Fabrication of Polymer Solar Cells Using Aqueous Processing for All Layers Including the Metal Back Electrode

Abstract

Utilization of sunlight as an energy source is one of the least exploited carbon-neutral methods available today. The potential is enormous, with a wide range of possible applications such as large scale energy production, small standalone energy production units in remote areas situated ‘off the power grid’ or tiny power production units aimed at recharging small electronic equipment that we surround ourselves with. Over recent years polymer and organic solar cells have been perfected at the laboratory level and the performance now approaches many of the inorganic thin fi lm solar cells. It has been argued that the ∼ 8% power conversion effi ciency recently reported for polymer solar cells might challenge polycrystalline silicon when projecting the steady increase in polymer solar cell performance a few years into the future. Many challenges remain that have to be addressed effi ciently before the vision of large scale manufacture and widespread usage of low cost polymer solar cells can be anticipated. Ideally the polymer solar cell should be manufactured in a fast, large-area, environmentally friendly process. The methodologies employed in typical laboratory studies do not represent this well. The most commonly employed fi lm forming technique is spin-coating which is incompatible with large areas, large volume, and low cost. Another troublesome aspect is the use of toxic organic solvents in the fi lm-forming process. In the large-scale application where production volumes corresponding to several GW peak are envisaged this is not a viable approach and alternative solvents will be a requirement. Until now, solvent-free or environmentally friendly solvent processing have not been studied to any signifi cant level. An explanation for this can possibly be sought in the delicate interplay between the processing solvent and the performance of the solar cell. In many ways the state-ofthe-art polymer solar cell has evolved around aromatic solvents such as chlorobenzene, dichlorobenzene, toluene, and xylene. Any change of solvent adversely affects the nanomorphology of the fi lm and the device performance. The ambition to use more benign solvents would require a redesign of the molecular structures and a re-establishment of the interplay between nanomorphology and processing for the new material-solvent