Energy from Semiconductors

N.J.Ekins-Daukes.

School of Physics, University of Sydney, NSW 2006.

ned@physics.usyd.edu.au

Conventional Energy Sources

The ever increasing living standards enjoyed by many in the modern world, is a product of technologi-cal innovation and social progress, fuelled by an incredible quantity of energy. Only 150 years ago, the energy consumption per capita was low, and derived from the limited natural resources available to our ancestors. Today, the undeniably welcome improvement in living standards and dramatic increase in population has resulted in a massive and unsustainable energy budget.

In order to discuss energy use in a meaningful way, it is convenient to introduce the concept of an “en-ergy slave”. Consider the energy that the human body can produce, for example peddling a bicycle. A physically fit individual can deliver a power of roughly 200W, and if this individual were to work a stan-dard 8 hour day, then the energy delivered would be 200W x 8H x 60min x 60secs = 5.76x106 J. The energy slave provides a unit of energy that is accessible to a young audience and gives a convenient scale for measuring energy consumption in the home. It also provides a perspective on global energy use. Table 1 compares some common household items and the number of energy slaves required to power them. A refrigerator would require one energy slave working their full 8 hours to provide suffi-cient energy for the full 24H day. Similarly a vacuum cleaner requires five energy slaves to run it, but only for the duration it is operating. A car (in this case a fuel efficient model) requires 150 energy slaves to keep it running. It is instructive to compare the cost of running these appliances, both in terms of conventional fuel cost and cost if powered by an energy “slave” who has been released from bondage and paid $12.30/H for their time.

Appliance Number of Energy slaves

Cost of conventional fuel

Cost if powered by an energy ‘slave’

Refrigerator 1 $0.15 per day $90 per day

Vacuum cleaner 5 for 20 minutes $0.03 for 20 mins $20.50 for 5 mins

Family car 150 $50 for 1 tank $18,500 for 1000km

Table 1. Estimates for the number of energy slaves required to power certain household appliances and the costs that this entails. Appendices A & B contain conversion tables to aid extension of this table to other household items.

The example of the car is particularly interesting. Even a small, low performance car engine requires a colossal quantity of energy, equivalent to over one hundred energy slaves. If we consider a trip from Sydney to Brisbane (approx 1000km), a fuel efficient model such as the Toyota Prius, would require one tank for the trip, costing $50. Paying the 150 energy slaves for the duration of the trip would cost sev-eral tens of thousands of dollars. The pricing seems completely outrageous, but reminds us how the energy that we take for granted in the modern world, is in fact an extremely valuable commodity. Our ancestors who lived prior to the industrial revolution had no means of harnessing such enormous quantities of energy. In those days, a horse was a valuable commodity, equivalent to almost four energy slaves and a well constructed wind or water mill could do the work of 30 energy slaves. It is telling that the average modern household “wastes” the power of approximately two energy slaves, simply by leaving appliances on standby, with the transformers powered.

Moving beyond the household, we find that a typical coal power station generates power on a colossal scale. The Eraring power station, the largest in Australia, houses the equivalent of 13 million energy slaves and delivers one quarter of the electricity supply in NSW. The largest power plant on Earth is the ITAIPU dam in Brazil, producing the equivalent power output of 62 million energy slaves. Austra-lia’s annual energy requirement is 9x1011 energy slave days, corresponding to approximately one third of the world’s population enslaved for 8H per day 365 days a year. On the global scale, Australia’s needs appear modest. Figure 1 shows historical energy usage separated by source: coal, oil, natural gas, nu-clear and renewable. Technological breakthroughs, such as the invention of the steam engine by James Watt in 1765 and the internal combustion engine by Etienne Lenoir in 1860, have lead to the massive demand for oil and natural gas. Nuclear power and renewable energy1 appear as minor contributors to the global energy budget. The worrying aspect of this trend is that our individual thirst for energy is increasing together with the global population. There is a strong link between gross domestic product (GDP) and energy use in developing nations, as their emerging industries and improved living stan-dards must be serviced by energy. In developed nations the link is still present but weaker, as old appli-ances are replaced with more energy efficient models, yet the general increase in electrically powered consumer goods leads to an ever increasing need for electrical power2.

Figure 1. Historical energy use for common fuels plotted together with global population. Modified from Y. Ha-makawa, JASP International, 5 (2002) p30 with additional data from BP’s statistical review of world energy3.

The response to the increased demand for energy has been to extract greater quantities of mainly fossil based fuel from the Earth. It is remarkable that these energy sources exist, they being the product of

1 Mainly hydroelectricity, but includes all forms of renewable energy, such as wind, solar, tidal etc...

2 http://www.eia.doe.gov/

3 http://www.bp.com/genericsection.do?categoryId=92&contentId=7005893

photosynthesis from sunlight that fell on the Earth many million years ago4. Nuclear fuel is older still, thought to be the product of exploding stars (supernova) in the early universe. While these sources of energy are convenient, we will never be able to renew them. Quite apart from environmental concerns regarding CO2 emission and nuclear waste disposal, our reliance upon such fuels will inevitably lead to an energy crisis. The problem essentially stems from a multi-million year time lag between the energy arriving on Earth in the form of sunlight and it being used. The only long term solution is to shorten this cycle by directly harnessing the energy that we receive daily from the sun. Here we consider direct forms of solar energy, but it should be noted that wind and biomass5 also derive their energy from the sun, albeit in an indirect fashion. Tidal and geothermal energy are the only renewable energy sources that are not derived from sunlight.

Solar Energy

In Central Australia, the sun delivers an energy equivalent to 6 energy slave days per square metre per day. In other words, each square metre has the potential to deliver the work of six energy slaves, each working their eight hour day. To put this in perspective, Australia’s annual energy expenditure falls as sunlight over an area of 395 km2, roughly half the area of the ACT. As solar panels typically have an energy pay-back time of one to three years and are guaranteed for up to thirty years, they offer the prospect of gaining at least ten times the energy required to make them.

The efficiency of solar energy collectors can be extremely high. For example, a solar thermal system that heats hot water can be 80% efficient. Considering that the average household typically employs 7 energy slaves just to heat water, the prospect of heating this water using sunlight is very appealing. The key features of an effective solar thermal system are a black absorber that absorbs most of the sunlight and a heat exchanger that efficiently transfers the thermal energy to water flowing through the absorber. The energy is then stored, as hot water, in a tank awaiting use. In general the systems avail-able commercially are mature, inexpensive, reliable products6.

Photovoltaic solar panels generate electricity from the sunlight that falls upon them. While power con-version efficiencies of up to 39% have been demonstrated in the laboratory, premium commercial panels attain an efficiency of 20% while more economical models operate around 15%. In practice there is usually sufficient roof area that even a 10% power conversion efficiency is sufficient to supply the electrical needs of most houses. The barrier to widespread photovoltaic deployment is the high cost of the panels and retrofitting into existing roofs. Photovoltaic solar panels are currently retailing at around AU$6/Wpeak7, meaning that a panel capable of generating 100W under full sunlight8 costs AU$600. A residential system would typically require 3kWpeak (~15 energy slaves), costing around AU$18,000 for the panels alone. The cost of retrofitting into an existing building often doubles this cost. Sustained market growth for photovoltaic panels of 30% per annum in Japan and Germany has lead to a reduction in the price of panels, mainly through mass production, although present panel de-signs are limited by the cost of the silicon wafers that they use at present. Several effective technologies

4 Coal dates back 280 million years, oil roughly 180 million years and gas about 100 million years.

5 Examples of biomass fuel include wood from sustainable forestry, ethanol from sugar cane, biodiesel.

6 For examples see, http://www.solahart.com.au/ or http://www.rheem.com.au

7 http://www.solarbuzz.com/

8 Full sunlight corresponds to approximately 1000W/m2

have been developed in Australia to either minimise the quantity of silicon required9 or that enable photovoltaic solar panels to be made inexpensively on glass10.

Photovoltaic solar cells are generally formed from a semiconductor p-n junction, although any system with the following properties is sufficient for making a photovoltaic solar cell: (A) There is an energy gap (band-gap) across which electrons can be excited. (B) There are selective contacts are made to the electron’s excited and ground state. In a typical semiconductor solar cell, the band-gap between the conduction and valance bands satisfies requirement A, while the n and p type doping ensures that ex-cited electrons are collected at one contact and fed back in at the other. Figure 2a below shows a sim-plified schematic of a general solar cell, where the negative (electron) contact is made to the conduction band and the positive (hole) contact is made to the valance band. The conduction band typically has few electrons it it, while the valance band is usually filled with electrons. Photons incident from the LHS are absorbed only if their energy is greater than the band-gap energy of the semiconductor. The energy of the photon is transferred to an electron, exciting it from the valance band to the conduction band, leaving a hole (absence of an electron) behind in the valance band. In the present discussion we adhere to the conventional language of electrons and holes, however, it is equally valid (but unconven-tional) to consider just the flow of electrons and this would entail reversing the lower horizontal arrow in figure 2a. The excited electron (and hole) will diffuse around the cell and one of two things will happen, either: (1) if the electron (and hole) reach the solar cell contacts, they will contribute to the current flowing in the cell, (2) the electron (and hole) recombine and transfer their energy back to a photon again. The likelihood of these two processes is determined by the bias voltage across the cell. As shown in the current-voltage curve of figure 2b, if the solar cell terminals are shorted (V=0), then process (1) domi-nates, most of the electrons (and holes) reach the contacts, leading to a large current called the short-circuit current (Isc). However, when the bias voltage is raised, the current drops rapidly corresponding to the onset of process (2). At some point all the electrons and holes that were generated by the sun-light, recombine radiatively again, giving no net current. This point is called the open-circuit voltage (Voc).

Figure 2, Schematic operation of a solar cell and the associated IV curve, showing the short-circuit current (Isc), open circuit voltage (Voc) and maximum power point.

Power is delivered by the solar cell between 0<V<Voc, but reaches reaches a maximum close to Voc, where the current x voltage product is maximum; this is called the maximum power point (Pmax). In general solar cells are operated at Pmax in order to extract power from them as efficiently as possible. Interestingly if the solar cell is biased beyond Voc (V>Voc) then the solar cell behaves as a light emit-

9 Sliver Solar Cell : http://www.originenergy.com.au/environment/environment_subnav.php?pageid=1233

10 CSG Solar : http://www.csgsolar.com/

ting diode. Current is fed into the solar cell, as indicated by the positive current in figure 2b and the horizontal arrows in figure 2a are reversed. Although photovoltaic solar cells and light emitting diodes are engineered differently, they are very similar from a physical standpoint. Shining light onto an ap-propriately biased LED will result in power generation! However the observation of light emission from common silicon solar cells is rare, mainly because the indirect band-gap of silicon makes light emission weak and what little light is emitted is in the infrared and therefore not visible.

It should be noted that although most common solar cells use p/n semiconductor junctions, it is not a pre-requisite for their operation. The discussion above assumes a p/n junction is used to achieve the electron and hole contacts shown in figure 2a, but there are alternative ways of achieving the same process. Appendix C contains notes written by Helen Smith in her first year of undergraduate studies, describing a dye sensitised solar cell that mimics photosynthesis. Here sunlight is absorbed by a dye, and electrons and holes separated by engineering energy levels that align with the ground and excited state of the dye. A particular merit of this solar cell is that a very simple variant can be fabricated by high-school students within one hour. Further details on the recipe for making this solar cell can be found in appendix D.

Solar cells such as the conventional silicon based devices and possibly advanced cell designs such as the dye sensitised solar cell offer hope that one day we will be able to generate the energy we require from the sunlight we receive. What is certain is that there is plenty of sunlight.

Acknowledgements

NJED is grateful to Dr Chris Dey for introducing him to the idea of an energy slave and for providing many useful energy statistics. Helen Smith is thanked for her enthusiasm in developing the dye sensi-tised solar cell demonstration.

Appendix A - Power unit conversion tableUnit Description SI Unit

Equivalent / WEquivalent Number of

Energy Slaves

1 W Watt 1 0.005

200 W Energy Slave 200 1

1 hp Horsepower 736 3.68

1 kW kilo-Watt 1000 5

Appendix B - Energy unit conversion table

Unit Description SI Unit Equivalent / J

Equivalent Number of Energy Slave Days

1 J Joule 1 1.74x10-7

1 kJ kilo-joule 1000 1.74x10-4

1 kCal kilo-Calorie 4187 7.27x10-4

1 MJ mega-Joule 1x106 0.174

1 BTU British Thermal Unit 1.055x106 0.183

1 kWh kilo-watt-hour 3.6x106 0.625

5.76 MJ Energy Slave Day 5.76x106 1

1 bboe Barrel of Oil Equivalent 6.12x109 1060

1 TCE Tonne of Coal Equiva-lent

2.93x1010 5090

1 TOE Tonne of Oil Equivalent 4.19x1010 7270

1 MTOE Million Tonnes of Oil Equivalent

4.19x1016 7.27x109

As oil prices hit a new all-time high and fossil fuels continue to

increase global warming, the quest to find a suitable renewable energy source is becoming increasingly urgent. Yet wind power is unreliable, hydroelectric systems spoil natural landscapes and nuclear power has associated heath risks. In central Australia greater than 24MJ/m2 day of solar energy is received (fig 1) and attention is now being directed at how we can use it [1].

The basis for all photovoltaic devices is the separation of charge at an interface of two materials that have different conduction methods (Grätzel M., 2000). In conventional cells, this is between n- and p- type semiconductor: although reasonable efficiencies of over 30% have been achieved in the laboratory (Cotal et al., 2000), typically the conversion rate is around 15-20% (Bignozzi, et al., 2000, Zweibel, 1993, 1990). However the widespread use of silicon and compound semiconductor solar cells is impracticable due to their expensive and complex manufacturing process. Toxic chemicals are used during manufacture, and they show a decrease of approximately 20% in the conversion of incident photons to electrons over 20-60°C temperature range (Nazeeruddin et al., 1993).

The dye-sensitised solar cell (DYSC) was developed by Gratzel and coworkers (O’Regan & Gratzel, 1991) and uses the principle of photosynthesis to generate power; the boundary in a DYSC is between a wide band gap semiconductor and electrolyte solution. In solid-state devices light absorption and charge movement both occur on the semiconductor, whereas the two functions are performed by different materials in the DYSC (Späth et al., 2003); this has opened up a new mechanism for capturing solar energy.

In a conventional solar cell, electron-hole pairs must travel a

considerable distance without recombining to contribute to the current in the external circuit. As a result, expensive high-purity materials must be used to avoid premature recombination (Hart, J. 2003). Conversely, a DYSC alters the wide band gap semiconductors by chemically attaching a redox dye. This dye absorbs light, and positive and negative charge separation occurs across the dye/semiconductor interface; confined within these materials, the charge carriers are transported. Hence cheaper, lower purity materials can be used and the conflicting requirements for the semiconductor band gap are avoided: an optimum band gap must be obtained – a narrow band gap often indicates relatively weak chemical bonding resulting in a solar cell with only a short lifetime that easily photocorrodes, whereas a large band can only absorb high-energy ultra-violet (UV) photons (Grätzel M., 2000).

With these limitations in mind, research has turned to the nature for inspiration: the DYSC mimics photosynthesis. DYSCs have achieved greater than 9% sunlight to electrical power conversion efficiencies and greater than 16 mA/cm2 photocurrents (Smestad et al., 1994, Nazeeruddin et al., 1993), which is a remarkable achievement compared to 1% for tropical rainforest flora (Smil, 1992) and 13% for the calculated upper limit of natural photosynthesis (Bolton, 1991). Over the past ten years conversion efficiencies have risen to 11.5% (Green, 2002), and efficiencies can be expected to increase with continued

Appendix: C

Blueberry Power ;

Pho tosynthe t i c Ele ctr i c i ty

By Helen Smith

Figure 1: Australian Government, Bureau of Meteorology: Australia’s Daily exposure to solar energy

INTRODUCTION: Shonky Solid-State Solar Cells

THE DYE SENSITISED SOLAR CELL (DYSC): a fruitful future?

research. The DYSC is a serious competitor to solid-state devices and is a commercially realistic option for using solar energy.

Using a photosynthetic mechanism, a plant converts the Sun’s

radiant energy into the chemical into carbohydrates (Knox et al., 2005), by directing an electron through a transport system and extracting useful energy as the electron falls from an excited state back to ground. Despite its complexity, photosynthesis can be summarised by the following equation: 6H2O + 6CO2 + light � C6H12O6 (glucose) + 6O2 (BIOL1101 Lab Notes, 2005). A photosynthetic pigment absorbs a specific wavelength of light and uses this energy to excite an electron. In turn these electrons synthesise dihydro-nicotinamide-adenine-dinucleotide phosphate (NADPH), a molecule that eventually produces a carbohydrate (fig 2). To regenerate the pigment to its original state, an electron is donated in the oxidation of water to produce oxygen (Bering, 1985).

Essentially a DYSC is photoelectrochemical cell containing an

electrolyte and two electrodes that produce electrical current by redox reactions that are driven by light. The operating principles of the DYSC in some respects parallel those of photosynthesis: both form a regenerative cycle that converts light into useful energy forms, and use a multilayer structure (similar to the thylakoid membrane) to enhance both the light absorption and electron collection efficiency. In the DYSC, the organic dye replaces light absorbing pigments; the wide band gap semi-conductor replaces oxidised NADPH and carbon dioxide as the electron acceptor; and the electrolyte replaces the water and oxygen as the electron donor and oxidation product, respectively (Smestad & Gratzel, 1998).

However, the key difference between the DYSC and plants is that plants store the energy in the form of starch for later use, whereas the DYSC cannot store energy. Currently research is being directed at inventing a device that incorporates both photoelectric and storage functions in a single cell structure or photocapacitor (Miyasaka & Murakami, 2004).

HOW THE DYSC WORKS: blueberry electricity A redox dye is chemically attached to the surface of the DYSC

(fig 3), and the absorption of incident light is determined by the number of dye molecules attached per unit volume of the semiconductor. If the dye is attached to a flat surface less than 1% of incident light is absorbed, and the conversion efficiency of light into useful energy is low (Sommeling et al.,, 2000). Light absorption is maximised by the use of sintered nanometre-sized anatase titanium dioxide; the surface area is increased by two or three orders of magnitude above the projected area of the film (Heij, 2002). This structure has pores in the range 20-500Å in diameter, and provides a huge surface area where absorption processes and electronic conduction can occur (fig 4). It is advantageous to use titanium dioxide because it is abundant, cheap, biocompatible and non-toxic (Gratzel & Hagfeldt, 2000). The anatase phase of titanium dioxide is used because it has a suitably wide band gap that it is transparent to visible light – this ensures that light is only absorbed by the dye – and can provide a useful cell voltage (Hagfeldt & Grätzel, 1995). The ideal titanium dioxide film thickness is between 5 µm and 20 µm (Grätzel, 2000): this is a compromise between maximizing surface area, and minimizing recombination losses. A larger film thickness means that the electrons must travel a greater distance before transferring to the conductive layer of the titanium dioxide. On the other hand, the film must be thick enough to give a sufficient surface area for good light absorption.

A photo-induced electron from the dye is injected into the

semiconductor conduction band; this induces a charge separation. The electrons travel in this conduction band, via an

Water

thyla-koids

Light

oxygen

ATP NADPH

NADP+ ADP + Pi

Carbon dioxide

CH2O Starch

stoma

Chloroplast envelope

Figure 2: Photosynthesis

Figure 3: DYSC construction

Figure 4: Electron micrograph of

Titanium dioxide in the nanocrystalline

form

PHOTOSYNTHESIS: power to the plants

DYSCs vs TREES: artificial photosynthesis DYSCs vs TREES: artificial photosynthesis

HOW THE DYSC WORKS: blueberry electricity

external circuit (where they can do work) to the electrolyte solution or ‘charge collector’ (fig 5). To restore the original state of the dye, and prevent the electron recapture by the oxidized dye, an electron is donated by the electrolyte solution: 3I-+I3-

+e-. Often this solution consists of an organic solvent containing a redox system, such as the iodide/triiodide couple. In turn, the iodide is restored by the reduction of triiodide at the counterelectrode (made from conductive glass coated with a catalyst-usually platinum), by an electron that has completed a circuit via the external load: I3-+e-3I-.

DYSC DYES: a healthy alternative Commercially produced DYSC use ruthenium bipyridyl–based

dyes1 (N3 dyes) and typically achieve conversion efficiencies of 10% (Nazerruddin, et al.,, 1993); they achieve excellent conversion of incident light photons to conduction band electrons in the titanium dioxide for wavelengths in the range of 510 nm to 570 nm (Nazeeruddin et al.,, 1993), and adequate conversion between 450-650 nm. This absorption spectrum of the dye overlaps well with the diffuse sunlight spectrum, which means that DYSC can be used indoors and in poor weather conditions. However, such dyes are hard to synthesise and are expensive (Cherepy et al., 1997).

It is also possible, and significantly cheaper2, to generate a significant photocurrent using natural anthocyanin dyes that are extracted from berries as natural water-based substitutes (Tennakone, 1995, 1997a, 1997b). Such dyes are responsible for the red and purple colours of fruit, and biologically serve to attract insects and protect leaves from UV damage (Martin, 1995). The adsorption of cyanin to the surface of TiO2 is a rapid reaction; an OH- counterion is displaced from the Ti(IV) site that combines a proton donated by the anthocyanin molecule (fig 6). This strong chemical affinity is one reason that the fruit dyes work effectively in the DYSC.

1 Ru(II)L2(NCS)2, where L is 2,2'-bipyridyl-4,4'-dicarboxylic acid. 2 Dysol supply 100mg of a ruthenium bipyridyl–based dye for $220,000! [2]

METHODS AND RESULTS: DYSC construction A commercially bought titanium dioxide coated glass slide was

stained with a berry dye (e.g. raspberries, blueberries, beetroot); the dye was made by crushing the fruit (or leaves) and purifying them in a solution of methanol, acetic acid and water (see Smestad & Gratzel (1998) for methods). This slide was washed with water, and dried with propanol, and a transparent conducting glass slide (tin dioxide coated) was secured over it using metal clips. The electrolyte solution (0.5M potassium iodide and 0.05M iodine in ethylene glycol) was drawn up into the porous titanium dioxide structure (see fig 7). Raspberry, blueberry, beetroot and orange leaf cells were constructed.

All cells produced a photocurrent when a voltage was applied (fig 8), but its magnitude varied between the dyes; the region of negative current and positive voltage represents photocurrent activity. It was found that blueberries were most efficient –they produced a photocurrent of 0.2mA (~0.02% efficiency) - followed by raspberries, then beetroot. The orange leaf dyed cells produced virtually no current. Initially the blueberries, raspberries and beetroot showed improvement, whereas there was none in the orange leaf dye. Figure 9 shows short circuit current verses time; there was significant improvement over the first 9 hours, before degradation began. There was improved photocurrent as the concentration of electrolyte solution was increased for raspberries, blueberries and beetroot, but not for the orange leaf cell.

Figure 5: How the DYSC works

Figure 6: Chelation process of dye to Titanium dioxide

Figure 7: A blueberry DYSC

DYSC DYES: a healthy alternative

METHODS AND RESULTS: DYSC construction

DISCUSSION: what’s going on? The 2nd half of figure 9 – the degradation process – is hard to

predict since many factors such as electrolyte degradation, oxidation of fruit and fruit decomposition by bacteria are contributing. However, it is likely that the improvement process is independent to the ruin degradation process, and the only reason a decrease in efficiency is observed is because this degradation becomes the dominant process (fig 10). The improvement process can be explained by looking at the energy levels associated with the titanium dioxide, the dye and the electrolyte solution (fig 11).

Consider the blueberry cell: the ease with which an electron moves through the system is determined by the energy barriers, or band gaps that must be overcome (fig 12). The electron must have a significant energy to transfer from the electrolyte to the dye. As the cell is exposed to the light from the lamp the cell heats up, causing evaporation of the electrolyte solution. This increases the concentration of KI and I2, and the energy level of the valence band is raised, making the transfer of electrons to the dye a far more favourable process. This explains why the photocurrent improved when placed underneath the light. Notice that the conduction band of the blueberry dye lies above that of the titanium dioxide conduction band. Hence it is energetically favourable for the electrons excited from the valence band into the conduction band of the dye to be transferred to the conduction band of the titanium dioxide.

Turning to the orange cell, the conduction band of the orange cell lies beneath that of the titanium dioxide (fig 13). Few electrons can transfer to the titanium dioxide, and a small photocurrent is produced. Even when placed underneath light, and the concentration of the electrolyte increases, this has no effect since the electrons still cannot make the transfer from the dye to the titanium dioxide.

Figure 8: Short circuit Current vs. Voltage

for a typical raspberry cell

Figure 9: Short circuit Current vs. Time for a typical blueberry cell

Figure 10: Model to predict improvement and degradation of

DYSC. In red is the predicted improvement if degradation was absent

Figure 11: Energy levels associated with the different components of the DYSC

DISCUSSION: what’s going on?

CONCLUSION: moving towards the light In choosing materials to construct a solar cell it is

important to consider the energy levels of such components. A cell that produces a large photocurrent has a dye that is energetically well suited to the titanium dioxide with a conduction band energy level that is slightly higher than the titanium dioxide conduction band level. Similarly, the concentration of the electrolyte solution is energetically well suited to the dye (Bisquert et al., 2004). For future work on anthocyanin dyes such as raspberries, blueberries, beetroot, the optimal electrolyte concentration of electrolyte needs to be established quantitatively by varying the concentration of potassium iodide or iodine and observing the changes in photocurrent. Also, to further confirm the band-gap model, it must be proved that temperature does not influence the improvement process in any other way than increasing the electrolyte concentration.

A significant shortcoming of the DYSC is leakage of the electrolyte, which reduces the lifespan of the cell, and technological problems associated with devices sealing up and long-term stability (Man Gu Kang, et al., 2003). Although a solid electrolyte replacement such as XXX is possible, where (on absorption of light) the dye would inject an electron into the titanium dioxide and a hole into the solid electrolyte, the efficiency has been very low. This is due to poor penetration of the solid into the pores of the titanium dioxide (Spiekermann, et al.,, 2001), allowing only a small surface area contact for electron transport. Currently there is research into the use of polymer gel

to quasi-solidify the liquid electrolytes (Ren et al.,, 2001; Kubo et al., 2001; Nogueira et al., 2001). The addition of Poly(viny1idene fluoride-co-hexafluoropropylene) to the KI/I2 electrolyte has improved the fill factors and the energy conversion efficiency of the DYSC by about 17 % (Man Gu Kang, et al.,, 2003).

The DYSC has the potential to become an economically viable

method of using solar energy commercially. Advantages of the DYSC over conventional solar cells include that it: does not need ultra-pure substances; uses low-cost materials and processes that have little environmental impact; achieves reasonable efficiencies which are expected to improve with more research; long-term stability of 10-20 years (Grätzel, 2000, Grätzel, 2001). There is flexibility in the choice of material for each component, allowing the properties to be adjusted and optimised for particular applications.

As fossil fuel resources dwindle, other methods such as solar energy must be considered: with improvements to increase their efficiency, extend their lifetime and reduce costs, the DYSC provides an economically viable solution, and could direct us to using solar power regularly.

ACKNOWLEDGEMENTS Special recognition must go Dr Nicholas Ekins-Daukes, my

supervisor, for all his efforts, support and patience throughout my project. I’d also like to thank Dr Mathew Boreland, George Brawley, David Young and Professor Dick Hunstead.

TiO2 Dye Electrolyte

e-

e-

TiO2 Dye Electrolyte

e-

e-

Figure 12: Energy levels in a blueberry DYSC

Figure 13: Energy levels in an orange DYSC

CONCLUSION: moving towards the light

ACKNOWLEDGEMENTS

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Longo, L., Nogueira, A. F., De Paoli, M.A., Cachet, H., 2002. Journal of Physical Chemistry B, 106(23), 5925. Martin, H.-D. Chimia 1995, 49, 45. Kang, M.G., Park, N-G., Kim, K-M., Ryu, K.S., Chang S.H., Kim, K.J., 2003. Highyl efficient polymer gel electrolytes for fye-sensitized solar cells. 3rd World Conference on Phorovolroic Emru Conversion . May 11-18, 2003 Osnh, Japan

Miyasaka, T., Murakami, T.N., 2004. The photocapacitor: An efficient self-charging capacitor for direct storage of solar energy. Applied Physics Letters, 85, number 17.

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Nogueira, A.F., De Paoli, M.A., Montanan, I., Monkhouse, R., Nelson, Durrant, I., 2001. J. Phys. Chem. B, 105,7417

FURTHER READING Kang, M.G., Park, N-G., Kim, K-M., Ryu, K.S., Chang S.H.,

Kim, K.J., 2003. Highyl efficient polymer gel electrolytes for fye-sensitized solar cells. 3rd World Conference on Phorovolroic Emru Conversion . May 11-18, 2003 Osnh, Japan

Miyasaka, T., Murakami, T.N., 2004. The photocapacitor: An

efficient self-charging capacitor for direct storage of solar energy. Applied Physics Letters, 85, number 17.

O'Regan, B., and M. Gratzel, 1991. A low-cost, high efficiency

solar cell based upon dye-sensitized colloidal TiO2 films. Nature 353, 737-740

Smestad, G., Bignozzi, C., Argazzi, R, 1994. Sol. Energy Mater.

Sol. Cells, 32 , 259. Smestad G.P., Gratzel. M., 1998. Demonstrating Electron

Transfer and Nanotechnology: A Natural Dye–Sensitized Nanocrystalline Energy Converter. Journal of Chemical Education, 75 (6).

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O'Regan, B., and M. Gratzel, 1991. A low-cost, high efficiency solar cell based upon dye-sensitized colloidal TiO2 films. Nature 353, 737-740.

Ren, Y., Zhang, Z., Gao, E., Fang, S., Cai, S., 2001. J. Appl. Electochem., 31,445

Smestad G.P., Gratzel. M., 1998. Demonstrating Electron Transfer and Nanotechnology: A Natural Dye–Sensitized Nanocrystalline Energy Converter. Journal of Chemical Education, 75 (6).

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WEBSITES

WEBSITES [1] http://www.bom.gov.au/gms/IDE3GS01.gif

[2]https://secure.ntechmedia.com/sites/dyesol/index.php?template=Main&catid=2

School of Physics, University of Sydney 2006.

METHOD – Part 1 1. Choose a fruit (blueberries, raspberries, or beetroot) 2. Crush fruit using mortar and pestle 3. Make up a solution of 25mL methanol, 4mL acetic acid and 21mL water

in the beaker 4. Add the methanol/water/acid solution to fruit and crush fruit again 5. Filter solution into a second beaker 6. Using tweezers, place slide titanium dioxide side up, and leave for an

hour NOTE: DO NOT TOUCH TITANIUM DIOXIDE PART WITH FINGERS!

Fruit Based Solar Cells

WHY GO SOLAR? As oil prices hit a new all-time high and fossil fuels continue to increase global warming, the quest to find a suitable renewable energy source is becoming increasingly urgent. In Central Australia up to

34x106 J/m2/day of solar energy is received and attention must be focused on using it.

WHY IS THE DSSC ADVANTAGEOUS? DSSC have a simpler production line, use cheaper materials, and have less environmental

impact than conventional silicon based solar cells

EQUIPMENT Mortar and pestle Measuring cylinder Fruit Filter Filter paper 2 x Beaker Titanium dioxide glass slide Pt coated Glass slide Tweezers

SUMMARY: The fruit dye has been purified and left to chelate (chemically bind) to the porous titanium dioxide structure. Once the dye is suitably attached, it can absorb sunlight, and initiate the charge

flow (a photocurrent) that can power electrical equipment.

Iodide electrolyte solution : prepared from 0.5M potassium iodide mixed with 0.05M iodine in anhydrous ethylene glycol. 2 x Crocodile Clips Beaker with water Tweezers

Appendix D

Isopropanol Methanol Acetic acid (99%) Multimeter Towelling paper Lamp

School of Physics, University of Sydney 2006.

METHOD – Part 2 1. Iodide electrolyte prepared from 0.5M potassium

iodide mixed with 0.05M iodine in anhydrous ethylene glycol.

2. Remove cell from dye solution using tweezers 3. Wash in water, then isopropanol 4. Dry, using towelling paper 5. Place glass slide partially over titanium dioxide slide

(fig 1) and squeeze edges together with fingers. 6. Put 3 drops of electrolyte solution on join of the

two glass slides: you will see the titanium dioxide soak up the electrolyte solution

7. Secure the glass slides together with clips 8. Connect cell to multimeter and measure short circuit

current and open circuit voltage - use aluminium foil to make contacts (see figure 1)

Figure 1: DSSC

I- I3-

Load

Titanium dioxide Dye

Electrolyte

TiO2 and glass

Glass electrode

EXERCISE: Draw the flow of electrons on figure 2 using this information:

1. Sunlight excites electrons in the dye 2. Electrons move into the conduction band of the titanium dioxide 3. Electrons move to counter electrode via an external circuit 4. This electron reduces tri-iodide to iodide at the counter electrode: 1/2I3

- + e- 3/2I- 5. The dye remains electrically neutral because the electrolyte oxidises at the dye: 3/2I- 1/2I3

- + e-

Figure 2: how the DSSC works

FIND OUT MORE: http://en.wikipedia.org/wiki/Dye-sensitized_solar_cells http://www.dyesol.com Smestad G.P., Gratzel. M., 1998. Demonstrating Electron Transfer and Nanotechnology: A Natural Dye–Sensitized Nanocrystalline Energy Converter. Journal of Chemical Education, 75 (6). O'Regan, B., and M. Gratzel, 1991. A low-cost, high efficiency solar cell based upon dye-sensitized colloidal TiO2 films. Nature 353, 737-740. http://www.solideas.com/

Figure 8: Short circuit Current vs. Voltage curve for typical DSSC

for a typical raspberry cell

Aim for open circuit voltage of 0.2V

Aim for short circuit current of 0.2mA

TiO2 coated glass & transparent Pt conductive glass from DyeSol, PO Box 6212 Queanbeyan, NSW 2620. Tel: 02 62991592 Fax: 02 62991698

All chemical reagents from Sigma-Aldrich Pty. Ltd. PO Box 970, Castle Hill, NSW 1765. Tel: 02 9841 0555

Suppliers in Australia: