In-situ defect detection systems for R2R flexible PV films

The atomic layer deposition technique (ALD) is
used to apply a thin (40-100 nm thick) barrier
coating of Al2O3 on polymer substrates for flexible
PV cells, to minimise and control the degradation
caused by water vapour ingress. However,
defects appearing on the film surfaces during the
Al2O3 ALD growth have been seen to be highly
significant in deterioration of the PV module
efficiency and lifespan [1]. In order to improve the
process yield and product efficiency, it is
desirable to develop an inspection system that
can detect transparent barrier film defects in the
production line during film processing. Off-line
detection of defects in transparent PV barrier
films is difficult and time consuming.
Consequently, implementing an accurate in-situ
defects inspection system in the production
environment is even more challenging, since the
requirements on positioning, fast measurement,
long term stability and robustness against
environmental disturbance are demanding. For
in-situ R2R defects inspection systems the
following conditions need to be satisfied by the
inspection tools. Firstly the measurement must
be fast and have no physical contact with the
inspected film surface. Secondly the
measurement system must be robust against the
environmental disturbance inspection. Finally the
system should have sub-micrometre lateral
resolution and nanometre vertical resolution in
order to be able to distinguish defects on the film
surface. Optical interferometry techniques have
the potentially to be used as a solution for such
application. However they are extremely sensitive
to environmental noise such as mechanical
vibration, air turbulence and temperature drift.
George [2] reported that a single shot
interferometry system “FlexCam” developed by
4D Technology being used currently to detect
defects for PV barrier films manufactured by R2R
technology. It is robust against environmental
disturbances; but it has a limited vertical range,
which is restricted by the phase ambiguity of the
phase shift interferometry. This vertical
measurement range (a few hundreds
nanometres) is far less than the normal vertical
range of defects (a few micrometres up to a few
tens micrometres). It is not possible to detect the
majority of defects in the R2R flexible PV barrier
films.