Shifting the Sun: Solar Spectral Conversion and Extrinsic Sensitization in Natural and Artificial Photosynthesis

Abstract

Solar energy harvesting is largely limited by the spectral sensitivity of the employed energy conversion system, where usually large parts of the solar spectrum do not contribute to the harvesting scheme, and where, of the contributing fraction, the full potential of each photon is not efficiently used in the generation of electrical or chemical energy. Extrinsic sensitization through photoluminescent spectral conversion has been proposed as a route to at least partially overcome this problem. Here, we discuss this approach in the emerging context of photochemical energy harvesting and storage through natural or artificial photosynthesis. Clearly contrary to application in photovoltaic energy conversion, implementation of solar spectral conversion for extrinsic sensitization of a photosynthetic machinery is very straightforward, and-when compared to intrinsic sensitization-less-strict limitations with regard to quantum coherence are seen. We now argue the ways in which extrinsic sensitization through photoluminescent spectral converters will-and will not-play its role in the area of ultra-efficient photosynthesis, and also illustrate how such extrinsic sensitization requires dedicated selection of specific conversion schemes and design strategies on system scale.

Keywords: artificial photosynthesis; photosynthesis; solar spectral conversion; solar‐to‐fuel; water splitting.

Figure 1

Solar spectral irradiance and spectral sensitivity of photochemical harvesting processes. a) Selected applications of artificial photosynthesis are depicted through the position of the optical bandgap of typical photocatalytic agents, TiO₂ (rutile), α‐Fe₂O₃ and CdSe. For comparison, we also show the characteristic absorption bands of some rare‐earth antenna which may be employed in up‐conversion phosphors. b) The same spectrum, here in comparison to absorption spectra of chlorophylls a and b in methanol solution, and in pumpkin thylakoids. Pigment data of (b) was adopted from Frigaard et al.138 thylakoid data from Antal et al.93 The solar spectrum and exemplary data shown in (a) have been adopted from Refs. 88, 92.

Figure 2

Action spectra of photosynthesis in a) cucumber leaves, b) the cyanobacterium Synechocystis sp. PCC 6803, and c) the chlorophyll‐a‐containing microalga Nannochloropsis oculata. d) Photochemical action spectrum of photosystem II in the chlorophyll‐d‐containing cyanobacterium Acaryochloris marina. The schematic data are redrawn from Refs. 23, 24, 25, 26, 27, 28.

Figure 3

Tandem machinery of an artificial photosynthetic process with intrinsic or extrinsic sensitization.

Figure 4

Principles of spectral conversion through photoluminescence. a) In up‐conversion, two low‐energy photons are converted into one high‐energy photon. Down‐conversion and down‐shifting rely on conversion of a high‐energy photon into one or more low‐energy photons. b) A simplistic scheme of the associated electronic transitions in the active center. UC, as a two‐photon process, involves coherence of ground state (GSA) and excited state absorption to generate high‐energy photons by direct relaxation or through energy transfer (ETU). In DC, emitted photons may derive from photon cascade emission (PCE) or from energy transfer reactions (ETD). For clarity, non‐radiative relaxation (NR) is depicted only for the DS reaction.

Figure 5

Normalized up‐conversion spectra in the UV–vis range for a variety of RE‐doped luminescent materials: a) NaYF₄ core‐shell nanoparticles,104 b,c) K₂YF₅ crystals105 d,i) ZBLAN fluoride glasses,88, 102 e) RE‐doped organic resins,107 f,h) oxyfluoride glasses,103 and g) nanocrystalline oxyfluoride glass ceramics.85 Spectra were taken at excitation with a 980 nm (a–f) or a 800 nm (g–i) laser diode, exciting the Yb³⁺ and/or the Er³⁺ and Nd³⁺ antenna ions. The corresponding sample photographs (right) illustrate the conversion of invisible IR light into visible light.

Figure 6

Conceptual approaches for implementing extrinsic spectral conversion with natural and artificial photosynthesis. a) The design of a frontlight (left) and a backlight (right) photoluminescent converter and its application with a flat‐panel microalgae reactor.7 b) A typical phosphor material (here: (Ca,Sr)S:Eu²⁺ for green‐to‐red downshifting (top). This phosphor can be coated on a light‐guiding and light‐concentrating optical fiber—example shown here (middle): 0.5 mm in diameter, PMMA, front‐end (left) coated with (Ca,Sr)S:Eu²⁺—for tailored light delivery. Large‐area fabrics can be manufactured from such fibers (bottom, showing un‐coated fiber). c) Summary of an exemplary concept of seawater‐splitting, where the intense solar irradiance of the Canary Islands (top, in kWh m⁻² per day) and the existing infrastructure of salt‐flats (middle) are used as the basis for H₂ generation in shallow, covered ponds which comprise slurries of photoconverters and photocatalysts in a combination of back‐ and frontlight converters. Reproduced with permission.88 Copyright 2013, Royal Society of Chemistry.