Science 336, 327–332 (2012)

Organic solar cells and light-emitting diodes are typically based on electrodes with dissimilar work functions. Whereas the high work-function electrode enables hole injection or extraction, access to the lowest unoccupied molecular orbitals in the organic semiconductor layer and electron transport is mediated by the electrode with the lower work function. However, electrodes with low work functions are not easy to prepare. Suitable materials are often unstable, and surface functionalization schemes require tailored chemistry. Yinhua Zhou and colleagues now report a general surface modification scheme that can lower the work function of a range of electrode materials by more than 1 eV, facilitating electron injection. It is based on polyethylenimine-type polymers, which can modify energy-level alignment through charge transfer and intrinsic molecular dipoles. The polymers are solution processable and stable at conditions typical of roll-to-roll processing schemes. The researchers demonstrate the functionality of the modification layers in organic solar cells, diodes and n-type field-effect transistors, and find that they are stable for more than four weeks.