Light Harvesting

The principles and recent progress in the research and development of photobiological hydrogen production are reviewed. Cyanobacteria produce hydrogen gas using nitrogenase and/or hydrogenase. Hydrogen production mediated by native hydrogenases in cyanobacteria occurs under in the dark under anaerobic conditions by degradation of intracellular glycogen. In vitro and in vivo coupling of the cyanobacterial photosynthetic system with a clostridial hydrogenase

DSSCs performance can be enhanced by modifying the morphology of photoelectrode. Titanium dioxide, TiO2, is the most used oxide semiconductors for DSSC photoelectrode. Anatase and rutile TiO2 are widely explored in DSSC for high photocatalytic activity and high light scattering ability, respectively. However, ZnO-based DSSC is interesting to be developed due to the higher position of the conduction band compared to the conduction band of TiO2 allowing possibility to generate higher photovoltage. In this study, bilayer and monolayer of TiO2-based and ZnO-based DSSCs were fabricated. TiO2 and ZnO nanoparticles were synthesized by co-precipitation method using precursor of TiCl3 and ZnAc dihydrate, respectively. All of TiO2-based DSSSs were sensitized with natural dye anthocyanin extracted from mangosteen percicarp and Ru-based dye (N719). ZnO-based DSSCs were only sensitized with N719 dye. The results show that bilayer TiO2-based DSSCs could enhance the light harvesting efficiency by ...

This work asks how light harvesting in photosynthetic systems can be optimised for economically scalable, sustainable energy production. Hierarchy theory is introduced as a system-analysis and optimisation tool better able to handle multiscale, multiprocess complexities in photosynthetic energetics compared with standard linear-process analysis. Within this framework, new insights are given into relationships between composition, structure and energetics at the scale of the thylakoid membrane, and also into how components at different scales cooperate under functional objectives of the whole photosynthetic system. Combining these reductionistic and holistic analyses creates a platform for modelling multiscale-optimal, idealised photosynthetic systems in silico.

Harvesting solar energy using photovoltaic (PV) cells is the most simple, efficient, and reliable approach to power marine electronics.  Installing PV above or under water provides cooling that increases power conversion efficiency and cleaning. Previous work quantified the performance of PV with different submerged environments and showed promising results in harvesting available underwater solar energy. These studies were limited to commercially-available silicon-based PV. Subsequent, theoretical studies point to enormous potential of using wide-band-gap PV in underwater conditions. With this motivation, for the first time, underwater measurements using dye-sensitized solar cells (DSSCs) employing wide-bandgap ruthenium sensitizers (1.8 eV) have been carried out here. The DSSCs were fabricated and characterized under submerged conditions up to 20 cm. Four replicates provided data detailing DSSCs potential for underwater PV applications when compared with the previously collected data for mono-crystalline, poly-crystalline and amorphous silicon PV. Although the light intensity under water decreases with an increase in depths, the rate of decrease in power output for DSSMs was only 40.68%, which was less than the traditional monocrystalline and polycrystalline silicon PV by approximately 20 to 25%. Also, compared to amorphous silicon PV, DSSCs showed a slightly better performance by 2-3%, which clearly displays the capability of DSSMs to harvest indirect/diffused lights in comparison to conventional Si/thin film PV. Compared to the conventional imported-semiconductor silicon-based PV, indigenously-fabricated DSSCs using locally-sourced materials showed tremendous relative increase in performance in underwater conditions. Further work is needed to optimize DSSCs to increase absolute PCE. Even given the studies limitations, it can be concluded that with the added advantages of simple fabrication process and cost-effectiveness, DSSMs have enormous future potential for underwater PV applications.

The production of H2 gas from water and sunlightusing microalgae, `biophotolysis', has been a subjectof applied research since the early 1970s. A numberof approaches have been investigated, but most provedto have fundamental limitations or requireunpredictable research breakthroughs. Examples areprocesses based on nitrogen-fixing microalgae andthose producing H2 and O2 simultaneously fromwater (`direct biophotolysis'). The most plausibleprocesses for future applied R & D are those whichcouple separate stages of microalgal photosynthesisand fermentations (`indirect biophotolysis'). Theseinvolve fixation of CO2 into storagecarbohydrates followed by their conversion to H2by the reversible hydrogenase, both in dark andpossibly light-driven anaerobic metabolic processes. Based on a preliminary engineering and economicanalysis, biophotolysis processes must achieve closeto an overall 10% solar energy conversion efficiencyto be competitive with alternatives sources ofrenewable H2, such as photovoltaic-electrolysisprocesses. Such high solar conversion efficiencies inphotosynthetic CO2 fixation could be reached bygenetically reducing the number of light harvesting(antenna) chlorophylls and other pigments inmicroalgae. Similarly, greatly increased yields ofH2 from dark fermentation by microalgae could beobtained through application of the techniques ofmetabolic engineering. Another challenge is toscale-up biohydrogen processes with economicallyviable bioreactors.Solar energy driven microalgae processes forbiohydrogen production are potentially large-scale,but also involve long-term and economically high-riskR&D. In the nearer-term, it may be possible tocombine microalgal H2 production with wastewatertreatment.

Photosynthesis, leaf structure, nitrogen content and nitrogen allocation in photosynthetic functions of Cypripedium flavum were studied in a naturally varying light regime. Light-saturated leaf net photosynthetic rate (A max) was strongly correlated with leaf dry mass per area (LMA), mesophyll conductance (g m) and area-based leaf nitrogen content (Narea), with all variables increasing with increasing irradiance. Such coordinate variation of all these parameters illustrates the plastic response of leaf structure to high light (HL). Leaf Narea was greater under HL than in low light (LL). The fractions of leaf nitrogen partitioning in carboxylation (P R) and bioenergetics (P B) were positively related to LMA. In contrast, P R and P B decreased with increasing mass-based leaf nitrogen content (Nmass). However, no correlation was found between leaf nitrogen investment in light harvesting (P L) and either LMA or Nmass. Like maximum rate of carboxylation (V cmax) and electron transport (J max), the J max/V cmax ratio, which was strongly correlated to LMA, also increased significantly with irradiance. Under HL, leaf maximum photosynthetic nitrogen efficiency (ANUE) and intrinsic water use efficiency (WUE) were greater than in LL conditions, despite a small difference in WUE. This suggests that a functional balance in the photosynthetic machinery favors leaf photosynthetic plasticity of C. flavum in response to different light conditions. Given an ample soil nitrogen supply, C. flavum may offset its susceptibility to HL by efficient nitrogen use and higher stomatal and mesophyll conductance against photoinhibition so as to keep leaf photosynthesis positive.

Recent evidence suggests that quantum coherence enhances excitation energy transfer (EET) through individual photosynthetic light-harvesting protein complexes (LHCs). Its role in vivo is unclear however, where transfer to chemical reaction centres (RCs) spans larger, multi-LHC/RC networks. Here we predict maximum coherence lengths possible in fully connected chromophore networks with the generic structural and energetic features of multi-LHC/RC networks. A renormalization analysis reveals the dependence of EET dynamics on multiscale, hierarchical network structure. Surprisingly, thermal decoherence rate declines at larger length scales for physiological parameters and coherence length is instead limited by localization due to static disorder. Physiological parameters support coherence lengths up to ∼5 nm, which is consistent with observations of solvated LHCs and invites experimental tests for intercomplex coherences in multi-LHC/RC networks. Results further suggest that a semiconductor quantum dot network engineered with hierarchically clustered structure and small static disorder may support coherent EET over larger length scales, at ambient temperatures.

Elucidating quantum coherence effects and geometrical factors for efficient
energy transfer in photosynthesis has the potential to uncover nonclassical design
principles for advanced organic materials. We study energy transfer in a linear light-
harvesting model to reveal that dimerized geometries with strong electronic coherences
within donor and acceptor pairs exhibit significantly improved efficiency, which is in
marked contrast to predictions of the classical Förster theory. We reveal that energy
tuning due to coherent delocalization of photoexcitations is mainly responsible for the
efficiency optimization. This coherence-assisted energy-tuning mechanism also explains
the energetics and chlorophyll arrangements in the widely studied Fenna−Matthews−
Olson complex. We argue that a clustered network with rapid energy relaxation among
donors and resonant energy transfer from donor to acceptor states provides a basic
formula for constructing efficient light-harvesting systems, and the general principles revealed here can be generalized to larger systems and benefit future innovation of efficient molecular light-harvesting materials.