Surface and bulk passivation of multicrystalline silicon solar cells by ...

CHAPTER 4
MINORITY-CARRIER LIFETIME MEASUREMENTS
4.1 Quasi-Steady-State Photoconductance Decay
One of the fundamental physical properties of a silicon solar cell is the minority-carrier
lifetime. In practice, measurements on wafers yield an effective lifetime which can then
be interpreted as a combination of bulk and surface recombination components. In order
to do this correctly, the measurements of the effective lifetime must have a strong
physical basis. Currently, there are three methods being deployed by the PV-Si industry
for lifetime measurements: quasi-steady state photoconductance decay (QSSPCD),
photoluminescence mapping and microwave reflection. In this thesis, QSSPCD was used
to measure the minority-carrier lifetime of PV-Si.
For long-lifetime wafers, the transient method (measurement after the light is
extinguished) is preferred since it does not require knowledge of the photogeneration in
the sample (reflection and absorption of photons in the sample) or the excitation
wavelength. The QSS method has been found to be useful for lower-lifetime materials
that are often used in the production of PV cells. The QSSPCD technique has been
developed by Sinton [100]. This award-winning technique uses inductive coupling
between a small coil that is placed under the sample platform and the sample under test.
The frequency used is 10 MHz, and the light source is a very long duration (several
milliseconds) flash lamp. The setup involves a zeroing procedure that accounts for the
dark conductivity of the sample. The user also inputs the estimated carrier concentration.
The sensor coil detects the photoconductivity produced by the flash lamp, and the
interfaced computer processes the data. A highly calibrated onboard silicon cell, that is
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