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Photocatalytic Water Splitting Enhancement by Sub-Bandgap Photon Harvesting Angelo Monguzzi, Amadeus Oertel, Daniele Braga, Andreas Riedinger, David Kim, Philippe Knüsel, Alberto Bianchi, Michele Mauri, Roberto Simonutti, David J. Norris, and Francesco Meinardi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10829 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 2, 2017
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ACS Applied Materials & Interfaces
Photocatalytic Water Splitting Enhancement by Sub-Bandgap Photon Harvesting Angelo Monguzzi*1, Amadeus Oertel2, Daniele Braga2, Andreas Riedinger2,3, David K. Kim2, Philippe N. Knüsel2, Alberto Bianchi1, Michele Mauri1, Roberto Simonutti1, David J. Norris2 and Francesco Meinardi*1 1
Dipartimento di Scienza dei Materiali, Università̀ degli Studi di Milano Bicocca, via R. Cozzi 55, 20125 Milano, Italy E-mail:
[email protected],
[email protected] 2
Optical Materials Engineering Laboratory, ETH Zurich Leonhardstrasse 21, LEE P 210, 8092 Zurich Switzerland
3
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Keywords: photon managing, up-conversion, triplet-triplet annihilation, fluorescent nanocrystals, nanocomposites
Abstract Up-conversion is a photon-management process especially suited to water-splitting cells that exploit wide-bandgap photo-catalysts. Currently, such catalysts cannot utilize 95% of the available solar photons. We demonstrate here that the energy-conversion yield for a standard photocatalytic water-splitting device can be enhanced under solar irradiance by using a low-power up-conversion system that recovers part of the unutilized incident sub-bandgap photons. The up-converter is based on a sensitized triplet-triplet annihilation mechanism (sTTA-UC) obtained in a dye-doped elastomer and boosted by a fluorescent nanocrystal/polymer composite that allows for broadband-light harvesting. The complementary and tailored optical properties of these materials enable efficient up-conversion at sub-solar irradiance, allowing the realization of the first prototype water-splitting cell assisted by solid-state up-conversion. In our proof-of concept device the increase of the performance is 3.5%, which grows to 6.3% if concentrated sunlight (10 suns) is used. Our experiments show how the sTTA-UC materials can be successfully implemented in technologically relevant devices while matching the strict requirements of clean-energy production.
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Introduction The efficiency of solar devices is often limited by the spectral mismatch between the distribution of the solar radiation and the absorption spectrum of the light harvesters.1-2 In order to overcome this issue, a new frontier of the research is focused on photon managing processes capable of converting the broad solar emission into monochromatic light that matches the spectral range where devices show the highest efficiency, instead of on the modification of their light-harvesting properties. In particular, the photon up-conversion by anti-Stokes emitters is extremely appealing to recover the huge amount of energy in the longwavelength tail of the solar emission.3-4 Trupke et al. have calculated the theoretical maximum efficiency of a solar cell equipped with an up-converter, an approach generally termed third-generation solar photon conversion, using a detailed balance model. In the case of a single-junction Si-based solar cell operating with non-concentrated sunlight, the model indicates that the use of an upconversion system can raise the maximum achievable efficiency up to 47.6%, which is well above the thermodynamic Shockley–Queisser limit for such devices of 34%.5 Even bigger improvements are expected for devices with large band-gap. Another crucial advantage of this strategy is that up-converters can be readily applied to existing devices, by the incorporation of a passive luminescent up-conversion layer.6 Hence, up-converters and solar cells can be developed in parallel, allowing for optimized performance to production-cost ratios. The up-conversion mechanism is especially suited to improving the photocatalytic water splitting (PCWS) performance of cells that exploit metal oxides as photo-catalysts. The lightdriven splitting of water into H2 and O2 (Figure 1) is an attractive technology that allows for conversion and storage of solar energy to chemical energy, and therefore it is of great economic and environmental interest. Since Fujishima and Honda demonstrated in 1972 water splitting (WS) with titanium dioxide photoanodes, many improved photo-catalysts have been 2 ACS Paragon Plus Environment
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developed.7 However, most of the metal oxides suitable for WS require high-energy light (λ
