Revealing the diffusion of aluminum in organic solar cells

It is well known that thermal annealing of region-regular poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells helps in improving their overall performance. When aluminum (Al) is used as the electrode, it has been found that annealing causes the Al atoms to diffuse into the organic layer and to form what is called a "mixed layer". In this work, we studied the diffusion of Al and using Fick's second law, calculated its diffusivity in a P3HT:PCBM heterojunctions layer. By employing energy dispersive X-ray spectroscopy elemental mapping, a diffusion constant of about 1.8 × 10−21 cm2 s−1 for devices annealed at 120 °C was obtained.