Microscopic Light-Beam Induced Current Measurement for High-Resolution Solar Cell Characterization

Transcription

1 Microscopic Light-Beam Induced Current Measurement for High-Resolution Solar Cell Characterization Susanne Richter, Stephan Großer, Tabea Luka, Marko Turek, Martina Werner, Christian Hagendorf Fraunhofer Center for Silicon Photovoltaics CSP Otto-Eißfeldt-St Halle (Saale)

2 Content Motivation Principle of µlbic Results of new devloped application Conclusion Comparison of EBIC and µlbic measurements Correlative LBIC measurements from macroscopic to microscopic scale

3 Motivation Quality control from module to cell level Non-destructive failure analysis, inline diagnostics on module and cell level EL Information of local structural, optical and electrical properties needed Current systems of electrical characterization on solar cell level limited in spatial resolution correlation between structural and electrical properties Separate measurement of 2D photo current images and topography and/or light microscopic images Non-destructive investigation of degradation mechanism Alternative method to EBIC

4 Principle of µlbic Laser beam absorption generates free electron hole pairs, which diffuse to and drift within an electric field photo current is induced The drift process induces a current for the duration in which carriers travel to their respective sinking nodes, i.e. the junction edges where they recombine with their opposite sign Scanning measurement of current enables generation of maps related to collection efficiency visualization of electrically active defect n + -Si p-si p + -Si Laser front I µlbic + rear Challenge: Establishment of software- and hardware interface Trilateral cooperation of Zeiss Microscopy, Point electronic and Fraunhofer CSP for µlbic measurement system

5 Principle of µlbic Experimental Setup LSM 700 (Zeiss) DISS 5 Scansystem (Point Electronic) Local measurement of light induced (photo) current on µm-scale and correlated visualization of topography

6 Principle of µlbic Experimental Setup Triggered: Line Frame Pixel LSM Measurement LSM 700 (Zeiss) DISS 5 Scansystem (Point Electronic) I SC Measurement of µlbic Local measurement of light induced (photo) current on µm-scale and correlated visualization of topography

7 Power density [W m - ² µm -1 ] Physical background - absorption depth UV visible range IR 1/α (Si) 405 nm 555 nm 639 nm 3,3 µm 1,6 µm 0,1 µm Wavelength [µm] [1] Absorption depth (1/α) corresponds to intensity drop of 1/e (~36%) Choice of wavelength influences information depth and power density [1] Data source:

8 Comparison of EBIC and µlbic measurement REM / SE-contrast EBIC n + -Si e - -beam front p-si p + -Si I EBIC µm 200 µm rear nm nm nm 200 µm 200 µm 200 µm Contrast and current values (in na range) vary for different wavelengths Results of EBIC and µlbic correlate A optical inactive/shadowed region B range with good life time C electrical volume losses due to grain boundaries D surface losses due to contaminations

9 Standard electrical characterization on cell level EL on Si solar cell Detail of EL image LBIC measurement on solar cell level (LOANA) 5 mm Different methods of electrical chararacterization on solar cell level (EL, LBIC, ) Spatial resolution of LBIC system (LOANA) >100 µm Microstructural analysis on µm-scale without sample preparation by µlbic on the example of light induced degradation (LID) test

10 Standard electrical characterization on cell level before and after LID test 980 nm 980 nm EL & LBIC detail of solar cell EL & LBIC detail of solar cell - after 19 h LID test Carrier life time decreases after LID test Some grain boundaries appear brighter compared to volume due to different degradation effects Effects on µm-scale?

11 Correlative LBIC measurements on µm-scale 980 nm (LOANA) 980 nm 555 nm (LSM700 / DISS 5) 500 µm 90 Positions of LBIC can be found and remeasured by µlbic for 405 nm, 555 nm and 639 nm in a higher spatial resolution (< 2,5 5,5 µm) LBIC (LOANA) uses also higher wavelengths with higher absorption depths

12 Correlative measurements on µm-scale 639 nm light microscopy topography (height map) topography (3D view) 200 µm 200 µm µlbic@639 nm (scaled) na Correlation of different material properties: Electrical characterization by µlbic Optical information by light microscopy Structural/topographic analysis by LSM

13 µlbic without and after LID test (neighbor cells) undegradated 405 nm 555 nm 639 nm µlbic measurement with equal parameters on neighbor cells After 19 h LID test 405 nm 555 nm 639 nm Comparison of electrical properties for different wavelengths Data evaluation of photo current values

14 µlbic without and after LID test (neighbor cell) 200 µm 200 µm undegradated (neighbor cell) After 19 h LID test Analyzed Grain structure in two neighbored cells Quantitative comparison of electrical properties on µm-scale

15 LBIC current [na] LBIC current [na] Quantitative comparison of µlbic without & after LID test undegradated (neighbor cell) After 19 h LID test na na undegradated after 19 h LID test position [a.u.] line 1 line 2 line 3 line position [a.u.] line 1 line 2 line 3

16 Conclusion New developed µlbic system shows good correlation to EBIC measurements No sample preparation necessary No vacuum condition needed Possibility of measurement of complete solar cells enables defect analysis on µm-scale e.g. for investigation of degradation mechanisms Quantitative (photo) current values in na-range Correlation of different material properties (optical, structural and electrical)

17 Acknowledgement Thank you for your attention! Financial support in project Fidelitas Contract no C Jörg Steinbach Stefan Groß Martin Beck Andreas Lutter Grigore Moldovan Uwe Grauel