|
Low work-function electrodes are required for efficient electron injection into semiconductor devices. However, electrodes with work functions shallower than ca. 4 eV suffer from fundamental incompatibility with ambient processing. Here, we provide an elegent solution to resolve this paradox using latent electron donors. Based on di- and higher-valent anions, including oxalate, carbonate and sulfite, which are stable in their native solution state, they provide surprisingly shallow donor energies when isolated as dehydrated ion clusters in organic matrices to electron-dope shallow electron-affinity semiconductor cores and reach previously inaccessible work functions as small as 2.4 eV. These shallow anion donors thus provide universal n-doping of most semiconductors of interest from ambient processable formulations. We observe spontaneous electron transfer to the semiconductor core upon removal of the stabilizing water molecules, or upon photo sensitization of the semiconductor core, or its hole sensitization by injection from opposite contact, while solvation serendipitously protects the anion from from premature doping and destruction. Using the recently developed self-compensated, doped polymer platform ω-trimethylammonioalkyl-functionalized polyfluorene, and silver as cathode, we demonstrated doping of the electron injection layer to provide unprecedented ohmic electron injection into the shallow electron-affinity organic semiconductor benchmark, poly(9,9-bis(4-octylphenyl)fluorene-2,7-diyl). We also show that higher performance can be realized generally with these highly-doped electron injection/extraction layers in organic solar cells and white light-emitting diodes, opening new device design latitudes.
|
