13 July 2010
Understanding the structure of plastic-based solar cells
Research into understanding the structure of plastics used in new-types of low-cost solar cell has been carried out by academics at the University of Sheffield, in a bid to improve their efficiency.
Led by Professor David Lidzey from the University of Sheffield´s Department of Physics and Astronomy, researchers from the University of Sheffield, Professor Athene Donald from the University of Cambridge and experts from Cardiff University and Diamond Light Source, the UK´s synchrotron facility, have carried out groundbreaking research looking at the real-time developments of the structure of a solar cell, to help increase the efficiency of photovoltaic panels.
Photovoltaics are semiconductor devices that are being used to generate low-cost renewable-energy. When sunlight hits a photovoltaic cell, it is absorbed and its energy is converted into an electrical current. Most photovoltaic devices are based on silicon, however researchers are now looking to make devices out of plastic. As plastic films can be deposited from solution by low-cost printing techniques, plastic solar cells could be much cheaper to produce resulting in a significant overall savings in energy and cost.
A key aspect of developing efficient plastic-based photovoltaic devices relies on the control of the nanoscale morphology within the thin organic semiconductor film (the most important part of a solar cell), as this plays a crucial role in maximising the efficiency of charge-generation and charge-extraction.
Previously, research has explored the structures developed in films after they have been created (usually by casting from solution). However the Sheffield-led team studied the dynamic processes that occur as the semiconductor thin-film blend is being deposited. This enabled the structure and morphology of the film to be determined in real-time as the solution dried. This `dynamic´ information permits a number of basic mechanisms in film-formation processes to be resolved; information that will help the design of new and improved materials for plastic solar cell applications.
Using the extremely bright X-rays generated by the Diamond synchrotron and combining this with the spectroscopic ellipsometry at the University of Sheffield, the team has been able to unravel new information. After a plastic-containing solution had been spread out onto a surface, a beam of polarised light or X-ray was directed onto the drying solution. This solution was composed of two different materials – the plastic (polymer) and a form of carbon called a fullerene – both of which are necessary in order to allow positive and negative electrical charges to be generated. The experiment enabled the crystallisation of the polymer to be observed as it dried, creating a more ordered structure.
The team observed that the drying process and consequent crystallisation could be divided into three main stages and it was discovered that the second stage in the drying process was the most important as it was at this point that the polymers began to rapidly crystallise. Crystallinity in these materials is very advantageous as it is associated with more rapid electrical charge-conduction and therefore more efficient device operation. Results of the experiment have been published in the journal Soft Matter and the team now hope to use these findings to develop means to manipulate the crystallisation of this and other polymers in order to create more efficient solar cells.
The news comes as the University of Sheffield launches a unique venture entitled Project Sunshine, led by the Faculty of Science. The Project aims to unite scientists across the traditional boundaries in both the pure and applied sciences to harness the power of the sun and tackle the biggest challenge facing the world today: meeting the increasing food and energy needs of the world´s population in the context of an uncertain climate and global environment change. It is hoped that Project Sunshine will change the way scientists think and work and become the inspiration for a new generation of scientists focused on solving the world´s problems.
Professor David Lidzey from the University of Sheffield´s Department of Physics and Astronomy, said: "This has been a very exciting experiment for us – we have used Diamond Light Source to carry out some important science on a technologically important class of materials. This has allowed us to understand a process that has so far remained unexplored in these materials. We will now capitalise on this by applying our techniques to new materials. This information will quickly feed into our solar-cell research programme and will hopefully allow us to develop more efficient plastic solar cell devices."
Post-doctorate researcher Dr Tao Wang at the University of Sheffield, said: "This is the first time we have unravelled the nanostructure evolution from random, long chains into densely-packed nanocrystals. The results will direct us to better control our device fabrication process for high efficient large-area plastics solar cells."
Notes for Editors: To read the paper entitled The development of nanoscale morphology in polymer: fullerene photovoltaic blends during solvent casting in full, visit the link below.
Diamond Light Source is the UK's national synchrotron facility, a powerful machine that produces very intense beams of X-rays, infrared and ultraviolet light. The light is created by accelerating electrons to almost the speed of light and passing them through special magnets, causing them to release energy in the form of incredibly bright synchrotron light, which can be used to look at liquid, solid and gas samples right down to the scale of molecules and atoms.
For more information about Diamond, visit the link below.
• Diamond generates extremely intense pin-point beams of synchrotron light of exceptional quality ranging from X-rays, ultra-violet and infrared. For example Diamond´s X-rays are around 100 billion times brighter than a standard hospital X-ray machine or 10 billion times brighter than the sun.
• Many of our everyday commodities that we take for granted, from food manufacturing to cosmetics, from revolutionary drugs to surgical tools, from computers to mobile phones, have all been developed or improved using synchrotron light.
• Diamond can bring benefits to:
o Biology and medicine. For example, the fight against illnesses such as Parkinson's, Alzheimer's, osteoporosis and many cancers are benefitting from the new research techniques available at Diamond.
o The physical and chemical sciences. For example, engineers will be able to image their structure down to an atomic scale, helping them to understand the way impurities and defects behave and how they can be controlled.
o The Environmental and Earth sciences. For example, Diamond can help researchers to identify organisms that target specific types of contaminant in the environment which can potentially lead to identifying cheap and effective ways for cleaning polluted land.
For further information please contact: Shemina Davis, Media Relations Officer, on 0114 2225339 or email shemina.davis@sheffield.ac.uk
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