research on the usability of low-cost materials in dye sensitized solar cells

Köchl Jürgen Strom aus der Farbstoffsolarzelle Seite 1 Wagner Andreas

Research on the usability of low-cost materials in dye sensitized solar cells

HTL Braunau am Inn Osternbergerstraße 55 5280 Braunau/Inn Austria Tel: 0043/7722/83690 E-mail: [email protected] www.htl-braunau.at

Students: Andreas Wagner (team leader) Jürgen Köchl

Project Teachers: Dr. Wolf Peter Stöckl

Date: 24.03.2010

Research on the usability of low-cost materials in dye sensitzied solar cells

Table of Contents:

1 Introduction ............................................................................................................................................................ 1

1.1 State of the Art ...................................................................................................................................................... 1 1.2 Intention ................................................................................................................................................................ 1 1.3 Theoretical background ........................................................................................................................................ 2

2 Materials and Methods .......................................................................................................................................... 5

2.1 Instruction sheet to build a dye sensitized solar cell .......................................................................................... 5 2.2 Experiment set-up for Long-Term Stability Research ........................................................................................ 10 2.3 DSC – Data logger ............................................................................................................................................... 12 2.4 Research on the chemical stability of the dye .................................................................................................. 12

3 Results .................................................................................................................................................................. 13

3.1 Measurement results ......................................................................................................................................... 13 3.2 Long term stability .............................................................................................................................................. 15 3.3 Regeneration of the cell after small area light exposure.................................................................................. 17 3.4 Stability of the dye .............................................................................................................................................. 18

4 Discussion ............................................................................................................................................................ 21

4.1 Efficiency of our low cost cells ........................................................................................................................... 21 4.2 Effect of decoloration ......................................................................................................................................... 21 4.3 Repeatability of measurement results .............................................................................................................. 22 4.4 Economic aspects ............................................................................................................................................... 22

5 Conclusions .......................................................................................................................................................... 23

5.1 Conclusions of the results .................................................................................................................................. 23 5.2 What would we do differently if we repeated this project? .............................................................................. 23 5.3 Future Prospects ................................................................................................................................................. 23

6 Personal Resume and acknowledgements ......................................................................................................... 25

7 Literature .............................................................................................................................................................. 26

7.1 Internet resources ............................................................................................................................................... 26 7.2 Scientific Journals ............................................................................................................................................... 26 7.3 Endnotes ............................................................................................................................................................. 28

©Andreas Wagner, Jürgen Köchl


©Andreas Wagner, Jürgen Köchl Page 1

1 Introduction The amount of CO2 in the air has been rising steadily and the impact on our climate is getting more and more serious. Our oil and gas resources are decreasing to an alarming extent and the price for valuable resources is growing. Many experts try to demonstrate the different problems of our energy supply and desperately look out for new ideas in the field of renewable energy. The energy problem is not only an ecological and economical problem; it is also a political one. Some people even say that governments start war because of fossil fuels. This issue alone would fill many reports, but we just wanted to use this information as food for thought to show how important the work on renewable energy is. Why did we actually choose to work on the field of dye sensitized solar cells (DSC)? Our project teacher Dr. Stöckl gave Andreas a construction manual for a DSC during his chemistry lab. He was very interested in this topic but was not able to build a cell because there was not even one substance that he would have needed in his laboratory. However he did not give up and decided to ask Jürgen to work with him on this topic.

1.1 State of the Art In 1991, as Prof. Grätzel presented the DSC the first time1, many hopes were raised because of this new technology. Unfortunately the hopes have not been satisfied yet. The highest efficiency that has ever been reached for a DSC was 11.1%. But the amount of work that is necessary to produce a DSC with an efficiency of over 10% has to be considered as well. While the silicon solar cell already reaches more than 17% in large scale the DSC has an efficiency of 11% at just 0,219cm² under laboratory conditions! The best cell with a larger photovoltaic area (26.5 cm²) has an efficiency of 6.3% but already uses many cells connected serially2. For this report only DSCs with a liquid electrolyte were studied because this type is by far the most common and most efficient. Some groups try to replace the liquid electrolyte with a gel, but because of the low mobility of the ions in the electrolyte and the bad diffusion into the TiO2 surface the efficiency is significantly lower than in the liquid electrolyte cells.

1.2 Intention The topic of dye sensitized solar cells is very complex and just specialized institutions work on it. Although our ideas sometimes are really simple, we tried to do our very best to improve the technology of DSCs and take it a step further to a commercial application, at least as much as it was possible for us. The major reasons why the DSC is still not available on the market are:

• sealing of the aggressive iodine electrolyte • efficiency • scaling up to larger cells • costs

Our Intention was to try new low cost materials in the cell and test them on their long-term stability and efficiency. Although the low cost cells probably will not have the same efficiency as expensive cells we might be able to build more economic cells concerning the costs in comparison to their efficiency. If the sealing is not dense, the electrolyte will evaporate, therefore, we tested new sealants like a rubber substance from one of our partner companies. To enable to study the cells over a long period, a special experiment set up with a very complex electronic measurement unit to monitor 30 cells at the same time was developed. Another intention of our work evolved over the time. We had very big problems to get detailed information about this topic or to filter essential information out of published papers. At the moment, only academics work with DSCs, moreover the information is shut away very often. We suppose more young and interested students should be able to work in this fascinating field of science, so we try to publish every step of our instruction sheet in detail. Furthermore we made a short movie to show how to build the cells (YouTube – “How to make a dye sensitized solar cell HTL Braunau”).

http://www.youtube.com/watch?v=qaGrHrLdRhs



1.3 Theoretical background The following chapter describes the working principle of the dye sensitized solar cell and the specific duties of the components in the cell. In addition, details of the problems with the long term stability and the efficiency limiting factors in the cell will be presented.

1.3.1 Working Principle

The DSC contains 2 TCO (transparent conductive oxide) glass electrodes. One of the electrodes is coated with a titanium dioxide layer (about 5-15µm thick) which is very porous. A huge number of dye molecules are absorbed into this layer. The photons (light) hit onto the dye molecules and the dye is lifted to an excited energy state. Because of this excited state, the dye loses an electron to the titanium dioxide (the dye oxidizes). From the TiO2 semiconductor the electron is transported over the conductive surface of the glass electrode and over the load back to the counter electrode. The graphite counter electrode now transfers the electron to the iodine redox couple which finally reduces the dye again. The titanium dioxide electrode is negative and the graphite electrode is positive.

Figure 1 working principle of the DSC Figure 2 components of the DSC

1.3.2 Efficiency limiting effects of the DSC

The following effects limit the efficiency of the DSC: • The internal resistance of the DSC is affected negatively by:

- resistance of the TCO glass - conductivity of the electrolyte and thickness of the electrolyte layer - parameters of the TiO2 layer (porosity, thickness)

• The excited dye molecule emits a photon instead of transporting an electron to the TiO2 • The already transferred electron at the TiO2 is transported back to the dye and is lost • The already transferred electron at the TiO2 is transmitted to the redox system in the electrolyte • The absorption spectrum of the dye is not using the whole spectrum available from the sun

1.3.3 Long term stability

The long term stability of the DSC is a very big hurdle on the way to commercial use. Three different problems can be distinguished in the cell:

• The DSC contains an aggressive liquid electrolyte and just very inert chemical substances are stable over a long time period in its presence. The sealing of the cell is very much related to the long term stability. This is one reason why our team tried to use new sealants based on rubber from our supporting company “Kraiburg Holding GmbH”.

• The titanium dioxide has a very strong photo catalytic effect under UV light, which destroys all

organic components in the cell. This problem can actually be prevented by the use of a UV-filter3.



• When the cell is used outdoors it may reach up to 80°C under sunlight. Furthermore, the temperature between night and day varies strongly. Those stresses cause other side reactions inside the cell. These effects even appear at a very good sealed cell with a UV-filter.

1.3.4 Titanium dioxide layer

The titanium dioxide layer in combination with the dye is the most important part of the DSC. Especially the porosity (particle size), thickness, homogeneity and the surface structure of the TiO2 layer are very important. These properties can only be studied by scanning electron microscopy (SEM). With the help of our supporting company “Amag rolling GmbH” we were able to make some pictures to study our different TiO2 coatings.

Figure 3 SEM pictures - TiO2 layer (left and middle); light microscope picture - layer thickness 6.7µm (right)

1.3.5 Counter electrode

Another very important point for the efficiency of the cell is the counter electrode. It is also made of a TCO glass coated with either platinum, graphite or gold. This layer has several duties:

• Increase conductivity • Allow more current to flow because of high porosity • Increase the voltage because of an improvement of the energy level (reduction of overvoltage)

1.3.6 Dye

The dye is responsible for collecting the photons and delivering the electrons to the TiO2. To do that in an efficient way it has to absorb as much light from the sun spectrum as possible. In addition there is a special characteristic that describes how many photons are actually able to lift the dye molecule on the certain energy level that is necessary to deliver an electron to the TiO2. This value is determined through the IPCE (incident photon to current efficiency) analysis. Unfortunately there are just a few values available in the respective literature – especially for Ruthenium-dyes. This is one of the reasons why we got in contact with the Ludwig-Maximilian University in Munich to measure these values for hibiscus as well.

1.3.7 Electrolyte

The electrolyte is responsible for the transport of the electron from the counter electrode to the dye. Iodine and several complexes are normally used as the redox couple in the electrolyte. To increase the efficiency of the cell, additives like 4-tert-Butylpyridine are often used. The main reaction in the cell is the balance reaction:

I3- + 2 e- ⇌3 I- Moreover, iodine can react with the oxygen solved in the electrolyte4. It can even react with water which might be the reason for a decrease of efficiency when water is used as the electrolyte solvent. Especially the following reactions are of importance: H2O⇌H+ + OH- I2 + 2OH-⇌I- + IO- + H2O

3IO- → IO3- + 2I- IO3

- + 5I- + 6H+ ⇌ 3I2 + 3H2O



In this paper ethanol was mainly used as solvent for the electrolyte (bought from the company “mansolar”). After studying that water and ethanol are very bad for the power output and stability5 of the cell, acetonitrile was used as solvent.

1.3.8 Mind map

This structure should help to visualize how many parts a dye sensitized solar cell has and what variations of components our team tried in this project.

Figure 4 different components of the dye sensitized solar cell and the variations that were applied in this report



2 Materials and Methods In the following chapter our practical work is shown. At first the self developed and optimized instruction sheet to build the dye sensitized solar cells is presented. Lots of tests had to be performed before the assembly instruction was good enough to start off with the experiments. As already mentioned our team had big problems to gain detailed information from the internet as well as scientific journals about the cell assembly. Just to improve the titanium dioxide coating, more than 200 tests and 60 suspensions were tested.

Figure 5 four of the first prototypes

2.1 Instruction sheet to build a dye sensitized solar cell Apart from the written instruction sheet there is a short movie which helps to understand every step in this manual. The film is in German and a few steps are a little different to this manual because a few production steps were slightly changed, nevertheless it should help to visualise the several steps. One can find it on YouTube – “How to make a dye sensitized solar cell HTL Braunau”. Because of this video young students, PhD students and researchers from different countries (Sweden, Australia, Ireland, India, China and Saudi Arabia) contacted our group.

2.1.1 List of all materials to build a DSC

• 2 transparent conductive glass plates (TCO glass) – our team used the product: “K-glass” from the company Pilkington which is a cheap but electrically conductive heat isolating window glass

• millimetre • sticky tape • ruler and a small knife • ultrasonic bath • isopropyl alcohol • titanium dioxide (company: Evonik Degussa GmbH; product name: P25) • spattle • mortar with pistil • solution for the TiO2 suspension made out of:

1,25ml acetyl acetone 2,5ml Triton X-100 (1/5 vol.) in H2O dest. 5ml polyethylene glycol 20000 (100g/l) in H2O dest. 50ml H2O dest.

• pipettor (25µl-1000µl) • muffle furnace (at least 450°C) • glass rod

http://www.youtube.com/watch?v=qaGrHrLdRhs



• dye solution (hibiscus extract in H2O/Ethanol 1/1) • graphite spray (company: Kontaktchemie; product name: Graphit 33) • electrolyte containing

0,5 mol/l lithium iodine or potassium iodine 0,05 mol/l iodine solvent: acetonitrile (as a easier (but less stable over a longer period of time) version:

distilled water or ethanol) 4-tert. Butylpyridin can be added to increase power output of the cell One may buy an electrolyte from these companies: solaronix, dyesol, mansolar

• sealant - two-component adhesive or hot melting foil (company: solaronix) – our group tried to use a new sealant (rubber solution from the company “Kraiburg Holding GmbH”) but it seems not to be dense

• brush • clamp • weighing machine • sticky copper tape (company: CMC Klebetechnik; product name: 91815), wire and a soldering

iron to contact the cells

2.1.2 Detect the conductive side

One side of the glass plates is conductive – it is coated with a transparent conductive oxide (TCO). Take an ohmmeter and test each side of each glass plate for conductivity. Afterwards mark those sides and from now on always work on the conductive side of the plates.

2.1.3 Place spacers

A small part of every glass plate has to be free for the electric contact and the sealing. Because of that a tape is used as a spacer for this area. The complete glass plate is covered with a sticky tape and afterwards the photoactive area is cut out with a small knife (called doctor blade method).

2.1.4 Clean the glasses

In order to ensure that the TiO2 layer and the graphite layer are able to adhere on the glass plates, it is necessary that the glass plates are free of any contamination. For this purpose the glasses are washed with the tape spacer in an ultrasonic bath with isopropyl alcohol. After five minutes the glass is taken out of the bath and dried. To simplify the coating procedure of the titanium dioxide layer the glass plate is put into distilled water and dried it again. This causes some sort of surface effect that helps to distribute the suspension on the glass more easily. This little step is not included in the movie so please do not wonder about that.

2.1.5 Coat the working electrode with titanium dioxide

Put 3g of TiO2 in a mortar and add 1ml of the solution mentioned above from time to time. Altogether 10ml are added to the solution. The suspension has to be stirred with a pistil for several minutes. Put

Tape

Area to contact the cell

Figure 6 drawing that shows how to place the tape



the solution on the top of the side with the tape. Use 5µl per square centimetre of photoactive area. If one rolled the glass rod, most of the suspension would stick on the rod itself and not enough suspension is left to coat the glass plate. In case more than one coating are produced at the same time, the glass rod with distilled water always need to be cleaned between every coating procedure. The sticky tape is used as a spacer and mask for the TiO2 film. Once the layers are dry the tape is carefully removed. Depending on the tape there are sometimes residues that have to be removed with petroleum.

Figure 7 Three coatings of TiO2

2.1.6 Temper the working electrode

To remove organic components that are included in the suspension, to optimize the crystal structure of the TiO2 and to electrically conduct the titanium dioxide, the glass plate has to be heated for 1 hour at 450°C. To avoid a very fast heating process the plates are put into the furnace when it is cold and the timer is started at this time. The slow heating procedure has the bonus effect of drying remained water. At 450°C the water would evaporate so fast that parts of the layer would be destroyed. To prevent the coating from damage, the cooling of the glass plate has to be slowly as well. The furnace is simply turned off and opened, but the plates are left in there. At about 80°C they are put out of the furnace.

2.1.7 Put the working electrode into the dye solution

After one removed the coated glass plates from the furnace they need to be put into a small box with the dye solution. About 12 hours later one has to wash them in a Petri dish with the same solvent as the dye solution. Pivot the dish carefully and dry the glass plate afterwards. This step is very important because otherwise dye molecules that are not absorbed to the titanium dioxide layer itself would increase the internal resistance of the cell.

2.1.8 Produce the counter electrode

The dye sensitized solar cells consists of 2 electrodes – one working electrode made out of titanium dioxide and a dye and a counter electrode made out of graphite (or platinum). This electrode has to be carried through step 1-3 as well as the TiO2 electrode. Following those steps a big amount of graphite is sprayed onto the glass plate with the help of a graphite spray (the spray needs to be shaken before using it). Afterwards one uses a glass rod to distribute the suspension evenly on the glass plate. The graphite layer is dried at room temperature, the tape spacer is removed and then one puts the electrode into the furnace for 2 hours at 200°C (again – started in a cold furnace). The last procedure is to polish the cooled layer with a pulp gently.



Figure 8 the graphite coating needs to be polished after the heating process

An electrolyte has to be put between the two plates in order to allow ions moving between the two electrodes. This as well as all the other working steps can be done just after the steps 1-7 are finished.

2.1.9 Fill the cell with electrolyte

The electrolyte is taken into the cell by dropping it onto the graphite electrode before the electrodes are put together. One just needs about 50µl of electrolyte because of the small distance between the two plates (30-70 µm).

2.1.10 Put the two electrodes together

After placing the electrolyte on the counter electrode, the cell is assembled by putting the two electrodes on one another shifted about 8mm. The shift between the two electrodes is needed because the area for the electrical contact has to be free.

2.1.11 Seal the cell

The cell has to be sealed, otherwise the electrolyte would evaporate. Coat all sides of the cell with the preferred sealant. Depending on the sealing method and sealant one or two clamps are used to press the two electrodes together until the sealant is dry.

Figure 9 Picture of a finished cell sealed with epoxy resin



Figure 10 Pictures of a finished cell sealed with a liquid rubber substance from one of our supporting companies

2.1.12 Contact the cell

Especially for long-term experiments the cell has to have a very good and stable electric contact. There are different ways to contact the cell (silver conductive paint etc.). After several tests our team concluded to use sticky conductive copper band on which one can braze a wire.

Figure 11 A completed cell electrically conducted with silver conductive paint



2.2 Experiment set-up for Long-Term Stability Research An extensive part of this project was the development of a complex electronic system to measure the long term stability of 30 DSCs at the same time. Additionally, a LabView software to monitor all the connected cells was developed.

Figure 12 block diagram of the experiment set up

Figure 13 experiment set up to measure 30 DSCs



Figure 14 circuit diagram of the multiplexer electronics

Figure 15 board connection diagram of the multiplexer electronics

Figure 16 LabView managing software for the experiment set up to measure the long-term stability



Unfortunately the electronics was destroyed when it was transported in a car to present the project at a local fair. The time to redesign and produce another blank was too short, so we had to cut down on our long-term stability research. But still these few results that we have from this experiment set up are very interesting (see chapter 3.2.1).

2.3 DSC – Data logger The logger was designed to observe a DSC outdoors. The device is powered by a battery pack, thus it is independent from the grid. A small 8 bit microprocessor is used as the controller of the whole electronic system. The cell is measured with eight different loads and the data is stored as an excel file on a SD card. Not only are the electrical characteristics of the cell recorded, but also the actual temperature and brightness. Since the long-term stability research electronics was destroyed at a local fair we mainly used the data logger to measure the I-V characteristics of the cells under a lamp.

Figure 17 Picture, circuit diagram and board connection diagram of the data logger

2.4 Research on the chemical stability of the dye Besides the variation of cell parameters, the production of several cells and the research on their long term stability, the chemical stability of the dye was investigated. Titanium dioxide is able to oxidize organic components with the help of UV light. This effect also happens in the dye sensitized solar cell. But there is a big difference between the use of the photo catalytic titanium dioxide in a suspension and as a coating. Because of that experiments were made to determine if there would still be the UV degradation of the dye when it was already absorbed into the TiO2 layer.



3 Results In the next chapter the most important and interesting results of this research project are pointed out. Apart from the presented results there are many more such as failed production procedures for every step of the self developed instruction sheet. But to bring up all of them would exceed an appropriate length of the report by far.

3.1 Measurement results As already mentioned the main intention of this project was to vary the components of the cell and to compare the results. All in all over 50 cells with different electrolytes, counter electrodes, TiO2 and other variations were produced.

3.1.1 Dependency on used dye

The most experiments and therefore the most time-consuming component to be changed in the cell is the dye. “Basacryl”, “Basilenrot” and “Cellitonscharlach” are dyes that are used in industry to color clothes.

Figure 18 the dye is responsible for the colour of the titanium dioxide layer

dye open circuit voltage

/mV short circuit current /mA light intensity mW/cm² Basacryl 20 0 16

Basilenrot 8 0 16 Cellitonscharlach 160 0,1 16 hibiscus extract in

ethanol 96% 225 0,31 16 hibiscus extract in

water 370 2,7 30 hibiscus extract in water/ethanol 1/1 445 3,35 30 red wine number 1 440 0,63 16 red wine number 2 290 0,16 16

N719 Ru 0,25 mmol/l in ethanol 96% 460

6,2 30 Unfortunately the lamp to illuminate the cells was changed between the tests so one needs to take into account that the light intensity is just half of some cells. After this first series of tests our team decided to stick to the ruthenium dye and the hibiscus dye and vary the other components.



3.1.2 Dependency on the counter electrode coating

Usually a TCO glass plate with a platinum coating is used as a counter electrode. In several instruction sheets for secondary schools a pencil is used to coat the glass with graphite. As alternatives a graphite spray was introduced and tested.

Coating material Open curcuit voltage

Voc [mV] Short curcuit current

Isc [mA] Illumination [mW/cm²]

Graphite spray 370 2,7 30

Uncoated TCO glass 370 0,125 30

pencil 420 0,5 30

In the process of improving the graphite coating, the solution that was in the graphite spray was diluted with isopropyl alcohol (1:1) and used to coat the counter electrode.

Quantity of graphite solution

Open curcuit voltage Voc [mV]

Short curcuit current Isc [mA]

Illumination [mW/cm²]

30µl 430 0,5 30

60µl 450 0,75 30

120µl 440 0,95 30

As another possibility to coat the counter electrode Gold-Palladium was used. With the help of our supporting company “Amag rolling GmbH” a ~100nm thick layer was sputtered on two glass plates. Even though the Gold-Palladium cells used TBP as an electrolyte additive to maximize the power output, the cells were not able to compete with the graphite coated counter electrodes.

dye counter electrode additive?Open curcuit

voltage Voc [mV] Short curcuit current

Isc [mA] Illumination [mW/cm²]

hibiscus in H2O/Eth 1/1 graphite no 455 1,9 30

hibiscus in H2O/Eth 1/1 gold+ palladium yes 393 0,23 30

N719 Ru 0,25 mmol/l gold+ palladium yes 530 2,1 30

N719 Ru 0,25 mmol/l graphite no 460 6,2 30

At last the graphite coating was compared to the widely-used platinum coating (5µl/cm² of 5mmol chloroplatinic acid in isopropyl alcohol were applied – coating method like graphite and titanium dioxide). Although the output power is lower it is fairly good in comparison to the platinum. Especially the third cell had a very high short circuit current considering that there was no TBP additive in the electrolyte.

dye counter

electrode additive?Open curcuit voltage

Voc [mV] Short curcuit

current Isc [mA] Illumination [mW/cm²]

N719 Ru 0,25 mmol/l platinum yes 830 8,6 30 N719 Ru 0,25 mmol/l graphite yes 650 4,8 30 N719 Ru 0,25 mmol/l graphite no 460 6,2 30



3.1.3 Dependency on the used titanium dioxide

At the beginning of the project the first prototypes were made with titanium dioxide from Merck (purity: p.a.). After very big problems (very small short circuit current, difficult to produce homogeneous coatings) our team decided to test the widely-used titanium dioxide from Degussa (product name: P25). Because of a different production procedure (the P25 is acidic with water and the Merck p.a. is neutral with water) completely different suspensions needed to be used. But because of the immensely smaller particle size the current output of the cell is a lot bigger.

titanium dioxide

Open curcuit voltage Voc [mV]

Short curcuit current Isc [mA]

Illumination [mW/cm²]

Merck p.a. 215 0,12 30

Merck p.a. 470 0,11 30

Degussa P25 370 2,7 30

3.1.4 Dependency on used electrolyte solvent

Most of the cells were made with an electrolyte from the company “mansolar” with ethanol as its solvent. Furthermore acetonitrile and propylene carbonate were used. The experiments of the cells with acetonitrile need to be proved in another series before we want to present them. The problem with propylene carbonate was, that the iodine salt (potassium iodine) was not 100% soluble and therefore the concentration of I- is less. Because of that the results are not comparable at the moment and need to be reviewed.

3.1.5 Dependency of the size of the illumination area

Besides the variation of the components, the area of illumination was varied too. A 1cm² mask was placed on top of the cells to ensure that just one sixth of the cell is illuminated. In the following table the percent of the voltage and current that the cell produces under these conditions are described. The second ruthenium-cell is better than the first because TBP was used as an additive.

dye voltage current 1cm² mask

voltage /mV 1cm² mask

current / mA percent voltage

percent current

hibiscus in H2O/Eth 1/1 433 2 364 0,5 84 25

hibiscus in H2O/Eth 1/1 455 1,9 390 0,47 86 25

Ruthenium N719 460 6,2 343 2,5 75 40

Ruthenium N719 605 7,4 500 3,5 83 47

The ruthenium dye was able to produce remarkably more current than the hibiscus cell with just one sixth of illumination. We are not really sure how to explain this result but it definitely shows that the ruthenium dye is capable of producing more electrons than the hibiscus dye. Further experiments in this field will follow.

3.2 Long term stability Besides the variation of the cell components the long-term stability of the cells was tested.



3.2.1 Electrical degradation

As already mentioned, a measurement unit to monitor 30 cells at the same time was developed. After one week one could already see an electrical degradation. The fill factor6 of this DSC is very small. The reason for this seems to be the graphite counter electrode – the cells will be measured at the University of Munich to determine whether this is true or not. A thesis was studied, which showed exactly that phenomenon7, but the other components of the cell were different too, so it is not directly similar. Besides the comparison of the two different types of solar cells the electrical degradation of the DSC in the time period of one week is shown. The open circuit voltage decreases and the short circuit current increases – all in all the output power stays the same, but there is some chemical reaction going on in the cell. This sort of degradation was already published in literature8, but there is still no explanation to this effect right now.

3.2.2 Optical Degradation

Apart from the electrical degradation an optical degradation after light exposure can also be observed.

Figure 19 cell 1 after 15 days light exposure; cells 2-4 after about 24 hours light exposure;

cell 4 uses a different dye than cell 1-3 It seems as if many factors combine their effects on the optical degradation. A very obvious explanation would be that the electrolyte evaporated, but this is not an explanation for the bleaching of several parts in the cell. Another interesting aspect is that the bleaching only happens when the cell is illuminated. Sometimes the effect of decoloration even stops. One more interesting effect is that the optical degradation and decoloration still happened when an UV filter is used.



To conduct more detailed research cell No. 1 was opened. On the area where there was still dye, there had been electrolyte too. Furthermore, one can see the yellow colour of the electrolyte – this seems to be elementary iodine that was oxidized in the cell. After the glass electrodes were washed with distilled water, yellow precipitate fell out.

After these first degradation processes were observed, experiments to find out what exactly happens in the cell were developed (see chapter 3.4.4)

3.3 Regeneration of the cell after small area light exposure With the help of the Amag rolling GmbH a small area of one cell was exposed to a very intense and focused light beam of an optical microscope. The cell was illuminated for half an hour and the current was measured under short circuit. The current was rising very strong in the first few minutes and then sinking nearly linear, while one could observe a very strong optical degradation on the illuminated area.

Figure 21 Degradation under spot illumination

Area of illumination

Figure 20 cell 1; picture of the optical degradation (left); opened (middle, right)



The interesting fact about this experiment is that only one day later the optical degradation (brown point) was gone. It looks as if the cell is able to “recover” from light exposure. Effects of regeneration of the cell have already been published9, but there is no explanation for it at the moment. As described in chapter 5.3 we want to see if these effects of degradation and regeneration also happen to the Ru-dye.

3.4 Stability of the dye

3.4.1 Stability test under UV-light

Six Petri dishes with the same amount of hibiscus dye solvent (10ml) were used. Three of the dishes (No. 1-3) were stored under UV light (λ=366nm) and the other three (No. 4-6) in an absolutely dark ambiance. In each 2 Petri dishes 0,5g and 2g of titanium dioxide (obtained from Merck, purity: p.a.) were added. After just 48 hours the Petri dishes with the hibiscus dye showed the following effect:

The dye solvent that was under exposure to UV light showed an extreme degradation. The more TiO2, the stronger the degradation (1:0g TiO2; 2:0,5g TiO2; 3:2g TiO2). After this experiment it is clear that the exposure to UV light has to be prevented by all means.

3.4.2 Stability test under sunlight

The glass already filters a lot of the UV light that comes from the sun. That is why the same experiment as above with sunlight and a glass bottle needed to be performed. 100ml of the hibiscus dye were put into 2 equal bottles and 10g of titanium dioxide (same as above) were added into one of the bottles. A stronger degradation with the use of TiO2 was determined but one could also observe a strong degradation without the use of the photo catalytic substance.

Sample 1 (with TiO2)

0

0,5

1

1,5

2

2,5

300 400 500 600 700 800

w avelength/nm

abso

rptio

n

0 days

5 days

140 days

Sample 2 (no TiO2)

0

0,5

1

1,5

2

2,5

300 400 500 600 700 800

w avelength/nm

abso

rptio

n

0 days

5 days

140 days

Figure 22 Degradation of the dye with TiO2 under UV light



Even after 140 days there is a dye with an absorption maximum at 350nm in the solvent. Surprisingly enough it is still in the solvent with TiO2 too. For the use in the DSC it would be really interesting to find biological dyes which are as stable as this yellow dye.

3.4.3 Degradation of the dye on the TiO2 layer

The dye itself gets already destructed just because of the sunlight. Our team was able to prove that on different glass plates. This degradation also happens due to the UV light of the sunlight.

Figure 23 Degradation of the dye on the titanium dioxide layer

3.4.4 Stability of dye-absorption on TiO2

As already mentioned our team could not really explain the effect of decoloration of the cells at certain areas (chapter 3.2.2). Especially the dependency on illumination was very interesting; therefore an experiment to analyze this phenomenon was developed. A TiO2 coated glass was sensitized with hibiscus dye and split into three pieces. One piece was put into a clear bottle of H2O/ethanol (1/1) and stored under sunlight. The other piece was stored in the same solvent in the darkness. The third piece was just a control standard and was stored dry and illuminated. The results were very interesting – it was observed that the color of the TiO2 layer depends on the exposure of light.

Figure 24 Comparison between the dye-absorption in H2O/Ethanol 1/1 in the dark (left) and under illumination

(right) for 2 hours under sunlight To analyze this phenomenon, a more detailed test was done. The solvent that the titanium dioxide layer lay in was varied: distilled water, ethanol, acetonitrile and petroleum were used.



Figure 25 Dependency of decoloration on used solvent and its polarity

As shown in Figure 25 the effect of decoloration depends on the used solvent and its polarity. The same experiment was done with the Ruthenium dye but no degradation was observed – the color stayed the same. This result will be discussed in chapter 4.2.



4 Discussion

4.1 Efficiency of our low cost cells Contrary to our expectations the power output of the produced low-cost cells was fairly good. Especially the graphite counter electrode showed very good results in comparison to the platinum coated electrode (see chapter 3.1.2). Since there are no certified and calibrated solar simulators in our labs at our school we contacted the Ludwig Maximilian University of Munich to test our cells in their labs. Unfortunately this has not been possible before we had to finish this paper. But those results will be presented at the fair in May in San Jose. Especially the dependency of the efficiencies on the resistance of the TCO glass and the size of the cells will be tested intensively. The company “solaronix” provided very low resistance TCO glass plates that we are going to test in comparison to the cheap heat-isolating window glass that we were using for this project.

4.2 Effect of decoloration Besides the variation of the cell components, a lot of work was done to identify the degradation processes in the cells. One of the most interesting results is the decoloration of the titanium dioxide layers (colored with a hibiscus extract) in a solvent under sun light. This effect depends very strongly on the polarity of the solvent. Our team thinks that there are two possible explanations for this result:

1. The bonding force over the carboxyl group to the titanium dioxide layer is weaker than the one of the ruthenium dye and the hibiscus dye simply dissolves into the solvent.

2. The dye molecule reacts with the solved oxygen (that is just available in polar solvents) and light to a keto form. Through a shift in the resonance frequency of the dye molecule it gets invisible (does not absorb visible light anymore) This theory seems to be more likely since we lately found a report in the literature that shows exactly this sort of degradation in an anthocyanin.

Figure 26 Degradation mechanisms that were reported in the literature10



One would be able to prove these explanations with the help of an IR spectrometer. If there was a substance in the solvent that has not been there before the experiment, the molecule would have to dissolve from the titanium dioxide layer.

4.3 Repeatability of measurement results To be able to compare components in the dye sensitized solar cell just by building a cell and comparing it to another one is very difficult. The fluctuation of the power output of 2 cells that were built in exactly the same way is sometimes too big to draw any conclusions out of the measurement. Not only the variation between two series of cells but also the variation in one series of cells is occasionally too large. This is mainly because of the deviation of the titanium dioxide layer and the thickness of the electrolyte layer. Because of that we often had to repeat one comparison a few times until we were able to draw a reasonable conclusion. In the following table our team wanted to show how big the deviation between two cell series and within one series is. All the cells were built completely identical. After looking for an answer why this first series is the best we have ever built we found out that the titanium dioxide plates were further in the back of our furnace and therefore maybe heated to a higher temperature than normal. Another explanation is that the graphite counter electrodes were heated in a drying oven because the furnace was occupied. Maybe this drying oven heats the plates in a more accurate way than the furnace does. The graphite coating is very sensible for higher temperatures than 200°C.

Date [dd.mm.yyyy] Dye

open circuit voltage /mV

short circuit current /mA

light intensity mW/cm²

02.02.2010 hibiscus in ethanol/water 1/1 460 4 30 02.02.2010 hibiscus in ethanol/water 1/1 450 5,2 30 02.02.2010 hibiscus in ethanol/water 1/1 460 5,3 30

05.02.2010 hibiscus in ethanol/water 1/1 440 2,7 30

05.02.2010 hibiscus in ethanol/water 1/1 450 3,95 30 05.02.2010 hibiscus in ethanol/water 1/1 445 3,35 30 05.02.2010 hibiscus in ethanol/water 1/1 450 2,55 30

4.4 Economic aspects A photovoltaic cell has to be stable for at least 15 years – from an economic point of view. At least this is the request to scientists who develop such cells. But many people simply forget about the most important characteristics of a solar cell:

- Cost for 1 watt electric power - Amortization time in comparison to life time - CO2 balance - Net energy gain (comparison of used energy to produced energy over the life time)

A silicon solar cell is more or less stable, but the production costs are extremely high and need a lot of energy. This is the reason why the characteristic numbers mentioned above are quite bad. Especially the price for 1 watt electric energy is still too high (1.5-2$) in comparison to fossil fuels. DSCs probably will not be stable for 10 years, but the net energy gain in comparison to the costs are responsible for the economical use of a solar cell. At the moment expensive Ruthenium-dyes and platinum counter electrodes are used in DSC’s. Although experts often say they are very cheap, our team does not believe that - this is the reason why low cost materials were used in this report to produce the cells. Prof. Dr. Peter Douglas from the University of Swansea, whom we got to know from the “International Youth Science Forum London”, described the future prospects of the DSC like that:

„All that said I am convinced work on alternative energy sources must continue; we see the price, reliability etc. against a background of very cheap energy. I think the really important analysis is the total energy required to make the cells compared to the energy they will produce over their lifetime. If this ratio is significantly greater than 1 then it is worth doing”



5 Conclusions After a lot of work for this project, we would like to reflect our work and our results:

5.1 Conclusions of the results Over 50 different cells were produced to determine their power output in dependency of the used components. Especially the graphite counter electrode showed very promising results (see chapter 3.1.2). Certified efficiency measurements will take place in April and will be presented at the fair. A very interesting new method of characterizing the dye was presented. The illumination is cut down to a small part of the photoactive area – the other cell characteristics stay the same (cell width, TCO glass resistance, surface area of the counter electrode). The open circuit voltage and short circuit current are measured and compared with the results under full illumination. This percentage showed that the ruthenium dye is capable of producing more electrons than the hibiscus dye under these conditions (see chapter 3.1.5). Furthermore UV stability tests of the dye have been done. It was shown that special anthocyanins are able to withstand the UV light (see chapter 3.4.2). More research needs to be done to find dyes with this stability. Long term stability experiments showed a very interesting electrical degradation (see chapter 3.2.1) in form of a rising short circuit current and falling open circuit voltage. Unfortunately our team had to cut back on these experiments because the electronics was destroyed while transporting the experiment set up to a local fair. Moreover optical degradation was shown as well – experiments were done that showed that there is a degradation of the dye in contact with a polar solvent and illumination. Two possible explanations for this effect were discussed (see chapter 4.2).

5.2 What would we do differently if we repeated this project? Unfortunately the project evolved different than we had expected because the time-consuming long-term experiment set up did not work as well as we expected because it was destroyed. If we repeated this project we would be even more persistent than we have already been. This should mean that we started to get in contact with experienced people were late and we would have saved a lot of time if we had contacted them sooner. We would also buy all the widely-used components sooner (hot-melting foil to seal the cells, expensive low resistance TCO glass) and after testing them, we would have tried our new low-cost components and not the other way around. Furthermore we would have designed a “master plate” with always for example 4 or 5 cells on one plate. It would be possible to calculate the average of those cells and make it easier to compare different components. But we think that all in all we can be very proud of what we have achieved.

5.3 Future Prospects There are lots of tests and experiments we have not done yet. A lot more variations of the cell would be possible, but it would take years to finish the project, if this would even be possible. Therefore we focused on several special issues, which we will investigate within the next four weeks: The first and maybe most important one is to specify the absolute efficiency of our cells. In March we got into contact with the Ludwig-Maximilian University (LMU) in Munich. In the Department of Physics we have the possibility to measure the efficiency with certified measurement equipment. Furthermore we can measure the IPCE value (Incident-Photon-to-electron Conversion Efficiency), which describes the efficiency of the cell in dependency of the wavelength of the irradiated light. We also had contact with the department of Organic and Macromolecular Chemistry. After signing a nondisclosure agreement, we got access to perylene dyes, which were developed by them. Our team will test them in the following weeks if they are capable of producing electrons in a dye sensitized solar cell.



As described in 3.3, a strong optical degradation occurs, when a small area is illuminated. It is not sure which components are responsible for this sort of degradation and regeneration. We will repeat this experiment with different components - especially the ruthenium dye will be tested intensively. The company “solaronix” donated some professional substances (TiO2 suspension, TCO glass, hot-melting foil), that we are going to compare to our low-cost substances.



6 Personal Resume and acknowledgements Well, what did we learn from that project? Of course, it is not always easy to work without money or bad possibilities for literature research. Nevertheless, we learned that the only two things that are absolutely necessary for a project like this are motivation and ambition. Even without lots of resources we worked in a very complex field of science and were able to present new results and ideas. In addition, what is special about this project is that we describe all our results and experiments in detail, hoping that more cooperation in science and a free know-how transfer between all scientists – from students to university professors – can be achieved. We really believe that this would accelerate the development of renewable energy sources. At this point we would like to thank everyone who helped us to get this far. First and foremost our project teacher – Dr. Wolf Peter Stöckl. He was always there for us when we had any questions, encouraged us and helped us to get in contact with experienced people. Thank you very much! Furthermore we would like to thank the following people:

• All chemistry professors at our school who always had an open ear for us • Mr. Casata from the Amag rolling GmbH who helped us making some great pictures of our

titanium dioxide coatings • Mr. Schneeberger who helped us with know-how in the field of glass and glass coatings. • Prof. Langhals, Dr. Esterbauer, Mr. Wiedemann from the Ludwig-Maximilian University • Prof. Peter Douglas from the University of Swansey • Dr. Toby Meier from the company solaronix

Moreover we would like to thank all universities and companys that we had contact with:



7 Literature

7.1 Internet resources • http://www.chemgapedia.de/vsengine/vlu/vsc/de/ch/6/ac/bibliothek/_vlu/iod.vlu/Page/vsc/d

e/ch/6/ac/bibliothek/iod/reaktivitaet.vscml.html • http://www.cmc-klebetechnik.ch/cmc-klebeband-metall.0.html • http://www.sigmaaldrich.com/catalog/ProductDetail.do?N4=142379|ALDRICH&N5=SEARCH_C

ONCAT_PNO|BRAND_KEY&F=SPEC • http://www.fmf.uni-freiburg.de/materialforschung/pg_life/solar/farbstoffsolarzellen • http://en.wikipedia.org/wiki/Dye-sensitized_solar_cell • http://de.wikipedia.org/wiki/Anthozyane • http://www.degussa.de/degussa/de/produkte/produktdatenbank/default.htm?action=details&

page=1&pid=31500&rno=1 • http://www.ise.fhg.de/geschaeftsfelder-und-marktbereiche/solarzellen/farbstoff-und-

organische-solarzellen/materialentwicklung/materialentwicklung • http://www.zukuenftigetechnologien.de/nanotecture/hinsch_praesentation.pdf • http://www.ipht-jena.de/fileadmin/user_upload/Journal/00_Lehre/Photovoltaik_WS08.pdf • https://www.fh-

muenster.de/fb1/downloads/laboratorien/ac/Versuchsvorschrift_Graetzel_Zelle.pdf • http://www.hs-

weingarten.de/home/studiengaenge/pt/de/labore/physikalisches_praktikum_2/pdf/va_solar_farbstoff1.pdf

• http://www.lehrer-online.de/dyn/bin/609426-609469-1-praktikumsanleitung.pdf • http://www.science-forum.de/download/graetzelmittel.pdf • http://www.physik.uni-bielefeld.de/didaktik/Experimente/Bezugsquellen.pdf • http://www.physik.uni-bielefeld.de/didaktik/Experimente/Solar1.pdf • http://www.univie.ac.at/pph/ecophys/photobio/doc/GraetzelZelle.pdf • http://deposit.ddb.de/cgi-

bin/dokserv?idn=968555535&dok_var=d1&dok_ext=pdf&filename=968555535.pdf • http://www.diss.fu-berlin.de/diss/receive/FUDISS_thesis_000000002568 • http://www.jufo-mbs.de/mediapool/50/505700/data/Die_Graetzelzelle_-

_Bundeswettbewerb.pdf • http://www.freidok.uni-freiburg.de/volltexte/2570/pdf/Dissertation_Uli_Wuerfel_2006.pdf • http://www.freidok.uni-

freiburg.de/volltexte/2623/pdf/Sastrawan_Photovoltaic_modules_of_dye_solar_cells_Dissertation.pdf

• http://www.freidok.uni-freiburg.de/volltexte/29/pdf/29_1.pdf • http://biblion.epfl.ch/EPFL/theses/2004/2955/EPFL_TH2955.pdf • http://www.helmholtz-

berlin.de/media/media/oea/web/pr_webseite/druckschriften/berichte/2000/jb_0401_se5u6.pdf

7.2 Scientific Journals • A.Hinsch et.al., Long term stability of dye sensitized solar cells for large area power applications

(LOTS-DSC). 16th European Photovoltaic Solar Energy Conference and Exhibition, Glasgow 2000. 2000

• Dongshe Zhang et. al., Room-Temperature Synthesis of Porous Nanoparticulate TiO2 Films for Flexible Dye-Sensitized Solar Cells. Adv. Funct. Mater. 2006, 16, 1228–1234.

• Raúl Pozas et. al., Building Nanocrystalline Planar Defects within Self-Assembled Photonic Crystals by Spin-Coating. Adv. Mater. 2006, 18, 1183-1187.



• Seigo Ito et.al., High-Efficiency Organic-Dye-Sensitized Solar Cells Controlled by Nanocrystalline-TiO2 Electrode Thickness. Adv. Mater. 2006, 18, 1202–1205.

• Hyung-Jun Koo et. al., Nano-embossed Hollow Spherical TiO2 as Bifunctional Material for High-Efficiency Dye-Sensitized Solar Cells. Adv. Mater. 2008, 20, 195-199.

• Thomas W. Hamann et. al., Aerogel Templated ZnO Dye-Sensitized Solar Cells. Adv. Mater. 2008, 20, 1560-1564.

• F.O.Lenzmann et.al., Recent Advances in dye-sensitized solar cells. Advances in OptoElectronics. 2007, 10 pages.

• Fan-Tai Kong et. al., Review of Recent Progress in Dye Sensitized Solar cells. Advances in OptoElectronics. 2007, 13 pages.

• P. de Almeida et. al., Microstructure Characterisation of titanium dioxide nanodispersions and thin films for dye-sensitized solar cell devices. Appl. Phys. A. 2004, 79, 1819-1828.

• Jinsoo Kim et. al., Sol-Gel Synthesis and Spray Granulation of Porous Titania Powder. Chem. Ing. Tech. 2001, 73, 461-468.

• Pan Xu et. al., Effects of TiO2 Film on the Performance of Dye-sensitized Solar Cells Based on Ionic Liquid Electrolyte. Chin. J. Chem. 2005, 23, 1579—1583.

• DDAI Song-Yuan et.al, Optimum Nanoporous TiO2 Film and Ist Application to Dye-sensitized solar cells. Chin. Phys. Lett. 2003, 20, 953-955.

• Yasuo Chiba et.al., Dye-sensitized solar cells with conversion efficiency of 11.1%. Jpn. J. Appl. Phys., Part 1 . 2006, 45, 638-640.

• P.M. Sommeling et.al, Long-term stability testing of dye-sensitized solar cells. J. Photochem. Photobiol., A . 2004, 164, 137-144.

• Tatsuo Toyoda et. al., Outdoor performance of large scale DSC modules . J. Photochem. Photobiol., A . 2004, 164, 203-207.

• Kenichi Okada et.al., 100mm*100mm large sized dye sensitized solar cells. J. Photochem. Photobiol., A . 2004, 164, 193-198.

• Yutaka Amao et. al, Preparation and properties of dye-sensitized solar cell using chlorophyll derivative immobilized TiO2 film electrode. J. Photochem. Photobiol., A . 2004, 164, 47-51.

• Peng Wang et.al., Charge Seperation and Efficient Light Energy Conversion in Sensitized Mesoscopic Solar Cells Based on Binary Ionic Liquids. J. Am. Chem. Soc. 2005, 127, 6850-6856.

• Daibin Kuang et.al., Co-sensitization of Organic Dyes for Efficient Ionic Liquid Electrolyte-Based Dye sensitized solar cells. Langmuir. 2007, 23, 10906-10909.

• Brian O'Regan and Michael Grätzel, A low-cost high efficiency solar cell based on dye sensitized colloidal TiO2 films. Nature. 1991, 353, 737-739.

• R. Kern et. al., Long term stability of dye-sensitised solar cells for large area power applications. Opto-Electron. Rev. 2000, 8, 284-288.

• Th. Dittrich, Porous TiO2: Electron Transport and Application to Dye Sensitized Injection Solar Cells. phys. stat. sol. (a). 200, 182, 447-455.

• Yoshikazu Suzuki et. al., Direct synthesis of an anatase-TiO2 nanofiber/nanoparticle composite powder from natural rutile. phys. stat. sol. (a). 2007, 204, 1757–1761.

• Hinsch et.al, Long term stability of Dye-sensitized solar cells. Prog. Photovoltaics Res. Appl. 2001, 9, 425-438.

• R. Grünwald et.al, Mechanisms of Instability in Ru-Based Dye Sensitization Solar Cells. J. Phys. Chem. B. 1997, 101, 2564-2575.

• P. M. Sommeling et.al., Influence of a TiCl4 Post-Treatment on Nanocrystalline. J. Phys. Chem. B. 2006, 110, 19191-19197.

• Qing Wang et.al., TiO2 Films in Dye-Sensitized Solar Cells. J. Phys. Chem. B. 2006, 110, 25210-25221.



7.3 Endnotes

1 B. O’Regan, M.Grätzel, Nature 353, 737-739 (1991) 2 L.Han et. al., “High Efficiency of dye-sensitized solar cells and module”, Proceedings of the 4th IEEE World Conference on Photovoltaic Energy Conversion, p. 178-182, Waikoloa, Hawaii, USA, May 2006 3 P.M. Sommeling et.al, Long-term stability testing of dye-sensitized solar cells, Journal of Photochemistry and Photobiology At Chemistry, Vol.164 (2004), p.137-144 4 Jander Blasius, Lehrbuch der analytischen und präparativen anorganischen Chemie, 16.Auflage, S.Hirzel Verlag Stuttgart, Seiten 280, 283, 284. 5 E. Rijnberg et.al, Long-Term Stability of Nanocrystalline Dye-Sensitized Solar Cells, www.ecn.nl/docs/library/report/1998/rx98033.pdf, 1998 6 The fill factor (FF) is the quotient of (Vmp*Imp)/(Voc*Isc) where Vmp and Imp describe the voltage and current at the maximum power point. 7 Hervé Nusbaumer, Alternative Redox Systems for the Dye-sensitized solar cell, thesis, École Polytechnique Fédérale de Lausanne 8 A. Hinsch et al., Long term Stability of Dye sensitized solar cells for large area power applications (LOTS DSC), 16th European Photovoltaic Solar Energy Conference and Exhibition, Glasgow 2000 9 R. Sastrawan, http://www.freidok.uni-freiburg.de/volltexte/2623/pdf/Sastrawan_Photovoltaic_modules_of_dye_solar_cells_Dissertation.pdf, 2006 10 Paxis der Naturwissenschaften – Chemie in der Schule: Anthocyane als Photosensibilisatoren für Titandioxid; Kress, S., Bohrmann-Linde, C., Aulis Verlag Deubner: Köln und Leipzig, 2005, pp. 24-30.

research on the usability of low-cost materials in dye sensitized solar cells

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