-type, CZ, 1-10 ?-cm, ~1015 cm-3, 500μm, both sides polished National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Semilab lifetime map PL image Lifetime to PL correlation Carrier Density Imaging National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future •? InSb infrared camera cooled to ~76 K •? Spectral response from 3.6 to 5.1 μm •? 640 x 512 array of 15 μm square pixels •? 100 Hz frame rate •?Lock-in detection, similar to averaged subtractions of dark background image from lit absorption image •? Free carrier absorption or emission of infrared radiation •? For absorption, hot plate is black body source: •? high emissivity with flat black high-temperature paint,  •?  ~50° to 100°C •?Square pulse 1-30 Hz •? ~30 W of 810nm laser diode excitation National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future  p-type CZ Si, 200 ?-cm, ~6x1013 cm-3, 310 μm thick PL image CDI (IR) image Image acquired in approximately 15 seconds, (~1 sun intensity of light turned on and off at ~20 Hz, camera: 100 frames per second),  longer acquisition time for shorter lifetimes and poor surface passivation National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future n-type, CZ, 1-10 ?-cm, ~1015 cm-3, 500μm, both sides polished Semilab lifetime map Carrier Density Image (IR camera) Lifetime to CDI correlation Electroluminescence Imaging National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future •? Si CCD camera cooled to -75°C •? 1024 x 1024 array of 13 μm square pixels •?Collect PL when forward biasing the cell •?With sample and camera in dark, and no filters, EL data collection in ~1 sec. EL – diffusion length comparison National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Electroluminescence, 1 s exposure time Semilab – LBIC scan, ~12 hour scan time, collect reflection, IQE data, too EL and LBIC comparison National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future Diffusion Length (μm) Dark Lock-in Thermography National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future •? InSb infrared camera cooled to ~76 K •?Spectral response from 3.6 to 5.1 μm •? 640 x 512 array of 15 μm square pixels •? 100 Hz frame rate •? Lock-in detection, similar to averaged subtractions of dark, no-bias background image from biased image •?Use ~1 to 30 Hz for bias pulses •? Total acquisition time of few to ~30 s •?Varying bias for shunt characterization •? Ohmic-type, Schottky-type, pre-breakdown, and recombination-induced Sense heat due to current flowing in shunts DLIT for shunting characterization National Renewable Energy Laboratory                                                                                                                                       Innovation for Our Energy Future 0.6 VF 2 VR 4 VR 6 VR Characterize shunts using varying voltage and frequency Zoom in using telescope lens with ~5 μm resolution National Renewable Energy Laboratory                                                                                         Innovation for Our Energy FutureProcess-Induced CrackingNational Renewable Energy Laboratory                                                                                         Innovation for Our Energy FutureSchottky-Type ShuntsSEMimagesNational Renewable Energy Laboratory                                                                                         Innovation for Our Energy FutureAluminum ParticlesSEMimagesSummary• Photoluminescence Imaging uses Si CCD camera• Infrared Carrier Density Imaging uses IR camera• Both show good correlation to Semilab microwave reflection lifetime• Electroluminescence compares to LBIC diffusion length maps• IR camera for shunt detection and cell characterization• Industry has shown interest in these imaging techniques due to their fast measurement speed for characterization and possibly in-line process control.National Renewable Energy Laboratory                                                                                         Innovation for Our Energy Future~150mm x 150mmwafersurfacepassivation annealing cellLifetimeimageLifetimeimageLifetimeimage(also, resistance, and shunts)Lifetimeimage(also, diffusion length, resistance, and shunts)

Evaluation of Four Imaging Techniques for the Electrical Characterization of Solar Cells

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Evaluation of Four Imaging Techniques for the Electrical Characterization of Solar Cells

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