Optimized light-driven electrochemical water splitting with tandem polymer solar cells

Article abstract of DOI:10.1039/c5ta10459a

Tandem polymer solar cells are used for light-driven electrochemical water splitting. To attain a high enough electrochemical potential a new wide band gap electron donor polymer (PTPTIBDT-OD) is developed and used in combination with [70]PCBM as an electron acceptor in a tandem device architecture with two identical photoactive layers. This homo-tandem device comprises an intermediate ZnO/PEDOT:PSS/MoO₃ charge recombination layer to connect the two subcells electrically and optically. The homo-tandem solar cell has an open-circuit voltage of 1.74 V and reaches a power conversion efficiency (PCE) of 5.3%. In combination with RuO₂ as the electrocatalyst for oxygen evolution and RuO₂ or Pt catalysts for hydrogen evolution, sunlight-driven electrochemical water splitting occurs with a solar-to-hydrogen conversion efficiency of η_STH = 4.3%. Owing to the very high fill factor of the polymer tandem cell (0.73), water splitting takes place near the maximum power point of the homo-tandem solar cell. As a consequence, the difference between PCE and η_STH is only due to the overpotential losses.

Full text of DOI:10.1039/c5ta10459a

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Text and Graphic for the table of contents
A wide bandgap polymer is used in homo-tandem solar cells for light-driven
electrochemical water splitting with a solar-to-hydrogen efficiency of 4.3%
4.5
η
STH =4.3%
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
LiF
PTPTIBDT-OD:[70]PCBM
MoO3
PEDOT:PSS
ZnO
PTPTIBDT-OD:[70]PCBM
PEDOT:PSS
ITO
Glass
0
2
4
6
8
10 12 14 16 18 20
Time (min)

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DOI: 10.1039/C5TA10459A
Journal of Materials Chemistry A
ARTICLE
Optimized light‐driven electrochemical water splitting with
tandem polymer solar cells†
Serkan Esiner,a Gijs W. P. van Pruissen,a Martijn M. Wienkab and René A. J. Janssen*ab
Received 00th January 20xx,
Accepted 00th January 20xx
Tandem polymer solar cells are used for light‐driven electrochemical water splitting. To attain a high enough electrochemical
potential a new wide band gap electron donor polymer (PTPTIBDT‐OD) is developed and used in combination with [70]PCBM
as electron acceptor in a tandem device architecture with two identical photoactive layers. This homo‐tandem device
comprises an intermediate ZnO/PEDOT:PSS/MoO3 charge recombination layer to connect the two subcells electrically and
optically. The homo‐tandem solar cell has an open‐circuit voltage of 1.74 V and reaches a power conversion efficiency (PCE)
of 5.3%. In combination with RuO2 as electrocatalyst for oxygen evolution and RuO2 or Pt catalysts for hydrogen evolution,
sunlight‐driven electrochemical water splitting occurs with a solar‐to‐hydrogen conversion efficiency of ηSTH = 4.3%. Owing
to the very high fill factor of the polymer tandem cell (0.73), water splitting takes place near the maximum power point of
the homo‐tandem solar cell. As a consequence, the difference between PCE and ηSTH is only due to the overpotential losses.
DOI: 10.1039/x0xx00000x
www.rsc.org/
potential is achieved, the solar to hydrogen conversion
efficiency (ηSTH) only depends on the current density of the solar
cell during operation. Hence tandem cells offer an advantage.
Of course, the electrochemical potential for sunlight‐driven
electrochemical water splitting poses a lower threshold for the
optical band gap of the semiconductors that can be used. The
minimum optical band gap is higher for tandem cells than for
triple junction cells, and this limits the use of the complete solar
spectrum.
In our recent example of a water splitting device, a triple
junction polymer solar cell operating at a voltage of 1.49 V was
used in combination with RuO2 electrocatalysts for hydrogen
and oxygen evolution.6 A tandem solar cell that is intended to
split water at this potential requires an open‐circuit voltage (Voc)
of about 1.75 V and only few examples of polymer tandem solar
cells with such high Voc exist, using absorber layers with a wide
optical band gap.5,9‐11 The higher Voc is generally achieved at the
cost of lower current densities and wide band gap materials that
can provide high current densities are scarce.12,13
One approach towards a high voltage tandem cell is a device
configuration in which both photoactive layers are composed of
the same wide band gap material. These homo‐tandem cells can
provide higher efficiencies compared to the corresponding
single junction devices because of improved absorption of light
and enhanced charge collection.9,14‐16 When light is distributed
over two identical but thinner layers, light absorption can be
equally high as in one thick layer, but bimolecular
recombination of charges decreases because the distance over
which charges are collected is reduced,9 resulting in an overall
higher efficiency.
Introduction
Electrochemical water splitting occurs at a standard potential of
1.23 V but in practice substantially higher potentials (1.4 to 1.8
V) are required due to overpotentials for the hydrogen and
oxygen evolution reactions. As a consequence, sunlight‐driven
electrochemical water splitting devices use multi‐junction cell
configurations in which two or more photovoltaic cells are
stacked and connected in series to reach the high potential
while still using an appreciable part of the solar spectrum. Triple
junction stacks of amorphous silicon solar cells are often used
for light‐driven electrochemical water splitting purposes,
because the summed voltage of the three cells provides the
required potential.1−4 Recently, we demonstrated that also
triple junction polymer‐fullerene solar cells can be used
successfully for this purpose.5,6 If the water splitting potential is
sufficiently reduced by using appropriate catalysts for hydrogen
and oxygen evolution, tandem solar cells may also be used.7,8
This is beneficial in terms of device fabrication, because there is
one less junction and photoactive layer, but also in terms of the
overall solar‐to‐hydrogen conversion efficiency. In principle, a
tandem solar cell can generate up to 50% higher current density
than a triple junction cell at the same optical band gap,7 because
the photon flux is distributed over only two instead of three
absorbers layers. Provided that the required electrochemical
a. Molecular Materials and Nanosystems, Institute for Complex Molecular Systems,
Eindhoven University of Technology, P. O Box 513, 5600 MB Eindhoven, The
Netherlands, E‐mail: r.a.j.janssen@tue.nl.
b. Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612 AJ Eindhoven,
The Netherlands
In this paper we demonstrate light‐driven electrochemical
water splitting using a series connected tandem polymer solar
† Electronic Supplementary Information (ESI) available: Synthetic procedures. See
DOI: 10.1039/x0xx00000x
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cell. In order to achieve a sufficiently high Voc, a new wide band
View Article Online
DOI: 10.1039/C5TA10459A
gap material PTPTIBDT‐OD (poly(4,10‐(‐octyldodecyl)‐4,10‐
dihydrothieno[2',3':5,6]pyrido[3,4‐g]thieno[3,2‐c]isoquinoline‐
5,11‐dione‐co‐benzo [1,2‐b:4,5‐b']dithiophene, Fig. 1) has been
developed. The polymer comprises an electron deficient
pentacyclic lactam unit that was recently introduced in donor‐
acceptor conjugated copolymers to afford optical band gaps
higher than 1.7 eV in combination with a Voc of about 0.9 V,
when copolymerized with different π‐conjugated donors.17‐20 To
ensure a loss free series connection of the two subcells, we
introduce a ZnO/PEDOT:PSS/MoO3 stack as the intermediate
recombination layer. Light‐driven electrochemical water
splitting performed with RuO2 – RuO2 and RuO2 – Pt electrodes
in 1.0 M KOH electrolyte shows solar to hydrogen conversion
efficiencies of ~4.3%.
(b)
1.00
0.75
0.50
0.25
0.00
Results and Discussion
Synthesis and characterization
400
500
600
700
800
Wavelength (nm)
PTPTIBDT‐OD was synthesized in a Stille polymerization of 2,8‐
dibromo‐4,10‐di(‐octyldodecyl)‐4,10‐dihydrothieno[2',3':5,6]‐
pyrido[3,4‐g]thieno[3,2‐c]isoquinoline‐5,11‐dione with 2,6‐
Fig. 1 (a) Structure of PTPTIBDT‐OD. (b) UV‐vis absorption (black) and fluorescence
(red) spectra of PTPTIBDT‐OD in chloroform solution (dashed line) and as thin film
(solid lines).
bis(trimethylstannyl)benzo[1,2‐b:4,5‐b']dithiophene
as
described in detail in the ESI†. The resulꢀng polymer had a Mn
= 35.4 kg mol−1. The optical band gap (Eg) of PTPTIBDT‐OD is 2.04
eV as determined from the onset of absorption in chloroform
solution and thin film (Fig. 1). PTPTIBDT‐OD shows a weak
fluorescence at 642 nm (1.93 eV) with a vibronic progression
towards lower energies. The oxidation and reduction potentials
of PTPTIBDT‐OD were measured using cyclic voltammetry on
spin‐coated thin films on ITO covered glass substrates in
acetonitrile containing 0.1 M TBAPF6 as electrolyte, resulting in
estimates for the frontier orbital energy levels of EHOMO = −5.75
eV, ELUMO = −3.19 eV. The electrochemical band gap, EgCV = 2.56
eV, is significantly larger than the optical band gap.
provides a Voc of 0.90 V in a cell configuration with transparent
ITO/PEDOT:PSS bottom and reflective LiF/Al top electrodes. The
optimized single junction PTPTIBDT‐OD:[70]PCBM cell has a PCE
of about 5.2% (Table 1). The J‐V characteristics and the EQE
measurement of this cell are shown in Fig. 2. The short‐circuit
current density (Jsc) determined from the EQE after integration
with the AM1.5G solar spectrum amounts to Jsc = 8.38 mA cm−2
and is slightly larger than the one determined from the J‐V
characteristics (Jsc = 8.19 mA cm−2). The optimized cell has a
notably high fill factor (0.70) which places the voltage of the
maximum power point (Vmax = 0.73 V) close to Voc (0.90 V).
Optimized PTPTIBDT‐OD:[70]PCBM films show narrow fibre‐like
structures and crystalline regions in transmission electron
microscopy (TEM) (Fig. 3). With such a high fill factor, a homo‐
Single and tandem junctions
When blended with [70]PCBM ([6,6]‐phenyl‐C71‐butyric acid
methyl ester), a single junction solar cell of PTPTIBDT‐OD
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(b)
(a)
2
0
-2
-4
-6
-8
-1.0
-0.5
0.0
0.5
1.0
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Fig. 2 Optimized single junction PTPTIBDT‐OD:[70]PCBM solar cell. (a) J‐V characteristics in the dark (dashed line) and simulated AM1.5G illumination (solid
line). (b) EQE.
2 | J. Mater. Chen. A, 2015, 00, 1‐3
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Paper
oxide (MoO3) onto pH neutral PEDOT:PSS VibewyArttichleeOrmnlinael
DOI: 10.1039/C5TA10459A
evaporation. Evaporated MoO3 is a well‐known hole collector
that can replace PEDOT:PSS in normal configuration single
junctions.23 Because of its high work function (5.40 eV), MoO3 is
expected to prevent voltage losses at high voltage back cells.
MoO3 is also commonly used as a hole extracting contact, in
inverted single junction cell configurations.24,25 Yang et al. have
also used MoO3 in the intermediate contact of an inverted
tandem solar cell.16
Homo‐tandem PTPTIBDT‐OD:[70]PCBM cells were made
with additional Nafion or MoO3 layers to modify the work
function of pH neutral PEDOT:PSS. The tandem device layout is
shown in Fig. 4a. In Fig. 4b and Table 1 we show the tandem
device characteristics. In these tandem cells, both photoactive
layers were around 110 nm and initially no optimization was
made for front and back cell thicknesses. As expected, the
tandem cell without any work function modification layer
resulted in a low Voc of 1.53 V. With the addition of the Nafion
layer, the Voc improved slightly to 1.58 V, but MoO3 is
significantly more effective in increasing the Voc up to 1.72 V.
After optimizing the tandem cell by reducing the front cell
thickness for a more balanced current generation in the front
and back cells of the series connected tandem solar cell,26 the
resulting cell had a PCE of about 5.3% (Table 1).
Fig. 3 TEM images showing the morphology of the optimized photoactive
layers, scale bars are 200 nm (a) and 50 nm (b).
tandem solar cell of the PTPTIBDT‐OD:[70]PCBM blend can have
a high voltage in the maximum power point, sufficient for light‐
driven electrochemical water splitting. However, reaching a
sufficiently high voltage in a tandem configuration is not trivial.
The reason is the nature of the recombination contact. In the
frequently used recombination contact of a bilayer of ZnO and
pH neutral PEDOT:PSS,21 the pH neutral PEDOT:PSS layer, which
serves as the hole extracting contact for the back cell, has a
lower work function (4.65 eV) than that of the common, highly
acidic, PEDOT:PSS (5.05 eV).22 The reduced work function limits
the maximum voltage of the second subcell. The use of pH
neutral PEDOT:PSS is dictated by the high solubility of ZnO in
water at low pH, which prevents spin coating acidic PEDOT:PSS
on top of a ZnO layer.21 If the back cell uses a polymer with a
deep HOMO level to reach a high Voc, the large difference
between the low work function of pH neutral PEDOT:PSS and
the deep HOMO of the polymer, causes a non‐Ohmic contact
and introduces losses in the Voc of the back cell.5,22 A partial
solution for increasing the work function of pH neutral
PEDOT:PSS is spin coating a thin Nafion layer on top.5,22
However, a considerable portion of the lost voltage still remains
to be recovered after adding the Nafion layer.5,22
To further characterize the tandem solar cell, the external
quantum efficiency (EQE) was measured. Because the solar cell
Table 1 J‐V characteristics of the PTPTIBDT‐OD:[70]PCBM homo‐tandem solar cells
with and without Nafion or MoO3 layers in the intermediate contact.
Device
Hole transport layer in
recombination contact
Jsc
Voc FF
PCE
(%)
[mA cm−2] (V)
Single
8.19
3.84
0.90 0.70 5.2
1.53 0.69 4.0
1.58 0.64 3.9
1.72 0.75 5.0
1.74 0.73 5.3
Tandem
Tandem
Tandem
Tandem
optimized
pH neutral PEDOT:PSS
pH neutral PEDOT:PSS + Nafion 3.91
pH neutral PEDOT:PSS + MoO3 3.91
pH neutral PEDOT:PSS + MoO3 4.19
Another approach to improve the work function of pH
neutral PEDOT:PSS is depositing a thin layer of molybdenum
(c)
0.6
(b)
3
2
(a)
EQE no optical bias
EQE with optical bias
EQE simulated front cell in tandem
EQE simulated back cell in tandem
pH neutral PEDOT
pH neutral PEDOT + Nafion
pH neutral PEDOT + MoOx
0.5
LiF / Al
1
pH neutral PEDOT + MoOx - optimized
PTPTIBDT-OD:[70]PCBM
MoOx / Nafion
pH neutral PEDOT:PSS
ZnO
0.4
0.3
0.2
0.1
0.0
0
-1
-2
-3
-4
PTPTIBDT-OD:[70]PCBM
PEDOT:PSS
ITO
-0.5
0.0
0.5
1.0
1.5
400
500
600
700
800
Glass
Voltage (V)
Wavelength (nm)
Fig. 4 (a) Layout of the PTPTIBDT‐OD:[70]PCBM homo‐tandem polymer solar cell with Nafion or MoO3 layers in the intermediate contact. (b) Comparison of the J‐V
characteristics of PTPTIBDT‐OD:[70]PCBM homo‐tandem solar cells with and without Nafion or MoO3 layers in the intermediate contact. (c) EQE measurement of a
PTPTIBDT‐OD:[70]PCBM homo‐tandem cell with and without optical bias (markers) in comparison with simulated EQEs of the subcells inside the tandem cell (solid
lines).
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consists of two identical absorber layers, the conventional limiting subcell, but this only holds when both suVbiecwelAlrsticelexOhnibliniet
tandem EQE measurement method described by Gilot et al.27 negligible dark current. Using the high‐cDoOnId: u10c.t1i0v3e9/pCH5TAn1e0u4t5r9aAl
cannot be applied.28 Due to the interference effects in the layer PEDOT in the intermediate contact, this condition is not always
stack, a different spectral response is expected for the front and met, and in such case the unbiased EQE can represent the
the back cell inside the tandem.28 The data markers in Fig. 4c response of the cell with the lowest dark current as explained in
show two different responses recorded when the EQE of the detail by Colsmann et al.29 This is what is seen in the un‐biased
homo‐tandem cell was measured without (black squares) and EQE experiment. The magnitude of the measured EQE is then
with (red circles) bias light (532 nm at an intensity to give a less than the actual EQE and consequently the actual current
Jsc equivalent to that for AM1.5G white light), both measured at generation of the subcell cannot be estimated in this case.
zero electrical bias for the tandem cell. The two measurements
The Voc of the optimized tandem cell reached up to 1.74 V.
clearly show different spectral responses. To gain more insight Surprisingly, the fill factor of the homo‐tandem cells with MoO3
into the origin of this difference, optical modelling using the layers were high (0.73‐0.75) likely in consequence of the
transfer matrix formalism was performed on the homo‐tandem reduced light intensity in the subcells compared to the single
device. The simulations use the wavelength dependent junction. Thus, Vmax of the optimized tandem was very high
refractive index (n) and extinction coefficient (k) for all layers in (1.48 V). In fact this Vmax enables water splitting to take place at
the stack. The n and k values determined for the PTPTIBDT‐ the maximum power point of this solar cell.
OD:[70]PCBM blend are shown in the ESI†. The simulaꢀons
provide the fraction of light absorbed by the front and back cell Light‐driven electrochemical water splitting
inside the tandem cell. The fraction of absorbed light at each
For a high solar‐to‐hydrogen energy conversion efficiency (ηSTH
)
wavelength was then corrected by the internal quantum
efficiency (IQE) to obtain an estimate for the EQE of each of the
two subcells (solid lines in Fig. 4c). The IQE used in the
simulations was determined from the ratio of a single junction
EQE and the fraction of absorbed photons, and was assumed to
be the same for both subcells. When the experimental and
simulated EQEs are compared, it is seen that the EQE
measurement without optical bias resembles the expected
response of the front cell, while the measurement with optical
bias coincides fairly well with the predicted response of the back
cell response (Fig. 4c). The latter is expected, because the front
cell absorption at 532 nm is significantly higher than the back
cell (solid black line in Fig. 4c), such that under 532 nm
illumination the back cell is current limiting and will be the one
that is measured in an EQE experiment (red circles in Fig. 4c).
The AM1.5 G integrated current density estimated from the
light biased‐EQE measurement is 4.00 mA cm−2 and corresponds
(in a first approximation) to the short‐circuit current of the
tandem cell. Consistently, the value corresponds with the Jsc =
4.19 mA cm−2, measured in the J‐V characteristics. Without light
bias, one would expect the EQE to always reflect the current‐
in light‐driven electrochemical water splitting, an optimized
tandem solar cell (0.0676 cm2) was connected to two RuO2
catalytic electrodes on Ti substrates (~1.2 cm2 catalyst) for
oxygen and hydrogen evolution in 1.0 M KOH. The reasons for
selecting RuO2 catalysts for both reactions are the low
overpotentials, the ease of preparation and the good stability of
the catalyst.6 The solar cell was illuminated with simulated
AM1.5G solar light for 20 min. The current density and voltage
characteristics measured during a 20 min. water splitting
experiment are shown in Fig 5. After 15 min. of water splitting,
the stabilized operating voltage (Vop) is 1.50 V, which is very
close to the Vmax of the solar cell (1.48 V). Hence, the homo‐
tandem cell operates almost at its maximum power point,
which is tailored to sunlight‐driven electrochemical water
splitting.
Fig. 5a shows the J‐V characteristics of the solar cell just
before, during and after the 20 min. experiment. The J‐V curves
of the tandem cell before and after the water splitting
experiment demonstrate the stability of the tandem cell during
the test. The current under operation, Jop = 3.46 mA cm‐2, shows
a minimal decrease with time (Fig. 5b), reflecting the stability of
4.5
(b)
(a)
1.6
1.4
1.2
1.0
0.8
before
1
4.0
3.5
3.0
2.5
2.0
after
during water splitting
0
Jsc = 4.14 mA cm-2
Voc = 1.76 V
-1
-2
-3
-4
FF = 0.73
Pmax= 5.28 mW cm-2
Vop = 1.50 V
op = 3.46 mA cm-2
STH = 4.3%
0.6
0.4
0.2
0.0
1.5
1.0
0.5
0.0
J

0
2
4
6
8
10 12 14 16 18 20
-0.5
0.0
0.5
1.0
1.5
2.0
Voltage (V)
Time (min)
Fig. 5 (a) JV curves of the PTPTIBDT‐OD:[70]PCBM homo‐tandem cell before, during, and after the 20 min. light‐driven electrochemical water splitting experiment.
(b) Simultaneous measurement of operating voltage and current density of the device during light‐driven electrochemical water splitting using RuO2 catalysts in 1.0 M
KOH. The light source is not chopped and the electrolyte is not stirred during measurements.
4 | J. Mater. Chen. A, 2015, 00, 1‐3
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Table 2 Operating conditions for light‐driven electrochemical water splitting
View Article Online
DOI: 10.1039/C5TA10459A
0.6
Catalyst area
1.2 cm2
with different catalysts.
RuO2 Total
0.12 cm2
Jop [mA cm‐2]
ηSTH [%]
4.3
4.3
0.5
0.4
0.3
0.2
0.1
0.0
OER
RuO2
RuO2
HER
RuO2
Pt
Vop [V]
1.50
1.48
3.46
3.49
RuO2 for O2
RuO2 for H2
for hydrogen evolution with a Pt plate (~1.5 cm2). Fig. 7 shows
the characterization of a 20 min. water splitting experiment.
The results are very similar to the previous experiment with two
RuO2 catalysts. Replacing RuO2 with Pt for hydrogen evolution
reduced the stabilized Vop to 1.48 V. The resulting Jop (3.49 mA
cm‐2) and ηSTH (1.23 × 3.49 / 100 = 4.3%) after 15 min. of
operation were very similar to those with RuO2 as catalyst for
hydrogen evolution. The stabilization of Vop and Jop in this case
was faster. A reason can be that the charging of the double layer
on Pt occurs faster than RuO2 for hydrogen evolution. The
stability of the homo‐tandem cell is evidenced in Fig. 6a.
For future and long term application the stability of the light‐
driven electrochemical water splitting stability is important. The
20 min. experiments depicted in Figs. 5b and 7b indicate no fast
degradation, but cannot be taken as evidence for stable
operation. For stable light‐driven water splitting the stability of
the polymer solar cells and the catalysts are important. The
stability of polymer solar cells has been discussed in several
reviews30‐32 and is determined by the stability of the active
layer, the electrodes, and the interface layers which can be
influenced by light, temperature, and the ingress of water and
oxygen. The stability of RuO2 for oxygen and hydrogen evolution
reactions in alkaline electrolytes has been described in detail,33
showing that dissolution of RuO2 during oxygen evolution is
limiting the stability but that it is stable during hydrogen
evolution. In the ESI† we demonstrate the stability of the RuO2
catalysts for 2 h, at current densities 20 times higher than in the
water splitting experiments.
-4
-3
-2
Electrochemical current density [log(A cm-2)]
Fig. 6 Tafel plots of RuO2 catalysts for hydrogen and oxygen evolution in 1.0
M of KOH. The vertical lines indicate the current densities reached for the
solar cell in Fig 5b for two different catalyst areas during light‐driven
electrochemical water splitting
the solar cell and the catalysts. Assuming 100% Faradaic
efficiency, the ηSTH = 1.23 × 3.46 / 100 = 4.3% after 15 min. of
operation (Table 2).
In the experiment we use a 0.0676 cm2 solar cell in
combination with catalyst areas of 1.1 and 1.3 cm2. The relative
areas of the solar cell and the catalyst are important for the
overpotential. The Tafel plot in Fig. 6 shows the overpotentials
for hydrogen and oxygen evolution for RuO2 catalysts as
function of the current density. At the current densities reached
in the solar cell and the catalyst, the expected total
overpotential is about 0.25 V, resulting in a total potential of
1.23 + 0.25 = 1.48 V, in very good agreement with the operating
potential of 1.50 V (Fig 5b). If the catalyst area would be
reduced, the overpotential would increase and this would move
the operating point away from the maximum power point of the
solar cell. A reduction of the catalyst area by e.g. a factor of ten
would increase the overpotential by 0.1 V and reduce ηSTH to
3.5%. Hence, balancing the nature and surface area of the
catalysts with the materials used in the solar cell is crucial to
achieve optimal performance.
Conclusions
Next to RuO2 we also tested Pt as catalyst for hydrogen
evolution reaction.3,7,8 For this purpose, the preceding
experiment was repeated using identical conditions and the
same homo‐tandem solar cell, only replacing the RuO2 catalyst
Light‐driven electrochemical water splitting has been
demonstrated using a new wide band gap semiconductor blend,
PTPTIBDT‐OD:[70]PCBM, in a homo‐tandem cell configuration.
4.5
4.0
3.5
3.0
2.5
2.0
(b)
(a)
1.6
1.4
1.2
1.0
0.8
before
1
after
during water splitting
0
Jsc = 4.14 mA cm-2
Voc = 1.76 V
-1
-2
-3
-4
FF = 0.73
Pmax= 5.28 mW cm-2
Vop = 1.50 V
op = 3.46 mA cm-2
STH = 4.3%
0.6
0.4
0.2
0.0
1.5
1.0
0.5
0.0
J

0
2
4
6
8
10 12 14 16 18 20
-0.5
0.0
0.5
1.0
1.5
2.0
Voltage (V)
Time (min)
Fig. 7 (a) JV curves of the PTPTIBDT‐OD:[70]PCBM homo‐tandem cell before, during and after a light‐driven electrochemical water splitting experiment of 20 min. (b)
Simultaneous measurement of operating voltage and current density of the device during light‐driven electrochemical water splitting using RuO2 and Pt catalysts in
1.0 M KOH. The light source is not chopped and the electrolyte is not stirred during measurements.
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Journal of Materials Chemistry A
To achieve a high enough voltage the homo‐tandem cell uses an Device preparation
intermediate recombination contact layer that consists of a thin
View Article Online
DOI: 10.1039/C5TA10459A
Photovoltaic devices were prepared by first spin casting a 40 nm
thick PEDOT:PSS (Clevios® P VP AI 4083, H. C. Starck) onto pre‐
cleaned glass substrates with indium tin oxide (ITO) patterns
(Naranjo Substrates). Prior to spin casting, PEDOT:PSS was
filtered through an Acrodisc® LC25mm syringe filter with
0.45μm PVDF membrane. Then, the active layer was spin cast.
Single junction devices were completed by thermally
evaporating a back contact of 1 nm LiF and 100 nm Al at 3 × 10−7
mbar. For the tandem devices with a Nafion layer, the
intermediate contact consisting of ZnO nanoparticles (~30 nm),
pH neutral PEDOT:PSS (~15 nm), and Nafion (~4 nm) was
deposited sequentially by spin casting. For devices with the
MoO3 layer, first ZnO nanoparticles (~30 nm), and pH neutral
PEDOT:PSS (~15 nm), were spin cast. Afterwards, a 10 nm thick
MoO3 film was deposited by thermal evaporation. For optimal
performance, ZnO nanoparticles were deposited inside a glove
box with nitrogen environment, while all other layers were spin
cast in air. No heat treatment was applied to any of the layers.
Layer thicknesses were measured with a Veeco Dektak 150
surface profiler.
layer of MoO3 deposited on a conventional ZnO/pH neutral
PEDOT:PSS junction to increase the work function at the hole
extracting side of the stack.
For light‐driven electrochemical water splitting, the
PTPTIBDT‐OD:[70]PCBM homo‐tandem cell was combined with
a RuO2 catalyst for oxygen evolution and RuO2 or Pt catalysts for
hydrogen evolution in 1.0 M KOH. The ηSTH is 4.3% and the
device operated almost at the maximum power point of the
solar cell due to its remarkably good fill factor (0.73). As a
consequence, the difference between the PCE of the solar cell
and the ηSTH water splitting is solely due to overpotential losses.
Further improvements in ηSTH will rely on improved polymer
semiconductors to increase PCE and better electrocatalysts,
rather than improving the design or optimization of light‐driven
electrochemical water splitting device.
Experimental Section
Materials
For PTPTIBDT‐OD:[70]PCBM homo‐tandems, both active
layers were spin cast from a solution of PTPTIBDT‐OD and
[70]PCBM (1:1.5 w/w) in chloroform containing 10 vol% o‐DCB
at 7 mg mL‐1 polymer concentration. ZnO nanoparticles34,35 of
~5 nm diameter were spin cast from a solution of 10 mg mL‐1
ZnO in isopropanol (IPA). The pH neutral PEDOT:PSS was
prepared by diluting Orgacon Neutral pH PEDOT:PSS from Agfa
with ultra‐pure water at a 1:1 volume ratio and adding 200 μL L‐
All commercial chemicals were used as received. Ruthenium(III)
chloride (35‐40% Ru) was obtained from Acros Organics.
Platinum plate was obtained from Drijfhout. PTPTIBDT‐OD was
synthesized as described in the ESI†. [70]PCBM was obtained
from Solenne BV. Water used in depositions and electrolytes
was purified in a Millipore system and has a resistance of at
least 18 MΩ.
1
IPA. The solution was then filtered with a 5.0 µm Whatman
Electrochemical measurements
Puradisc FP30 syringe filter. Nafion® (Aldrich chemistry,
perfluorinated ion exchange resin 5 wt.% in mixture of lower
aliphatic alcohols and H2O, containing 45% water) was first
diluted in ethanol with a 1:200 volume ratio. The resulting
solution was spin cast directly onto pH neutral PEDOT for an
optimized thickness of 2‐5 nm.
Cyclic voltammetry (CV) was conducted under inert atmosphere
using an Autolab PGSTAT30 with a three electrode setup
equipped with an ITO working electrode covered with a spin
coated polymer film, a silver counter electrode, and a silver
electrode coated with silver chloride (Ag/AgCl) as quasi
reference
electrode
in
combination
with
ferrocene/ferrocenium (Fc/Fc+) as internal standard. A 1 M
solution of tetrabutylammonium hexafluorophosphate
(TBAPF6) in acetonitrile was used as the electrolyte. HOMO and
LUMO levels were calculated from the oxidation and reduction
potentials Eox/red vs. Fc/Fc+ using EHOMO/LUMO = – 5.23 – qEox/red eV.
The Tafel plot was constructed from the cyclic voltammetry (CV)
measurements performed in the stationary current mode as
described in detail in Ref. 6.
Characterization
A Keithley 2400 source‐measurement unit was used to measure
current density to voltage (J‐V) characteristics of the devices.
The illumination was carried out with ~100 mW cm‐2 white light
from a tungsten‐halogen lamp filtered by a Schott GG385 UV
filter and a Hoya LB120 daylight filter. No mismatch correction
was performed. The measurements were performed inside a
glove box with a nitrogen atmosphere. The tandem devices
were exposed to UV illumination (with a Spectroline EN‐160L/F
365 nm lamp from Spectronics Corporation) for about 10 min.
to provide an Ohmic contact between the ZnO and pH neutral
PEDOT layers before being measured. To prevent parasitical
charge collection due to the high lateral conductivity of pH
neutral PEDOT, tandem and triple junction devices were
measured with a mask that is slightly smaller than the device
area, which is determined by the overlap of the ITO and Al
electrodes. The actual device area was 0.09 cm2, while
corresponding mask size was 0.0676 cm2. For single junction
devices, more accurate short circuit currents under AM1.5G
RuO2 catalysts from thermal decomposition
Ti substrates (thickness 0.5 mm, 1.5 cm × 2 cm) were sonicated
in acetone and then treated with air plasma in a Femto PCCE
low pressure plasma system (Diener Electronic). A 270 W
plasma was applied for 2 min. The RuO2 catalysts was prepared
via thermal decomposition of RuCl3. For this purpose an
aqueous RuCl3 solution (200 μL of a 0.2 M) was placed onto a
titanium substrate to form a catalyst area of about 1.3 cm2. The
substrate was first dried at 90 °C on a hot plate for 20 min. and
then oxidized at 350 °C in air in an oven for 3 h.
6 | J. Mater. Chen. A, 2015, 00, 1‐3
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were calculated by convolution of the EQE measurements with for simultaneous measurement of current and voltage during
View Article Online
DOI: 10.1039/C5TA10459A
the AM1.5G solar spectrum.
water splitting.
EQE measurements were performed in a homebuilt set‐up.
Mechanically modulated (SR 540, Stanford Research)
monochromatic (Oriel Cornerstone 130) light from a 50 W
tungsten halogen lamp (Osram 64610) was used as a probe light
together with continuous bias light from a solid state laser
(B&W Tek Inc., λ = 532 nm, 30 mW) through an aperture of 2
mm diameter. The intensity of the bias laser was adjusted using
a variable neutral density filter. The response was recorded
using a lock‐in amplifier (Stanford Research Systems SR830),
over a resistance of 50 Ω. For all the single junction devices, the
measurement was carried out under representative
illumination intensity (AM1.5G equivalent, provided by the 532
nm laser). A calibrated silicon solar cell was used as reference.
Devices were kept behind a quartz window in a nitrogen filled
box during the measurements.
Solar to hydrogen conversion efficiencies were determined
using a home‐built setup. As the water splitting experiments
took place in air, the solar cells were kept behind a quartz
window in a nitrogen filled box and connected to the catalysts
through external cables. The solar cell was illuminated with
white‐light from a tungsten‐halogen lamp (~100 mW cm‐2)
filtered by a Schott GG385 UV filter and a Hoya HMC 80A 72 mm
daylight filter. The solar cell was positioned by assuring that the
short‐circuit current in this setup corresponds to AM1.5G power
standards. A Keithley 2600 source‐measurement unit was used
Optical Simulations
The optical modelling was done in SetFos version 3.2 (Fluxim
AG, Switzerland). The optical constants for PTPTIBDT‐
OD:[70]PCBM were determined from reflection/transmission
measurements on active layers on quartz.
Acknowledgements
This project was carried out within the research programme of
BioSolarCells, co‐financed by the Dutch Ministry of Economic
Affairs. This work is part of the research programme of the
Foundation for Fundamental Research on Matter (FOM), which
is part of the Netherlands Organization for Scientific Research
(NWO). The research leading to these results has further
received funding from the European Research Council under the
European Union's Seventh Framework Programme (FP/2007‐
2013) / ERC Grant Agreement No. 339031 and the Mujulima
project, Grant Agreement No. 604148. The research is part of
the Solliance OPV programme and received funding from the
Ministry of Education, Culture, and Science (NWO Gravity
program 024.001.035).
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