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Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode Bernhard Kaiser a,*, Wolfram Calvet a, Eswaran Murugasen a, Ju¨rgen Ziegler a, Wolfram Jaegermann a, Sascha E. Pust b, Friedhelm Finger b, Sascha Hoch c, Matthias Blug c, Jens Busse c a €t Darmstadt, Institute of Materials Science, Jovanka-Bontschits-Straße 2, Darmstadt 64287, Technische Universita Germany b Forschungszentrum Ju¨lich GmbH, Institut fu¨r Energie-und Klimaforschung (IEK-5), Ju¨lich 52425, Germany c Evonik Industries AG, Creavis Technologies & Innovation, Paul-Baumann-Straße 1, Marl 45722, Germany
article info
abstract
Article history:
Thin film tandem solar cells based on amorphous and microcrystalline silicon (a-Si:H/mc-
Received 5 March 2014
Si:H) are employed as the cathode in a photoelectrochemical converter for solar water
Received in revised form
splitting. It is setup in such a way that the silver back contact of the cell is directly con-
29 October 2014
nected to the electrolyte and the light enters the cell through the glass substrate. This
Accepted 2 November 2014
arrangement offers a number of distinct advantages compared to the conventional de-
Available online xxx
signs. The cathode is further optimized by the deposition of platinum nanoparticles to achieve higher conversion efficiencies. The front contact of the photovoltaic cell is con-
Keywords:
nected to a standard platinum counter electrode in a three-electrode arrangement. Photon
Photoelectrochemistry
to current conversion efficiencies can reach up to 3% for our design, which has not been
Nanoparticles
optimized to the requirements of the water splitting reaction, yet. The optimization of such
Photovoltaic converter
tandem devices made from abundant silicon in combination with nanoparticle catalysts
Silicon
offers an affordable pathway for direct solar-to-fuel conversion devices in form of an
Tandem cells
artificial inorganic leaf. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Artificial photosynthesis using semiconductor materials in a liquid environment to directly split water into hydrogen and oxygen is the “holy grail” of photoelectrochemistry since the
discovery of the photovoltaic effect by E. Becquerel and the first practical demonstration presented by Fujishima and Honda in 1972 [1,2]. The reaction requires an electrochemical potential of at least 1.6 eV, which has only been delivered so far by wide band-gap semiconductors, e.g. TiO2, with bandgaps larger
* Corresponding author. E-mail address: [email protected] (B. Kaiser). http://dx.doi.org/10.1016/j.ijhydene.2014.11.012 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
than 3 eV, where the intensity of solar light is scarce [2e6]. Other successful designs delivering a sufficient photovoltage made use of tandem or multi-junction arrangements [7e9]. Recent work by Reece et al. showed that triple cells made from abundant silicon produce enough power to act as an artificial leaf generating hydrogen with an efficiency of 2.5%e4.7% [10]. Since each junction reduces the achievable maximum current, a tandem device producing a large enough photopotential would be a highly desirable goal. Previous studies have investigated solely amorphous silicon tandem and triple cells, tandem cells made from III/V semiconductor materials or tandem cells combined with a photoanode [11e21]. Here we present first studies of tandem cells made from amorphous and microcrystalline silicon (a-Si:H/mc-Si:H) as possible devices for PEC water splitting. Such a system has the added advantage (compared to cells made just from a-Si) that the two materials absorb sunlight in two different wavelength regimes, thereby employing a broader range of the solar spectrum, which could possibly lead to higher overall efficiencies of up to 28% as has been theoretically predicted by Bolton et al. [22]. The tandem cell acts as a superstrate cell (see Fig. 1), which is illuminated through the glass substrate with a transparent conductive oxide (TCO) layer as the front contact and an Ag layer as the rear contact and working electrode, which is in direct contact to the electrolyte [23]. The tandem cell therefore works as the photocathode. This has several distinct advantages: The light must not be transmitted through several materials with different index of refraction minimizing intensity losses, and there is no scattering of the light rays by hydrogen bubbles in the electrolyte. In addition, the highly sensitive absorber material is not in direct contact with the electrolyte, instead the metal layer may provide a shielding layer. There are two absorber regions based on p-i-n device
structures made of amorphous silicon (a-Si:H) and microcrystalline silicon (mc-Si:H), each possessing different spectral absorption properties as shown in Fig. 1c). The top cell made of a-Si:H starts to absorb at a wavelength of 800 nm (1.55 eV), which is about the band gap of amorphous silicon. The bottom cell starts to absorb at 1100 nm (1.24 eV) according to the band gap of bulk silicon. This design allows open circuit photovoltages up to 1.4 V and electric power efficiencies of greater than 13% when used as pure photovoltaic device. In order to reduce overpotentials, platinum is used as catalyst material for hydrogen generation. It is applied in the form of nanoparticles to reduce material costs and consumption.
Experimental section All reagents used were p.a. grade from Merck Millipore, unless otherwise noted. The samples (active area 0.5 cm2) were cleaned with acetone, isopropanol and Millipore™ water before being used for the electrochemical studies. Platinum nanoparticles were electrochemically deposited in the dark from a 0.5 mM K2PtCl6 (Mateck) solution in 0.1 M H2SO4 under cathodic current conditions cycling the potential from 0.5 V to 0.8 V twice at scan rates of about 30 mV/s. Electrochemical characterization took place in a modified test cell using a three-electrode setup with an Ag/AgCl (3 M) reference electrode and a Pt wire coil as counter electrode at a scan rate of 20 mV/s. The rear contact of the tandem cell with the silver layer was brought into direct contact with the electrolyte. Hydrogen bubbles are formed at this surface. The oxygen evolution takes place at the Pt counter electrode (see Fig. 1a) and supporting information). A white light LED was used as light source in front of the cell. Its intensity was adjusted to an
Fig. 1 e a), b) Sketch of the a-Si:H/mc-Si:H tandem cell device. The TCO layer is connected to the Pt counter electrode, while the silver and catalyst layer makes the direct contact to the electrolyte. Reference electrode omitted for clarity. c) Schematical presentation of the optical absorption characteristics of the tandem cell. Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
3
Fig. 2 e a) CV of the tandem cell in 0.1 M H2SO4. b) Calculated efficiency from a) using formula (1). MPP designates the maximum power point.
illumination power of 100 W/m2 (see Supporting Information). All photoelectrochemical measurements presented here were done with a commercially available setup from Zahner Elektrik (PECC-2, Zennium, CIMPS) using 0.1 M H2SO4 as the working electrolyte. As the photovoltaic converter a-Si:H/mcSi:H tandem cells have been used, which are optimized in its photon to power conversion efficiency [24].
Results and discussion Fig. 2a) shows the cyclic voltammogram (CV) of a tandem cell sample under illumination. The onset of the hydrogen evolution is shifted clearly to the anodic regime yielding an OC photovoltage of 1.21 V vs. RHE. The calculation of the maximum photon to current conversion efficiency yields a value of 2.6%. All efficiencies reported here are the efficiencies of a single photoelectrode [25] for the conversion of lightenergy to chemical energy and are calculated according to the formula. h¼
j4a 40 j$iph $100% pi
photoelectric efficiency of 5.2% are extracted. In contrast, for the photoelectrochemical (PEC) measurement an OCP of 1.21 V and a F.F. of about 42% are obtained. This implies, that nearly half of the photo-generated power is lost in the PEC setup without optimized cell to electrolyte contacts. In general the kinetics of the water splitting reaction is hindered by high overpotentials on the cathodic as well as on the anodic side. Therefore, we deposited Pt-nanoparticles electrochemically onto the silver backside of the solar cell, since platinum is a much better water reduction catalyst than silver [26]. Fig. 4a) shows a scanning electron microscope (SEM) picture of the surface after Pt deposition. Pt particles agglomerate to form larger clusters with a mean diameter of 500 nm. The dark grey background corresponds to the morphology of the silver layer on the tandem cell. The chemical surface composition has also been validated by energy dispersive x-ray (EDX) analysis (see Fig. 4b). Small circular holes in the surface might be due to corrosion of the Ag
(1)
with 4a operation potential of the photocathode, 40 the reference potential of the reaction occurring on the photocathode (in our case 40 is chosen to be zero according to the potential of the RHE, see supporting information S4), iph the photocurrent density obtained for the applied potential 4a and Pi the power density of the light incident on the photoelectrode. Losses at the counter electrode as well as light intensity and wavelength dependencies were not taken into account in the present setup, but will certainly have to be considered for a future characterization. Measured efficiencies vary according to the evaluated tandem cell batch between 1 % and 5 % [24]. The photoelectric efficiency of the tandem cell, as measured under the same LED illumination, is shown in Fig. 3 for comparison. From the measurement an open circuit potential (OCP) of 1.17 V and a fill factor (F.F.) of 73% yielding a
Fig. 3 e Currentevoltage plot for the silicon tandem cell under LED illumination.
Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
Fig. 4 e a) SEM picture and b) EDX spectrum of the Ag side of the tandem cell after electrochemical deposition of platinum.
surface during the Pt deposition process and can limit the final PEC efficiency. To achieve a better insight into the kinetic limitations responsible for the photoelectrochemical behavior of the electrode, we chopped the light source during the PEC measurements at a frequency of 1 Hz Fig. 5a) shows the I/V-curve of the untreated Ag electrode, its shape similar to the one shown in Fig. 2a). Additionally, the curve shows large spikes in the anodic regime at potentials between 0.5 V and 1.5 V. This transient behavior is a clear indication of initial charge carrier separation followed by a hindered charge transfer limiting the obtainable efficiency of the device. These processes may be attributed to an accumulation of photo generated charge carriers in the surface region or to the trapping of charge carriers in surface states reducing band bending effects in the semiconductor structures of the device and enhancing recombination of electrons and holes. The first wave at around 1.3e1.5 V vs. RHE is related to cathodic as well as
anodic redox reactions involving Ag and H2O on the surface. At higher cathodic potentials (1.2e0.6 V) the silver layer does not provide sufficient electrocatalytic properties for the H2 evolution and hence the kinetics of the charge transfer is reduced. After the addition of the platinum catalyst the PEC characteristics improve considerably: almost all transient signals below the OCP have disappeared and the curve shows a very smooth behavior under illumination. At the MPP values of 0.8 V and 350 mA/cm2 are reached giving an efficiency of 2.8%. The sample without Pt yields values of 0.5 V and 350 mA/ cm2 and an efficiency of 1.8%, respectively. The addition of Pt eliminates the transients, i.e. charge accumulation and recombination in the surface region due to slow electron transfer from the silver back contact to the electrolyte is reduced. Therefore it is assumed that a hindered charge transfer is responsible for the observed transient behavior of the samples without a catalyst. The limitation of the photocurrent density of the samples with platinum may
Fig. 5 e Chopped light CVs for the a) standard silicon tandem cell and b) after electrochemical deposition of the platinum catalyst. Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
be either due to the size of the active area of the nanoparticles or to the contact behavior between the silver layer, the catalyst and the electrolyte. The step in the photocurrent at 0.1 V in Fig. 5b) is explained by an onset of corrosion of the silver layer due to the Pt deposition (see Fig. 4a). Further studies on how the size and distribution of catalyst particles influences the efficiency of the water splitting device are underway. For a tandem cell working under AM 1.5 illumination we would expect a short-circuit current of more than 6 mA/cm2 (see supporting information). Assuming a Faradaic efficiency of 95% this will yield a hydrogen production rate of about 0.1 mmol/(h cm2). Please bear in mind, that this would be the rate where the power output of this tandem cell would be assisted by an external power source.
Summary and outlook We have demonstrated the feasibility of a slightly modified silicon based tandem cell device to be used as an integrated power driver for direct light driven water splitting. The tandem cell has been successfully used as a photoelectrochemical cell in 0.1 M H2SO4 electrolyte providing a maximum photon to current conversion efficiency of about 2.6%. The application of platinum as catalyst material in a nanoparticle form improves the overall efficiency by shifting the maximum power point to a higher potential in the anodic direction. The chosen arrangement of the water splitting cell allows the unobstructed illumination of the device from the glass side and it makes it possible to avoid the direct contact of the semiconductor material with the aqueous electrolyte. For future studies the silver layer will be replaced with even more stable layers of oxide materials, which have been used quite successful for the shielding of photoelectrodes made from silicon and copper oxide [27,28]. Using just two absorber layers yields an overall simpler and more economic setup than previously reported triple cell devices. Additionally, the absorption of light over a wide range of the solar spectrum yields large theoretically possible efficiencies for the here presented setup. Nevertheless, further work is required to improve the voltage output of the tandem cell towards a free-running device and to replace the expensive Pt catalyst by a more abundant and cost-effective material [15,29e31]. To overcome the voltage requirement it would also be possible to combine the tandem cell with a stable photoanode delivering a maximum power point voltage of about 0.6 V [21].
Acknowledgments Partial funding by Evonik Industries AG (part-financed by the State of North Rhine-Westphalia and co-financed by the European Union “Investing in our Future, European Regional Development Fund”) is gratefully acknowledged. B.K. and W.J. acknowledge funding by the DFG excellence program in the frameworks of the cluster “Smart Interfaces” (EXC 259) and the graduate school “Energy Science and Engineering” (GSC 1070).
5
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.11.012.
references
moire sur les effets e lectriques produits sous [1] Becquerel E. Me l'influence des rayons solaires. Comptes Rendus 1839;9:561. [2] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972;238:37. [3] Nozik AJ. p-n photoelectrolysis cells. Appl Phys Lett 1976;29:150. [4] Lewerenz H-J, Jungblut H. Photovoltaik. Berlin: Springer; 1995. [5] Grimes CA, Varghese OK, Ranjan S. Light, water, hydrogen. New York: Springer; 2008. [6] Rajeshwar K, McConnell R, Licht S. Solar hydrogen generation. New York: Springer; 2008. [7] Heller A, Vadimsky RG. Efficient solar to chemical conversion: 12% efficient photoassisted electrolysis in the [ptype InP(Ru)]/HCl-KCl/Pt(Rh) cell. Phys Rev Lett 1981;46:1153. [8] Khaselev O, Turner JA. A monolithic photovoltaicphotoelectrochemical device for hydrogen production via water splitting. Science 1998;280:425. [9] Licht S, Wang B, Mukerji S, Soga T, Umeno M, Tributsch H. Efficient solar water splitting, exemplified by RuO2- catalyzed AlGaAs/Si photoelectrolysis. J Phys Chem B 2000;104:8920. [10] Reece SY, Hamel JA, Sung K, Jarvi TD, Esswein AJ, Pijpers JJH, et al. Wireless solar water splitting using silicon-based semiconductors and earth-abundant catalysts. Science 2011;334:645. [11] Appleby AJ, Delahoy AE, Gau SC, Murphy OJ, Kapur M, Bockris JOM. An amorphous silicon-based one-unit photovoltaic electrolyzer. Energy 1985;10:871. [12] Delahoy AE, Gau SC, Murphy OJ, Kapur M, Bockris JOM. A one-unit photovoltaic electrolysis system based on a triple stack of amorphousesilicon (pin) Cells. Int J Hydrogen Energy 1985;10:113. [13] Sakai Y, Sugahara S, Matsumura M, Nakato Y, Tsubomura H. Photoelectrochemical water splitting by tandem type and heterojunction amorphous silicon electrodes. Can J Chem 1988;66:1853. [14] Lin GH, Kapur M, Kainthla RC, Bockris JOM. One step method to produce hydrogen by a triple stack amorphous silicon solar cell. Appl Phys Lett 1989;55:386. [15] Rocheleau RE, Miller EL, Misra A. High-efficiency photoelectrochemical hydrogen production using multijunction amorphous silicon photoelectrodes. Energy Fuel 1998;12:3. [16] Khaselev O, Bansal A, Turner JA. High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production. Int J Hydrogen Energy 2001;26:127. [17] Miller EL, Rocheleau RE, Deng XM. Design considerations for a hybrid amorphous silicon/photoelectrochemical multijunction cell for hydrogen production. Int J Hydrogen Energy 2003;28:615. [18] Yamada Y, Matsuki N, Ohmori T, Mametsuka H, Kondo M, Matsuda A, et al. One chip photovoltaic water electrolysis device. Int J Hydrogen Energy 2003;28:1167. [19] Kelly NA, Gibson TL. Design and characterization of a robust photoelectrochemical device to generate hydrogen using solar water splitting. Int J Hydrogen Energy 2006;31:1658.
Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
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[20] Walter MG, Warren EL, McKone JR, Boettcher SW, Mi QX, Santori EA, et al. Solar water splitting cells. Chem Rev 2010;110:6446. [21] Abdi FF, Han L, Smets AHM, Zeman M, Dam B, van de Krol R. Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode. Nat Comm 2013;4:2195. [22] Bolton JR, Strickler SJ, Connolly JS. Limiting and realizable efficiencies of solar photolysis of water. Nature 1985;316:495. [23] Patent pending. [24] Lambertz A, Smirnov V, Merdzhanova T, Ding K, Haas S, Jost G, et al. Microcrystalline silicon oxygen alloys for application in silicon solar cells and modules. Sol Energy Mater Sol Cells 2013;119:134. [25] Pleskov YV. Solar energy conversion. Berlin: Springer; 1990. [26] Conway BE, Jerkiewicz G. Relation of energies and coverages of underpotential and overpotential deposited H at Pt and other metals to the ‘volcano curve’ for cathodic H2 evolution kinetics. Electrochim Acta 2000;45:4075.
€ tzel M, Thimsen E. [27] Paracchino A, Laporte V, Sivula K, Gra Highly active oxide photocathode for photoelectrochemical water reduction. Nat Mater 2011;10:456. [28] Chen YW, Prange JD, Duhnen S, Park Y, Gunji M, Chidsey CED, et al. Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation. Nat Mater 2011;10:539. [29] Kanan MW, Nocera DG. In situ formation of an oxygenevolving catalyst in neutral water containing phosphate and Co2þ. Science 2008;321:1072. € rketun ME, Herbst K, [30] Hou Y, Abrams BL, Vesborg PCK, Bjo Bech L, et al. Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nat Mater 2011;10:434. [31] Chen W-F, Sasaki K, Ma C, Frenkel AI, Marinkovic N, Muckerman JT, et al. Hydrogen-evolution catalysts based on non-noble metal nickelemolybdenum nitride nanosheets. Angew Chem Int Ed 2012;51:6131.
Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode Bernhard Kaiser a,*, Wolfram Calvet a, Eswaran Murugasen a, Ju¨rgen Ziegler a, Wolfram Jaegermann a, Sascha E. Pust b, Friedhelm Finger b, Sascha Hoch c, Matthias Blug c, Jens Busse c a €t Darmstadt, Institute of Materials Science, Jovanka-Bontschits-Straße 2, Darmstadt 64287, Technische Universita Germany b Forschungszentrum Ju¨lich GmbH, Institut fu¨r Energie-und Klimaforschung (IEK-5), Ju¨lich 52425, Germany c Evonik Industries AG, Creavis Technologies & Innovation, Paul-Baumann-Straße 1, Marl 45722, Germany
article info
abstract
Article history:
Thin film tandem solar cells based on amorphous and microcrystalline silicon (a-Si:H/mc-
Received 5 March 2014
Si:H) are employed as the cathode in a photoelectrochemical converter for solar water
Received in revised form
splitting. It is setup in such a way that the silver back contact of the cell is directly con-
29 October 2014
nected to the electrolyte and the light enters the cell through the glass substrate. This
Accepted 2 November 2014
arrangement offers a number of distinct advantages compared to the conventional de-
Available online xxx
signs. The cathode is further optimized by the deposition of platinum nanoparticles to achieve higher conversion efficiencies. The front contact of the photovoltaic cell is con-
Keywords:
nected to a standard platinum counter electrode in a three-electrode arrangement. Photon
Photoelectrochemistry
to current conversion efficiencies can reach up to 3% for our design, which has not been
Nanoparticles
optimized to the requirements of the water splitting reaction, yet. The optimization of such
Photovoltaic converter
tandem devices made from abundant silicon in combination with nanoparticle catalysts
Silicon
offers an affordable pathway for direct solar-to-fuel conversion devices in form of an
Tandem cells
artificial inorganic leaf. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Artificial photosynthesis using semiconductor materials in a liquid environment to directly split water into hydrogen and oxygen is the “holy grail” of photoelectrochemistry since the
discovery of the photovoltaic effect by E. Becquerel and the first practical demonstration presented by Fujishima and Honda in 1972 [1,2]. The reaction requires an electrochemical potential of at least 1.6 eV, which has only been delivered so far by wide band-gap semiconductors, e.g. TiO2, with bandgaps larger
* Corresponding author. E-mail address: [email protected] (B. Kaiser). http://dx.doi.org/10.1016/j.ijhydene.2014.11.012 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
than 3 eV, where the intensity of solar light is scarce [2e6]. Other successful designs delivering a sufficient photovoltage made use of tandem or multi-junction arrangements [7e9]. Recent work by Reece et al. showed that triple cells made from abundant silicon produce enough power to act as an artificial leaf generating hydrogen with an efficiency of 2.5%e4.7% [10]. Since each junction reduces the achievable maximum current, a tandem device producing a large enough photopotential would be a highly desirable goal. Previous studies have investigated solely amorphous silicon tandem and triple cells, tandem cells made from III/V semiconductor materials or tandem cells combined with a photoanode [11e21]. Here we present first studies of tandem cells made from amorphous and microcrystalline silicon (a-Si:H/mc-Si:H) as possible devices for PEC water splitting. Such a system has the added advantage (compared to cells made just from a-Si) that the two materials absorb sunlight in two different wavelength regimes, thereby employing a broader range of the solar spectrum, which could possibly lead to higher overall efficiencies of up to 28% as has been theoretically predicted by Bolton et al. [22]. The tandem cell acts as a superstrate cell (see Fig. 1), which is illuminated through the glass substrate with a transparent conductive oxide (TCO) layer as the front contact and an Ag layer as the rear contact and working electrode, which is in direct contact to the electrolyte [23]. The tandem cell therefore works as the photocathode. This has several distinct advantages: The light must not be transmitted through several materials with different index of refraction minimizing intensity losses, and there is no scattering of the light rays by hydrogen bubbles in the electrolyte. In addition, the highly sensitive absorber material is not in direct contact with the electrolyte, instead the metal layer may provide a shielding layer. There are two absorber regions based on p-i-n device
structures made of amorphous silicon (a-Si:H) and microcrystalline silicon (mc-Si:H), each possessing different spectral absorption properties as shown in Fig. 1c). The top cell made of a-Si:H starts to absorb at a wavelength of 800 nm (1.55 eV), which is about the band gap of amorphous silicon. The bottom cell starts to absorb at 1100 nm (1.24 eV) according to the band gap of bulk silicon. This design allows open circuit photovoltages up to 1.4 V and electric power efficiencies of greater than 13% when used as pure photovoltaic device. In order to reduce overpotentials, platinum is used as catalyst material for hydrogen generation. It is applied in the form of nanoparticles to reduce material costs and consumption.
Experimental section All reagents used were p.a. grade from Merck Millipore, unless otherwise noted. The samples (active area 0.5 cm2) were cleaned with acetone, isopropanol and Millipore™ water before being used for the electrochemical studies. Platinum nanoparticles were electrochemically deposited in the dark from a 0.5 mM K2PtCl6 (Mateck) solution in 0.1 M H2SO4 under cathodic current conditions cycling the potential from 0.5 V to 0.8 V twice at scan rates of about 30 mV/s. Electrochemical characterization took place in a modified test cell using a three-electrode setup with an Ag/AgCl (3 M) reference electrode and a Pt wire coil as counter electrode at a scan rate of 20 mV/s. The rear contact of the tandem cell with the silver layer was brought into direct contact with the electrolyte. Hydrogen bubbles are formed at this surface. The oxygen evolution takes place at the Pt counter electrode (see Fig. 1a) and supporting information). A white light LED was used as light source in front of the cell. Its intensity was adjusted to an
Fig. 1 e a), b) Sketch of the a-Si:H/mc-Si:H tandem cell device. The TCO layer is connected to the Pt counter electrode, while the silver and catalyst layer makes the direct contact to the electrolyte. Reference electrode omitted for clarity. c) Schematical presentation of the optical absorption characteristics of the tandem cell. Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e6
3
Fig. 2 e a) CV of the tandem cell in 0.1 M H2SO4. b) Calculated efficiency from a) using formula (1). MPP designates the maximum power point.
illumination power of 100 W/m2 (see Supporting Information). All photoelectrochemical measurements presented here were done with a commercially available setup from Zahner Elektrik (PECC-2, Zennium, CIMPS) using 0.1 M H2SO4 as the working electrolyte. As the photovoltaic converter a-Si:H/mcSi:H tandem cells have been used, which are optimized in its photon to power conversion efficiency [24].
Results and discussion Fig. 2a) shows the cyclic voltammogram (CV) of a tandem cell sample under illumination. The onset of the hydrogen evolution is shifted clearly to the anodic regime yielding an OC photovoltage of 1.21 V vs. RHE. The calculation of the maximum photon to current conversion efficiency yields a value of 2.6%. All efficiencies reported here are the efficiencies of a single photoelectrode [25] for the conversion of lightenergy to chemical energy and are calculated according to the formula. h¼
j4a 40 j$iph $100% pi
photoelectric efficiency of 5.2% are extracted. In contrast, for the photoelectrochemical (PEC) measurement an OCP of 1.21 V and a F.F. of about 42% are obtained. This implies, that nearly half of the photo-generated power is lost in the PEC setup without optimized cell to electrolyte contacts. In general the kinetics of the water splitting reaction is hindered by high overpotentials on the cathodic as well as on the anodic side. Therefore, we deposited Pt-nanoparticles electrochemically onto the silver backside of the solar cell, since platinum is a much better water reduction catalyst than silver [26]. Fig. 4a) shows a scanning electron microscope (SEM) picture of the surface after Pt deposition. Pt particles agglomerate to form larger clusters with a mean diameter of 500 nm. The dark grey background corresponds to the morphology of the silver layer on the tandem cell. The chemical surface composition has also been validated by energy dispersive x-ray (EDX) analysis (see Fig. 4b). Small circular holes in the surface might be due to corrosion of the Ag
(1)
with 4a operation potential of the photocathode, 40 the reference potential of the reaction occurring on the photocathode (in our case 40 is chosen to be zero according to the potential of the RHE, see supporting information S4), iph the photocurrent density obtained for the applied potential 4a and Pi the power density of the light incident on the photoelectrode. Losses at the counter electrode as well as light intensity and wavelength dependencies were not taken into account in the present setup, but will certainly have to be considered for a future characterization. Measured efficiencies vary according to the evaluated tandem cell batch between 1 % and 5 % [24]. The photoelectric efficiency of the tandem cell, as measured under the same LED illumination, is shown in Fig. 3 for comparison. From the measurement an open circuit potential (OCP) of 1.17 V and a fill factor (F.F.) of 73% yielding a
Fig. 3 e Currentevoltage plot for the silicon tandem cell under LED illumination.
Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
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Fig. 4 e a) SEM picture and b) EDX spectrum of the Ag side of the tandem cell after electrochemical deposition of platinum.
surface during the Pt deposition process and can limit the final PEC efficiency. To achieve a better insight into the kinetic limitations responsible for the photoelectrochemical behavior of the electrode, we chopped the light source during the PEC measurements at a frequency of 1 Hz Fig. 5a) shows the I/V-curve of the untreated Ag electrode, its shape similar to the one shown in Fig. 2a). Additionally, the curve shows large spikes in the anodic regime at potentials between 0.5 V and 1.5 V. This transient behavior is a clear indication of initial charge carrier separation followed by a hindered charge transfer limiting the obtainable efficiency of the device. These processes may be attributed to an accumulation of photo generated charge carriers in the surface region or to the trapping of charge carriers in surface states reducing band bending effects in the semiconductor structures of the device and enhancing recombination of electrons and holes. The first wave at around 1.3e1.5 V vs. RHE is related to cathodic as well as
anodic redox reactions involving Ag and H2O on the surface. At higher cathodic potentials (1.2e0.6 V) the silver layer does not provide sufficient electrocatalytic properties for the H2 evolution and hence the kinetics of the charge transfer is reduced. After the addition of the platinum catalyst the PEC characteristics improve considerably: almost all transient signals below the OCP have disappeared and the curve shows a very smooth behavior under illumination. At the MPP values of 0.8 V and 350 mA/cm2 are reached giving an efficiency of 2.8%. The sample without Pt yields values of 0.5 V and 350 mA/ cm2 and an efficiency of 1.8%, respectively. The addition of Pt eliminates the transients, i.e. charge accumulation and recombination in the surface region due to slow electron transfer from the silver back contact to the electrolyte is reduced. Therefore it is assumed that a hindered charge transfer is responsible for the observed transient behavior of the samples without a catalyst. The limitation of the photocurrent density of the samples with platinum may
Fig. 5 e Chopped light CVs for the a) standard silicon tandem cell and b) after electrochemical deposition of the platinum catalyst. Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012
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be either due to the size of the active area of the nanoparticles or to the contact behavior between the silver layer, the catalyst and the electrolyte. The step in the photocurrent at 0.1 V in Fig. 5b) is explained by an onset of corrosion of the silver layer due to the Pt deposition (see Fig. 4a). Further studies on how the size and distribution of catalyst particles influences the efficiency of the water splitting device are underway. For a tandem cell working under AM 1.5 illumination we would expect a short-circuit current of more than 6 mA/cm2 (see supporting information). Assuming a Faradaic efficiency of 95% this will yield a hydrogen production rate of about 0.1 mmol/(h cm2). Please bear in mind, that this would be the rate where the power output of this tandem cell would be assisted by an external power source.
Summary and outlook We have demonstrated the feasibility of a slightly modified silicon based tandem cell device to be used as an integrated power driver for direct light driven water splitting. The tandem cell has been successfully used as a photoelectrochemical cell in 0.1 M H2SO4 electrolyte providing a maximum photon to current conversion efficiency of about 2.6%. The application of platinum as catalyst material in a nanoparticle form improves the overall efficiency by shifting the maximum power point to a higher potential in the anodic direction. The chosen arrangement of the water splitting cell allows the unobstructed illumination of the device from the glass side and it makes it possible to avoid the direct contact of the semiconductor material with the aqueous electrolyte. For future studies the silver layer will be replaced with even more stable layers of oxide materials, which have been used quite successful for the shielding of photoelectrodes made from silicon and copper oxide [27,28]. Using just two absorber layers yields an overall simpler and more economic setup than previously reported triple cell devices. Additionally, the absorption of light over a wide range of the solar spectrum yields large theoretically possible efficiencies for the here presented setup. Nevertheless, further work is required to improve the voltage output of the tandem cell towards a free-running device and to replace the expensive Pt catalyst by a more abundant and cost-effective material [15,29e31]. To overcome the voltage requirement it would also be possible to combine the tandem cell with a stable photoanode delivering a maximum power point voltage of about 0.6 V [21].
Acknowledgments Partial funding by Evonik Industries AG (part-financed by the State of North Rhine-Westphalia and co-financed by the European Union “Investing in our Future, European Regional Development Fund”) is gratefully acknowledged. B.K. and W.J. acknowledge funding by the DFG excellence program in the frameworks of the cluster “Smart Interfaces” (EXC 259) and the graduate school “Energy Science and Engineering” (GSC 1070).
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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.11.012.
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Please cite this article in press as: Kaiser B, et al., Light induced hydrogen generation with silicon-based thin film tandem solar cells used as photocathode, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.11.012