Cyanine dye polyelectrolytes for organic bilayer solar cells

Polyelectrolytes composed of an anionic sulfoethyl methacrylate/methacrylate backbone and cationic cyanine dyes counter ions are synthesized. The effective reaction sequence is confirmed by NMR spectroscopy and consists of a thermal radical copolymerization to obtain the polymer backbone (Mn ∼ 40 kg mol−1), followed by deprotonation of the sulfoethyl group and a salt metathesis reaction with elimination of silver halide to incorporate the dye. With increasing cyanine content (20–50% of polymer repeating units) polyelectrolytes become insoluble in apolar solvents. This allows the fabrication of simple solution-processed bilayer organic solar cells, where an electron acceptor fullerene layer is coated from the orthogonal solvent chlorobenzene onto an underlying polyelectrolyte electron donor layer. The optimization of solar cells is described, and photo-CELIV measurements are used to identify the low hole mobility in the polyelectrolyte layer as the main reason that currently limits solar cell performance to ∼0.9%.