| Summary: | In contrast to p-dopants, highly efficient molecular n-dopants are prone to degradation in air due to their low ionization potentials, limiting the processing conditions of doped functional organic devices. In this contribution, we investigate the air-stability of pure films of the n-dopant tetrakis(1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato)ditungsten(II) (W<sub>2</sub>(hpp)<sub>4</sub>) and of C<sub>60</sub> layers doped by W<sub>2</sub>(hpp)<sub>4</sub>. We find that 1/3 of the initial conductivity of the doped C<sub>60</sub> thin films can be restored by thermal annealing in vacuum after a drop by 5 orders of magnitude upon air exposure. Furthermore, we show by ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements that the Fermi level shift toward the lowest unoccupied molecular orbital (LUMO) of C<sub>60</sub> remains after air exposure, clearly indicating a conservation of n-doping. We explain these findings by a down-shift of the W<sub>2</sub>(hpp)<sub>4</sub> energy levels upon charge-transfer to a host material with deeper lying energy-levels, facilitating a protection against oxidation in air. Consequently, the observed recovery of the conductivity can be understood in terms of a self-passivation of the molecular n-doping. Hence, an application of highly efficient n-doped thin films in functional organic devices handled even under ambient conditions during fabrication is feasible. © 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.
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