Organic photovoltaic technologies in general, and full organic polymeric technology in particular compare favourably with other PV technologies regarding the embedded energy of the PV module even for a far from optimum laboratory fabrication procedure. The main reason is that there are not high temperatures involved in the process. For laboratory cell production, the final embedded energy per square meter of module is 2800.79 MJ/m2. This value is half of the average value calculated for crystalline silicon technologies, it is of the same order of magnitude of thin film technologies and slightly higher than dye-sensitized solar modules. We can expect bigger reductions for the organic technology in the large scale industrial process, therefore giving a clear advantage to organic technologies provided that the efficiency of these industrial modules is similar to that of actual laboratory cells (about 5% now and an expected 10% in the coming years).
For a typical organic solar module, the energy embodied in the materials is 726.26 MJ/m2 including both the materials of the solar cell and the materials used in the process. The direct process energy is 1973.78 MJ/m2 which is more than double, indicating that there is plenty of room for further reduction if the optimization of an industrial production process is accomplished. In particular, the nitrogen use is a key factor: in the laboratory-fabrication process it accounts for 48.19% of the embodied energy as an input material and for 38.56% of the process energy required to keep the N2 atmosphere in the globe box. The spin-casting method widely used in the laboratory is very inefficient regarding the usage of materials. More than 99% of the polymer is misused with this coating technology. Also the lack of control of the nanostructure of the spin cast layer should be avoided. This result makes compulsory the shift to ink-jet, spray-coating or screen-printing technologies for a scale-up in production.
The fabrication of the electrodes is the most energy costly factor. For ITO, it accounts for 50.39% of total material embodied energy, which adds to the fact that Indium is a scarce and expensive material. Alternative materials for cathodes are therefore needed to make this technology more competitive, the problem risen by the use of ITO in organic low-cost technologies is also enhanced by the fact that it accounts for more than half of all the materials embodied energy. The evaporation of Calcium and Aluminium for the anode fabrication accounts jointly for 33.77% of direct process energy input in the fabrication of laboratory cells.
(these results will appear in “Progress in Photovoltaics: Research and Applications”)
Reported by: Rafael GarcĂa-Valverde, Nieves Espinosa, Javier Padilla, Antonio Urbina (Universidad PolitĂ©cnica de Cartagena)
