Air-stable dimers of sandwich compounds including rhodocene and (pentamethylcyclopentadienyl)(arene) ruthenium and iron derivatives can be used for n-doping electron-transport materials with electron affinities as small as 2.8 eV. A p-i-n homojunction diode based on copper phthalocyanine and using rhodocene dimer as n-dopant shows a rectification ratio of greater than 106 at 4 V.