N-doping of an electron-transport material, a tris(thieno)hexaazatriphenylene derivative (1), with the strongly reducing molecule bis(cyclopentadienyl)-cobalt(II) (cobaltocene, CoCp2), is investigated using ultra-violet, X-ray, and inverse photoemission spectroscopies, and current-voltage measurements. Condensed CoCp2 films show a 4 eV ionization energy, which is unusually low for vacuum-deposited molecular material and suggests that cobaltocene is promising as a molecular n-dopant. Efficient n-doping of 1 by CoCp2 is confirmed by a 0.56 eV shift of the Fermi level toward the unoccupied states of the host, and by a three orders of magnitude current increase in devices where compound I is interfacially doped with cobaltocene. (c) 2006 Elsevier B.V. All rights reserved.