STUDY ON THE EFFECT OF POTASSIUM ALUMINUM SULFATE TREATMENT ON ZnO FILM FOR PHOTOANODE OF DYE SENSITIZED SOLAR CELL USING SHOREA ROBUSTA AND GARUGA DYE�

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A Dissertation

Submitted to the Department of Physics, Patan Multiple Campus in the partial fulfillment of the Requirement of theMaster’s Degree of Science in Physics

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Presented by Sharad Puri.

Objectives

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• To study the effect of potassium aluminum sulfate treatment on absorbance of ZnO photo electrode.

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• To fabricate natural dye sensitized solar cell using dye extracted from inner bark of Shorea robusta and Garuga and study the effect of treatment on performance of DSSC.

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Introduction

Dye sensitized solar cell(DSSC):

It is a photovoltaic device which uses dye for the generation of charges and converts solar energy into electrical energy.

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1.The incident photons are absorbed by dye sensitizers.

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2.The dye sensitizers are excited from the ground state (S) to the excited state (S^∗). The excited electrons are injected into the conduction band of the TiO₂ or ZnO nanostructured electrode.

S + hν → S^∗

S^∗→S⁺+e⁻ (TiO₂ or ZnO)

3.The injected electrons in the conduction band of TiO₂ or ZnO are transported between TiO₂ or ZnO nanoparticles and electrons finally reach the counter electrode through the circuit.

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4. The oxidized dye sensitizer (S⁺) accepts electrons from the I⁻ ion redox mediator leading to regeneration of the ground state (S), and the I⁻ is oxidized to the oxidized state, I₃⁻.

S⁺ + e⁻ → S

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5. The oxidized redox mediator, I₃⁻, diffuses toward the counter electrode and then it is reduced to I⁻ ions.

I₃⁻ + 2 e⁻ → 3 I⁻

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Fig: Mechanism of DSSC

Our work includes�

• Fabrication of ZnO nano structured photo anode.

(spray pyrolysis technique for fabrication of FTO, spin coating method for ZnO seed layer deposition and Hydrothermal method for ZnO growth)

• Treatment of ZnO photoanode with potassium aluminum sulfate.
• Extraction of dyes from barks of Shorea robusta and Garuga and dye loading.
• Fabrication of counter electrode.
• Solar cell assembling and measurement of I-V characteristics.

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Why ZnO?

• it is wide band gap (~3.36 eV) semiconductor.
• highly transparency in visible spectrum.
• low-cost
• non toxic
• high electron mobility
• easily forms thin films and nanostructures by room temperature solution processing.

Why potassium aluminum sulfate treatment of photo anode?

To increase over all performance of DSSC.

( we observed increase in transparency of ZnO photoanode and better dye adsorption after treatment)

Uses of potassium aluminum sulfate

•  water purification 
• leather tanning
•  dyeing
•  fireproof textiles
•  baking powder etc.

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Characterization

• Effect of Potassium aluminum sulfate treatment on absorbance of ZnO photo anode was observed by using UV-Visible spectrophotometer
• I-V characteristics of DSSC was observed by using multimeters,potentiometer and amprobe solar-100 power meter.

Experimental

Fabrication of FTO thin films:

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2^nd step

5gm SnCl₂.2H₂O+ 5ml HCl (After dissolving SnCl2)+ 30 ml ethanol

0.1234gm NH4F + 30 ml distilled water

3^rd step

Stirred for 2hrs and aged for 24 hrs

4^th step

Kept in solution container of spray pyrolysis kit and then sprayed over glass substrate with temperature 450° C

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Glass substrates were put into the water bath with detergent and ultrasonically treated at 60°C about 30 minutes. Then, substrates were kept in ultrasonic bath with distilled water for 10 minutes and then washed with acetone and then washed with distilled water then dried.

1^st Step

ZnO Seed layer deposition:

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1^st Step

0.4M solution was prepared by dissolving zinc acetate dehydrate [Zn (CH₃COO)₂.2H₂O] in ethanol and DEA (Molar ratio 1:1)

2^nd Step

Stirred for 2hrs to get homogeneous solution and aged for 1 day

3^rd Step

Solution dropped on FTO and spinning at rate 3000 rpm.

4^th Step

Pre heated at 350° C for 15 minutes

5^th Step

Repeat steps 3^rd and 4^th for next coat

6^th Step

Annealed at 500°C for 1 hour.

Hydrothermal growth of ZnO:

1^st Step

1.7849 gm Zn(NO₃)₂.6H₂O+ 30 ml distilled water in R.B.flask ‘1’

Then,

Stirred for 30 minutes

2^nd Step

0.8411 gm HMTA + 30 ml distilled water in R.B. flask ‘2’

Then

Stirred for 30 minutes

3^rd Step

Mix solution from R.B. ‘1’ and R.B. ‘2’ and stirrer for 30 minutes then filter out

ZnO seeded electrodes were immersed facing downward solution obtained from step ‘2’ and kept in water bath at 90° C for 3 hours.

4^th Step

Washed with distilled water and dried then annealed at 350° C for 1 hr

Chemical treatment

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1^st Step

1.4231 gm [KAl(SO₄)₂.12H₂O] + 30 ml distilled water

2^nd step

Stirred for 30 minutes then filtered out

3^rd Step

ZnO nanostructured samples were kept in solution obtained from step ‘2’ for 5 minutes

4^th Step

Washed with distilled water then dried

Dye extraction

• First, barks were torn into small pieces then washed with distilled water. Then, these pieces of barks were dried in hot air oven at 60^°C for two days. In between two days, pieces of barks were washed with distilled water again. Dried pieces of barks, then, cursed into fine powder using grinding and pestle.
• For dye extraction, 1g fine powder of each barks were immersed in 60 ml of ethanol separately in air tight borosilicate bottles and kept in dark place at room temperature for 24 hours and then filtered out.

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Fig: Dye extraction

Dye loading

• For dye loading,dye solution was taken in airtight vessel and ZnO nanostructured electrodes were immersed in it and kept in hot air oven at 60⁰ C for 3 hours.

Fig: Dye loading

Fabrication of Counter electrode

• Graphite was coated over FTO coated glass substrate with the help of pencil, writing over it in same direction back and forth until the substrate was filled completely.

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Fig: Fabrication of counter electrode

Solar cell assembling

• Paraffin paper strip cleaned with ethanol and elongated 5 times more than initial length. Then fixed as like rectangular frame on four edges of working electrode with making two small hole in it. Counter electrode, then, placed on the top of the frame facing graphite coating surface towards working electrode. Small pressure was applied on paraffin strips, sandwiched between two electrodes. Then these two electrodes were hold tightly using two clips. Electrolyte (I^-/I₃^-) was introduced from one of two small holes using dropper.

Fig: Electrolyte loading

Result and discussion

Resistance of FTO samples used for photo electrode

Resistance of FTO samples used for counter electrode

FTO Samples having nearly same resistance values are grouped for fabrication of electrodes and counter electrodes.

Transmission curves of FTO used as Photoelectrode

Transmission curves of FTO used as Counter electrode

From above graph, we see slight variation in the transmission spectra of samples used for electrodes and counter electrodes.

Effect of potassium aluminum sulfate treatment on absorption spectra.�

(a)

(b)

(c)

Samples ‘e3’, ‘e4’ and ‘11a’ was cut into two parts where one part was treated with potassium aluminum sulfate and another was not treated after ZnO growth. Above Fig. shows average absorption curves of ZnO photo anode used for fabrication of DSSC (a) loading shorea robusta dye,(b) loading Garuga dye and (c) anode treated in low temperature, before and after treatment with potassium aluminum sulfate. From Fig. (a) and (b),absorbance of ZnO photo electrodes have decreased which may be due to clearance of powder form of ZnO trapped in nanostructures or the reduction of thickness of grown nanostructures but result is different at low temperature which is shown in figure (c).

Absorbance curves of Natural dyes

Figure shows absorbance spectra of (a) dye extracted from bark of Garuga pinnata(dabdabe), (b) dye extracted from bark of shorea robusta (sal) in ethanol, (c) N3 dye

Effect of [KAl(SO₄)₂.12H₂O] treatment on dye loading

(a)

(b)

Fig: Average absorbance curves for (a) untreated anode and (b) 0.1M [KAl(SO₄)₂.12H₂O] treated anode before and after dye loading.

Effect of [KAl(SO₄)₂.12H₂O] treatment on dye loading

(a)

(b)

Figure above shows average absorbance curves for (a) untreated anode and (b) 0.1M [KAl(SO₄)₂.12H₂O] treated anode before and after dye extracted from bark of Garuga pinnata loading. The absorbance of treated sample, after dye loading, has increased in greater amount than that of untreated sample. This may be due to good adsorption of dye on treated ZnO electrode than that of untreated ZnO electrode.

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Effect of potassium aluminum sulfate treatment on DSSC performance�

(a)

(b)

Figure shows the I-V characteristics of DSSC(S) fabricated with (a) untreated ZnO anode (b) treated with 0.1M [KAl(SO₄)₂.12H₂O] using shorea robusta dye and DSSC(G) fabricated with (c) untreated ZnO anode (d) treated with 0.1M [KAl(SO₄)₂.12H₂O] loading Garuga dye.

(c)

(d)

Comparison of IV characteristics of DSSC prepared with untreated and treated ZnO anode using dye extracted from inner bark of (a) shorea robusta and, (b) Garuga pinnata.

(a)

(b)

From table (a) and (b), it is seen that the performance of DSSC fabricated using treated ZnO photoanode with potassium aluminum sulfate is better than DSSC fabricated using untreated ZnO photoanode.

Conclusion

• The absorbance of ZnO photo electrode was decreased little a bit after treatment with potassium aluminum sulfate in room temperature.
• We observed better dye loading on treated ZnO Photo electrode than untreated ZnO photo electrode.
• DSSCs fabricated using treated ZnO photo electrode showed good performance (for shorea robusta dye loaded DSSC, Voc=358mV, Isc=46.6μA, FF=0.503, for Garuga loaded DSSC, Voc=186.6mV, Isc=29.8μA, FF=0.258) than DSSCs fabricated using untreated photoanode (for shorea robusta dye loaded DSSC, Voc=225.4mV, Isc=15.8μA, FF=0.238 and for Garuga dye loaded DSSC, Voc=180.1mV, Isc=13.8μA, FF=0.206).

Future work

• We can fabricate DSSC with optimizing efficiencies by lowering sheet resistance and enhancing optical transparency of ZnO photo electrode.
• Nepal is rich in biodiversity. Natural dyes extracted from various sources can be loaded to enhance performance of DSSC.

Thank You.