EXPERIMENTAL SECTION

Boosting Photovoltaic Performance of SnO 2 Based Dye Sensitized Solar Cells by Utilizing 2D MgO

7.2 EXPERIMENTAL SECTION

7.2.1 SYNTHESIS OF SK1 DYE

Synthesis of the dye molecule is carried out by following the steps presented in the scheme 7.2.1. In a typical procedure, the aldehyde was reduced by reacting with sodium borohydride in the presence of in tetrahydrofuran at 5 ^°C, followed by stirring at room temperature for 6 h. After that the reaction mixture was poured into cold water and extracted using dichloromethane. The organic layer was dried using anhydrous sodium sulfate, and the solvent was removed under vacuum to yield the oil.

Scheme 7.2.1 Synthesis procedure for 2-Cyano-3-(4-(2-(9-p-tolyl-9H-fluoren-6-yl)vinyl)phenyl) acrylic acid or SK1 dye.

[Reagents and conditions: (a) 0.4 equivalent of NaBH4 in THF followed by triphenylphosphine hydrobromide in dichloromethane, (b) terephthaldehyde, sodium ethoxide in ethanol at 5 °C, and (c) cyanoacetic acid and catalytic amount of piperidine in acetonitrile.]

The alcohol obtained in the previous step was then converted into Wittig salt by using triphenylphosphine hydrobromide in dichloromethane and separated out it as a white colored salt by adding of diethyl ether. Further it was condensed with terephthaldehyde in ethanol using sodium ethoxide as a base under cold conditions (5−10 °C) and the reaction mixture was stirred at room temperature for 2 h, poured into water followed by the neutralizing with dilute hydrochloric acid to obtain a yellow colored solid crude. As obtained yellow solid product was dissolved in tetrahydrofuran in the presence of a catalytic amount of iodine and refluxed for 4 h. Then the reaction mass was poured into dilute alkali solution to separate the solid product which is purified by column chromatography (EtOAc: n-hexane, 3:7, v/v). In the next step, purified aldehyde was dissolved in acetonitrile and cyanoacetic acid, and then added a catalytic amount of piperidine.

(D-π-A) type SK1 Dye (a) (b) (c)

Donor πLinker

Acceptor

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The reaction mass was allowed to reflux for 3 h, after which solvent was removed under vacuum.

Finally the dried crude product was purified by column chromatography (EtOAc: n-hexane, 1:1, v/v) to obtain an orange solid product.

The ¹H NMR (in CDCl3) and ¹³C NMR (in DMSO-d6) spectra of SK1 dye are shown in figures 7.2.1 and 7.2.2 respectively.

Figure 7.2.1 400 MHz ¹H NMR spectrum of metal-free carbazole SK1 dye recorded in CDCl3 as the solvent.

1H NMR (CDCl3, 400 MHz) δ (ppm): 11.00 (s, 1H), 8.30 (s, 2H), 8.20 (s, 2H), 7.86 (s, 1H), 7.72 (s, 3H), 7.42 (d, 4H), 7.41 (d, 2H), 7.40 (d, 2H), 7.20 (d, 2H), 1.90 (s, 3H).

Figure 7.2.2 400 MHz ¹³C NMR spectrum of metal-free carbazole SK1 dye recorded in CDCl3 as the solvent.

13C NMR (DMSO-d6, 400 MHz) δ (ppm): 21.09, 64.05, 116.90, 125.24, 126.52, 127.55, 127.66, 128.41, 128.69, 129.47, 130.28, 130.38, 130.87, 131.09, 132.10, 132.19, 134.04, 145.79, and 153.23.

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The mass spectrum of SK1 dye is shown in Figure 7.2.3. MS m/z: [M]+ for C31H22N2O2, calculated, 454.16; observed, 453.80. Elemental analysis for C31H22N2O2: calculated C, 81.83; H, 5.10; N, 6.16; O, 7.03. Observed: C, 81.70; H, 5.13; N, 6.10; O, 7.07.

Figure 7.2.3 Mass spectrum of SK1 dye.

7.2.2 SYNTHESIS OF 3D HIERARCHICAL SnO2 MICROSPHERES

3D hierarchical SnO2 was synthesized by following a previously reported protocol.³² To prepare 3D SnO2 microspheres, 2.25 g tin dichloride dihydrate (10 mmol) and 5.6 g oxalic acid (44.4 mmol) were dissolved in Milli-Q water (20 mL) followed by the addition of 0.4 mL hydrochloric acid (35 wt %). Aqueous solution (80 wt %) of hydrazine monohydrate, 2.16 g (41.5 mmol) was added dropwise under magnetic stirring until the solution became clear. Finally the solution was transferred to a Teflon-lined stainless steel autoclave and maintained at 180 C for 14h. After that the autoclave was naturally cooled to room temperature and the obtained product was centrifuged and rinsed thoroughly with distilled water and absolute ethanol several times, dried in hot air oven at 60 C for 24 h and calcined at 500 C for 2 h to obtain the final product.

7.2.3 SYNTHESIS OF POROUS 3D HIERARCHICAL MgO

In a typical procedure, the MgCl2.6H2O (0.5mmol) was dissolved in 25 mL distilled water, followed by the addition of sodium bicarbonate (5 mmol) into the solution. This solution was slowly added into vigorously stirring ethanol (100 mL, in a 250 mL two-necked flask) at 80 °C.

After 10 min, the stirring was stopped and the solution was maintained at 80 °C for 2 h. After cooling down to room temperature, the obtained precipitate was centrifuged and washed with

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water and absolute ethanol several times followed by drying in a hot air oven at 60 °C for 12 h.

Finally, the obtained precursor [Mg5(CO3)4(OH)2.4H2O] was annealed in air at 400 °C for 4 h to obtain the porous hierarchical MgO.

7.2.4 FABRICATION OF PHOTOANODES AND DEVICES

Pristine SnO2 and SnO2-MgO based photoanodes were fabricated as follows. For the fabrication of bare SnO2 based photoanode, synthesized hierarchical SnO2 microsphere powder (0.5 g) was added to a mixture of 1.0 mL of terpineol and 0.2 g of PEG-PPG-PEG and ground well in an agate mortar until a homogeneous paste was obtained. This paste was coated on the conductive glass substrates by using doctor blade technique. These films were then dried in hot air oven at 70 °C and calcined at 450 °C for 30 min to remove the polymer. SnO2-MgO photoanodes were fabricated by following the same procedure used for pristine SnO2 based photoanode except that the ultra-sonication of the mixture of porous hierarchical MgO powder and SnO2 microspheres with different weight ratios in ethanol (5 mL) for 1 h, prior to making the paste. Step-by-step fabrication process of the photoanodes is schematically shown in the scheme 7.2.2.

Scheme 7.2.2 Step-by-step fabrication process of SnO2MgO photoanode.

The paste was coated by using doctor blade technique over the precleaned FTO substrates. The substrates were dried in air and then calcined at 450 °C for 30 min to obtain SnO2-MgO films.

These substrates were then sensitized with SK1 dye solution in anhydrous ethanol:

dichloromethane (1:1, v/v) mixed solvents for 6 h, washed with ethanol and dried under hot air blow. The thickness of all the photoanode films were found to be in the range of 12−15 m by

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using a surface profilometer. For the preparation of counter electrode, a 50 mM H2PtCl6 solution prepared in isopropanol was spin coated on pre-cleaned, ozonized FTO substrates, followed by calcination at 450 C for 30 min. The photovoltaic devices were fabricated by sandwiching the photoanodes and counter electrodes and then electrolyte solution was injected after sealing (using low-temperature thermoplastic sealant, thickness ∼50 μm) to complete the device. In this work, we also studied the effect of two electrolytes on the PCE of fabricated devices. To prepare I^/I3

electrolyte solution, 0.5 M LiI, 0.05 M I2, 0.1 M guanidium thiocyanate, and 0.5 M 4-tert- butylpyridine dissolved in a solvent mixture of acetonitrile: valeronitrile (9:1, v/v) and similarly Co(II/III) electrolyte solution was prepared by dissolving 0.2 M [Co(bpy)3](PF6)2, 0.02 M [Co(bpy)3](PF6)3, and 0.5 M 4-tert-butylpyridine in acetonitrile: valeronitrile (9:1, v/v) solvent mixture. Before the photovoltaic measurements, the fabricated devices were kept under dark condition for 24 h. It should be noted that an active surface area of ∼0.25 cm² was fixed for all the devices.