Sequence of annealing polymer photoactive layer influences the air stability of inverted solar cells

The sequence with which the polymer photoactive layer is annealed during inverted solar cell fabrication not only affects device characterization, it also influences the stability of devices dramatically. Devices in which the photoactive layer is annealed after top electrode deposition outlast devices in which the photoactive layer is annealed before top electrode deposition. This difference in device stability is traced to the grain structure and grain size of the top electrode, which in turn shows strong dependence on the underlying morphology of the photoactive layer. Annealing the photoactive layer causes significant roughening of the film; subsequent metal deposition results in electrodes with small grains. It is moisture penetration through the grain boundaries of the metal electrode that results in the deterioration of photovoltaic characteristics in devices in which the photoactive layer is annealed before top electrode deposition.