Solar Emission Spectrum Research Articles

Structural and opto-electronic engineering with ZnS ETL via PVD technique for efficient and durable heterojunction perovskite solar cells

This study presents a comprehensive investigation into fabrication and characterization of HJPSC leveraging chalcogenide Zinc Sulphide (ZnS) thin films as ETL and Selenium (Se) as a protective capping layer through PVD technique. ZnS thin films annealed at 200 °C, 300 °C and 400 °C were systematically analyzed for their structural and opto-electronic properties. Among them, ZnS-1, annealed at 200 °C, demonstrated superior performance with high transmittance, optimal crystallite size, wide bandgap, lowest photoluminescence intensity, high conductivity and low resistivity. Consequently, ZnS-1 was selected as the ETL for HJPSC fabrication. The HJPSC device was fabricated with ZnS/MAPbI3--Se/Ag as device structure with Se as capping layer. SEM images confirmed the smooth and continuous nature of the ZnS-1 film, while X-Ray diffraction analysis validated the purity and crystallinity of MAPbI3 perovskite layer. The successful alignment of the perovskite absorption band with the solar cell emission spectrum obtained by UV–Vis spectra highlighted the potential for efficient solar energy conversion. J-V characteristics of the HJPSC device exhibited a high short-circuit current (Jsc) of 20.18 mA/cm2, an open-circuit voltage (Voc) of 0.99 V, a power conversion efficiency (PCE) of 9.1 % and fill factor (FF) of 45.7 % was accredited to improved electron conductance and of ZnS-1 as an ETL.

DFT modeling of new composite based on PANI/PVK functionalized With single-walled carbon nanotubes for optoelectronic applications

We present a comparative experimental and theoretical study of the vibrational, optical, and electronic properties of polyaniline at both emeraladine leuco-emeraldine forms and PVK. Our objective is first to select the appropriate form of PANI to be linked with PVK to form a hybrid polymer matrix. Then, on the basis of quantum chemical calculations, we present a theoretical study of a hybrid composite PANI/PVK functionalized with lower radius (5, 5) chiral single-walled carbon nanotubes (SWCNTs). The relationship structure-properties of the resulting nano-composite are proved based on the conformational and optical study made from Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) using B3LYP method with 6-31-G(d) basis set. First, it has been demonstrated that the resulting interaction between PANI and PVK leads to more advantageous properties, such as good optical absorption compatibility with the solar emission spectrum. It has also been found that after the incorporation of SWCNTs into the matrix of PANI/PVK hybrid polymer, a good organization of SWCNTs via π-staking interaction is imposed by polyaniline sequences. However, PVK moieties are grafted on the sidewalls of SWCNTs. Finally, the electronic structure of the resulting hybrid nano-composite PANI/PVK/SWCNTs was carried out with the use of different electrodes on both sides.

Statistical analysis for EUV dynamic spectra and their impact on the ionosphere during solar flares

The X-rays and extreme ultraviolet (EUV) emitted during solar flares can rapidly change the physical composition of Earth’s ionosphere, causing space weather phenomena. It is important to develop an accurate understanding of solar flare emission spectra to understand how it affects the ionosphere. We reproduced the entire solar flare emission spectrum using an empirical model and physics-based model, and input it into the Earth’s atmospheric model, GAIA to calculate the total electron content (TEC) enhancement due to solar flare emission. We compared the statistics of nine solar flare events and calculated the TEC enhancements with the corresponding observed data. The model used in this study was able to estimate the TEC enhancement due to solar flare emission with a correlation coefficient greater than 0.9. The results of this study indicate that the TEC enhancement due to solar flare emission is determined by soft X-ray and EUV emission with wavelengths shorter than 35 nm. The TEC enhancement is found to be largely due to the change in the soft X-ray emission and EUV line emissions with wavelengths, such as Fe XVII 10.08 nm, Fe XIX 10.85 nm and He II 30.38 nm.Graphical

Solar water purification with photocatalytic nanocomposite filter based on TiO2 nanowires and carbon nanotubes

Water contamination due to environmental conditions and poor waste management in certain areas of the world represents a serious problem in accessing clean and safe drinking water. This problem is especially critical in electricity-poor regions, where advanced water purification methods are absent. Here, we demonstrate that titanium dioxide nanowires (TiO2NWs)-based photocatalytic filters assisted only with sunlight can efficiently decontaminate water. Moreover, interweaving TiO2NWs with carbon nanotubes (CNTs) leads to the formation of a TiO2NWs/CNTs composite material and offers an additional water decontamination channel, that is of pasteurization with the visible part of the solar emission spectrum. Our results demonstrate that this nanoporous filter can successfully intercept various types of microbial pathogens, including bacteria and large viruses. In addition, photo-catalytically generated reactive oxygen species (ROS) on the surface of the TiO2NWs/CNTs-based filter material under exposure to sunlight contribute to an efficient removal of a broad range of organic compounds and infective microbes. A pilot study also yielded encouraging results in reducing traces of drugs and pesticides in drinking water.

Electronic gap stability of two-dimensional tin monosulfide phases: Towards optimal structures for electronic device applications

It is well known that tin monosulfide is a potential environment-friendly solar cell material that maximizes light-energy conversion efficiency, given its band gap at the peak of the solar emission spectrum. However, the presence of Sn and S vacancies at the surface of SnS hinders the performance of these solar cells. In the present work, we have prepared tin sulfide platelets on graphite using vapor phase deposition under a known temperature gradient. Remarkably, we observe two types of growth modes: (i) a dominant one with platelet-like flat crystals and (ii) a less favorable one, formed by spiral terraces. Both structures present a chemical composition compatible with SnS. Using scanning tunneling microscopy (STM) and spectroscopy (STS), we observe that, whereas flat SnS platelets exhibit fluctuations in the band gap and doping, spiral platelets exhibit stable, homogeneous band gap, and negative doping. Using selected area electron diffraction (SAED), we determine that, whereas the platelets of the unstable phase are associated with the common orthorhombic structure of SnS, the minor stable spiral platelets are polycrystalline and are in the metastable cubic phase. The production of this less favorable phase on large surface areas is of potential interest for the production of more efficient tin sulfide-based solar cells. In addition, our analysis of solar cell materials using STM/STS and SAED provides a framework for the rapid determination of the suitability and applicability of 2D materials for other potential optoelectronic applications.

Optimizing the Cosensitization Effect of SQ02 Dye on BP-2 Dye-Sensitized Solar Cells: A Computational Quantum Chemical Study.

Cosensitization of the semiconducting electrode in dye-sensitized solar cells (DSCs), with two or more light-harvesting dyes, is a chemical fabrication method that aims to achieve a panchromatic absorption spectrum emulating that of the solar emission spectrum. In this paper, SQ02 and BP-2 cosensitizers have been investigated, as isolated monomers/dimer and adsorbed monomers/dimer on the TiO2 (101) anatase surface, by employing density functional theory (DFT) and time-dependent DFT calculations. Computed results showed that the dominant electron injection pathway is direct injection from each dye into the conduction band of TiO2. The almost complete spectral overlap between the simulated absorption spectrum of BP-2 and fluorescence emissions of SQ02 implies that excitation energy transfer occurs between cosensitizers via the trivial reabsorption mechanism. However, the results showed very limited unidirectional intermolecular charge transfer (CT) from SQ02 dye to BP-2 dye (0.04 |e-|). Therefore, this study also presents a stepwise molecular engineering of BP-2 dye, aiming at optimizing the cosensitization functionality. First, 14 redesigned dye candidates are reported to identify dyes with photophysical properties matching the requirements for efficient DSCs. Second, the four most promising dyes are shortlisted for testing as cosensitizers with the SQ02 dye. The molecular design factors of cosensitization that need validation are chemical compatibility, availability of CT between cosensitizers, and complementarity of the absorption spectra. This screening suggests the judicious choice of the modeled difluorenyl amine donor-based dye (BP-D4) as a very promising cosensitizer. In particular, the SQ02/BP-D4 dimer showed 10 times larger (0.53 |e-|) unidirectional CT than that of SQ02/BP-2 dimer, in addition to the maximum increased electron population of acceptor moieties upon photoexcitation.

Comparative Investigation of Fullerene PC71BM and Non-fullerene ITIC-Th Acceptors Blended With P3HT or PBDB-T Donor Polymers for PV Applications

Fundamentally, organic solar cells (OSCs) with a bulk-heterojunction active layer are made of at least two electronically dissimilar molecules, in which photoabsorption in one (donor) generates Frenkel excitons. The formation of free charge carriers emerge after exciton dissociation at the donor:acceptor interface. In the past decade, most of the progress in enhanced device performance has been steered by the rapid development of novel donor and acceptor materials and on device engineering. Among these donor materials, regioregular poly(3-hexylthiophene) (P3HT) produced better performance despite the mismatch of its absorption coefficient with the solar emission spectrum. Comparatively the donor PBDB-T exhibits an outstanding absorption coefficient with a deeper-lying highest occupied molecular orbital (HOMO) level. Previously most of the efficient acceptors were based on fullerene molecules characterized by limited photoabsorption and stability. In contrast, the recently developed non-fullerene OSCs have a tunable absorption spectrum and exhibit improved stability. In this work, we explore the fundamental sources of the differences in the device performance for different blend compositions made of fullerene derivative (PC71BM) and non-fullerene (ITIC-Th) when paired with the polymer donors P3HT and PBDB-T. The characteristic changes of the optical properties of these blends and their roles in device performance are also investigated. We also studied charge generation where PBDB-T:PC71BM showed the highest maximum exciton generation rate (Gmax) of 3.22 × 1028 s–1 while P3HT: ITIC-Th gave the lowest (0.96 × 1028 s–1). Also noted, PC71BM based counterparts gave better charge transfer capabilities as seen from the lower PL quenching and higher charge carrier dissociation plus collection probability P(E,T) derived from a plot of Jph/Jsat ratio under short-circuit conditions against the effective voltages.

Cosensitization in Dye-Sensitized Solar Cells.

Dye-sensitized solar cells (DSCs) are a next-generation photovoltaic technology, whose natural transparency and good photovoltaic output under ambient light conditions afford them niche applications in solar-powered windows and interior design for energy-sustainable buildings. Their ability to be fabricated on flexible substrates, or as fibers, also makes them attractive as passive energy harvesters in wearable devices and textiles. Cosensitization has emerged as a method that affords efficiency gains in DSCs, being most celebrated via its role in nudging power conversion efficiencies of DSCs to reach world-record values; yet, cosensitization has a much wider potential for applications, as this review will show. Cosensitization is a chemical fabrication method that produces DSC working electrodes that contain two or more different dyes with complementary optical absorption characteristics. Dye combinations that collectively afford a panchromatic absorption spectrum emulating that of the solar emission spectrum are ideal, given that such combinations use all available sunlight. This review classifies existing cosensitization efforts into seven distinct ways that dyes have been combined in order to generate panchromatic DSCs. Seven cognate molecular-engineering strategies for cosensitization are thereby developed, which tailor optical absorption toward optimal DSC-device function.

High Nd3+→Yb3+ energy transfer efficiency in tungsten‐tellurite glass: A promising converter for solar cells

AbstractThis work reports on the energy transfer efficiency for Nd3+/Yb3+ co‐doped tellurite glasses (80TO2‐20WO3, in mol%,). The correlation between Yb3+ ion concentration and the downconversion mechanism was investigated using optical and thermal lens spectroscopies, which enabled investigation of the radiative and nonradiative processes, respectively, involved in energy transfer from neodymium to ytterbium. The Nd3+ near‐infrared fluorescence disappeared almost entirely when the maximum concentration of Yb3+ ions (4 mol%) was doped into the host. In contrast, there was a corresponding increase in the ytterbium emission at around 980 nm. When ytterbium was added, there was also a simultaneous reduction in the amount of heat generated by the sample due to a reduction in the nonradiative decay rate, corroborating the suspected high energy transfer efficiency of Nd3+→Yb3+. The results indicate that tungsten‐tellurite glasses may be of potential use in solar cells for matching the solar emission spectrum to the semiconductor cell.

Fate and implication of acetylacetone in photochemical processes for water treatment

Acetylacetone (AA), due to the peculiar enol-keto structures, has attracted wide scientific interests. In terms of photo-decolorization, it works much more efficiently than the well-known H2O2. However, there is very limited information on the photochemistry of AA in aqueous solutions. Herein, the photolysis kinetics, quantum yield, mass balance, decomposition pathway, and bioavailability of AA during UV irradiation were systematically investigated. It seems that photophysical processes predominated over photochemical ones when AA was irradiated with UV light. Although the quantum yield of AA (0.116) was much lower than that of H2O2 (1.0), the stronger light absorption of AA and the better overlap of the AA absorption spectrum with the solar emission spectrum, as well as the direct energy/electron transfer mechanisms, ensured its high efficiency in photochemical processes. The main degradation products of AA in photochemical processes were similar to the metabolic products in bio-fermentation. Besides, the irradiated AA solution showed a high bioavailability to the cells in activated sludge. Therefore, the UV/AA process might be a promising pre-treatment approach for bio-treatment. The results provide new insights into the photochemical fate and implication of β-diketones in aqueous solutions.

Functionally graded poly(dimethylsiloxane)/silver nanocomposites with tailored broadband optical absorption

In this work, we produce functionally graded nanocomposites consisting of silver (Ag) plasmonic nanoparticles (PNPs) supported in a poly(dimethylsiloxane) (PDMS) matrix. PDMS was selected due to its high optical transparency, nontoxicity and ease of use. The Ag PNPs were formed by annealing sputtered Ag ultra-thin films and were subsequently capped by a spin-coated PDMS layer. We investigate the factors that affect their plasmonic behavior, such as the PNP size, the annealing conditions and the surrounding environment. In order to achieve broadband absorption, we developed PDMS/Ag(PNPs) multilayers with graded PNP size. Thus, we demonstrate the significance of the stacking sequence of various plasmonic layers sandwiched between PDMS layers and its potential for tailoring the plasmonic response of multilayer structure. As a demonstration of this approach, we deposited a specially designed multilayer structure, whose optical extinction resembles the solar emission spectrum.

Strategy to Modulate the Electron-Rich Units in Donor–Acceptor Copolymers for Improvements of Organic Photovoltaics

We present an effective strategy to modulate the electron-rich capability in donor–acceptor (D-A) polymers for improving the performances of organic solar cell (OSC) devices. In order to confirm this strategy, based on a series of the reported D–A polymers ((PCPDTBT(Pa1), PCPDTFBT (Pa2), and PCPDTDFBT (Pa3)) which contain the electron-donating cyclopentadithiophene (CPDT) and differently electron-withdrawing units of benzo[c][1,2,5,]thiadiazole (BT), 5-fluorobenzo[c][1,2,5]thiadiazole (FBT), and 5,6-difluorobenzo[c] [1,2,5]thiadiazole (DFBT), we replace CPDT with electron-donating dithienogermolodithiophene (DTTG) in polymers Pa1–Pa3, respectively, and design a series of new D–A polymers Pb1–Pb3. Compared with the polymers Pa1–Pa3, the new designed polymers Pb1–Pb3 not only yield a greater red-shift of the absorption spectrum of the donor polymer and result in a larger absorption region within the solar emission spectrum and an improved light-absorbing efficiency but also exhibit much better electron transfer efficiency in active layer, larger hole transport rates and higher open circuit voltage. Moreover, the estimated power conversion efficiency of the designed polymers in OSC applications reaches up to ∼8.4%. Conclusively, the approach based on modulating the electron-donating capability in D–A polymer chain is a feasible way to enhance their intrinsic properties of donor polymers and thereby achieving the purpose that improves the performances of the OSC devices.

Synthesis and photoluminescence properties of green-emitting BaSi3O4N2:Eu2+ phosphors

A series of green-emitting phosphors with the formula Ba1−xEuxSi3O4N2 (x=0.02, 0.04, 0.06, 0.08, 0.10, 0.15, 0.25) were synthesized successfully by solid state reactions. The phosphors exhibited strong green emission upon the excitation in the range of 250–480nm, matching with the emission bands of the near ultraviolet and the blue LED chips. Combined Ba0.94Eu0.06Si3O4N2 with Sr2Si5N8:Eu2+ and YAG:Ce3+ phosphors for the blue LED, the obtained emission spectrum was analogous to the solar emission spectrum. Because evolution has produced human eyes being optimized to match the solar emission spectrum, Ba1−xSi3O4N2:xEu2+ is a promising candidate of green-emitting phosphor for eye health white LEDs.

Luminescent materials based on nanoparticles of silicon dioxide for solar cells

The report considers luminescent mesoporous silica dioxide particles of activated RE ions and lead sulfide nanocrystals of different sizes. For the purpose of assessing the possibility of converting by lead sulfide nanoclusters of solar radiation into the red-orange light, the following methods will be drawn to the attention: mesoporous silica particles sol-gel synthesis, additional light scattering method as an additional approach for the determination of particle size. The element composition of a particle size of 300 ± 40 nm was identified using the scanning electron microscope with the system of energy disperse analysis. Particle images were obtained using the field scanning electron microscope. Zeta Potential Analyzer was used to measure the surface charge of the particles. Particles in solution were excited by near ultraviolet and visible light. The intensity of intrinsic emission was registered in the range of 400–550 nm. The diagnosis made visible the electron induced energy transmission from the matrix to the Eu3+ and Tb3+. Emission of Eu3+ and Tb3+ has been registered at 300K. It is assumed that the identical matrixes could work as converters of solar emission spectrum, to which some solar cells are sensitive.

Investigation of luminescent properties of ZnO nanoparticles for their use as a down‐shifting layer on solar cells

AbstractCommercially available solar cells (e.g. CdS/CdTe, CIGS, c‐Si) have a spectrally narrow absorption band when compared to the solar emission spectrum. The UV‐wavelength response of a solar cell (SC) can be improved by the application of a luminescent down‐shifting layer (LDSL) on the SC front side, permitting the conversion of short‐wavelength photons to the longer wavelength photons better matching the SC’s absorption spectrum. The ideal down‐shifting material must possess a large Stokes shift and have a high luminescence quantum yield. We propose the use of the LDSL containing ZnO nanoparticles of less than 5 nm in diameter able to absorb UV light (λ < 400 nm), where the solar cell spectral response (SR) is low, and re‐emitting at longer wavelengths (λ > 425 nm), where the typical SC’s SR increases. ZnO nanoparticles were synthesized by a low energy cluster beam deposition (LECBD) technique and their luminescent properties were studied as a function of the oxygen partial pressure (OPP) applied during the deposition process. The stoichiometry and crystallinity of ZnO nanoparticles can be controlled via the adjustment of the OPP. It was also observed that there exists an optimal value of the oxygen pressure introduced during the LECBD process, which permits to obtain the highest visible photoluminescence emission, necessary for an efficient down‐shifting. The yield of the down‐shifting in ZnO nanoparticle layer was determined varying the excitation wavelength using the photoluminescence excitation technique.

Broadband optical absorption of amorphous carbon/Ag nanocomposite films and its potential for solar harvesting applications

The emergence of Localized Surface Plasmon Resonance (LSPR) in nanocomposite films consisting of a hydrogen-free amorphous Carbon (a-C) matrix and Ag is considered theoretically and experimentally. While in theory it could be manifested for highly tetrahedral (>90% sp3) matrices, Auger electron spectroscopy and neutron reflectivity have shown that the incorporation of Ag into the a-C matrix induces severe graphitization that eliminates the LSPR; nonetheless, the dielectric damping of graphitized a-C, in combination with the π−π⁎ electronic transitions of carbon and the defect states introduced by Ag, cumulatively result in a strong broadband optical absorption in the near infrared, visible and UVA/UVB spectral ranges, revealed by optical reflectivity spectra, that coincides with the solar emission spectrum. The incorporation of Ag into a-C at room temperature thus proves to be an energy-efficient pathway for the controlled graphitization and the tailored optical absorption of novel nanocomposite films for solar photothermal applications.

Brown carbon formation from ketoaldehydes of biogenic monoterpenes

Sources and chemical composition of brown carbon are poorly understood, and even less is known about the mechanisms of its atmospheric transformations. This work presents molecular-level investigations of the reactive compound ketolimononaldehyde (KLA, C9H14O3), a second-generation ozonolysis product of limonene (C10H16), as a potent brown carbon precursor in secondary organic aerosol (SOA) through its reactions with reduced nitrogen compounds, such as ammonium ion (NH4+), ammonia, and amino acids. The reactions of synthesized and purified KLA with NH4+ and glycine resulted in the formation of chromophores nearly identical in spectral properties and formation rates to those found in similarly-aged limonene/O3 SOA. Similar chemical reaction processes of limononaldehyde (LA, C10H16O2) and pinonaldehyde (PA, C10H16O2), the first-generation ozonolysis products of limonene and alpha-pinene, respectively, were also studied, but the resulting products did not exhibit the light absorption properties of brown carbon, suggesting that the unique molecular structure of KLA produces visible-light-absorbing compounds. The KLA/NH4+ and KLA/GLY reactions produce water-soluble, hydrolysis-resilient chromophores with high mass absorption coefficients (MAC = 2000-4000 cm2 g(-1)) at lambda - 500 nm, precisely at the maximum of the solar emission spectrum. Liquid chromatography was used to isolate the light-absorbing fraction, and UV-Vis, FTIR, NMR and high-resolution mass spectrometry (HR-MS) techniques were used to investigate the structures and chemical properties of the light-absorbing compounds. The KLA browning reaction generates a diverse mixture of light-absorbing compounds, with the majority of the observable products containing 1-4 units of KLA and 0-2 nitrogen atoms. Based on the HR-MS product distribution, conjugated aldol condensates, secondary imines (Schiff bases), and N-heterocycles like pyrroles may contribute in varying degree to the light-absorbing properties of the KLA brown carbon. The results of this study demonstrate the high degree of selectivity of organic compound structures on the light-absorbing properties of SOA.

A solar-driven UV/Chlorine advanced oxidation process

An overlap of the absorption spectrum of the hypochlorite ion (OCl−) and the ultraviolet (UV) end of the solar emission spectrum implies that solar photons can probably initiate the UV/chlorine advanced oxidation process (AOP). The application of this solar process to water and wastewater treatment has been investigated in this study. At the bench-scale, the OCl− photolysis quantum yield at 303 nm (representative of the lower end of the solar UV region) and at concentrations from 0 to 4.23 mM was 0.87 ± 0.01. Also the hydroxyl radical yield factor (for an OCl− concentration of 1.13 mM) was 0.70 ± 0.02. Application of this process, at the bench-scale and under actual sunlight, led to methylene blue (MB) photobleaching and cyclohexanoic acid (CHA) photodegradation. For MB photobleaching, the OCl− concentration was the key factor causing an increase in the pseudo first-order rate constants. The MB photobleaching quantum yield was affected by the MB concentration, but not much by the OCl− concentration. For CHA photodegradation, an optimal OCl− concentration of 1.55 mM was obtained for a 0.23 mM CHA concentration, and a scavenger effect was observed when higher OCl− concentrations were applied. Quantum yields of 0.09 ± 0.01 and 0.89 ± 0.06 were found for CHA photodegradation and OCl− photolysis, respectively. In addition, based on the Air Mass 1.5 reference solar spectrum and experimental quantum yields, a theoretical calculation method was developed to estimate the initial rate for photoreactions under sunlight. The theoretical initial rates agreed well with the experimental rates for both MB photobleaching and CHA photodegradation.

Synthesis and applications of 2,7-carbazole-based conjugated main-chain copolymers containing electron deficient bithiazole units for organic solar cells

A series of low-band-gap (LBG) donor-accepor con- jugated main-chain copolymers (P1-P4) containing planar 2,7- carbazole as electron donors and bithiazole units (4,4 0 -dihexyl- 2,2 0 -bithiazole and 4,4 0 -dihexyl-5,5 0 -di(thiophen-2-yl)-2,2 0 -bithia- zole) as electron acceptors were synthesized and studied for the applications in bulk heterojunction (BHJ) solar cells. The effects of electron deficient bithiazole units on the thermal, optical, electrochemical, and photovoltaic (PV) properties of these LBG copolymers were investigated. Absorption spectra revealed that polymers P1-P4 exhibited broad absorption bands in UV and visible regions from 300 to 600 nm with opti- cal band gaps in the range of 1.93-1.99 eV, which overlapped with the major region of the solar emission spectrum. More- over, carbazole-based polymers P1-P4 showed low values of the highest occupied molecular orbital (HOMO) levels, which provided good air stability and high open circuit voltages (Voc) in the PV applications. The BHJ PV devices were fabricated using polymers P1-P4 as electron donors and (6,6)-phenyl-C61- butyric acid methyl ester (PC61BM) or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptors in different weight ratios. The PV device bearing an active layer of polymer blend P4:PC71BM (1:1.5 w/w) showed the best power conver- sion efficiency value of 1.01% with a short circuit current den- sity (Jsc) of 4.83 mA/cm 2 , a fill factor (FF) of 35%, and Voc ¼ 0.60 V under 100 mW/cm 2 of AM 1.5 white-light illumination.

Molecular Factors Controlling Photosynthetic Light Harvesting by Carotenoids

Carotenoids are naturally occurring pigments that absorb light in the spectral region in which the sun irradiates maximally. These molecules transfer this energy to chlorophylls, initiating the primary photochemical events of photosynthesis. Carotenoids also regulate the flow of energy within the photosynthetic apparatus and protect it from photoinduced damage caused by excess light absorption. To carry out these functions in nature, carotenoids are bound in discrete pigment-protein complexes in the proximity of chlorophylls. A few three-dimensional structures of these carotenoid complexes have been determined by X-ray crystallography. Thus, the stage is set for attempting to correlate the structural information with the spectroscopic properties of carotenoids to understand the molecular mechanism(s) of their function in photosynthetic systems. In this Account, we summarize current spectroscopic data describing the excited state energies and ultrafast dynamics of purified carotenoids in solution and bound in light-harvesting complexes from purple bacteria, marine algae, and green plants. Many of these complexes can be modified using mutagenesis or pigment exchange which facilitates the elucidation of correlations between structure and function. We describe the structural and electronic factors controlling the function of carotenoids as energy donors. We also discuss unresolved issues related to the nature of spectroscopically dark excited states, which could play a role in light harvesting. To illustrate the interplay between structural determinations and spectroscopic investigations that exemplifies work in the field, we describe the spectroscopic properties of four light-harvesting complexes whose structures have been determined to atomic resolution. The first, the LH2 complex from the purple bacterium Rhodopseudomonas acidophila, contains the carotenoid rhodopin glucoside. The second is the LHCII trimeric complex from higher plants which uses the carotenoids lutein, neoxanthin, and violaxanthin to transfer energy to chlorophyll. The third, the peridinin-chlorophyll-protein (PCP) from the dinoflagellate Amphidinium carterae, is the only known complex in which the bound carotenoid (peridinin) pigments outnumber the chlorophylls. The last is xanthorhodopsin from the eubacterium Salinibacter ruber. This complex contains the carotenoid salinixanthin, which transfers energy to a retinal chromophore. The carotenoids in these pigment-protein complexes transfer energy with high efficiency by optimizing both the distance and orientation of the carotenoid donor and chlorophyll acceptor molecules. Importantly, the versatility and robustness of carotenoids in these light-harvesting pigment-protein complexes have led to their incorporation in the design and synthesis of nanoscale antenna systems. In these bioinspired systems, researchers are seeking to improve the light capture and use of energy from the solar emission spectrum.