Interpreting the results for OPVNieves Espinosa Martinez
The last step in the realization of the LCA is the evaluation and report of LCIA results to gain a better understanding of the reliability of the results. The first OPV process analyzed by LCA, called ProcessOne, was found to have large embodied energy, and the majority of it was coming from a dominant material, the ITO-electrode, as seen in Figure 1, depicted in the pie chart in green. Moreover, the sputtering of this component makes the largest impacts in fossil fuels, respiratory inorganics and carcinogens, shown in Figure 1. Behind the ITO, contributing the most are silver and polyethylene terephthalate, used respectively as electrode and substrate.
Indium is moreover a scarce material. Therefore finding a replacement for the ITO electrode was a matter of time. In Figure 1 a progressive optimization of the energy requirements has been performed since 2010 where 360 MJ/m2 and 3 years of EPBTDOI:10.1039/c1ee02728j, clearly having an unbalanced inventory with one component representing the majority of the equivalent primary energy. ProcessTwo uses a silver electrode, reaching the currently EPBT of ca. 90 days, possible for polymer solar cells that embed ca. 40 MJ/m2, with an efficiency of around 2% on flexible ITO-free modulesDOI:10.1039/c1ee02728j. Polymer solar cells potentially enable an EPBT of 1 day provided that higher feasible and challenging large-scale efficiencies are reached as seen on the right side in Figure 2.
When the cells are mounted in a solar park (see Figure 2 left) they only need roughly 300 days to pay back the energy. Or if mounted in plastic tubes that could be placed on shore or offshore, the EPBT is 50 days lower than the one in the solar parkDOI:10.1002/adma.201302031DOI:10.1039/C3EE43212B. These low values are possible thanks to a light installation, they do not need frame or heavy structures, the can be simply rolled out and attached to a wooden or a plastic structure.
The last task in the interpretation of the LCA results is the evaluation: to complete information, to check the consistency (results and methods) and to make sensibility studies and consider data uncertainties. LCA has proved as a good tool to make of OPV a competitive technology, and to anticipate solutions to the problems that might be created.