Water Splitting with Series-Connected Polymer Solar Cells - ACS

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Water Splitting with Series-Connected Polymer Solar Cells Serkan Esiner, Harm van Eersel, Gijs W.P. van Pruissen, Mathieu Turbiez, Martijn M. Wienk, and René A. J. Janssen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06381 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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ACS Applied Materials & Interfaces

Water Splitting with Series-Connected Polymer Solar Cells Serkan Esiner,† Harm van Eersel,‡ Gijs W. P. van Pruissen,† Mathieu Turbiez,§ Martijn M. Wienk†,|| and René A. J. Janssen*,†,|| †

Molecular Materials and Nanosystems, Institute for Complex Molecular Systems, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands.

‡

Simbeyond B.V., P.O. Box 513, NL 5600 MB Eindhoven, The Netherlands.

§

BASF Schweiz AG, Schwarzwaldallee 215, CH-4002 Basel, Switzerland.

||

Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612 AJ Eindhoven, The Netherlands.

E-mail: [email protected]. Abstract: We investigate light-driven electrochemical water splitting with series-connected polymer solar cells using a combined experimental and modeling approach. The expected maximum solar-tohydrogen conversion efficiency (ηSTH) for light-driven water splitting is modeled for two, three, and four series-connected polymer solar cells. In the modeling we assume an electrochemical water splitting potential of 1.50 V and a polymer solar cell for which the external quantum efficiency and fill factor are both 0.65. The minimum photon energy loss (Eloss), defined as the energy difference between the optical band gap (Eg) and the open-circuit voltage (Voc) is set to 0.8 eV, which we consider a realistic value for polymer solar cells. Within these approximations, two seriesconnected single junction cells with Eg = 1.73 eV or three series-connected cells with Eg = 1.44 eV are both expected to give a ηSTH of 6.9%. For four series-connected cells the maximum ηSTH is slightly less at 6.2% at an optimal Eg = 1.33 eV. Water splitting was performed with seriesconnected polymer solar cells using polymers with different band gaps. PTPTIBDT-OD (Eg = 1.89 eV), PTB7-Th (Eg = 1.56 eV), and PDPP5T-2 (Eg = 1.44 eV) were blended with [70]PCBM as absorber layer for two, three, and four series-connected configurations respectively, and provide ηSTH of 4.1%, 6.1%, and 4.9% when using a retroreflective foil on top of the cell to enhance light 1

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absorption. The reasons for deviations with experiments are analyzed and found to be due to differences in Eg and Eloss. Light-driven electrochemical water splitting was also modeled for multijunction polymer solar cells, having vertically stacked photoactive layers. Under identical assumptions, an ηSTH of 10.0% is predicted for multi-junction cells. KEYWORDS:

light-driven

electrochemical

water

splitting,

organic

photovoltaics,

semiconducting polymer, fullerene, solar to hydrogen conversion, overpotential 1. Introduction Reaching high efficiencies in light-driven electrochemical water splitting involves generating a high current density at the required electrochemical potential, which is determined by the standard potential of 1.23 V and the overpotentials for the hydrogen and oxygen evolution reactions. Single wide band gap absorber materials, such as TiO2,1 can provide sufficient voltage but generally give low solar-to-hydrogen conversion efficiencies (ηSTH) because they only absorb UV-light and consequently generate low current densities in sunlight. Hence, wide band gap single junction solar cells are less feasible for solar water splitting purposes. Stacking solar cells with different optical band gaps on top of each other is a well-known strategy to increase the photovoltaic conversion efficiency. Such multi-junction cells are also able to convert low energy photons and, simultaneously, reduce thermalization losses of high-energy photons.2 If the sub cells are stacked in a series connection, the potentials of the sub cells add up and can exceed the threshold for electrochemical water splitting. Accordingly, light-driven electrochemical water splitting generally involves the use of tandem or triple junction solar cells with two or three absorber layers.3-10 Besides stacking multiple layers on top of each other, the necessary high voltage for water splitting can also be obtained via series connection of single junction solar cells positioned next to each other. Depending on the optical band gap of the absorber material, two, three, or four single junction solar cells of the same material can be connected in series and water splitting can be tuned to take place at the maximum power point of these series-connected solar cells. Lately, several examples of a high ηSTH have been reported for light-driven electrochemical water splitting using series-connected single junction solar cells. Well-known examples comprise the work of Jacobsson et al., where three CuInxGa1−xSe2 cells are connected in series and combined with two platinum electrodes for a ηSTH of over 10%.11 Grätzel et al. have demonstrated light-driven water splitting 2


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with two series-connected CH3NH3PbI3 perovskite solar cells in combination with NiFe layered double hydroxide catalysts at a ηSTH = 12.3%.12 In the work of Cox et al., four series-connected crystalline silicon solar cells are combined with earth abundant nickel-borate and nickelmolybdenum-zinc catalysts resulting in a ηSTH of ~10%.13 Jacobsson et al. have studied the theoretical efficiency limits of light-driven electrochemical water splitting using series-connected single junction inorganic solar cells in comparison with multi-junction solar cells.14 Next to inorganic semiconductors also organic and polymer semiconductors can be used as active layer in photoelectrochemical water splitting. Organic dyes in combination with a suitable catalysts have successfully used as photoanodes for oxygen evolution,15-18 and there are also recent examples of photocathodes consisting of an organic semiconductor covered with MoS3, TiO2, or CdSe, that enable photoelectrochemical hydrogen evolution together with an external potential.1925

Also organic dye-sensitized tandem photoelectrochemical cells for light driven water splitting

using molecular catalysts have been reported.26,27 Alternatively, organic or polymer solar cells can be used to provide the required potential for water splitting, especially when using organic multi-junction solar cells.28-33 Light-driven electrochemical water splitting with tandem and triple junction polymer solar cells has recently been demonstrated.34-38 With the use of transition metal and metal oxide catalysts, water splitting potentials were reduced to ~1.50 V, resulting in ηSTH up to 6%. In these examples either different34,35 or identical36-38 polymer solar cells where stacked using appropriate recombination layers to optically and electrically connect the different sub cells. In organic solar cells, the use of multiple identical layers stacked on top of each other can have an advantage compared to single junction cells,31,32 mainly because of improved charge collection. When two or more identical sub cells are stacked, the sub cells can be thinner, while still absorbing a large fraction of the light, which reduces the electronic losses due to recombination and space charge effects and provides an enhanced power conversion efficiency in organic solar cells.39 However, for organic materials in which the charge collection is less thickness dependent,40 such advantage would not be expected. Of course, another way the reach the required potential for water splitting is by connecting multiple organic solar cells in series. Series-connected polymer solar cells eliminate the electrical or optical losses that may occur in the recombination layers between the sub cells of a multijunction solar cell. An example of light-driven electrochemical water splitting with seriesconnected polymer solar cells has been reported by Aoki et al. who used six series-connected 3



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devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:[60]PCBM) active layer in combination with platinum electrodes.41 The six seriesconnected solar cells had a power conversion efficiency (PCE) of 1.47%, but no ηSTH was reported. Even though multi-junction cells are expected to outperform single junction cells, the highest PCEs reached for single and multi-junction polymer solar cells are presently very similar at 1012%.31,42−45 Moreover, manufacturing a multi-junction device requires more effort than a single junction solar cell. Therefore, it is of interest to compare the efficiency for light-driven electrochemical water splitting of series-connected single junctions to that of multi-junctions. While connecting polymer solar cells in series itself is trivial, it is not if one aims at a high ηSTH in water splitting. The design of efficient light-driven electrochemical water splitting depends on the catalysts used, the optical band gap of the polymers, and the number of the cells used, which can be only an integer number. As an example, by lowering the band gap a larger fraction of the solar spectrum can be absorbed, but eventually the open-circuit voltage will drop to the point that an additional cell is needed to achieve the required voltage for water splitting. Starting with n cells in series, an additional cell, will reduce the current density and, hence, ηSTH by a factor n/(n+1). Hence, optical band gap and number of cells and catalysts form a subtle balance. In this paper we first investigate the theoretical limits for ηSTH in series-connected single junction polymer solar cells using a fixed potential of 1.50 V for water splitting, which can be achieved by common catalysts for hydrogen and oxygen evolution. We use a combination of two, three, or four series-connected single junction cells for water splitting and compare this with the highest efficiencies for tandem and triple junction polymer solar cells. Subsequently, three different photoactive layers with different optical band gaps are selected and used in two, three, or four of series-connected cells for light-driven electrochemical water splitting. The ηSTH was improved by applying a retroreflective foil to increase light absorption.46 The results are compared with the modeling and the reasons for the differences are discussed and compared to those for multi-junction polymer solar cells.34-38

2. Results and discussion

2.1 Modeling

4


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The expected ηSTH for two, three, and four series-connected single junction polymer solar cells was calculated as a function of the optical band gap (Eg) of the photoactive layer using a number of assumptions. First, the maximum achievable open-circuit voltage (Voc) is assumed to be related to the optical band gap (Eg) via qVoc = Eg – Eloss, with q the elementary charge and Eloss the minimum photon energy loss in polymer solar cells. Eloss is an adjustable parameter. Previously we have argued that for polymer solar cells the lowest possible Eloss equals about 0.6 eV.47 Although this is not a fundamental limit, only few examples are known for which Eloss ≤ 0.6 eV.48-52 On the other hand, most examples of high efficiency polymer solar cells often contain photoactive materials where Eloss ≈ 0.8 eV, mainly because of a more efficient charge generation at higher energy loss.49 As an example the most efficient single junction polymer solar cell reported to date has Eloss ≈ 0.84 eV at a PCE of 11.7%.44 Therefore, the calculations were performed using Eloss = 0.8 eV. In the modeling, the short circuit current density (Jsc) of the solar cell was calculated by assuming a fixed external quantum efficiency (EQE) of 0.65 below the optical band gap and integrating the EQE with the AM1.5G solar spectrum. The fill factor (FF) was fixed at 0.65. Both assumptions are not theoretical maxima,53 but are rather realistic achievable estimates. The water splitting potential was fixed at 1.50 V based on the electrochemical potential required for electrochemical water splitting using ruthenium oxide (RuO2) and platinum (Pt) catalysts at the current densities in the experiment.36 The current density at 1.50 V (J1.50V) was determined using a normalized typical J−V curve of a polymer solar cell with FF = 0.65 which was then scaled with the calculated Jsc and Voc. To estimate ηSTH, the J1.50V was determined for two, three, or four seriesconnected single junction solar cells and converted into ηSTH in sunlight using ηSTH = 1.23×J1.5V/PAM15G, assuming 100% Faradaic efficiency and an AM1.5G solar radiation PAM15G of 100 mW cm−2. The assumed 100% Faradaic efficiency is of course an upper limit, but close to the experimental value as shown in the Supporting Information, and the assumption is often used in this field to convert current density into hydrogen produced.11-14 The solid lines in Figure 1 show the expected ηSTH versus optical band gap of the photoactive material for two, three, and four series-connected single junction polymer solar cells using these assumptions and Eloss = 0.8 eV. The steady increase in ηSTH values with decreasing Eg values is caused by the fact that a lower band gap allows more photons from the solar spectrum to be converted. This provides a higher current and hence a higher ηSTH, until a maximum is reached because the potential, rather than the current becomes limiting. The steep decrease of ηSTH when 5



the band gap is reduced below the optimal value is caused by the fact that beyond the maximum the potential of the cells is no longer sufficient to sustain 1.5 V. The reason for the steepness lies in the steepness of the J-V curve of the solar cell between the maximum power point and opencircuit conditions, where the photocurrent, and hence ηSTH, drops rapidly. These calculations suggest that ηSTH can reach 6.9% (Eg = 1.73 eV), 6.9% (Eg = 1.44 eV), and 6.2% (Eg = 1.33 eV) for two, three, and four series-connected single junction cells, respectively. If it is possible to reduce Eloss to 0.6 eV with future and improved materials, significantly higher ηSTH can be expected. For two series-connected wide band gap solar cells ηSTH can reach 9.0% for Eg = 1.53 eV, while for three and four series-connected cells, the highest possible ηSTH would be 8.4% and 7.6% at band gaps of 1.24 and 1.12 eV, respectively (dashed lines in Figure 1).

10 Solid: Eloss = 0.8 eV

9

Dashed: Eloss = 0.6 eV

8

2 series 3 series 4 series

7

STH [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

6 5 4 3 2 1 0 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Band gap Eg [eV]

Figure 1. Predicted ηSTH for a light-driven electrochemical water splitting cell operating at 1.50 V with two, three, or four series-connected single junction polymer solar cells with varying band gaps. Single junction cells are assumed to have an EQE of 0.65 and a FF of 0.65. Solid lines correspond to calculations based on Eloss = Eg − qVoc = 0.8 eV, dashed lines are based on Eloss = 0.6 eV. 2.2 Materials selection The solid lines in Figure 1 suggest photoactive materials with band gaps of 1.72, 1.44, and 1.31 eV for optimized two, three, and four series-connected cells. The actual selection of the materials was done by also taking into account the actual Voc of single junction solar cells made with these materials. Polymers chosen for two, three, and four series-connected cell configurations are PTPTIBDT-OD

(poly(4,10-(2'-octyldodecyl)-4,10-dihydrothieno[2',3':5,6]-pyrido[3,46


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g]thieno[3,2-c]isoquinoline-5,11-dione-alt-benzo[1,2-b:4,5-b']dithiophene), PTB7-Th (poly[(4,8bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b0](dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)thieno[3,4-b]thiophene)-2,6-diyl]), and PDPP5T-2 (poly[3,6-bis(4-hexyl[2,2' -bithiophen]-5-yl)2,5-bis(2-hexyldecyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-l,4-dione-alt-thiophene-2,5-diyl]) respectively. Figure 2 shows the molecular structures of the selected polymers. Optical band gaps of the selected materials are listed in Table 1.

Figure 2. Structures of PTPTIBDT-OD, PTB7-Th, and PDPP5T-2 polymers. Single junction polymer solar cells were made by blending these polymers with [6,6]-phenylC71-butyric acid methyl ester ([70]PCBM). Figure 3a shows the J-V curves for single junction solar cells of each material with optimized thicknesses. The corresponding EQE measurements of the single junction cells are shown in Figure 3b. The maximum EQEs for all solar cells are around 0.65. The PTPTIBDT-OD:[70]PCBM cell has a Voc of 0.90 V. Based on the optical band gap of PTPTIBDT-OD (1.89 eV) the cell has a high energy loss Eloss = 0.99 eV (Table 1). It is, however, important to realize that charge transfer can also take place from the acceptor to the donor material. In this case, the Eloss would be 0.85 eV based on Eg = 1.75 eV for [70]PCBM. In the maximum power point two series-connected PTPTIBDT-OD:[70]PCBM cells have an operating voltage close to 1.50 V, which is sufficient for water splitting,. The PTB7-Th:[70]PCBM cell has a lower Eloss (0.77 eV) with Eg = 1.56 eV and Voc = 0.79 V. A Voc of ~2.4 V is expected when three PTB7Th:[70]PCBM cells are connected in series. This high Voc might result in the cell to operate below its maximum power point during water splitting. PDPP5T-2 also has a band gap (Eg = 1.44 eV) lying above the optimal value suggested by the modeling (Eg = 1.31 eV) for four series-connected cells. In fact, the band gap of this polymer is exactly the same as what is suggested for three series-

7



connected cells. However, the Voc of PDPP5T-2:[70]PCBM cell is only 0.55 V. This voltage is not high enough for efficient water splitting with three series-connected cells. (b) 0.7

(a) 0

PPTPTIBDT-OD:[70]PCBM PTB7-Th:[70]PCBM PDPP5T:[70]PCBM

0.6 0.5

-5

EQE

Current density [mA cm-2]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.3

-10 0.2

-0.2

PTPTIBDT-OD:[70]PCBM PTB7-Th:[70]PCBM PDPP5T:[70]PCBM

0.1

-15 0.0

0.2

0.4

0.6

0.8

0.0

1.0

400

500

600

700

800

900

Wavelength [nm]

Voltage [V]

Figure 3. (a) J-V curves of single junction solar cells made with three polymers with different optical band gaps. (b) EQE measurements of the same cells. Table 1. Polymers used for Light-Driven Electrochemical Water Splitting with Series-Connected Single Junction Polymer Solar Cells Polymer

Eg [eV]

Voc [V]

Eloss [eV]

PTPTIBDT-OD

1.89

0.90

0.99

PTB7-Th

1.56

0.79

0.77

PDPP5T-2

1.44

0.55

0.89

2.3 Light-driven electrochemical water splitting with polymer solar cells Water splitting experiments were performed in a 1.0 M KOH aqueous electrolyte with RuO2 and Pt catalysts for oxygen and hydrogen evolution reactions, respectively. The RuO2 catalyst was deposited onto a titanium substrate by thermal decomposition of RuCl3. To reduce current density and overpotentials on catalyst surfaces, large-area catalysts were combined with relatively smaller sized solar cells. The solar cell surface areas were 0.12 cm2 per single series-connected cell. The catalyst surface areas were ~1.2 cm2 for RuO2 and ~1.5 cm2 for Pt. As one substrate consists of four equally sized (0.12 cm2) solar cells, series connection of the single junction cells was easily done via external wiring. Solar cells and catalysts were also connected externally. While integrated water splitting devices in which the catalysts are directly placed on the contacts of the polymer solar cells35 and monolithic interconnection54 have been demonstrated a wired configuration has 8


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the practical advantage that allows to measure current and voltage in time.11,12 The electrochemical experiment took place in air, but the solar cells were kept behind a quartz window in a nitrogen filled box during the experiment, to exclude the effect of moisture and oxygen on the solar cell, which can affect the aluminum and PEDOT:PSS electrodes and the organic semiconductor. The box was aligned precisely, allowing every cell on the substrate to perform under identical illumination conditions. The solar cells were then illuminated with simulated AM 1.5G solar light for 20 min. water splitting. 2.3.1 Two series-connected PTPTIBDT-OD:[70]PCBM cells A single junction PTPTIBDT-OD:[70]PCBM cell has a PCE of ~5.5% (Table 2). In a first approximation, the efficiency remains the same when two cells with the same area are connected in series because the Voc doubles and the Jsc halves. In this case the two series-connected cells have an efficiency of ~5.2%. The Jsc and PCE can enhanced by placing a textured retroreflective foil on top of the polymer solar cell.46 The surface textures are designed to change the angle of incidence of the light and enhance light absorption.55 When the retroreflective foil is applied onto the substrate, the efficiency of the series-connected cells increases by 14.5%, due to an increased Jsc as a result of more efficient light absorption. Table 2. Characteristics of Single Junction and Two Series-Connected PTPTIBDT-OD:[70]PCBM Solar Cells.

a

Cell type

Retroreflective foil

Jsc [mA cm−2]

Voc [V]

FF [-]

PCE [%]

Single junction

No

8.87 a

0.90

0.69

5.49

2 Series-connected

No

4.42 b

1.80

0.66

5.24

2 Series-connected

Yes

4.95 c

1.83

0.66

6.01

Determined by integrating the EQE of the solar cell with the AM1.5G spectrum. b White light

intensity adjusted such that Jsc matches the current of the AM1.5G integrated EQE for the current limiting cell. c Same illumination as without the retroreflective foil. The simultaneous current density and voltage characteristics of two series-connected cells, measured during the 20 min. water splitting experiment are shown in Figure 4b. Solid lines 9



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correspond to measurements without the retroreflective foil, while dashed lines belong to the measurements with the foil. The operating voltage quickly converges to the value of about 1.50 V which is expected for the RuO2/Pt catalysts at current densities employed.35,36 There is a clear difference between the current generation with and without the foil throughout the entire 20 min. period. After 15 min. of operation, the stabilized operating current density (Jop) without the foil is 3.49 mA cm−2, while this value increases to 3.77 mA cm−2 when the foil is applied. The operating voltages (Vop) with and without foil are very similar because the overpotential is only weakly dependent on the current density.36 Figure 4a shows the J-V characteristics of the two series-connected solar cells with a retroreflective foil just before, during and after the 20 min. experiment. J-V curves of the solar cells before and after the water splitting experiment show a slight reduction in Jsc and Voc. Taking the stabilized Jop after 15 min. and assuming 100% Faradaic efficiency, ηSTH = 4.29% without retroreflective foil and ηSTH = 4.64% with retroreflective foil as shown in Figure 4b. 2.3.2 Three series-connected PTB7-Th:[70]PCBM cells The single junction and the three series-connected PTB7-Th:[70]PCBM cells both had a PCE of about 7.8%. The PCE can be increased to 8.7% by applying the retroreflective foil (Table 3). The simultaneous recorded current density and voltage characteristics of the three series-connected cells for 20 min. water splitting (Figure 4d) show stable output for cells with (dashed lines) and without (solid lines) the retroreflective foil. After 15 min. of operation, the stabilized Jop without the foil is 4.46 mA cm−2, while this value increases to 4.95 mA cm−2 when the foil is applied. Assuming 100% Faradaic efficiency, we find ηSTH = 5.49% without retroreflective foil and ηSTH = 6.09% with retroreflective foil. The J-V curves of the solar cells with the foil before and after the water splitting experiment (Figure 4c) are virtually identical. Figure 4c shows that the three seriesconnected solar cells operate at a significant lower voltage (Vop = 1.50 V) while splitting water, than the maximum power point voltage (Vmax = 1.90 V). The large difference of 0.4 V between Vop and Vmax is consistent with the fact that the selected polymer has a larger band gap (Eg = 1.56 eV) than the value suggested by the modeling for three series-connected cells (Eg-opt. = 1.44 eV) in combination with an Eloss = 0.77 eV that is close to the value assumed in the modeling.

10


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Table 3. Characteristics of Single Junction and Three Series-Connected PTB7-Th:[70]PCBM Solar Cells.

a

Cell type


Jsc [mA cm-2]

Voc [V]

FF [-]

PCE [%]

Single junction

No

14.04 a

0.79

0.70

7.79

3 Series-connected

No

4.72 b

2.38

0.70

7.87

3 Series-connected

Yes

5.26 c

2.38

0.69

8.65

Determined by integrating the EQE of the solar cell with the AM1.5G spectrum. b White light

intensity adjusted such that Jsc matches the current of the AM1.5G integrated EQE for the current limiting cell. c Same illumination as without the retroreflective foil. 2.3.3 Four series-connected PDPP5T-2:[70]PCBM cells The PCEs of a single junction and of four series-connected PDPP5T-2:[70]PCBM solar cells are both about 5.6%, which increases to 6.1% by application of the retroreflective foil (Table 4). Lightdriven water splitting with and without the foil stabilizes in the first few minutes (Figure 4f). Figure 4e shows that the J-V characteristics are unchanged after the experiment. The Jop after 15 min. of light-driven electrochemical water splitting results in ηSTH = 4.48% without retroreflective foil and ηSTH = 4.94% with retroreflective foil. With Vop = 1.51 V, the four series-connected cells operate slightly below the maximum power point (Vmax = 1.61 V). Table 4. Characteristics of Single Junction and Four Series-Connected PDPP5T-2:[70]PCBM Solar Cells.

a

Cell Type


Jsc [mA cm-2]

Voc [V]

FF [-]

PCE [%]

Single Junction

No

16.23 a

0.55

0.62

5.57

4 series-connected

No

4.09 b

2.24

0.62

5.65

4 series-connected

Yes

4.57 c

2.23

0.60

6.13

Determined by integrating EQE of the solar cell with the AM1.5G spectrum.

b

White light

intensity adjusted such that Jsc matches the current of the AM1.5G integrated EQE for the current limiting cell. c Same illumination as without the retroreflective foil.

11



0

5

Jsc = 4.95 mA cm-2

-1

Voc = 1.83 V FF = 0.66 Pmax= 6.01 mW cm-2

-2

6

(b) 1.5

before after during water splitting

1

Operating voltage [V]


(a)

-3 -4

4

1.0

0.5

Without foil Vop = 1.46 V

With foil Vop = 1.47 V

3

Jop = 3.49 mA cm-2

Jop = 3.77 mA cm-2

2

STH = 4.29%

STH = 4.64% 1

-5 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0

2

4

6

8

Voltage [V]

(d)

Jsc = 5.26 mA cm-2

-1

Voc = 2.38 V FF = 0.69 Pmax= 8.65 mW cm-2

-2

16

18

20

0

6 1.5 5



0

14

Time [min]


1

12

-3 -4

4

1.0

Jop = 4.46 mA cm

0.5

3

With foil Vop = 1.50 V

Without foil Vop = 1.49 V -2

Jop = 4.95 mA cm

-2

2

STH = 6.09%

STH = 5.49%

1

-5 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0

2

4

6

8

-1


(f) 1.5

Jsc = 4.57 mA cm-2



0

16

18

20

0

6 5

Voc = 2.23 V FF = 0.60 Pmax= 6.13 mW cm-2

-2

14

-3 -4

4

1.0

0.5

Without foil Vop = 1.51 V

With foil Vop = 1.51V

3

Jop = 3.64 mA cm-2

Jop = 4.02 mA cm-2

2

STH = 4.48%

STH = 4.94% 1

-5 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0

2

4

6

8

10

12

14

16

18

20

0

Operating current density J [mA cm-2]

1

12

Time [min]

Voltage [V]

(e)

10

Operating current density [mA cm-2]

(c)

10

Operating current density [mA cm-2]

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Time [min]

Voltage [V]

Figure 4. (a, c, e) JV curves of series-connected polymer solar cells with retroreflective foil before, during and after a light-driven electrochemical water splitting experiment of 20 min. (b, d, f) Simultaneous measurement of operating voltage and current density of the solar cells with (dashed lines) and without (solid lines) retroreflective foil during light-driven electrochemical water splitting. Two, three, or four cells of 0.12 cm2 were connected in series and then connected to RuO2 – Pt catalysts in 1.0 M KOH. The light source is not chopped and the electrolyte is not stirred during measurements. 12


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2.3.4 Comparing two, three, and four series-connected cells Figure 5 and Table 5 compare the experimental ηSTH values to the predictions for light-driven electrochemical water splitting with two, three, or four series-connected single junction cells. Since the selected polymers all had energy losses of Eloss ≥ 0.77 eV, the predictions with Eloss = 0.8 eV were used for comparison. The experiments gave lower ηSTH values than the modeling. The first reason for this deviation is the mismatch between the modeled and actual energy losses Eloss. Only for the best performing PTB7-Th:[70]PCBM cells the actual Eloss = 0.77 eV is close to the value assumed in the modeling (0.80 V). For the PTPTIBDT-OD:[70]PCBM and PDPP5T-2:[70]PCBM cells the experimental Eloss is significantly larger: 0.99 and 0.89 eV (Table 1), respectively. This implies that to meet the required potential the optical band gap has to be increased compared to the optimum value found in the modeling. In fact, all three selected polymers have optical band gaps higher than the optimum band gaps suggested by the modeling (Table 5). The second reason for lower performance is that the assumption of a flat EQE of 0.65 below the band gap is not met. Even though the solar cells reach maximum EQEs of 0.65 without the retroreflective foil, the EQE remains less than 0.65 in various parts of the spectrum (Figure 3b). By applying the retroreflective foil, Jsc increases by ~11%, which results from an increased EQE especially in the spectral regions where absorption of light is limited.46 These factors result in a current density and a ηSTH are less than the values expected from the modeling. The experimental fill factors however do not deviate much from the assumed value of FF = 0.65. The highest ηSTH of 6.1% belongs to the three series-connected PTB7-Th:[70]PCBM cells. As can be seen in Figure 5 and Table 5, the experimental ηSTH (6.1%) for PTB7Th:[70]PCBM is only slightly less than the expected value (6.4%) at this band gap.

13



8 2 series modeled 3 series modeled 4 series modeled expt w/ foil expt w/o foil

7 6

STH [%]

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5 4 3 2 1 0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

Band gap Eg [eV]

Figure 5. Predicted and experimental ηSTH for light-driven electrochemical water splitting with two, three, or four series-connected single junction polymer solar cells as function of the optical band gap. The predictions assume that EQE = 0.65 and FF = 0.65, while Vop = 1.50 V for water splitting. The solid lines represent ηSTH for Eloss = 0.8 eV for all single junction cells. The dashed lines represent ηSTH taking the actual Eloss for the individual single junction cell (Table 1) into account. Table 5. Summary of Modeled and Measured ηsth of Light-Driven-Electrochemical Water Splitting With Series-Connected Single Junction Solar Cells. Optimum

Maximuma

Experiment

Maximum a,b

Predicted b,c

Experimentd,e

Eg [eV]

ηSTH [%]

Eg [eV]

ηSTH [%]

ηSTH [%]

ηSTH [%]

2 series

1.72

6.9

1.89

6.1

5.3

4.6

3 series

1.44

6.9

1.56

6.4

6.4

6.1

4 series

1.31

6.2

1.44

5.7

5.5

4.9

Type

a

Based on Eloss = 0.8 eV.

b

At the selected Eg. c Based on actual Eloss in Table 1. d Using the

retroreflective foil. e Differences between columns Predicted and Experiment are due to the fact that in the experiment the EQE and FF are not perfectly equal to 0.65. 2.4 ηSTH with multi-junction polymer solar cells In order to compare the potential of the two methods for series-connected cells, a modeling study was performed to estimate the theoretical ηSTH for light-driven electrochemical water splitting with tandem and triple junction solar cells with vertically stacked, series-connected photoactive layers. This result can be compared to ηSTH with the series-connected single junction cells we just 14


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demonstrated. Our modeling for the multi-junction polymer solar cell efficiency is based on the methodology described by Dennler et al. for tandem solar cells.56 The model can be extended to triple junction cells. In our description, the open-circuit voltage of each sub cell is given by Voc = (Eg − Eloss)/q, where Eloss is an adjustable parameter. The open-circuit voltage of the total cell is assumed to be the sum of the Vocs of the front, middle, and back cells: Voc  Voc,f  Voc,b for tandem and Voc  Voc,f  Voc, m  Voc, b for triple junction cells. The short-circuit current density of the front cell is determined by integrating the EQE with the AM1.5G photon flux density Φ(λ) (expressed in photons s-1 cm-2 nm-1):



J sc,f  q ( )  EQEf ( )  (1  M )d

(1)

In this expression, M is a mirror loss factor empirically set to M = 0.15 that accounts for the fact that the front cell in the tandem has no direct reflective back contact.56,57 For the short-circuit current density of the middle cell, an additional factor is introduced to correct for the loss of the reflection of the back electrode:

 EQEf ( )  J sc, m  q ( )  EQE m ( )  1   (1  M )  (1  M )d IQEf ( )  



(2)

Here the IQE represents the internal quantum efficiency and the ratio EQE/IQE is the fraction of absorbed photons. For the back cell, the incident light has to be corrected for both the front and the middle cell. This results in the following equation for the short-circuit current density of the back sub cell:

  EQEf ( )   EQEm ( ) J sc, b  q ( )  EQEb ( )  1   (1  M )d  (1  M )  1  IQEf ( ) IQEm ( )    



(3)

For triple junction cells Eqns. (1), (2) and (3) are used, for tandem cells Eqns. (1) and (3) are used in combination with EQEm = 0. The short-circuit current density of the multi-junction cell is assumed to be equal to the smallest short-circuit current density of the sub cells:

J sc  min( J sc,f , J sc, b ) or J sc  min( J sc,f , J sc, m , J sc, b ) . In the modeling we assumed constant value for the IQE of 0.85 and for the EQE of 0.65 below the optical band gap and zero above. The electrochemical potential for water splitting was set to 1.50 eV. Based on Jsc and Voc for the multi15



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junction cell, the current density at the electrochemical potential of 1.50 V (J1.50V) was determined using a normalized J−V curve with FF = 0.65. J1.50V was then converted into ηSTH in sunlight using ηSTH = 1.23×J1.5V/PAM15G. Using this model, ηSTH was calculated for band gap energies between 300 and 1240 nm. The modeling was performed for Eloss = 0.8 and 0.6 eV. Figure 6 shows the theoretical predictions for ηSTH of light-driven electrochemical water splitting with multi-junction polymer solar cells. For Eloss = 0.8 eV, the highest predicted ηSTH equals 9.4% for tandem and 10.0% for triple cells (Figure 6). The result that ηSTH is not strongly increased going from two to three junctions as would occur for the PCE of a conventional solar cell is primarily related to the fact that light-driven electrochemical water splitting is primarily determined by the current density J1.5V at the water splitting potential of 1.50 V. Triple cells can convert sunlight at higher PCEs than tandem cells because of a higher Voc rather than because of a higher Jsc (dividing a photon flux over three rather than two cells reduces the current density by two thirds). The modeling reveals that to reach ηSTH = 9.4% at Eloss = 0.8 eV, the sub cells in the tandem should have absorption onsets at 664 nm (1.87 eV) and 789 nm (1.57 eV). To reach the 10.0% efficiency with a triple junction cell, absorber layers with optical band gaps at 700 nm (1.77 eV), 880 nm (1.41 eV), and 1060 nm (1.17 eV) are required. By reducing the optical band gap of the sub cells compared to the tandem cell, the triple junction cell can convert a broader range of the solar spectrum and thereby lessen its intrinsically lower current density. When the modeling is performed for Eloss = 0.6 eV, the highest achievable ηSTH increases to 12.2% for both tandems and triple cells. The lower Eloss allows using sub cell absorber materials with even lower optical band gaps, which increases the current density at 1.50 eV. The modeling shows that at Eloss = 0.6 eV tandem cells can reach ηSTH = 12.2% at optical band gaps at 731 nm (1.70 eV) and 898 nm (1.38 eV). The corresponding triple junction solar cell should consist of materials with optical band gaps at 750 nm (1.65 eV), 990 nm (1.25 eV), and 1230 nm (1.01 eV) to reach ηSTH = 12.2%.

16


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Figure 6. Theoretical predictions for ηSTH for light-driven electrochemical water splitting operating at 1.50 V with tandem (top) and triple (bottom) junction polymer solar cell as a function of the onset of optical absorption (in nm) of front, middle, and back cells. All sub cells are assumed to have EQE = 0.65, IQE = 0.85, and FF = 0.65. The photon energy losses are assumed to be Eloss = 0.6 eV (left) and Eloss = 0.8 eV and (right). The modeling shows that light-driven electrochemical water splitting with multi-junction polymer solar cells is expected to outperform water splitting with series-connected single junction polymer solar cells. At Eloss = 0.8 eV the modeled ηSTH for tandem (9.4%) and triple (10.0%) junction cells is significantly higher than ηSTH for two and three series-connected single junction solar cells (6.9%). Presently few experimental results are available for direct comparison. Lightdriven electrochemical water splitting using tandem polymer solar cells with two identical absorber layers have reached ηSTH = 4.3%36 and 6.1%37 and while for triple junctions ηSTH = 5.4%35 (based on two different absorbers) and 5.6%38 (based on one absorber) have been reported. These values 17



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are comparable to the ηSTH = 4.6% and 6.1% reached in this study for two and three seriesconnected single junction devices. As explained in the introduction, the use of multiple identical layers stacked on top of each other in one tandem or triple cell can have an advantage compared to thick single junction cells by reducing recombination losses, but ultimately, higher efficiencies are expected for absorber layers with different optical band gaps. At present, the differences between experimental and modeled ηSTH values are predominantly determined by the larger Eloss in the experiment. Hence, future polymer semiconductors with low Eloss49-52 can improve the efficiency of light-driven electrochemical water splitting 3. Conclusions The solar-to-hydrogen conversion efficiency ηSTH for light-driven electrochemical water splitting with series-connected single junction polymer solar cells was modeled and investigated experimentally. Assuming a realistic photon energy loss of Eloss = 0.8 eV in each cell, a ηSTH of 6.9% is expected for two and three series-connected cells, while with four cells the maximum ηSTH equals 6.2%. Based on the modeling, three polymers with different optical band gaps were selected for two, three, and four series-connected configurations. Experimentally, the highest ηSTH of 6.1% was obtained for three series-connected PTB7-Th:[70]PCBM cells. The PTB7-Th:[70]PCBM layer has an optical band gap (Eg = 1.56 eV) higher than the optimal value modeled (Eg = 1.44 eV). As a result, the three series-connected PTB7-Th:[70]PCBM cells operate below the maximum power point during water splitting. For the other two materials, PTPTIBDT-OD:[70]PCBM and PDPP5T2:[70]PCBM, the actual photon energy losses were higher than the assumed value of 0.8 eV, and hence the ηSTH of 4.6% and 4.9% are less than the modeled values. The ηSTH was also modeled for a multi-junction polymer solar cell with vertically stacked absorber layers with different optical band gaps. The highest modeled ηSTH (10.0%) at Eloss = 0.8 eV is higher than the expected ηSTH for series-connected solar cells (6.9%). This shows that the efficiency of light-driven electrochemical water splitting can be higher with triple junction polymer solar cells than with series-connected single junction polymer solar cells. Moreover, the modeling shows that ηSTH is strongly dependent on Eloss. Hence, reducing Eloss is crucial to reach high solarto-hydrogen conversion.

18


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4. Experimental Materials. PTB7-Th was obtained from 1-Material. The syntheses of PTPTIBDT-OD and PDPP5T-2 have been described in Refs. 36 and 58. [70]PCBM was obtained from Solenne BV. Platinum plate was obtained from Drijfhout. The deposition of RuO2 on Ti has been described in Ref. 35. Water used was purified in a Millipore system and has a resistance of at least 18 MΩ. Device preparation. Solar cells were fabricated as described in Ref. 36 by depositing the photoactive layer on glass/ITO/PEDOT:PSS and evaporating LiF(1 nm)/Al(100 nm) back contact at 3 × 10-7 mbar. 115 nm of PTPTIBDT-OD:[70]PCBM (1:1.5 w/w) was spin cast from chloroform containing 10 vol% o-DCB at 17.5 mg mL-1 total concentration. 104 nm of PTB7-Th:[70]PCBM (1:2 w/w) was deposited from chlorobenzene containing 4 vol% DIO at 24 mg mL-1 total concentration in inert atmosphere. 125 nm of PDPP5T-2:[70]PCBM (1:2 w/w) was spin cast from chloroform containing 10 vol% o-DCB at 18 mg mL-1 total concentration. Retroreflective foil. Detail on the manufacturing of the have been described in Ref. 46. The present experiments were performed by using textures with 90 μm height. Characterization. J-V characteristics and EQE of the polymer solar cells were measured as described previously.34,36 Electrochemical water splitting. The Pt and RuO2/Ti electrodes were put into contact with a 1.0 M KOH aqueous electrolyte in air. The solar cells were kept in inert N2 atmosphere. Each substrate included four solar cells with 0.12 cm2 device area. Depending on the type of the experiment two, three, or four of these cells were electrically connected in series. The solar cells were illuminated with simulated AM1,5G light. The solar cell was positioned by making sure that the short-circuit current in this setup corresponds to AM1.5G power standards determined from the EQE. The short circuit current density of two, three, or four series-connected cells were based on the current limiting cell among the series-connected cells on the substrate, which was precisely measured by convoluting its EQE measurement with AM1.5G solar spectrum. Current and voltage were measured simultaneously during water splitting. Measurements with the retroreflective foil were performed by using a reflecting mirror placed behind the solar cell as described in Ref. 46 . Supporting Information Measurements on the Faradaic efficiency. This material is available free of charge via the Internet at http://pubs.acs.org. 19



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Acknowledgements This project was carried out within the BioSolarCells research programme, 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. 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). References 1

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Water Splitting with Series-Connected Polymer Solar Cells - ACS

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