Incorporation of Kojic Acid-Azo Dyes on TiO 2 Thin Films for Dye Sensitized Solar Cells Applications

Sensitization of heavy metal free organic dyes onto TiO2 thin films has gained much attention in dye sensitized solar cells (DSSCs). A series of new kojic acid based organic dyesKA1–4were synthesized via nucleophilic substitution of azobenzene bearing different vinyl chains A1–4 with kojyl chloride 4. Azo dyes KA1–4 were characterized for photophysical properties employing absorption spectrometry and photovoltaic characteristic in TiO2 thin film. The presence of vinyl chain in A1–4 improved the photovoltaic performance from 0.20 to 0.60%. The introduction of kojic acid obtained from sago waste further increases the efficiency to 0.82–1.54%. Based on photovoltaic performance, KA4 achieved the highest solar to electrical energy conversion efficiency (η = 1.54%) in the series.


Introduction
Dye sensitized solar cells (DSSCs), a third-generation solar cell discovered in early 1990s, have gained intensive attention and been considered as promising alternative for fossil fuel energy [1,2].DSSCs offer advantages over the conventional silicon based solar cell due to cost effectiveness, flexibility in shape, easy accessibility of dye resources, and noteworthy performance [3,4].DSSCs convert sunlight into electrical energy mimicking the photosynthesis process employing synthetic or natural dye as light harvesting pigments [5].Sensitizer in the cell absorbs photons and induces excitation of electron to the wide bandgap semiconductor, dyes, and electrolyte [6].Ruthenium complex is one of the most effective light harvesting sensitizers reported in DSSCs; however, the drawbacks of these ruthenium sensitizers are complicated procedure, limited source, and being expensive, environmentally unsafe, and carcinogenic [7][8][9].
Over the years, tremendous efforts have been made to explore natural and organic dye as DSSCs sensitizer due to being nontoxic and environmentally friendly, low cost, and easy modification for functionalization.Extracted natural dye, however, has low yields of extract and scarce resources [10].Organic dye such as azobenzene has outstanding chromophores with strong absorption in visible region and intrinsic advantages of good photo and thermal stability [11,12].Azobenzene is a reactive precursor for functional group conversion and attachment such as vinyl and alkoxy chain with enhanced optical properties [13,14].
Development of natural and organic dyes as sensitizer on TiO 2 thin film for DSSC performance has been widely reported [15][16][17].Anthocyanin in natural dye which has carbonyl and hydroxyl groups has been reported for its anchoring ability to TiO 2 surface in DSSC applications [18][19][20].Binding of C=O and OH groups to the TiO 2 surface promotes better electron transfer mechanism to the conduction band of TiO 2 .Kojic acid [21,22], a natural pyrone which carries one C=O and two OH groups, is envisaged to have similar properties of natural anthocyanins dyes for its ability to bind with TiO 2 .Kojic acid is a nonhazardous and biodegradable natural product, which was earlier reported for tyrosinase inhibition and colorimetric determination [23][24][25][26].To the best of our knowledge, no studies reported on the applications of kojic acid derivatives as DSSCs sensitizer.

Experimental
2.1.Materials.Commercially available reagent grade chemicals were used without further purification.All solvents were of analytical grade and used as received unless specified as dry, in which case they were dried and distilled before being used under oxygen free nitrogen as follows: acetone was distilled over magnesium sulphate anhydrous and stored over 4 Å molecular sieves.
Sago hampas (an abundant agricultural waste in Sarawak, Malaysia) was collected from Herdson Sago Mill in Pusa, Sarawak.The substrate was oven dried and ground before use.ITO-coated (Indium-doped tin oxide) conductive glass slides (surface resistivity 15 Ω /sq, thickness 2.2 mm) obtained from China were used as substrates for preparing TiO 2 thin film electrode and platinum counter electrode. 13C NMR spectra were recorded on a JEOL ECA 500 spectrometer at 500 MHz ( 1 H) and 125 MHz ( 13 C) with the chemical shifts  (ppm) reported relative to DMSO-d 6 as standard.Tetramethylsilane (TMS) was used as the internal reference.Infrared (IR) spectra (] cm −1 ) were recorded as KBr pellets on a Perkin Elmer 1605 FTIR spectrophotometer.The coupling constants (J) are given in Hertz (Hz).The current to voltage characterization was carried out by varying the value of load resistance under the light source of 120 W/240 V and was used and illuminate at 120 W/m 2 .The electrical characteristics of DSSCs were measured on two-wire current-voltage analyzer using National Instrument Educational Laboratory Virtual Instrumentation Suite NI ELVIS II+ 100 MS/s Oscilloscope.The CHNS elemental analyses were using Thermo Sci-entific6 FLASH 2000 CHNS/O Analyzer.Supplementary data for FTIR, 1 H NMR, 13 C NMR, and UV spectra and photovoltaic measurement of synthesized compounds are available online (in Supplementary Material available online at https://doi.org/10.1155/2017/2760301). [27].Sago hampas (5 g) was supplemented with 3% (w/v) urea and 10% mineral salts solution containing KH 2 PO 4 , (w/v) MgSO 4 ⋅7H 2 O and yeast.Mixed strains of Aspergillus Flavus NSH9 and Aspergillus Flavus Link 44-1 were employed for fermentation.The culture was incubated at 30 ± 2 ∘ C in static condition for 18 days.The slurry suspension culture (40 mL) was extracted with ethyl acetate (2 × 15 mL).The organic layer was evaporated in vacuo to form crude brown solid and recrystallized from ethanol to afford as yellowish needle like crystal (0.15 g, 30%): IR (KBr Pellet) V max in cm  [28]

Ethyl 4-[E-(4-Hydroxyphenyl)azo]benzoate ( ).
A solution of 4-aminobenzoate (5 g, 30.0 mmol) in methanol (50 mL) was cooled in an ice bath (0-5 ∘ C) and HCl (8 M, 100 mL) was added dropwise to the solution at such a rate that the reaction temperature was maintained below 5 ∘ C. A solution of sodium nitrite (3.1 g, 45.0 mmol) in distilled water (10 mL) and phenol (3.4 g, 36 mmol) in methanol (50 mL) were cooled to 0-5 ∘ C and added dropwise to the reaction mixture.NaOH solution (1 M, 100 mL) was cooled in ice bath and added dropwise to pH 8.5−9.5.The reaction mixture was stirred for 4 h under nitrogen atmosphere.Methanol (50 mL) and ice (20 mL) were added to the reaction mixture and were acidified with HCl (8 M, 50 mL) until yellow precipitate formed.The solid formed was filtered, washed, and recrystallized from ethanol to give , an orange colored solid (6.7 g, 83%); V max (KBr, cm

4-[(E)-(4-Hydroxyphenyl
)azo]benzoic Acid ( ).KOH (0.13 g, 2.4 mmol) was added to the solution of (0.2 g, 0.8 mmol) in methanol (50 mL) and refluxed for 4 h under nitrogen.The reaction was monitored by TLC analysis.Water (30 mL) and crushed ice (10 mL) were added and the reaction mixture was acidified with acetic acid (8 M) until precipitate formed.The crude product was filtered and recrystallized from ethanol to give (0.12 g, 62%) as bright yellow solid; V max (KBr, cm −1 ) 3208 (OH), 1686 (COO), 1594 (aromatic), 1198 (C-O), 1140 (C-N).[29].Azo precursor was added to the solution of bromoalkene derivatives, t-BuoK, and KI in dried acetone.The reaction mixture was refluxed for 48 h under nitrogen and the reaction was monitored using TLC analysis.DCM (50 mL) and water (30 mL) were added and the layers separated.The aqueous layer was extracted with DCM (2 × 30 mL) and the combined organic layers were dried and evaporated under reduced pressure to give the title compounds.

2.7.
General Procedure for the Synthesis of KA -.Azo intermediate Awas added to the solution of in dried acetone with TEA and refluxed for 24 h.The reaction mixture was washed with dilute HCl (1 M, 2 × 20 mL).The organic layer was dried, filtered, and concentrated under reduced pressure to give title compound.

Fabrication of DSSCs. Indium Tin Oxide (ITO) substrate
was cleaned with deionized water and treated in water bath for 10 min to remove ionic impurities.The ITO was soaked in methanol for 5 min and dried.Scotch tape was used to cover two corners of the conducting side of ITO. 3 cm 2 of area was used for TiO 2 film.TiO 2 (1.0 g) was transferred into a crucible and annealed at 450 ∘ C in a furnace for 2 h and cooled to room temperature.TiO 2 electrode was prepared by mixing dried TiO 2 (1.0 g) with acetic acid (1 mL) and Triton X-100 (0.5 mL) at room temperature for 5 min.The TiO 2 paste was applied onto ITO and coated using spin coater (1000 rpm) for 15 sec, heated at 250 ∘ C for 30 min and immersed in 0.5 mM of Aand KAdyes solution and rinsed with ethanol after 24 hr.The conducting side of another ITO glass was coated with graphite and clamped tightly in a sandwich pattern so that the dyed TiO 2 is facing down onto the coated graphite anode.A solution of iodide/tri-iodide (5 mL) in acetonitrile acted as electrolyte was added to the cell.The fabricated DSSCs were stored in dark for 24 h prior to testing.

Photovoltaic Measurement.
A voltage and current measurement of DSSC Aand KAwere applied via connection of anode and cathode to DUT+ and DUT− of The National Instruments Educational Laboratory Virtual Instrumentation Suite (NI ELVIS), respectively.LabVIEW was used to display the current and voltage during a voltage sweep from 0 to 5 V.The voltage spacing was set to 0.04 V.The electrical properties of the DSSC were determined via the potential difference between both electrodes in DSSC.The percentage efficiency () of DSSC is the ratio of power output ( out ) versus power input ( in ), where  in is the total radiant energy incident on the active area of the cell calculated by (1). in for efficiency measurement using 3 cm 2 is 120 W/m 2 .were employed to support the growth of bacteria strain and to boost the production of kojic acid [31].Aspergillus Flavus was used for a higher yield of [32].Yeast acted as a nitrogen source to enhance kojic acid production through metabolic activation.It also contains high levels of essential components to support growth and fermentation such as vitamins and oligoelements [33].Extraction with ethyl acetate afforded with 30% yield.Low yield obtained was due to the properties of kojic acid which is soluble in both water and ethyl acetate [34,35].The NMR and IR spectra corresponded to structure of kojic acid.

Synthesis.
The synthesis pathway of kojic acid based organic dye is shown in Scheme 1. Ethyl 4-aminobenzoate underwent diazotization with sodium nitrite in the presence of hydrochloric acid and coupled with phenol to afford azobenzene dye [28].Hydrolysis of ester bond in with strong base KOH was performed to give .Introduction of vinyl group gives optical properties [29].The alkylation of with series of bromoalkenes was carried out in the presence of tBuOK and a catalytic amount of KI in refluxing acetone to afford A -. Compound was treated with thionyl chloride to obtain kojyl chloride [30] followed by nucleophilic substitution of Awith in TEA to afford organic dye KA -.

Characterization.
Characterization of KAwas performed using FTIR, 1 H, and 13 C-NMR spectroscopies.FTIR showed the absorption band for olefinic group at 3223-3218 cm −1 .IR spectra revealed a strong band at 1713-1708 cm −1 attributed to the presence of ester group and a sharp band at 1654-1651 cm −1 corresponded to C=O of .The absorption band at 1620-1606 cm −1 is attributed to the aromatic group.The synthesized KAwas further characterized using 1 H NMR spectroscopy.The 1 H NMR showed two singlets at  8.13-8.11ppm and  6.60-6.58ppm representing olefinic protons of .The presences of four doublets peaks at  8.19-7.13ppm were attributed to eight protons in aromatic ring.The peak of oxymethyl proton of was observed at  5.26-5.24ppm as singlet indicated 7-Osubstituent [36].The CH=C resonated at  5.90-5.78ppm as multiplet and C=CH 2 proton of vinyl chain was observed as doublet of doublet at  5.15-4.95ppm.The OCH 2 of vinyl chain resonated as doublet at  4.16-4.06ppm as triplet.The 13 C NMR spectra showed all the important peaks supported to the formation of the desired product.The

UV-Vis Absorption Spectra.
The UV-vis spectra of Aand KAin ethanol are shown in Figure 1.Absorption spectra of Aexhibit an absorption band at 358-363 nm while KAexhibited two distinct absorption bands.The absorption band at 346-392 nm in the UV region was attributed to the - * transition and another strong absorption band at 489-494 nm in the visible region was due to the n- * transition [37].The red shift in maximum absorption for KAcompared to Ais due to expansion of overall  conjugation systems after incorporation of kojic acid moieties and suggested some increased electron delocalization [38,39].The corresponding data are presented in Table 1.The bandgap (  ) is estimated from the onset of absorption edge for Adid not have significant difference, indicating that the length of vinyl chain only showed slight influence on  National Instruments Educational Laboratory Virtual Instrumentation Suite (NI ELVIS) under irradiation 120 W/m 2 .The photovoltaic properties of the dyes are summarized in Table 2.
The kojic acid-azo dye, KA -, exhibits enhancements in efficiency,  (0.82-1.54%) compared to A -(0.20-0.60%).The KAbased DSSCs showed higher open circuit voltage and current density with fill factor 68-75%.The high fill factor is comparable to typical first-generation silicon based solar cell [40].This phenomenon is ascribed to the anchoring ability of kojic acid moieties to the TiO 2 photoanode.The carbonyl and hydroxyl group of acted as chemical binding anchors by either monodentate or bidentate bridging to the TiO 2 surface which enhanced the binding of KAto the substrate [41].Anchoring ability of dye is critical for DSSCs sunlight harvesting where the efficiency is dependent on dye deposition [42].Incorporation of also expands the conjugation in KAand promotes better electron transfer mechanism to the conduction band of TiO 2 , resulting in higher absorption in the visible region [21].
13C-NMR spectrum displayed two deshielded carbon signals at  174.4-173.7 ppm and  168.9-164.4ppm indicated for C=O of kojic acid and ester group.The aromatic carbons and vinyl group were represented by absorption peak at  163.3-112.7 ppm.The signals at  68.6-67.7 ppm and  62.8-56.3ppm were designated as OCH 2 of and vinyl chain, respectively.

Table 1 :
Absorption properties of the dyes.