FAU researchers develop new electrode
How to make organic solar cells light-transmissive without using the expensive raw material indium? This is the question researchers from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have asked, and they found their answer in fine silver nanowires. They replaced the indium tin oxide (ITO) electrodes used so far with silver nanowires, which are less expensive both in terms of raw material and processing. The online version of the journal Advanced Energy Materials1 recently published the researchers’ findings.
Organic solar cells are very different from the regular silicon cells known from ground-mounted and rooftop photovoltaic systems. The layers responsible for the photoelectric effect in organic solar cells consist entirely of synthetically produced materials, special polymers and fullerenes. The synthetic material comes with many advantages: the solar cells are as thin as envelopes and flexible. They can be produced in a translucent version or in different colours. These special qualities mean that they, unlike crystalline solar cells, are specially suited for use in fabrics or as a design element in architecture, for example for cladding or windows.
So far, the stumbling blocks in the way of the commercial breakthrough of organic solar cells have been costs, efficiency and service life. Especially the costs will be reduced thanks to the FAU researchers’ findings – semitransparent solar cells in particular will see a development boost. The manufacture of these cells previously depended on the use of ITO as the material for the electrodes: it was the only material that had the good conductive and light-transmissive properties needed for the electrodes. The electrodes discharge the electric charges generated in the photoactive layer as a current. Not only is indium a cost-intensive raw material, but the manufacture of indium tin oxide involves an expensive vacuum process. This step consumes most of the energy needed in the manufacture of ITO.
FAU scientist Fei Guo has now succeeded in replacing ITO with fine silver wires as electrodes. Guo is a member of the research group co-ordinated by Prof. Dr. Christoph Brabec, Chair of Materials Science (Materials for Electronics and Energy Technology) and supported by the working groups of Prof. Dr. Marcus Halik, Prof. Dr. Dirk Guldi and Prof. Dr. Erdmann Spiecker. The researchers placed a wire mesh with nanoscale thickness over the photoactive layer of the solar cells: electroconductive and yet wide-meshed enough to let plenty of light into and through the cell. The reference cell tests showed that the new variant has the same efficiency as the conventionally-produced organic solar cells. With a filling factor (one of the indicators of a solar cell’s efficiency) of 63 percent, the researchers have even achieved the highest value documented so far for organic solar cells produced with a printing process. Over 50 of the other cells produced scored only slightly lower with a filling factor between 58 and 62 percent.
However, besides comparable efficiency, the new material offers significant advantages: silver nanowire is considerably cheaper than ITO and manufacture is less expensive and energy-intensive – semi-transparent solar cells can now be entirely produced with a printing process. The individual components are dissolved in liquid and are printed as ink onto a thin plastic foil and let dry layer by layer. The entire vacuum process is avoided. This means that huge solar sheets can be produced with comparative ease.
The new research findings are furthermore relevant for organic LEDs, currently used to light up phone displays among other things, as well as for intransparent organic solar cells. For instance, laptop users may be able to charge their computer directly via their carry case, which could have such an intransparent solar cell integrated into its fabric. This is another area where the silver nanowire electrodes have the potential to shine: they are significantly more flexible than ITO electrodes, which can break more easily under strain.
The research was conducted within the Cluster of Excellence Engineering of Advanced Materials (EAM), which bridges fundamental research and practical applications. The FAU researchers’ results enable savings that make the technology attractive for new mass markets. In the long term, their work could even spell competition for the conventional photovoltaic systems on roofs and outdoors.
1Advanced Energy Materials (2013)/doi: 10.1002/aenm.201300100
Prof. Dr. Christoph Brabec
Chair of Materials Science and Director of the Bavarian Center for Applied Energy Research (ZAE)
Phone: +49 (0)9131 85 25426