DSC Report

Investigation of Different Natural Dyes asPhotosensitizers for Dye Sensitized Solar Cells

S.Palamakumbura

2009/07/31

Acknowledgements

My period of training at the Institute of fundamental studies would not bea successful one if not for the guidance and encouragement given by manyoutstanding professionals. The most enjoyable part of this project report isto thank them all from the depths of my heart.

I am very grateful to Prof.C.B.Dissanayake, Director of the Institute offundamental studies for granting me permission to work as a volunteer/trai-nee. A special person to me is Dr.G.K.R.Senadeera under whom I workedthroughout my period at IFS. Thanks you sir, for your guidance. Its trulyan honor to work under your supervision.

My heart filled gratitude also goes out to Mr.C.A.Thotawatthage. Like ateacher you advised me in my work at IFS. I appreciate your helpfulness. Itwas a pleasure to work with Mr.T.R.C.K.Wijayarathna and Mr.Y.P.Y.P.Ari-yasinghe. Their vast knowledge and experience gave strength to my work.I am truly indebt to Mr.I.P.L.Jayarathna and Mr.I.G.C.K.Kumara for thevaluable time they spent in helping me.

I am grateful to my beloved parents whose help cannot be expressed inwords. They are always behind me in my work. Throughout my trainingperiod there were many friends whose company I whole heartedly enjoyed.Their generosity and kindness would always be remembered.

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Contents

0.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Experimental techniques and method . . . . . . . . . . . . . . 3

0.2.1 Preparation of natural dye sensitizers . . . . . . . . . . 30.2.2 Preparation of TiO2 electrode . . . . . . . . . . . . . . 40.2.3 Preparation of liquid electrolytes . . . . . . . . . . . . 40.2.4 Fabrication of dye sensitized solar cel . . . . . . . . . . 50.2.5 Characterization and measurement . . . . . . . . . . . 5

0.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . 60.3.1 The effect of 4-tert-butylpyridine (TBP) on the effi-

ciency of dye sensitized solar cells using natural dyes . 60.3.2 Photoelectrical parameters of dye sensitized solar cells

fabricated using natural dyes . . . . . . . . . . . . . . . 70.3.3 Methods to improve the conversion efficiency of dye

sensitized solar cells fabricated using Oxalis hedyseroidesas photo sensitizer . . . . . . . . . . . . . . . . . . . . 11

0.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130.5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

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0.1. INTRODUCTION 1

0.1 Introduction

Fossil fuel is the main energy supplier of the modern world having an 86%share in primary energy production. It had been estimated the depositedfossil fuel is sufficient for only fifty years. To meet the increasing demand ofenergy, fossil fuel is no longer a cheap and reliable energy source. In thatcontext new methods are being innovated and implemented in recent times.One such attempt is the dye sensitized solar cell invented by Michael Gratzeland Brian ORegan in 1991.

Dye sensitized solar cell is a new class of solar cells which converts solarlight into electrical energy. Comparing to the semiconductor solar cells, dyesensitized solar cells are made through a simple and low cost manufacturingprocedure. This is the main reason for the interest it has gained during thepast few years.

As depicted in Figure 1 dye sensitized solar cell mainly consists of threecomponents.

1. The anode is a glass plate coated with a conducting material (Fluorinedoped Tin Oxide or Indium doped Tin Oxide) on one side. On top ofthe conducting surface a highly porous TiO2 layer is coated.

2. A glass plate coated with platinum metal as the cathode.

3. The electrolyte which is mainly an organic solvent containing a redoxcouple. (Usually I−/I−3 )

The anode is immersed in the dye solution to absorb a thin layer of dyemolecules into the TiO2 surface. Then the electrolyte is spread over thecathode. The two electrodes are then joined together.

When the dye sensitized solar cell is exposed to sunlight photons whichenter through the transparent anode strike the dye molecules so as to createan excited state of the dye. (See Figure 2)

D + hv −→ D∗

The excited dye molecules will release electrons to the conduction band ofTiO2.

D∗ −→ D+ + e

The dye cations would quickly regain their electrons from Iodide ions in theelectrolyte oxidizing them into Tri iodide ions.

3I− −→ I−3 + 2e

0.1. INTRODUCTION 2

Figure 1: Schematic Diagram of Dye sensitized solar cell

D+ + e −→ D

This reaction occurs quickly preventing the recombination of dye cations andelectrons which would short-circuit the solar cell. The ejected electrons flowthrough the external circuit and combine with Tri iodide ions reducing theminto iodide ions.

I−3 + 2e −→ 3I−

As seen from the above reactions Iodide and Tri iodide ions are kept atequilibrium thus keeping the cell active as long as there are Photons withsufficient energy to excite dye molecules.

From the time dye sensitized solar cells were invented; improving the ef-ficiency had been the primary objective. Cell performance mainly dependson the dye used. Synthesized various photosensitizes were tried out to im-prove the efficiency of dye sensitized solar cells. Among them Rutheniumpolypyridyl complexes were found out as one of the most effective sensi-tizers. However, Ruthenium polypyridyl complexes contain heavy metals.Therefore Ruthenium polypyridyl complexes are not suitable in terms of en-vironmental concerns. In addition the complicated process that has to followin synthesizing Ruthenium polypyridyl complexes makes them costly thuseliminating from commercial use. As a result interest is focused on naturaldyes as sensitizers. In this study dye sensitized solar cells were fabricated us-ing dyes extracted from forty eight different plants which are widely availablein tropical countries.

0.2. EXPERIMENTAL TECHNIQUES AND METHOD 3

Figure 2: Working mechanism of dye sensitized solar cells.

0.2 Experimental techniques and method

0.2.1 Preparation of natural dye sensitizers

Dyes were extracted from plants using the following methods.Betel leaves (Piper betle) and Areca nuts (Areca catechu) were mixed

together in similar portions and grinded thoroughly. Afterwards the mix-ture was kept in 95%wt Ethanol for twenty four hours. A clear dye Beteland Areca nut mixture was obtained via removing the solid residues aftercentrifuging.

Powdery form of Red Sandalwood (Pterocarpus santalinus), Turmeric(Curcuma longa), Coscinium tenestratum (locally called Venivalgata), Com-mon Madder/Walmadata (Rubia cordifolia) and Ebony (Diospyros ebenum)were kept in 95%wt Ethanol for twenty four hours. Dye solutions were ex-tracted by removing solid residues through centrifuging.

Mustard (Brassica nigra) was grinded vigorously and then kept in 95%wtEthanol for twenty four hours. Solid residues were filtrated out.

The following plant samples were cut into small pieces and kept in 95%wtEthanol and or Acetonitrile for twenty four hours. Then solid residues werefiltrated out. Garden balsam (Impatiens balsamina), Hibiscus rosa sinensis(orange colour), Brunfelsia gandiflora (locally known as yesterday,today andtomorrow), Begonia Rex(Black beauty), Nerium Oleander (locally known asKaneru), Parrot beak (Heliconia Pendula), Hibiscus schizopetalus, Crownof thorns (Euphobia milii), Butterfly pea (Clitoria ternatea), Cockscomb


(Celosia cristata), Canna indica (colour red), Canna indica (yellow with redcolour dots), Grapes (Vitis vinifera), Mangoostin (Garcinia mangostana),Fire fern(Oxalis hedysaroides), Orange (Citrus sinensis), Madagascar peri-winkle (Catharanthus roseus), Prosopis juliflora, Heliconia bihai, Creepingwoodsorrel (Oxalis corniculata), False shamrock (Oxalis regnellii), Averrhoabilimbi (locally known as Biling), Starfruit (Averrhoa carambola), Eupho-bia caracasana, Peacock flower (Caesalpinia pulcherrima), Fiddlehead jat-ropha (Jatropha pandurifolia), Kings mantle (Thunbergia erecta), Rose apple(Syzygium samarangense), Hibiscus mutabilis(pink colour), Nutmeg (Myris-tica fragrans), Rangoon creeper (Quisqualis indica), Egyptian starcluster(Pentas lanceolata), Nephelium lappaceum (locally known as Rambutan),Poinciana (Delonix regia), Garcinia cambogia (locally known as Goraka),Lily of the incas (Alstroemeria aurantiaca), Flame lily (Gioriosa superba),Exacum trinervium (locally known as Binara)

0.2.2 Preparation of TiO2 electrode

TiO2 paste was prepared by blending 0.2g of TiO2 power, 12 drops of Aceticacid (99.74%wt), 1 drop of Triton X-100 and 2ml of Ethanol (95%wt). Themixture was ground vigorously for 30 minutes until a uniform TiO2 pastewas formed. Conductive glass plates (FTO, Fluorine doped Tin Oxide) of2cm x 0.5cm area were used to apply the TiO2 paste. TiO2 paste was spreadon about 0.5cm x 0.5cm area (doctor blade method). Then the glass plateswere sintered at 450C for 45 minutes. After cooling to room temperaturethe TiO2 electrodes were immersed in the dye solutions for appropriate time.Dyed films (electrodes) were then removed from the dye solutions and driedwith a hot air flow.

0.2.3 Preparation of liquid electrolytes

In this study two different electrolytes were prepared.

1. Electrolyte with 4-tert-butylpyridine.

0.783g of Tetrapropylammonium Iodide, 0.06g of Iodine, 3.6ml of Ethy-lene carbonate, 0.35ml of 4-tert-butylpyridine and 1ml of Acetonitrilewere mixed together and kept without exposing to the direct sunlight.

2. Electrolyte without 4-tert-butylpyridine.

0.783g of Tetrapropylammonium Iodide, 0.06g of Iodine, 3.6ml of Ethy-lene carbonate and 1ml of Acetonitrile were mixed together and keptwithout exposing to the direct sunlight.


Figure 3: Typical I-V curve of a solar cell.

0.2.4 Fabrication of dye sensitized solar cel

Dye sensitized solar cells were fabricated by sandwiching the liquid electrolytebetween the TiO2 electrode (anode) and counter electrode (cathode). Thenthe two electrodes were clipped using crocodile clips.

0.2.5 Characterization and measurement

Current-Voltage (I-V) curves of the cells were plotted under standard solarirradiation of 100mWcm-2 supplied by a Xe-lamp. A multimeter (Keithley2000) and a Potentiostat (Hokuto Denko, HA301) coupled with a computerwas used for data acquisition. The absorption spectrums were acquired by us-ing a UV-3000 UV-VIS spectrophotometer. (Shimadzu Corporation, Japan)

Based on the I-V curve (Figure 3) the fill factor (FF) is defined as follows.

FF =Imax × Vmax

Isc × VocWhere Imax and Vmax are current and voltage at the maximum power outputlevel. Isc and Voc are short circuit current density and open circuit voltagerespectively.

When the Intensity of the incident light is Pin (Wm−2), the power con-version efficiency of the cell (η) can be defined by the following equation.

η =Jsc × Voc × FF

Pin

0.3. RESULTS AND DISCUSSION 6

Figure 4: Performance of dye sensitized solar cells fabricated using naturaldyes based on two different electrolytes. (Electrolyte with TBP and elec-trolyte without TBP) 1) Fire fern 2) Mangoostin skin 3) Mangoostin pulp4) Grapes 5) Orange 6) Mangoostin/Orange/Grapes)

Where Voc is open circuit voltage (V) and Jsc is short circuit current density(Am−2).

0.3 Results and Discussion

0.3.1 The effect of 4-tert-butylpyridine (TBP) on theefficiency of dye sensitized solar cells using nat-ural dyes

Six dye sensitized solar cells were fabricated using the following natural dyes;Fire fern (Oxalis hedysaroides), Grapes ((Vitis vinifera), Mangoostin skin((Garcinia mangostana), Mangoostin pulp ((Garcinia mangostana), Orange((Citrus sininsin) and a mixture of Mangoostin / Grapes / Orange in similarproportions. Two cells were fabricated for each dye using electrolyte withTBP and electrolyte without TBP. Then conversion efficiency of solar powerto electrical power was measured and depicted in Figure 4.

It was observed that higher efficiencies can be achieved by cells withelectrolyte containing no TBP. Therefore electrolyte without TBP was usedhereafter in the purpose of testing natural dye sensitized solar cells.


0.3.2 Photoelectrical parameters of dye sensitized so-lar cells fabricated using natural dyes

Table 1 shows the photoresponses of dye sensitized solar cells prepared withdifferent dyes. Betel leaves (Piper betle) and Areca nuts (Areca catechu) mix-ture, Red Sandalwood (Pterocarpus santalinus), Turmeric (Curcuma longa),Coscinium tenestratum (locally called Venivalgata), Common Madder (Ru-bia cordifolia), Ebony (Diospyros ebenum), Mustard (Brassica nigra), Gar-den balsam (Impatiens balsamina), Hibiscus rosa sinensis (orange colour),Brunfelsia gandiflora (locally known as yesterday,today and tomorrow), Be-gonia Rex(Black beauty), Nerium Oleander (locally known as Kaneru), Par-rot beak (Heliconia Pendula), Hibiscus schizopetalus, Crown of thorns (Eu-phobia milii), Butterfly pea (Clitoria ternatea), Cockscomb (Celosia cristata),Canna indica (colour red), Canna indica (yellow with red colour dots), Grapes(Vitisvinifera), Mangoostin (Garcinia mangostana), Fire fern (Oxalis hedysaroides),Orange (Citrus sinensis), Madagascar periwinkle (Catharanthus roseus), Prosopisjuliflora, Heliconia bihai, Creeping woodsorrel (Oxalis corniculata), Falseshamrock (Oxalis regnellii), Averrhoa bilimbi (locally known as Biling), Star-fruit (Averrhoa carambola), Euphobia caracasana, Peacock flower (Caesalpiniapulcherrima), Fiddlehead jatropha (Jatropha pandurifolia), Kings mantle(Thunbergia erecta), Rose apple (Syzygium samarangense), Hibiscus muta-bilis(pink colour), Nutmeg (Myristica fragrans), Rangoon creeper (Quisqualisindica), Egyptian starcluster (Pentas lanceolata), Nephelium lappaceum(locallyknown as Rambutan), Poinciana (Delonix regia), Garcinia cambogia (locallyknown as Goraka), Lily of the incas (Alstroemeria aurantiaca), Flame lily(Gioriosa superba), Exacum trinervium (locally known as Binara) were usedto fabricate dye sensitized solar cells.

Table 1: Conversion efficiency (η), Fill factor (FF), Short circuitcurrent density (Jsc) and Open circuit voltage (Voc) of dye solarcells sensitized with different dyes

Dye Voc(mV ) Jsc(mAcm−2) FF% η%

Fire fern (Oxalis hedysaroides) 422 1.528 60.78 0.392Egyptian starcluster (Pentas lanceolata) 562.7 0.88 62.04 0.307Begonia Rex (Black beauty) 467.3 0.804 71.17 0.267Turmeric (Curcuma longa) 579.7 0.56 70.2 0.229Hibiscus schizopetalus 575.4 0.137 72.59 0.229Poinciana (Delonix regia) 574 0.444 70.08 0.178Mangoostin skin (Garcinia mangostana) 609.9 0.404 68 0.167Garcinia cambogia (locally known as Goraka) 419.1 0.804 47.91 0.162Hibiscus rosa sinensis (orange colour) 621.7 0.396 64.76 0.16Common Madder(Rubia cordifolia) 517.2 0.4 69.93 0.144

Continued on the next page


Dye Voc(mV ) Jsc(mAcm−2) FF% η%

Crown of thorns (Euphobia milii) 544 0.101 65.76 0.145Parrot beak (Heliconia Pendula) 602.5 0.324 72.43 0.141Hibiscus mutabilis(pink colour) 599.6 0.081 70.56 0.137Betel leaves (Piper betle) 625 0.3 67.9 0.127and Areca nuts (Areca catechu) mixtureMangoostin pulp (Garcinia mangostana) 625.3 0.296 66.9 0.123Butterfly pea (Clitoria ternatea) 618.2 0.244 71.27 0.108Orange (Citrus sinensis) 627.1 0.232 74.06 0.108Euphobia caracasana 551.7 0.28 67.84 0.105Rangoon creeper (Quisqualis indica) 561.4 0.264 65.74 0.097Madagascar periwinkle (Catharanthus roseus) 575.9 0.228 72.8 0.096Canna indica (colour red) 591.7 0.272 58.16 0.093Averrhoa bilimbi leaves (locally known as Biling) 663.5 0.208 61.86 0.085Cockscomb (Celosia cristata) 602.1 0.2 69.12 0.083Rose apple (Syzygium samarangense) 519.2 0.22 73.26 0.083Averrhoa bilimbi flowers (locally known as Biling) 531.1 0.204 71.24 0.077Exacum trinervium (locally known as Binara) 614.4 0.18 68.88 0.077Grapes(Vitis vinifera) 492.8 0.244 63.2 0.076Lily of the incas (Alstroemeria aurantiaca);(Red colour) 601.6 0.172 68.95 0.071Lily of the incas (Alstroemeria aurantiaca);(Pink colour) 595.8 0.156 73.48 0.069Fiddlehead jatropha (Jatropha pandurifolia) 527.5 0.204 62.27 0.067Flame lily (Gioriosa superba) 562.5 0.168 70.53 0.067Canna indica (yellow with red colour dots) 616.1 0.192 50.66 0.06Coscinium tenestratum (locally called Venivalgata) 531.3 0.16 68.67 0.059Nephelium lappaceum(locally known as Rambutan) 495.6 0.196 60.9 0.059Red Sandalwood (Pterocarpus santalinus) 426.6 0.276 47.99 0.057Prosopis juliflora 592.2 0.136 66.9 0.054Garden balsam (Impatiens balsamina) 553.7 0.12 73.95 0.048Starfruit (Averrhoa carambola) 627 0.116 65.24 0.048Kings mantle (Thunbergia erecta) 631.1 0.096 79.88 0.048Peacock flower (Caesalpinia pulcherrima) 600.4 0.12 65.75 0.047False shamrock (Oxalis regnellii) 382.3 0.168 66.51 0.043Ebony (Diospyros ebenum) 502.9 0.116 67.47 0.04Brunfelsia grandiflora 618.4 0.096 68.01 0.04(locally known as yesterday,today and tomorrow)Mustard (Brassica nigra) 689.2 0.12 45.46 0.038Heliconia bihai 598.7 0.072 69.4 0.03Nutmeg (Myristica fragrans) 580.7 0.06 61.52 0.022Creeping woodsorrel (Oxalis corniculata) 378.4 0.064 66.87 0.016Nerium Oleander (locally known as Kaneru) 641.1 0.004 17.51 0

Among the dye solar cells tested, cells fabricated with Oxalis hedysaroides,Pentas lanceolata and Begonia Rex (Black beauty) showed best photoelec-trical characteristics. The current-voltage curves obtained from these cellswere shown in Figure 5.

The highest conversion efficiency was recorded from Oxalis hedysaroides


Figure 5: Current-Voltage characteristics of the dye sensitized solar cellsfabricated with different dyes. (a) Oxalis hedysaroides (b) Pentas lanceolata(c) Begonia Rex (Black Beauty)

(Table 1). Figure 6 show the absorption spectra of dye solutions (a) Oxalishedysaroides (b) Begonia Rex (Black Beauty). Both dyes show absorbancein the visible region. But Oxalis hedysaroides absorbs more light than Bego-nia Rex (Black Beauty) especially in the wavelength range from 550 nm to700 nm. The relatively high conversion efficiency of Oxalis hedysaroides isprobably due to this higher absorption in the above region than other dyes(other absorption spectra were not included for clarity). The dark I-V curveof the cell fabricated using Oxalis hedysaroides as photo sensitizer is shownin Figure 7.


Figure 6: Absorption spectrums of (a) Oxalis hedysaroides (b) Begonia Rex(Black Beauty)

Figure 7: Dark I-V curve of solar cell sensitized with Oxalis hedysaroides


0.3.3 Methods to improve the conversion efficiency ofdye sensitized solar cells fabricated using Oxalishedyseroides as photo sensitizer

The following methods were investigated to improve the efficiency of cellsfabricated using Oxalis hedysaroides dye.

1. ZrO2 was added to the TiO2 paste in the molar ratio [TiO2] : [ZrO2] =95 : 5

2. Added 10 drops of 0.025M CuCl2 to the TiO2 paste.

3. Five drops of 1.55x10−3M Trisodium citrate was added to the TiO2

paste.

4. Addition of 0.2M Guanidinium Thiocyanate to the electrolyte.

5. Increasing the thickness of the TiO2 paste by coating several times.

6. Immersing time of the TiO2 electrode in Oxalis hedysaroides dye solu-tion is changed.

Addition of ZrO2, CuCl2, Trisodium citrate, Guanidinium Thiocyanate, in-creasing the thickness of the TiO2 paste, did not have expressive impact oncell performance. It was observed that changing the immersing time of TiO2electrode in Oxalis hedysaroides dye solution significantly affects cell effi-ciency. As shown in Figure 8 the highest conversion efficiency was achievedwhen the TiO2 electrode was immersed for two minutes.

The current-voltage curve of the cell which was immersed for two minutesin the dye solution is shown in Figure 9.

Considering the above results it can be concluded that, the highest effi-ciency of dye solar cell sensitized with Oxalis hedysaroides can be obtainedby immersing the semiconducting film in the dye solution for two minutes.


Figure 8: Variations of cell efficiency due to the immersing time of TiO2

electrode.

Figure 9: Current-Voltage curves of Oxalis hedysaroides when the immersingtime of TiO2 electrode is 2 minutes.

0.4. CONCLUSION 13

0.4 Conclusion

Dye sensitized solar cells were fabricated using forty eight different naturaldyes as photosensitizes. Among them Fire fern (Oxalis hedysaroides), Egyp-tian starcluster (Pentas lanceolata) and Begonia Rex (Black beauty) reportedbest photoelectrical characteristics, with conversion efficiencies of 0.392%,0.307%, 0.267% respectively. Dye solar cells sensitized with Turmeric (Cur-cuma longa), Hibiscus schizopetalus, Poinciana (Delonix regia), Mangoostinskin (Garcinia mangostana), Garcinia cambogia (locally known as Goraka)and Hibiscus rosa sinensis (orange colour) showed moderate performance.Oxalis hedysaroides which had the highest conversion efficiency was furtherimproved by changing the immersing time of TiO2 electrode in the dye so-lution. An optimum short circuit current density of 3.088mAcm−2 and aconversion efficiency of 0.823% were recorded when the immersing time wasadjusted to two minutes.

Moreover the effect of 4-tert-butylpyridine (TBP) in the electrolyte, onthe performance of dye sensitized solar cells was investigated. The resultsindicated that electrolyte without TBP is more suitable for use with naturalphotosensitizes as it gives higher conversion efficiencies than electrolyte withTBP.

Further studies should be carried out to enhance the cell performance ofFire fern (Oxalis hedysaroides), Egyptian starcluster (Pentas lanceolata) andBegonia Rex (Black beauty) sensitized solar cells. Especially investigationsshould be carried out to improve film morphologies of the semiconductorTiO2 layer.

0.5. REFERENCES 14

0.5 References

1. Natural anthocyanins as photo sensitizers for dye-sensitized solar de-vices, J.M.R.C. Fernando and G.K.R. Senadeera, Current Science, Vol.95, No.5, 10 September 2008, 663-666

2. Natural dyes as photo sensitizers for dye-sensitized solar cell, SancunHao, Jihuai u, Yunfang Huang, Jianming Lin, Solar Energy Materialsand Solar Cells, Volume 80, Issue 2, February 2006, Pages 209-214

3. Dye-sensitized solar cell using natural dyes extracted from rosella andblue pea flowers, Khwanchit Wongcharee, Vissanu Meeyoo, SumaethChavadej, Solar Energy Materials and Solar Cells, Volume 91, Issue 7,16 April 2007, Pages 566-571.

4. On the photophysical and elelectrochemical studies of dye-sensitized so-lar cells with the new dye CYC-B1, Jian-Ging Chen, Chia-Yuan Chen,Shi-Jhang Wu, Jheng-Ying Li, Chun-Guey Wu, Kuo-Chuan Ho, SolarEnergy Materials and Solar Cells, Volume 92, Issue12, December 2008,Pages 1723-1727

5. TiO2 − ZrO2 Mixed Metal Oxide Electrode for a Dye-sensitized SolarCell, Athapol Kitiyanan, Sorapong Pavasupree, Taku Kato, YoshikazuSuzuki and Susumu Yoshikawa, Sustainable Energy and Environment,December 2004.

6. http://en.wikipedia.org/wiki/Dye-sensitized solar cell

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