Aarohi Vijh

1
Measurement of Component Cell IV Characteristics
in Multijunction Solar Cells

• Aarohi Vijh
• Midwest Optelectronics LLC
• Chandan Das and Xunming Deng
• University of Toledo

NREL a-Si Meeting San Francisco, April 17, 2006
2
Objectives

• To obtain IV characteristics of individual
  component cells in tandem and triple junction
  solar cell stacks
• To build instrumentation to automate this process

3
Procedure developed at UT
(Tandem Cells)

• Get Voc-Isc relationships for top and bottom
  cells
• Measure Iscs under illumination of interest
• Obtain Voc values from Voc-Isc relations
• Scan IV of tandem cell under illumination of
  interest with top and bottom cells limiting
• Subtract Voc of other cell (shift the IV curve)
  to the left

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Obtaining Voc-Isc (Tandem)

• With a blue probe light ON, measure Voc
• Add a stronger red bias light, measure Isc
• Turn red bias off, step up blue probe, repeat
• Gives Voc-Isc for top cell

(Tandem Cell)
Top Cell Voc-Isc
Bottom Cell Voc-Isc
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Obtaining Cell VOCs and IV curves

• Measure Iscs at given illumination, get
  corresponding VOCs
• Reduce red light slightly, measure top-cell
  limited IV, shift to the left by an amount equal
  to VOC (bot)

(Bottom)
(Top)
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Response of Tandem Cell
Different Cell, Expected Voc1.4V, Voc(bot)0.4V,
Voc(top)1.0V
Blue ON Blue OFF
Blue light 400nm from monochromator Red light
800nm longpass filtered incandescent light
Red ON Red OFF
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Response of Tandem Cell
Blue ON Blue OFF
Blue light 400nm from monochromator Red light
940nm IR LED
Red ON Red OFF
800 nm/940 nm fluxes were comparable
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Test setup with LED optical bias
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Bottom Cell IV with New System
Isc1.24mA
Voc0.55V
Voc 0.39 V Isc 1.24mA FF 49.5
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Top Cell IV
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Status

• Have applied UT method for component cell IV
  determination in tandem cells
• Have assembled a computerized system for applying
  biases and acquiring data
• Trying to improve the determination of Vocs in a
  tandem cell

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Plan

• Use filters with LEDs to eliminate any tails, or
  better, use lasers
• Move further into IR for bottom cell probe light
  to reduce top cell response
• Instead of a fixed bias light intensity, use one
  that tracks the expected Isc of the probed cell
• If Voc-Isc relationship is accurately obtained
  for one cell, use that information to correct the
  Voc estimate for the other cell(s)
• Test procedure for higher intensities
• Test procedure for triple-junction cells