Considerations for the Accurate
Measurement of Incident Photon to
Current Efficiency in
Photoelectrochemical Cells
David S. Ellis
1
*, Yifat Piekner
2
, Daniel A. Grave
1
,
3
, Patrick Schnell
4
,
5
and Avner Rothschild
1
*
1
Department of Materials Science and Engineering, Technion–Israel Institute of Technology, Haifa, Israel,
2
The Nancy & Stephen
Grand Technion Energy Program (GTEP), Technion–Israel Institute of Technology, Haifa, Israel,
3
Department of Materials
Engineering and Ilse Katz Institute for Nanoscale Science and Technology, Ben Gurion University of the Negev, Be’er Sheva,
Israel,
4
Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany,
5
Institute of
Chemistry, Technische Universität Berlin, Berlin, Germany
In this paper we review some of the considerations and potential sources of error when
conducting Incident Photon to Current Efficiency (IPCE) measurements, with focus on
photoelectrochemical (PEC) cells for water splitting. The PEC aspect introduces
challenges for accurate measurements often not encountered in dry PV cells. These
can include slow charge transfer dynamics and, depending on conditions (such as a white
light bias, which is important for samples with non-linear response to light intensity),
possible composition changes, mostly at the surface, that a sample may gradually
undergo as a result of chemical interactions with the aqueous electrolyte. These can
introduce often-overlooked dependencies related to the timing of the measurement, such
as a slower measurement requirement in the case of slow charge transfer dynamics, to
accurately capture the steady-state response of the system. Fluctuations of the probe
beam can be particularly acute when a Xe lamp with monochromator is used, and longer
scanning times also allow for appreciable changes in the sample environment, especially
when the sample is under realistically strong white light bias. The IPCE measurement
system and procedure need to be capable of providing accurate measurements under
specific conditions, according to sample and operating requirements. To illustrate these
issues, complications, and solution options, we present example measurements of
hematite photoanodes, leading to the use of a motorized rotating mirror stage to solve
the inherent fluctuation and drift-related problems. For an example of potential pitfalls in
IPCE measurements of metastable samples, we present measurements of BiVO
4
photoanodes, which had changing IPCE spectral shapes under white-light bias.
Keywords: IPCE, EQE, photoelectrochemical, device characterisation, measurement technique
1 INTRODUCTION
A standard measure to gauge the performance of photoactive devices, whereby an electron-hole pair
is generated by a photon, leading to useful electrical current, is the Incident Photon to Current
Efficiency (IPCE), synonymous with External Quantum Efficiency (EQE). IPCE is a measurement of
the output current for a given number of incident photons at a given wavelength. Physically, this
Edited by:
Chengxiang Xiang,
California Institute of Technology,
United States
Reviewed by:
Anja Bieberle-Hütter,
Dutch Institute for Fundamental
Energy Research, Netherlands
Vinod Singh Amar,
South Dakota School of Mines and
Technology, United States
*Correspondence:
David S. Ellis
Avner Rothschild
Specialty section:
This article was submitted to
Hydrogen Storage and Production,
a section of the journal
Frontiers in Energy Research
Received: 16 June 2021
Accepted: 08 December 2021
Published: 05 January 2022
Citation:
Ellis DS, Piekner Y, Grave DA, Schnell P
and Rothschild A (2022)
Considerations for the Accurate
Measurement of Incident Photon to
Current Efficiency in
Photoelectrochemical Cells.
Front. Energy Res. 9:726069.
doi: 10.3389/fenrg.2021.726069
Frontiers in Energy Research | www.frontiersin.org January 2022 | Volume 9 | Article 7260691
ORIGINAL RESEARCH
published: 05 January 2022
doi: 10.3389/fenrg.2021.726069
photocurrent spectrum comprises a combination of key
characteristics of the device. These include the ability to
concentrate incident photons in the active area (a function of
the optical architecture of the whole device), the absorption
coefficient within the active area, the nature of the excited
electronic states as a result of the absorption (i.e., mobile
charges, vs. non-mobile localized excitations), the ability to
separate and transport these charges through the bulk to reach
the surfaces, and efficiency of transferring charges across the
surface interface vs. surface recombination. For methods of
extracting these physical parameters from the IPCE spectrum
and optical measurements, please refer to Piekner et al. and
references therein (Piekner et al., 2021). Because of its
importance in accessing and comparing the performance of
photovoltaic and photoelectrochemical solar cells, previous
works have sought to establish standards and protocol for the
IPCE measurement, and examine various issues that can affect it
(Chen et al., 2013;Reese et al., 2018;Saliba and Etgar 2020;Bahro
et al., 2016;Timmereck et al., 2015;ASTM Standard E1021-15
2019). Chen et al. (2013) (Chen et al., 2013) were focused on
measurements of perovskite solar cells in particular. They
highlighted frequency and time-scale as issues affecting the
accuracy, and concluded that a 10–20% consistency between
IPCE and observed photocurrent was “reasonably accurate.”
Saliba and Etgar (2020) (Saliba and Etgar 2020) also dealt with
mismatch between observed photocurrent and IPCE spectra in
perovskite, citing both settling time and frequency dependences,
as well as possible non-stability of the sample during
measurements. Bahro et al. (2016) paper (Bahro et al., 2016)
examined IPCE measurements of organic tandem devices, with
emphasis in their discussion about the effect of light bias on the
IPCE spectrum due to different charge carrier interactions.
In this paper we discuss these issues, and demonstrate with
many actual examples from measurements of
photoelectrochemical (PEC) cells for water splitting, based on
(mostly) hematite photoanodes, that we accumulated over the
course of the last few years. Many of the aforementioned
problems we were able mitigate by modifications to the basic
IPCE measurement system and procedure, which are presented
herein. We include and expand upon several of the issues of time-
scale and frequency considerations, sample stability, and white
light bias. Since there can be significant differences between
specific devices and material systems earmarked for IPCE
measurements, we do not aim to provide a set of master rules
and priorities–but rather provide illustrative examples, from our
specific case, and leave it to the reader to decide how relevant each
issue may be for their own type of cell and material.
Our paper is organized as follows. Section 2 begins with the
basic definition of IPCE and a general outline of a measuring
system with a Xenon lamp and monochromator. The notable
alternative of “flash”IPCE technology is also briefly described.
Next, the validity of applying the small-signal IPCE spectrum to
predict the large signal performance (photocurrent) is discussed
at length, with an illustrative example of non-linear response of
photocurrent to light intensity, and, for those latter cases, the
strategy of measuring in white-light bias as a means to best
capture the efficiency spectrum which most represents the actual
performance. Section 3 is about mitigating sources of error and
noise. We emphasize that it is based on our own experiences with
photo-electrochemical cells with hematite photoanodes, and
using our Xe-lamp/monochromator light source, and not all of
the issues might necessarily apply universally, but certainly (from
the above literature survey) many are prevalent in other systems
as well. The section is divided into subsections which deal with
Section 3.1 a detailed description and rationale for our IPCE
measurement setup and procedure, Section 3.2 optical power
measurements including calibrated detectors, averaging,
background subtraction, and the additional issue of
harmonics in the case of monochromator-based system,
Section 3.3 example of photocurrent drift and our way of
dealing with it, and the issue of system frequency response
which would be important in the case of the lock-in amplifier
approach, Section 3.4 Xe-lamp drift and noise, including our
(to our knowledge) unique solution and demonstration of
repeatable and consistent results achieved in our “final”
measurement system, and discussion of alternative the
“flash”approach. Section 3.5 is of a slightly different nature
in that it deals with the “human error”factor and how interface
software can be designed to mitigate it. While not directly
related to evaluating the accuracy IPCE spectra itself, the
sub-section may be skipped without loss of continuity, but
contains user interface considerations of that can promote
(or hinder, if neglected) more fruitful measurement sessions
and may be of interest to system developers. The final Section 4
isacasestudyofameasurementofanew(tous)sample,BiVO
4
,
which unexpectedly exhibited strong dependence on both bias
light and time. While this presented a grave challenge, the fact
that the final measurement system and technique was accurate
and robust, as demonstrated in the prior sections, allowed us to
rule out measurement error and attribute the observed
systematic spectral evolution to the sample itself, that could
lead to unique physical insightintothedeviceoperation.
2 BASIC DEFINITIONS AND THEIR
SUBTLETIES
IPCE is not a single figure of merit, but rather a spectrum as a
function of photon wavelength λ. Thus, the measurement
usually takes the form of scanning the wavelength of a light
source or bandpass filter, and measuring the current increase
from that monochromatized light at each wavelength point. The
two key observables, for each wavelength, are 1) the
photogenerated current δj
ph
(λ), which is proportional to the
amount of photogenerated holes (electrons) that reach the
surface of the anode (cathode) and contribute to the
photocurrent, which we denote (for holes) as δn
h
(λ), and 2)
the incident optical power δP(λ), which is proportional to the
amount of photons δn
ph
(λ), multiplied by each photon’senergy,
the latter proportional to 1/λ. Thus, we arrive at the unitless
expression:
IPCE(λ)δnh(λ)
δnph(λ)Kδjph(λ)
λδP(λ)(1)
Frontiers in Energy Research | www.frontiersin.org January 2022 | Volume 9 | Article 7260692
Ellis et al. Incident Photon to Current Efficiency
where Kis comprised of fundamental physical constants: Kh.c/
e, where his Planck’s constant, cthe speed of light, and ethe
charge of an electron. All of the above variables are written in
small signal form, preceded by a “δ”, because as a matter of
practice the IPCE is measured only as small signals, even though
this is not mentioned in most definitions that one might look up
for IPCE or EQE. We also note that “per unit time”of the
observables δj
ph
(λ) and δP(λ) cancel out between numerator and
denominator, and likewise “per unit area.”Therefore, if δP(λ)is
actual power reading, then δj
ph
(λ)inEq. 1 likewise has units of
current, as opposed to the usual current density; we avoided the
usual “I”nomenclature for current in order to reserve that
variable for optical intensity. It is therefore ideal that the
effective areas accepting the incident light for optical power
detection and current detection respectively be the same,
which includes the center position relative to the beam
profile. A typical IPCE setup is schematically depicted in
Figure 1A, showing the light source, PEC cell with the
sample and electrolyte. A potentiostat is used to measure
δj
ph
(λ)in3-electrodemode(Figure 1C)aswellas
characterize the J
ph
-U curves, where Uis the applied
potential, and J
ph
is the large-signal photocurrent. The
latter is determined from the current in the total incident
light (usually dominated by the white-light bias), minus the
current measured in the dark. The dark current can become
significant at high enough U. Also included is the optical
power meter for δP(λ)(Figure 1B) with aperture in front to
maintain a light beam area consistent with that on the sample
side, and a white-light bias source contributing broadband
optical power Pto the sample, whose importance is discussed
below. The resultant IPCE spectrum is shown in Figure 1D,
which is essentially Figure 1C normalized by Figure 1B,asper
Eq. 1.ThesharppeaksinFigures 1B,C are a result of the Xe
lamp’s spectrum, but are seen to cancel out in the IPCE shown
in Figure 1D.
From the definition of IPCE as described by Eq. 1,itis
common to relate the total expected photocurrent J
ph
to the
IPCE spectrum (where both J
ph
and IPCE are measured at the
same applied potential, U) by integrating the IPCE over the full
wavelength range of the incident light spectrum S(λ). This is used
to validate the consistency between the IPCE and photocurrent
voltammetry measurements. Then, the spectrum of the light
source can be replaced by the standard spectrum of the
sunlight (typically the NREL AM1.5G standard) to calculate
the expected photocurrent under 1-sun illumination
conditions. Omitting unit conversion factors, for S(λ) units of
photons per seconds per unit area per wavelength, we write for
simplicity:
Jph(U)eS(λ)IPCE.(λ,U
)dλ(2)
While Eq. 2 in many cases has been demonstrated to be exact
within experimental error, and is indeed used as a standard check
for consistency between the IPCE measurement and measured
FIGURE 1 | (A) Basic IPCE setup. The three-electrode mode voltammetry measurement setup of the current –potential (J
ph
-U) curve of the PEC cell (using a
potentiostat) is illustrated schematically in the bottom left, with the sample serving as the working electrode (WE), a reference electrode (RE) and counter electrode (CE),
all in contact with the electrolyte. Typical measurements of optical power (B) and photocurrent (C) spectra, resulting in IPCE spectrum (D), computed from the above
with Eq. 1 Note that the bias light (of power P) from the LED leads to a bias photocurrent level (J
ph
) produced by the PEC cell, but the IPCE is determined from only
the small signal values.
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Ellis et al. Incident Photon to Current Efficiency
photocurrent, there are a number of subtleties to consider. In
practice, because the monochromized (or sometimes also
modulated) light signal P(λ) for the IPCE measurement could
be relatively weak, Eq. 1 for IPCE should be considered to be a
small-signal equation, measuring a small increase in current for a
small increase in optical power. In contrast, J
ph
in Eq. 2 is a large
signal quantity. Eq. 2 would be exact if IPCE were a constant
(for a given applied potential U), independent of light intensity,
but in some cases the small-signal IPCE spectrum could be
dependent on light intensity [8]. This light-bias dependence is
readily apparent simply by inspecting the dependence of J
ph
-U
curves normalized by light bias. Figure 2 shows an example of
J
ph
-U curves measured at different light intensities, and
normalized by them, which cannot be explained by an
intensity-independent IPCE spectrum at all applied
potentials, even if one allows for series resistance to account
for the photocurrent onset potential shifts. This issue regarding
Eq. 2, has been highlighted previously (Christians et al., 2015;
Shi et al., 2015), and up to 20% discrepancy can be commonly
expected (Zimmermann et al., 2014). Before attempting to
improve on Eq. 2, we should note that the mathematical
complexity is increased because IPCE(λ)maynotonlybe
dependent on the light intensity at a given wavelength λ,but
also on the total light intensity across all photon energies
exceeding the bandgap. Physically, this determines the steady
state photogenerated carriers, which can, in turn, affect the
internal sample and surface conditions, and therefore have an
influence on the IPCE.
Notwithstanding, we can proceed to a more exact variation of
Eq. 2 if we consider a total-intensity-dependent IPCE (λ,U,β)
where IPCE now also depends on total light intensity Ivia an
intensity scaling factor βsuch that IβS(λ)·dλ. In this way, we
can envision a “thought-experiment”in which we gradually
increment the total intensity, while maintaining the spectral
shape of the light source, by increasing βcontinually from 0
to 1, which in turn gradually increases the photocurrent
accordingly until it reaches the final value. With this picture
in mind, we write:
Jph(U)
β1
β0
S(λ)IPCE.λ,U,βdλdβ(3a)
where the inner integral is over the full wavelength range
determined on either side by the IPCE or S (λ) limits,
whichever is more limiting. Re-writing to a form resembling
the original Eq. 2:
Jph(U)S(λ)IPCE.(λ,U
)dλ(3b)
where IPCE(λ,U)1
0IPCE(λ,U,β)dβis the intensity-averaged
IPCE spectrum, at applied potential U. Strictly speaking, this
requires measuring IPCE at a number of different light biases to
find the average. However, in the interest of saving time and
reducing tedium, it may be an acceptable approximation to
measure at half of the actual operating light bias, β0.5, to
represent a kind of average. Many in the community may feel more
comfortable intuitively to know the IPCE at 1-sun conditions β1,
even though it may arguably be less representative of the required
FIGURE 2 | Photocurrent normalized by light intensity vs. applied
potential, for various intensities of the LED white light source. The light intensity
was determined by a fitted calibration curve of light intensity vs. LED current,
the former measured with a spectroradiometer and integrated over
wavelength for the total irradiance. The sample was a 26 nm thick epitaxial 1%
Ti-doped hematite deposited on platinized sapphire.
FIGURE 3 | Integrations of Eq. 3b for IPCE of a hematite photoanode
(measured in white LED light bias) vs. potential, compared to the J
ph
-U curve
measured, on a different date, under solar simulator illumination. The sample
was a 1% Sn-doped hematite layer, having 85 nm thickness, deposited
on ITO, on an eagle glass substrate.
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Ellis et al. Incident Photon to Current Efficiency
average of Eq. 3b than would be β0.5 light bias. This may
nevertheless be acceptable in many cases, especially when the IPCE
of the sample is almost independent of light intensity, which could
also depend on applied potential and other conditions. Figure 3 is
an example of good agreement of integrations as compared to the
photocurrent–potential curves for a hematite photoanode, even
though the light bias for the IPCE measurement (white LED light
bias) was different from the photocurrent measurement light bias
(commercial solar simulator). The integration, of course, used the
spectrum for the latter, under which the photocurrent was
measured. Excellent agreement between photocurrent and
integrations was also seen for our BiVO
4
photoanode
measurements presented in Section 3, but as will be shown,
stability of the IPCE within the spectral range of S (λ)iscrucial.
Especially for new and relatively untried types of photoanodes,
we highly recommend to at least once characterize the typical
light-bias dependence of the current–potential curves (as in
Figure 2) to begin with (and if warranted, measurement of
IPCE at a number of different light biases), in order to avoid
potential misuse or mis-interpretation when attempting to
validate the IPCE and/or J
ph
measurement with Eq. 2. So-
motivated to measure IPCE under light bias, we caution that
this introduces additional challenges in the measurement, but can
be dealt with as shown in the next section. A final note regarding
Eq. 2 or Eq. 3b is that ideally, S(λ) should be measured in the
same sitting as the J
ph
–Umeasurement, to ensure that the actual
S(λ) is used to integrate with the IPCE, not affected by possible
changes in sample distance from the source, etc. This could be
measured, for example, by a spectroradiometer put in place of the
PEC cell. The IPCE measurement itself need not be done in the
same sitting as the J
ph
–Uor light source measurements (for
example, see the dates between measurements in Figure 3), as
long as the sample is stable over time and conditions such as
applied potential and electrolyte are well replicated.
3 MITIGATING SOURCES OF ERROR AND
NOISE
3.1 Description of Basic System and
Measurement Procedure
In the following sub-sections we list or state a number of
measurement trouble-spots and possible solutions, some of
which are straightforward and self-explanatory (but
nevertheless warrant brief attention as a reminder), but others
requiring more elaboration which we provide with specific
demonstrations and examples. As a starting point of reference,
we begin with a description of our base system and procedure
(before introducing modifications in subsequent sections), whose
elements are likely typical for many IPCE systems, with only
minor variations (a majorly different scheme, however, will be
briefly touched upon in Section 3.4). Our measuring system,
depicted schematically in Figure 1A, is based around the Oriel
QV-PV-SI “Quantum Efficiency Measurement Kit,”by Newport.
The work area at the IPCE station is enclosed in a black curtain
and ambient light (as can be monitored on the power meter) is
kept to a minimum; ambient light from instrumentation panels
and computer screens is low enough for the power meter reading
to be at its noise floor and likewise does not produce significant
photocurrent (as a comparison, the few 100 μWof
monochromatic light in Figure 1B, which in the dark looks
quite bright on the sample, produces a few μA of photocurrent,
but the ambient light is relatively dim, perhaps several hundred
times less). The probe light is generated from a broadband Xe
lamp source (up to 1 kW input power) monochromatized by a
Cornerstone 260 monochromator. The output light is collimated
and focused with the appropriate optics. Collimation of the beam
plays the important role of decreasing the sensitivity of the beam
cross-sectional profile to small changes that could inadvertently
occur in distance between mirror and sample or power meter, in
spite of efforts made to make these distances consistent. The
aluminum mirror shown at the center of the system illustration
presented in Figure 1 could be manually rotated so as to direct the
light to either the power meter or PEC cell (i.e., the sample), with
grooves on the mirror stage restricting the possible angles to
discrete values to ensure repeatability and proper placement of
the angle. On the PEC cell side, the photoanode is placed in a
“cappuccino”cell (Cesar 2007), and connected in 3-electrode
mode to a potentiostat (Zahner Zennium in our system). Our
typical sample has a transparent current collector layer (typically
a doped tin oxide layer, fluorine-doped FTO or niobium-doped
NTO, or a tin-doped indium oxide layer ITO), with the
photoanode layer (i.e. hematite, and possible underlayers and
overlayers) deposited over part of it. Typical hematite samples
produced in our group were deposited by pulsed laser deposition
either on a tin-doped indium oxide (ITO) (Piekner et al., 2018),
niobium doped titanium oxide (NTO) conductive layer (Grave
et al., 2016b), or platinum conductive layer (Grave et al., 2016a)
on sapphire, or on eagle glass substrates for polycrystalline films.
The working electrode lead is connected with an alligator clip to
the exposed transparent correct collector layer, with the hematite
layer partially in contact with the electrolyte (1M NaOH aqueous
solution, pH 13.6), through the aperture (3.7 mm when
measuring samples with sapphire substrates, or 6 mm diameter
for larger samples having eagle glass substrates) of the cappuccino
cell. An Hg/HgO/1M NaOH reference electrode (ALS model RE-
61AP, appropriate for use in alkaline solutions), and platinum
counter electrode (ALS model 012961), are immersed in the
electrolyte and likewise connected to the potentiostat with
alligator clips. This is shown schematically in the bottom left
of Figure 1A.
Prior to making electrical connections, the optics is inspected
to ensure that the (monochromatic) beam profile on the aperture
to the power meter is identical to the beam profile incident to the
aperture on the cappuccino cell. After connecting the alligator
clips, the electrical connection is checked by, in our case, making
an impedance measurement (typically at 10 kHz frequency, to
bypass usual capacitive effects from electrochemical interfaces)
using the a.c. capabilities of the potentiostat which is coupled to a
frequency response analyzer (FRA) in our Zahner Zennium
potentiostat. In the case of our typical samples, we consider
200 Ωand below to indicate a reasonably good connection; if
our typical photocurrent is maximum 200 μA, then this amounts
to an error of ∼40 mV at most, which could represent a significant
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Ellis et al. Incident Photon to Current Efficiency
offset (i.e. see Figure 2), but on the other hand could be accounted
for and does not qualitatively impact the measurement, as
compared to a “bad connection”which is typically 1 kΩand
higher, and is usually a sign of an ill-placed or corroded alligator
clip. The next step after the probe beam optics and electrical
connections are set, is to arrange the white-light bias so as to
produce an expected amount of photocurrent for a given applied
potential. In our setup, a broadband, high power LED (Mightex
Systems, 6500 K “glacial white”spectrum, 300 mW maximum
radiant flux, which could output an order of ∼1 sun equivalent
intensity on the sample) is positioned to obliquely shine on the
sample, as depicted in Figure 1A, so as not to block the incident
monochromatic beam. The usual procedure is to modify the
incident angle of the light bias to center its circular profile on the
sample, monitoring the photocurrent in real time to maximize it.
The light level can be subsequently adjusted by changing the LED
current via software.
After the above preliminary checks and setups, a current-
potential scan under light bias is performed. These may be
repeated until the sample stabilizes (i.e. the current-potential
curves are repeatable), and further adjustments to the light bias
current may be made to achieve the desired condition for the
IPCE measurement (i.e., typically to replicate a J
ph
-U scan made
elsewhere, or set βto a specific value). For our hematite samples,
we found that 20 mV/s was an adequately slow sweep-rate, but
this may vary depending on the sample. We note that the
photocurrent is obtained by subtracting the dark current vs.
applied potential scan from the scan made under light. Once
the applied potential and white light bias are set for the IPCE
measurement, a certain time may be allotted for stabilization,
which may also be monitored in real time by continuous
measurement of the photocurrent.
Having set the operating condition, including fixing the
applied potential and light bias, the basic IPCE procedure is
relatively simple. First the mirror is rotated to direct the
monochromatic beam to the power meter, and a scan of
optical power vs. wavelength is performed. Then, the mirror is
rotated for sample illumination and the same wavelength scan
repeated, but this time measuring current on the potentiostat. The
baseline current, measured with the same light-bias, but without
any monochromatic beam, is subtracted to produce δj
ph
(λ), and
Eq. 1 applied for the IPCE spectrum. Usually, another P(λ) scan is
done afterwards to check stability of the monochromator,
discussed in Section 3.4 below. We found this method to be
adequate in the case of zero light bias (β0), whereby the LED
would only be used to check the J
ph
-U curve for the purpose of
setting the applied potential to a certain operating point, but then
turned off for the IPCE scans. However, in the case of light biases
approaching 1-sun intensity (β1), it fails spectacularly. In the
sub-sections below we discuss some of the finer points of different
aspects of the measurement.
3.2 Optical Power Measurement
The basic optical power measurement is fairly straightforward.
Here we just briefly review a few points. As described above, one
should be careful to keep the optical power (i.e. beam-profile) the
same on the power meter as on the sample, by use of appropriate
apertures, and symmetric geometry etc., which should be
confirmed by appropriate distance and height measurements
and visual inspection, to verify proper centering of the
incident beam profile on the sample and photodetector. The
power meter should be calibrated according to wavelength,
ideally NIST traceable, and informed of the correct wavelength
at each point in the scan. As the power measurement time could
be rather quick (several readings per second), fluctuations can be
reduced by repeated averaging (or longer integration times) of
measurement without much additional cost in scan time. To be
optimal, this could be adjusted to be comparable to the overhead
time of each wavelength point, or some fraction thereof. If the
ambient environment is dark, there will usually be negligible
background for most of the scan, but as can be seen from the Xe
lamp (plus monochromator grating/filter) spectrum in
Figure 1B, the power can fall off significantly at the extrema
of the wavelength range. If the scan must necessarily include such
low-power ranges, the electronic noise-floor of the power meter
may dominate the reading. In this case, a background (in the
dark) scan of the power meter should be done first and subtracted
wavelength-per-wavelength. We found, however, that a few
wavelength points plus linear interpolation, lasting only a few
seconds total, are adequate for the background scan. Lastly in this
section, although perhaps not solely a power issue, we emphasize
the importance of correct use of optical filters (which cut off low
wavelengths) appropriate to the wavelength range (or vice-versa)
during the monochromatic scan, in order to block higher order
diffraction passed by the monochromator gratings. Although the
higher harmonics may have a relatively small effect on the total
power, they can have a significant impact on the
photocurrent–for example, if at a photon energy below the
sample’s optical bandgap, there should be practically no
photocurrent, but a second harmonic could have energy above
the bandgap, and produce significant (and misleading)
photocurrent. Additionally, the typically wide-bandgap
transparent conducting layers (i.e., FTO, ITO or NTO) may
also activate from short-wavelength second harmonics.
3.3 Current Measurement and Current Drift
Current drift during measurement is the main killer when it
comes to measuring in light bias, and is the likely reason many
published works to date have avoided presenting IPCE
measurements in light bias. Very simply, the optical power of
the monochromatic probe beam is typically much weaker than
the ∼1 sun white-light bias intensity, and thus the resultant
photocurrent signal is correspondingly weaker. A factor of 100
is not unusual, depending on the wavelength and lamp condition.
Therefore, merely a few percent drift of the current under bias
over the time of a typical wavelength scan (which could be ∼½h
or more, depending on the wavelength stepsize, etc., and could
easily be caused by gas bubbles or other effects) could produce a
significant error, in either direction, to the measured δj
ph
after
subtracting the light bias contribution, and thus to the IPCE. This
is illustrated in Figure 4A, which shows reasonable looking IPCE
values for zero light bias (0 mA LED current), but an artifact
decrease, even to negative IPCE values, at low wavelengths when a
∼1-sun bias (1,000 mA LED current) is applied. The inset shows
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Ellis et al. Incident Photon to Current Efficiency
the measured total current under light bias, which looks relatively
stable on an absolute scale, but the slight decrease near the end of
the scan (from high to low wavelength) causes a precipitous
decrease of the calculated IPCE, which is clearly an artifact.
Onesolutiontotakeintoaccountthiscurrentdriftisto
subtract the baseline current (the current under the light bias
used, but without the monochromatic small signal light), such
that the “mono-light on”and “mono-light off”current
measurements are done at almost the same time (only a few
seconds apart) at each wavelength. We initially implemented
this by simply using the slits in the monochromator as an
effective shutter: opening and closing them at each wavelength,
and measuring current as a function of time for each of the
shutter-open and shutter-closed states, as shown in the inset of
Figure 4B. The improvement of using the shutter on-off
scheme is immediately apparent in Figure 4B,wherethe
measured IPCE spectrum does not change with light bias,
even up to ∼1 sun at LED current of 1,000 mA. Figure 4C
shows that when the applied potential is lowered to be below
the plateau region of the J
ph
-U curve, indicated in the inset,
there appears to be a systematic drop in the IPCE for this
sample as the light bias is lowered. Given the consistency of the
measurement in the plateau region shown in Figure 4B,wecan
plausibly make the distinction that this is truly a sample
condition effect of non-linear sensitivity to light intensity in
the near-onset region, and not a measuring artifact like in
Figure 4A.
Toexaminemorecloselythedynamicsthatcouldoccurin
the PEC cell during the IPCE measurement, the current vs.
time response upon opening or closing the monochromator
shutter (λ582 nm), for another hematite sample, is shown in
Figures 5A,B for zero light bias and ∼1sunwhitelightbias,
respectively. We note that j
ph
(λ)inFigure 5B is less than 1% of
the current due to the light bias. For the case without light bias,
Figure 5A, we see a relatively slow decay in the current
response to opening and closing the monochromator
shutter, lasting several seconds before the current reaches
steady state. The photocurrent j
ph
(λ) is taken as the
difference between the (approximately) steady state values.
The dynamics with light bias, Figure 5B is somewhat faster,
but still with a settling time of the order of ∼1s.Thisvariation
in settling time could be understood in terms of charging and
loss mechanisms at the surface, which has also been
demonstrated by modulated techniques and related
distribution of relaxation time (DRT) analysis (Peter 2013;
Klotz et al., 2016;Klotz et al., 2018). This sometimes-slow
dynamics is a reason for caution related to using the popular
phase-locked loop (PLL) technique with a lock-in amplifier for
measuring the small-signal IPCE, which indeed can overcome
the drift (and other) problems, but if the modulating frequency
is higher than 1 Hz (which is typical for PLL), may miss such
slow loss mechanisms, erroneously measuring the higher
δj
ph
(λ) seen at the beginning of the transient response. This
is more likely to occur at lower applied potentials where
surface recombination dominates. Frequency dependence
issues of IPCE have also been studied for perovskite
materials (Ravishankar et al., 2018) and dye-sensitized solar
cells (Xue et al., 2012).
FIGURE 4 | IPCE scans on a 30 nm Ti-doped hematite film deposited on a platinized sapphire substrate, for different light bias intensities (1000 mA LED current is
comparable to 1 sun). (A) Measured by the base system and method, before the shutter on-off method was implemented. In this case, the current is measured as the
monochromator is incremented from high to low wavelengths. The inset shows the total measured current (including from the light bias) of such a scan under strong light
bias. The applied potential was set to 1.7 V RHE, near the plateau region as indicated in the inset of (C).(B) Using the shutter closed-open scheme. The inset shows
the current vs. time measurements for one of the wavelength points, with shutter open in red (upper), shutter closed in black (lower).(C) Using the shutter open-closed
scheme, but for the applied potential lowered to 1.5 V, which is below the plateau region of the J
ph
-U curve, shown in the inset; dark current is shown as the black
dashed line.
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Ellis et al. Incident Photon to Current Efficiency
3.4 Light Source Drift and Noise
Another source of error in the IPCE measurement, usually less
dramatic than photocurrent drift, but still potentially significant,
is drift and/or noise of the light source for the monochromatic
beam. In many IPCE setups including our own, the light source is
a Xe lamp. However, it is a fitting juncture to make mention of an
alternative technology which was developed at the National
Renewable Energy Laboratory (Young et al., 2008). In this
concept, light from an array of several LED’s, each emitting a
different wavelength spanning the spectral range, is
simultaneously focused on the sample. The LED’s are each
modulated with a different frequency, so data processing can
isolate the contributions to the generated photocurrent from each
wavelength via the Fourier transform of the photocurrent, which
in Fourier space will have a peak for each modulation frequency.
In this way, the whole spectral range is measured at once,
drastically reducing the time required to measure each IPCE
spectrum, and can be as short as a second (for high frequencies).
This method is currently employed in a number of commercial
systems. Many of the drift-related problems can be totally
avoided, however the modulation frequencies (and thus speed)
would still subject to the system frequency limitation described in
the previous section. Anticipated disadvantage of an LED array as
opposed to Xenon source plus monochromator, would be less
access to the UV portion of the spectrum, and also somewhat less
wavelength resolution (or sharpness) based on the inherent LED
bandwidth limitations as compared to the monochromator, and
discrete separation of the LEDs. Evaluating these quantitatively is
beyond the scope of this paper. The approach certainly deserves
serious consideration, depending on the system being measured,
but we here we proceed to deal with the problems and procedures
for using the conventional Xe lamp and monochromator.
A common practice is to allow an hour or half an hour time
after turning on for the lamp to stabilize before commencing
the IPCE scans. However, as shown in a sequence of repeated
lamp scans over 6 hours in Figure 6,fluctuations in lamp power
from one scan to another, can occur well past this initial
warmup time. While several scans in a row show intensity
that is relatively stable (∼2% or better), occasionally larger
shifts of lamp power also occur, of the order of ∼10%. While
this allows the viability of checking the lamp stability
immediately before and after each δj
ph
(λ) measurement,
should one be unlucky enough to have measured between
the big jumps of the lamp, it would mean repeating the
measurement/waiting until the lamp is stable again,
amounting to a considerable cost in time.
The above problem would be largely circumvented if, similar
to the shutter on-off measurements in the previous section, the
optical power were also measured at every wavelength point.
There are a number of different approaches to achieve this, each
having advantages and drawbacks. One approach (Palma et al.,
2015) used the optical architecture of a Lambda 35
spectrophotometer, which featured a beam splitter to divide
the incident beam to simultaneously measure the incident
power. A slight complication of the beam splitter approach is
that, from surveying vendor’s websites, even beam splitters
designed for broadband applications seldom exhibit completely
uniform or consistent splitting ratios over a wide wavelength
range, so a careful calibration, requiring two calibrated (ideally
identical) photodetectors, would be needed to characterize the
FIGURE 5 | Effect of light bias on small-signal photocurrent response to a step in the light intensity. The sample was a 1 μm thick, Ti-doped hematite layer over an
FTO conductive layer, measured with monochromatic light incident on the back of the sample. The applied potential was set to 1.6 V RHE, well above the photocurrent
onset (at ∼1.2 V RHE, not shown), but near the onset of dark current (A) Without light bias (B) with ∼1 sun white light bias, incident on the front of the sample. Note that
the photocurrent scale spans the same 4 μAasin(A), but offset by the current contribution from the LED bias (∼166.5 μA).
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Ellis et al. Incident Photon to Current Efficiency
splitting ratio as a function of wavelength. In principle, however,
this calibration need be done only once.
As an alternative method, we automated the rotation of the
mirror in our setup which was previously done by hand between
scans, allowing the mirror to switch angles to direct the light
between the power meter and the PEC cell for every wavelength.
This has the advantage of measuring (in principle) the exact same
light beam incident on the PEC cell, aside from possible errors in
motor motion, or power fluctuations that could occur on a few-
second scale, which in practice was usually not large (with
exceptions, discussed below). The main challenge with this
method is precise and consistent control of the motor angle.
Our rotary stage was a HR-3 model from Newmark Systems, with
an NSC-A1 stepper motor controller. Inclusion of an encoder
which independently measured the angle was crucial for ensuring
the correct angles, and we incorporated stabilization routines and
checks in our software control. In practice, a 0.1°repeatability in
angle was achieved with relatively fast motions (∼1 s position
switching time), resulting in negligible variation in detected
power or current. Each time upon startup of the IPCE system,
we performed a brief (few minutes) but effective angle zeroing
procedure whereby a small aperture is temporarily placed in front
of the monochromator output, and the mirror angle zeroed by
back-reflecting the beam back unto the aperture. Even relatively
tiny IPCE’s could be resolved with relatively little fluctuation.
This sensitivity is also largely thanks to the resolution of the
Zennium potentiostat (by Zahner), which can accurately resolve
down to the level of nA currents. The ability to measure low
IPCE’s could be useful, for example, to characterize device
behavior outside the normal operational range, such as
energies near or below the band-edge, or a low applied
potentials. Figure 7, adopted from (Grave et al., 2021), shows
IPCE spectra (Figure 7B) measured at different applied potentials
along the J
ph
-U curve (Figure 7A). As the cyan spectrum around
500 nm and scaling factors in the legend of Figure 7B show, IPCE
spectra with values down to ∼0.001% can be at least roughly
measured. An added benefit of the motorized mirror method is
that, in addition to measuring the optical power at each
wavelength, the light-off current may also be measured by
directing the beam away from the PEC cell, thus also filling or
replacing the “shutter on/shutter off”role described in the
previous section.
One situational disadvantage of the motorized mirror stage
scheme, which might be in some respects better handled with the
beam splitting approach, is if lamp fluctuations occurred over the
same timescale as the delay between mirror rotations (typically a
number of seconds). Ideally the Xe lamps are designed to not
normally exhibit such fast intensity fluctuations, but we
nevertheless found this to be the case from time to time. We
noticed that in these situations, the spectra markedly improved
and the noise disappeared in the evening hours, when other
electronic devices in the building or vicinity were likely not as
active. We consequently attributed the short-term fluctuations to
electromagnetic interference (EMI) or other noise coming from
the operation of other devices in the vicinity of the experiment,
perhaps from adjacent rooms or floors, or outside. bXe arc lamps
and similar plasma-based devices can be particularly sensitive to
electrical noise including from EMI, in part due to thermal
stresses and turbulent convection flows within the lamp, and
there have been studies of these issues (Green et al., 1968;Rolt
et al., 2016). Therefore, EMI could be a possible issue to consider,
which could be mitigated by additional shielding or regulation, or
FIGURE 6 | Series of monochromator scans of a Xe lamp over a period of 6 h. Some typical shifts, during the periods of relative stability between the larger jumps of
intensity, are indicated as a percent error.
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Ellis et al. Incident Photon to Current Efficiency
more simply by measuring in a time and/or place where less EMI
or other external noise is likely to occur. Sensitivity to electronic
noise may also be a function of the age of the lamp, which should
be periodically replaced.
3.5 On Software: Handshaking With
Machines but Holding Hands With Humans
We conclude this section by briefly digressing from the science/
instrumentation side to discuss a nonetheless important part of
effective IPCE measurements: the softwareinterface.Thecoregoalof
IPCE software is to synchronize the actions of the rather diverse
collection of instrumentation participating in an IPCE
measurement–for the system outlined here this includes
monochromator, potentiostat, optical power meter, and
motorized mirror stage–integrating their control into a single
program capable of properly orchestrating the measurement
according to the experimenter’s specifications. This could be
implemented by such programs as Labview, as was done in this
work. But to truly be effective, the “user friendliness”aspect should
not be neglected. This includes an intuitive and easy-to-learn
interface, active anticipation and prevention of common human
error (such as checking if the typed data-save directory or filename
exists before the scan starts, and prompting the user if there is a
suspected problem with either files or devices to take corrective
action), accommodation of situations such as the need to pause the
scan mid-way to remove bubbles (especially important for
measurements under light bias) and bubble-related spike-removal
options, communicating and updating status (such as the applied
potential) to the user so they can easily spot problems in real time,
and convenient (but controlled) access to peripheral functions, such
as opening popups that allow manual control of the monochromator
or rotary stage, etc. In the case of Labview, a “stop”button is default
to all programs, yet can present a hazard in that it allows users to
arbitrarily and easily interrupt the program at any time, typically for
reasons that could have been prevented or otherwise handled, and
with potentially costly end-results of very possibly requiring resets of
machines (and in the case of our rotary motor, even re-zeroing of the
position). This button can be replaced with a more controlled abort
and shutdown mechanism. In short, what we describe amounts to
pro-active prevention of problems by tight control of the measuring
process flow, but at the same allowing user flexibility in a “safe”
manner. These features were implemented in our software, largely
motivated by the experience (and frustrations) of ourselves and other
colleagues and students measuring IPCE at various stages of
development of the system. Figure 8 shows some sample
screenshots of our Labview interface including such features.
Visual cues and audio cues (such as end-of-scan announcement)
were also incorporated for enhanced awareness (and, a little bit, fun
variety) during the measurements.
4 INCIDENT PHOTON TO CURRENT
EFFICIENCY MEASUREMENTS OF BIVO
4
ULTRATHIN FILM PHOTOANODES UNDER
WHITE LED BIAS
Recently, some of us undertook to measure the IPCE of BiVO
4
photoanodes under light bias. Light intensity dependence and
photodegradation of BiVO
4
photoanodes under PEC operation is
a well-known issue with these types of photoanodes, and has been
studied by various groups of authors for more than a decade (and
counting), including but not limited to references (Sayama et al.,
2006;Abdi and van de Krol 2012;Toma et al., 2016;Zhang et al.,
2019;Kou et al., 2020;Zhang et al., 2020). Nevertheless, the
photocurrent under our white LED bias appeared to be relatively
stable, decreasing ∼3% during a typical scan. From this
(deceptively) stable photocurrent, one might reasonably
assume as a corollary of Eq. 2 or Eq. 3b that the IPCE
spectrum should likewise be stable. As later became apparent,
this was not to be the case. Nevertheless, it makes for an
FIGURE 7 | IPCE measurements at different applied potentials along the J
ph
-U curve of a 1% Sn-doped 7 nm hematite film deposited on ITO-coated glass
substrate (A) J
ph
-U curve, with circles indicating the applied potentials at which IPCE measurements were made and their corresponding integrations with the LED
spectrum according to Eq. 3b. The inset plots the same, but with the photocurrent axis on a log scale. (B) IPCE spectra measured at the different applied potentials, in
white light bias shown in the legend. The measured spectra were scaled up by multiplying by the factors indicated in the legend. Adapted from Supplementary
Figure S4 of (Grave et al., 2021).
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Ellis et al. Incident Photon to Current Efficiency
interesting case study of an unexpected situation that can occur
when measuring IPCE on unknown samples, and the IPCE data
obtained may offer some new lines of investigation for further
studies of the IPCE decrease, although the latter is beyond the
scope of this manuscript, and from this point of view we present
this final section.
The samples were 30 and 10 nm thick BiVO
4
films deposited by
pulsed laser deposition (Kölbach et al., 2020) for which zero-light bias
IPCE measurements of the 10 nm film were recently published
(Grave et al., 2021). Our original intention for the latter work was
to measure under light-bias, as was done for hematite photoanodes.
We describe the procedure, which was approximately identical for
both samples. The J
ph
-U curves were measured with the LED current
set to 1,000 mA, shown in Figure 9A as solid lines. This LED
intensity roughly corresponded to an overall photon flux of the
same order of magnitude as 1 sun intensity. The first IPCE
measurements were done under the same light bias, with the
applied potential set to the plateau region (as shown by the
crosses in Figure 9A, which also correspond to Eq. 3b
integrations). These initial IPCE spectra in strong light bias are
shown as yellow lines in Figures 9B,C for the 30 and 10 nm
samples, respectively. Then, the PEC cell was rotated 180°so as to
illuminate the back of the sample with the monochromator light,
whilealsomovingtheLEDaroundsoastocontinuetoilluminate
with the light bias on the front, and IPCE measurements were done
with back illumination (not shown). Additional UV measurements
were performed (not shown), and the sample and LED once more re-
positioned to repeat the front (monochromator) illumination scans.
By the time the 2nd front-illumination IPCE scan was started, the
sample had been under the LED illumination for approximately 2 h
(with brief interruptions rotating the sample, etc). The 2nd IPCE
scans with front illumination are shown as purple lines in Figures
9B,C.The10nmfilm especially showed dramatic decrease of the
hump at low wavelengths between the two front illumination
measurements. For the 30 nm film there is a significant change
between 1st and 2nd measurements, but not as dramatic. This is likely
because even before the first measurement, the sample was already
under illumination for some time while trouble-shooting a technical
issue with the alligator clip, so was already in a decreased-IPCE state
(possibly light-soaked; see for example, Wing et al., 2015)bythetime
the first measurement commenced. After observing these dramatic
changes, the electrodes and electrolyte were removed, and sample
allowed until the next day (∼14 h) to recover to their original
surface state.
When the measurements commenced the following day, it was
without exposing the sample to a strong light bias, in order to avoid
the time-dependent effects observed above. Even the J
ph
-U curves,
usually done before each IPCE measurement to verify a consistent
operating potential relative to them, were done with the LED current
set to only 25 mA. These low-intensity curves are shown as dotted
FIGURE 8 | Screenshots of the software interface used for the IPCE system: (A) Interface with scan settings and large buttons for the main, often-used functions,
such as beginning the scan and pausing. Plots of the on-off transient current, photocurrent (before automatic spike removal, which can be user-specified), optical power,
and resultant IPCE spectra update point-by-point during the scan. The large buttons also dynamically display messages such as status messages (like “setting
potential,”etc.), and/or instructional messages (like “press to resume scan”after pausing), or open pop-ups, such as (B) a pop-up window for manual control and
calibration of the mirror angle.
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Ellis et al. Incident Photon to Current Efficiency
lines in Figure 9A, scaled as indicated in the legends by a factor of
∼34 which was similar for both samples so the shapes and onset
potentials may be compared. We attribute the larger potential shift for
the 30 nm sample to a larger series resistance, but otherwise the shape
of the J
ph
-U curves were fairly consistent with the previous day’shigh-
intensity scans. The IPCE curves for 25 and 0 mA LED biases,
respectively, are plotted as red and blue curves in Figures 9B and a
large low-wavelength hump appears, qualitatively similar to that of
the 10 nm sample, approximately the same for both low intensities.
For the 10 nm film in Figure 9C,thezerolight-biasIPCE
measurement (the 25 mA was skipped) showed very similar IPCE
to the 1st light bias measurement the previous day. Repeated zero
light-bias IPCE measurements for both films were found to be
consistent, so the sample was stable in zero light-bias conditions.
These results together indicate that not only is there a light-
bias effect, but also a slow decrease of parts of the IPCE spectrum
in sustained light bias. But what appears to be the most
unexpected and, in some sense, perplexing/counter-intuitive
behavior, is that the spectral portion of the IPCE which is
encompassed by the spectrum of the LED is relatively stable,
while the unstable part is outside of the LED spectral coverage.
The latter is plotted as dashed lines in Figures 9B–D, showing
that the LED itself causes a dramatic decrease of the higher-
energy (lower wavelength) part of the IPCE, which is well outside
its own spectral range. A physical model that could explain this
behavior is outside the scope of this manuscript. One additional
indicator could perhaps be in the time constants of the
photocurrent step-response (i.e., like plotted in Figure 5A for
example). The step responses, recorded for each wavelength as
part of the IPCE measurements described in Section 3.3, were
fitted to an exponential decay for each wavelength and plotted in
Figure 9D. The time resolution of our measurements was
relatively poor, only 0.5 s intervals, and the 10 nm sample’s
response became stable too quickly for us to fit, but the 30 nm
sample was slower (consistent with a larger series resistance, as
indicated by the onset potential shift in Figure 9A), and the decay
could be somewhat resolved and fitted at most wavelengths (far
outliers were excluded from the plot). As Figure 9D shows, while
the time constant is steady between 350 and 400 nm, there
appears to be a distinct increase above ∼410 nm and then falls
off again beyond ∼450 nm. Comparing against the IPCE curves in
Figure 9B above, there might be some correlation between the
FIGURE 9 | Measurements of 30 and 10 nm thick BiVO
4
samples measured in a neutral buffer solution with a hole scavenger; details in (Grave et al., 2021). (A) J
ph
-
U curves for 30 nm (blue) and 10 nm (red) thicknesses, under white-light LED bias of ∼1 sun (solid) and low-intensity (dotted). The x’s at 1.2 V indicate the applied
potential where the IPCE was measured as well as the resultant integrations of Eq. 3b (B) IPCE spectra for the 30 nm film at various light intensities. The measurement
sequence is described in the text. The LED spectrum is overlaid on the right y-axis (teal color). Zero-bias IPCE data obtained from (Grave et al., 2021). (C) IPCE
spectra for the 10 nm film at various light intensities. The measurement sequence is described in the text. The LED spectrum is overlaid on the right y-axis (teal color).
Zero-bias IPCE data obtained from (Grave et al., 2021). (D) Fitted time constant of the step-response vs. wavelength, measured during the IPCE measurement without
light bias, for the 30 nm film. The LED spectrum is overlaid on the right y-axis (teal color).
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Ellis et al. Incident Photon to Current Efficiency
time constants, and the stable and unstable regions of the IPCE
spectrum. This could be an avenue to investigate in further
studies using smaller time steps and more systematic
measurements, which should also include dependence on
applied potential as well. We note that similarly prepared
samples, but with 90 nm thick BiVO
4
layers, were used in a
recent photocorrosion study (Zhang et al., 2020). In our case, the
relative stability of the IPCE within the LED spectrum gave rise to
the observed stability of the photocurrent, which only changed a
few percent during the scans. The crosses in Figure 9A, at 1.2 V,
are the integration of the (10 and 30 nm) IPCE spectra with the
LED spectrum according to Eq. 3b and which works well because
the IPCE in the LED’s spectral region is mostly independent of
light-bias intensity of the LED, and is stable over time. Under a
spectrum more resembling the solar spectrum, which has lower-
wavelength spectral weight, the photocurrent would be expected
to decrease with time more dramatically, as has indeed been
observed in the literature. Subsequent to this preliminary
measurement, we observed similar time dependence of the
IPCE spectrum for thicker BiVO
4
samples under white light
bias, which will be presented elsewhere.
In summary, we reviewed the basics and expanded on some of
the finer technical points of IPCE measurements in PEC systems,
using primarily hematite as a model system for examples.
Emphasis was placed on demonstrating the importance
(depending on the linearity of the photo-response) and
difficulties of measuring the sample under white light bias,
which along with applied potential sets the operating point to
a large-signal photocurrent. The wavelength-resolved light
source used for the small-signal probe was based on a Xe
lamp with scanning monochromator. Because of the
considerable amount of wavelength points for the scan,
combined with upper limit on the photo-electrochemical
frequency response (or lower limit on measurement time
required), the total measurement time for an IPCE scan can
take several minutes or even half an hour. During this time, two
distinct problems can occur, drift of the large-signal
photocurrent under white light bias, due to small changes in
the sample or its environment, or change in the Xe lamp’s
output. These can be remedied by measuring the optical
power, and the small-signal photocurrent (both with and
without monochromatic light) at each wavelength point. We
accomplished this by using a rotary mirror, but it is by no means
the only possible solution, and the advantages and potential
drawbacks of various alternatives, including the lock-in
technique, beam-splitter, and the “flash”IPCE technique,
were discussed. The IPCE spectra measured with our final
system was demonstrated to be consistent (when integrated
over the incident light spectra) with large signal photocurrent
(Figures 3,7A), and spectral shape repeatable under light bias
for different LED intensities (in the linear regime, Figure 4B),
and applied potentials (and able to resolve miniscule signals,
Figure 7B). In the case study of BiVO
4
photoanodes, the
measuring system so-modified to minimize drift effects
allowed us to attribute the observed time-dependence under
light bias to actual behavior of the photo-electrochemical system
under test, rather than to the measuring system.
DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in
the article, further inquiries can be directed to the corresponding
authors.
AUTHOR CONTRIBUTIONS
DE wrote the first draft of the manuscript and did the system
development work described herein, and measured or closely guided
assistants measuring the presented data. DG and YP. grew the
various hematite samples measured, helped with some of the
IPCE measurements, and helped with the subsequent drafts of
the manuscript. PS grew the BiVO
4
samples and helped with the
subsequent drafts of the manuscript. AR is the principle investigator
who initiated this work and worked carefully especially on the initial
editsofthismanuscript.
FUNDING
The research leading to these results has received funding from the
PAT Center of Research Excellence supported by the Israel Science
Foundation (grant no: 1867/17). The IPCE measurements were
carried out at the Technion’s Photovoltaics Laboratory (HTRL),
supported by the Russell Berrie Nanotechnology Institute (RBNI),
the Nancy and Stephen Grand Technion Energy Program (GTEP)
and the Adelis Foundation. Part of this research was carried out
within the Helmholtz International Research School “Hybrid
Integrated Systems for Conversion of Solar Energy”(HI-SCORE),
an initiative co-funded by the Initiative and Networking Fund of the
Helmholtz Association. Part of the work was funded by the
Volkswagen Foundation. DG and DE acknowledge support from
the Center for Absorption in Science at the Ministry of Aliyah and
Immigrant Absorption in Israel. YP acknowledges support by the
Levi Eshkol scholarship from the Ministry of Science and
Technology of Israel. AR acknowledges the support of the L.
Shirley Tark Chair in Science.
ACKNOWLEDGMENTS
DE would like to thank Yossi Levi who acquainted him with IPCE
measurements and initial system, which was in large part set up
by Gideon Segev, and introduced him to some of the issues
discussed herein, and is grateful to Dr. Guy Ankonina for his
constant technical support and willing assistance whenever
needed or asked for in the Photovoltaics Laboratory at the
Technion. Many thanks to SofiYanru, Ortal Tiurin, Rotem
Yaniv and Alon Inbar for assisting with the measurements
throughout the various stages of this work.
Frontiers in Energy Research | www.frontiersin.org January 2022 | Volume 9 | Article 72606913
Ellis et al. Incident Photon to Current Efficiency
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Conflict of Interest: Author PS is employed by Helmholtz-Zentrum Berlin für
Materialien und Energie GmbH. All authors declare no other competing
interests.
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Frontiers in Energy Research | www.frontiersin.org January 2022 | Volume 9 | Article 72606914
Ellis et al. Incident Photon to Current Efficiency
