Our Energy Future: Lecture by Prof Tom Meyer

Prof Tom Meyer, Energy Frontier Research Centre, University of North Carolina, was in Dublin to participate in a Dublin Region Higher Education Alliance Master Class on Solar Energy. Afterwards, he gave a public lecture on “Our Energy Future: Science, Technology and Policy Challenges for the 21st Century – A US Perspective“. The lecture was held at TCD, and was sponsored by the Royal Society of Chemistry Republic of Ireland Local Section. It considered the various current and future world energy demands, and the role renewable energies have to play in providing this energy. My summary is given below.

Prof Thomas J Meyer has been researching the photochemistry of ruthenium complexes since the late 1960’s. Much of what we know about electron transfer in ruthenium polypyridyl complexes today is due to work conducted by Meyer and others in this period. Meyer worked with Henry Taube, who won the Nobel Prize in 1983 “for his work on the mechanisms of electron transfer reactions, especially in metal complexes”, publishing a paper with him in Inorganic Chemistry (1968) on excited state oxidation potentials of ruthenium-amine complexes. This work was an important pre-cursor to a 1973 paper published by Taube, Meyer and co-workers on the reduction of oxygen by these complexes. In the mid-1970’s, at a time when the oil crisis of the time was reaching a peak, Meyer published a series of important papers in Journal of American Chemical Society on the nature and kinetics of quenching of ruthenium amine complexes (including ruthenium – tris-bipyridyl) which gave great kinetic and mechanistic insight into the electron transfer between the metal complexes and an array of quenchers. Meyer reiterated in an article written in 1975 the importance of understanding electron transfer in the study of energy conversion, especially so with metal complexes as these absorb strongly at wavelengths of solar interest.

Prof Meyer, speaking at TCD on "Our Energy Future"

A surge of interest in these systems was observed the oil crisis, which faded somewhat in the 80’s and it wasn’t until Gratzel’s work on dye-sensitised solar cells, reported in 1990, that generated efficiencies that would allow for devices to become realistic contributors to energy supply. Since that itme, work has been concentrating on enhancing light absorption capacity, currently champoined by a ruthrnium dy “N3” (see DSSC post), as well as considering and optimising electron transfer processes in the solar cell devices.

Meyer’s lecture in TCD considered the current and future status of energy demands. It was a message he has delivered to the american political system, across administrations, during his tenure at the Los Alamos National Laboratory. Meyer reported that in the US, energy costs make up 7 – 10% of the cost of living, and 7% of overall world trade. A large demand in energy increase has been observed since 1900’s and this surge is expected to continue until at least 2100. While current stable economies’ energy usage will level off, emerging and transitional ecomomies (China, India, etc) will place major demands on the world’s energy supply. In the six years since 1999, China and India increased their energy usage by 80% and 25%, respectively (Cicerone). (A presentation by Cicerone, Preseident of the National Academy of Sciences is reference below and places thes enegy demands in context). In summation, >100 TW of additional ‘clean’ energy will be required by 2100.

The US currently uses 26% of the world’s oil supply, greater than the next five net using countries combined. 26% of the world’s oil is in the middle-east. Globally, the cost of oil is increasingly expensive to extract, as reserves become more and more difficult to source. Therefore additional energies from alternate sources is required to factor the loss in and increasing expensive of oil production; as well as the surge in energy demand from emerging economies. In addition, this energy supply must be in the context of envrironmental considerations, primarily global warming.

Meyer outlined several strategies to large scale energy production. Principal among these were nuclear, solar, and clean hydrocarbons. These and others are considered below.

Coal currently supplies 27% of the world’s energy demands, including half of US energy needs. It is also responsible for 35% of US carbon dioxide emissions. In principle, it could provide increased energy requirements until 2050, if 1% of GDP was used in dealing with carbon dioxide sequestration. The story of coal usage inclues the story of FutureGen – an initiative announced by the Bush administration in 2003. This was aimed at using coal as a clean fuel, with achieved targets of 275 MW of energy production with 90% carbon dioxide sequestration. However, the project was cancelled by the Bush administration in Jan 2008, due to massive cost overruns ($900M). The Obama adminsitration has restarted this work (June 2009), recognising that clean coal will be a crucial element to supplying energy demands in the forseeable future. Oil shale and tar sands are estimated to contain 2 trillion barrels of oil. However, it expensive (requireing a lot ofwater) and enviornmentally damaging to extract oil from these reserves.

Hyrdogen fuel is obtained from a variety of sources – primarily methane, but also from coal extraction and water electrolysis. In the latter case, electrolysis of water to produce hydrogen (and oxygen) is utililised by photochemical processes. Meyer identified the Idaho National Laboratory hydrogen programme as one which was making good progress in the production of hydrogen as a mass fuel. The advantages of hydrogen were good efficiency, and water and heat as emission products. However, the current costs (for transportation) are ca. $3500/kW, with a target of $35/kW. Another significant problem with the use of hydrogen was storage and transportation, which were expensive because of the nature of the fuel.

Nuclear energy provids ~20% of US energy, and increased usage would result in a significant decrease in greenhouse gases. There are 44 nuclear reactors currently being built internationally, and therefore these will be significnat contributors in to the future. The issues, well know, of nuclear power are what to do with waster, control (political issue), reprocessing and general safety issues.

Renewable energies provide an alternative approach to the solution. It is estimated that wind could provide 20% of US energy requirements. However, solar energy is a real viable option, given that 26,000TW per year of sunclight isiincident on the Earth’s surface (net amount after absorption etc). the technology is on the cusp of mass implementation, with some lingering problems regarding efficiencies. (In the US, there are also problems regardingthe arrangement of the national grid (see Grid 2030 project). Current estimates are that solar generation of 3 TW, assuming 10% efficiency solar cells, would cost approximately $60 Trillion (covering an area of 57k sq – miles). Current and future work will be focussed on reducing this cost.

Meyer reiterated the point in his talk, and again in questions, that there must be a political will to drive this work forward. Solar energy could have emerged as a major player much earlier, if work started after the oil crisis had continued apace. 6% of US energy is currently sourced from renewable sources; with 85% from coal, oil and gas. The hope is that by 2059, these numbers can be reversed!

References

C. R. Bock, T. J. Meyer, D. G. Whitten, Photochemistry of transition metal complexes. Mechanism and efficiency of energy conversion by electron-transfer quenching, J. Amer. Chem. Soc., 1975, 97, 2909 – 2911.

R. J. Cicerone, National Academy of Sciences, Address to the 145th Annual Meeting, available at: http://www.nationalacademies.org/includes/NASmembers2008.PDF [Oct 2009]

Las Alamos National Lab: National Security Science: http://www.lanl.gov/ [Oct 2009]

T. J. Meyer and H. Taube, Electron transfer reactions of ruthenium ammines, Inorg. Chem., 1968, 7, 2369 – 2371.

J. R. Pladziew, T. J. Meyer, J. A. Broomhea, and H. Taube, Reduction of oxygen by hexamammineruthenium(II) and by tris (ethylenediamine) ruthenium (II), Inorg. Chem., 1973, 12, 639 – 643.

H. Taube, Nobel Prize Lecture Nobel Prize 1983, http://nobelprize.org/nobel_prizes/chemistry/laureates/1983/taube-lecture.html [Oct 09]

R. C. Young, T. J. Meyer and D. G. Whitten, Kinetic relaxation measurement of rapid electron-transfer reactions by flash photlysis – conversion of light energy into chemical energy using Ru(bpy)3(3+)-Ru(bpy)3(2+*) couple, J. Amer. Chem. Soc., 1975, 97, 4781 – 4782.

Ruthenium polypyridyl photochemistry

Ruthenium polypyridyl complexes certainly rank amongst the most researched family of compounds in inorganic photochemistry. They are interesting complexes to study, having relatively long (100’s ns) emission lifetimes and a range of applications. It was the oil crisis of the 1970’s that sparked interest in these compounds, as potential hydrogen fuel generators by the photochemical splitting of water, and as seen in other posts, they are currently at the forefront in terms of efficiency in dye-sensitised solar cells. In addition, they have been used as DNA probes and oxygen sensors. The photochemistry of these complexes is discussed below. Readers are recommended to be familiar with the concepts in the “Light Absorption and Fate of the Excited State” article before studying this material.

Like so many aspects of modern photochemistry, Ireland has some key researchers in ruthenium photochemistry and the article below draws from a recent perspective by John Kelly (TCD) and Han Vos (DCU). The fundamentals are discussed here with applications discussed in a forthcoming article.

1. Introduction to Inorganic Photochemistry

We have looked elsewhere at Jablonski diagrams for organic molecules. Inorganic molecules, or more specifically d-block complexes, add an extra layer of molecular orbitals to this Jablonski diagram, between the ground state (HOMO) of the organic compound (which is now the ligand) and the excited state (LUMO). This opens up a range of new transitions, aside from the HOMO-LUMO transition observed in organic chromophores. This latter transition in inorganic photochemistry is called a ligand-field or ligand-ligand transition, as in the excited state the electron is located on the ligand.  As well as this, because of the presence of the metal’s molecular orbitals, three other transitions are available – a d-d transition, where an electron is excited from a metal orbital to an unoccupied metal orbital (this is usually referred to as a metal centred (MC) transition as well as transitions between the metal and the ligand. These can be either an electron excited from the ligand to the metal, called Ligand to Metal Charge Transfer (LMCT) or from the metal to the ligand (MLCT). Because of the energy differences between the various types of transitions, ligand field transitions are usually in the near-UV region (analogous to where we would expect organic molecules to absorb light), charge transfer transitions are in the visible region. The resulting emission from charge-transfer states is often highly coloured.

Light absorption in d-block (octahedral) complexes resulting in from left: metal centred (MC), ligand to metal charge transfer (LMCT), metal to ligand charge transfer (MLCT) and ligand-ligand transition (L-L)

Light absorption in d-block (octahedral) complexes resulting in from left: metal centred transition (MC), ligand to metal charge transfer (LMCT), metal to ligand charge transfer (MLCT) and ligand-ligand transition (L-L)

In order to discuss these transitions in context, we will focus on the, that is, the, inorganic photochemistry complex: Ru(II)(bpy)32+.

2. Fundamentals of ruthenium polypyridyl photochemistry

2.1 Absorption and Emission

Because of the incorporation of metal orbitals, the Jablonski diagram needs to incorporate the notation discussed above. Ruthenium in oxidation state II is d6, and so as an octahedral complex its electrons are in the low-spin t2g6 configuration. Incident light at about 450 nm promotes one of these electrons to a ligand anti-bonding orbital, a metal to ligand charge transfer. (We’ll discuss this, but you might consider how this was established.) Therefore we modify the S0 – S1 notation used in the Jablonski diagrams of organic molecules to one which denotes the type of excited state in inorganic ones – in this case 1MLCT. Transfer to 3MLCT is efficient (heavy atom effect) and so ruthenium complex’s photochemistry generally happens from here. [Remember intersystem crossing is effectively an electron flip, from a situation where electrons are paired to one where they are unpaired.]

Jablonski diagram for ruthenium polypyridyl complexes.

Jablonski diagram for ruthenium polypyridyl complexes. Solid lines and dashed lines are radiative and non-radiative processes respectively.

Absorption (top, source unknown) and emission (bottom, author's results) of Ru(bpy)3 complex in water

Absorption (top, source unknown) and emission (bottom, author's results) spectra of Ru(bpy)3 (2+) complex in water

The absorption and emission data are shown. Ruthenium absorbs at 450 nm (2.8 eV) and emits strongly at ~620 nm (~2.0 eV) in water. This emission is caused by radiative process from the 3MLCT state to the ground state. Emission lifetimes are approximately 200 ns in water in aerated solution and 600 ns in deaerated water. The oxygen in water is a very efficient quencher, and quenches emission with a rate of ~ 109 M-1 s-1. It is possible to map out the various deactivation processes of the excited state to investigate its kinetics:

Deactivation processes of an excited state M* in the presence of a quencher (oxygen)

Deactivation processes of an excited state M* in the presence of a quencher (oxygen)

The quantum yield of emission is therefore affected by how efficient the rate of emission is compared to the rates of deactivation and quenching. This is quantified by the Stern-Volmer relationship (oxygen quenches according to the dynamic quenching model) as discussed in the Quenching section, according to the equation below:

Stern Volmer Equation for Quenching with oxygen as quencher

Stern Volmer Equation for Quenching with oxygen as quencher

The rate constants, in particular the rate constant for deactivation, are dependent on how close the ground and excited states are. The excited state of this complex is a charge-transfer state (charge has moved from one region of the molecule to another), and therefore is very sensitive to solvent polarity – it will be stabilised in more polar solvents. Therefore, changing solvent polarity will affect the energy of the emitting state. It is found that on changing the solvent from water to acetonitrile, the emission lifetime increases from 635 ns to 870 ns, and the quantum yield of emission increases by 50% from 0.o4 to 0.o6. The emission maximum increases in energy from 627 nm to 615 nm.

These results can be explained as follows: on decreasing polarity of the solvent, the emitting state is destabilised by about 12 nm. This increase in energy difference between ground and excited state means that there is poorer overlap of the vibrational levels of the ground and excited state, so the deactivation process is not as efficient. Therefore the deactivation rate constant term is lower in the expression for the emission quantum yield in the presence of quencher, above, indicating a larger emission quantum yield. All of this is based on the assumption that the radiative rate constant remains unchanged, which is found to be true in practice. This observation is generally summarised as the Energy Gap Law – the larger the gap between ground and excited state, the less efficient deactivation processes are.

2.2 Nature of the Excited State

Absorption and emission spectra give initial information on the excited state, and are the photochemist’s initial tools to probe the excited state chemistry of molecules. To delve further, flash photolysis/transient spectroscopy give more detailed information. Flash photolysis, as mentioned elsewhere on this site, allows us to study the excited state by obtaining its lifetime and absorption spectrum. An experimental set-up is outlined below (more details onthe general details of flash photolysis in the Experimental article on Flash Photolysis). Excitation using, for example a Nd:YAG laser at 355 nm, generates the excited state which quickly equilibrates to the 3MLCT state. At this stage, a Xe or Hg/Xe obtains an absorption spectrum of the excited state. This was traditionally acquired point by point (i.e. measuring the change in absorption at 400, then 410, then 420 nm, etc) but iCCD (intensified charge coupled device) detectors are now the norm – these acquire information across a broad spectral range (~600 nm) at once. As well as providing structural information on the nature of the excited state by generating its absorption spectrum, flash photolysis also allows for the lifetime of this state to be measured, by acquiring a spectrum at intervals after the laser flash, therefore monitoring the decay of the excited state.

Schematic of Transient Absorption Spectroscopy Experiment: Laser excites sample and change in absorption is monitored by a xenon lamp. The simulated transient spectrum (top right) is the difference in absorption after laser flash, showing negative (dissapearance of ground state) and positive (formation of transients) absorbances. The absorption spectrum is shown on the bottom for comparison. Inset shows a kinetic trace of any of the transient peaks from which lifetime information can be gleaned.

Schematic of Transient Absorption Spectroscopy Experiment: Laser excites sample and change in absorption is monitored by a xenon lamp. The simulated transient spectrum (top right) is the difference in absorption after laser flash, showing negative (disapearance of ground state) and positive (formation of transients) absorbances. The absorption spectrum is shown on the bottom for comparison. Inset shows a kinetic trace of any of the transient peaks from which lifetime information can be gleaned.

The transient spectrum is shown with the accompanying ground state absorption spectrum. In the transient spectrum, it can be seen that some peaks have negative changes in absorbance whereas others have positive changes. The negative changes in absorbance (“bleaching”) occur where the molecule shows absorbance bands in the ground state. Hence, with a transient spectrum, the lash flash results in the formation of the excited state, and the xenon lamp records the loss of ground state chromophores – any absorbance that was present because of these chromophores is now registered as negative changes in absorbance in the transient spectrum. On formation of excited/transient state, new chromophores are present, which are monitored by the xenon lamp, and hence appear as positive changes in absorption (remember ground and excited states are chemically different species). To generate a true transient spectrum, the differences in absorption is subtracted from the absorption spectrum, although this is rarely necessary. The decay curve, in the inset is the rate of decay of one of the peaks – e.g. the transient peak at 390 nm. Fitting this curve to an exponential function allows for the rate constant (and hence lifetime) of the transient state to be easily determined. For example, if the decay was found to be mono-exponential, the curve of intensity (I) versus time (t) would be fitted to the expressionand allow for calculation of k.

emission_decay

The above experiment discusses results from a nanosecond experiment, but if we were to push faster, into the picosecond and femtosecond domain, the processes of intersystem crossing and relaxation in the triplet state would be observed. These kind of experiments are how information such as charge injection rates  in dye-sensitized solar cells can be determined.

The extent of positive absorbances in transient spectroscopy provide information on the nature of the transient species or excited state. Like conventional UV/vis spectroscopy, broad featureless bands very often don’t provide much direct information. However, considering the various types of transitions available, why is the excited state assigned as a MLCT state? This state, as indicated above, results in an extra electron residing on the bipyridyl (bpy) ligand, after an electron was transferred from the metal to it. Therefore, the transient spectrum should show characteristics of this bpy radical (called “bpy dot minus”). How can this be done? Well with the assistance of our electrochemical friends, we can electrochemically generate the bpy radical, and obtain its UV/vis spectrum (this technique is called spectroelectrochemistry). If it has characteristics similar to those in the transient spectrum (which in this case it does, the band at 368 nm), we can conclude that they must be attributed to the same chromophore.

3. Conclusion

In this first of two articles, we have looked at basic photophysical properties of a ruthenium complex and examined how absorption, emission and transient spectroscopic studies provide information on their excited state. In the second article, we will look at how these properties are used in a variety of applications.

4. References and Further Reading

Photochemistry of polypyridine and porphyrin complexes, K. Kalyanasundaram, Academic, London: 2002. Very comprehensive book on the area with excellent introduction covering theory in much more detail than above.

Vos, J. G. and Kelly, J. M., Ruthenium polypyridyl chemistry: from basic research to applications and back again, Dalton. Trans., 2006, 4869 – 4883. Good ooverview of the synthesis of these complexes and their variety of applications, especially looking at the role of Irish researchers in the area

Dye-Sensitized Solar Cells (DSSC)

Increasingly, solar energy has a vital role to play in providing energy to cater for an ever increasing demand. This article looks at what dye-sensitised solar cells are and their current technological status, as well as what needs to be done to make them big hitters in the energy game. It summarises some recent reviews on the topic, interested students are pointed to the source material and other references at the end of the article. [Aug 2009]

1. Introduction

A FEW YEARS AGO when talking about dye-sensitised solar cells in lectures, a student asked me what the problem with them was. Gratzel had published his seminal paper in 1991, and now, over 15 years later, from an outside observer’s perspective, there wasn’t much progress in terms of developing a commercial applicable devide. It’s a good question, and one worth asking periodically of any innovation. We hear that drugs often take 10 – 15 years from bench to patient, so one might reasonably ask with DSSC – “What’s the delay?“.

Here’s a short podcast (with audio) introducing this section outlining what this article will cover:
Vodpod videos no longer available.

2. Context

Now that we are near the end of the first decade of the 20th century, future energy demands make for sobering reading. The world currently uses about 13 terawatts (TW) of energy, and it is predicted by 2050, an additional 10 TW will be required. Not only that, the additional energy required will be have to be carbon neutral. Not only that, we are heading for peak oil. Considering all of this, what is the role of solar energy?

Kamat (2007) has summarised the role of solar energy very clearly with data that… ahem… blows other alternative/renewable energy sources out of the… ahem… water. Of the 10 TW additional energy required, building 1 GW nuclear power station every day for the next 50 years would meet the demand. (Nuclear isn’t strictly speaking renewable.) Hydroelectric could provide about 0.5 TW, tides and oceans could chip in 2 TW and wind power could blow 2 – 4 TW our way. Solar energy striking the earth amounts to 120,000 TW, yet only 0.01 – 0.04% of current energy usage is derived from solar sources. (Approximately 13% of current energy needs being supplied by renewable sources.) In theory, one hour’s solar irradiation is enough to supply a year’s global energy demands.

How would increasing the role of solar energy manifest itself on the ground? Solar flux at ground level is approximately 340 W/m2 in the world’s sunniest areas. Assuming 10% efficiency, each metre-squared of solar cell could generate 34 W. Plugging in the numbers, you’d be looking at a mere 4 x 10^11 m2, or about 618 km square to address the increase in world energy needs by 2050. While large, it’s not unrealistic. (Dublin county has an area of approximately 115 km square). The map below shows how, based on average sunlight irradiance measured over 1993-1994, how placement of solar energy “stations” at various locations around the planet would provide a substantial amount of solar-derived energy.

Total Primary Energy Supply: Required Land AreaRequired land area to supply an average of 18 TW (by Matthias Loster, 2006, reproduced with permission, full details on source website)

3. Overview

Dye-sensitised solar cells are as a concept, ingeniously simple. The idea was first conveived in the late 1970’s but since a Swiss photochemistr, Michael Gratzel, published a Nature paper in 1991 reporting 7% efficiency, the interest in the systemhas grown enormously. The outline of a DSSC is shown below and discussed in detail in section 4. Light harvesters gather in energy from a solar/light source and pass on the energy to an electrical circuit which does work (how do you think this compares with nature’s way of generating energy from sunlight?!).

Schematic of a Dye-Sensitized Solar Cell (DSSC) showing energy levels and electronic transitions

Schematic of a Dye-Sensitized Solar Cell (DSSC) showing energy levels and electronic transitions

Light harvesting dyes absorb solar radiation incident on them. This results in excitation of these molecules, who pass the energy obtained by means of transferring electrons onto a nanocrystalline TiO2 substrate onto which they are adsorbed. The electrons, now in the conduction band of titanium dioxide, conduct around a circuit and do work. At some counter electrode, a redox couple is utilised (usually iodide-triiodide) to regenerate the dye so the process can occur all over again. Assuming the dye is efficient at harvesting light, the transfer of electrons to titanium dioxide is efficient, the conduction of electrons in the circuit displays good potential and the dye can regenerate multiple-million times before being degraded, the concept works well with reasonable efficiency (~7 – 11%).

Slideshow: Electronic Pathways in Dye-sensitised Solar Cells:

The chemistry involved is sandwiched between two sheets of conducting glass, coated with a conductive layer (e.g. ITO) which is transparent. One plate of glass (working electrode) is coated with titanium dioxide nanoparticles that have the dye adsorbed onto the surface and the other (counter electrode) is coated with a catalyst (platinum or carbon). The plates contain electrolyte solution between them with the redox couple to regenerate the dye.

gratzel cell schematic showing forward and reverse processes

Forward and reverse electronic pathways in a dye-sensitized solar cell

Here’s a video of a DSSC in action:

4. Developing the idea further

The efficiency of dyes can be measured by consdering how many absorbed photons result in electron injection and how many of these injected electrons are collected to be used in the electrical circuit. This is expressed according to Equation (1), where IPCE(λ) is the incident photon to current efficiency – a measure of how many photons translate into electrical current.

IPCE(λ) = LHE(λ) × Φ(inj) × η(c)

The light harvesting efficiency (LHE) is the fraction of photons absorbed by the dye at a particular wavelength. The electron injection efficiency (Φ(inj)) is a measure of how many absorbed photons result in an injected electron into the semi-conductor and the charge collection efficiency is a measure of how many of these injected electrons are collected for electrical use. The equation essentially maps out each of the processes in the cell and considers their efficiency. All of the processes in the DSSC are kinetic – their efficiency is determined by how fast they occur relative to competing processes. We’ll consider below the various components of the dye and identify where any efficiencies could be improved upon in future developments. This section is based mainly on Hupp (2008 and subsequent more recent articles).

4.1 Dye Characteristics

The light harvesting dye is clearly a crucial component of the cell design and needs to fulfil several criteria; adsorption onto metal surface, overlap effectively with solar spectrum, inject electrons efficiently into metal oxide and be stable for many million cycles.

Adsorbtion of the dye onto the metal oxide surface is facilitated by incorporating a substituent that will adsorb readily. The most efficient studied are ruthenium dyes with carboxyl-substituted ligands – these carboxyl substituents adsorb onto the dyes surface.

run3-tio2

Solar Spectrum and Ru-N3 dye (left-most red spectrum) Additional spectra are predicted overlap from changing dye energygap (Hupp el at, 2008 Reproduced by permission of the Royal Society of Chemistry - original article linked below)

Top - Ruthenium based "N3" dye adsorbed onto a titanium dioxide surface; Bottom: Solar Spectrum and Ru-N3 dye (left-most red spectrum) Additional spectra are predicted overlap from changing dye energygap (Hupp el at, 2008 Reproduced by permission of the Royal Society of Chemistry - original article linked below)

The spectral overlap with the solar spectrum should be maximised so that as much of the sun’s energy as possible is utilised in exciting the dye, and promoting a high density of electrons into the excited state. In practice, dyes absorb in the visible and near infrared region (about 400 – 700 nm), capturing about half the available power and a third of the available photons from solar source. The ruthenium complexes which are currently “best in show” do have a limitation in that their exctinction coefficients are comparatively low (1 – 2 x 10e4 M-1 cm-1), requiring several hundred monolayers which in turn requires the metal oxide support to have a very high surface area. To achieve efficiencies of >15%, DSSCs will need to absorb bout 80% of light between 350 – 900 nm. Therefore using materials that have a higher absorption capacity may be a useful future strategy. Research here has included using osmium in place of ruthenium, which extended the absorption further into the red and enhanced the response of the cell to light relative to the ruthenium analogue. The 1MLCT to 3MLCT transition in osmium is much more intense than in ruthenium. Organic dyes have also been used successfully as attested by the very many articles and school projects on using fruit berries as the dye in these cells. A range of dyes are shown below, and while they vary in chemical structure, you should note a common factor between them and between these and the ruthenium dye shown above. Organic dyes have much larger extinction coefficients (5 – 20 x 10^5 M-1 cm-1), albeit across a narrow range than the ruthenium counterparts. (It seems that dyes will either absorb moderately well across a broad range or very well across a narrow range!)

dssc_dyes

Organic dyes utilized in (a) 9% efficient indoline, (b) 6.5% efficient coumarin, (c) 5.2% efficient hemicyanine, (d) 4.5% efficient squarine, (e) 7.1% efficient porphyrin, and (f) 3.5% efficient phthalocyanine-based DSSCs. (From Hupp et al, Reproduced by permission of the Royal Society of Chemistry - Original Article linked below)

In order for electronic transfer to be energetically favourable, the excited state energy of the dye should be higher in energy than the conduction band of the semiconductor. As well as this, the kinetics of electron injection into titanium dioxide should be faster than recombination of the dye (by luminescence or non-radiateive decay). This isn’t a problem with the N3 dye, above which injects on a femtosecond timescale and decays at a much more leisurely sub-picosecond timescale. (You might consider in your studies how this data would be determined experimentally). Electrons are injected from both the 1MLCT (on a sub-ps timescale) and the 3MLCT, which is formed within 100 fs by intersystem crossing (due to the heavy atom effect of ruthenium). Recent research (Durrant, J. Am. Chem. Soc., 2009, 131, 4808) has questioned whether similar difference in rates are observed in real DSSC (as opposed to model systems), based on results showing that in these real systems, the electron injection slowed down to ps timescale, allowing recombination to be competitive. Nevertheless, it is considered that this process is efficient, although caution is required in ensuring that efficiency isn’t reduced by other design factors on the cell.

Timescales involved for electron injection

Timescales involved for electron injection from Ru-dye to TiO2

Considering the discussion above regarding extending the dye’s absorption further into the red, this could be achieved by lowering the LUMO of the dye, although researchers have been reluctant to do this as since the LUMO-CB transition is so fast, the energy levels must be very well matched. (Goldilocks Principle: Not too high, not too low, just right).

Very recent research at Stanford University has coupled luminescent chromophores which absorb high energy photons and pass their energy on to the sensitising dye (Nature Photonics, 2009, 3, 406 – 411) – I’ll put more on this in a future article.

4.2 Metal Oxide Support

Following sucessful injection into metal oxide, the next phase is for the elctron to percolate through the oxide layer onto the working electrode. TiO2 is the most common substrate used. As a chemical, it is relatively inert, cheap and can be synthesised via the sol-gel process (offering flexibility and scalability). Unlike silicon solar cels, very high purity is not required. It exists in various forms, mainly anatase, rutile and brookite. Anatase has a high band gap (3.2 eV compared to rutile’s 3.0 eV) which gives several advantages. It absorbs very little of the solar spectrum, meaning it is transparent to incoming light source (so that the dyes rather than the metal oxide is activated by light). In addition, the larger band gap than rutile means that recombination is slower (ref the energy gap law) – some reports have determined a 30% lower efficiency with rutile.As with the dye, there are several factors to consider to maximise efficiency.

Gratzel’s innovation in his 1991 paper was among other things to use nanocrystalline TiO2. The nanomaterial’s much greater surface area meant that many more molecules of dye could adsorb onto the surface and pushed efficiency to the best reported at the time: 7%. Nanocrystalline surfaces in these cells are reported to have a surface: geometrical area of ca. 1200. Best performers actually have two distinct layers of metal oxide: a 12 micron thick transparent layer of 10 – 20 nm sized particles covered with a 4 micron thick layer of much larger (400 nm) particles. The larger particles scatter photons back into the film.

One significant limitation of this model is the very slow time taken for electrons to percolate through the material and onto the transparent conducting electrode; which is of the order of 100’s of microseconds, in comparison with conduction band-dye recombination which is of the order of a couple of microseconds. (It is however faster than the other decay process: conduction band-redox couple, which is in the millisecond range). Because of this, researchers are looking at improving the dynamics of electron percolation through the film. One strategy is to use nanotubes/rods rather than particles. These have the disadvantage of having lower surface area, but an advantage of very much improved electron transport because they provide an physical electron pathway from the particle to the electrode. Pagliaro (2009) gives a nice summary of this and this and other work on ZnO nanorods will be summarised in a future article.

4.3 Electrolyte and Regeneration

The electrolye contins the redox couple which regenerates the oxidised dye whcih were formed byinjection of electron from the dye to the titanium dioxide layer, leaving D+. The redox couple has a difficult job – it must be very efficient at reducing the dye cation back to the original state for another cycle, but not intercept or capture one of the electrons being injected in the first place! The latter process, which would result in inefficiency involves transfer of an electron from the conduction band of titanium dioxide to the redox couple (to the triiodide ion I3-). Of the range of redox couples studied, iodide-triiodide has proved to be in the most efficient  cells, mainly because the transfer of electron from titania to trioidide is, as mentioned above, very slow (in the millisecond time range). One of the problems in trying to look for improved systems is that the redox chemistry of this redox couple is not well understood, so it is difficult to plan effective alternative dyes. One interesting approach is to use solid state redox couples, which allow for higher concentrations of the redox couple and extending the applicability of the device (because of removal of the liquid electrolyte).

5. Summary and Review

Dye-sensitized solar cells offer enormous potential as an alternative renewable energy provider. The principle of operation is for a light harverster to absorb light efficiently and pass on the energy to a metal oxide surface, which links in to a circuit generating current. The dye is regenerated at the counter-electrode via a redox couple. In reviewing dye sensitised solar cells, you should consider the two main themes covered in this article:

  1. Describe the principle of operation outlining the pathway the electron follow from light absorption to dye regeneration. Using the diagram below, represent these processes on an energy level diagram and for each of the stages outline the counter process available resulting in efficiency. By using time constants/scales, indicate whether these processes are likely to significantly impact on the efficiency of the cell.
  2. For each of the processes, outline improvement strategies that are being considered in current research in the area, indicating the rationale for such approaches.
Kinetic Processes in DSSC - you should consider the relative times for the various processes involved

Kinetic Processes in DSSC - you should consider the relative times for the forward(green) and reverse processes (purple arrows) involved

Addendum:

This diagram below, from Hupp et al., Chem. Eur. J., illustrates effectively the rates of competing processes:

Normalised rates of competitive processes in each stage of the DSSC. Note the "double hump" for charge injection - you should relate this to the schematic for charge injection above. (J. T. Hupp et al, New architechtures for dye-sensitized solar cells, Chem. Eur. J., 2008, 14, 4459. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Normalised rates of competitive processes in each stage of the DSSC. Note the "double hump" for charge injection - you should relate this to the schematic for charge injection above. (J. T. Hupp et al, New architechtures for dye-sensitized solar cells, Chem. Eur. J., 2008, 14, 4458. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

Further Reading/References

  1. Meeting the clean energy demand: nanostructure architechtures for solar energy conversion, P. V. Kamat, J. Phys. Chem. C, 2007, 111, 2834 – 2860. Excellent article on the wide variety of roles nanotechnology may play in providing future energy needs. This paper was used to provide much of the context. Prof. Kamat’s website is also worth looking at. Google “Light Energy Conversion” and this is the web-page that comes up, with good reason.
  2. Advancing beyond current generation dye-sensitized solar cells, Hamann, T.W., Jensen, R. A., Martinson, A. B. F., Van Ryswyk, H and Hupp, J. T., Energy Environ. Sci., 2008, 1, 66 – 78. An excellent paper on the properties of DSSC and how they may be technically advanced in future developments. Section 4 is substantially based on this article. Link
  3. Dye-sensitized solar cells: a safe bet for the future, Gonçalves, L. M., de Zea Bermudez, V., Aguilar Ribeiro, H. and Magalhães Mendes, A, Energy Environ. Sci., 2008, 1, 655 – 667. A concise overview on the device aspect of DSSC, considering current and future implementation requirements.
  4. Nanochemistry aspects of titania in dye-sensitized solar cells, Pagliaro, M., Palmisano, G., Ciriminna, R. and Loddo, V., Energy Environ. Sci., 2009, 2, 838 – 844. Good overview of the photoanode requirements and research developments.
Now more than ever, solar energy has a vital role to play in providing energy to cater for an ever increasing demand. This post looks at what dye-sensitised solar cells are and their current technological status, as well as what needs to be done to make them big hitters in the energy game. It summarises some recent reviews on the topic, interested students are pointed to the source material and other references at the end of the article.

Published August 2009. Subsequent additions/amendments/corrections will be logged in comments.