Characterization and classification of irradiation
Light accounts for the main part of the energy input in
photoelectrochemical measurements. It is common for photoelectrochemical
experiments to use the global standard spectrum AM1.5G (IEC 60904-3) and
to calibrate its intensity to 100 mW cm–2 with a
reference solar cell. The quality of a solar simulator is defined in IEC
60904-9 by three parameters which are spectral match, spatial
non-uniformity and temporal instability. Since the test setup contains a
mirror which possibly affects the irradiation quality, the parameters
were determined and compared with the characteristics of the solar
simulator.
The spectral distribution was measured with a spectroradiometer on four
points in the plane of the photoelectrode. The classification is based
on the cumulative intensity in six spectral ranges between 300 and 1200
nm. The measurements found that the deviation is maximal in the
predefined spectral range of 772–919 nm, where it is 14.8 %. This
resulting classification of »A« is lower than the classification of the
solar simulator (»A+«), which is probably due to a spectral dependence
of reflection of the mirror.
The spatial non-uniformity of the light is especially important for the
investigation of large photoelectrodes. It was be measured with an array
of intensity-calibrated cSi solar cells. The classification is based on
the ratio of the largest and smallest irradiance according to
(I max–I min)/(I max+I min).
Depending on the investigated area, classifications of »A«, »B« and »C«
are obtained in the investigated setup (Table 1). The degrading
uniformity on larger areas is a result of the limited mirror size.
Temporal instability of irradiance is defined by short term and the long
instability, of which only the latter is relevant for PEC measurements.
It can be determined from the light intensities before and after a PEC
experiment by
(I max–I min)/(I max+I min).
A value of below 0.02 was found in the investigated setup, which allows
a classification of »A«.
It has to be noted that the use of Si solar cells for the measurement
light intensity, spatial non-uniformity and temporal instability is
legitimate only for absorbers with a similar band gap.[11] Since
many semiconductors for water splitting experiments have higher band
gaps (e.g ., WO3, 2.8 eV) an uncertainty is
induced which will be especially prominent when solar simulators with
poor spectral fit are used.