Charge of the light brigade

Photovoltaic cells are not widely used because of their high cost. Simon Hadlington explores some of the work under way to produce cheaper, more efficient cells

For decades, the possibility of generating large quantities of electricity from photovoltaic cells has been raised as one of the main ways the world might be able to reduce its reliance on dwindling fossil fuel resources. So, at the beginning of the 21st century, how much does this clean, green and ’free’ source of power contribute to global energy production? The answer is less than 0.1 per cent. 

The principal reason for this tiny figure is cost. In the UK, for example, an array of rooftop solar cells capable of generating around 1500 kilowatt hours of electricity a year - less than half the average household’s consumption - costs about ?10 000. At that price the payback period for the installation - the amount of time it would take for the savings on the ’free’ electricity generated by sunlight to negate the capital outlay - runs to several decades, making it difficult for even the ’greenest’ householder to justify economically. 

The more encouraging news is that photovoltaic technology is advancing rapidly. 

Governments around the world are under pressure from international agreements to reduce emissions of greenhouse gases and they recognise the potential benefits of photovoltaic technology as a clean, sustainable energy source. Consequently, they are showing increased commitment to helping make the technology more competitive with conventional energy sources.   

This is typified by the EU’s recently established photovoltaics research advisory council, which describes itself as a ’high-level discussion group’ aimed at developing a common strategic approach to photovoltaic research across the EU. 

The UK government, through the Engineering and Physical Sciences Research Council, has set up a new multidisciplinary research programme under its renewable energy theme Supergen. The research, which is being carried out by a consortium of academic and industrial partners, is laying the foundations for photovoltaic technologies of the future based on so-called excitonic systems, such as organic dyes.   

The work complements an existing Supergen photovoltaic programme, Photovoltaic materials for the 21st century. This involves research centres at Bangor, Bath, Durham, Loughborough and Southampton universities and aims to develop new techniques and technologies to produce cheaper solar cells based on inorganic semiconductors. Both programmes are highly multidisciplinary, involving chemists, physicists and electronic engineers. 

Exciting stuff 

The excitonic solar cell consortium comprises the universities of Bath, Edinburgh, Cambridge and Imperial College London, together with industrial partners. Consortium leader Laurie Peter of the University of Bath predicts: ’The synergies generated by bringing the consortium together will continue to generate new ideas and strategies for developing flexible, low-cost solar cells. We hope that our collaborative efforts will consolidate the UK’s competitive position in this emerging research area.’ 

James Durrant, a reader in the chemistry department at Imperial, explains the general concept of excitonic solar cells. ’When you shine light on a conventional inorganic semiconductor such as silicon, you put an electron from a valence band into a conduction band, generating free carriers,’ he says. ’The positive and negative charges - the holes and electrons - can be separated relatively easily to create a current. 

’However, when you shine light on certain molecules you can induce an excited state but the electron does not separate from the molecule. These excited-state entities are often called excitons.’   

To separate the charge - to pull the electron from the molecule - you need to have an easily-reduced compound nearby. The system is broadly analogous to photosynthesis, says Durrant, where electrons in chlorophyll are excited by light energy, then siphoned off by adjacent quinone molecules. 

The key to these systems is to have the electron donor in intimate contact with the acceptor and to connect both up to external contacts to allow the electron to flow around an external circuit and regenerate the system. 

The most successful excitonic solar cells so far have been based on dyes, such as certain ruthenium-based complexes. The light-sensitive dye coats a film of nanocrystalline titanium dioxide, which acts as the acceptor. A redox couple, such as triiodide/iodide, dissolved in an organic solvent, connects the dye molecule to a counter electrode, thus completing the cell.   

The first such cell to produce reasonable conversion efficiencies was reported by Gr?tzel in 1991. Refinements to the Gr?tzel cell has produced efficiencies - the proportion of solar energy converted to electrical energy - of more than 10 per cent. This compares to around 17 per cent that can be routinely achieved with conventional silicon-based solar cells. 

Overcoming obstacles 

’The attraction of such dye-based cells is that they are relatively easy and cheap to manufacture,’ says Durrant. ’The problem is that because they contain liquid components that are corrosive, the long-term stability of the system tends to be compromised, with issues of leakage as well as toxicity.’ 

To overcome these obstacles, researchers have adopted a variety of strategies, including replacing the electrolyte with molecular hole conductors, and both the dye and the electrolyte with a single polymer that both absorbs light and transports the positive charge. 

The Imperial team has had some success with polymers such as polyphenylvinylenes as light absorber and hole transporter but efficiencies remain relatively low. Other research groups have experimented with fullerenes as the electron transporter in conjunction with polymers. 

’With the titania systems the structural detail is reasonably well characterised and we have a pretty good idea of what is going on and how the system works,’ says Durrant. ’With the fullerene/polymer blend, the morphology is less defined and we really want to be able to control these things at the nanoscale level.’ 

Other approaches using fullerenes involve replacing the polymer with small light-absorbing molecules, such as a phthalocyanin. ’One advantage with the small molecule systems is that you fabricate the structure by evaporation methods, rather than solution processing as you do with the polymer,’ Durrant says. ’In this way it is possible to obtain much more control over the fabrication process, so you can dictate the profile of the mixture on the surface much more closely.’   

Furthermore, with small molecules the charge can be transferred over greater distances within the film, so the degree of mixing between the donor and acceptor need not be so intimate. 

The researchers at Imperial are examining various functional aspects of exciton systems, using laser spectroscopy to monitor electron kinetics. ’We can measure the motion of the electrons, how fast the electrons and holes move apart and how long they stay apart or recombine,’ says Durrant. ’This is important because it is key to determining the function and efficiency of the device. If we can correlate electron kinetics with the materials that we are using, we will be in a position to start controlling the dynamics of the process with much greater certainty.’ 

Another angle, which is being pursued by Laurie Peter’s group at the University of Bath, is to replace the dye with quantum dots - nanoscale particles of coloured semiconductors such as cadmium selenide. ’Due to a phenomenon called quantum confinement, the optical properties of quantum dots can be tuned simply by changing their size,’ says Peter. ’They could replace the dyes currently used in solar cells.’ 

To dye for 

One member of the excitonics consortium is the University of Edinburgh. Here, Neil Robertson’s group is focusing on the design of new dyes. 

’There has been a lot of effort to find alternatives to the liquid electrolytes in dye-based cells to produce a system with better stability,’ says Robertson. ’Our view is that you cannot necessarily assume the current best dyes will still give the highest efficiency if you change the electrolyte. We think it is important to consider the system much more as a whole. We want to explore new designs of dyes in the context of new designs of cells. The key to such an approach is that it must be a multidisciplinary effort - no one can do everything on their own.’ 

One of the main issues, for example, is that when polymers replace liquid electrolytes as hole-transporters, there can be problems in ensuring a sufficiently intimate contact between the polymer and the dye. ’Often you have a situation where a hydrophobic polymer is in contact with hydrophilic, dye-coated titanium dioxide, so that the process of regenerating the dye with an electron transported from the polymer is inefficient,’ says Robertson.   

It is possible to modify the dye by incorporating hydrophobic groups to provide better communication between the polymer and the dye. Another idea is to covalently link thiophene oligomers, which are electronically conductive, to ruthenium- or platinum-based 
dyes.   

’This would result in much greater chemical affinity between the hole-conducting polymer and the surface of the titania,’ suggests Robertson. 

The Edinburgh team also intends to embark on more experimental, speculative work on the modification of dyes.   

Already, the laboratory has demonstrated that subtle changes to the chemical architecture of dyes can produce potential benefits. ’Many of these dyes possess bipyridyl rings which are anchored to the titanium dioxide surface by means of ester linkages with acidic carboxylate groups,’ says Robertson. ’We have been experimenting with the position of these groups on the bipyridyl ring. Whereas most systems have the carboxylate at the 4,4? position on the ring, we placed them at the 3,3? and 5,5? position. We found that with the carboxylate at 3,3? the efficiency of our dye was increased.’ 

By repositioning the carboxylate, the energy of the lowest unoccupied molecular orbital of the dye is changed. This is crucial to control the absorption wavelength of the dye and provides the driving force for electron transfer to the titania. 

Meanwhile, the first Supergen photovoltaic consortium is now entering its second year. Its aim is to investigate ways of producing inorganic semiconductor solar cells as cheaply and efficiently as possible to drive down costs. 

’For photovoltaics to become sufficiently cost-effective to be a significant contributor to the world’s energy needs involves an immense technological challenge and one that is generally hugely underestimated. With photovoltaic technology it is not a question of whether you can do it, it is whether you can afford it,’ says Stuart Irvine at the University of Wales Bangor and leader of the consortium.’ 

The bulk of photovoltaic cells are based on crystalline silicon, which is an inherently expensive material to produce, requiring high temperatures and a large energy input. 

The most efficient cells use single-crystal silicon - usually offcuts from the computer chip industry. Typically such cells can reliably achieve an efficiency of around 17 per cent. A cheaper form of silicon is multicrystalline. However, the boundaries between the crystal grains reduce the efficiency of the device to around 15 per cent. 

’Silicon is an indirect band gap semiconductor, which means that you need a relatively thick section of material to absorb the light efficiently,’ says Irvine. ’Typically this might be 100 m m. Making it this thick is expensive.’ 

One way of reducing costs, therefore, would be to make the material thinner while retaining its efficiency. Researchers at the University of Southampton, for example, are investigating the possibility of incorporating sophisticated diffraction gratings on the surface of the cells to cause the light to slew into the crystals at a shallower angle. This would increase the path length of the light through the silicon, therefore allowing the semiconductor to have a thinner section. 

Low efficiency 

One key area of research is into materials whose electronic properties enable them to be much thinner than silicon. These are direct band gap semiconductors. Copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe) are receiving most attention. 

’Unfortunately things are not as straightforward as we would like,’ says Irvine. ’While the electronic properties of these thin film materials should enable them to beat silicon hands down, the big stumbling block is low efficiency - something around 10 per cent." 

While the thin films can absorb light well and create charge efficiently, collecting the charge is the hurdle. ’These materials are highly polycrystalline, with many grain boundaries,’ Irvine says. The boundaries are traps for mobilised electrons, causing them to recombine with positive holes. 

The researchers need to understand precisely what is happening at the electronic scale to devise strategies to overcome the obstacles. At the University of Durham, researchers have microscopic techniques that can measure the extent of electron-hole recombination at grain boundaries, for example. 

At Bangor, scientists systematically scan lasers across the surface of the thin film and monitor where in the structure the charge is generated efficiently and where it is not. The charge generation can then be correlated with the fine structure of the film, and analysed with techniques such as x-ray diffraction, electron microscopy and secondary ion mass spectroscopy. ’If we can identify those characteristics that make a film efficient, we are then in a position to start experimenting with fabrication techniques to ensure the end product incorporates the desired structural features,’ says Irvine. 

The new generation of thin film devices are currently made by chemical vapour deposition techniques. These are energy intensive and one strand of the new programme is to investigate whether cheaper film fabrication methods might be possible. 

At the University of Bath, members of Peter’s team are investigating the possibility of using more traditional wet-chemical methods to deposit the various layers needed for a solar cell. 

Phil Dale and Anura Samantilleke are two of the Bath researchers. ’Essentially to make a CIGS-based cell the idea would be to place the substrate - molybdenum-coated glass typically - into a solution containing copper, indium, gallium and selenium ions and carry out an electrolytic reaction in which the ions migrate to the substrate,’ says Dale. ’There are many variables that you can manipulate that will have an effect on the layer of CIGS that you build up; things like voltage, relative concentration of the ions, temperature and pH, and even whether the solution is stirred or not.’ 

The team is looking at how the process can be controlled to produce films that have the appropriate qualities. ’Ideally we are looking for a film that contains large crystals with uniform grain sizes and as few grain boundaries as possible,’ says Dale. 

CIGS cells have been made using electrodeposition for certain stages of the process but vapour deposition methods are needed to modify the layer, and other processes are required to deposit the various 
other layers needed for the complete cell. 

’Our aim is to use only wet chemical methods to fabricate the whole cell structure,’ says Samantilleke. ’This idea is to make these complex structures much more cheaply than is currently possible.’ 

Simon Hadlington is a science writer based in York, UK