Dye sensitized solar cells based on Antarctic

Environmental Sustainability https://doi.org/10.1007/s42398-018-0007-1

ORIGINAL ARTICLE

Dye sensitized solar cells based on Antarctic Hymenobacter sp. UV11 dyes Tatiana Montagni1 · Paula Enciso1 · Juan José Marizcurrena2 · Susana Castro‑Sowinski2 · Carolina Fontana3 · Danilo Davyt4 · María Fernanda Cerdá1 Received: 24 March 2018 / Revised: 10 May 2018 / Accepted: 10 May 2018 © Society for Environmental Sustainability 2018

Abstract Xanthophylls pigments extracted from Hymenobacter sp. UV11 (a bacterium that produces reddish colonies on agar) collected at Fildes Peninsula, King George Island, Antarctica, were tested as sensitizers in dye sensitized solar cells (DSSC). Experiments were performed in the presence and in the absence of a co-adsorbent, a slimy substance produced by UV11 and identified as a polysaccharide during this work. Results suggest that the highest conversion efficiency (0.03%) was obtained when using the orange-xanthophylls pigment in the presence of the α-1,4-glucan both co-extracted from UV11. This work highlights the importance of using co-adsorbents as co-adjuvants in the production of more efficient DSSC manufactured with bacterial dyes. These results may contribute to the development of an exploration program of Antarctic resources, and offer the possibility to start the change in the energetic matrix in that remote area of the Planet, decreasing the environmental impact associated with the use of fossil fuels. Keywords Xanthophylls · Co-adsorbent · DSSC · Impedance · Antarctica · Hymenobacter sp

Introduction The U.S. Energy Information Administration’s latest International Energy Outlook 2017 (IEO2017) has projected that from 2015 to 2040 the worldwide energy consumption will grow by 28% (https://www.eia.gov/todayinenergy/ detail.php?id=32912). In this scenario, the use of renewable energy sources has been considered as an environmental friendly alternative to the use of carbon-intensive energy systems. Among different potential renewable energy sources, * María Fernanda Cerdá [email protected] 1

Laboratorio de Biomateriales, Facultad de Ciencias, Universidad de la República (UdelaR), Iguá 4225, 11400 Montevideo, Uruguay

2

Sección Bioquímica y Biología Molecular, Facultad de Ciencias, UdelaR, Iguá 4225, 11400 Montevideo, Uruguay

3

Laboratorio de Espectroscopía y Fisicoquímica Orgánica, Departamento de Química del Litoral, Facultad de Química & CENUR Litoral Norte, UdelaR, Ruta 3 km 363, 60000 Paysandú, Uruguay

4

Departamento de Química Orgánica, Facultad de Química, UdelaR, General Flores 2124, 11800 Montevideo, Uruguay

the solar energy represents a few thousand times the primary energy that humans consume. Thus, solar power might be a crucial component of a renewable energy portfolio. Dye sensitized solar cells (DSSC) are an interesting alternative to conventional photovoltaic-silicon based cells, with conversion efficiency values up to 12% (quite similar to conventional ones) (O’Regan and Grätzel 1991; Bisquert et al. 2004; Gao et al. 2008; Chen et al. 2009; Yella et al. 2011; Yum et al. 2012; Grätzel and Zakeeruddin 2013). However, compared to photovoltaic-silicon based cells, DSSC are produced at lower cost, they are easily manufactured and their tuneable optical properties are better (Mathew et al. 2014). The light-to-electric energy conversion in DSSC is based on the ability of coloured dyes used as photo sensitizers, for harvesting the light in the visible range and leading a flow of electrons (Govindaraj et al. 2015). They contain a nanoporous oxide layer, normally titanium dioxide under the form of anatase, to which the dyes are attached. When photo excitation of the pigment takes place, the injection of an electron into the conduction band of the oxide does occur. Then, the redox status of the dye is restored by electron donation from the electrolyte, where a redox system such as the iodide/triiodide couple is contained. Iodide is finally regenerated by the reduction of

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triiodide at the counter electrode, and the circuit is completed through an electron migration at the external load (Bisquert et al. 2004). Since the first publication of a high efficient DSSC in 1991, the search for high performance dyes have showed an explosive growth, mainly due the world wide interest of finding and/or developing new clean energy technologies (Grätzel and Zakeeruddin 2013; Nazeeruddin et al. 2011). Initial attempts were focussed on the search of plant pigments obtained from flowers, leaves and fruits (anthocyanins, chlorophyll, xanthophylls, flavonoids and carotenes). They have showed efficiency values up to 2% (Calogero et al. 2012; Shalini et al. 2015), but recently the use of bacterial pigments has also been studied. The information supports that bacterial dyes have the potential to be used for manufacturing DSSC (Órdenes-Aenishanslins et al. 2016; Molaeirad et al. 2015; Fu et al. 2014). With the aim of contributing to the environmental sustainability of Antarctica, we worked out on the search of bacterial dyes, produced by Antarctic microorganisms, which might be used in DSSC to generate energy in this hostile environment. The search for alternative and clean sources of energy is of great importance in the Antarctic areas, where Scientific Bases receive very sporadic fuel incomes. Thus, DSSC could represent an interesting alternative that partially may solve the need of energy at Antarctica. We propose the following hypothesis: pigments produced by Antarctic bacteria probably evolved to produce high performance capture light-energy pigments, ones that could even work under the Antarctic cloudy sky and non-direct sunlight conditions. Thus, Antarcticdye-sensitized solar cells might be assembled indoors as photovoltaic located at windows or even inside a room, protected from climate inclemency and low temperatures. As previously reported by many authors, Antarctica is a source of pigment-producing algae and bacteria. In particular, the development of DSSC using pigments from red algae and bacteria has been reported before (Woronowicz et al. 2012; Calogero et al. 2014; Calogero et al. 2015; Enciso and Cerdá 2016; Órdenes-Aenishanslins et al. 2016). The aim of this work was to analyze the potential of the pigment produced by Hymenobacter sp. UV11 as sensitizers in the presence or in the absence of co-adsorbents. Hymenobacter sp. UV11 was isolated in January 2010 near the Artigas Antarctic Scientific Base (62°11′4″S; 58°51′7″W) at the Fildes Peninsula, King George Island (Morel et al. 2015). Marizcurrena et al. (2017) further characterized UV11 as an ultraviolet (UV) resistant bacterium that produces reddish colonies on agar. During the present work, we extracted and partially characterized a red/orange-xanthophylls pigment from UV11 and then we tested this pigment in DSSC.

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Environmental Sustainability

Materials and methods Hymenobacter sp. UV11 growth conditions The UV-resistant bacterium Hymenobacter sp. UV11 (Marizcurrena et al. 2017) was grown in R2 broth (Reasoner et al. 1979) at 20 °C and 220 rpm for 5 days, or on R2A (R2 medium supplemented with 1.5% agar) medium for 2 weeks at 20 °C. UV11 cells were stored in 15% glycerol at – 80 °C.

Pigment extraction The reddish pigment produced by UV11 was obtained from cells grown in R2 broth. A 100 mL pre-inoculum of bacterial cells was obtained in R2 medium, transferred to 1 L of R2 and grown as described above. The bacterial culture was centrifuged at 6000g for 20 min, and three phases (pellet, slime and supernatant) were obtained as showed in Fig. 1. The uncoloured supernatant was discarded. Reddish pellet and slime were suspended in three volumes of 98% ethanol and heated at 60 °C for 1 h. When both slime and pellet turned white and the ethanol solutions were fully orange (at this moment the reddish pigment turned to orange), samples were centrifuged at 15,000g for 30 min at 4 °C, and the supernatants were filtered with a 0.45 μm sterile membrane. All extractions were kept at − 20 °C in darkness (wrapped in aluminium foil). The pigment obtained was called RAW orange.

Transmission electron microscopy (TEM)—negative staining of Hymenobacter sp. UV11 An aliquot of 10 µL of UV11 strain growth in R2 broth was placed on a formvar carbon (200 Mesh) grid for 2 min and

Fig. 1 Picture of Hymenobacter sp. UV11 after growth on R2. a Picture of an eppendorf containing 1 mL UV11 growth medium after centrifugation; b a magnification of the bottom of the eppendorf, showing up the different phases obtained after centrifugation. These phases are: (1) bacterial pellet, (2) slime, (3) supernatant


excess water was dried with filter paper. One drop of uranyl acetate was added and sample was incubated for 2 min. The sample was completely dried using filter paper and introduced on the TEM. The analysis was performed on a JEOL Transmission Electron Microscope, JEM 1010 model at an acceleration voltage of 100 kV, in the Electron Microscopy Unit, Faculty of Sciences, Universidad de la República, Montevideo, Uruguay.

Purification process The purity of the samples of dyes was checked using TLC plates (silica gel on aluminium foils, 70644 SIGMAALDRICH) and a mixture acetonitrile/methanol 4/1 as mobile phase. Solutions of the RAW orange dye were purified using disposable silica columns (SUPELCLEAN LC-Si, SUPELCO) with the same mobile phase. After that, samples were dried under nitrogen and re dissolved in ethanol. The purified dye solutions were called PURE orange.

Spectroscopic measurements UV–Vis measurements were carried out on a SPECORD 200 Plus spectrometer from Analytic-Jena, in the 200–700 nm range. FTIR spectra in the range 400–4000 cm−1 were obtained at room temperature employing a Shimadzu infrared spectrometer model IR-Prestige 21, averaging 10 scans at a nominal resolution of 4 cm−1. Dried samples were thoroughly mixed with KBr in an agate mortar, and 13 mm-discs were prepared in a Pike CrushIR at a pressure of 10 ton. For comparison purposes, FTIR spectra were recorded for different samples: the orange RAW dye (i.e. as obtained), the purified dye (PURE), and both pigments after incubation during 24 h onto TiO2 electrodes. For these last situations, 100 µL of the dyes containing solutions were deposited on the FTO/TiO2 electrodes. After an adsorption time of 24 h in darkness, the electrode surface was scratched and mixed with KBr, following the above described procedure to prepare the disc. The NMR spectra of the polysaccharide preparation (4.9 mg) were recorded in D2O solution (0.7 mL) at 25 °C on a Bruker Avance III 500 MHz spectrometer equipped with a 5 mm Z-gradient TXI (1H-13C/15N) probe.

Electrochemical measurements In order to evaluate the redox behavior of the orange dye, voltammetric profiles were recorded by using DROPSENS SPELEC1050 potentiostat in a mixture of ethanol/NaClO4 0.1 M (50/50) as supporting electrolyte. Potential scan rates routines within the range 0.03–0.07 Vs−1 were applied. Cyclic voltammetries were carried out at disposable electrodes (DROPSENS), where Au-pc was the working, Ag/

AgCl (0.195 V vs. SHE) the reference and carbon the counter one respectively. All potentials in the text are referred to this one. For DSSC, FTO/TiO 2 electrodes (DYESOL, screen printed with Dyesol’s DSL 18NR-AO Active Opaque Titania paste) and FTO/Pt (screen printed with Dyesol’s Pt1 Platinum Catalyst) were used as working and counter electrodes. The selected electrolyte was 50 mM iodide/tri-iodide in acetonitrile (SOLARONIX Iodolyte AN-50). Photoanode was sensitized by immersion in the dye-containing solution in darkness, overnight. Three different dyes solutions were used: RAW and PURE orange, and a mixture of PURE orange plus chenodeoxycholic acid (SOLARONIX), in a molar ratio (20 to 1). After the solar cells were assembled, current-voltage measurements were performed with a CHI 604E potentiostat at potential scan rate v = 0.05 Vs−1, at room temperature (in the dark and under illumination using a solar simulator from ABET Technologies, 1 sun, 1.5 AM). Electrochemical impedance spectroscopy measurements were carried out in the dark, in the frequency range 0.1 Hz to 3 MHz, with potentials between 0 and 0.6 V.

Results Transmission electron microscopy (TEM) analysis TEM images of UV11 cells grown in R2 broth showed that the bacteria are surrounded by a slimy substance, probably a polyelectrolyte gel composed of complex carbohydrates observed as dark-grey zones around bacteria (Fig. 2). The external light-grey zones probably correspond to the polysaccharides secreted by the organisms.

Spectroscopic measurements Visible spectra of both ethanolic solutions (PURE orange and RAW orange) were identical. Only small differences were detected at the UV range, and these are irrelevant for the use of such solutions to sensitize the DSSC. Figure 3 shows the recorded spectra for the RAW sample. According to the measured data (A = 0.44) and to the reported molar attenuation coefficient (141 × 103 L mol−1 cm−1 at 453 nm in ethanol for canthaxanthin), it is possible to estimate the xanthophylls concentration at extracted samples. The obtained concentration value was approximately 3 µM. With respect to FTIR data, Fig. 4 shows the measured results. Some common factors, detected at all spectra, can be highlighted.

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Fig. 2 Transmission electron micrograph of Hymenobacter sp. UV11. A and B show different sections of the slide. Cells were grown on R2 agar for 3 days at 20 °C. Arrows indicate a few bacteria, they are observed as electron-dense rods. Scale bar represents 2 μm

Fig. 3 Recorded visible spectra for the RAW sample in ethanol as obtained and recorded UV spectra for a 5 times diluted solution

Fig. 4 FTIR spectra for samples prepared with KBr (1% w/w). The black line shows results coming from T iO2 + orange RAW dye. Red line shows measured results for T iO2 + orange PURE dye. Blue line corresponds to RAW orange dye

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The signal at 3420 cm −1 (broad and strong) could mainly be ascribed to the presence of water but also can be assessed to -OH stretching due to the presence of alcohol groups. This peak is particularly intense in samples prepared from the RAW orange dye, and probably due to the polysaccharides present in such samples. The bands at 1630 cm−1 were assigned to the presence of conjugated C=O and C=C groups, as expected in xanthophylls molecules. The strong signals at 2920 cm−1 correspond to characteristic stretching frequencies of the C–H bond, belonging either to the C H2 and C H3 groups of the xanthophylls and/ or the CH and CH2 groups of the polysaccharide. It is important to mention that according to the literature two different xanthophylls are proposed as the main compounds produced by Hymenobacter sp. UV11 (Klassen and Foght 2008; Órdenes-Aenishanslins et al. 2016): canthaxanthin (without OH groups) or 2′-Hydroxyflexi xanthin (with an alkene and OH groups at the structure). Structure are displayed in Scheme 1. Data obtained from FTIR measurements does not allow a clarification of this point. The presence of carbonyl groups is detected but could be raised from the presence of this group at the extracted xanthophylls or due to oxidation of OH groups due to the oxygen of the air. Moreover, OH signals can be detected at a region of the spectra where water presence also can be assessed. Additionally, the existence of polysaccharides at the RAW samples contribute to complicate the assignation of the signals. At this point it is necessary to make an important stop: the successful use of a dye as a sensitizer in DSSC depends on many factors. Among them, the structure of the compound must allow the coordination to Ti of the semiconductor. So, the existence of available OH moieties could be a significant point to take into account when assembling the cells.

Environmental Sustainability Scheme 1 Main xanthophyll isolated from Hymenobacter sp. UV11 according to literature as explained in the text

Fig. 5 1H NMR spectrum of the polysaccharide preparation

Finally, bands at 1220 and 1078 cm−1 at RAW free samples can be assigned to the C–O stretching frequencies from polysaccharides and move towards 1130 and 1070 after binding to the Ti. Signals within the 750–400 cm−1 range can be attributed to the TiO2 semiconductor. The 1H NMR spectrum of the polysaccharide preparation revealed a material of high complexity (Fig. 5) and, employing 2D NMR spectroscopy, its major component could be readily identified as an α-1,4-glucan. The multiplicity-edited 1H,13C-HSQC spectrum (Fig. 6) shows a single peak in the region of the anomeric resonances (δH/δC 5.63/100.5 ppm), indicating that the polymer is composed of monosaccharide repeating units. The 1 H and 13C chemical shifts of the anomeric resonance indicate that the monosaccharide residue is in the pyranose form. From the analysis of 1H,1H-TOCSY spectra (recorded with mixing times ranging from 20 to 100 ms),it was

Fig. 6 Selected region of the multiplicity-edited 1H,13C-HSQC NMR spectrum of the polysaccharide preparation showing one-bond carbon-proton correlations from the mayor component. The cross-peaks from the hydroxyl methyl group of the Glcp residue appear in red

revealed that the monosaccharide residue has the gluco-configuration. The substitution position (C-4) was identified from 13 CNMR glycosylation shifts, since its resonance is shifted 6.98 ppm downfield with respect to that of the corresponding unsubstituted sugar; thus, it was concluded that the monosaccharide are bound through (1→4) linkages. In addition, the experimental 1H and 13C chemical shifts of the PS are almost identical to those predicted using the CASPER program for a polymer composed of →4)-α-Glcp-(1→ repeating units (Jansson et al. 1989).

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Electrochemical measurements Prior to the use of a dye for sensitizer, some characteristics are checked. One of them is the electrochemical behaviour: if the pigment has not a suitable oxidation potential it is not later used as a sensitizer. Cyclic voltammetry measurements constitute a fast technique of evaluation and using screenprinted electrodes consume a little amount of dye’s solution. Gold electrodes are very useful to perform this test, because of their high sensitivity. Figure 7a shows the voltammetric profile obtained for a PURE orange solution in the supporting electrolyte at 0.05 Vs−1. For RAW orange containing solutions recorded profiles were almost the same. As can be observed, a main anodic contribution at 1.21 V is clearly detected at the first potential scan, whereas in the following swept is not so obvious. This is consistent with a process where the electrode surface is occupied by the electroactive compound in the first scan. The presence of electroactive groups at the dye could explain this observation. On the other hand, the observed cathodic peak can be easily understood when the voltammetric behaviour coming from the supporting electrolyte is taken into account. The cathodic peak located at ca. 0.5 V arises mainly from the O-desorption process ascribed to oxide monolayer produced in the anodic scan. The intensity current of this peak depends on the upper potential value reached in the scan (Trasatti and Petrii 1992). As stated above, two main structures are proposed for the main xanthophylls produce by Hymenobacter sp. UV11. Electroactive groups are C=O in case of canthaxanthin, and C=O or OH in case of 2′-hydroxyflexixanthin. It is reported that the ketones could be reduced, whereas alcohols could be oxidized (Zuman 2006; Enciso et al. 2017). Therefore, the presence of the anodic peak at 1.21 V in the first potential


scan could be explained by the oxidation of the OH moieties. Oxidation of this group occurs irreversibly, and as a consequence, the OH is no longer available in the electrode interface for further potential scans. Oxidation of the OH takes place involving adsorption of this group onto the electrode surface; on the other hand, xanthophyll molecules are big and diffuse slowly. Summarizing, at the first potential scan xanthophyll diffuses towards the electrode surface, adsorbs to the metal using the OH groups and this group is oxidized. In the following scans, new fresh molecules are not able to reach the electrode interface, and anodic peak ascribed to OH oxidation is not detected. Therefore, voltammetric profiles allow to conclude that the main compound produced from bacterial growths is 2′-hydroxyflexixanthin, with the OH groups available for the TiO2 electrodes used to assemble the DSSC. With respect to the cells, DSSC were sensitized using PURE and RAW orange dyes, as well as cocktails prepared with PURE orange and chenodeoxycholic acid in a molar ratio (20 to 1).

Fig. 8 Photocurrent density-voltage curves for the dye sensitized cells using different pigments: PURE orange (black line), RAW orange (red line) and cocktail orange + co adsorbent (blue line) Table 1 Photovoltaic properties for cells assembled using different sensitizers

−2

Jsc/mA cm Voc/V FF η/% Fig. 7 Voltammetric profiles on Au-pc disposable electrodes for a PURE orange solution in a mixture ethanol/NaClO4 0.1 M (50/50) as supporting electrolyte or b in the supporting electrolyte. Room temperature, 0.05 Vs−1

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PURE orange

RAW orange

Cocktail

0.078 0.26 0.39 0.008

0.127 0.46 0.51 0.03

0.098 0.26 0.38 0.009

All measurements were performed under one sun light intensity of 100 mW cm−2, AM 1.5G and the active areas were 0.7 cm2 for all the cells. Jsc is the current density, Voc the open circuit potential, FF is the fill factor and η is the conversion efficiency. Showed results arise from 5 independent experiments performed for each dye


Main results are presented in Fig. 8 and Table 1. As can be observed, cell efficiency increased in the presence of polysaccharides or (but in minor extent) the selected co-adsorbents. This is in full agreement with our previous considerations with the structure of the orangexanthophylls: a good sensitizer needs the presence of functional groups able to coordinate with the Ti of the semiconductor. The presence of co-adsorbents as polysaccharides or chenodeoxycholic acid helps to a better fixation of the xanthophylls to the electrode surface. Coadsorbents have a good bond capability with Ti because of the presence of OH or C OO- groups. Once fastened to the surface, they can establish interactions with xanthophylls chains working as an anchor. These results can also be confirmed by impedance electrochemical spectroscopy measurements. For this case, the results are evaluated using a circuit like the one shown in Scheme 2. This transmission line-based model was successfully used to describe the electrochemical behaviour of DSSC in the dark. DSSC involves different electronic processes: photogenerated carriers must travel towards the external contacts, while the recombination process with the redox couple of the electrolyte competes (Bisquert and FabregatSantiago 2010). Circuit showed at Scheme 2 takes into account this possibility, because the transmission-line element contains the recombination as well the transport resistance. Additionally, also considers the behaviour at the counter electrode. Figure 9 shows an example for the recorded impedance measurements for one assembled DSSC. In the case of this Figure, the cell was sensitized with RAW orange, and results are displayed on a Nyquist plot. The linear interval at higher frequencies is predicted for a transmission-line based model. Table 2 shows the values obtained after fitting the experimental results, where: Rct is the charge transport resistance related to recombination of electrons at the TiO2/dye/electrolyte interface Rt is the electron transport resistance in the photo anode Cμ is the chemical capacitance at the TiO2/dye/electrolyte interface, an equilibrium property that relates the variation of the electron density to the displacement of the Fermi level.

Γt = Rt × Cμ is the time constant for the transport of the injected electrons that diffuses through the nanoparticle network. Γrec = Rct × Cμ is the recombination time that reflects the lifetime of an electron in the photo anode. High values of the recombination resistance are mainly responsible for a better efficiency performance. Especially when considering the ratio Rct to Rt, the impact could be better understood. A big ratio shows that electrons at the photoanode follow the path involving the transference across the semiconductor towards the FTO/TiO2 interface instead of following recombination between the promoted electrons (after photoexcitation of the dye) with the triiodide couple of the electrolyte. In the EIS analysis of the DSC devices, several features can be observed in Fig. 10. The recombination resistance is lowering devices with PURE compared to devices sensitized with RAW orange. On the other hand, for devices prepared with PURE orange, the chemical capacitance indicates a lower lying conduction band edge of the T iO2 (~ 70 mV lower when compared to devices with RAW orange). According to Eq. (1) differences at observed Voc values can be estimated (Enciso et al. 2017):

ΔVOC = ΔEF + ΔVOC,𝛤 = ΔEF +

kB T 𝛤 1 ln , q 𝛤2

(1)

Fig. 9 Nyquist plot recorded for an assembled DSSC sensitized with RAW orange dye measured at 400 mV in darkness

Table 2 Obtained results from measured data at E = 0.4 V (in darkness), using the transmission line model

Scheme 2 Transmission line based model used to fit the measured EIS results

Rct/Ω Rt/Ω Γt = Rt × Cμ/s Γrec = Rct × Cμ/s

PURE orange

RAW orange

Cocktail

612 236 0.009 0.020

4100 12 0.001 0.330

1072 83 0.003 0.032

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Fig. 10 Evolution of the charge transfer resistance of the cell, after subtracting platinum and series resistance, and of the capacitance of the cell without the contribution of the platinum capacitance. Using PURE orange (red dots) or RAW orange (black squares) dyes as sensitizers of the assembled cells

where ΔVoc represents the expected differences between Voc for the compared DSSC, taking into account the Boltzmann constant k B, the elementary charge q, the temperature T and the calculated Γt = Rt × Cμ. Differences at the Fermi level are ca. 70 mV, as stated before. From Fig. 10 and Table 2, it is possible to calculate a Voc difference of ca. 0.13 V between cells assembled using RAW and PURE sensitizers. This is in line with measured values shown in Fig. 8 and Table 1.

Discussion Pigments extracted from Hymenobacter sp. UV11were easily purified using disposable silica columns. Polysaccharides (mainly α-1,4-glucan) and xanthophylls (most probably 2′-hydroxyflexixanthin) were main compounds present in such samples. The use of UV11 produced xanthophylls as sensitizers in DSSC produces very low-efficiency cells. However, our results suggest that polysaccharides improve DSSC performance. This is a fact to consider when using the pigments produced in the bacterium as sensitizers in DSSC. The chemical structure of xanthophylls is not the most adequate to establish bonds with Ti, but the polysaccharide might have a helper function for improving xanthophylls bounding to the Ti, probably favouring the anchorage of xanthophylls to the photoelectrode surface. Polysaccharides have OH groups that could be adsorbed onto the FTO/ TiO2 and constitute a suitable way to anchor the xanthophylls (Ananth et al. 2015; Musser et al. 2015). Results confirm this assumption since control experiment carried out using only the co-adsorbent showed no detectable efficiency at all, and FTIR experiments (from scratched TiO2 dipped overnight into dyes solutions) displayed strong

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signals coming from these compounds. Therefore, the presence of the co-adsorbent could yield a large amount of adsorbed dye to the photo anode. The use of natural co-adsorbents as sugar or protein or mixtures of different dyes extracted from natural sources is widely reported and show promising results (Gao et al. 2008; Hemalatha et al. 2012; Lim et al. 2015; Prabavathy et al. 2018). Nevertheless, a large number of polysaccharides in comparison with xanthophylls could represent a problem itself, because competition for surface space at FTO/TiO2 electrodes occurs. Isolation and fully characterization of the polysaccharides produced by Hymenobacter sp. UV11 is underway. Further experiments could be done to tune up a high-performance Antarctic-DSSC. The addition of controlled amounts of xanthophylls and polysaccharides could represent a better choice to sensitize the photoelectrode and increase the conversion efficiency of the cells. Acknowledgements Authors are grateful to Instituto Antártico Uruguayo (IAU). MFC, DD, CF, JJM, PE and SCS are ANII researchers. MFC, DD, CF and SCS are PEDECIBA researchers.

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