Self-Assembled Poly(ethylene glycol) Buffer Layers in Polymer Solar Cells: Toward Superior Stability and Efficiency

In this study, we have systematically investigated the mechanism behind the formation of nanoscale self-assembled polymer buffer layers at the cathode interfaces of polymer solar cells. Poly(ethylene glycol) (PEG) molecules in a polymer blend, comprising poly(3-hexylthiophene) and [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester, spontaneously migrated to the surface where they reacted with the Al cathode to form ohmic contacts. In terms of thermodynamics, the surface energy of the substrates played an important role in triggering the vertical-type morphology. From a kinetics point of view, PEG polymers having lower molecular weights readily underwent vertical phase separation prior to solidification of the polymer films, due to their higher mobilities, whereas PEG polymers of higher molecular weights tended to become trapped in the active layer. Employing this knowledge, we prepared organic photovoltaic cells exhibiting both high efficiency and appreciable improvement in stability.